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Airspeed Controller

Sensor to control airspeed - Scale model flying

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George110/03/2012 04:06:05
5 forum posts

G'Day,

Does anyone have information regarding a sensor and throttle control unit to enable setting an airspeed just above stall, ( point found by experience).

It is fine to tear around to keep away from the stall, but....

No telemetry wanted, unless comes with the airspeed unit. Set off for takeoff, on for scale speed flying and approach, off at touchdown to allow stall. Many aircraft get into stall flying over trees and I feel that it is an airspeed problem, which of course the pilot can not see until too late. Can not blame dumb thumbs or radio.

Any ideas?

George.

Cyclicscooby10/03/2012 04:43:09
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Hi George..

I've never heard of an off the shelf unit, that works like that, but can't see why it couldn't be done with a PIC... You'd obviously have to investigate the outputs from the AS sensor...

I think to compensate for 'real' wind, and for it to work the way you want it to, you'll need a rear facing tube too, and differentiate between the two,,,

I could be over complicating it, but thats the way i'd go..

HTH's

Luv

Chrisie.. xx

Wingman10/03/2012 07:35:31
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I've said before in another thread that with all this telemetry stuff going on now the "killer app" (in the modern idiom) would be stall warning - much much more useful to us than battery or fuel tank info or even airspeed - it would certainly persuade me to get into telemetry.

George110/03/2012 09:48:53
5 forum posts

Now I know that there is interest in helping with a problem that many flyers don't seem to want to know about. Cyclicscooby, to me PIC means "Pilot in Charge" maybe you could explain why we need a rear facing pitot, this is all a bit new to me.

Wingman, there is a system available here

but includes all the telemetry modules, which I agree doesn't help much in real terms. But even at this price, if it saves a model worth $4,000 it is cheap enough. The trouble is, trying to get info out of them is like drawing hen's teeth. More detail about the speed control is mentioned in the PDF User manual, which can be downloaded.

Edited to correct link....

Edited By Steve Hargreaves - Moderator on 11/03/2012 07:14:50

Martin Harris - Moderator10/03/2012 10:00:12
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Surely it's far more satisfying to control the model's speed/flight path with attitude and throttle yourself than simply allowing it to be flown for you?

Former Member10/03/2012 15:35:30
6538 forum posts

[This posting has been removed]

George111/03/2012 04:41:23
5 forum posts

Thank you for contributing, but what we are looking for is an airspeed control system, irrespective of whether we know how to fly an aeroplane or know how to use the throttle. The idea is not to fly about just above the stall, but to have a throttle over-ride in case the stall condition approaches. A pilot is completely unable to accurately anticipate airflow and angle of attack in disturbed conditions, on occasions many get it wrong. Hence a device to open and close the throttle. Why shouldn't a scale model have a bit of auto control?

Steve Hargreaves - Moderator11/03/2012 07:17:26
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Hi Geoge1....welcome to the forum....

Please can you take a moment to read this tutorial about posting links....thumbs up

Thanks

Biggles' Elder Brother - Moderator11/03/2012 10:47:24
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Hi George,

I'm afraid there is a fundamental, and very common, flaw in your analysis here.

Aircraft do not stall because the airspeed is too slow.

So an airspeed detector/control unit will not protect you from a stall.

Aircraft stall becuase their angle of attack is too high - and that is the only reason.

BEB

George111/03/2012 23:27:43
5 forum posts

BEB,

Well, I don't dispute the point. What you are saying is that a stall can happen at any airspeed.

In certain cases the AOA can increase in level flight without elevator input, and it seems to me that an airspeed device would detect a decrease in air speed and open the throttle well before the stall approaches. That is, before the AOA becomes critical. Assuming that as the AOA increases, the change in angle of airflow on the pitot will lower pressure, and indicate a decrease in airspeed. So that airspeed is an indicator of an increasing AOA = approaching stall.

With a glider for instance, the answer is to put in down elevator. Easy.

Different aircraft stall at different AOA, so the idea is not a panacea for the casual flyer.
Or maybe anyone. Possibly the whole concept is invalid, but until it is tried out, we will never know.

Daithi O Buitigh12/03/2012 00:53:16
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In every tutorial I've read (and as was taught to me years ago) - throttle controls ALTITUDE - the ELEVATOR controls speed

If you are at a critical AoA and on the verge of a stall the solution is not to increase the engine rpm but to bring the nose down (ie - feed it down elevator)

George112/03/2012 02:02:04
5 forum posts

Daithi, that is true, but it's one thing to be in an aeroplane, and another to be flying a model at what seems more than a good airspeed, and is so up to the point of seeing a wing dip.

What I am looking for is an airspeed detector and controller, not a stall warning or an AOA device..

While groundspeed seems more than adequate to the model flyer, airspeed can decay to the point that AOA goes critical, and airspeed can decay so very rapidly.

Assuming level flight, at low altitude it is too late to push down elevator. The idea is to have the throttle open well before AOA goes critical, airspeed increases, all is fine. Once through the disturbance, the throttle reduces back to the ideal speed. From the above it could be assumed that there may be airspeed hunting, I don't know, case of try it and see. Telemetry can send airspeed back to the flyer, but for mine he should be watching the plane, not a screen, and there is a time delay.

I have seen so many models go in, the flyer is left with a puzzled mind, the gear checks out and it is put down to dumb thumbs or interference.

There may be another cause.

Ben B12/03/2012 08:26:39
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Tutorials for beginners say (especially when it comes to landing) throttle for height, elevator for speed. Of course the other way round is also true. You have to balance it.

Whilst it's true that stalls occur due to a AoA it is also linked in with throttle because the flight of the plane relative to airflow is partially dependent on the pull of the engine (when there is one!!!). Also the prop-wash over the wing (unless it's a pusher) helps prevent stalls etc etc. IE quite a few reasons why saying it's only to do with AoA is not entirely true.

So it would be quite simple to get a barometer IC and a PIC and create a little device but what would you have it do when it senses a low air-speed? Dip the nose or increase the throttle?

Martin Harris - Moderator12/03/2012 13:19:28
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I suspect that you and BEB are both correct in a way. I'm sure BEB is totally correct in saying that the AoA relates directly to the stall but it can vary locally due to factors that you've mentioned plus others such as the interraction of slip, dihedral and sweepback.

Our little aircraft are subject to larger proportional variations in turrbulence than full size have to cope with so we need a larger margin to cope with them.

Do any of our contributors have first hand knowledge of full size stall avoidance systems - does a modern airliner's stick pusher also add throttle? (I've never heard of it doing so)

I understand that they can all be over-ridden by a pilot's control input which would be something rather necessary near the ground!

Gary Binnie12/03/2012 14:47:52
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I could bore you to death with the description of some full size systems and recovery techniques!!

At the small end of general aviation (gliders and single-engine jobs) there is often no stall warning device fitted, pilots rely on airflow breaking away from the top surface of the wing and hitting the tailplane/elevator, this shakes the stick and the airframe and is difficult to ignore. Flight training emphasis is on 'recovery with minimum height loss' which in a 'puddle jumper' involves adding power and preventing the nose from pitching down.

Many small GA aircraft that do not give good aerodynamic stall warning are fitted with a small vane on the leading edge of the wing, this moves upwards as the AoA increases and operates a switch connected to a horn or light (or both) in the cockpit. Others, like the DH Chipmunk' have small strips (Toblerones) fitted to the wing leading edges near the wing root to make the airflow break away. The Tiger Moth does not give any stall warning, you have to use some Jedi powers to know when it is going to stall!

I worked on F-4 Phantoms, they had an eccentric weight fitted just below the control column grip that span round and shook the stick on the approach to a stall, it was driven by angle of attack vanes on each side of the fwd fuselage, these vanes also sent a signal to the angle of attack gauge up on the glareshield, pilots flew approaches using that (units of  Alpha) and a reference speed.

The F-104 Starfighter had the same system plus a 'stick kicker', a hydraulic ram that moved the stick forward if the pilot did not, we used to test it on the ground on start-up by moving the vanes up and down. It had a nasty habit of operating at low level and high speed, not good so pilots turned it off!

On more modern jets like Tornado it is done electronically, the system is called 'SPILS' (Spin Prevention and Incidence Limiting System).

None of these systems add any engine power as far as I am aware.

Some more full size guff if you are interested, the approach reference speed is often set by manufacturers at 1.4 times the straight and level stalling speed.

Stalling speed increases with the square root of the 'g' load applied, sounds complicated but if we take an aircraft that stalls at 40 knots in level flight then it will stall at 80 knots in a 4 g turn or pull-out manouevre, (square root of 4 is 2, doubles the stalling speed).

It's an interesting thought for a model, only kind of stall prevention system I could see working is an angle of attack vane mixed to some down elevator and you would need to turn it off for landing otherwise flaring would be interesting! Seem to remember that the full-size systems are disabled with the landing gear down or full flap selected.

I don't remember in my model flying that stalling is a particular problem, for F3J gliders we habitually add two clicks of down trim prior to landing or switch it into a landing flight mode that does the same. For sport power jobs (Wots-Wot is one that I fly a lot) then I make sure it is trimmed fairly well above the stall, I know that coming back from downwind can make you hold your breath but if it's trimmed it should come home, if it gets grabbed by a gust then there is not an awful lot you can do about it (except add power, climb out of the turbulence and try for a normal a circuit).

GB

Edited By Gary Binnie on 12/03/2012 14:48:39

Edited By Gary Binnie on 12/03/2012 14:49:29

Peter Beeney12/03/2012 14:56:42
1595 forum posts
59 photos

Yes, I too have to admit that I’d have thought that the speed of the plane and the AoA go hand in hand from the point of view of George’s OP. If an aeroplane is flying in level flight and at a certain higher speed I’m sure it won’t be near the stall, if the speed is then reduced toward zero, by closing the throttle, and if it’s still kept in level flight at some point the wing will stall, I guess, because the AoA is too high, even though it’s still in level flight. But surely now this is as a direct result of the speed being reduced? What else would induce the wing to stall under these circumstances? That’s leaving out wind effects and so on. So I think I would be very tempted to say that the aircraft would stall because it’s AoA was too high but also relative to the speed. It’s possible that the same may apply at higher speed too, if the elevator is operated too fiercely, causing the AoA to increase sufficiently to stall the wing.

If I were the pilot of a full sized plane and I suddenly found myself flying slowly and very low near the stall over a large swamp full of man eating crocodiles I think I’d be very convinced that I needed to open the throttle fully to gain speed, rather than pushing forward on the stick. However, when later I came to land it, I’m sure the ‘down elevator for speed’ ruling then comes into it’s own. So, again, this must be largely conditional, everything is relative to everything else in each set of circumstances.

If you practise regularly flying models really slowly it’s surprising how effective you can become. Then when you get into more unknown situations it’s not so difficult. Flying over trees low and near the stalling speed in a wind must be a rather special sort of adrenaline rush anyway, I’m not really into that. A gadget to open the throttle might be ok, but then you’d have to remember to switch it in and out, forget on one occasion and it might be the cause of a mishap.

I’ve often read the stalling reports in model reviews, but it occurred to me long ago that if someone really wanted to know how well or otherwise a model responded to stalling conditions there is a slightly different test. Fly downwind in a stiff breeze, in claggy wet conditions, low over some trees with an engine that appears not to be entirely happy and you soon find out what the true recovery situation is likely to be. I found this once long ago when demonstrating at a fete, with a large Cub, the model fell without the slightest warning into an uncalled for spin, although it recovered ok, I’d practised many times, fortunately, but the speed at which it happened was incredible. And it was definitely much more by luck than judgement.


Also I’m not a fan of telemetry, because of watching the screen, I’d not be able to do that, and I’m still on 35 anyway, but how about an onboard ASI with an audio alarm at the tx? That would be instantaneous. I’m not sure about 2.4 manufacture’s in general, but I’m sure I’ve read somewhere that MPX at least have such a system.

Still trying to keep the wings level……

PB

Martin Harris - Moderator12/03/2012 15:35:09
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Thinking out loud, adding power usually results in a pitch up (zero/zero models possibly excepted) and cimb (from level trimmed flight) so the action of opening throttle and reducing pitch gives the desired reduction in AoA without deviating from the desired flight path.

Any system that simply opens the throttle would still seem to need an elevator input to avoid stalling from flight close to the maximum AoA. It is much easier to think of airspeed as being the critical factor as this is more easily measured but it is only an indication to a pilot of his approximate AoA (and subject to some lag as well) during relatively stable flight conditions.

As far as I'm aware, the primary action in stall recovery is always to apply down elevator commensurate with the situation and then add power to aid in the recovery process. If the 2 actions are simultaneous, all well and good but the more important action is the reduction in AoA to unstall the wing and regain flying and control "speed".

Steve W-O12/03/2012 16:11:53
2775 forum posts
310 photos
Posted by Biggles' Elder Brother - Moderator on 11/03/2012 10:47:24:

Hi George,

I'm afraid there is a fundamental, and very common, flaw in your analysis here.

Aircraft do not stall because the airspeed is too slow.

So an airspeed detector/control unit will not protect you from a stall.

Aircraft stall becuase their angle of attack is too high - and that is the only reason.

BEB

Therefore if the model is flying at one mm a year, but zero angle of attack, it won't stall? Of course it would be sinking, so the angle of attack would be high, just thought your explanation could lead to confusion.

Steve W-O12/03/2012 16:26:13
2775 forum posts
310 photos

I think the Eagletree sensor will measure your speed OK, the other way is through GPS.

Eagletree will probably help you with the output and how to read it on the I2C bus, and I'm sure you could use their RPM sensor as well for a brushless motor, or the optical one for the other type of motor.

As has been suggested, a PIC will be the easiest way to go, as it can be prgrammed in Basic, the compiler is very good, so no extra overhead. It might be a good idea to include gyros, and a 3 axix accelerometer (again the Eagletree one comes to mind) as this will enable you te quite easily sense the attitude of the plane, apply correction, add throttle as needed.

The other way is to look at Ardupilot, if you can get someone in the east to order it for you, it can be had for less than $100 ( the price wis a lot more when bought from here). Then you can set it up at home, put the plane on the runway, and sit down and watch as it takes off, flies around and lands

Martin Harris - Moderator12/03/2012 16:37:10
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It's a lot cheaper and easier to ask one of your clubmates to do the honours if you just want to watch it fly!

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