What is a unit of AoA?

[ArkhamAsylum]

I've asked this question of everybody I know, and haven't found an answer. I refuse to believe the size of the unit is arbitrary. My two guesses are (1) they are simply angles, and (2) it is a percentage from zero to max AoA (i.e. 90 degs). Any input?

 

[TurnandBurn55]

I refuse to believe the size of the unit is arbitrary.

You won't like the answers on this board, then

If it makes you feel better, in the Rhino it actually is a degree

 

[mmx1]

It's an angular measure, so yes, the choice of unit is arbitrary. Tough to compute fractionals on the fly, so we just pick the whole circle to equal something.

We picked 360 since it has a lot of factors.

 

[TheBubba]

Yeah... AoA is a completely arbitrary unit... it actually differs between different airframes...

Just press the I BELIEVE button. Its on the PFM panel right below the Aoa indexer.

 

[ArkhamAsylum]

NO! I don't believe! More specifically, I'm interested in the T-34, which stalls at 29.5 units. There has to be a reason it wasnt 30 units.

 

[flygal22]

AOA stands for Angle of Attack. If the unit for the T34 is 29.5 then the Critical AOA (angle at which the aircraft will stall [stop producing lift]) is 29.5 degrees. The AOA is the angle between the chord line and the relative wind. Chordline being an imaginary line drawn from the leading edge of the wing to the trailing edge (front to back).

While you can change the AOA in flight you cannot change the Critical AOA (the airplane will always stall at 29.5 degrees)

*This is assuming the military measures the same as civilian.*
Hope that helps.

 

[usmcecho4]

I never had trouble with pressing the IBB regarding AOA. What I don't like is that max N1 (compressor turbine rotation) is 101.5% or 102.6% of...100%? Good times. I hear that we just put gauges on that were not purpose designed and at max N1 they just happened to read above 100%. Maybe it's a "but this one goes to eleven" type thing.

Semper Fi,
usmcecho4

 

[Scamahmrd]

As far as the T-34 stalling at 29.5 units AOA, that does not necessarily mean 29.5 degrees. It's all dependent on airspeed. You can easily raise the nose 29.5 degrees at 180 KIAS without stalling, but at 90 KIAS, it's a different story. Again, a "unit" of AOA is arbitrary dependent upon the airframe.

 

[usmcecho4]

AOA stands for Angle of Attack. If the unit for the T34 is 29.5 then the Critical AOA (angle at which the aircraft will stall [stop producing lift]) is 29.5 degrees. The AOA is the angle between the chord line and the relative wind. Chordline being an imaginary line drawn from the leading edge of the wing to the trailing edge (front to back).

While you can change the AOA in flight you cannot change the Critical AOA (the airplane will always stall at 29.5 degrees)

*This is assuming the military measures the same as civilian.*
Hope that helps.

Military does not measure the same as civilian.
"The AOA indicators do not display absolute angles of attack, but are arbitrary units grouped around the optimum with specific areas of interest. These are stall warning and optimum approach and they are depicted on the face of the instrument as special indices. While on the ground, the AOA system will not represent any particular angle of attack..."
NATOPS 2.14.1.2
Hence the original question. [edit too slow]

Semper Fi,
usmcecho4

 

[sevenhelmet]

If the unit for the T34 is 29.5 then the Critical AOA (angle at which the aircraft will stall [stop producing lift]) is 29.5 degrees.

Nope, sorry... 29.5 units in the T-34 does NOT equal 29.5 degrees AOA. The units are indeed, completely arbitrary. The reason the T-34 stalls at 29.5 vice 30 units is instrument and engineering error. Someone designed the instrument to peg at 30 units in a stall, and instead it reads 29.5 units in a stall. That's less than a 2% error; close enough for government work. Ever wonder why the maximum continuous N1 RPM is 101.5% instead of 100%? Same reason. It's easier (read: cheaper) to just know your sh!t in the airplane than it is to constantly re-engineer everything to be a PERFECT round number and thus satisfy your sensibilities.

 

[TheBubba]

NO! I don't believe! More specifically, I'm interested in the T-34, which stalls at 29.5 units. There has to be a reason it wasnt 30 units.

Same reason the T-6 stalls at 18 units vice 20 or any other number...

The engineers picked a scale, and that's just where the stall fell on the specific airframes... at least thats the way it was explained to me... by an engineer...

I still say look for the PFM panel and press the I BELIEVE button... It helps... alot...

 

[usmcecho4]

edit i'm an idiot

 

[nittany03]

Maybe it's a "but this one goes to eleven" type thing.

Well you must be jealous that the Goshawk's goes up to a hundred and FOUR percent then . . .

 

[sevenhelmet]

I'm jealous... but not because of that.

 

[Harrier Dude]

Well you must be jealous that the Goshawk's goes up to a hundred and FOUR percent then . . .

The AV-8B goes to 120%.