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True or False: The stick determines elevator position and elevator position determines angle of attack. When the critical angle of attack is exceeded, the wing stalls. So, given a design with enough elevator authority, if the stick is in your lap, then the wing is stalled. If the stick is, say an inch farther forward, the wing will not be stalled. Let's stipulate no flaps/slats, calm air, normal CG, positive G, and plenty of elevator authority. Maybe use a plane such as the Luscombe 8A for the sake of this discussion. That will keep things straightforward.

I guess the crux of what I'm asking is: why couldn't the stick be used as an angle of attack indicator? Let's assume a pilot is able to very precisely detect the stick position, or that maybe an indicator is installed in the panel which is connected to the bottom of the stick under the cabin floor.

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    $\begingroup$ You can (and probably have in training) fly to the edge of a stall with the stick at a certain position, then apply some power to keep flying, with the stick (or control wheel) at pretty much the same position. (I say "pretty much" because you don't really pay that much attention to the exact position, you just fly the plane :-)) $\endgroup$ – jamesqf May 21 at 4:17
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    $\begingroup$ There is another parameter involved: Pitch rate. A higher pitch rate will need a more backward stick position for maintaining AoA. $\endgroup$ – Peter Kämpf May 21 at 5:22
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    $\begingroup$ The direction of the inflow to the wing plays an important role, too. In the course of a mushing descent, or in a sideslip, for example, the attitude and horizontal airspeed of the plane may remain more or less constant, but the sink speed tends to rise. Hence, the AoA increases, and may easily reach the critical, stall value. $\endgroup$ – xxavier May 21 at 11:33
  • $\begingroup$ @PeterKämpf What do you mean by "pitch rate"? $\endgroup$ – birdus May 21 at 17:35
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    $\begingroup$ Stick transducers exist already and are not difficult to mount, in fact they provide inputs to Flight Data Recorders already. It is one of the inputs to AoA. There is no substitute for direct measurement of the variable. $\endgroup$ – Koyovis May 21 at 20:45
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For a given set of configuration parameters, CG, weight, power, G loading, you could theoretically use stick position as an approaching stall indication. Problem is, the actual location would vary with all those parameters, so if you had a stall indicator device that was measuring and displaying stick position, you would constantly have to vary the calibration with power setting, flap position, G load, all up weight, and center of gravity.

So, take the apparatus you would have developed to go under the floor to measure where the stick is at, and move it out to the wing with a vane that gets blown on in the clean air flow, have THAT drive your indicator, and voila! No need to adjust for any configuration variables and you know exactly where you stand relative to stall at all times and in all flight regimes.

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  • $\begingroup$ So you must be saying that a given stick/elevator position doesn't necessarily set the wing to a specific angle of attack. You mention "CG, weight, power, G loading." Are there others? How does each affect what angle of attack is achieved for a given stick/elevator position? $\endgroup$ – birdus May 21 at 1:10
  • $\begingroup$ I like your answer better... $\endgroup$ – Michael Hall May 21 at 1:38
  • $\begingroup$ There are many other inputs into the AoA. Stabiliser deflection. Landing gear extension. Flap deflection. Vertical speed. $\endgroup$ – Koyovis May 21 at 1:52
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    $\begingroup$ @birdus as Koyovis and Peter mention there r just so many variables that change elevator deflection required to get a given airplane AOA/speed.You COULD predict AOA for a given stick position,for a fixed set of physical parameters,in level,un-accelerated,1G flight,in smooth air,but the instant any of those things changes your prediction would have to change. Almost all light aircraft use indicated airspeed,which gives a reliable indication of AOA margin for a given physical config,but only in stabilized un-accelerated flight at constant power.U have to measure AOA directly cover all the bases. $\endgroup$ – John K May 21 at 13:57
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I'm going to contradict everyone just a bit and say that the idea has some merit.

If we consider the problem mathematically, in perfect conditions (calm air, straight and level flight, const everything), then it may be useful to think of elevator control as an AoA control. Many people, including pilots, fail to realise the strong link here. So at least for educational purposes we can say yes.

However, even in such conditions we must stress that we are only talking about steady state conditions, when everything settles on steady/const values again. When you move the stick (which you can do very quickly), the aircraft doesn't react instantly (nor strictly proportionally). There will be transition effects of varying complexity, and much of the Flight Dynamics science studies just that. Nevertheless, a 'good' stable airplane will settle on something (as long as it has capacity for that, e.g. power), and this new AoA will reflect the new stick position.

For this reason, in practice it is more useful to think of the elevator trim controlling the steady AoA than the elevator itself: we normally use trim for exactly such steady conditions, and use elevator for 'dynamics'.

Of course, as everyone said, in practice there are many more variables (which actually vary), and a proper AoA sensor is not so complicated. Even the stick/trim position for a given steady state AoA will vary depending on the CG location, as it will need a different amount of travel for the same effect. Nevertheless, there are some practical cases when the stick position makes a reasonable proxy for AoA, even despite having an AoA sensor.

First, the stick allows to separate intentional AoA changes from accidental ones (e.g. due to gusts). Sometimes it matters. Second, it may be faster (for intentional changes): the stick will show the desired AoA before one is actually reached. These two things are used in at least one control system I know: the nose cone control of MiG-21bis.

The cone needs to be moved forward at higher AoAs, so that the supersonic shockwave didn't get inside the intake. Yet you don't want to move it constantly with every little gust; at the same time, the drive is not that fast and you want to pre-empt it before the dangerous AoA is reached. So the stick position is used as an input for this system on this aircraft, instead of the true AoA (which is available).

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  • $\begingroup$ Stick position is indeed a rapid input, AoA and cone deflection are responses to the input. Good point. $\endgroup$ – Koyovis May 22 at 13:22
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    $\begingroup$ Phrasing this a bit more pithy: If you are flying straight and level, and pull the stick back beyond the "stall line," the plane will not stall immediately. Likewise, if you are in a stall, and let go of the stick, the plane does not immediately recover. $\endgroup$ – Cort Ammon May 22 at 22:10
  • $\begingroup$ Good answer which could be improved by mentioning the cg position: A more forward cg requires more stick travel for trim, so the stick positions will differ at the same AoA and different cg. $\endgroup$ – Peter Kämpf May 22 at 23:19
  • $\begingroup$ Good point @Peter, added. $\endgroup$ – Zeus May 23 at 2:55
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The aircraft's dynamics certainly relate angle of attack and stick position, but they are not one-to-one related. There are complex dynamics involved. Some examples:

  1. You can take the stick and at a slow aircraft speed rapidly cycle it back and forth to the stops, and the aircraft's AOA will not track with it. The response of the AOA to the stick position is not instant (at slow speeds).
  2. You can fix the stick and fly into turbulence, and the AOA will be all over the place while the stick is fixed. Again, its a dynamic model relationship between stick position and AOA.
  3. You can shift the CG position by burning off fuel, and the stick position/AOA relationship will change.
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Langewiesche's "Stick and Rudder" emphasizes the idea that the elevator is fundamentally an angle-of-attack control, and that limiting the aft motion of the control stick will prevent the wing from reaching the stall angle-of-attack.

But here is a fundamental problem with the idea of using the control stick as an angle-of-attack indicator-- it may work fine in wings-level, non-accelerated (non-looping) flight, but in turning flight, the stick must often be positioned MUCH FURTHER AFT to set the wing at a given angle-of-attack than in wings-level flight.

For example when a sailplane is thermalling, the stick is often quite far aft-- at a position that would produce a stall in wings-level flight. This is especially true if the CG is rather far forward.

There are several sailplanes (example: Slingsby Swallow) that have been designed to have rather limited elevator throw in the interest of stall prevention, in which heavy pilots flying near the forward edge of the allowable CG envelope find that in a thermal turn, even placing the stick full aft against the aft stop produces an angle-of-attack that is clearly lower than the angle-of-attack that would yield the minimum sink rate. In other words, they are forced to fly too fast. Even though those same pilots could slow down well below the minimum sink-rate speed, and perhaps even all the way to stall speed, in wings-level flight.

Several faulty explanations have been offered as to why this is so. The truth is, if the flight path is curving, then the relative wind is also curving. Or to put it another way, since the aircraft is rotating in both pitch and yaw, as well as translating linearly, the rotational motion induces a difference in the direction of the local relative wind between the nose of the aircraft and the tail of the aircraft.

Loosely speaking, in a moderate to steep-banked turn, in the aircraft's reference frame the nose is constantly "rising" and the tail is constantly "falling", and so the curving relative wind tends to "push up" on the tail and create a nose-down pitch torque, placing the wing at a lower angle-of-attack than we'd see with the same stick position in wings-level flight.

This can also be described as a "pitch damping" effect-- the aircraft has an inherent aerodynamic resistance to pitch rotation, and this aerodynamic resistance is expressed as a nose-down pitch torque that causes the wing to fly at a lower angle-of-attack than we'd see for the same elevator position in wings-level linear flight.

These effects are much more pronounced in slow-flying aircraft than in faster-flying aircraft with the same linear dimensions, because the radius of curvature of a turn is inversely proportional to the square of the airspeed.

If this all seems a bit implausible to you, you might want to read the article "Circling the Holighaus way", which deals with the effects of the curving relative wind in the yaw (not pitch) dimension.

http://www.wisoar.org/Documents/Holighaus%20-%20Thermalling%20Efficiency.pdf

Also note that in a pitch "phugoid", either with the elevator allowed to float freely or with the elevator firmly held in a completely fixed position, it can happen that the stall horn sounds as the flight path is arcing downward near the top of each oscillation, but is silent as the flight is arcing upward near the bottom of each oscillation. Again this is a manifestation of the way that the curvature in the flight path and relative wind causes an increase or decrease in the wing's angle-of-attack.

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    $\begingroup$ Also as another answer implied, there are issues of inertia and lag. There's no doubt that you can place an aircraft in a very nose-high climbing trajectory where it will eventually run out of energy and whipstall, even with the stick not too far aft. Again though this may be viewed as a function of the radius of curvature of the flight path. $\endgroup$ – quiet flyer May 21 at 15:02
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    $\begingroup$ As an aside, I once did some experiments involving a device that could be clamped on to the tube that comes out of the instrument panel of a Cessna 152/72, to which the yoke is attached. This device created an artificial stop limiting how far forward the control yoke could be pushed. When the yoke was held firm forward against this stop, the elevator position was fixed in the fore-and-aft sense yet the ailerons could still be used. I carried out experiments involving rather rapid transitions from 45 or 60 degree banked flight to wings-level. The results were a bit interesting (alarming)! $\endgroup$ – quiet flyer May 21 at 16:44
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    $\begingroup$ The device was basically the type of vice grip that is used to hold sheet metal, with rubber padding glued on. Like this. amazon.com/IRWIN-VISE-GRIP-Original-Locking-23/dp/B0000TFGFU The most interesting/ alarming results involved pitch "phugoid" dynamics in which changes in angle-of-attack did not necessarily play a key role. However, if one set up the plane so that the stall horn was barely sounding in wings-level flight, the stall horn would go silent once the plane was established in a banked turn, with the elevator in the exact same position. $\endgroup$ – quiet flyer May 21 at 16:48
  • $\begingroup$ Also note that for a given fixed position of the control stick or yoke, and assuming no cable stretch, the position of the elevator trim tab will have some small effect on angle-of-attack, acting in a "reverse" sense-- nose-up trim will cause the wing to fly at a slightly lower angle of attack than nose-up trim, for any given position of the elevator. This only pertains to cases where pitch trim is accomplished by a trim tab on the elevator and not some other means. $\endgroup$ – quiet flyer May 22 at 13:36
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Relative stick position is a good "seat of the pants" gauge for knowing when you are close to a stall, but stick feel will change with trim and therefore isn't reliable indicator of stick position. So yes, there is a correlation between the two that can be useful, but position alone should not be used as a primary means of determining critical AOA. Also, Angle of Attack has a precise definition associated with it. While stick position influences AOA, it doesn't meet that definition so the two should not be confused.

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  • $\begingroup$ But trim just moves the elevator (and thus stick) preemptively. How does that change anything? I believe all trim would do is change the force you'd need to apply to the stick to get it to whatever position. Let's be precise. You said trim changes "feel." Can we agree that we're not talking about feel here? Second, you said position can change with trim. Do you mean that trim will change the angle of attack for a given elevator position since the elevator has a very slightly different shape for varying degrees of trim? $\endgroup$ – birdus May 20 at 23:36
  • $\begingroup$ OK, I edited my answer a little to be more precise. I guess I don't have a complete understanding of what you are suggesting. Do you think a stick or yoke position indicator would be useful for a pilot to know when he/she is approaching an impending stall? Because I don't agree with that. I think there are more and better options. $\endgroup$ – Michael Hall May 21 at 0:10
  • $\begingroup$ When trim is implemented with trim tabs, and the elevator controlled by simple pulley, the same elevator coefficient of lift (and thus trimmed AoA) will correspond to the same stick position. However it is no longer the case when trim is implemented with a movable stabilzer (as airliners do), the coefficient of lift will change more when the whole stabilizer moves. And then the pilot can judge the force on the stick quite well, but judging the position is much harder as there is not much of a reference for it. $\endgroup$ – Jan Hudec May 23 at 18:56
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False.

The problem is, even with all your stipulations, there are still things that can affect the AoA besides stick input. For instance, you stipulated "normal CG" without defining what, exactly, that means. Most pilots on hearing that phrase would assume you meant "somewhere between the fore and aft limits" -- i.e., where the CG "normally" is. But the CG can vary within that range, changing the airplane's pitching moment, thereby changing the relationship between the stick position and the AoA.

But what if we stipulated that the CG was at a fixed, known location? There's still the issue of airspeed. The more air flowing across the elevator, the more control authority you have, so the same input creates a bigger AoA change.

But what if we stipulated that the airspeed was fixed? Then there are probably several other factors that I can't think of right off the top of my head that also affect the AoA.

But what if we stipulated that all these other factors are fixed as well? Then you still have the issue that the AoA would lag behind the stick position due to the inertia of the airplane. That changes your theoretical AoA indicator into more of a what-the-AoA-would-be-if-you-haven't-moved-the-stick-recently indicator.

But, if you add all these other stipulations, and find some way to compensate for the lag, then the answer to your question becomes true: you can use stick position to calculate AoA.


Let's try a concrete example. Suppose I'm flying along, straight and level, at a 3° Angle of Attack. I want to climb, so I pull the stick back. What happens to the AoA? All I've done is rotate the plane, I haven't actually changed the direction of flight (yet), therefore the AoA starts increasing. Increasing the AoA increases lift (as long as you haven't stalled, that is), so now the lift is greater than the weight, so the airplane starts to climb. This, in turn, changes the angle of the relative wind up until it reaches an equilibrium based on the speed of rotation of the airplane.

The AoA is maintained at this equilibrium point by aerodynamic forces. If the AoA decreases below this point, then the airplane loses lift, so the relative wind can't keep up with the rotation of the plane, which increases AoA. If the AoA goes above this point, then the airplane gains extra lift, so the relative wind starts to catch up with the rotation of the plane, which reduces AoA.

But anything that rotates the plane will have the exact same effect. Let's say a big guy gets up from the front seat and walks to the back. This produces a rotating moment that tries to pitch the plane up. If nothing is done to balance this, then the plane will start to rotate, producing the exact same effect of changing the AoA, causing a climb, which changes the relative wind until it finds its new equilibrium point.

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  • $\begingroup$ For the sake of this discussion, I guess I assumed people wouldn't be imagining that the CG was moving around, but was at only a single, specific location. $\endgroup$ – birdus May 21 at 17:42
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    $\begingroup$ @birdus The CG does change during flight. If nothing else, the fuel draining from the tanks will cause the CG to slowly wander away from where it was. But I was referring more to the change in CG from one flight to the next. If you're trying to create a general-purpose AoA indicator, it would have to work regardless of if you have passengers in the front, passengers in the back, cargo, etc., etc., etc. $\endgroup$ – HiddenWindshield May 22 at 4:24
  • $\begingroup$ I'm trying to understand physics, not design an angle-of-attack indicator. For the sake of argument, then, use a glider or an electric airplane. And why would CG have to change for a gas powered plane? If the tank(s) is over the CG, then CG might not change at all. $\endgroup$ – birdus May 23 at 0:12
  • $\begingroup$ For a gas-powered airplane, the odds that the gas tank would be directly over the CG for any given flight is vanishingly small. But if that did happen, then, yes, the CG wouldn't change as the gas drained. As far as understanding the physics: I think maybe a concrete example would help, but that's a little long for a comment, so I'm going to edit my answer instead. $\endgroup$ – HiddenWindshield May 23 at 17:15
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The trouble of using the stick position as stall indicator is that even with the stick at neutral, I mean not touching it, you may get stall just by acting on the trim. Several solutions to prevent stall do exist; in addition to the stall warning the Boeing philosophy on B777 is to modify the stick load. Airbus too has a solution by which the stick instead of giving a proportionate deflection to the elevator, it gives a load factor order that becomes limited when you get close to the critical AOA, the critical AOA being function of the configuration that is flaps, slats etc, by this technique pulling fully the stick does not cause stall; Airbus uses 3 AOA sensors to isolate a faulty sensor.

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FALSE

This actually is a very dangerous assumption. Firstly, there is a relationship between CG and elevator pitch that can be learned from paper airplanes and simple balsa gliders. Learning the basics of unpowered flight is essential to understanding AOA and stalling, as well as appreciating the importance of being within CG limits.

AOA is dependent on the relationship between Clift, CG, and pitch (stick position AND trim), and airspeed. A plane set up with forward CG and up elevator by design will raise its AOA at too high a speed and lower it as speed bleeds off from climbing and increased drag.

Notice as the plane climbs the relative wind also changes on the tail, raising the nose even higher, but the forward CG and lowering airspeed lessen the pitch up effect. Eventually, the plane stops climbing and the relative wind again changes on the tail, pushing the nose down as it sinks.

Now move the weight back for the same elevator setting past the aft CG limit. As the plane climbs and slows, the nose keeps rising. AOA will increase to the point of stall. In models this produces a smooth level flight as the aircraft obeys the laws of physics: higher AOA increasing lift cancels lower speed decreasing lift, up to the point of stall, where it will abruptly pitch down.

So really airspeed is what you need to keep an eye on. At a CONSTANT airspeed, a given pitch setting will give a given AOA assuming no change in relative wind (see Quiet Flyers work describing thermals).

Too many factors in real flying to make stick position useful beyond a general approximation.

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