Angle of Attack from pitch, TAS, and vertical speed?

wildwobby

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wildwobby
I had an idea recently that angle of attack can be calculated from the pitch angle minus the the climb angle. Since the pitch angle can be given by the AHRS in an aircraft with a G1000 or similar, and the climb angle can be calculated from TAS and vertical speed which the G1000 also knows... why can't the G1000 display angle of attack information (or can it?).

It seems it would be not require any additional equipment if it is calculated in this manner rather than measured with a separate AoA probe. What is the reason this is not done? I understand that the AoA is not very important in most GA aircraft where stalls are pretty easily predicted based on flying characteristics, stall horn, and speeds at various known configurations, but if the information is there, why not have the ability to display it?

Maybe because this wouldn't be measuring AoA directly, the error in the calculations to the AoA reading wouldn't be accurate enough to provide any meaningful information, but has it even been tried? It seems obvious enough that it would have been done before but I can't find anything online about it.

Anyone have any insight that cares to explain?
 
You know... that actually makes sense to me. The only contributing factor I can think of that's missing is the updraft or downdraft component. If your plane can climb 800ft/min at Vy and you're just holding altitude because of a big downdraft, the VSI will read 0 but you'll still have a large AoA.

Maybe you don't really want the vertical speed so much as you want the vertical wind component, because vertical speed is ground-referenced and vertical wind would be wing-referenced like TAS? I don't even know if there's a term for that.
 
Maybe due to the lag of VSI?

As I understand it, actual AoA indicators measure the air pressure differential between the top of the wing and the bottom of the wing. That's a pretty instantaneous measurement.

VSI is more of a 'trend' measurement. A 'calculated' AoA might not be as useful in low/slow/dirty air scenarios as an actual AoA measurement would be.
 
Maybe you don't really want the vertical speed so much as you want the vertical wind component, because vertical speed is ground-referenced and vertical wind would be wing-referenced like TAS? I don't even know if there's a term for that.

Yup, that's it.

And the instrument to measure vertical wind is ... an AoA sensor.

The vertical wind can be very, very significant, especially if you hang around mountains. This will limit the device's usefulness rather substantially.

There really isn't that much to a real AoA sensor. You have a form of one in the stall warning.
 
Dynon has AOA on some of its EFIS systems

Pitot tube has two ports
 
Yup, that's it.

And the instrument to measure vertical wind is ... an AoA sensor.

The vertical wind can be very, very significant, especially if you hang around mountains. This will limit the device's usefulness rather substantially.

There really isn't that much to a real AoA sensor. You have a form of one in the stall warning.


Actually it's not that significant. But...small changes make a BIG difference.


For example: a 1,000 fpm downdraft is huge, right? Well compared to your 80kt's through the air (8,101 fpm) it's a fraction. Also that vertical component would be enough to introduce an error into the display of a few degrees. A few degree difference equates into a large error.
 
You still need the incidence angle of the wing with reference to the longitudinal axis of the aircraft.
 
You can stall with the nose pointed straight at the ground. Angle of attack has nothing to do with the parameters you mention.
 
Pilots need to fly their plane by the seat of the pants and skill.... Having AoA gauges and all the other {gotta have} stuff in the panel will kill you fast... IMHO...
Staring at a gauge will NOT overcome instant windsheer..:no::(
 
Pilots need to fly their plane by the seat of the pants and skill.... Having AoA gauges and all the other {gotta have} stuff in the panel will kill you fast... IMHO...
Staring at a gauge will NOT overcome instant windsheer..:no::(
Au contraire. An aoa gage trumps everything in the plane including your pants.
 
Au contraire. An aoa gage trumps everything in the plane including your pants.


I will fly my plane..........................

You can stare at your life saving gauge........
 
I think that the upward and downward moving air could be solved for with GPS data, just like winds are.

I think think the lack of precision, lack of accuracy, and instrument lag would be the primary reasons this would be difficult to accomplish.

You can stall with the nose pointed straight at the ground. Angle of attack has nothing to do with the parameters you mention.

I would think the opposite.

All you need to know is what direction air is hitting the aircraft from. If you know your attitude (pitch/roll/yaw) and the path through the air (TAS, and "true" climb/descent rate) you could easily calculate AoA.

The difficult part is doing it it accurately and with no lag.

Pilots need to fly their plane by the seat of the pants and skill.... Having AoA gauges and all the other {gotta have} stuff in the panel will kill you fast... IMHO...
Staring at a gauge will NOT overcome instant windsheer..:no::(

Most people that say this fly aircraft with more instruments than is required by regulation, and I doubt they ignore them in flight.

IAS is good for knowing how long it's taking you to get somewhere. AoA is the most critical information when aerodynamic limits are of concern. From what I understand, it works pretty well for carrier pilots, and high altitude pilots.
 
You know... that actually makes sense to me. The only contributing factor I can think of that's missing is the updraft or downdraft component. If your plane can climb 800ft/min at Vy and you're just holding altitude because of a big downdraft, the VSI will read 0 but you'll still have a large AoA.

Maybe you don't really want the vertical speed so much as you want the vertical wind component, because vertical speed is ground-referenced and vertical wind would be wing-referenced like TAS? I don't even know if there's a term for that.

I see what you are getting to. I guess "true" VSI is needed to do this accurately in practice.

Uncertified avionics equipment, maybe.

Certification or not, that doesn't change how the math works.

Maybe due to the lag of VSI?

As I understand it, actual AoA indicators measure the air pressure differential between the top of the wing and the bottom of the wing. That's a pretty instantaneous measurement.

VSI is more of a 'trend' measurement. A 'calculated' AoA might not be as useful in low/slow/dirty air scenarios as an actual AoA measurement would be.

I'm not sure thats how AoA meters work, but the VSI lag makes sense. As Ted said, I should be referring to "true" vertical speed.

Dynon has AOA on some of its EFIS systems

Pitot tube has two ports

I just looked at that. Is that actually measuring AoA though? I could be wrong, but that seems to be more like a LRI with a pitot port that is aligned with the known critical angle of attack? Wouldn't an AoA meter need to be physically movable to align with the free stream direction for all phases of flight?

You still need the incidence angle of the wing with reference to the longitudinal axis of the aircraft.

That is true.

You can stall with the nose pointed straight at the ground. Angle of attack has nothing to do with the parameters you mention.

Angle of attack has everything to do with the parameters I mentioned... in fact it is defined as the angle between the wing chord (which is the pitch angle plus the constant angle which the wing is mounted with reference to the longitudinal body axis) and the velocity vector (stability axis).

Pilots need to fly their plane by the seat of the pants and skill.... Having AoA gauges and all the other {gotta have} stuff in the panel will kill you fast... IMHO...
Staring at a gauge will NOT overcome instant windsheer..:no::(

As I said, I was not recommending this be the best solution for small aircraft. In larger aircraft, it is very important though. The flight conditions are not as constant with larger aircraft and experience is much more difficult to rely upon. The stall speed depends on the weight of the aircraft which drastically changes in bigger aircraft. Additionally, the obvious warnings in larger aircraft are not present to the extent they are in most smaller aircraft. This is why they have stick shakers and such and actual AoA meters.

I will fly my plane..........................

You can stare at your life saving gauge........

Ok... you use "the seat of the pants and skill", I'll use instrumentation that tells me.

I think that the upward and downward moving air could be solved for with GPS data, just like winds are.

I think think the lack of precision, lack of accuracy, and instrument lag would be the primary reasons this would be difficult to accomplish.



I would think the opposite.

All you need to know is what direction air is hitting the aircraft from. If you know your attitude (pitch/roll/yaw) and the path through the air (TAS, and "true" climb/descent rate) you could easily calculate AoA.

The difficult part is doing it it accurately and with no lag.



Most people that say this fly aircraft with more instruments than is required by regulation, and I doubt they ignore them in flight.

IAS is good for knowing how long it's taking you to get somewhere. AoA is the most critical information when aerodynamic limits are of concern. From what I understand, it works pretty well for carrier pilots, and high altitude pilots.

Good points, except I think you mean to say ground speed is good for knowing how long it takes to get somewhere.
 
I just bought the Alpha AOA unit instead.
 
Pilots need to fly their plane by the seat of the pants and skill.... Having AoA gauges and all the other {gotta have} stuff in the panel will kill you fast... IMHO...
Staring at a gauge will NOT overcome instant windsheer..:no::(
Ben, having had AoA on my glareshield for more than 10 years I disagree with your assessment. First of all I never "stare" at the instrument, it's simply part of my scan during certain maneuvers. Second, there are times when knowing how close you are to stalling can be very useful such as on short final, climbing at Vx to clear obstacles, or even the dreaded "impossible turn".
 
Ben, having had AoA on my glareshield for more than 10 years I disagree with your assessment. First of all I never "stare" at the instrument, it's simply part of my scan during certain maneuvers. Second, there are times when knowing how close you are to stalling can be very useful such as on short final, climbing at Vx to clear obstacles, or even the dreaded "impossible turn".

I'll go you one better. I've been flying jets that have AoA indicators for nearly 30 years. Assuming you understand what they're telling you, they are the single most important flight instrument on the panel.
 
I have one of each (original chevron and circle display on vertical stick) and the horizontal colored light bar. I installed both for demo use but prefer the vertical stick on top of the glare shield since it's the same as most of the jets.

Installed it yet? Which display presentation did you opt for?
 
Maybe due to the lag of VSI?

As I understand it, actual AoA indicators measure the air pressure differential between the top of the wing and the bottom of the wing. That's a pretty instantaneous measurement.

VSI is more of a 'trend' measurement. A 'calculated' AoA might not be as useful in low/slow/dirty air scenarios as an actual AoA measurement would be.

The Lear 20-series had a vane on the left side of the forward fuselage that drove the AOA indicator...don't know where you got the pressure idea.

Bob Gardner
 
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If your airplane doesn't have an installed sensor.....just try to visualize the AOA.

images
 
The Lear 20-series had a vane on the left side of the forward fuselage that drove the AOA indicator...don't know where you got the pressure idea.

Bob Gardner

On the AoA on the RV-10 (the only AoA setup that I am familiar with), there is a small hole on the top of the wing and a small hole on the bottom of the wing.

That's where I got the pressure idea.
 
On some newer planes (my DA20, for example), the stall warning is actually a crude AoA sensor. A 1/4" hole on the leading edge is normally an area of high pressure, but when the critical AoA is exceeded, it becomes low pressure and "squaawk". Works great, lasts a long time.;)
 
On the AoA on the RV-10 (the only AoA setup that I am familiar with), there is a small hole on the top of the wing and a small hole on the bottom of the wing.

That's where I got the pressure idea.

I learn something new every day. I have no experience with AoA indicators on piston airplanes in general and no knowledge whatsoever of the RV-10 specifically.

Bob
 
On the AoA on the RV-10 (the only AoA setup that I am familiar with), there is a small hole on the top of the wing and a small hole on the bottom of the wing.

That's where I got the pressure idea.
That's Jim Franz's system which also ties into the airplane's pitot-static lines. I have a different setup which uses a separate probe. But both of these "AoA" sensing systems only approximate the actual AoA. To truly measure AoA requires a probe with 3 ports or a moving vane AFaIK.
 
I'll go you one better. I've been flying jets that have AoA indicators for nearly 30 years. Assuming you understand what they're telling you, they are the single most important flight instrument on the panel.

Personally I believe that an AoA indicator is a better tool than an ASI for any "performance" monitoring except cruise speed (and even then the ASI requires compensation for DA). It's too bad someone decided to label their dynamic pneumatic pressure gauge with "airspeed" numbers. Had they left the scale to read in Pascals or some other unit of pressure we wouldn't have to explain that you can stall a wing at any "airspeed" and AoA would be taught in primary flight training.
 
What difference could the units alone make? If you can stall at any airspeed, you can stall at any dynamic pressure. It wouldn't make any difference.
 
What difference could the units alone make? If you can stall at any airspeed, you can stall at any dynamic pressure. It wouldn't make any difference.

My point is that we wouldn't be taught about stall "Speed", we'd learn about the critical AoA instead. The POH wouldn't list performance airspeeds like Vx and Vy, it would provide the AoA for best rate, max range, etc. But you are right, the problem isn't so much the units on the dynamic pressure gauge it's the lack of an AoA indicator.
 
The AoA would be quite interesting as a focus, but relabeling the ASI in pressure units would just have people discussing stall pressures. Pressure is not AoA; it has exactly the same faults as speed. Because it's the same measurement.
 
You still need the incidence angle of the wing with reference to the longitudinal axis of the aircraft.

Which is fixed. You also need the airspeed which presumably the G1000 also has. The big question is to what precision is all this information? If we're talking about a barometric vertical speed (as in the case of a standard steam gauge VSI), we might not quite have the accuracy to compute this with sufficient accuracy to be useful.
 
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You can stall with the nose pointed straight at the ground. Angle of attack has nothing to do with the parameters you mention.

Eh? If you have pitch angle, vertical speed, airspeed (all variable) and angle of incidence you can compute AOA.

If you are nose pointed straight at the ground and falling vertical you are not stalled (in any aircraft I'm aware of, the angle of incidence is never THAT far off horizontal). You'd need to translating horizontally in addition to cause the relative wind to sufficient to exceed the critical angle. All this can be calculated from the parameters above.
 
Eh? If you have pitch angle, vertical speed, airspeed (all variable) and angle of incidence you can compute AOA.

If you are nose pointed straight at the ground and falling vertical you are not stalled (in any aircraft I'm aware of, the angle of incidence is never THAT far off horizontal). You'd need to translating horizontally in addition to cause the relative wind to sufficient to exceed the critical angle. All this can be calculated from the parameters above.
Not falling vertically. Flying vertically. Thats the error in this line of thinking. Relative wind has nothing to do with the relative angle of flight vs the ground.
 
Angle of flight relative to the ground has nothing to do with angle of attack.
 
Eh? If you have pitch angle, vertical speed, airspeed (all variable) and angle of incidence you can compute AOA.

If you are nose pointed straight at the ground and falling vertical you are not stalled (in any aircraft I'm aware of, the angle of incidence is never THAT far off horizontal). You'd need to translating horizontally in addition to cause the relative wind to sufficient to exceed the critical angle. All this can be calculated from the parameters above.

No. A sudden change in attitude, such as trying to pull out of that dive quickly, will cause a stall. No horizontal motion is necessary.
 
No. A sudden change in attitude, such as trying to pull out of that dive quickly, will cause a stall. No horizontal motion is necessary.

His stipulation was "nose down". If you are in a sustained vertical descent, not nose down, then of course you could be in a stalled condition.
 
I've been trying to wrap my head around this and I do believe that if you know precisely (to the degree you want to measure AoA) what your path is (normal to the wings) and what your pitch attitude is the AoA will be equal to the difference plus the wing's angle of incidence in still air. But if you throw in vertical air current which you have no independent means of measuring the AoA becomes uncertain at least dynamically. Consider an airplane flying level in still air with an AoA of +3° (Pitch=0, path angle = 0, incidence = 3). If that plane flies into a rising column of air, the AoA will almost instantly increase yet the pitch, path, and incidence haven't changed. If those conditions remain steady long enough for the airplane to return to it's trimmed AoA the airplane will eventually reach a new equilibrium with a vertical speed equal to the velocity of the rising air thus again making it possible to calculate the AoA from pitch attitude and path.

It might be possible to compute AoA during that transition by adding acceleration (g-force) and weight to the mix but if you know the weight, acceleration and CAS you can compute AoA indirectly from just that using the wing's lift vs AoA curve.
 
EDIT:

I don't think that what I posted is right. Off to think some more.
 
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I had an idea recently that angle of attack can be calculated from the pitch angle minus the the climb angle. Since the pitch angle can be given by the AHRS in an aircraft with a G1000 or similar,
The 'pitch angle' you refer to is an angle measured relative to the plane of the earth, right?

Your angle of attack measurement would only be correct when the AoA is measured relative to the plane of the earth.

Your idea would work in a practical sense during normal level no gee climbs and descents, but when g forces or outside forces of up/down draft occur, these measurements would not work,..I don't think.
 
I see what the OP was thinking,

If you know exactly where the nose is pointed, and exactly where the plane is actually going you could figure out what angle the relative wind is coming from, at least in still air.

However here is another scenario where the calculating AOA would be difficult. I can hang on my prop behind the curve and get the ASI to indicate zero. That would sure throw a wrench at the computer trying to figure out TAS.
 
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