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Diyguy

Stall warning device?

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Wondering if anyone has an idea how one could build a stall warning device, which, I presume, would transmit an audible warning over your audio tx. I have limited full time hours and don't remember how the 'real' planes warning system works, air flow mechanical 'flapper' thingy- I think. Now that the MAHI concept is a reality, seems like this would be a nice addition. You would have to have a method of establishing the stall threshhold of each plane. This sound plausible? Maybe there are other 'nice to have' projects you have in mind that would enhance UAV craft.

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Full scale planes use a sort of tab located just under the leading edge that will flip when air doesn't flow fast enough on the wing. Basically if the concept still works at our scale a switch and a few metal parts should do it...

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Yea! I remember on a Cessna that metal tab. Should be easy to replicate, a pivoting piece of metal, spring loaded and a micro switch connected to a piezzo buzzer and/or led light might work. I guess the airflow has to 'get under' the flap and push it backward or up as the AOA increases otherwise it would activate at taxi speed, I think. Think I'll drop in the 'full size' forum over at RCU and see if anyone can explain more.

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If you use telemetry you could set up a warning at a specific air speed ?

Im not sure you need it though ??

Terry

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Hey,

Well... if you cut a hole in the leading edge and put a thick bit of card board or metal flap in a tunnel with in the wing, with a button and part of the flap overhanging the button, as the airspeed increases the flap will hold down the button and disable any warning device you want use, as the airspeed drops off or the angle of attack reduces the airflow within the cut out tunnel the flap will release pressure on the button and the stall warning will go off, you just have to tighten or lossen the flap hinge as required so that you know the stall speed of the aircraft.

I hope this helps...

- Philip

P.S I apologise for the image, its just to give you the general idea...

post-8-1110286505_thumb.jpg

Edited by goose

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Another good idea! I dropped this question over at RCU ( Aerodynamics forum)and got some ideas there too. One was mention of a ball and/or use of a vane at the wing tip. I was thinking last night of a variation of your idea. Let me see if I can explain. Drill a hole thru LE inboard ( the few full full size I recall seemed to be well inboard), elongate the hole up and down to allow adjustment, a piece of tube soldered to a 'plate' with 2 elongated holes or slots to allow adjustment. The consensus seems to be with models, we're talking very small movements. The starting point appears at what they call the 'stagnation point'. Making the tube long enough would then allow your flap/switch combo essentially well within the wing interior. Not sure what size tube to start with, think maybe 1/4" as I think there may be a volume issue. Not sure of the pressure exerted between say 30mph and a stall configuration. The micro switch, I think would have to be super sensitive. I've got one somewhere in my shop I'll give atry. One fellow referred me to an AOA system-very cool- But I think that's another good project. Maybe with a 'Nagging Nadia' voice!

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Hey,

I am glad I could be of some help, the pilot tube seems like a rock solid idea, your right in relation to the small movements in relation to RC aircraft aerodynamics, with a vane you would get a more "correct" reading, as the airflow wouldn't be spoiled by the aircrafts structure.

I suppose it could be set up so that if your RC aircraft has Telemetery, you could set up a very simple algorthm so that once the aircraft goes to a certain angle of attack, using the vane, and then an a predefined airspeed, the telemetery could start screaming...

Some "Bitching Betty" promps would be fun for RC telemetery. :)

- Philip

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Just want to point out that stall is only dependent on AoA, not velocity. Of course, as lift depends on AoA and velocity, a low velocity will require greater AoA to maintain proper lift. The little "flap" on a full size plane actuates when the AoA gets too great (maybe 14-15deg), not when speed is too low.

Aslo, since the mass of the plane will differ some from flight to flight (and during flying due to fuel burn) the "stall speed" will differ. "Stall AoA" will not change, since it depends on the wing itself.

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I'll come back and try to think through the comments. Look at this for a AOA/stall warning device- www.hciaviation.com ( How do get this to create a link?)

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Maybe I'm approaching the concept from the wrong direction, i.e., perhaps an AOA device similar to the one in the above site, incorporating a stall warning is more useful. From the users's standpoint, the value of a negative AOA landing approach is, maybe, just as useful. This assumes one is using video. I initially was thinking in terms of my camera inside which takes in part of the instrument panel. If a high powered aircraft, prop or jet, goes vertical, does it stall, or just run out of airspeed? Just based on memory, seems like most of my planes start mushing somewhere around 50 degrees of climb. The telemetry is frankly beyond my ability to figure out! I have no idea how accurate a simple device might be capable of. Looking at the pics of the MAHI device, if you were climbing, with no visible horizon, I guess you could establish a line or reference point on the vertical plane whereat your particular a/c approaches a stall-or I should say, shortly before. Some of this stuff may be less precise than a full size, but heck that's the fun of this, I think!

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I am not sure if I quite understand you now. Angle of attack should not be mixed with flight path angle, the latter one being the angle indicated on a horizont indicator. The AoA is the relative angle between the wing and the free flow. If you have a negative angle of attack on landing you most sure have negative lift (or close to zero for a non-symmetric profile), but negative flight path angle is common until flaring out for touch down.

If I will make myself an AoA indicator I will make a small flap that will be bent down, actuating a microswitch, at leading edge. When approachin stall AoA the flap should be "grasped" by the flow and flip over, releasing the switch, the stall alarm should sound. The angle of the flap would probably have to be greater than your expected stall angle, but I will not go into the that theory..... Anyway, I would start with a flap angle of, say 15 deg, then do a flight test, adjust, test....and so on. Mechanically very easy, and should be a good indicator.

Edited by Flutter

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The stall warning device on the link works after the same principles. More advanced you can make a little fin on a potmeter, place it somewhere where the airflow is relatively undisturbed and measure the angle of attack. Very commonly used. Some fighters use probes with two sets of pressure holes. When the aoa changes there will be a pressure difference measured, the probe adjusts so the pressure equals, and the amount of adjustment needed gives the AoA. Not very practicall for my Cub I guess..... :).....but a "bitching betty" would be cool..... Warning warning.....pull up.....

Edited by Flutter

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Flutter, I can't tell how the stall warning is incorporated in the above AOA device. I'm still confused about the "relative angle between angle the wing and free flow". Without going back to some of the sites I looked at, I got the impression some of these AOA indicators could be used for setting up one's glide path. If we get that far, maybe we can have a 'balling Bruce" for those so inclined!

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Diyguy. Imagine that you are in a car driving on the highway. You put your hand (flat) out the window, so the palm is paralell to the road. Now the AoA is zero of the hand relative to the airflow, and the road, no lifting force felt, only the force paralell to the road, pushing your hand backwards. The flight path angle is also zero, the car moving paralell to the road (earth fixed system). Now, tilt your hand a little , exposing the palm to the wind, you will feel the lifting force. The hand has still the same "flight path" angle, following the car down the road, but the AoA has changed, positive, resulting in a lifting hand (or wing). Tilting the hand down again, past neutral, will push your hand downwards, due to negative AoA, but the flight path is still unaltered. Go for a ride :=)

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Just a thought or two.

Angle of attack is better than airspeed for stall warning. The stall angle remains constant while stall speed changes with G loading, aircraft weight etc.

I have flown the Piper Cherokee and recall its stall warning system.

It used a tab I would guess about 2cm (3/4”) square. It was installed in the lower leading edge of one wing, was angled slightly down and possibly held with a very light spring tension or weight. Its travel was limited to a few degrees so if lifted by a high angle of attack it would reset as the angle was reduced again.

I wouldn’t think that a reduced model sized tab could provide the force needed actuate a micro switch.

An alternate idea could be to use an optical device that could detect the small lifted tab. Look at an old roller ball mouse and the optical setup that uses an LED and sensor divided by a rotating wheel with slits to detect motion. Just apply the idea and parts. The needed electronics could be simple.

Mark

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