Measuring Stall AOA

[David-aviator]

I had an inspiration today to attempt measuring stall AOA .

The basic premise is that wing incidence is set to a level longeron and that incidence represents zero AOA. If that is not true, the data is flawed from the beginning and useless.

What I did was take off with a Smart Level to measure the angle of the longeron when the airplane stalls. That was easier said than done. Beside clearing the area, the level had to be held in position on the longeron while trying to keep the airplane straight and level during the stall. There is a freeze feature with level so the button was pushed when the stall occurred.

The first attempt resulted in a reading of 10.4?. That did not seem right and later at lunch with some guys, including a retired F-18 test pilot, it was noted that if there was any drift down, that angle had to be added to the reading. The idea of measuring the angle at the longeron was valid but only if the airplane was level and not sinking.

Later, on the way home, I tried it again. This time I went to great effort to keep the V/V at zero going into the stall. I almost succeeded but the Dynon still said 100 fpm. The longeron measurement at stall was 12.1?. I tried one with 20? flaps it came in at 8.8?. To do this right, I really need a co-pilot to read the level while I concentrate on keeping sink rate at zero.

I took a few measurements at a set IAS with the altitude hold engaged. The trend with increasing speed obviously would result in a negative AOA. I'm not sure what to make of that.

130 KIAS = .3?
110 KIAS = 1.6?
100 KIAS = 2.4?
90 KIAS = 2.7?
80 KIAS = 4.0?
70 KIAS = 5.5?

Not sure what all this is worth but it is a diversion from searching for enemy Cessnas and Pipers.

[rv7charlie]

but increasing speed means increasing lift; if you're in level flight then the AOA must go down to keep lift the same.

Charlie

[plehrke]

Originally Posted by David-aviator

Not sure what all this is worth but it is a diversion from searching for enemy Cessnas and Pipers.

Dave,
Head over to Creve Coeur as we have lots of enemy bi-planes to search for but it can be like shooting fish in a barrel (although mythbusters showed that shooting fish in a barrel can be tricky). Some of the big and slow bi-planes are even in attack formation 2 to 4 across. If you really feel cocky, you may even find a stray Mustang or Sky Raider to try and deal with.

[BlndRvtr]

Stall AOA is a fixed value for each airfoil in a given configuration. Vans should know that value. Of course trying to discover it is much more fun than just calling Vans. But you might want to rethink that longeron/wing incidence/zero AOA premise. Again, Van should know that correlation.

George

[Andy Hill]

Just to add some complication If you are trying to use a "level" then this will measure the angle of the longeron relative to local gravity. In order for the local gravity to be correct, the aircraft must be in unaccelerated flight i.e. it's G/S constant.

This will be difficult - since at the stall drag will be rising fast - the power will need to be low to decelerate to the stall, and then smartly increased to hold it constant in the stall. As you state above, V/S also needs to be zero, which again requires power... so all you are determining is the AoA at that power setting

Here's an idea... you need a device which measures pitch angle and does not require unaccelerated flight, and might even be able to cope with some V/S. I note you say 'but the Dynon still said 100 fpm'. The Dynon AH Pitch scale will give you the pitch angle, and records a series of data values for later analysis. Something along the lines of:

1. Establish the Dynon "zero pitch" angle relative to the Longeron on the ground (with your angle device).
2. Fly S&L/unaccelerated for some time prior to the stall to ensure the Dynon has stabliised it's attitude.
3. Do a few power off stalls trying to hold the aircraft at constant V/S, but above all smoothly
4. Use the V/S from the Dynon date at the stall to determine the flight path just prior to the stall
5. The V/S to fly (descent) is that which at idle gives you a slow decrease in IAS enabling the approach to the stall to be as slow as possible

As you say, the outcome is valueless, but then most of the flying we do is anyway

Andy

[Bill Wightman]

Originally Posted by David-aviator

The basic premise is that wing incidence is set to a level longeron and that incidence represents zero AOA.

David, isn't the wing in the 7 is mounted at a positive incidence angle relative to the longerons? If so, all you need to do is add that incidence angle to your flight test results.

If wing incidence isn't accounted for, then you're actually measuring stall pitch attitude (or body angle) -vs- AOA.

[David-aviator]

Originally Posted by Bill Wightman

David, isn't the wing in the 7 is mounted at a positive incidence angle relative to the longerons? If so, all you need to do is add that incidence angle to your flight test results.

If wing incidence isn't accounted for, then you're actually measuring stall pitch attitude (or body angle) -vs- AOA.

We do not know if the 7 wing has a positive incidence.

The procedure for checking whatever it is, is a dimensioned riser near the aft wing edge forward to the wing top aft of the leading edge. That span needs to be level when the longeron is level.

If the wing has an incidence, Vans has not told us. I just assumed it was zero. You are a correct, if there is an incidence it would have to be accounted for by adjusting the in flight measurement.

A friend commented at lunch the other day, you've got too much time on your hands. Actually, I don't. But one needs to have a mission when going flying.