Airfoilsand Lift
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The angleof incidence is measured by the angle at which the wing is attached to thefuselage.
An airfoil isa device which gets a useful reaction from air moving over its surface. When anairfoil is moved through the air, it is capable of producing lift. Wings,horizontal tail surfaces, vertical tails surfaces, and propellers are allexamples of airfoils.
Generally thewing of small aircraft will look like the cross-section of the figure above.The forward part of an airfoil is rounded and is called the leading edge. Theaft part is narrow and tapered and is called the trailing edge. A referenceline often used in discussing airfoils is the chord, an imaginary straight linejoining the extremities of the leading and trailing edges.
Angle ofIncidence: The angle of incidence is the angle formed by the longitudinal axis ofthe airplane and the chord of the wing. The longitudinal axis is an imaginaryline that extends lengthwise through the fuselage from nose to tail. The angleof incidence is measured by the angle at which the wing is attached to thefuselage. The angle of incidence is fixed --it normally cannot be changed bythe pilot. (An exceptionis the Vought F8U Crusader.)
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Bernoulli'sPrinciple: To understand how lift is produced, we must examine a phenomenondiscovered many years ago by the scientist Bernoulli and later calledBernoulli's Principle: The pressure of a fluid (liquid or gas) decreases atpoints where the speed of the fluid increases. In other words, Bernoulli foundthat within the same fluid, in this case air, high speed flow is associated withlow pressure, and low speed flow with high pressure. This principle was firstused to explain changes in the pressure of fluid flowing within a pipe whosecross-sectional area varied. In the wide section of the gradually narrowingpipe, the fluid moves at low speed, producing high pressure. As the pipenarrows it must contain the same amount of fluid. In this narrow section, thefluid moves at high speed, producing low pressure.
Animportant application of this phenomenon is made in giving lift to the wing ofan airplane, an airfoil. The airfoil is designed to increase the velocity ofthe airflow above its surface, thereby decreasing pressure above the airfoil.Simultaneously, the impact of the air on the lower surface of the airfoilincreases the pressure below. This combination of pressure decrease above andincrease below produces lift.
Lift:Probablyyou have held your flattened hand out of the window of a moving automobile. Asyou inclined your hand to the wind, the force of air pushed against it forcingyour hand to rise. The airfoil (in this case, your hand) was deflecting thewind which, in turn, created an equal and opposite dynamic pressure on thelower surface of the airfoil, forcing it up and back. The upward component ofthis force is lift; the backward component is drag.
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Pressureis reduced is due to the smaller space the air has above the wing than below.Air cannot go through the wing, so it must push around it. The surface airmolecules push between the wing and outer layers of air. Due to the bump of theairfoil, the space is smaller and the molecules must go faster. According to Bernoulli'sLaw, faster air has lower air pressure, and thus the high pressure beneath thewing pushes up to cause lift. HowAirplanes Fly: A Physical Description of Lift cDavid Anderson
Fermi NationalAccelerator Laboratory
Batavia IL60510
dfa@fnal.gov&Scott Eberhardt
Dept. ofAeronautics and Astronautics
University ofWashington
Seattle WA91895-2400
scott@aa.washington.edu
Almosteveryone today has flown in an airplane. Many ask the simple question«what makes an airplane fly»? The answer one frequently gets ismisleading and often just plain wrong. We hope that the answers provided herewill clarify many misconceptions about lift and that you will adopt ourexplanation when explaining lift to others. We are going to show you that liftis easier to understand if one starts with Newton rather than Bernoulli. Wewill also show you that the popular explanation that most of us were taught ismisleading at best and that lift is due to the wing diverting air down.
Letus start by defining three descriptions of lift commonly used in textbooks andtraining manuals. The first we will call the Mathematical AerodynamicsDescription which is used by aeronautical engineers. This description usescomplex mathematics and/or computer simulations to calculate the lift of awing. These are design tools which are powerful for computing lift but do notlend themselves to an intuitive understanding of flight.
Thesecond description we will call the Popular Explanation which is based on theBernoulli principle. The primary advantage of this description is that it iseasy to understand and has been taught for many years. Because of itssimplicity, it is used to describe lift in most flight training manuals. Themajor disadvantage is that it relies on the «principle of equal transittimes» which is wrong. This description focuses on the shape of the wingand prevents one from understanding such important phenomena as invertedflight, power, ground effect, and the dependence of lift on the angle of attackof the wing.
Thethird description, which we are advocating here, we will call the PhysicalDescription of lift. This description is based primarily on Newton’s laws. Thephysical description is useful for understanding flight, and is accessible toall who are curious. Little math is needed to yield an estimate of manyphenomena associated with flight. This description gives a clear, intuitiveunderstanding of such phenomena as the power curve, ground effect, andhigh-speed stalls. However, unlike the mathematical aerodynamics description,the physical description has no design or simulation capabilities. The popular explanation of lift
Students ofphysics and aerodynamics are taught that airplanes fly as a result ofBernoulli’s principle, which says that if air speeds up the pressure islowered. Thus a wing generates lift because the air goes faster over the topcreating a region of low pressure, and thus lift. This explanation usuallysatisfies the curious and few challenge the conclusions. Some may wonder whythe air goes faster over the top of the wing and this is where the popularexplanation of lift falls apart.
Inorder to explain why the air goes faster over the top of the wing, many haveresorted to the geometric argument that the distance the air must travel isdirectly related to its speed. The usual claim is that when the air separatesat the leading edge, the part that goes over the top must converge at thetrailing edge with the part that goes under the bottom. This is the so-called«principle of equal transit times».
Asdiscussed by Gail Craig (Stop Abusing Bernoulli! How Airplanes Really Fly,Regenerative Press, Anderson, Indiana, 1997), let us assume that this argumentwere true. The average speeds of the air over and under the wing are easilydetermined because we can measure the distances and thus the speeds can becalculated. From Bernoulli’s principle, we can then determine the pressureforces and thus lift. If we do a simple calculation we would find that in orderto generate the required lift for a typical small airplane, the distance overthe top of the wing must be about 50% longer than under the bottom. Figure 1shows what such an airfoil would look like. Now, imagine what a Boeing 747 wingwould have to look like!
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Fig1 Shape of wing predicted by principle of equal transit time.
Ifwe look at the wing of a typical small plane, which has a top surface that is1.5 — 2.5% longer than the bottom, we discover that a Cessna 172 would have tofly at over 400 mph to generate enough lift. Clearly, something in thisdescription of lift is flawed.
But,who says the separated air must meet at the trailing edge at the same time?Figure 2 shows the airflow over a wing in a simulated wind tunnel. In thesimulation, colored smoke is introduced periodically. One can see that the airthat goes over the top of the wing gets to the trailing edge considerablybefore the air that goes under the wing. In fact, close inspection shows thatthe air going under the wing is slowed down from the «free-stream»velocity of the air. Somuch for the principle of equal transit times.
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Fig2 Simulation of the airflow over a wing in a wind tunnel, with colored «smoke»to show the acceleration and deceleration of the air.
Thepopular explanation also implies that inverted flight is impossible. Itcertainly does not address acrobatic airplanes, with symmetric wings (the topand bottom surfaces are the same shape), or how a wing adjusts for the greatchanges in load such as when pulling out of a dive or in a steep turn.
So,why has the popular explanation prevailed for so long? One answer is that theBernoulli principle is easy to understand. There is nothing wrong with theBernoulli principle, or with the statement that the air goes faster over thetop of the wing. But, as the above discussion suggests, our understanding isnot complete with this explanation. The problem is that we are missing a vitalpiece when we apply Bernoulli’s principle. We can calculate the pressuresaround the wing if we know the speed of the air over and under the wing, buthow do we determine the speed?
Anotherfundamental shortcoming of the popular explanation is that it ignores the workthat is done. Lift requires power (which is work per time). As will be seenlater, an understanding of power is key to the understanding of many of theinteresting phenomena of lift. Newton’s laws and lift
So, how does awing generate lift? To begin to understand lift we must return to high schoolphysics and review Newton’s first and third laws. (We will introduce Newton’ssecond law a little later.) Newton’s first law states a body at rest willremain at rest, and a body in motion will continue in straight-line motionunless subjected to an external applied force. That means, if one sees abend in the flow of air, or if air originally at rest is accelerated intomotion, there is a force acting on it. Newton’s third law states that forevery action there is an equal and opposite reaction. As an example, anobject sitting on a table exerts a force on the table (its weight) and thetable puts an equal and opposite force on the object to hold it up. In order togenerate lift a wing must do something to the air. What the wing does to theair is the action while lift is the reaction.
Let’scompare two figures used to show streams of air (streamlines) over a wing. Infigure 3 the air comes straight at the wing, bends around it, and then leavesstraight behind the wing. We have all seen similar pictures, even in flightmanuals. But, the air leaves the wing exactly as it appeared ahead of the wing.There is no net action on the air so there can be no lift! Figure 4 shows thestreamlines, as they should be drawn. The air passes over the wing and is bentdown. The bending of the air is the action. The reaction is the lift on thewing.
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Fig3 Common depiction of airflow over a wing. This wing has no lift.
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Fig4 True airflow over a wing with lift, showing upwash and downwash.The wing as a pump
As Newton’slaws suggest, the wing must change something of the air to get lift. Changes inthe air’s momentum will result in forces on the wing. To generate lift a wingmust divert air down, lots of air.
Thelift of a wing is equal to the change in momentum of the air it diverts down.Momentum is the product of mass and velocity. The lift of a wing isproportional to the amount of air diverted down times the downward velocity ofthat air. Its that simple. (Here we have used an alternate form of Newton’ssecond law that relates the acceleration of an object to its mass and to theforce on it, F=ma) For more lift the wing can either divert more air (mass) orincrease its downward velocity. This downward velocity behind the wing iscalled «downwash». Figure 5 shows how the downwash appears to thepilot (or in a wind tunnel). The figure also shows how the downwash appears toan observer on the ground watching the wing go by. To the pilot the air iscoming off the wing at roughly the angle of attack. To the observer on theground, if he or she could see the air, it would be coming off the wing almostvertically. The greater the angle of attack, the greater the vertical velocity.Likewise, for the same angle of attack, the greater the speed of the wing thegreater the vertical velocity. Both the increase in the speed and the increaseof the angle of attack increase the length of the vertical arrow. It is thisvertical velocity that gives the wing lift.
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Fig5 How downwash appears to a pilot and to an observer on the ground.
Asstated, an observer on the ground would see the air going almost straight downbehind the plane. This can be demonstrated by observing the tight column of airbehind a propeller, a household fan, or under the rotors of a helicopter, allof which are rotating wings. If the air were coming off the blades at an anglethe air would produce a cone rather than a tight column. If a plane were to flyover a very large scale, the scale would register the weight of the plane.
Ifwe estimate the average vertical component of the downwash of a Cessna 172traveling at 110 knots to be about 9 knots, then to generate the needed 2,300lbs of lift the wing pumps a whopping 2.5 ton/sec of air! In fact, as will bediscussed later, this estimate may be as much as a factor of two too low. Theamount of air pumped down for a Boeing 747 to create lift for its roughly800,000 pounds takeoff weight is incredible indeed.
Pumping,or diverting, so much air down is a strong argument against lift being just asurface effect as implied by the popular explanation. In fact, in order to pump2.5 ton/sec the wing of the Cessna 172 must accelerate all of the air within 9feet above the wing. (Air weighs about 2 pounds per cubic yard at sea level.)Figure 6 illustrates the effect of the air being diverted down from a wing. Ahuge hole is punched through the fog by the downwash from the airplane that hasjust flown over it.
Sohow does a thin wing divert so much air? When the air is bent around the top ofthe wing, it pulls on the air above it accelerating that air down, otherwisethere would be voids in the air left above the wing. Air is pulled from aboveto prevent voids. This pulling causes the pressure to become lower above thewing. It is the acceleration of the air above the wing in the downwarddirection that gives lift. (Why the wing bends the air with enough force togenerate lift will be discussed in the next section.)
Asseen in figure 4, a complication in the picture of a wing is the effect of«upwash» at the leading edge of the wing. As the wing moves along,air is not only diverted down at the rear of the wing, but air is pulled up atthe leading edge. This upwash actually contributes to negative lift and moreair must be diverted down to compensate for it. This will be discussed laterwhen we consider ground effect.
Normally,one looks at the air flowing over the wing in the frame of reference of thewing. In other words, to the pilot the air is moving and the wing is standingstill. We have already stated that an observer on the ground would see the aircoming off the wing almost vertically. But what is the air doing above andbelow the wing? Figure 7 shows an instantaneous snapshot of how air molecules aremoving as a wing passes by. Remember in this figure the air is initially atrest and it is the wing moving. Ahead of the leading edge, air is moving up(upwash). At the trailing edge, air is diverted down (downwash). Over the topthe air is accelerated towards the trailing edge. Underneath, the air isaccelerated forward slightly, if at all.
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Fig7 Direction of air movement around a wing as seen by an observer on the ground.
Inthe mathematical aerodynamics description of lift this rotation of the airaround the wing gives rise to the «bound vortex» or«circulation» model. The advent of this model, and the complicatedmathematical manipulations associated with it, leads to the directunderstanding of forces on a wing. But, the mathematics required typicallytakes students in aerodynamics some time to master.
Oneobservation that can be made from figure 7 is that the top surface of the wingdoes much more to move the air than the bottom. So the top is the more criticalsurface. Thus, airplanes can carry external stores, such as drop tanks, underthe wings but not on top where they would interfere with lift. That is also whywing struts under the wing are common but struts on the top of the wing havebeen historically rare. A strut, or any obstruction, on the top of the wingwould interfere with the lift. Air has viscosity
The naturalquestion is «how does the wing divert the air down?» When a movingfluid, such as air or water, comes into contact with a curved surface it willtry to follow that surface. To demonstrate this effect, hold a water glasshorizontally under a faucet such that a small stream of water just touches theside of the glass. Instead of flowing straight down, the presence of the glasscauses the water to wrap around the glass as is shown in figure 8. Thistendency of fluids to follow a curved surface is known as the Coanda effect.From Newton’s first law we know that for the fluid to bend there must be a forceacting on it. From Newton’s third law we know that the fluid must put an equaland opposite force on the object that caused the fluid to bend.
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Fig8 Coanda effect.
Whyshould a fluid follow a curved surface? The answer is viscosity: the resistanceto flow which also gives the air a kind of «stickiness.» Viscosity inair is very small but it is enough for the air molecules to want to stick tothe surface. The relative velocity between the surface and the nearest airmolecules is exactly zero. (That is why one cannot hose the dust off of a carand why there is dust on the backside of the fans in a wind tunnel.) Just abovethe surface the fluid has some small velocity. The farther one goes from thesurface the faster the fluid is moving until the external velocity is reached(note that this occurs in less than an inch). Because the fluid near thesurface has a change in velocity, the fluid flow is bent towards the surface.Unless the bend is too tight, the fluid will follow the surface. This volume ofair around the wing that appears to be partially stuck to the wing is calledthe «boundary layer». Lift as a function of angle of attack
There are manytypes of wing: conventional, symmetric, conventional in inverted flight, theearly biplane wings that looked like warped boards, and even the proverbial«barn door.» In all cases, the wing is forcing the air down, or moreaccurately pulling air down from above. What all of these wings have in commonis an angle of attack with respect to the oncoming air. It is this angle ofattack that is the primary parameter in determining lift. The lift of theinverted wing can be explained by its angle of attack, despite the apparentcontradiction with the popular explanation involving the Bernoulli principle. Apilot adjusts the angle of attack to adjust the lift for the speed and load.The popular explanation of lift which focuses on the shape of the wing givesthe pilot only the speed to adjust.
Tobetter understand the role of the angle of attack it is useful to introduce an«effective» angle of attack, defined such that the angle of the wingto the oncoming air that gives zero lift is defined to be zero degrees. If onethen changes the angle of attack both up and down one finds that the lift isproportional to the angle. Figure 9 shows the coefficient of lift (liftnormalized for the size of the wing) for a typical wing as a function of theeffective angle of attack. A similar lift versus angle of attack relationshipis found for all wings, independent of their design. This is true for the wingof a 747 or a barn door. The role of the angle of attack is more important thanthe details of the airfoil’s shape in understanding lift.
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Fig9 Coefficient of lift versus the effective angle of attack.
Typically,the lift begins to decrease at an angle of attack of about 15 degrees. Theforces necessary to bend the air to such a steep angle are greater than theviscosity of the air will support, and the air begins to separate from thewing. This separation of the airflow from the top of the wing is a stall. The wing as air «scoop»
We now wouldlike to introduce a new mental image of a wing. One is used to thinking of awing as a thin blade that slices though the air and develops lift somewhat bymagic. The new image that we would like you to adopt is that of the wing as ascoop diverting a certain amount of air from the horizontal to roughly theangle of attack, as depicted in figure 10. The scoop can be pictured as aninvisible structure put on the wing at the factory. The length of the scoop isequal to the length of the wing and the height is somewhat related to the chordlength (distance from the leading edge of the wing to the trailing edge). Theamount of air intercepted by this scoop is proportional to the speed of theplane and the density of the air, and nothing else.
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Fig10 The wing as a scoop.
Asstated before, the lift of a wing is proportional to the amount of air diverteddown times the vertical velocity of that air. As a plane increases speed, thescoop diverts more air. Since the load on the wing, which is the weight of theplane, does not increase the vertical speed of the diverted air must bedecreased proportionately. Thus, the angle of attack is reduced to maintain aconstant lift. When the plane goes higher, the air becomes less dense so thescoop diverts less air for the same speed. Thus, to compensate the angle ofattack must be increased. The concepts of this section will be used tounderstand lift in a way not possible with the popular explanation. Lift requires power
When a planepasses overhead the formerly still air ends up with a downward velocity. Thus,the air is left in motion after the plane leaves. The air has been givenenergy. Power is energy, or work, per time. So, lift must require power. Thispower is supplied by the airplane’s engine (or by gravity and thermals for asailplane).
Howmuch power will we need to fly? The power needed for lift is the work (energy)per unit time and so is proportional to the amount of air diverted down timesthe velocity squared of that diverted air. We have already stated that the liftof a wing is proportional to the amount of air diverted down times the downwardvelocity of that air. Thus, the power needed to lift the airplane isproportional to the load (or weight) times the vertical velocity of the air.If the speed of the plane is doubled the amount of air diverted down doubles.Thus the angle of attack must be reduced to give a vertical velocity that ishalf the original to give the same lift. The power required for lift has beencut in half. This shows that the power required for lift becomes less as theairplane's speed increases. In fact, we have shown that this power to createlift is proportional to one over the speed of the plane.
But,we all know that to go faster (in cruise) we must apply more power. So theremust be more to power than the power required for lift. The power associatedwith lift, described above, is often called the «induced» power.Power is also needed to overcome what is called «parasitic» drag,which is the drag associated with moving the wheels, struts, antenna, etc.through the air. The energy the airplane imparts to an air molecule on impactis proportional to the speed squared. The number of molecules struck per timeis proportional to the speed. Thus the parasitic power required to overcomeparasitic drag increases as the speed cubed.
Figure11 shows the power curves for induced power, parasitic power, and total powerwhich is the sum of induced power and parasitic power. Again, the induced powergoes as one over the speed and the parasitic power goes as the speed cubed. Atlow speed the power requirements of flight are dominated by the induced power.The slower one flies the less air is diverted and thus the angle of attack mustbe increased to maintain lift. Pilots practice flying on the «backside ofthe power curve» so that they recognize that the angle of attack and thepower required to stay in the air at very low speeds are considerable.
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Fig11 Power requirements versus speed.
Atcruise, the power requirement is dominated by parasitic power. Since this goesas the speed cubed an increase in engine size gives one a faster rate of climbbut does little to improve the cruise speed of the plane.
Sincewe now know how the power requirements vary with speed, we can understand drag,which is a force. Drag is simply power divided by speed. Figure 12 shows theinduced, parasitic, and total drag as a function of speed. Here the induceddrag varies as one over speed squared and parasitic drag varies as the speedsquared. Taking a look at these curves one can deduce a few things about howairplanes are designed. Slower airplanes, such as gliders, are designed tominimize induced drag (or induced power), which dominates at lower speeds. Fasterairplanes are more concerned with parasitic drag (or parasitic power).
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Fig12 Drag versus speed.Wing efficiency
At cruise, anon-negligible amount of the drag of a modern wing is induced drag. Parasiticdrag, which dominates at cruise, of a Boeing 747 wing is only equivalent tothat of a 1/2-inch cable of the same length. One might ask what affects theefficiency of a wing. We saw that the induced power of a wing is proportionalto the vertical velocity of the air. If the length of a wing were to bedoubled, the size of our scoop would also double, diverting twice as much air.So, for the same lift the vertical velocity (and thus the angle of attack)would have to be halved. Since the induced power is proportional to thevertical velocity of the air, it too is reduced by half. Thus, the liftingefficiency of a wing is proportional to one over the length of the wing. Thelonger the wing the less induced power required to produce the same lift,though this is achieved with an increase in parasitic drag. Low speed airplanesare affected more by induced drag than fast airplanes and so have longer wings.That is why sailplanes, which fly at low speeds, have such long wings.High-speed fighters, on the other hand, feel the effects of parasitic drag morethan our low speed trainers. Therefore, fast airplanes have shorter wings tolower parasite drag.
Thereis a misconception held by some that lift does not require power. This comesfrom aeronautics in the study of the idealized theory of wing sections(airfoils). When dealing with an airfoil, the picture is actually that of awing with infinite span. Since we have seen that the power necessary for liftis proportional to one over the length of the wing, a wing of infinite spandoes not require power for lift. If lift did not require power airplanes wouldhave the same range full as they do empty, and helicopters could hover at anyaltitude and load. Best of all, propellers (which are rotating wings) would notrequire power to produce thrust. Unfortunately, we live in the real world whereboth lift and propulsion require power. Power and wing loading
Let us nowconsider the relationship between wing loading and power. Does it take morepower to fly more passengers and cargo? And, does loading affect stall speed?At a constant speed, if the wing loading is increased the vertical velocitymust be increased to compensate. This is done by increasing the angle ofattack. If the total weight of the airplane were doubled (say, in a 2-g turn)the vertical velocity of the air is doubled to compensate for the increasedwing loading. The induced power is proportional to the load times the verticalvelocity of the diverted air, which have both doubled. Thus the induced powerrequirement has increased by a factor of four! The same thing would be true ifthe airplane’s weight were doubled by adding more fuel, etc.
Oneway to measure the total power is to look at the rate of fuel consumption.Figure 13 shows the fuel consumption versus gross weight for a large transportairplane traveling at a constant speed (obtained from actual data). Since thespeed is constant the change in fuel consumption is due to the change ininduced power. The data are fitted by a constant (parasitic power) and a termthat goes as the load squared. This second term is just what was predicted inour Newtonian discussion of the effect of load on induced power.
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Fig13 Fuel consumption versus load for a large transport airplane traveling at aconstant speed.
Theincrease in the angle of attack with increased load has a downside other thanjust the need for more power. As shown in figure 9 a wing will eventually stallwhen the air can no longer follow the upper surface, that is, when the criticalangle is reached. Figure 14 shows the angle of attack as a function of airspeedfor a fixed load and for a 2-g turn. The angle of attack at which the planestalls is constant and is not a function of wing loading. The stall speedincreases as the square root of the load. Thus, increasing the load in a 2-gturn increases the speed at which the wing will stall by 40%. An increase inaltitude will further increase the angle of attack in a 2-g turn. This is whypilots practice «accelerated stalls» which illustrate that anairplane can stall at any speed. For any speed there is a load that will inducea stall.
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Fig14 Angle of attack versus speed for straight and level flight and for a 2-gturn.Wing vortices
One might askwhat the downwash from a wing looks like. The downwash comes off the wing as asheet and is related to the details of the load distribution on the wing.Figure 15 shows, through condensation, the distribution of lift on an airplaneduring a high-g maneuver. From the figure one can see that the distribution ofload changes from the root of the wing to the tip. Thus, the amount of air inthe downwash must also change along the wing. The wing near the root is«scooping» up much more air than the tip. Since the root is divertingso much air the net effect is that the downwash sheet will begin to curl outwardaround itself, just as the air bends around the top of the wing because of thechange in the velocity of the air. This is the wing vortex. The tightness ofthe curling of the wing vortex is proportional to the rate of change in liftalong the wing. At the wing tip the lift must rapidly become zero causing thetightest curl. This is the wing tip vortex and is just a small (though oftenmost visible) part of the wing vortex. Returning to figure 6 one can clearlysee the development of the wing vortices in the downwash as well as the wingtip vortices.
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Fig15 Condensation showing the distribution of lift along a wing. The wingtipvortices are also seen. (from Patterns in the Sky, J.F. Campbell and J.R.Chambers, NASA SP-514.)
Winglets(those small vertical extensions on the tips of some wings) are used to improvethe efficiency of the wing by increasing the effective length of the wing. Thelift of a normal wing must go to zero at the tip because the bottom and the topcommunicate around the end. The winglets blocks this communication so the liftcan extend farther out on the wing. Since the efficiency of a wing increaseswith length, this gives increased efficiency. One caveat is that winglet designis tricky and winglets can actually be detrimental if not properly designed. Ground effect
Another commonphenomenon that is misunderstood is that of ground effect. That is theincreased efficiency of a wing when flying within a wing length of the ground.A low-wing airplane will experience a reduction in drag by 50% just before ittouches down. There is a great deal of confusion about ground effect. Manypilots (and the FAA VFR Exam-O-Gram No. 47) mistakenly believe that groundeffect is the result of air being compressed between the wing and the ground.
Tounderstand ground effect it is necessary to have an understanding of upwash.For the pressures involved in low speed flight, air is considered to benon-compressible. When the air is accelerated over the top of the wing anddown, it must be replaced. So some air must shift around the wing (below andforward, and then up) to compensate, similar to the flow of water around acanoe paddle when rowing. This is the cause of upwash.
Asstated earlier, upwash is accelerating air in the wrong direction for lift.Thus a greater amount of downwash is necessary to compensate for the upwash aswell as to provide the necessary lift. Thus more work is done and more powerrequired. Near the ground the upwash is reduced because the ground inhibits thecirculation of the air under the wing. So less downwash is necessary to providethe lift. The angle of attack is reduced and so is the induced power, makingthe wing more efficient.
Earlier,we estimated that a Cessna 172 flying at 110 knots must divert about 2.5ton/sec to provide lift. In our calculations we neglected the upwash. From themagnitude of ground effect, it is clear that the amount of air diverted isprobably more like 5 ton/sec. Conclusions
Let us reviewwhat we have learned and get some idea of how the physical description hasgiven us a greater ability to understand flight. First what have we learned:
· Theamount of air diverted by the wing is proportional to the speed of the wing andthe air density.
· Thevertical velocity of the diverted air is proportional to the speed of the wingand the angle of attack.
· Thelift is proportional to the amount of air diverted times the vertical velocityof the air.
· Thepower needed for lift is proportional to the lift times the vertical velocityof the air.
Now let uslook at some situations from the physical point of view and from theperspective of the popular explanation.
· Theplane’s speed is reduced. The physical view says that the amount of airdiverted is reduced so the angle of attack is increased to compensate. The powerneeded for lift is also increased. The popular explanation cannot address this.
· Theload of the plane is increased. The physical view says that the amount of airdiverted is the same but the angle of attack must be increased to giveadditional lift. The power needed for lift has also increased. Again, thepopular explanation cannot address this.
· Aplane flies upside down. The physical view has no problem with this. The planeadjusts the angle of attack of the inverted wing to give the desired lift. The popular explanation implies thatinverted flight is impossible.
As one cansee, the popular explanation, which fixates on the shape of the wing, maysatisfy many but it does not give one the tools to really understand flight.The physical description of lift is easy to understand and much more powerful. Axisof Rotation
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Axis of anAirplane in Flight.
An airplanemay turn about three axes. Whenever the attitude of the airplane changes inflight (with respect to the ground or other fixed object), it will rotate aboutone or more of these axes. Think of these axes as imaginary axles around whichthe airplane turns like a wheel. The three axes intersect at the center ofgravity and each one is perpendicular to the other two.
LongitudinalAxis: Theimaginary line that extends lengthwise through the fuselage, from nose to tail,is the longitudinal axis. Motion about the longitudinal axis is roll and isproduced by movement of the ailerons located at the trailing edges of thewings.
LateralAxis: Theimaginary line which extends crosswise, wing tip to wing tip, is the lateralaxis. Motion about the lateral axis is pitch and is produced by movement of theelevators at the rear of the horizontal tail assembly.
VerticalAxis: Theimaginary line which passes vertically through the center of gravity is the verticalaxis. Motion about the vertical axis is yaw and is produced by movement of therudder located at the rear of the vertical tail assembly. DETAILSOF MODERN AIRSHIPS — 1927
Advantagesof Rigid Type Airships--Airship Frame Construction--Large AirshipsProjected--Army Non-rigid Dirigibles--Requirements of Airships for CivilianFlying.
Advantagesof Rigid Type Airship. Before describing typical lighter- than-air craft or airshipsthat have received actual commercial as well as military usage, it may be wellto briefly review some of the advantages of the rigid type, which is the onethat lends itself most easily to large structures and which is also the safestof the three types we have previously reviewed in Chapter II which is devotedto a consideration of the elementary principles underlying airship constructionand application. Rigid airships have made longer single flights than othertypes and have flown more hours and miles without refueling than any otherform. The rigid airship is said to be the fastest large vehicle oftransportation that engineering ability of man has yet evolved. The NavyAirship Los Angeles, shown near the mooring mast at Lakehurst, N. J. to whichit may be anchored is depicted at Fig. 315. A design of the new 6,500,000 cubicfoot capacity ship recently authorized by Congress is shown at Fig. 316 flyingover a battleship at an elevation of about 1,500 feet. The rigid airship, owingto its large size and light weight can carry more load than any other type ofaircraft. It is independent of topography as oceans and continents are butareas to fly over. Land vehicles must stop when they reach water, watertransport must stop when the ship is docked.
AirshipFrame Construction. The rigid airship, because of its bulkhead system, in whichthe lifting gas is carried in 16 to 20 cells, has a much greater safety factorthan the types in which the gas is carried in only one or two containers. Inevent of damage to one or two cells, the ship can continue its journey andrepairs can be made to a leaky gas cell while in flight.
Therigid ship has a complete metal framework. Girders extend from nose to tail, orin nautical parlance, from stem to stern. Ring girders set at intervals bracethe longitudinals and are themselves internally reinforced by cross girders andtension wire bracing. The entire framework is enclosed by a network of wiringand the whole is streamlined or faired to minimize air resistance with a fabriccovering.
Theview of the crew's quarters on the Bodensee, a German air liner at Fig. 317,shows the triangular keel member with the cat-walk by which the crew can travelfrom one end of the ship to the other and gain access to the different gasbags. The character of the longitudinal duralumin girders and the way they arebraced by the ring girders is clearly shown at Fig. 318. This depicts thatportion of the hull where one set of fuel tanks are located. The view at Fig.319 shows the interior with the deflated gas cells hanging from the top-mostlongitudinal ready for inflation.
Theouter skin is in place and the large size and extreme lightness of thestructure is clearly shown. The passenger cabin of the Deutschland, anotherrigid dirigible of the Zepellin series is shown at Fig. 320. Wicker chairs areused because of their light weight and the interior structure of the cabin canbe determined by study of the illustration.
Thecontrol of a Zepellin type airship is not as simple as that of an airplane andno one man is at the controls. Special controls are provided for the elevatorsand still another set for the vertical rudders. The elevator control of the L59with the instruments for altitude navigation is shown at Fig. 321. Control isby a large wheel similar to the steering wheel of a ship. Directional controlis by a similar wheel at another part of the control car.
LargeAirship Projected. The largest of the United States Navy airships, theShenandoah was 600 feet long with a capacity of 2,115,000 cubic feet. Theprojected airship designed by the engineers of the Goodyear- Zepellin Company,while it has over three times the capacity of the Shenandoah will be only 100feet longer and will be of such size that it may be housed in the Lakehursthangar. The illustration at Fig. 322 shows how the new ships authorized bycongress will compare with the Shenandoah. The control car will be built into thehull and streamlined. Engines of 4,800 horsepower, giving a speed of 90 milesper hour with fuel for from 5,000 to 8,000 miles will drive the ship. The airscrews will be fitted in tilting mountings, which will turn in a 90 degree arcto help force the ship upward or downward as desired and greatly aid incontrolling the huge vessel.
Itwill embody the proved structural advantages of some 135 ships built in thepast.
(a)Multiple gas cells which function like bulk-heading on a steamship, so that ifone or more cells fail the ship will still remain aloft: (b) The triple coversystem, one cover to hold the lifting gas, one consisting of the shape-formingduralumin frame-work, and an outer cover to shed rain and snow, to reflectrather than to absorb heat, and to present a fair surface; (c) invulnerabilityagainst lightning; (d) accessibility to inspection and repair.
Itwill however present certain new features as well of far reaching importance:(a) A double or triple keel giving added longitudinal strength comparable tothe breaking strength of one length of metal, as against two or three boltedtogether; (b) a new type of ring girder each internally braced and structurallyself sufficient, which (c) will permit the control car and even the power carsto be built within the hull; (d) even fuller accessiblity to continuousinspection and permitting repairs to be made even in flight; (e) the use of newfuels to conserve helium and reduce weight. Army Non-Rigid Dirigibles. The non-rigid dirigible is the smallest of thethree types as the largest now being built in the United States for the Armyand Navy service have a gas capacity of about one-tenth that of the LosAngeles. Under ordinary conditions a 230,000 cubic foot non-rigid has acruising radius of from 500 to 1,000 miles and an air endurance of from 18 to24 hours. Such airships are essentially motorized free balloons and the enginesare carried in a car attached to the lower side or bottom of the bag. ThePilgrim, a small non-rigid previously described with a gas capacity of 50,000cubic feet has a speed of 50 miles per hour and is propelled by a Wright«Gale» three-cylinder engine as shown at Fig. 323. This small shipwas built to carry four passengers. The gas in non-rigid ships, as in the armyTC types, as shown at Fig. 324 is contained in a single bag, but an inner twocompartment bag, called the ballonet, is filled with air to keep the maincontainer properly distended because the air pressure can be made to compensatefor variations in gas pressure in the bag. These ships have a capacity of about200,000 cubic feet, are 196 feet long overall and 47 feet in extreme height.The hull diameter is 33.5 feet. The fineness ratio is 4.4 to 1. The total liftis 11,584 pounds of which the useful lift is about 4,000 pounds. The grossweight per horsepower is 38.6 pounds. Two Wright Type I water-cooled engines of150 horsepower each were provided on the first ships of this series but thesehave been replaced on later types with two Wright J1 engines, which are nine-cylinderradial air-cooled types driving tractor propellers 9 feet 10 inches indiameter. It is claimed that the saving of 400 pounds over the water-cooledinstallation permits an increase of speed from 54 to 60 miles per hour; with anincrease in range of 10 per cent.
FlightControl Surfaces — Elevons
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Deltawinged aircraft use elevons as primary flight controls for
roll andpitch. />
Elevon:Deltawinged aircraft can not use conventional 3 axis flight control systems becauseof their unique delta shape. Therefore, it uses a device called an elevon. Itis a combination of ailerons and elevators.
Theelevon is used as an aileron. Ailerons control motion along the longitudinalaxis. The longitudinal axis is an imaginary line that runs from the nose to thetail. Motion about the longitudinal axis is called roll.
Theelevon is also used as an elevator. Elevators control motion along the lateralaxis. The lateral axis is an imaginary line that extends crosswise, fromwingtip to wingtip. Motionabout the lateral axis is called pitch. Fokker DR.I — Thoughts on Wing Failures
by Mike Tate
© 2000
The recent WWI AERO article (#165, Aug, 1999) concerning wing failures in the Nieuport 28 prompted me to put some ideas to paper, regarding those more familiar failures of the Fokker triplane.
The reputation of Fokker aircraft for fragility was mainly the result of structural problems with the Dr.I triplane and D.VIII cantilever monoplane. The D.VIII wing problem was due to flexural failure (ie, they broke in bending); and the evidence indicates that this was due to production quality-control inadequacies rather than deficiencies of design or technical understanding. The Dr.I, however, is a different «kettle of fish» in that it experienced failures very like those of the Nieuport 28, namely that of «wing stripping.» Unlike the D.VIII, the triplane was grounded not because of spar failure, but because of the disintegration of the secondary structure- wing ribs etc- whilst the spars remained intact. The similarity of the failures in the N28 and Dr.I is intriguing because the 2 aircraft are fundamentally different: one a biplane of almost sesquiplane proportions, the other a triplane of equal-chord wings. The N28 had thin-section wings, wire-braced; the Dr.I had thick sections and a cantilever structure very different animals. For me, the most interesting fact of all (and the most difficult to explain) has been that the failures always occurred in the upper wings of either aircraft — to my knowledge there are no reported incidents of failures in the lower planes.
In the case of the 2 most notable triplane failures, the extent of the upper wingstripping was almost total, with fatal consequences for Lieutenants Gontermann and Pastor. It is of particular interest that, after the triplane was reissued with modified wings, the same type of failure still occurred — but to a more limited (and survivable) extent.
At the time of the Dr.I grounding, after the 2 crashes mentioned, various theories were proposed to account for the failures. Sand loading of the Fokker F5 (the Dr.I prototype) had shown that the triplane cantilever wing cellule had excellent strength for its period; and it fell to those interested to create new (and unlikely) aerodynamic phenomena to account for the fatal discrepancy between experiment and practice. Because the ailerons of both Gontermann's and Pastor's aircraft were seen to detach, interest centered on the aileron supporting-structure and related internal componentry.
Various reinforcements were introduced, and emphasis was placed on better internal protection of the glued structure by varnishing. (The peculiarity of upper wing failures had not, of course, gone unnoticed at the time. The possibility of the casein glue deteriorating, due to weathering, gave cause for concern — the lower wings being considered to be somewhat protected — debatable, of course.) Also poor workmanship was extensively uncovered in grounded aircraft and Fokker was urged to improve on this aspect of his production of further aircraft. However, as noted, failures continued to occur in the reissued aircraft.
In the case ofthe Nieuport 28, the fabric of the upper-wing top-surface together with theentire leading edge would detach. On this aircraft, however, damage appears tohave been selflimiting at this point: the rib tails and undersurface, forinstance, always seem to have held up. This is just as well for the pilotsconcerned, since the (almost) sesquiplane proportions of the N28 could not havetolerated complete loss of the upper lifting area. Fortunately, the Nieuportcarried its ailerons on the lower plane so that roll control was available — nodoubt this helped survivability.
Of all WWIaircraft, these 2 are the only ones I am aware of that suffered this type offailure as a generic fault. «Ballooning» of wing fabric was a knownrisk resulting from wing leading-edge damage. Wings failed simply through lackof strength. Wings failed due to a lack of stiffness. (True sesquiplanes-V-strutters, notably other Nieuport and Albatros models- are known to haveoccasionally lost a lower plane due to a lack of torsional stiffness) — butwingstripping seems mainly recorded for the 2 models in question. Since wingreinforcement better weather protection and better-built quality did not fullycure the triplane ills, then there was another factor at work. So what was it?
I began bylooking for a common factor. What is it that both aircraft possess which cancause almost identical failure in a wing- and why only the top plane? There arein fact, 2 unusual structural features present in both. Firstly, the main sparsare very closely spaced so that the rib noses project unusually far forward ofthe spar group. The N28 spars are closely spaced, but maintain an orthodoxdrag-bracing arrangement of steel tube and piano wire. The Dr.I located thespars with a small separation, so that plywood closing-skins top and bottomformed a single-spar system, accounting for both drag and to a limited extent,torsion.
The othercritical feature present in both aircraft was the use of a plywood leading-edgecontour panel. This was relatively unusual in WWI. British aircraft seem not tohave used it at all, preferring intermediate riblets as leading-edge support;and from a quick appraisal of my library, I have identified only 5 aircraftwhich had this feature (I don't suppose this to be at a definitive.). These arethe Pfalz D.XII Fokkers Dr.I, D.VI and D.VII, and the Nieuport 28 (possiblyalso the 27).
Some aircraftwings were, of course, totally skinned in sheet plywood or aluminum; but withthese exceptions, at least, complete fabric cover was the norm. The use ofplywood leading-edge covering presents a problem in the attachment of fabricsince stringing (ie, the through-wing stitching normally used) would berequired to stop at the plywood-covered surface. This may account for the factthat both the triplane and N28 are reported as originally having the fabrictacked to the rib flanges rather than being sewn (which was considered to bethe correct way). The fabric attachment itself is therefore suspect but thetest still remains; why only failures of the upper wing? If the fabricattachment was the critical factor, then failures could have occurred in anywing with this feature, which would have included lower planes of both thetriplane and the N28.
Both aircrafthave structurally suspect features in their wing leading-edges. In the case ofthe N28, the long rib-noses would produce large bending stresses (duringviolent manoeuvres) at their main-spar attachment locations. Large bendingstresses can have attendant large shear stresses; and on the N28, these wouldexist in the thin poplar rib-webs (typical of the period). This is a very riskyarrangement, since timber is not particularly strong when subject to shearloading along the grain — plywood is much better. (The N28 rib-noses had verylittle shear material anyway)
The othersuspect feature is that of the omission of rib-capping referred to in therecent WWI AERO article. These details appear peculiar to the N28, and are atthe most extreme in the upper wing. There is little doubt that the upper wingleading edge was simply of marginal strength; and at first sight it seems odd thatsandloading did not reveal this weakness. But of course this reveals a weaknessof sand-loading. The chordwise distribution of lift, at high angles of attack,will not normally be represented by a heap of sand, since dry sand slumps toapproximately 45 deg- forming a triangular load distribution with acentrally-located center of gravity. (This can be modified within limits byconstructing walls along the wing edges.) Sandloading therefore successfullytests the wingspar adequacy, but is insufficient to the task of testing the ribnose strength (and remember that here we have 2 aircraft which resolutely heldon to their spars, whilst liberally shedding secondary structure). Thisproof-loading problem is exacerbated by the fact that wing lift (particularlyat large angles of attack) is largely generated by the negative pressure zoneexisting on the forward upper surface (see Fig 18- taken from SIMPLEAERODYNAMICS (1929), by Charles N Monteith.).
The criticalstructural requirement under these loading conditions is to have adequate«peel» strength between the upper skin and the substructure (ribsand/or stringers etc). Both the N28 and the Dr.I were deficient here. TheNieuport was devoid of rib cap-strips or spanwise stringers at the criticallocation; the Dr.I leading-edge plywood was severely cut away at each rib, hadno supporting stringers, and had only minor connection to the main spar. Withthis arrangement, a significant amount of the local lift- would have beentransmitted in a peel condition from the plywood skin to the supporting ribs — there was no other load-path. Again, this is a very unreliable form of joint.Today, the attachment of wing skins to substructure remains a critical factor;in fact, where fuel is carried inside a wing much of the wing design isoverridingly determined by this consideration.
So, theNieuport had a weak upper-wing leading edge and larger chord to boot. Thiscould (as suggested in the WWI AERO article) be the complete answer to the N28failures. But the Triplane had the same design condition on all wings, but onlythe top wing ever failed. So there was something else.
It is notcommon to see a biplane or triplane wing cellule in which equal-chord wings areof differing span, although some famous aircraft such as the BE- 12, RE-8 andCurtiss Jenny are exceptions. Typically, where an upper wing is of greaterspan, it is often of greater chord also. This has the virtue of approximatelymaintaining constant aspect-ratio for each wing in the complete wing system.(To what extent this represented a design objective at the time I have noinformation.)
The fact thatreal wings are of finite span (as opposed to the theoretically infinite spanwing which is implicit in aerofoil section data) means that a real wing willattain a particular lift coefficient at an angle of attack somewhat greaterthan that apparent from he section-data. It also follows that wings ofdiffering aspect ratio, but identical section, will generate differentlift-intensities, to one another, when operating at the same angle of attack.
The Dr.1 hadaspect ratios of 6.8, 5.9 and 5.1 for the upper, middle and lower planesrespectively. The wing section (tested as the Gottingen 289 section after the war)had a maximum lift coefficient of about 1.4. Making estimates for each of thetriplane wings (working as independent surfaces), the planes would require19.2, 20 and 21 degrees respectively to reach the maximum lift coefficient.When working at the same angle of attack (as in the aircraft alignment), theupper wing would produce a lift intensity about 9% greater than the lower wing.So could aspect-ratio be the cause of the Triplane wing failures? Well no, I amafraid not. A 9% increased lift intensity cannot be considered sufficient toalways fail the upper wing before one or the other planes. Variations inmaterial strength and build quality would both have similar (or greater)tolerance, which would occasionally bias the failure to one of the other planes.There has to be something else – something more emphatic.
I found theanswer by chance, and I found it in a ‘history’ book. Whilst flipping through acopy of SIMPLE AERODYNAMICS (1929), by Charles N Monteith, (Chief Engineer,Boeing), looking for data on the Gottingen 289 section, I came across aparticularly relevant passage under Item 70, p89, “Pressure distribution testson MB-3A Airplane”, which is reproduced in facsimile here:
Paragraphs Band C are telling. The loading distribution noted is very significant over thebiplane system described. A factor of 1.6 at high-lift coefficients cannot beignored. The Triplane system with its relatively smaller wing gaps andpronounced stagger would almost certainly have a greater value than this.Together with aspect-ratio effects it is not unreasonable to suggest that thelift intensity of the upper wing of the Dr.I approached twice that of thebottom wing. This is certainly enough to test the upper wing integrity beforethe rest of the system.
Conclusion
I wouldsuggest that the Dr.I wing failures (and almost certainly those of the N28,too) occurred because lift-grading (particularly), together with aspect-ratioeffects, caused the upper surface of the upper wing to be subject to muchgreater lift intensity than the rest of the system. This tested a leading-edgedesign of marginal strength, poorly made, to the point of collapse inparticular aircraft. The leading edge failure continued back across the wingdue to design details. Where rib tails, for example, were connected by a wiretrailing edge, ballooning fabric will exert tensile loading in this wire whichwill then tend to «gather up» the rib tails and strip the wing. Thiswould also destabilize the area of the aileron support structures, and so on.The strengthening of the wing aft of the spars and the improvements to buildquality, carried out after the original failures, would have acted to preventthis catastrophic failure. But the root cause of the failure lift-grading) wentunappreciated until after the war when investigations like those at NACA wereconducted.
It would befascinating to know to what extent these factors were understood prior to 1918.I expect that the concentration of lift forces (as an intense negative pressurezone at the upper surface LE) was reasonably well appreciated by wind-tunnelinvestigators- if only by the application of Bernoulli's theorem to the visibleflow patterns around test sections. Probably the effects of aspect ratio wereunderstood- even if only qualitatively; but lift-grading would require muchmore complex investigation. Regarding the aspect-ratio issue; advocates ofmultiplanes (Horatio Phillips, for example) appear to have worked from theunderstanding that high aspect-ratio is a «good thing» (true) but notto have had evidence of the detrimental effects of interference betweenclosely-spaced multi-plane wing systems.
But such isthe nature of progress — the testing of ideas. It took the lives of airmen todrive the investigations which led to today's understanding of these mattersand which allow our complacent and sometimes arrogant review of history.
A finalthought. It is theoretically possible for the Fokker triplane to remainairborne on its 2 lower planes alone (of 9.9 square metres area). The stallspeed would be about 64mph. No doubt, when both Gontermann and Pastor foundthemselves in dire straits, they did the natural thing: to pull back on thestick even though the aircraft was deeply stalled. Maybe if they had first pushed… ?Forces Acting on an Airplane
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Theairplane in straight-and-level unaccelerated flight is acted on by four forces.The four forces are lift, gravity, thrust and drag.
The airplanein straight-and-level unaccelerated flight is acted on by four forces--lift,the upward acting force; weight, or gravity, the downward acting force; thrust,the forward acting force; and drag, the backward acting, or retarding force ofwind resistance.
Liftopposes gravity.
Thrustopposes drag.
Drag andweight are forces inherent in anything lifted from the earth and moved throughthe air. Thrust and lift are artificially created forces used to overcome theforces of nature and enable an airplane to fly. The engine and propellercombination is designed to produce thrust to overcome drag. The wing isdesigned to produce lift to overcome the weight (or gravity).
Instraight-and-level, unaccelerated flight, (Straight-and-level flight iscoordinated flight at a constant altitude and heading) lift equals weight andthrust equals drag, though lift and weight will not equal thrust and drag. Anyinequality between lift and weight will result in the airplane entering a climbor descent. Any inequality between thrust and drag while maintainingstraight-and-level flight will result in acceleration or deceleration until thetwo forces become balanced. FlightControl Surfaces
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The threeprimary flight controls are the ailerons, elevator and rudder.
Ailerons: The two ailerons, one atthe outer trailing edge of each wing, are movable surfaces that controlmovement about the longitudinal axis. The movement is roll. Lowering theaileron on one wing raises the aileron on the other. The wing with the loweredaileron goes up because of its increased lift, and the wing with the raisedaileron goes down because of its decreased lift. Thus, the effect of movingeither aileron is aided by the simultaneous and opposite movement of theaileron on the other wing.
Rods or cablesconnect the ailerons to each other and to the control wheel (or stick) in thecockpit. When pressure is applied to the right on the control wheel, the leftaileron goes down and the right aileron goes up, rolling the airplane to theright. This happens because the down movement of the left aileron increases thewing camber (curvature) and thus increases the angle of attack. The rightaileron moves upward and decreases the camber, resulting in a decreased angleof attack. Thus, decreased lift on the right wing and increased lift on theleft wing cause a roll and bank to the right.
Elevators: The elevators control themovement of the airplane about its lateral axis. This motion is pitch. Theelevators form the rear part of the horizontal tail assembly and are free toswing up and down. They are hinged to a fixed surface--the horizontalstabilizer. Together, the horizontal stabilizer and the elevators form a singleairfoil. A change in position of the elevators modifies the camber of theairfoil, which increases or decreases lift.
Like theailerons, the elevators are connected to the control wheel (or stick) bycontrol cables. When forward pressure is applied on the wheel, the elevatorsmove downward. This increases the lift produced by the horizontal tailsurfaces. The increased lift forces the tail upward, causing the nose to drop.Conversely, when back pressure is applied on the wheel, the elevators moveupward, decreasing the lift produced by the horizontal tail surfaces, or maybeeven producing a downward force. The tail is forced downward and the nose up.
The elevatorscontrol the angle of attack of the wings. When back pressure is applied on thecontrol wheel, the tail lowers and the nose raises, increasing the angle ofattack. Conversely, when forward pressure is applied, the tail raises and thenose lowers, decreasing the angle of attack.
Rudder: The rudder controlsmovement of the airplane about its vertical axis. This motion is yaw. Like theother primary control surfaces, the rudder is a movable surface hinged to afixed surface which, in this case, is the vertical stabilizer, or fin. Itsaction is very much like that of the elevators, except that it swings in adifferent plane--from side to side instead of up and down. Control cablesconnect the rudder to the rudder pedals.
Trim Tabs: A trim tab is a small,adjustable hinged surface on the trailing edge of the aileron, rudder, or elevatorcontrol surfaces. Trim tabs are labor saving devices that enable the pilot torelease manual pressure on the primary controls.
Some airplaneshave trim tabs on all three control surfaces that are adjustable from thecockpit; others have them only on the elevator and rudder; and some have themonly on the elevator. Some trim tabs are the ground-adjustable type only.
The tab ismoved in the direction opposite that of the primary control surface, to relievepressure on the control wheel or rudder control. For example, consider thesituation in which we wish to adjust the elevator trim for level flight.(«Level flight» is the attitude of the airplane that will maintain aconstant altitude.) Assume that back pressure is required on the control wheelto maintain level flight and that we wish to adjust the elevator trim tab torelieve this pressure. Since we are holding back pressure, the elevator will bein the «up» position. The trim tab must then be adjusted downward sothat the airflow striking the tab will hold the elevators in the desiredposition. Conversely, if forward pressure is being held, the elevators will bein the down position, so the tab must be moved upward to relieve this pressure.In this example, we are talking about the tab itself and not the cockpitcontrol.
Rudder andaileron trim tabs operate on the same principle as the elevator trim tab torelieve pressure on the rudder pedals and sideward pressure on the controlwheel, respectively. LaminarFlow Airfoil
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LaminarFlow isthe smooth, uninterrupted flow of air over the contour of the wings, fuselage,or other parts of an aircraft in flight. Laminar flow is most often found atthe front of a streamlined body and is an important factor in flight. If thesmooth flow of air is interrupted over a wing section, turbulence is createdwhich results in a loss of lift and a high degree of drag. An airfoil designedfor minimum drag and uninterrupted flow of the boundary layer is called alaminar airfoil.
The Laminarflow theory dealt with the development of a symmetrical airfoil section whichhad the same curvature on both the upper and lower surface. The design wasrelatively thin at the leading edge and progressively widened to a point ofgreatest thickness as far aft as possible. The theory in using an airfoil ofthis design was to maintain the adhesion of the boundary layers of airflowwhich are present in flight as far aft of the leading edge as possible. onnormal airfoils the boundary layer would be interrupted at high speeds and theresultant break would cause a turbulent flow over the remainder of the foil.This turbulence would be realized as drag up the point of maximum speed atwhich time the control surfaces and aircraft flying characteristics would beaffected. The formation of the boundary layer is a process of layers of airformed one next to the other, ie; the term laminar is derived from thelamination principle involved.
History ofLaminar Flow The P-51 Mustang is the first aircraft every intentionally designed touse laminar flow airfoils. However, wartime NACA research data I have showsthat Mustangs were not manufactured with a sufficient degree of surface qualityto maintain much laminar flow on the wing. The RAE found that the P-63, despitebeing designed with laminar airfoils, also was not manufactured with sufficientsurface quality to have much laminar flow.
The Mustangwas a mathematically designed airplane and the wing foil that was to beclassified as a «semi-empirical venture» by the British was clearedfor adoption on the new design. The wing section would be the only part of thefighter which would be tested in a wind tunnel prior to the first test flight.Due to the speculation of the success of the radical foil, the engineeringdepartment was committed to adopt a more conventional airfoil within thirtydays of the tests in the event the wing did not come up to specifications. Aone quarter scale model of the wing was designed and constructed for tests inthe wing tunnel at the Caiifornia Institute of Technology.
The use ofthis airfoil on the Mustang would greatly add to the drag reducing concept thatwas paramount in all design phases of the airplane. The few applications ofthis foil, prior to this time, had been handbuilt structures which werefinished to exacting tolerances. An absolutely smooth surface was necessary dueto the fact that any surface break or rough protrusion would interrupt theairflow and detract from the laminar flow theory. Because of the exactnessrequired, the foil had been shelved by other manufacturers due to theclearances and tolerances which are used in mass production. The engineers atNAA approached this problem with a plan to fill and paint the wing surface toprovide the necessary smoothness. The foil which was used for the Mustang had athickness ratio of 15.1 percent at the wing root at 39 percent of the chord.The tip ratio was 11.4 percent at the 50 percent chord line. These figuresprovided the maximum thickness area at 40 percent from the leading edge of thewing and resulted in a small negative pressure gradient over the leading 50-60percent of the wing surface.
The B-24bomber's «Davis» airfoil was also a laminar flow airfoil, whichpredates the Mustang's. However, the designers of the B-24 only knew that theirairfoil had very low drag in the wind tunnel. They did not know that it was alaminar flow airfoil.
There wereseveral aircraft modified by NACA, in the late 1930s, to have laminar flow testsections on their wings. Hence, such aircraft as a modified B-18 were some ofthe first aircraft to fly with laminar flow airfoils.
The boundarylayer concept is credited to the great German aerodynamicist, Ludwig Prandtl.Prandtl hyposthesized and proved the existence of the boundary layer longbefore the Mustang was a gleam in anyone's eye.
Example: First, lets get morespecific about what laminar flow is. The flow next to any surface forms a«boundary layer», as the flow has zero velocity right at the surfaceand some distance out from the surface it flows at the same velocity as thelocal «outside» flow. If this boundary layer flows in parllel layers,with no energy transfer between layers, it is laminar. If there is energytransfer, it is turbulent.
All boundarylayers start off as laminar. Many influences can act to destabilize a laminarboundary layer, causing it to transition to turbulent. Adverse pressuregradients, surface roughness, heat and acoustic energy all examples ofdestabilizing influences. Once the boundary layer transitions, the skinfriction goes up. This is the primary result of a turbulent boundary layer. Theold «lift loss» myth is just that — a myth.
A favorablepressure gradient is required to maintain laminar flow. Laminar flow airfoilsare designed to have long favorable pressure gradients. All airfoils must haveadverse pressure gradients on their aft end. The usual definition of a laminarflow airfoil is that the favorable pressure gradient ends somewhere between 30 and75% of chord.
Now Considerthe finish on your car in non-rainy conditions. Dust and leaves have settled onthe hood's paint. We go for a drive. At once the leaves blow off. But the dustremains. We speed up. Even if we go very fast, the dust remains because of thethin layer of air that moves with the car. If you drive with dew on your car,the dew will not so quickly be blown dry where the air flow has this thinlaminar layer. Downstream, where the laminar flow has become turbulent, the airflow quickly dries the dew.
In the fiftiesthis was dramatically shown in a photograph of the top of a sailplane wing(inflight) that had dew on it. A few tiny seeds had landed on forward area thewing while on the ground. In flight these seeds, tiny though they were, reachedthrough the laminar layer and caused micro-turbulence causing the dew to beblown dried in an expanding vee shaped area down stream of each tiny seed.
Additionalinformation
Profile drag
This comprisestwo components: surface friction drag and normal pressure drag (form drag).
Surfacefriction drag
This arisesfrom the tangential stresses due to the viscosity or «stickiness» ofthe air. When air flows over any part of an aircraft there exists, immediatelyadjacent to the surface, a thin layer of air called the boundary layer, withinwhich the air slows from its high velocity at the edge of the layer to astandstill at the surface itself. Surface friction drag depends upon the rateof change of velocity through the boundary layer, i.e. the velocity gradient.There are two types of boundary layer, laminar and turbulent, the essentialfeatures of which are shown in Fig 8. Although all combat aircraft surfacesdevelop a laminar boundary layer to start with, this rapidly becomes turbulentwithin a few per cent of the length of the surface. This leaves most of theaircraft immersed in a turbulent boundary layer, the thickness of whichincreases with length along the surface. The velocity and hence pressurevariations along the length of any surface can have adverse effects on thebehavior of the boundary layer, as will be discussed later.
Surfacefriction drag can amount to more than 30% of the total drag under cruiseconditions.
Normalpressure drag (form drag)
This alsodepends upon the viscosity of the air and is related to flow separation. It isbest explained by considering a typical pressure distribution over a wingsection, as shown in Fig 4, first at low AOA and then at high AOA.
At low AOA thehigh pressures near the leading edge produce a component of force in therearward (i.e. drag) direction, while the low pressures ahead of the maximumthickness point tend to suck the wing section forward, giving a thrust effect.The low pressures aft of the maximum thickness point tend to suck the wingrearwards, since they act on rearward-facing surfaces. Without the influence ofthe boundary layer, the normal pressure forces due to the above drag and thrustcomponents would exactly cancel.
There is afavorable pressure gradient up to the minimum pressure point, with the pressurefalling in the direction of flow. This helps to stabilize the boundary layer.Downstream of the minimum pressure point, however, the thickening boundarylayer has to flow against an adverse pressure gradient. Viscous effects reducemomentum within the boundary layer, and the thickness of the layer furtherincreases so that the external flow «sees» a body which does notappear to close to a point at the trailing edge. A narrow wake is formed as theboundary layer streams off the section. This prevents the pressures on theaft-facing surface of the wing section from recovering to the high valueobtaining near the stagnation point on the leading edge, as they would havedone if a boundary layer had not formed. There is thus a lower than expectedpressure acting on the aftfacing surface, giving rise to normal pressure drag.In the low-AOA case this component is small, most of the profile drag beingmade up of surface friction drag.
As the AOA ofthe wing section is increased, the point of minimum pressure moves towards theleading edge, with increasingly high suction being achieved. This means thatthe pressure then has to rise by a greater extent downstream of the minimumpressure point and that the length of wing surface exposed to the risingpressure is increased. The resulting adverse pressure gradient becomes moresevere as AOA is increased. This has serious implications for the boundarylayer, which is always likely to separate from the wing surface under suchconditions.
The Swept Wing The whole idea of sweepingan aircraft's wing is to delay the drag rise caused by the formation of shockwaves. The swept-wing concept had been appreciated by German aerodynamicistssince the mid-1930s, and by 1942 a considerable amount of research had goneinto it. However, in the United States and Great Britain, the concept of theswept wing remained virtually unknown until the end of the war. Due to theearly research in this area, this allowed Germany to successfully introduce theswept wing in the jet fighter Messerschmitt ME-262 as early as 1941. Early British and Americanjet aircraft were therefore of conventional straight-wing design, with ahigh-speed performance that was consequently limited. Such aircraft includedthe UKGloster Meteor F.4 , the U.S. Lockheed F-80 Sooting Starand the experimental U.S. jet, the Bell XP-59A Airacomet. After the war Germanadvanced aeronautical research data became available to the United States ArmyAir Force (USAAF) as well as Great Britain. This technology was thenincorporated into their aircraft designs. Some early jets that took advantageof this technology were the North American F-86 Sabre, the HawkerHunter F.4 and the Supermarine Swift FR.5. Not to be outdone, theSoviet Union introduced the swept wing in the Mikoyan Mig-15 in 1947.This aircraft was the great rival of the North American F-86 Sabreduring the Korean War.
Jet Engine Theory
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Centuriesago in 100 A.D., Hero, a Greek philosopher and mathematician, demonstrated jetpower in a machine called an «aeolipile.» A heated, water filledsteel ball with nozzles spun as steam escaped.
Overthe course of the past half a century, jet-powered flight has vastly changedthe way we all live. However, the basic principle of jet propulsion is neithernew nor complicated.
Centuriesago in 100 A.D., Hero, a Greek philosopher and mathematician, demonstrated jetpower in a machine called an «aeolipile.» A heated, water filledsteel ball with nozzles spun as steam escaped. Why? The principle behind thisphenomenon was not fully understood until 1690 A.D. when Sir Isaac Newton inEngland formulated the principle of Hero's jet propulsion «aeolipile»in scientific terms. His Third Law of Motion stated: «Every actionproduces a reaction… equal in force and opposite in direction.»
Thejet engine of today operates according to this same basic principle. Jetengines contain three common components: the compressor, the combustor,and the turbine. To this basic engine, other components may be added,including:
· A nozzleto recover and direct the gas energy and possibly divert the thrust forvertical takeoff and landing as well as changing direction of aircraft flight.
· An afterburneroraugmentor, a long «tailpipe» behind the turbine into whichadditional fuel is sprayed and burned to provide additional thrust.
· A thrustreverser, which blocks the gas rushing toward the rear of the engine, thusforcing the gases forward to provide additional braking of aircraft.
· A fanin front of the compressor to increase thrust and reduce fuel consumption.
· Anadditional turbine that can be utilized to drive a propeller or helicopterrotor.
The Turbojet Engine
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A turbojetengine.
Theturbojet is the basic engine of the jet age. Air is drawn into the enginethrough the front intake. The compressor squeezes the air to many times normalatmospheric pressure and forces it into the combustor. Here, fuel is sprayedinto the compressed air, is ignited and burned continuously like a blowtorch.The burning gases expand rapidly rearward and pass through the turbine. Theturbine extracts energy from the expanding gases to drive the compressor, whichintakes more air. After leaving the turbine, the hot gases exit at the rear ofthe engine, giving the aircraft its forward push… action, reaction!
Foradditional thrust or power, an afterburner or augmentor can be added.Additional fuel is introduced into the hot exhaust and burned with a resultantincrease of up to 50 percent in engine thrust by way of even higher velocityand more push.
TheTurboprop/Turboshaft Engine
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Aturboprop, or turboshaft engine.
Aturboprop engine uses thrust to turn a propeller. As in a turbojet, hot gasesflowing through the engine rotate a turbine wheel that drives the compressor.The gases then pass through another turbine, called a power turbine. This powerturbine is coupled to the shaft, which drives the propeller through gearconnections.
Aturboshaft is similar to a turboprop engine, differing primarily in thefunction of the turbine shaft. Instead of driving a propeller, the turbineshaft is connected to a transmission system that drives helicopter rotorblades; electrical generators, compressors and pumps; and marine propulsiondrives for naval vessels, cargo ships, high speed passenger ships, hydrofoilsand other vessels.
The Turbofan Engine
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Ahigh bypass turbofan engine.
Aturbofan engine is basically a turbojet to which a fan has been added. Largefans can be placed at either the front or rear of the engine to create highbypass ratios for subsonic flight. In the case of a front fan, the fan isdriven by a second turbine, located behind the primary turbine that drives themain compressor. The fan causes more air to flow around (bypass) the engine. This produces greater thrust andreduces specific fuel consumption.
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Alow bypass turbofan engine.
Forsupersonic flight, a low bypass fan is utilized, and an augmentor is added foradditional thrust.