Stall/Spin and V-tails

[HELModels]

This is related to downwind turn, as it is one place I experienced it. The plane is a V-tail pusher. A few times and when 4-5 mistakes high and once much lower while fighting to turn from downwind to upwind, the main wing has stalled while steeply turning and banked. I didnt understand what I was seeing for awhile, but what I see is the spin is opposite to what I would expect. Banked sharply left, stall, spin briefly left, spin sharply right. Am I seeing things? The V is the only thing flying and now acts like ailerons? :eek:

[Sparky Paul]

This is a rudder-vator plane, no ailerons?
A left command on the rudders doesn't reverse its effect on the plane when the wing stalls, it's still left rudder.. and the left side should push the nose down, if it isn't also stalled. The right side will be a nose-up but still a left turn, again,if it isn't also stalled.
I haven't seen this yet in my vee-tails.

[HELModels]

I've seen it a few times and been able to recover with power so the spin hasnt been more than about 30-90 degrees, but still scary. This evening was eyeopening as the treetops were approaching. It is briefly in the direction of the turn and then sharply opposite. :eek: Release stick, full power.

[Sparky Paul]

Sounds more like an aft c.g. problem than something unique to a v-tail.

[Gary Warner]

I think that something unique is going on with V's that are on the raged edge of tail heavy.

I fly sailplanes mostly and we tend to setup the CG as far back as we can get away with. I have (read: had) several V's and they get setup with the same basic CG 'feel'. But when a V stalls, it's always been more unforgiving than the conventional tail sailplanes. It's my experience that something is going on that’s different between the two tails.

Sparky Paul, do you have thoughts about this observation of mine?

Gary
--

[davidfee]

Gary,
It has been my experience that many sailplanes (and pylon racers) on the market with V-tails have tails which are only marginally big enough. Basically, they are too small... but they are big enough when the model is flown correctly. If there is a bad launch, a nasty gust at low speed, stall, etc, the model can go out of control.

The dynamics of a V-tail certainly are different than for a conventional tail, but I think the biggest problem is marginal sizing. The "rule of thumb" of making the horizontal projection equal in area to a normal horizontal tail results in an undersized V-tail.

-David

[Gary Warner]

Gary,
It has been my experience that many sailplanes (and pylon racers) on the market with V-tails have tails which are only marginally big enough. Basically, they are too small... but they are big enough when the model is flown correctly. If there is a bad launch, a nasty gust at low speed, stall, etc, the model can go out of control.

The dynamics of a V-tail certainly are different than for a conventional tail, but I think the biggest problem is marginal sizing. The "rule of thumb" of making the horizontal projection equal in area to a normal horizontal tail results in an undersized V-tail.

-David

That's what I thought. Since the lift vectors are off to the side, a V should have a larger area in order to have the same effective forces as conventional tail projections.

So what you are saying is I'm flying with too small of a tail, right?

My Asprite died on launch because of a stall right after launch. I also killed George P.'s Super V in the same fashion. George said it just committed suicide.

Gary
--

[davidfee]

Gary, yes... that sounds right.

-David

[HELModels]

In the situation I described, the spin starts as expected and then reverses until adrenalin kicks in and power is applied. I was thinking my effective area was too big and not too small. It behaves very nicely overall and this oddity has been rare. I'll try to repeat it at safe altitude.

[davidfee]

E-Storch,
What you were experiencing, per your description, was an asymmetric stall (commonly called a "tip-stall"). When that happens, all bets are off when it comes to control. The best bet is to let go of the controls and pray you have enough altitude for the model to start flying again. Your own words betray that you are a victim of the "downwind turn" school of thought. You say "while fighting to turn from downwind to upwind, the main wing has stalled while steeply turning and banked." You simply pulled too hard on the elevator and the model stalled. It was drifting downwind, giving you the impression that it was not turning... so you were "fighting" to bring it around.

You just need to put yourself in the shoes of the airplane instead of the guy standing on the ground. The wind does not push on the top of the wing when you make such a turn... the model is flying faster than the wind, or else it would fall... not fly. Airplanes are not kites (which are tied to the ground). Let go of the string and the kite stalls and falls, since it is now moving with the wind at ~zero airspeed.

[Sparky Paul]

I think that something unique is going on with V's that are on the raged edge of tail heavy.

I fly sailplanes mostly and we tend to setup the CG as far back as we can get away with. I have (read: had) several V's and they get setup with the same basic CG 'feel'. But when a V stalls, it's always been more unforgiving than the conventional tail sailplanes. It's my experience that something is going on that’s different between the two tails.

Sparky Paul, do you have thoughts about this observation of mine?

Gary
--
.
Gary, I size my v-tails according to the equations at Charles River..
Which results in what appears to me a larger than needed tail.
I've had some unusual things occur in flight, but nothing like any unexplained stalls or instant snaps.
I have a Seniorita for AP almost complete iwith a vee-tail, which looks a lot larger than the same tail surfaces on a normal Seniorita. I usually go for a forward c.g. anyway, not needing to extract all the performance the plane is capable of, when flying mostly straight and level for AP.
I wouldn't be surprised that setting a plaen up more sensitivily would result in odd behavior at the edge of the envelope.

[HELModels]

I dont doubt all the words of wisdom when they apply.

As usual, there may be something more sinister at work here. I yapped about flying, described a flight condition I observed and I crashed while experiencing an acute version of what I described. Very repairable, but more than irritating :mad: I had minimal control of where it went and no wind testing my perception. It just spiralled for no apparent reason and was unresponsive. I'd understand if I were up to no good, but I'm not. That is downright mean. :mad: :mad: :mad: I dont need any baptism to willingly fight the devil. :mad: :mad:

You hear me Devil, you cant have my soul.

Everyone else, peace be with you.

[Sparky Paul]

Can you duplicate this thing up close? Something that can be done "on command" has a better chance of being fingered out than something that occurs rarely, at a distance.

[Gary Warner]

I dont doubt all the words of wisdom when they apply.

As usual, there may be something more sinister at work here. I yapped about flying, described a flight condition I observed and I crashed while experiencing an acute version of what I described. Very repairable, but more than irritating :mad: I had minimal control of where it went and no wind testing my perception. It just spiralled for no apparent reason and was unresponsive. I'd understand if I were up to no good, but I'm not. That is downright mean. :mad: :mad: :mad: I dont need any baptism to willingly fight the devil. :mad: :mad:

You hear me Devil, you cant have my soul.

Everyone else, peace be with you.

Well, since no one has address this (I figured a 'master' would have caught this) I'll give it a try.

The problem is that the V tail is directly effected by the pusher configuration. You have way more control with the motor on. Cut the motor and the V's are less effective. This happens on tractor setups too, but it's worse in pushers. Also, the angle up, down, right or left of the thrust line can cause the plane to be in one trim with the motor on, and a totally different trim when off.

Since you describe a stall/spin with the motor off, it suggests that the thrust line is angled upwards, causing an down elevator trim when power is on and a up elevator trim when the power is cut. In most V's with a high thrust line motor, two things are effecting a down trim with the motor on. First is the higher airspeed over the top of the V's, reducing the surface pressure on the top of the V's, giving a down trim. Second is with an high thrust line behind the CG, the pitching moment of the thrust applies a downward pitching to the airframe (pitch torque).

All of this might be getting the plane too slow with the power off, setting it up for the stall. Either trim the thrust line, or trim the elevator down when the motor is off. Keep the airspeed up when motor is off.

You really should consider moving the CG forward a bit. And you need to be aware that you will need to input more control movement to get the same motor-on performance. Check and correct for ANY wing warps too.

I flew one of those Hobby Zone pushers for a novice. It was well enough with the power on, but turned like an 18 wheeler with all tires flat when the motor was cut. Correcting bank was almost imposable. I recommended more control throw in general and get used to flying a bit more sensitive elevator when the motor is on.

Hope this helps and good luck.

Gary
--

[HELModels]

Thanks Gary! The JBWeld has dried, the paranoia subsided, and rationality will prevail. I have to wonder if I have a bad servo. When it went down, my orientation was poor but it was flying straight in a gentle descent and then all of a sudden started spiraling. Alot of possibilities, I've managed to fly it out of trouble before but this time was pure evil.

[HELModels]

Can you duplicate this thing up close? Something that can be done "on command" has a better chance of being fingered out than something that occurs rarely, at a distance.

Well, the devil didnt do it. Rational thought is winning. I took it out yesterday and had horrible time - flew like the CG was too rearward, and right turn tendency. Took it home in one piece and examined the boom and the control surfaces carefully. There was tiniest amount of uptrim. How it got there? No, the devil didnt put it there. When I repaired the boom, I increased the boom length by about 1/16. This could only be seen from the bottom of the boom and so I also increased incidence and uptrim ever so slightly. I corrected the pushrod and controls to neutral and took it out tonight. Pretty close to back to where it was before I went Bonkers. It did not have any right turn tendency or too much uptrim, but I did fly in close a bit and got it to spiral without any damage. Too much rudder and not enough elevator and it will spiral, which is how it crashed before. Not exactly the same thing I observed before with the spin switching direction and not really a stall/spin.

I will try it with the CG a little more forward - hate to add the weight though.
A 3s 340 lipo would prabably do it and be functional weight.

Gary, yes it has a tendency to pitch down when throttle is cut, but has plenty of control surface and movement, probably too much for me.

Peace be with you,
Harry

[HELModels]

Well, since no one has address this (I figured a 'master' would have caught this) I'll give it a try.

The problem is that the V tail is directly effected by the pusher configuration. You have way more control with the motor on. Cut the motor and the V's are less effective. This happens on tractor setups too, but it's worse in pushers. Also, the angle up, down, right or left of the thrust line can cause the plane to be in one trim with the motor on, and a totally different trim when off.

Since you describe a stall/spin with the motor off, it suggests that the thrust line is angled upwards, causing an down elevator trim when power is on and a up elevator trim when the power is cut. In most V's with a high thrust line motor, two things are effecting a down trim with the motor on. First is the higher airspeed over the top of the V's, reducing the surface pressure on the top of the V's, giving a down trim. Second is with an high thrust line behind the CG, the pitching moment of the thrust applies a downward pitching to the airframe (pitch torque).

All of this might be getting the plane too slow with the power off, setting it up for the stall. Either trim the thrust line, or trim the elevator down when the motor is off. Keep the airspeed up when motor is off.

You really should consider moving the CG forward a bit. And you need to be aware that you will need to input more control movement to get the same motor-on performance. Check and correct for ANY wing warps too.

I flew one of those Hobby Zone pushers for a novice. It was well enough with the power on, but turned like an 18 wheeler with all tires flat when the motor was cut. Correcting bank was almost imposable. I recommended more control throw in general and get used to flying a bit more sensitive elevator when the motor is on.

Hope this helps and good luck.

Gary
--

I evaluated the Neutral point and CG is well ahead of it. Too far forward CG could contribute to stall/spin too as more up elevator would be required in turns to maintain altitude. power off glide is slightly nose down, as it should be. Control surface and throw is more than needed. There is a small band of throttle settings around 1/3 - 1/2 throttle which provide level flight - no real surprise there. This "level flight band" shifts as the battery voltage drops, requiring just a little more juice to maintain altitude. My plane is better than this pilot, but this pilot has learned what the plane wants to do. stall/spin only happened throttle off when trying to stretch the glide and turn - classic. This aint no firebird.

I doubt I'll see any control reversal again as I learn this plane and avoid the stall/spin.

Peace!

[Gary Warner]

I evaluated the Neutral point and CG is well ahead of it. Too far forward CG could contribute to stall/spin too as more up elevator would be required in turns to maintain altitude.

Eeah... no. It won't make stalls easier to get into. It will make them harder to get into.


power off glide is slightly nose down, as it should be. Control surface and throw is more than needed.

That's good, except too much control throws IF CG is too far back will make pitch very sensitive and easier to stall.


There is a small band of throttle settings around 1/3 - 1/2 throttle which provide level flight - no real surprise there. This "level flight band" shifts as the battery voltage drops, requiring just a little more juice to maintain altitude. My plane is better than this pilot, but this pilot has learned what the plane wants to do. stall/spin only happened throttle off when trying to stretch the glide and turn - classic. This aint no firebird.

Understood. Still you say it stalls/spins in turns and when stretching the glide. Both of which require up elevator, increasing the angle of attack. My money is still on an induced stall. Move the CG forward a bit.

I doubt I'll see any control reversal again as I learn this plane and avoid the stall/spin.

Peace!

That's a good thing. Learning to 'fly' the plane will teach you a lot about aerodynamics. When you feel comfortable with your skills, move the CG forward a bit (2-3% of cord). You might find it flies better. But don't worry about making stalls easier to get into. The effect will be to soften the elevator feel. Too far forward and you might not be able to adequately slow down for landings and you might run out of elevator in steep turns. As you move the CG forward you'll need to add a bit of 'up' elevator trim.

Gary
--

[Wiker]

I evaluated the Neutral point and CG is well ahead of it. Too far forward CG could contribute to stall/spin too as more up elevator would be required in turns to maintain altitude.Eeah... no. It won't make stalls easier to get into. It will make them harder to get into.
I had a sailplane that would tip stall easily if pulled too hard in turns. I got the advice, by someone who seemed to know what he was talking about, to move the CG back. The reason for this was that a nose heavy plane has to fly at a higher AoA, and then it's obviously easier to put the plane at a too high AoA (stall) when pulling elevator in a turn. I don't know if the theory is correct, but moving the CG backwards cured the stall problems on my plane.

[HELModels]

...And you didnt get your answer from a pilot training manual, but from real experience. Bravo!

I dont know of too many designs that wont stall in a power off turn - I'd be interested in how that would look and work. I have yet to get it to stall power on. It just kind of hangs there and then drops the nose real gentle and starts flying again.

[Sparky Paul]

You're most likely to encounter a tip-stall with an aileron plane. The downgoing aileron increases the angle of attack on that side.. and when the plane is close to the stall angle, the wing in front of the aileron stalls.. and the plane rolls into that side.. which results in -more- aileron command to stop the roll.. which results in splinters in a hole in the ground.. :(
My no-aileron SAE Lifter will tip-stall, but self-recover almos instantly.

[HELModels]

I've got quite a few more recent flights on it now. It doesnt like to turn left as easily. Left turns require more rudder and with polyhedral it can go into steep bank, pull enough elevator and it spins. Turn right is much smoother, requiring alot less rudder and elevator. I wonder if it has do with the wing coming off an old free flight which was designed to circle - cant remember but it must have been gliding right turn power off. It can be a real bear debugging these things.

[Sparky Paul]

Is the "spin" a true departure, or just tightening up?
On some of my planes without ailerons, I've decided to add ailerons for the wings-leveling ability lacking in a rudder plane, having lost two recently to serious nose up-can't recover and nose down-can't recover manuvers.

[HELModels]

It is really pretty self correcting - neutral stick and wings level. When it spun hard was when I could see the "break" - it also spirals pretty tight without any break. The spiral taught me to use up elevator in tight turns, but not too much to cause a break and spin. If a V is slanted to one side, say with the V pointing off vertical to right from front view, would that cause a right or left turn? I've tried visualizing vectors but I cant seem to convince myself which is correct. I also have to watch motor alignment as it is only attached by wing rubber bands. I might have to resort to trim to get rid of any right turn - Duh!

I must be doing somethings right as I can now consistently land at my feet, where at first it was looks like I'm gonna have to land it over there. Tonight I managed to catch some lift in an updraft of some sort. all of a sudden wing banked hard without input and I went back for a sniff and woosh up it went - throttle was way back and still going up. I used the spiral tendency to escape it - tight turn no elevator.

Maybe a little more wing span would decrease the spiral. Then again, slightly more forward CG?

[HELModels]

Is the "spin" a true departure, or just tightening up?
On some of my planes without ailerons, I've decided to add ailerons for the wings-leveling ability lacking in a rudder plane, having lost two recently to serious nose up-can't recover and nose down-can't recover manuvers.

Like I said earlier, these things can be tough to debug. I took it out again tonight when the air was very still, different night, different conditions. throttled up to full and tossed it. I didnt touch the controls for a good 5 sec. and noticed it going slightly right, so I used the trim tab and moved it until it was going straight and kept on going straight. It flies like a different plane now, better. When I turn left and release it doesnt go into any kind rolling back and forth. Same thing when turning right, stop turn and it goes straight. All the fighting I was doing when turning left had very little to do with wind conditions and everything to do with bad trim. This right turn tendency whereever it came from was causing alot of problems, even visually.
I lost orientation once and spun because it really wanted to go right.

I aint touching the CG!

I suggest anyone wondering how hard it can be to fly with a built in turn tendency go out and purposely set your trim to turn gently left or right and then try and fly turns in the opposite direction and see. You might see alot of stuff that is hard to explain.

[HELModels]

It Figures, I moved the CG ahead 5% - added 5.5 grams nose lead - and It Is much easier to handle. If I had one of those fancy computer radios, I would reduce the amount of throw and add exponential. As it is right now, it rarely needs big deflections - slight stick pressure is usually enough. I even flew it in some pretty strong winds. Of course, at the moment of launch it was calm, but winds aloft were much different. Full throttle and it just hovered in place once up there. Didnt have any weird perception problems on downwind turns either, no tip stalls, no spins while racking that sucker around.

Conclusion to all this weirdness is:

1. Build a proven design first to learn to fly on.
a) dont fly around trees
b) carry a broom handle for plucking from trees
c) expect to crash some, but try like crazy not to

2. Go ahead and build your own design - It is fun - but dont depart far from reality.

3. Use the trim on the transmitter - it will reduce pilot workload.

4. Play with the CG, but try it forward, then back gradually, not back and then forward. Use the dive test at safe altitude. Stability is a good thing in a trainer.

5. Strive to learn more.

Adios My Amigos

[Sparky Paul]

Electro, we always have things to learn .
I used to say..."This is Flight Test, not Flight Assured".. when getting on a Tristar to go out and do some very weird things... :)