Turbulence

[Michael Massey]

Went flying in very cold, damp and calm air.  Had to deal with my own turbulence to an excessive degree.  Stepping back of course helps but this is a fairly new airplane and the turbulence was very large.  That raises the question, is there a particular airplane design, weight, size...color...that is more or less turbulence prone or handles turbulence better or worse?

[Bill Little]

Went flying in very cold, damp and calm air.  Had to deal with my own turbulence to an excessive degree.  Stepping back of course helps but this is a fairly new airplane and the turbulence was very large.  That raises the question, is there a particular airplane design, weight, size...color...that is more or less turbulence prone or handles turbulence better or worse?

HI Michael,

I am sure that there will be input from much more qualified posters that I am on this subject.  But for whatever reason, I know that some models deal with it better than others.

BIG Bear
AMM

[Brett Buck]

Went flying in very cold, damp and calm air.  Had to deal with my own turbulence to an excessive degree.  Stepping back of course helps but this is a fairly new airplane and the turbulence was very large.  That raises the question, is there a particular airplane design, weight, size...color...that is more or less turbulence prone or handles turbulence better or worse?

   It's counter-intuitive but the big factor seems to be prop diameter. I would have expected it to be the weight, since there should be a lot of effect of turbulence generated by the wing in the corners, and the heavier it is the more induced drag and lift and the more the air is disturbed. But the prop diameter seems to matter a lot more.

   I think the issue is this - the turbulence from the wake moves out of the track of the airplane, but the prop wake hangs in the track. In humid conditions or when you have exhaust hanging in the air, you can actually see where the wing wake goes, and it seems to move something like a foot or so off the track (in the direction the air is displaced to create lift, e.g. towards the bottom of the airplane on an inside loop). There's no particular reason that the prop turbulence would move out-of-track, it seems to mostly spiral around on-track. When flying in dead calm, I will make sure that if I miss the track, I miss it to the *inside* of the maneuvers, like, make the loops progressively smaller (or at least make extra-sure they don't grow).

  Paul Walker provided the insight to this, both intentionally and unintentionally. The first was intentional - running a 9-4 Rev-Up on his 45FSR/Bad News at the Reno NATs. He can add but he had essentially no issues with turbulence. The second (and when I hit on the idea) was unintentional, either the 99 or 01 TT, where Paul was first up in the morning on qualifying day and it was *dead*, and I mean *dead*. By the time he was done there was a hemisphere of castor mist hanging in the air. With that, you could see the turbulence coming off the wing, and it always went off-track about a foot or so, and stopped. Every time he overshot the track, you could see the airplane fly through the turbulence, and every time, he got a big bump. The rest of it was an almost continual low-level rattling around through the prop wake. This was with a PA61/Aussie Mod and *probably* a 12-4 or something like that.

    Brett

[Andrew Borgogna]

I can only provide empirical data to support what Brett said.  I have a big heavy Top Flite Score with a 13" prop that will tumble and fall out of the sky when it flies through wake turbulence.  I avoid wake turbulence like the plague!.  With a decent breeze the Score flies great but in dead air it can be dangerous.
Andy

[Dennis Moritz]

Fat airfoil, constant chord wing appears to exaggerate the issues. At least as I observe stuff, as well as big prop prop wash. Dan Banjok's Vista, which fits that description, was sadly pulled to the ground one late afternoon. A plane that meets all the above criteria. Fat airfoil, constant chord (just a hair of leading edge sweep back) and big prop. In most planes I've flown turbulence is more of a startling surprise than a fatal suck down. Planes right themselves without loosing too much altitude. Avoid panic, if possible. You can anticipate that turbulence is likely when putting the bird up in still air. Where the turbulence happens during maneuvers, a mystery to me. Can be anywhere. Dan's plane was his in the lower right corner of the triangle. His five foot bottoms, or lower, made for the splat. I've often experienced turbulence in the upper quadrant of round maneuvers. Elsewhere as well. These days flying the birds I fly, turbulence is more of an amusing diversion than a threat for a crash. Reminds me that the air is calm, real calm, and I can concentrate on shape without the interference of the weird winds that usually blow at our home field.

[Brian Massey]

Had this happen to me today; twice in the same pattern. First leg of the outside square, I pointed the plane down and the nose dropped, the outboard wing swung up so I could see the entire panel, and then it just fell. I was lucky enough to catch it about 3 feet off the ground by back peddeling like crazy, but that was my last flight of the day. Running an 11/5 Xoar on an LA 46. Not sure if a smaller prop would be in order.

Brian

[Dennis Moritz]

I doubt the prop would make much of a difference. I've had turbulence issues with sport planes turning smaller props. It just happens. That's as small a prop as I would use on an LA 46.

[PJ Rowland]

" I doubt the prop would make much of a difference.  "

Yikes....... 

Obviously empirical evidence from past national winners means nothing ?

[Dennis Moritz]

The gentleman is already using a prop on the small diameter size for an LA46. At least in my experience. Perhaps he could go to an 11x4. But that doesn't seem to be much of a difference in diameter.

[PJ Rowland]

Prop size is relivant to airfram size and engine displacment.. if the prop is too big for that sized frame for example, it will still be suject to turbulence.

[Larry Cunningham]

It seems that certain models generate more wake turbulence than others. I've watched big models cut a di-do and nearly crash flying through their own wake in dead calm air. If props have a lot to do with that, I wonder why the problem simply isn't there most of the time? 90 degree roll maneuvers are caused by the wing experiencing some kind of major up- or down- draft in a local area. And wing tip turbulence is usually a vortex, right?

Maybe we just need some winglets on our stunters? ;->

L.

"A life spent making mistakes is not only more honorable but more useful than a life spent doing nothing." -George Bernard Shaw

[Brett Buck]

I doubt the prop would make much of a difference.

     I did, too, until it was pretty definitely proven otherwise.

     Brett

[Larry Cunningham]

Well, the prop DOES generate a vortex, and perhaps it acts to decrease the tip turbulence vortex.. It would be interesting to see how different the same prop in opposite rotations would feel in regard to this problem. I know the prop's vortex swirls around the fuselage, but I didn't think it had so much influence on the wing tips.

About here I think we need a full simulation in 3D to actually predict what the effects are. I'll bet Boeing, MacDoug, and AirBus all have a lot of data, buy probably none appropriate to a control line stunter.. (I've noticed the airliners are using "pointier" and thinner wings than we do..)

I take it back, perhaps we need a wing airfoil with a dimpled surface (a la golf bar) instead. Or perhaps a prop with that.

Merry Christmas, youse guys!

L.

"You moon the wrong person at an office party and suddenly you're not 'professional' any more." -Jeff Foxworthy

[Dennis Moritz]

Certainly it's possible that prop size would make a difference in turbulence. My thought is that an 11.5 prop is about as small as I would go for an LA46. I fly models with that engine often, usually with a 12.25x3.75 APC cut down perhaps a 1/4 of an inch. 11x4 or 5, don't drive as well. I thought that the difference in turbulence that Brett observed was a comparison of props that varied in diameter more than 1/2 inch.