AOA units to degrees conversion

Just got my F-14B after a free trial. Really appreciate the quality of this product.

Found myself struggling with understanding of AOA units. Basically I don’t need to overthink this question, and use this indication as it is. But still, just theoretically:

According to the manual : “ Tape indicating angle of attack (AOA) on a scale of 0 to 30 units. (Equivalent to -10° to +40° rotation of the AoA probe.)“

So 50 degrees of total AOA probe range corespondents to 30units of AOA tape. Which makes around 1 unit=1,67 degrees.

According to this proportion 15units= -10 + (15*1,67)= -10+25=15 degrees of AOA, but in fact in window of external view (F2) we can see AOA= 10.

Basically we don’t know what does AOA shows to us. Is it a AOA of the longitude axis of an airplane, or it’s AOA of a airfoil chord (which is more likely).

When the F14 is on the ground with the pitch of “0” AOA is 0.7 degrees (on the external view). This probably means that wing to fuselage attachment angle is about +0.7.

If the proportion above is correct we might think that AOA tape supposed to show us indication of around 6.4. But in fact we can see only 4.5 instead.

So that proportion isn’t correct for both cases.

So how does it work in reality? Is this proportion linear? And what is the “zero” position of probe to fuselage attachment? Or what I’m missing?

For example F15 on the runway shows pitch of 0 degrees and AOA =0 (in external view window F2). AOA tape shows 10units, (whole range from 0 to 40) which make me think that it’s kinda the same system but with proportion 1unit =1degree of AOA. And range 0 to 40 units= -10 to +30 degrees of AOA.

Please let me know what you think about all of this. Thank you!

I think the formula is dynamic, that is it depends on your airspeed/mach number. I have only found a conversion rate for a landing configuration.

It' a system to keep things easy to read for the pilot. It's easier to read "15 units" than to try and find "25.65 degrees" on the instrument (numbers are made up for argument's sake). I don't know if it's linear or not.

From Boeing, "Many AOA indicators used in the past have been of the "normalized" type, where AOA is shown in arbitrary units and scaled so that zero load factor is shown as an AOA of zero and stall is shown as an AOA of one. Normalized AOA on a commercial jetliner would require that Mach number be introduced into the calculation of AOA because stall AOA and buffet margins are a function of Mach number." Except in our case it's 0-30 rather than 0-1.

http://www.boeing.com/commercial/aeromagazine/aero_12/attack_story.html

Modern aircraft they dont actually show AOA to you even they do have AOA sensors. What you can see its a "caution bars" which appear when you're getting closer to the critical or max AOA. Lets say Embraer, I'm flying Emb-135BJ, shows those bars to you in green when you're 10deg to stall, yellow when 5, and red when its 2 or less. Hawkers and CJ they do have a separate gauge for the AOA. So it's marked from 0 to 1. One '1' is a stall. So normally Vref is always at 0.6. And in all these examples indication is linear. And because of it and also because they mentioned range of AOA probe in the manual, I thought it supposed to be some kind of proportion between units and AOA in degrees. Im pretty much sure that load factor and a Mach number (at list below speed of sound ) is only affecting stall speed but not the max AOA.

You are calculating wrong thing and that is AOA vane deflection according to AOA instrument, that is your 1,667°.

You have to calculate AOA instrument scale to real wing AOA position.

You know that angle of incidence of the wing is 0,74°, when you are on the ground and AOA instrument scale is 4,5 units,

You also know that 15 units AOA is real 10° wing AOA. (if those values are not right somebody correct me)

Based on that according to my calculations (1 units AOA = 0,883° wing AOA):

0 units AOA = -3,23° wing AOA

4,5 units AOA = 0,74° wing AOA

15 units AOA = 10° wing AOA

30 units AOA = 23,25° wing AOA

From the f14 wing rock model:

AOA(true) = 0.8122*AOA(ARI)+0.7971

F14 operating at Mach number less than 0.4:

AOA(units)=1.0989*(AOA(true)+3.01)

‘The net effect of this conversion is to subtract five from the indicated AOA expressed in units to get AOA in degrees (i.e. 25units = 20 degrees)’.

Wow! That info is what I've been looking for. Thank you very much Gentlemen you've saved my day!

Now it looks reasonable and understandable for me. Thank you!

‘The net effect of this conversion is to subtract five from the indicated AOA expressed in units to get AOA in degrees (i.e. 25units = 20 degrees)’.

So is the manual incorrect with its -10° to +40° range?

So is the manual incorrect with its -10° to +40° range?

No its correct, it is the angular deflection limits of the AOA sensor itself. From what I know this sensor is registering "local AOA" at the place it is installed.

Local AOA at the sensor location will be different from true wing AOA due to airflow disturbance around the nose where the sensor is installed.

You have posted that link to that Boeing AOA article, Im sure its mentioned there.

So is the manual incorrect with its -10° to +40° range?

Probably not :)

This formula and simple math is for speeds below mach 0.4. Wings are full forward then and critical aoa hardly above 25 units.