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Joined: Fri May 04, 2018 1:27 pm

Negative AoA

Mon May 07, 2018 1:28 pm

A long time ago there was a thread about the Google Earth flight simulator on this forum, but it did not give me any answer to the question I have about the artificial horizon of this simulator, or more precisely, a certain element within it. And I also believe that my question could be pertinent not only to this artificial horizon in particular, but to other artificial horizons as well (even though in images of other artificial horizons on the internet I could not find this particular element in question in the Google artificial horizon which I am having trouble with).

The Google Earth flight simulator offers two aircraft to choose from: A General Dynamics F-16 fighter and a Cirrus SR22 light aircraft. What I will say below will refer itself above all to the (comparatively low speed) Cirrus.

Now I know, that the little upside down “T” with a large horizontal bar in the artificial horizon symbolizes the aircraft and indicates the direction of flight. But then there is that large arrowhead pointing upward and around which the roll indicator arc is rotating, which to me indicates that the point of this arrowhead is pointing in the same direction as the nose of the aircraft. While the plane is parked on the airfield, only the arrowhead is visible at the center of the artificial horizon. The aircraft symbol becomes visible only at take off.

But if my assumption that this arrowhead is the nose of the airplane is correct, then something weird is happening with my flight simulator: When I fly the Cirrus at very low speed, like say 85kn, the arrowhead or aircraft nose(?) points -about +7º in this case- above the flight direction indicator, giving a positive AoA (or angle of attack for beginners like myself) just like I would expect. And when I increase my velocity to medium speed, like say 120kn, the arrowhead and the flight direction indicator pretty much coincide.

But if I increase my velocity even more, like say to 210kn, the arrowhead is pointing a good 5º BELOW the flight direction indicator, giving a negative AoA(!?). Now if the Cirrus were a helicopter, this wouldn´t surprise me much. But the Cirrus is is a fixed wing aircraft. And if I put flaps, the presumed AoA becomes negative even below that speed.

In a fixed wing aircraft, I would expect that the AoA gets smaller and smaller as the aircraft becomes faster and faster, but never for it to actually become negative. And this is exactly what happens with the F-16. Here, the AoA is positive up to about 820kn and then the arrowhead and the direction indicator stay together all the way up to 1300kn, just as I would expect. But not so with the Cirrus.

So how can a -preferrably slow, as I suspect- fixed wing aircraft fly horizontally or even climb(!as I found out) with its nose pointing down, i.e. having a negative AoA with its flight direction? This issue utterly baffles me, and I hope that someone on this forum will save me from my confusion.

Thanks for your help

(P.S.: For some reason I believed, that it is possible to download images from my computer to this server so I can show screen shots of my flight simulator in the above mentioned situations, but now it seems to me that I was wrong)
Posts: 73
Joined: Thu Jun 16, 2016 8:41 pm

Re: Negative AoA

Tue May 08, 2018 10:20 am

It's possible. Cambered wings (as on a Cirrus and most aircraft) will generate some lift at zero AOA. You actually have to go to negative AOA to get zero lift. Throwing out flaps accentuates that (it increases camber).

But you also need to look at the incidence of the wing--that is, the angle that the wing is mounted at relative to the fuselage. On many (most?) aircraft, the wing will be at some positive incidence, so when the fuselage is aligned with the airflow the wing will be at positive AOA.

That HUD "arrowhead" doesn't represent the camber line of the wing, it represents the "boresight" or "waterline" of the fuselage. When it is level with the horizon, your fuselage is level. One would expect you'd set your incidence such that your fuselage would be level for cruise, but sometimes you have other considerations (e.g. takeoff and landing) that might drive you to a higher or lower incidence. The B-52, for example, cannot really rotate for takeoff, so the wing incidence was set to allow it to take off and land with the fuselage essentially level.

Now, having that reference 5 degrees below the horizon at high cruise speed sounds a little excessive, but also remember this is just a flight sim tucked away inside a GIS program. They may have their coefficients largely correct, but they'll be simplified and there's no guarantee they got the attitude reference right in the first place. I've never flown a Cirrus, but my usual ride (RV-6) will seem like you're way nose-low in high speed level flight, at least until you get used to the airplane.

A couple interesting examples of this are the B-52 (at low altitudes and high speed, the fuselage will be visibly nose-down because of the incidence of the wing; see above) and the old DeHavilland Caribou (with full flaps they used to do a trick they called "wheelbarrowing" where they could fly along at negative deck angle and touch down on the nosewheel only).

The F-16 wing isn't really cambered except as provided by the leading- and trailing-edge flaps (and actually, those drive to negative camber when you get to supersonic--I forget the exact reason why). Its wing is also set with no incidence.
Topic Author
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Joined: Fri May 04, 2018 1:27 pm

Re: Negative AoA

Tue May 08, 2018 11:00 pm

Thanks a lot for this complete (and coherent) explanation!
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Re: Negative AoA

Thu May 31, 2018 2:41 pm

gtae07 wrote:
The B-52, for example, cannot really rotate for takeoff, so the wing incidence was set to allow it to take off and land with the fuselage essentially level.

Myth....2:58 and 4:40 good examples.......

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