Does it really have all information? It needs to determine the angle of the airflow in relation to a reference line. I don’t see how it could do that.
I can’t see that either. They probably just compute load factor and from there stall speed and compare it to airspeed. At least in the video they say they use the accelerometers.
It probably works on the difference between pitch (from AHRS) and the trajectory relative to horizontal (from GPS or baro altitude).
The airflow is assumed to always be horizontal. OK; that’s not a good assumption sometimes! But this device should work in steady state air, away from convective wx or terrain. Maybe they worked out that the airflow is “horizontal enough” in practice.
It has all the information available, which are CAS, OAT, groundspeed, magnetic compass direction, pitch and vertical acceleration, cross track angle. To calculate AOA, the only missing info, is to find out at which exact AOA the plane stalls, for which an initial calibration flight is needed. It should work even in wind shear, since wind shear immediately is registered as changes in TAS, and down draft, since immediately registered in changes of vertical acceleration.
What I don’t like is that it shows two indications always (at least judging from the pictures), one for flaps extended and one for retracted, since it does not “know”, whether flaps are extended or not.
What’s with the stupidity of doing a video production whilst flying a circuit???
Fake AoA indeed! But that doesn’t mean it can’t be accurate. The Aspen’s G-sensors are really quite good. The display looks a bit archaic. Also the dual indication isn’t very intuitive. I could imagine it would be easy to look at the wrong one. I’m not sure how that would work with a conventional AoA indicator either. Does the wing stall at different AoA’s and if so how does a conventional AoA indicator know the difference?
The main reason I did not fit one was because I could not be at all sure if they actually work at various altitudes all the way up to the aircraft ceiling. The reason I would want one is to optimise the configuration to reach the ceiling, not to stop me stalling at low level.
how does a conventional AoA indicator know the difference?
I don’t think it does, unless there is a flap position sensor which feeds into the AoA algorithm.
Fake AoA indeed!
What’s fake about it? It’s applied physics and math expressed in software, as opposed to faking AOA by using pressure differential.
It’s applied physics and math expressed in software
Can you describe the physics and math in detail, Lucius?
I can think of at least one very good mathematician who posts here.
FWIW, I suspect the other sensors (which mostly measure the AoA by the pressure in two little holes) probably don’t work well over altitude. Whenever I asked the mfgs this Q they avoided replying. The company I was going to install (Alpha Systems) stopped talking to me after that (which might just mean they lost interest in answering questions). A search term in the search box of
alpha AND systems
digs out the old thread(s).
What’s fake about it? It’s applied physics and math expressed in software, as opposed to faking AOA by using pressure differential.
Because it doesn’t know anything about the air that affects the wing. It just measures how the aircraft is moving in 3D space. AoA is defined as the angle between the chord of the wing and the undisturbed airflow that hits the wing. The Aspen doesn’t know anything about the latter factor. It just ‘assumes’. But as I said, that doesn’t mean it doesn’t work.
Because it doesn’t know anything about the air that affects the wing.
Well, it knows quite a little bit about it. From the air data computer it knows the air density (static pressure and temperature) and the velocity of the air.
Estimating the mass of the aircraft and measuring the load factor it can calculate the weight of the aircraft and – for a state of equilibrium where lift is equal to weight – solve the lift equation for Cl, the coefficient of lift:
(I hope it doesn’t create any problems if I paste other peoples graphics here once again…)
With Cl and stored parameters of the airfoil (the polar here is only an example) the angle of attack can be determined:
By definition the AOA is the angle between the airflow and the chord of the wing. Extending flaps influences the cord of the wing and changes the surface area (“S” in the equation above), therefore different values for the AOA result.
The dynamic case where the energy state of the aircraft changes (climb, descent, acceleration, deceleration, change of load factor) is somewhat tricky to calculate and requires additional input which in this case comes from GPS data and some guesses and assumptions making the derived “AOA” value rather uncertain.
AOA is an angle, a pure geometric property. Geometric properties are best measured with geometric methods (= AOA vane!). Everything else introduces errors and uncertainties.
Anyway, an AOA value by itself is totally meaningless for the pilot. Which is why many AOA indicators are not even labelled with angle figures (like 10, 20, 30 degrees) but with either dimensionless numbers (0-1) or color codes or both. I wonder why Aspen bothers to call their system “AOA” indicator at all. (Actually, I don’t wonder, they just jump the train of AOA add-ons that have been en vogue among light type owners for a couple of years after some clever salesperson started the whole thing by placing an article in a flying magazine). As the guy in the video says, what it really shows to the pilot is some kind of energy reserve. And this is what he needs to worry about, not the angle itself. They should simply call it “energy reserve indicator” and everything will be fine.
And finally, regarding the video itself: This is by a vast margin the most stupid flying video I have ever seen. There is zero need to do this interview in the air. They show nothing meaningful about the device they try to sell. And the talking should better be done on the ground. Instead they talk while the camera flies backwards without even bothering to look at the listener/customer. In my part of the world this is considered to be very very unpolite. No one would buy a car from somebody who keeps looking the other way while praising his merchandise. Stupid and bad and totally useless. Wasted fuel, wasted breath and wasted time.
