Theoretical tip stall reduction using mixing

[Rob Thomson]

So...

Been thinking about how to prevent some high speed tip stalls on my fox glider, by application of some mixing.

Where I often see the top stall is when pulling high speed turns, and loops. Too tight... And I get a tip stall.

So... Some magic mixing idea.

If elevator is greater than 80% slow raise the ailerons by 2mm.

The result : an artificial increase of washout, that is only applied when using the elevator aggressively. Then slow application is used to reduce the impact of a sudden reverse snap flap.

Still need to test this.. But it should help in this sort of stall scenario.

What I really should do is build an arduino angle of attack sensor. Then auto apply the mix based in that input!

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[tilmanb]

Tell me when you got a usable AOA sensor for model planes. could use that.

[Rob Thomson]

That Tilman.. Will be a long way off!

I say an interesting solution using a rotary encoder, and a small wind vane attached to it.

Biggest issue is the size of the unit!

Even the pipet tube options are IMHO too large for a clean and simple solution!

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[Flaps 30]

One small hinged metal tab on the leading edge. Add one 2mm by 1mm magnet to the tab that works in conjunction with a hall effect device (as mentioned in the potentiometer thread) that goes into say A1 or A2 of the RX. There you have your AoA unit with a voltage output that you can use in whatever way you like at the TX..

Easy innit!

[Rob Thomson]

You going to make one first?

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[Flaps 30]

No need.. My flying skills are that good, that I have no need of such things.. Yet! I have some hall effect devices heading this way from China. I might have a go at it with an old foamie just for fun..

IIRC if it is the Fox glider that I know of.. It had/has a tendency to snap in a violent way into a tip stall/spin without much provocation.

[tilmanb]

Problem is the scale look really.

[G550Ted]

Interesting idea Rob. I've not flown nor even seen a Fox in person, but I have an FMS ASK-23 that is the only model I have flown or ever seen that has a true tip stall. And it's NASTY! I don't fly it much because of that.

It's problem has nothing to do with scale looks, or "thin wings" or any of the other issues put forth in some threads. It is, as you said, essentially all to do with improper physical or aerodynamic wash out in the design. Here are some of my thoughts based on my ASK-23 which would probably also apply to your Fox as they have similar designs.

Without an AOA sensor the mix of aileron reflex and elevator needs to begin at a much lower elevator deflection than 80%. It also needs to vary with increasing elevator deflection, either linearly or some curve based on flight test. My guess is that the amount of reflex at high elevator deflection will need to be greater than 2mm. I'm thinking a start of 2mm at perhaps 20% elevator going to 6mm (?) at full deflection. Yea, the ASK-23 is that bad. It would be better (quicker) to start with too much reflex and adjust based on testing than the other way around.

Thanx for the inspiration.

Ted

[Rob Thomson]

Flaps...

My flying us plenty capable of avoiding the situation. Going on 20 years of air time.

But reducing this tendency in a simple way would make a lovely model, even more of a joy to fly.

The annoying thing about the tip stall on this model is that it tends to ruin my loops. If you don't take it a enough speed, rather than a simple flop at the top, you drop a wing and spin. Easy enough to recover - jut annoying.

If applying artificial washout in a mix helps. Then it is a good result.

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[Kilrah]

One small hinged metal tab on the leading edge.

Hmm that would only give you an on/off signal (like the stall warning on most planes that is done that way)

[jhsa]

Does it stall always to the same side or it can be any side?

Maybe the stall strips at the wing root as mentioned here before.. You know, That glider would look good with winglets.

[Rob Thomson]

Always stalls different.

I am going to be installing stall strips on my next outing - but will have to wait and see if that works.

Rob

[jhsa]

I mentioned before that winglets would look good on that plane.. But thinking better, maybe they could help the situation a bit as they could help the airflow to stick to the wing at the wing tip..
Just a thought..

[Rob Thomson]

Maybe... but then that would not be very scale like would it

[jhsa]

true.. but it would look nice.. let's say that if it works you could sell the idea to the manufacturer

[jhsa]

I don't think this one has problems with tip stall, at least the pilot seems not to
Nice flying by the way..

[BBvideo 425,350]https://www.youtube.com/watch?v=9mGTNffSEc4[/BBvideo]

[Flaps 30]

I take it that you have already tried a bit of up aileron on both sides to give the effect of washout.

One small hinged metal tab on the leading edge.

Hmm that would only give you an on/off signal (like the stall warning on most planes that is done that way)

Maybe a bit of light spring tension would help give some sort of intermediate signals, given that a hall effect device is being used. I did have thoughts of a small hole on the leading edge that is coupled to a tube that goes to one of Rainers vario units to give an indication of the AoA..

I have a model which is a Reichard Proxima II.. It tip stalls in a violent way and it has a rather alarming wing flex on recovery. It doesn't like slow flight one little bit. I don't like it which is why it stays in the corner of the room for most of its life. I should get rid of it and let someone else have all the fun. Of course I could do the AoA bit and that will be it sorted.

[Kilrah]

Maybe a bit of light spring tension would help give some sort of intermediate signals, given that a hall effect device is being used.

Not if the tab is sticking out the leading edge. Either the air hits it from the top and it lies down, or it hits from the bottom and it flips it up. It's an unstable condition.

You need a tab that is held from the front, not from the back. So something like a real AoA sensor, mounted on the fuse

[Rob Thomson]

You could use a small vane, and a rotary encoder.

The vane could be made easy from a strip of trailing edge sheet!

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[Flaps 30]

Hmm.. Yes,.. I forgot about that Kilrah... Not sure about having the AoA sensor on the fuselage. It works due to the dead air at the leading edge of the wing when the airflow is correct.

The vane could be made easy from a strip of trailing edge sheet!

Is that real wood

[Kilrah]

It works due to the dead air at the leading edge of the wing when the airflow is correct.

No, this would be a stall warning!

You're thinking of this:



That's a stall warning tab. Just a little piece of metal at the leading edge. In normal flight the air pushes on the top of it and keeps it down. When AoA approaches the limit airflow becomes disturbed and flicks up. That triggers the stall warning, but it's only ON/OFF.

If you want a proportional device that actually gives you an angle reading you use one of those things I posted above, that is always in clean air. You can see one mounted on that 757's nose:





The air goes past and the tab naturally keeps aligned with the airflow. As the wing is (usually ) rigidly attached to the fuselage, you just need to align the zero so that it's matches 0° AoA on the wing.

[tilmanb]

This guy seems to have no problems with his Fox. https://www.youtube.com/watch?v=_zdOfhp ... hHefsuACNA
Could the Pilot be the problem. :p

But seriously. Watch the videos, it's amazing.

Rob, how about having something like this dangling off your slick Fox?


The paparazzi research folks probably care a bit less for scale look.
http://lists.gnu.org/archive/html/papar ... 00048.html
The list archive has some details on which encoder they used though.
Also useful http://www.charlesriverrc.org/articles/ ... launch.htm

[tilmanb]

Nice rotary encoder. http://usdigital.com/products/ma3
There are pure digital ones as well. But PWM can be dealt with...

[Rob Thomson]

Lol.

Did I look like I was having a problem with it last week?

For me... I just like to reduce the risk. In practice - it makes little difference.

I was wondering if one could use a gyro as an angle sensor, plus an accellerometer (tri access) and some fancy maths to make a pseudo aoa detector. That way it would be contained!

Rob

[Flaps 30]

Thanks Kilrah.. Yes you are right.. Mixed up thinking here. Too early combined with a lot of multitasking has this effect.

[Kilrah]

I was wondering if one could use a gyro as an angle sensor, plus an accellerometer (tri access) and some fancy maths to make a pseudo aoa detector. That way it would be contained!

In theory yes, in practice it's mega-complicated maths, and if you want to have a decent accuracy you'll also need military-grade sensors

You need to not only keep track of attitude (which is already not very easy) but also of 3D velocity (in which direction the airplane is moving) to be able to take the difference between them.

[jhsa]

3D velocity? errrr.... you mean forward? planes move forward? Or am I missing something?
Last time I tried to go backwards I couldn't find the gear
It said, only available on the ground and only sometimes

Just find a way to measure the bl***y airspeed and you don't have to do complicated math.. and do what all instructors shout at you all the time when you're learning..

WHATCH YOUR AIRSPEED A****LE..

they don't shout

WATCH YOUR ANGLE OF ATTACK

[Rob Thomson]

Soo... Do we need to get to romollo and mike to talk :mrgreen::mrgreen:

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[Rob Thomson]

Jhsa....

It is not air speed that worries me.

My tip stalls occur at speed. Joy of the wing design, and high wing loading

Pull to hard in a turn, instant drop!

But to be fair. The plane is a blast to fly!

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[jhsa]

I had a mustang doing the same many years ago..