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I started doing some simulations with XPlane and could observe a very interesting effect when doing turns in the presence of constant wind:

All simulations are done with the XPlane Cessna 172 (?) with the motor turned off in gliding. (I want to simulate an emergency approach)

I control my plane to have a constant glide path angle and a constant roll angle. No coordinated turn. When I do so without wind, I get perfect circles with a true airspeed (TAS) that is decreasing with decreasing elevation. My glide angle controller has not too much work to keep the gliding angle perfectly constant.

However, if I turn on some constant wind, I can observe,

  • firstly: my circles change to trochoids over ground
  • secondly: my glide angle controller needs to counteract depending on the relative heading to the wind. When I am in a sidewind situation where the wind attacks the outer (higher) wing, the true airspeed is decreasing and when after a half circle the wind attacks the inner, lowered wing, the true airspeed needs to be increased to hold the gliding angle. In the quarters of turning with straight headwind or tailwind, the true airspeed is equal. So this results in a sine-like true airspeed profile with respect to the actual turning heading.

So in principle, I can imagine, that wind attacking the higher wing increases lift as it raises the whole plane from below and when it attacks on the lowered wing, it pushes the whole plane down from above.

What I don't really understand is why I experience this relative sidewind at all? My plane flies trochoids over ground instead of circles, as the wind blows the whole wind frame away. But shouldn't be the windframe kind of windless with regards to the plane?

My naive idea was, that the whole plane with its own wind frame is moved constantly by the wind, and calculating true airspeed within this wind frame is the same as in the no-wind condition. (see e. g. https://aviation.stackexchange.com/a/26332/30632)

Where do I over-simplify?

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    $\begingroup$ How does your glide path angle controller work, and how do you calculate the glide path angle and target glide path angle? $\endgroup$ – Cpt Reynolds Jun 28 '18 at 22:23
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    $\begingroup$ Holding "constant glide path angle and a constant roll angle" does not use any ground reference (except the horizontal plane reference). If the wind is constant, there should be no difference; the airplane will simply move with the wind. But if you measure "glide path angle" with respect to ground, and try to control pitch to hold it (not a very wise idea for gliding), then you should see the biggest difference in up/downwind segments and none in crosswind. Something is wrong there. What exactly do you control? $\endgroup$ – Zeus Jun 29 '18 at 1:40
  • $\begingroup$ I use the X-Plane dataref named "sim/flightmodel/position/vpath" which is described as "The pitch the aircraft actually flies. (vpath+alpha=theta)". So my assumption was, that this is exactly the tangens of true airspeed divided by the sink rate. However, when exactly analyzing my logdata, this is again only true for the straight flight and not for turning in the wind. When turning in the wind, I see again some oscillations in the difference of the vpath dataref and the angle calculated from TAS and sink rate. I have to investigate that further. $\endgroup$ – opt12 Jun 29 '18 at 7:41
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Wind cannot 'attack' your wing, because there is no wind relative to your aircraft, only relative to the ground. You are flying in still air; the still air in which you are flying is moving over the ground.

Your path over the ground is affected by the wind, so your first bullet point is correct. 'Wind' (in the weather sense) is the parcel of air in which you are flying, moving relative to the ground.

In your second bullet point, if your speed is varying, it must be due to a change in the controls, resulting in a pitch attitude change, or a change in power setting. I'm not sure what you mean by "glide angle controller" but that would usually be the pilot. Your speed cannot vary as a result of the wind, as there is no wind relative to the aircraft. Only by referencing an external frame of reference (looking at the ground, looking at GPS) can you tell that there is any wind.

So if you are experiencing any sidewind (or any other wind), it is caused by whatever is controlling the aircraft (pilot or something in the program) referencing something other than the air. This experience cannot be caused by a constant wind.

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    $\begingroup$ Thanks for the explanation! Meanwhile, I found out, that this strange effect was caused by the use of "sim/flightmodel/position/vpath" which is described as "The pitch the aircraft actually flies. (vpath+alpha=theta)" in X-Plane. This vpath is obviously not correctly described, but is is the angle resulting from the forward ground speed of the airplane divided by the sink rate. When changing my controller to control the angle resulting from TAS divided by sink rate and keeping this constant, this effect does not occur which is totally in line with your explanation. $\endgroup$ – opt12 Aug 17 '18 at 10:02

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