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I am making a flight sim and would like some help. If I am correct, the angle of attack on the horizontal stabilizer should look something like this:

aoa(Hstab) = aoa(Wing) - downwash + Hstab incidence

From research, the AOA of the Hstab should be negative most of the time, however, that just doesn't seem to happen. For example, if the plane is flying at an AOA of 8 degrees, the downwash won't be more than 3 or 4 degrees, and the horizontal stabilizer incidence won't be less than -1 or -2 degrees. So how is the aoa of the Hstab ever negative, if 8 - 4 - 2 is positive?

DeltaLima
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SpiralingTessaract8164
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    For stability the stab's AoA need not be negative, it just needs to be less than the wing's. – Camille Goudeseune Oct 24 '23 at 15:29
  • Is the stab's own incidence really no lower than -2 degrees? – Camille Goudeseune Oct 24 '23 at 15:31
  • @CamilleGoudeseune , so let's say for instance this scenario. The AOA of the wing is 8 degrees. The downwash angle is subsequently 3 degrees, and the horizontal stabilizer incidence is -1 degree. 8 - 3 - 1 is 4 degrees. This means that the AOA of the horizontal stabilizer is 4 degrees, therefore it will produce lift upwards in the same direction of the wing, therefore it will contribute to the pitching forwards moment of the wing, pitching the plane forwards. How is that stable? – SpiralingTessaract8164 Oct 24 '23 at 19:20
  • Why are you considering the AoA of the wing? The AoA of the horizontal stabilisers is... well, the AoA of the horizontal stabilisers. It's easier if you use the fuselage as reference and not the AoA of the wing so that the AoA of the hstab is AoA of the fuselage + its incidence - downwash. As a rule of thumb the downwash is half of the AoA of the wing. – sophit Oct 24 '23 at 20:25
  • @sophit If you use the AoA relative to the zero-lift angle it is indeed more relevant than any angle of the fuselage. And your downwash rule-of-thumb needs a rather low aspect ratio to be in the ballpark. – Peter Kämpf Oct 25 '23 at 01:22
  • @PeterKämpf: sure, one can definitely use the AoA relative to the wing's zero-lift, the important thing is to be consistent which it doesn't seem to be the case here since the incidence of the horizontal stabilisers seems to be given in respect to the fuselage in the question. – sophit Oct 25 '23 at 02:41
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    I see confusion from the lack of a distinction between stability and trim in a lot of the discussion. I can set the stab AOA to be in line with the wing's zero lift AOA and my airplane will be a very stable ballistic dart. To make it fly I have to force the wing to a positive AOA, by trimming it with the rear surface to oppose the wing's pitching moment until an equilibrium is reached at some wing AOA. Stability forces do their thing by acting with restorative forces on either side of that equilibrium point as required. Thus stability and trim are separate but interrelated things are they not? – John K Oct 25 '23 at 04:56
  • @zander: btw when you write that "the AOA of the Hstab should be negative" do you mean that the stabilisers is generating a negative lift? – sophit Oct 25 '23 at 06:04
  • @sophit yes that's what I mean. Otherwise both the wing and horizontal stabilizer will be producing "positive" lift and the plane will just pitch forwards. – SpiralingTessaract8164 Oct 25 '23 at 12:13
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    @zander: perfect. If you choose the AoA of the wing to do your calculations then just remember that the incidence of the horizontal stabilisers have to be given in respect to the one of the wing. And no, positive lift from the horizontal stabilisers doesn't imply an unstable airplane (think about a canard as an extreme example). – sophit Oct 25 '23 at 12:34

1 Answers1

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Your formula is correct, and there is no need for the AoA of the horizontal tail to be negative. It only needs to be less than that of the wing (relative to the zero-lift angle). Stability is determined not by the direction of the lift force on the tail, but its gradient over AoA.

Peter Kämpf
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