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How can I tell what a reasonable control surface deflection is?

For example, is a 10 deg aileron deflection reasonable? Or a 0.5 degree aileron deflection? Is that too little? Too much? Same question concerning stab/rudder. I'm not a pilot, I've never seen these surfaces actually in action, so it's kind of difficult to tell what (roughly) a reasonable surface deflection would be. As I go through flight data, I find myself seeing this type of data a lot, so an idea of what's reasonable would be really useful!

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    $\begingroup$ This question is extremely broad. A reasonable control surface deflection at one speed may tear the plane apart at a higher speed, as was the case with American Airlines 587. Likewise, this varies by plane; an Extra aerobatic plane might need to deflect surfaces by almost 70º which is not desirable on most other types of aircraft. $\endgroup$ – verandaguy Feb 18 at 17:41
  • $\begingroup$ "I'm not a pilot" So go search a local small airport and take a lesson or two. Flying is a lot of fun. I can't say how much deflection I use in normal turns, same as I couldn't say how much steering wheel turn correlates to how much the front wheels actually turn - you just steer until it's enough for where you're trying to go. Plane yoke controls turn a lot less than a car steering wheel - the amount of turn is more like a Formula 1 wheel's amount of turn, or even less. I don't think the yoke on my Cessna turns as much as 90 degrees. $\endgroup$ – CrossRoads Feb 18 at 17:41
  • $\begingroup$ I'm curious as to exactly where you are encountering this data. Accident reports? Is that why you can't assume that actual data you are seeing is "reasonable"-- because they represent non-normal situations? $\endgroup$ – quiet flyer Feb 18 at 18:04
  • $\begingroup$ @quietflyer Yes, the plane was in an accident. It's unmanned. I'm going over some data for work, and as an inexperienced engineer, I really don't have much to go on. $\endgroup$ – synchh Feb 18 at 18:15
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    $\begingroup$ Can you get some similar data from the same aircraft type from flights that were successful? That seems to be what you need to do. $\endgroup$ – quiet flyer Feb 18 at 18:17
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The maximum useable deflection angle depends on the relative chord of the control surface. A good first-order value is ±20° for a 20% chord. With increasing chord, the deflection range will become smaller, like ±15° for a 30% flap.

With ailerons, things are a bit different because they are part of a lifting surface. This gives them some pre-loading so the negative range (trailing edge up) will become larger than the positive range (trailing edge down). This is also useful to reduce control forces (aileron differential) so the useable range will be +10°/-25° for a 20% chord aileron.

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This is thinking a little "outside the box", but one thing you could do is use to the internet to locate some manuals for some ready-to-fly radio-controlled model airplanes such as those produced by HobbyZone, etc. The manuals will typically list "low rates" and "high rates" for each surface. The "low rates" have to provide the minimum control throw that the pilot would likely want to use in any flight mode, including high-speed flight. The "high rates" have to provide the control throw that the pilot would likely want to use during the most extreme maneuvers. The pilot can switch back and forth between these two different modes at will during flight. Forget about any model airplane that is promoted as being capable of "3-D" flying, because during this style of extreme flying, extreme throws are used that would not be representative of anything used in full-scale aircraft. Obviously, you will find that aircraft intended to fly very fast will generally employ much smaller control throws than aircraft intended to fly slowly, and aircraft intended for aerobatics will employ larger control throws than aircraft not so intended.

The point here is that because the pilot can switch back and forth between low rates and high rates at will, the "low rates" do not have to be configured to provided all the possible deflection that the pilot might ever need, and therefore might reasonably be assumed to be based on deflection that the pilot actually might regularly employ in ordinary non-aerobatic flight.

With this method, you'd have to measure the control surfaces to come up with an angular deflection in degrees, because the values provided in these manuals will typically be a linear measurement of how far the surface moves from the neutral (centered) position, when the control stick is fully deflected.

Anyway, you'll find a tremendous variation in the recommended control throws, depending on the type of aircraft and the way it is intended to be flown-- just as is the case with full-scale aircraft.

I suspect what would really better suit your needs would be some actual flight-test data from full-scale aircraft, but I can't tell you where to easily find that.

You also could consider playing around with a flight simulator such as can run on a personal computer. Some of them can be configured to display control surface deflections. Some also have modes where they will fly through maneuvers for you, as a lesson.

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There is no concrete answer for this. It all depends on aircraft, airspeed, air density, attitude, configuration, wing loading, and weight. For example, a Piper Archer at cruise airspeed in straight, level, unaccelerated flight will have very little deflection necessary in flight controls to change or maintain attitude of flight.

As you transition to slow flight, the control surfaces will become progressively “mushy”. They will become less effective, requiring more deflection to change or maintain attitude.

One caveat is that at slower air speeds, less change in attitude is required to change, maintain direction of flight. Another caveat is that at lower aircraft weights, full deflection of control surfaces will result in structural damage at lower air speeds. But, that is a whole other discussion.

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