What is a unit of AoA?

[HueyCobra8151]

I thought 100% NT was just wherever the (g)Avi mech w/ a hangover put the green sticker on the gauge.

 

[Squid]

i vaguely remember hearing something about 112.4% +/- 4??? i think that's the egt/rpm light...

the t-45 optimum aoa is 17, stall warning at 21.5, l/d max at 24, stall at 28+ depending on configuration, etc, etc. It's MAGIC! It's all about placement on the gauge (i.e. optimum for the landing pattern is always 3 o' clock,regardless of number)

 

[ArkhamAsylum]

Ok, the instrument error theory makes quite a bit more sense than the proverbial "I believe" button. I extrapolate this to assume that most aircraft that use the "units" measure have important numbers (i.e. stalling AoA) that come quite close to nice round numbers. Follow-on question: does 18 units in a T-34 roughly correspond to 18 units in a T-6, or any other airplane? Is it more manufacturer specific?

 

[sevenhelmet]

No, it is airframe specific.

 

[pdx]

The reason that UNITS of AoA are used instead of degrees or some other geometric unit is configuration. A T-34C with a properly calibrated gauge always stalls at 29.5 +/- 0.5 with flaps up or flaps down. The AoA in DEGREES is much greater for full flaps, but in UNITS it is exactly the same. Likewise, 20 units provides a 25% airspeed excess above stall. This is regardless of weight or flaps.

If the mechs wanted to, they could recalibrate the gauge to set stall at 30, but they don't. Instead, they set optimum AoA to be 20. You will have to memorize so many non-round number limits that you will get very used to this.

The engineers design the system, the gauges, and the calibration proceedure, but Navy test pilots determine the actual limits. Sometimes, the limits even change as the NATOPS does.

 

[gregsivers]

If it bothers you that much, you could just go helos. No AOA here.

 

[KSUFLY]

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

Wrong...well at least the first thing about the military does not measure the same as civilian. Even in civilian aircraft the AOA is an arbitrary number and has nothing to do with degrees.

 

[ArkhamAsylum]

Such wisdom and beauty.

 

[pdx]

If it bothers you that much, you could just go helos. No AOA here.

Plenty of AoA, you just can't tell what it is.:icon_wink

 

[usmcecho4]

Wrong...well at least the first thing about the military does not measure the same as civilian. Even in civilian aircraft the AOA is an arbitrary number and has nothing to do with degrees.

I was referencing a previous post by FLYGAL22 who postfaced her response with "*This is assuming the military measures the same as civilian.*" As my civilian aircraft time is limited to the mighty Cessna 172 (which I'm not sure even had instruments) I assumed this was correct. Thanks for the correction.

Incidentally I did find a civilian aircraft with direct measurements of AOA in degrees for you to put in your hip pocket. The space shuttle:

"Alpha displays vehicle angle of attack, defined as the angle between the vehicle plus X axis and the wind-relative velocity vector (negative wind vector). Alpha is displayed by a combination moving scale and moving pointer. For angles between minus 4 degrees and plus 28 degrees, the scale remains stationary and the pointer moves to the correct reading. For angles less than minus 4 degrees or greater than plus 28 degrees, the pointer stops (at minus 4 or plus 28 degrees) and the scale moves so that the correct reading is adjacent to the pointer. The alpha tape ranges from minus 18 to plus 60 degrees with no scale changes. The negative scale numbers (below zero) have no minus signs; the actual tape has black markings on a white background on the negative side and white markings on a black background on the positive side."

http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/avionics/dds/ami.html

I know they do a lot of DOD stuff but they claim to be civilians. I guess this all reinforces the post topic that AOA indicators are in no way standardized.

Semper Fi,
usmcecho4

 

[ChunksJR]

Learn it, use it, love it

The sooner the better...just believe.

 

[Schnugg]

Try this...

You can also fly on the back side of the power curve....just takes lots more power than you got.

 

[Chubby]

Damn, I think I'm just going to go with Chunks version ... sorry Shnugg.

 

[nmund11]

...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.

When the manufacturer produces an engine, they create a power setting at which if fully used it will not hurt the engine or shorten its life span. If the engineers underestimated the engine, and realize after production a couple of years down the line it is more capable and durable of what they initially rated it, then they will increase the opperating parameters. Therefore, you can now increase power beyond what was once "100%."

This is what I understand of the operating parameters which are beyond 100%... Correct me if I'm wrong...

 

[ChunksJR]

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?

I'll take "Reasons to fly helos" for $1000.

Alright, the answer is: "This is a great reason to fly helos"...

Cicco rings in first...

"What is we never talk about AOA in helos."

Correct!