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Old Apr 21, 2012, 09:07 AM
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Joined Mar 2010
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Idea
Solution to vicious tip stall!

Let me start by saying that I have been flying models for a long time but I don't have much technical knowledge of airodynamics.

I recently purchased the 36" Great Planes PT-19 ARF from Tower Hobbies. Contrary to the wonderful reports on this plane elsewhere on this site I found mine to be a real bear to fly and almost impossible to land. The problem being that at anything less than about 50% throttle it would rapidly and unpredictably dip a wing and go into a downward spiral which was difficult to recover from. The landing speed and stall speed were very similar so most landings ended up as a spiral into the ground. Even from straight and level flight at normal speed, a turn would have to be very gentle or the wing down spiral was instant.

Even after installing a great lump of lead to correct the CG and correcting a few degrees of upthrust built into the engine mount, the plane was still horrible to fly.

I searched this and other forums for answers but found little, and no final definitive solution. I tried the "twist the wing and heat with heat gun" ides but after not much twist there was some cracking and creaking so I decided not to take that too far. Used the heat gun to remove ripples caused while twisting but could notice no obvious washout (with incidence meter) after this exercise.

I was sceptical of removing the covering and heating the wing with steam as this 'plane appears to have been built using mostly cyanoacrylate glue, which I don't think would take kindly to this treatment. Also it seems to me that the results could be kind of unpredictable.

In my ignorance I believe I have a solution. Out of sheer frustration I decided to physically add washout to the ends of the wings. I simply used a cardboard flat folder and designed a contraption that would give a concave curve from the highest point of the wing profile back towards the trailing edge. I decided to exaggerate the extent of the "lift" at the trailing edge of the wing because washout would normally be built in along the entire length of the wing, whereas this contraption only covers the last four ribs, so I guessed that a but of extra "twist" was required.

Since this experiment has worked I will now remove the covering from the last four ribs, add balsa formers to the ribs to imitate the cardboard washout and then recover. It should then not look too bad, and should be hardly noticeable.

Hope this idea may help some who have similar problems
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Last edited by skybolt123; Apr 29, 2012 at 02:05 PM. Reason: Additional info
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Old Apr 21, 2012, 09:14 AM
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Joined Mar 2010
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Solution to vicious tip stall.

I must add that the plane is now a pussycat. Easy to fly and easy to land.

When flying straight and level, if I cut the speed too much it just puts it's nose down gently. Landings are a breeze and I can even "touch and go" with confidence.

I did have to dial in a bit more movement on the ailerons as the cardboard washout devices definitely interfere with the last section of the ailerons.

A big advantage of this approach is that it is quick and easy to try, easy to modify and easy to undo completely if it did not work for you.

Generally I would regard this experiment a great success.
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Last edited by skybolt123; Apr 22, 2012 at 01:20 AM. Reason: Final conclusion
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Old Apr 21, 2012, 09:39 AM
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Don't go to the trouble twist some washout into the current wing using a heat gun....
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Old Apr 21, 2012, 09:59 AM
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Tried that...didn't help
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Old Apr 21, 2012, 10:27 AM
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vonJaerschky's Avatar
Canada, BC, Comox
Joined Nov 2003
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Funny that such an "Elegant" solution helped so much! Thanks for sharing the fix. Could be the ticket for other nasty fliers, too.
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Old Apr 21, 2012, 10:28 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Aberdeen
Joined Mar 2006
12,170 Posts
What you have done there isn't creating washout at all. The wings on the PT-19 dont have any taper so should not suffer from tip stall, the fact no one else has the problem backs this up. I'm guessing that you probably have a 'wash-in' twist in one or both wings that the 'ramp contraption' is off-setting. Whatever you do though dont go and build anything remotely like that into the wing, there are much better ways to sort it out.

As Evan said, twist some proper washout into the wing. If you cant do it by heating the covering then remove the covering and steam the wing while holding in some twist.. this will do it for sure. Make sure both wings have identical twist.
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Old Apr 21, 2012, 10:57 AM
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Boise, ID
Joined Dec 2008
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skybolt123: There's something else you might try. There's a P-51 model, offered by one of the Horizon Hobby companies, Hangar 9 or eflite, I believe, that has clear plastic "NACA droops," I believe they are called, that come off and on. I'm not an aerospace engineer or anything, but I believe NACA droops work to eliminate tip stalls and make stalls easy to recover from. My Apprentice wing has them. The P-51 can be flown with the droops on at first, and turns it into a trainer, essentially. Then you can take them off as you get more experienced. They stick to the leading edge of the wing, out near the tips, with double-backed tape I believe, and taking them off is easy. I believe you can buy those NACA droops separately, for $10 or so. Being clear plastic, you hardly see them in flight.

Might work for you. Just a thought, and sorry to be so vague about the specifics.

Jetplane flyer might be on to the best permanent fix, however.

Found it: http://www.hangar-9.com/PTS/PTSSystem.aspx

Oops again. At the Parts and Accessories page, says they aren't available anymore, but contact the sales and support staff. I bet Horizon has some left somewhere, so might call them up. If you haven't called Horizon customer service number before, they are usually very helpful, in my experience.
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Old Apr 21, 2012, 10:58 AM
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Hi Guys;

Seeing the pictures, looks to me that he had an "up twist" in one of the wing halves (creating bassically a tip stall at low speed); (if I remember correctly PT-19's should be "0's" across all surfaces with just only downtrust (1 degree) and right trust (1 degree) and motor level. Jst my two cents.
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Old Apr 21, 2012, 11:01 AM
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United States, FL, The Villages
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That "fix" involves too many degrees of change. Washout would only be a very few degrees anyway.
I agree with JetPlaneFlyer, I think there was/is another issue to start with. I had a PT-19 and it was a very easy plane to fly and land.
I wonder if you were just trying to fly it too slow. Not every plane is as easy as "pull the power off and let it glide to a landing".
Good luck and let us know what you finally do.
Don
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Old Apr 21, 2012, 11:02 AM
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Tip stall solution

I did mention that I have been flying for many years? Used an incidence meter at root and near the tip, identical - no twist either way, also after trying the heat gun idea, no discernable effect - incidence meter again.

As for removing the covering, twisting with steam - sounds like a lot of trouble for a possibly hit and miss solution to me.
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Old Apr 21, 2012, 11:16 AM
We got some flyin' to do!
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United States, CA, Fort Irwin
Joined Apr 2012
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Expert advice....

slybolt123,

Reading your story and solution to your problem makes me say "Great job!"

It's sad you have to reiterate that you've been flying for a long time...I find that lots of times people only look at the number of posts you have as an indicator of your level of competence...as if that has anything to do with it.

I'll bet your solution is based on your years of experience, which is why you thought of it in the first place, so hats off to you.

Happy flying!
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Old Apr 21, 2012, 11:21 AM
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Evan D's Avatar
Charlotte, NC
Joined Jan 2004
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Still say twist some washout into it... but it is your airplane. Hope it works out for you, we all need better flying planes....
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Old Apr 21, 2012, 11:30 AM
We got some flyin' to do!
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United States, CA, Fort Irwin
Joined Apr 2012
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Wingtip Washout...

Lots of famous, groundbreaking aircraft designs have had this feature--the Piper J-3 Cub and the Mitsubishi A6M Zero among some of the notables. (Just a couple of little known aircraft)

I'd almost bet that the full-size PT-19 had washout (It was a Primary Trainer after all, and tip-stall was a big killer of trainee pilots) and maybe the model designers didn't feel it was necessary.

Just my .02

Happy flying!
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Old Apr 21, 2012, 12:06 PM
Bye Bye VP Aug 2010 - Aug 2012
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Joined Jan 2005
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Quote:
Originally Posted by skybolt123 View Post
The landing speed and stall speed were very similar so most landings ended up as a spiral into the ground
That would be pilot error. Why come in to land at near stall speed in the first place?

Clearly you were flying approaches that were too shallow and too slow.
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Old Apr 21, 2012, 01:05 PM
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Quote:
Originally Posted by Justwingit View Post
Lots of famous, groundbreaking aircraft designs have had this feature--the Piper J-3 Cub and the Mitsubishi A6M Zero among some of the notables. (Just a couple of little known aircraft)

I'd almost bet that the full-size PT-19 had washout (It was a Primary Trainer after all, and tip-stall was a big killer of trainee pilots) and maybe the model designers didn't feel it was necessary.

Just my .02

Happy flying!
.
The first versions of the T-6.. the BC-2s were very bad fliers! There was a significant redesign to add washout among other things.
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