I'm going to throw this question in here and see if someone has an idea. I remember on a full size Cessna a pivoting metal 'flap' device that activated the stall warning buzzer. This was on the LE. I don't know what other types may be out there. To replicate this on a model, I'm wondering how to create this. I'd like to do this on a model with tv tx and an audio channel. What puzzles me is the relation of the airflow to the flap at high AOA. Any thoughts?

I honestly don't know your immediate answer but I do have an indicator that a stall has occured -

....... my airplane stuck in the ground.

What is your building technique? Mine usually just explode in little matchstick size fragments!

Cessna most likely determined the position by empirical means. It probably was some variation of mounting it on the leading edge and moving it around until they got consistant performance at stall.
I do remember from over 40 years ago that it was part of the preflight to turn on the master switch and push the little tab upwards to chech for reeedom of movement and that the stall warning horn was functioning.
In models the onset of stall is usually more rapid as the operate in lower Reynolds numbers and lower levels of inertia.

I suspect you're correct as far as the placement is concerned though there may be more sophisticated methods for designing these in larger A/C. As I recall, the Cessna's 'flap' was more inboard. I had some hours in a Piper Cherokee but recall nothing as to how it worked. Can you explain the inertia aspect, and how that translates to models. With my limited flying experience in the above planes, you 'feel' the stall coming on. Trying to compare the relationship of the speed curve between, e.g., 'slow' flying and a stall. I recall years ago flying an old Schweitzer sailplane wherein the the difference between speed at level flight and stall was maybe 2or 3 knots.

There is a line along the wing leading edge where the flow divides with some going over the top of the wing and some going underneath. That line is called the stagnation point. It is close to the nose of the airfoil at low angles of attack and moves downward as the angle is increased. The stall warning vane is located such that as the wing approaches the stall angle the stagnation point moves past the vane causing it to be blown the other way by the change in flow from below to above the wing. The warning must actuate not less than five nor more than ten knots above the stall speed.

The switch is positioned to the initial location during production and it is checked during production flight test. There is an adjustment available of about 3/8 inch.

For an aircraft like the Cessna 172 and Piper Cherokee, the stagnation point moves a sizable fraction of an inch. For wings the size of a typical R/C model the range would be almost micrometer size. Although the same principle applies, in practice it would require a level of precision that would be difficult to say the least.

Gosh, I love this place! I have never heard of 'stagnation point' before, most informative. It shouldn't be too difficult to make such a device, the results may be less than desirable. I guess an air blast, fan, compressed air, or such, would allow simple testing. ASK here and YE shall receive the answer! Now about the Lottery................

II have had similar thoughts. I was thinking of measuring pressure at the top and bottom of the wing close to the leading edge.
If a slight negative pressure exists at the top we have lift. If the pressure differential falls then it would activate the stall device. This could be horn, or switch . for high lift device etc. Is this feasible?

I don't know why it's not feasible. Whether it's practical may be another story. I thought it might be useful flying with video where the field of blue sky doesn't give an indication of attitude. For example, the video cameras I have both have audio channels. The circuitry seems simple enough. a mechanical device-'flap'- triggering a micro switch, a power supply and a piezzo speaker and/or led light. If you try to 'sense' the air flow electronically, might be a bit more complicated. The fellows at www.rc-cam.com have developed an attitude indicator which overlays on the video. Glyphs give attitude and bank indication of your plane. This seemed like a natural progression or addition.

What you are looking for is something like this system made for homebuilt airplanes (like my RV-4).

http://advanced-control-systems.com/

I think you are missing the point. To be of any use, the device must warn of an impending stall. Once the stall has occurred, the plane is falling and there is no mistaking that the stall has occurred. When operating near, but not yet at the stall angle of attack, there is still lift (i.e. a normal pressure differential between upper and lower surfaces.)

As far as a vane to sense stagnation point, on a wing sized for a .40-. 60, airplane the movement would likely be somewhere around 1/16 inch. Though such a small and precision device might be made, it’s not likely to be rugged enough to withstand the rigors of model airplane operation.

Another possibility might be a vane mounted on a boom near the wing tip that would align itself with the relative wind. Such a vane is frequently used during flight test of full-scale aircraft and would be a little simpler to construct and calibrate than a stagnation point sensor. It would still require precision (sort of watchmaker) skill in fabrication and would be a challenge to be made rugged enough for routine flight. Sometimes a ball is used on the end of the boom rather than a vane. The ball has two holes about 90 degrees apart connected to a differential pressure sensor. The sensor can be correlated with angle of attack. The instrumentation normally used costs many hundreds of dollars but perhaps with some ingenuity something could be adapted to work.

From your signature line you might be talking about model turbines. If so, the boom could be mounted directly in front of the nose, and the larger size would permit a little more room for mechanism.

Although as an academic exercise the subject is interesting, It seems as though the craft you are describing might be more akin to a military RPV rather than an around the patch fun machine. All of these problems have been addressed and solved in the commercial arena, and you may find all you need to know in unclassified, published sources.

Sometimes, I miss a response to a post, this being the case here. Yes, Lou, ultimately, the idea is incorporation into a 'for fun' UAV airplane. Using video, gps, autopilots and the rest of that stuff can be fun for 'flyin' around the patch'. The vane idea might be more practical, as well as the ball. I'm not sure what the axis of the 90 degree holes would be, I'm guessing one forward and one upward, but maybe 45 degress to level? I'm also trying to imagine how the 'guts' would work. Here again, I assume each 'hole' would connect to one end of a pressure sensor. Any idea of the math involved for the pressures generated, or differential. I'll have to pull out an old Mouser catalog and look at whats available. High Plains, the AOA device is also another idea! The one photo shows a pressure hole at what I assume to be wingtip. I normally think of AOA as a positive climb angle, but I guess it refers to downward angle too, apparently used to establish one's glide angle.

The inertia factor comes from the fact that weight doesn't scale with size. That is fortunate for us. Imagine a 1/4 scale Cub weighing over 200# empty.
On earth, lower weight translates to lower inertia which in turn means less kinetic energy. Or, models accelerate, deccelerate, and change angles of attack more rapidly than full scale man carriers.
Stall warning devices could be put on a model but the benefit to the pilot could be questionable in that the information may come at a time too late for positive action.
The 60s vintage Cessna would start to activate the stall warning just as you were half way through the flair and go to full on usually a second or two before touchdown. That was providing you had made a textbook full stall landing.
As for the Schweizer sailplanes the speed you are referring to is the minimum sink rate/best thermal climb rate which was 3 to 5 knots above stall.
I learned to fly full scale sailplanes in one of their old 222s serial number 11. Later I instructed in tha 222, 223, 232 the Schleicher ASK-13 and KA-7 plus the Grob.
Each had their own peculiar sounds when they approached stall.

]ORIGINAL: wsmalley

I suspect you're correct as far as the placement is concerned though there may be more sophisticated methods for designing these in larger A/C. As I recall, the Cessna's 'flap' was more inboard. I had some hours in a Piper Cherokee but recall nothing as to how it worked. Can you explain the inertia aspect, and how that translates to models. With my limited flying experience in the above planes, you 'feel' the stall coming on. Trying to compare the relationship of the speed curve between, e.g., 'slow' flying and a stall. I recall years ago flying an old Schweitzer sailplane wherein the the difference between speed at level flight and stall was maybe 2or 3 knots.
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Gremlin, I think I learned in a 232. I remember doing loops one day and when I came down an instructor said "I don't think I would do that!" I asked why, he said that plane wasn't stressed to fly upside down. Never got to fly in any of the really neat stuff, but it was the greatest thrill in flying. I do remember making some 'perfect' stall warning landings in the Cessna. Latest though-couple of years ago now-was a Flightstar ultralight that was great! Second only to a sailplane. As for model devices, close may be good enough to accomplish the intended purpose. I have autopilots on almost all my planes for that split second when I become disoriented or lose sight. Try not to out- fly my eyes, but it does happen. Thanks for the explanation of the inertia factor!

Angle of attack is the angle between the direction in which the wing is moving relative (relative wind) to the chord line of the wing. It bears no relation to the horizon. It can be low in a climb and high in a glide depending on the elevator position. During my full-scale aerobatic training, I would occasionally pull too hard at the top of a loop and stall the wing while the airplane was inverted.

The two holes in the ball would be located as shown. At zero AOA, pressure would be equal at both holes. At a positive AOA, the lower hole would have a greater pressure and the upper one would have less. It would be possible to calculate the difference but I suspect that for any real accuracy, it must be calibrated with a measured angle. For model speeds, the pressure difference would quite small and very good accuracy would be required to give meaningful results.

Your instuctor didn’t have his thinking cap on that day. In a loop the g-forces are always positive. The fact that the aircraft is upside down is not relevant. During a loop the stress at that point is no more than in normal level flight.

Thanks for the explanation, and the drawing. I looked at Digikey's sensors last night and a differential pressure sensor is about $14 for a low pressure model. I think using the ball configuration would allow easier rotation to align the 'neutral' point on a model, or any plane I presume. The circuitry needed to trigger a light or buzzer shouldn't be too difficult.

Probably what his instructor was saying was in context to the type of sailplane involved which was likely a Schweizer SGU 233 rather than the 232. Since kinetic energy is all you have to get you through the loop in a sailplane the loop can go to 0 or even a slight negative. If you spend too much energy in the climb portion the over the top gets pretty weak. I used to watch the tabs on my shoulder harness flip straight up on my shoulders when things got too slow over the top.

As far as load ratings on airframes most sailplanes are rated +/- 6gs or more while most production light aircraft, Cessna, Piper etc. are 4.8 positive and 3.5 negative.
Sailplanes are an order of magnitude cleaner in parasitic drag and as such will accelerate from stall to red line at rates appproaching that of gravity.
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Your instuctor didn’t have his thinking cap on that day. In a loop the g-forces are always positive. The fact that the aircraft is upside down is not relevant. During a loop the stress at that point is no more than in normal level flight.

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Not to mention that the wings have to take negative loading on most landings.

I would read the patent on the device, since the idea of a patent is to teach others as well as protect invention.

Just type in USPTO on your search function.

Electronicly speaking... an audible "buzzer" is not at all hard to design... (the design of the "electronic switch" is very easy)

The only real problem I could see is, every plane will have different stall conditions, depending on airfoil design. You could easily get an estimated area that you would have the stagnant air (as the airfoil sees it at slower speeds). The problem sits at where the calibration of the "switch" is. The "tab" that resides on the wing also needs to be small, but have an area that will give you accurate activation of stall audible sound.

I don't see where a "one fits all" type of device could be manufactured without having some way of getting an accurate speed of the particular aircraft to have the device installed in. So, I would think that the individual would have to design it them selves, and just keep doing trial an error to find that "magic" spot right before stall.

Now, if there is a device out there (I've never seen one, and might be a seriously great idea to get a group of people together and design one... I design electronics for a living, so I'll help if there is interest) that you can mount to your aircraft to give you vectors on the performance of the aircraft in real time, showing actual speed, and g-force that could be plugged into your USB port of your computer, and show you a performance graph of the flight.

next post, I'll post the idea I'm thinking of. (kinda like an accelerator G-meter that they install in indy cars)

In my opinion, a stall alert device is a really cool idea...

Danoman...

How does a wing take negitive G's during landing? If wheels are on wing then its poitive. If wheels are on fuselage its basicaly 0 unless you really slam it into the ground and even then its only the wing weight so probably fairly low negitive.