A-7 AOA Demystified. Sort of...

[PRB]

The first mystery in A-7 AOA is that it is measured in “units”, not simply degrees. Nobody knows why, except the engineers who designed the plane. But it turns out that 1 unit = 1.5 degrees. AOA is measured by a “weather vane” device sticking out of the left side of the plane, and indicated to the pilot by a gauge calibrated in “units”, 0 to 30, on the instrument panel.

The problem, until now, is that I had no idea where 0 units is on this system. You can look at the chart in the NATOPS that says you will be at 13.5 units of AOA if you are 25,000 lbs gross weight, at sea level, at 200 KIAS, but what does that translate to in terms of actual aircraft pitch attitude (assuming level flight)? Without knowing where 0 units is, there is no way to tell. We know 0 cannot be a straight line running through the airframe, because we know that optimum AOA for landing is 17.5 units, and if 1 unit = 1.5 degrees, that would mean the plane would be assuming a pitch angle of 23.25 degrees nose high during landing, assuming a -3 degree glide slope ((17.5 X 1.5) – 3 = 23.25). Seems a bit nose high to me... The NATOPS does say that the AOA vane is “bore sighted to the Waterline 100 reference”, which is useless without knowing 1) what the heck “WL 100” is, and 2) what “unit” should be indicated when the AOA vane is lined up with WL 100.

Then I got my hands on a PDF copy of a USAF maintenance pub on general airframe “stuff” for the A-7D, and inside, I found the “AOA Rosetta Stone.” Turns out that WL 100 is a line running straight down the airframe (see pic). Also turns out that the indicated AOA when the vane is lined up with WL 100 is 6.2 units! Now we can see that 17.5 units (optimum landing AOA) is 11.3 units from WL 100, and that comes out to 13.95 degrees offset from WL 100, assuming a -3 degree glide slope. Still seems a little high, but they did land with quite a bit of nose up attitude.

Now we can look at that chart in the NATOPS and make some sense of it. Using the “clean aircraft” chart, 10,000 feet, 25,000 lbs, 300 KIAS = 10.3 units. 10.3 – 6.2 = 4.1 units nose high, which comes out to 6.15 degrees of “true AOA.”

 

[Snave]

I think you misunderstand AOA.

It has never been an absolute measure. Instead it is a relative measure based on the lift:drag component and the resulting vector of lift.

AOA represents the ratio between the chord of the wing and the vector representing the relative motion - or in simpler terms the difference between the angle of the wing and the angle of the direction of travel.

From this one can calculate the closeness to the onset of stall. AOA is thus a calculated value based on several factors.

Stall is relative not only to the absolute velocity but also incipient drag on the body in motion. If you stay n the green on the AOA you will not crash and die, no matter the IAS or the angle of incidence.


Obviously, from the forementioned it follows that AOA is a function of airspeed, drag and G-force. There is no need for absolute measures as they have no bearing on what the gauge registers...

 

[bstolle]

GREAT find Paul!! Thanx very much. I was looking for these data as well since a long time :mix-smi:

 

[ZEUS67]

Thank you very much Paul. I wish I had this information a few weeks ago, since it would have prevented a few sessions of my head hitting a hard object in frustration over what was the relationship between "unit" and "true" AOA.

It would have also prevented few voodoo sessions in which I burned wax effigies of Vought engineers for creating such a system and not providing accurate info on how it was built.

 

[warchild]

umm, not that i can add anything to the conversation really, but i did learn one interesting thing when building the FM for it.. The AOA recommended in the NATOPs for a carrier landing is 8 degrees, and not 3. That might bring your nose high position down to a more realistic level especially in light of the amount of drag the plane can generate during the glide..

 

[Sundog]

I think you misunderstand AOA.

It has never been an absolute measure. Instead it is a relative measure based on the lift:drag component and the resulting vector of lift.

AOA represents the ratio between the chord of the wing and the vector representing the relative motion - or in simpler terms the difference between the angle of the wing and the angle of the direction of travel.

From this one can calculate the closeness to the onset of stall. AOA is thus a calculated value based on several factors.

Stall is relative not only to the absolute velocity but also incipient drag on the body in motion. If you stay n the green on the AOA you will not crash and die, no matter the IAS or the angle of incidence.


Obviously, from the forementioned it follows that AOA is a function of airspeed, drag and G-force. There is no need for absolute measures as they have no bearing on what the gauge registers...

If you're just talking about the wing, this is true. However, when talking about an aircraft, alpha is usually the difference between the fuselage longitudinal axis and the velocity vector, since the angle of incidence of the wing is usually fixed.

However, I believe PRB didn't misunderstand such definitions, I think he is referring to the relationship between the A-7's actual AOA and how the A-7's system actually measures it. I never would have guessed the Vought engineers would have done it the way they did, but they must have had a very good reason. I just wish I knew what their reasoning was at the time.

 

[SkippyBing]

The Buccaneer and Phantom also had a gauge measuring AoA in units rather than degrees. I believe the reason for this is that it's easier to make a sensor that will measure AoA according to the definition Snave gave, rather than one that'll give the angle between mean chord and airflow.
I 'think' the Sea Vixen originally had one that measure in degrees, either way it was so good they replaced it with a clock.
Again I 'think' the reason units and Snave's definition of AoA are used are because they're useful when flying the plane, i.e. you will always be good flying X units irrespective of speed. It saves the aircrew trying to memorise a load of curves for the right value of AoA vs airspeed/AUM etc.

 

[PRB]

umm, not that i can add anything to the conversation really, but i did learn one interesting thing when building the FM for it.. The AOA recommended in the NATOPs for a carrier landing is 8 degrees, and not 3. That might bring your nose high position down to a more realistic level especially in light of the amount of drag the plane can generate during the glide..

Pam, where did you find 8 degrees in the NATOPS? All I could find was this magic 17.5 "units" of AOA when landing.

@Snave: Basically, if the plane is flying such that the relative wind is parallel to the WL 100 line, what the book called "zero AOA", the AOA gauge in the cockpit will read 6.2 units. This, by the way, isn't possible in actual flight, because the plane will always have some positive AOA, even at it's maximum speed of 500 something knots. Knowing this allows you to figure out what the airframe's AOA, in degrees, ought to be for a given value of "units" as measured from the AOA gauge.

 

[PRB]

The Buccaneer and Phantom also had a gauge measuring AoA in units rather than degrees. I believe the reason for this is that it's easier to make a sensor that will measure AoA according to the definition Snave gave, rather than one that'll give the angle between mean chord and airflow.
I 'think' the Sea Vixen originally had one that measure in degrees, either way it was so good they replaced it with a clock.
Again I 'think' the reason units and Snave's definition of AoA are used are because they're useful when flying the plane, i.e. you will always be good flying X units irrespective of speed. It saves the aircrew trying to memorise a load of curves for the right value of AoA vs airspeed/AUM etc.

I could go along with that, but the book says that 1 unit = 1.5 degrees. So it's a linear relationship. In the A-7, your landing approach is done at 17.5 units, regardless of landing weight. That comes out to different KIAS, depanding on weight, but the AOA will be the same actual angle, in degrees or units. The A-6 is this way too, and has almost the same approach AOA (17.0 units).

 

[PRB]

Thank you very much Paul. I wish I had this information a few weeks ago, since it would have prevented a few sessions of my head hitting a hard object in frustration over what was the relationship between "unit" and "true" AOA.

It would have also prevented few voodoo sessions in which I burned wax effigies of Vought engineers for creating such a system and not providing accurate info on how it was built.

Understand. I started thinking about how to get this kind of info as soon as we started talking about this a couple weeks ago. I even wrote the National Air & Space Museum and asked if they had "engineering and design data on the Vought A-7 series aircraft." They responded quickly and said they would put somebody on it and get back to me. They haven't yet. Then I thought to look for maintenance pubs, thinking some of this data has to be in them. The only ones I could find were from some outfit called Sicuro Publishing, and had USAF T.O. 1A-7D-2-1, General Info and Airframe Group, and T.O. 1A-7D-2-8, Flight Control Systems. They arrived in the mail yesterday. The first of these had the stuff I posted here.

 

[Sundog]

The Buccaneer and Phantom also had a gauge measuring AoA in units rather than degrees. I believe the reason for this is that it's easier to make a sensor that will measure AoA according to the definition Snave gave, rather than one that'll give the angle between mean chord and airflow.
I 'think' the Sea Vixen originally had one that measure in degrees, either way it was so good they replaced it with a clock.
Again I 'think' the reason units and Snave's definition of AoA are used are because they're useful when flying the plane, i.e. you will always be good flying X units irrespective of speed. It saves the aircrew trying to memorise a load of curves for the right value of AoA vs airspeed/AUM etc.

Actually, we engineers can make it measure in any units, one unit isn't easier than the next. Although, you could be right in terms of a mechanical system. It may have just been a "gauge stability" issue. Maybe 1 degree is too fine for the gauge. However, in all my time writing aircraft performance programs and analyzing stability and control, we've never used a wing reference. Granted, I only had a couple of years of doing that, but we never used the wing as the reference. It isn't standard, at least in aeronautics in the U.S.

However, I'm sure there is a good reason they use "units." I just don't know what that is yet.

As for the pilot, the pilot doesn't know if it's fuselage referenced or wing referenced. It could be set to anything, as long as we give them the proper numbers in the manual to look for on the gauge, that's all that will matter to them. So, if the pilot see's it will stall at "14 units" in the manual that's what he has to watch for if he is trying to avoid stalling. It doesn't matter to him/her what it's referenced to as long as it keeps them in a safe operating envelope.