Stall warning sensor

[Discharger]

Would anyone have any info on constructing a stall warning sensor vane similar to that used on some full size light a/c?

[green66]

If you are referring to an angle of attack sensing vane, this project that utilizes one might give you some ideas; don't know if it's anything like what F/S aircraft use. Here are the vane figures (http://www.charlesriverrc.org/articles/asfwpp/lelke_launch.htm); here's the main page (http://www.charlesriverrc.org/articles/asfwpp/helmutlelke_asfwpp.htm) (click the pic to proceed into the project). Text for the AoA sensor begins here (http://www.charlesriverrc.org/articles/asfwpp/lelke_activepitch.htm).

[JimDrew]

I would recommend using an accelerometer. It wouldn't take much coding to determine you were no longer moving forward. See:

http://www.dimensionengineering.com/DE-ACCM3D.htm

[Miami Mike]

Jim, it sounds like you don't know what "stall" means. Do you?

[JimDrew]

Yes, I certainly do... which is why I linked to a 3 axis accelerometer.

[Brandano]

two problems with that approach:
1) what you propose is a stall detecting device, not a stall warning device. There's little point in knowing that a stall HAS happened, you can see that by yourself by looking at the plane sinking
2) in any case is overkill.
You can just as well check if the airflow has separated from the upper surface of the wing by checking at what AOA the wing is flying. You will see that the flow separation happens quite predictably at a certain AOA for each plane. Just rig a vane to trim a switch a few degrees before the critical AOA is reached. On full scale planes this is often a small tab on the wing leading edge bent downwards at an adjustable angle. This is from a Cessna 310: http://www2.tech.purdue.edu/at/Courses/AEML/airframeimages/stallswitch.jpg In normal flight the airflow pushes the tab down, while at high AOA it's pushed up against the warning switch. Simple, reliable and easy to check in a preflight.
Oh, yes, I gforgot: Google http://images.google.com/images?&q=stall+warning+switch

[Miami Mike]

what you propose is a stall detecting device, not a stall warning device.Not even that. A stall has nothing to do with whether or not you're moving forward, and it doesn't necessarily mean that you're sinking either. A snap roll is an example of a stall that involves neither of these things. As you just wrote, it has to do with the angle of attack of the wing and the airflow separating from the upper surface. That's why it's incorrect to call a hammerhead turn a "hammerhead stall".

Your methods are perfectly valid, of course, but I can't see any way that Jim's idea of using an accelerometer could work to detect a stall.



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[Gary Warner]

Woops - I see my comments were already conered in using the 'tab' thing.

[Discharger]

Thank you all for your comments. The sensor tab/vane is what I was looking for and could be simple to construct but maybe a bit fiddly setting the correct position to detect the impending stall. Please feel free to continue the discussion on what is a stall!

[Miami Mike]

But first, how is this stall warning device going to communicate with you down on the ground?

[Discharger]

But first, how is this stall warning device going to communicate with you down on the ground?
Trying to keep things simple, one of many ways could be to use something like this piezo reversing alarm. Probably sound very much like the real thing if this one is loud enough.

[Discharger]

Apologies, couldn't get the photo to load, but device is commonly available.

[bravokilo]

put a small transmitter of a different band which will send only a signal in bursts and not continously. A tiny 3 transistor tx will do. You must have a receiver with a good antenna on the ground. The small flap near the leading edge with a switch underneath looks right.

BravoKilo

[Brandano]

I'd just put a buzzer or a few bright LED's on the plane. You got to watch it while it flies anyway.
My perception of a stall is that a wing section is stalled when it stops producing useful lift as a consequence of the flow detaching from the "upper" surface. This happens at a precise AOA, and not at a precise speed. An aircraft will stall at a higher speed in a turn, for example, because it will need a higher AOA to produce the additional lift needed for the turn. Let's simplify things, and try to think of the aircraft as flying straight. The heavier the aircraft is, the higher will be the AOA to keep level flight at a set speed. In a turn the airgraft has a higher apparent weight, due to inertia wanting it to keep flying off at a tangent (some people call this "centrifugal force", but it's just dear old inertia). The same sort of thing happens when you pitch violently, for example to initiate a snap roll, and it any high G manouver. The stall can be progressive or sudden, depending on the wing profile, and can happen on only portions of the wing. Stalling doesn't necessarily mean losing control either, since there's planes that can keep under control with large portions of the wing or the entire wing stalled, like the F18.

[Miami Mike]

Good description! That sounds perfectly accurate to me, Brandano.

[green66]

My perception of a stall is that a wing section is stalled when it stops producing useful lift as a consequence of the flow detaching from the "upper" surface. This happens at a precise AOA, and not at a precise speed...... The heavier the aircraft is, the higher will be the AOA to keep level flight at a set speed. Yes, stall occurs at a precise AoA, but a plane flies at essentially fixed AoA regardless of its weight (the AoA corresponding to the design lift coefficient). The heavier an aircraft, the faster it must fly (by the square root of the weight ratio) in order to generate lift equal to its weight for equilibrium level flight. A tab-type stall warning device has the advantage, besides its simplicity, of being independent of airspeed, weight, acceleration, orientation, air density.

[JimDrew]

Once the AoA is known, you can determine a stall using a 3 axis sensor. This in conjuntion with a pitot tube, used as a reduntant check, is how full scale airliners determine when a stall warning should occur (according to Boeing).

[green66]

Once the AoA is known, you can determine a stall using a 3 axis sensor. This in conjuntion with a pitot tube, used as a reduntant check, is how full scale airliners determine when a stall warning should occur (according to Boeing). I can see how a 2-axis (fore/aft and vertical) accelerometer could be used in a stall-sensing scheme, but I don't see how the 3rd (lateral) axis comes into play.

JimDrew - Is this something that XPS might consider for its telemetry system? An r/c stall sensor / mitigator would be an awesome feature if you can get it to work accurately & reliably, esp if the output could be fed back to the elevator servo, basically a pitch gyro that only comes into play when stall is impending.

I'm into electric-assisted gliders which typically launch and thermal well above 1000 feet where it's nearly impossible to tell if the plane has stalled until well after the fact, i.e. after the plane has fallen 50+ feet :( I think r/c thermal glider pilots would really be receptive to a practical / lightweight stall sensor since the desired trim for minimum sink rate is right on the verge of stall..... and there's the rub - - For a thermal glider the sensor would have to operate within a very narrow band - above the AoA for minimum sink to avoid false alarms but before the point of actual stall. Adding another challenge to a glider stall sensor based on pitch or speed is the natural phugoid oscillation (http://en.wikipedia.org/wiki/Phugoid) of a glider in its typical thermalling trim.

In my glider case, I typically dial-in a little down trim when at high altitude to be on the safe (stall avoidance) side, which is not so good because then the plane isn't flying at minimum sink. So a device that can maintain that fine line between min sink and stall I think would get some raves from the thermal glider crowd, in addition to jets and higher-speed powered planes.

Here is a post (http://www.rcgroups.com/forums/showthread.php?t=32132&highlight=glider+gyro) from a while back re using a gyro for holding optimal pitch trim of a glider, which I think may be of interest.

[Brandano]

Yes, stall occurs at a precise AoA, but a plane flies at essentially fixed AoA regardless of its weight (the AoA corresponding to the design lift coefficient). The heavier an aircraft, the faster it must fly (by the square root of the weight ratio) in order to generate lift equal to its weight for equilibrium level flight. A tab-type stall warning device has the advantage, besides its simplicity, of being independent of airspeed, weight, acceleration, orientation, air density.
Both assumptions are correct, I think. You are thinking in terms of optimal cruise, while I am trying to generalize. A plane can fly extremely fast at an extremely small AOA, or slow at a greater AOA, as you can see there's both terms in the equation to obtain level flight.

[edit] btw, I don't know if the added drag is worth the effort, but you could have a vane activate directly a pot and read the value off that, or use an optical encoder or hall sensor. Then use a microcontroller to limit the elevator servo excursion so that you never reach the stall angle, or even have it setting the proper AOA for the maximum efficience/speed (in this case deducting speed from AOA and weight. It's "IAS", but that's what you need anyway) I doubt it will keep you absolutely from stalling, but it might reduce it enough, and you could always have a stick pusher option set to get you out of the stall automatically. Or you could design the aircraft in such a way that the tail will always stall earlier than the wing (ever wondered why flat plate elevators seem to be more difficult to stall?) so that it will recover on its own.

[JimDrew]

Left/right/forward/backward/up/down... those are all positions relative to an aircraft's current location and flight direction. It is fairly easy for a computer to predict a stall if the parameters concerning the flight characterstics are available for comparison. This is in fact a module that we could add to the sensor array. However, although a possibility, I have not really toyed too much with the idea of the sensor array being able to change servo output.

[xtal]

What if you had a triangle shaped vane hinged at the point and made such that the upper side of triangle catches the airflow over the top of the wing and the bottom side of triangle catches the airflow across the bottom of the wing.. would give a Delta value?
then by somehow measuring the angle of the dangle of the triangle compute some
magic stall # related to the type of plane

just another silly thought!

[Andrew McGregor]

I'd just put a buzzer or a few bright LED's on the plane. You got to watch it while it flies anyway.
My perception of a stall is that a wing section is stalled when it stops producing useful lift as a consequence of the flow detaching from the "upper" surface. This happens at a precise AOA, and not at a precise speed.

Almost... it's not that it stops producing useful lift past stall AoA, it's just that stall AoA is that AoA that creates the absolute MAXIMUM amount of lift that wing can create... so with further increasing AoA the lift starts decreasing again, and the drag increases really fast.

It can be that the lift generated is still plenty to fly the plane, provided you have enough power to counter the drag; this is what a 'harrier' is. You might think that a harrier is 'hanging on the prop', but in fact you need much less power than for a hover, and that's because you're still getting lots of wing lift as well.

Everything else you said is bang on though :D

[Andrew McGregor]

Once the AoA is known, you can determine a stall using a 3 axis sensor. This in conjuntion with a pitot tube, used as a reduntant check, is how full scale airliners determine when a stall warning should occur (according to Boeing).

Um, yes, but this is way more complicated than necessary, and the software is massively airframe dependant. Also airliner stall warnings aren't required to work inverted or during aerobatics, whereas I think we'd want AoA warnings in any attitude for models.

An AoA sensor can be just a semicircular pitot vane with three holes (one forward, one 30 degrees up, one down) and two differential pressure sensors. Then it's just a matter of deciding what AoA to use for the warning and a little bit of trigonometry to figure out the AoA given those two dynamic pressure differences.

[Andrew McGregor]

However, although a possibility, I have not really toyed too much with the idea of the sensor array being able to change servo output.

It would be great... here's some functions I think someone might find useful:

Current limiter... measure motor current, throttle back (or reduce prop pitch) so as not to exceed a preset limit.

A real constant speed prop unit for folks with variable pitch... this would, in essence, vary the pitch to govern the RPM given a set power level (and do the right thing as the power comes back, that is allow the RPM to reduce somewhat).

AoA limiter... basically limit the elevator throw dynamically to whatever gives a particular AoA.

Automatic engine-out rudder trim for twins... this would make flying glow or gas twins practical (electric twins basically don't have to deal with engine-out).

RPM governor/limiter for helis... it would be a BIG selling point for the heli folks if this were built in to the telemetry unit, especially if it had pitch hinting.

EPR governor for EDFs (ok, this is getting exotic...) the efficiency of an EDF could be greatly increased by varing the tailpipe diameter (there's a simple mechanical way to do this) to set an exhaust pressure ratio based on the throttle input... but the only way to do this is a differential pressure sensor between the tailpipe and a static port, and have a servo driven off that sensor trying to achieve a pressure ratio that is related to the throttle setting.

All of these are linear mixes between some set of channels and sensor inputs, or limiting behavours relating a sensor to a channel.

[JimDrew]

Certainly all possible, it's just a matter of coding it. I am making the sensor protocol public information so that anyone can make their own sensor. I will look into making some kind of hook function to feed back into the channels.