Turbulence when flying in calm air

Question for the Aero Engineers out there (you know who you are, Paul,Ted ,Bret) or anyone else with the knowledge.What causes most of the plane killing turbulence, the prop or wing? I've been flying for many years and long ago learned to step backward with each repetition of maneuver but I've always wondered what the main culpret was. Everyone points to props but I suspect the wing is a larger player in this opera. Any tips? (wingtips that is, pun intended).

Gary,

Any pilot can answer that question. "Wing tip vortices". They spin inward twoards the fuselage and separate as they drop down behind the airplane. There usually no problem doing level laps but are a problem in repetitve maneuvers. I've lost a couple of planes this way. That's why I'm experimenting with some trailing edge tip plates on the Twister III I'm building.

>Gary,
>
>Any pilot can answer that question. "Wing tip vortices".
>They spin inward twoards the fuselage and separate as they
>drop down behind the airplane. There usually no problem
>doing level laps but are a problem in repetitve maneuvers.
>I've lost a couple of planes this way. That's why I'm
>experimenting with some trailing edge tip plates on the
>Twister III I'm building.
>
>Jim Pollock

Hi Jim.

The tip vortices are not just at the tip but rotate between the
wing root and half the span out from each tip. The whole
vortex structure is around 2 wing spans wide. The vortices are
not just caused by air flowing around the tips. It is the result
of combined cross flows along the whole span. For these reasons,
no fancy tip design has ever had an appresiable effect on the size
or strength of tip vortices.

Despite what some think, those winglets on commercial jets do not
reduce vortex or induced drag. They are carefully aligned to
the crossflows at the tip to gain some thrust from the vortex
crossflows. As such, they only really work at the aircrafts
optimum crusing speed.

The wing. Funny wingtips won't help.

This'd be an interesting thing to do, I think. Choose a day where there's a very, very light breeze. Get one of those smoke generating things (the sort skydivers use), position it halfway between the pilot and the downwind edge of the circle. When smoke gets generated, it will be carried by the breeze downwind into the area where you're performing aerobatics. It should show the turbulence as it is created, and as it travels downwind. You probably don't want the smoke too thick - you want to be able to see the plane, after all, so it'll be a balance between the density of the generated smoke and the strength of the breeze.

Be sure to videotape it, of course.

VGS on the end plates.

Reduced Wing tip vortices, by controlling the flow.

Alas, you will never completly cure this tho, In my opinion anyway.

AMEN on that!! I have tried them!!

Yep the wing--you can see the proof of this since it is the points in manuevers where the wing begins to make big lift that the turbulence appears--transitions from inside to out, bottom corners, etc. If it were primarily the prop, it would be everywhere.

Anyone ever fly a stunter with one of the rc smoke systems?

>Question for the Aero Engineers out there (you know who you
>are, Paul,Ted ,Bret) or anyone else with the knowledge.What
>causes most of the plane killing turbulence, the prop or
>wing? I've been flying for many years and long ago learned
>to step backward with each repetition of maneuver but I've
>always wondered what the main culpret was. Everyone points
>to props but I suspect the wing is a larger player in this
>opera. Any tips? (wingtips that is, pun intended).

Actually it's Roger after the "All U Can Eat Bean Burrito Bar"

>Actually it's Roger after the "All U Can Eat Bean Burrito
>Bar"

You makin' fun of me again FOWLer?
Wait 'till the next time we go someplace in your truck!

Reminds me, at Wichita I launched for Doc and walked back a bit from the circle, right after he pulled out of the reverse wingover, which was right in front of me, I got hit with a slap of breeze. Don't think it was prop blast, didn't smell good, it was either the wing of the vortexes, though I didn't detect any twist, it was a straight shot of air like someone was about 5 feet from me with a good size airhose and pulsed the flow out of it.

Gary, since none of the designated heavies have shown up so far...

One theory describes lift generation as displacing a mass of air equal (more or less) to, and in the opposite direction from, the "weight" -- read g loading -- in the path observed.

At the bottom of loops, stunters are pulling around 10 g for the path, plus the 1 g we always have (gravity.) So that's 11 g. A 4 lb stunter, then, 'displaces' at least 44 lbs of air away from its "pilot's" head (imaginary li'l guy sitting in the cockpit) on inside bottoms, and t'other way on outsides.

Wild Bill Netzeband suggests that sharp corners may reach 35 g for the path flown. So, that model in low corners to (or from)level should 'displace' 35 + 1 = 36 g, or 144, lbs of air in that fraction of a second!

Stand a safe distance (10'-20')outside the pullout zone and watch a few loop bottoms, or square recoveries. Sharp spank of air, no? I doubt even the best of the new engines could punch that hard. Also note: NO downwash as the model flies by level (g load = 1.0) ...

My vote is wing "down"wash, due to g loads.

Although I have never done it with a C/L model you can visualise the trailing vortices by taping cassette tape streamers to the wing trailing edge.
I have done it to both R/C gliders and sport models. In level flight the vortices are small in diameter and spin slowly. If you pull some G's the radius and rotational speed increase. At the root there is no vortex, and the further out on the span you get the larger they become.
Sounds like an "experiment" worth repeating for a C/L model.
Pat MacKenzie

I'll keep on truckn'backwards in the calm as always.Best wind ship I ever saw (at least for a non piped plane) was Wynn Paul's ST46 Fantasy. It had a round cross section fuse.like a baseball bat with a cabin and a needlenose spinner. Wing,stab and rudder all had really pointy tips. Flew unhindered through the heavy turb. at the early 1990s meets at Lunkin airport circles in Cincinatti (sunkin lunkin). Never did get to see his piped version fly. Makes me think there is still something to be seen in wing tip exp. Wonder how they were in the calm air? I guess I ought to call Wynn. I certainly have experienced the heavy wind blast while standing at the pullout section of the circle.By the way Rog., The next time we go somewhere in Fowler's truck, include me out.

Go canoeing. Observe the little whirlpools around each edge of the paddle blade as you pull it through the water (tip vortices) There is no way to eliminate that when you fly through it again. The solution is not to fly through it. I suppose we ought to issue CAUTION, WAKE TURBULANCE on squares, triangles and the last inside our outside loop. Sometimes you can step backward and step right in a hole. I did on a turbulating day when the wind (in a dead calm) turbulated and I put that plane right in the middle of the hole. SMACK!!!!!!!. YEP, IT'S NOT FUN.

It comes with the turf and is like running a computer and hard disk crashes. There are those persons who have crashed and those who will.

"One theory describes lift generation as displacing a mass of air equal (more or less) to, and in the opposite direction from, the "weight" -- read g loading -- in the path observed. "

You must be thinking of boats or balloons.

The aforementioned heavies can check my ciphering, but I figure that the power that the airplane puts into wake vortices is equal to induced drag x airspeed. Therefore, wake vortex energy per foot along the airplane's path is equal to induced drag. Not counting the lift needed to overcome gravity, induced drag is 2 x (airplane mass)^2 x speed^2 / ((loop radius)^2 x (air density) x wingspan^2 x Pi x (efficiency factor)). Randy Powell has an advantage here, because elliptical wings have an efficiency factor of 1.0, rather than the .9 or so of tapered wings. His airplanes also have ridiculously high wingspan. So you can reduce vortices by reducing weight, increasing wingspan, turning wider corners, and flying slower.

The propensity of an airplane to get upset by turbulence is greater as wingspan increases and probably as aspect ratio increases, thereby steepening the lift curve slope.

The moment needed to recover from a turbulence-caused upset is proportional to line tension x half the wingspan (not counting aerodynamic rolling moment due to roll rate opposing the recovery).
Line tension, though, is proportional to airplane weight and the square of speed.

Dave Pellerin was wondering this weekend how airplane weight affects performance in turbulence. Beats me.

F = MA. We have an airplane flying along horizontaly at 60 MPH. Over a distance of 15 horizontal feet (15 foot radius turn) it decellerates from 60 MPH to 0 MPH, whilst accellerating from 0 to 60 mph in a vertical direction within 15 feet. Say the airplane weighs 3 pounds. So for the airplane to do this it has to move a sufficient mass of air with sufficient acceleration to give MA airplane = MA air. I don't think there is any 3 pounds of air involved, so the air has to be accellereated at a much greater rate than the airplane.

I made C's and D's in Physics, 50 years ago, so you cannot hurt my feelings by correcting my errors.

OK, Howard, agreed...

I have seen mass displacement description used re: wing lift. But then, I am older than some of the dirt here in Arizona... <g>

I like your formulae using matters other than lift. It is an easier way, to refer directly to the FORCES involved in moving the model through its path. And it works with what is actually observed, not theoretically "predicted."

I still credit the idea that lift (which equals accelerated weight at any infinitesimal moment) allows finding a momentary C(L)-- the basic aerodynamic term finding C(D(i)).

Both should give similar results.

Brett, does it matter whether wash or prop blast is the major factor? I expect what we hit is what endures longest: -- energetic vortices, however they are formed. The increased lift load for the turn forms vortices with force and energy levels much above those for level or straight flight conditions.

(Simplified, for those unfamiliar: Air rolls 'over' the wingtips because pressure 'below' the wing is higher than that 'above.' Since the wing moves forward through "still" air, the only available direction air 'below' can slide due to the pressure difference is toward the wingtips. The spiraling flow off the tips is the vortex.)

A downwash pulse may dissipate or drift off fairly quickly. Vortices maintain energy efficiently, and dissipate slowly. They stay in place, pretty much, after the disturbance that formed them ceases (or eases significantly). The more energetic, the longer to dissipate.

In consecutive figures, that's plenty of reason to step back a wingspan per rep in still or near calm air.

Howard,

Hmm, I'd plan to stay well out of this, but...

>>Randy Powell has an advantage here, because elliptical wings have an efficiency factor of 1.0, rather than the .9 or so of tapered wings. His airplanes also have ridiculously high wingspan. <<

But not at the same time. The high aspect planes I've built are all straight taper wings and all long (68 to 74 inches generally. One was 83 inches). The elliptical planes I've built are relatively small (~640 squares) and of average AR at around 5 to 1. The longest about 58 inches.

>>The propensity of an airplane to get upset by turbulence is greater as wingspan increases and probably as aspect ratio increases, thereby steepening the lift curve slope.<<

Well, this is certain, in my mind anyway. This is why I haven't built
anymore very high aspect ratio planes. While I could migtigate some of the problems like airspeed changes radically changing control sensitivity and difficulty in getting turns to stop where you wanted, in the end, I couldn't overcome the ridiculously high sensitivity to wake turbulence without giving up all the things that induced me to build the things in the first place. Highly tapering the wing to reduce the wing tip area helped to a limited extent, but caused other problems. Most planes in that series tended to be pretty good flyers in the wind, even very high winds (as long as it's relatively undistrubed wind), but were death on calm, hot days.

One of the best flights I've ever put in was on a very large, open grass field in pretty high wind (15-20mph) using one of the high aspect planes. Nothing around for miles to distrub the wind. Perfect conditions for that design. But let the wind die and it get hot and it was a dog.

>Question for the Aero Engineers out there (you know who you
>are, Paul,Ted ,Bret) or anyone else with the knowledge.What
>causes most of the plane killing turbulence, the prop or
>wing? I've been flying for many years and long ago learned
>to step backward with each repetition of maneuver but I've
>always wondered what the main culpret was. Everyone points
>to props but I suspect the wing is a larger player in this
>opera. Any tips? (wingtips that is, pun intended).

Well. I don't know for certain. I always felt it almost had to be the downwash from the wing. But my observation has been that the prop makes more difference. Diameter matters, for sure, but the prop design (presumably the efficiency) effects it as well.

Other credible people think that, too.

>Other credible people think that, too.

OK, That's my cue!!

I did an experiment back in 1985 time frame to help answer just this question, wing or prop. The plane was my light Reno plane, Bad News at 725 in^2 at 43 ounces. It was powered by an OS 45 FSR. The starting prop was a 11.3 * 6, 2 blade.

Flying in the absolute dead air, there were very clear "hits" at the bottom of the square corners, and backing up was very necessary. Lap times were 5.3 to 5.3 sec / lap. Result: Plane flew through "disturbed" air and caused "Bad Things" to happen.

Then switched to a smaller prop: A 9 * 4 cut from a 12 * 4 and the engine cranked to 15 k. Lap times a little faster, 4.2 sec / lap, but quite flyable! Result: Absolutely no "hits" from wake. Could stand dead still without backing up and was never "hit". I could not believe it, and switched back immediatly to the original prop to see if the weather conditions had changed.

With the original prop, was "hit" regularly with turbulance once again. Back to the small prop, no "hits" what-so-ever...Period.

Conclusion: Prop was the dominant factor in that equation. If that combination would fly an overhead eight in the wind, everyone would have seen that set-up years ago, but alas, it wouldn't stay tight through that SINGLE maneuver, in the wind. Great everywhere else.

Note: This conclusion is valid for the LIGHT plane flown. Does it still hold for a more NORMAL wing loading configuration? Tried it on a more normal plane, and the 9*4 was in no way adequate to fly a competition pattern.

Some have asked in the past why I flew an 11.3 dia prop for so long. This prop diameter was my best compromise for performance in the wind and performance in the dead air. I spent many years developing just the right combination for my design (Bad News and Impact). That combination has worked very well over the years!

And yes..I violated that significantly when I used the 4-strokes, and clearly paid the price on Saturday at the NATS this year. Heavy plane (bad) and 13in prop (bad) and a nose heavy configuration (bad) in still air (at least for my flights it was still) made for big problems, and I faired poorly. That won't happen again, trust me.

Soooo, in conclusion, it is my OPINION (from my tests and contest experience) that the primary factor is the prop, with the wing being secondary, but not a totally non-existent effect.

It's 11.3 for me.

:)

It is always funny to find that "counter theory". So here is one:

The wing worticles needs some power. It can come only from engine and thus by prop. So if that small prop does not convert power to thrust under load in maneuvers well, it could be that the wing produces much less worticles as it slows down in maneuvers. It means the prop is a factor, but worticles still comes from wing tips

So, based on your experiment, other inputs here, I have come to the conclusion that a small prop equals slower airspeeds which in turn generates smaller vortices, less lift, less drag. Larger props equal more airspeed, more lift, more drag and larger vorticie. (all else being equal). I have been smashed into the ground twice during the indside loops, but never square loops. The plane makes a definite jump and I know then I'm in the same track. Good news, bad looking for points. So I noticed several flyers backing up and thought it was just an affectation. But learned how to avoid vortices. Just like my CFI told me to never land until two minutes after a heavy has touched down and same for take off, to allow the turbulence to drift away from the runway. Big or little, horizontal whirl winds will smash your plane to the ground.

>So, based on your experiment, other inputs here, I have come
>to the conclusion that a small prop equals slower airspeeds
>which in turn generates smaller vortices,

I think you misread Paul's post.
With the smaller prop, he had higher RPM and lower laptimes;
that is, faster airspeed.

Right on Preston. Seems I got it backwards. Fact is at lower speeds of landing and takeoff is the worst for vortices on large planes, so should be the same for our smaller ones in the same ratio, perhaps.

I think that it's more related to angle of attack (AOA) of the wing rather than speed. The AOA is usually higher for a given weight at slower speeds. So a heavy slow flying airplane will have a higher AOA thus create more turbulence. An aircraft lightly loaded will produce less turbulence than the same aircraft heavily loaded at the same speed. The AOA is greater on the heavy plane to produce adequate lift. Thus is much more apparent during take-off and landings of real aircraft.

As mentioned in an earlier post, our models are seeing much higher AOA during abrupt turns creating tip turbulence than in level flight..

Well, I was reluctant to speak up because I am far from an expert but one thing jumped out at me in the comparison: all things were equal except for the flying speed, and the prop, of course. Faster flying speed means lower coefficient of lift, which means the shedded vortex is weaker and faster to dissipate. In fact, the second C-sub-L is about 65.2 percent of the first with the times listed and the ratio of the coefficients should be similar in maneuvering loads. Wouldn't this be the predominant factor in the absence of the bump instead of the prop?

RK

Speaking about maneuvers (not lift against the gravity) the lift coefficient is invariant on speed. Higher speed -> higher centrifugal force (square of speed) -> needs higher lift (comes with square of speed)

Chris...

Yes, for an airplane flying in 1-g, level flight, I figure that vortex energy per unit length along the flight path is inversely proportional to the square of airspeed.

Years ago, my boss and I were arguing over lunch about trailing-vortex intensity of an airplane going fast vs. that of the same airplane going slow. I showed him the calculation, but he was still unconvinced. We continued the argument all the way back from lunch. As we got out of the car, two of the USAF Thunderbirds flew over with their smoke on. One was flying slow, the other fast. I just pointed up and smiled. (Have I told you this story before?)

Rollin,

Good observation! Yup, faster times equal less problems with turbulence!

I will second Paul's observation here. My current light plane(43 oz. 750 squares) will hit itself very hard with turbulence in calm air. Something like a 60 deg. roll. It will even take hits from the round loops. I haven't experimented with smaller props much recently because it loses too much in maneuvers to be worth flying.

The improvement with a smaller prop, flying faster does leave quite a few variables though. Higher speed means more line pull, which will damp upsets better. When flying slower with a larger prop, the plane quite likely takes more control movement/more flap, which would increase the turbulence. You'd have to keep the airspeed the same during the maneuver to really control the experiment.

Phil,

One of the things with a smaller diameter prop (vs a smaller prop, which doesn't necessarily mean the same thing) is how much air are you displacing? And what is the effect on vortex?

I've been testing several props. The best for the plane I'm flying (so far) is pretty small diameter compared to what I've been using (11" diameter 3 blade vs a 13" diameter 3 blade). I believe the reason it's working with quite a lot of power in the maneuvers is not the diameter, but I believe, the displacement. The prop was cut from a 14" 3 blade prop, so although the diameter is pretty small, the blade are very wide.

Makes a difference.

Randy,

Thus the reason that GMA made some paddle blade carbon props - he knew they worked great. Gee, reminds me of the old song. "Everything old is new again!"

The prop makes more difference in plane killing turbulence? I don't think so, from my own observations both as a modeler and as a pilot the tip vortex is "BY FAR" the greatest contributor to plane killing turbulence. I base this on several model and real experiences that are too lengthly to describe on this forum.

>The prop makes more difference in plane killing turbulence?
>I don't think so, from my own observations both as a modeler
>and as a pilot the tip vortex is "BY FAR" the greatest
>contributor to plane killing turbulence.

I don't know what to tell you, that's what I used to think, too. I base my opinion on having flown the same airplane at the same weight with many different props. Turbulence in dead air was a minor problem with a 11.5-3.5 Bolly 2-blade on a 40VF. It was a much larger problem with a 12-3.75 Eather UCT 3-blade on a PA or RO-Jett 61.

I don't know why, it doesn't seem reasonable, but that's been my observation.

Perhaps its the way the propellor reacts to the turbelence vice what it CONTRIBUTES to the turb. that is the culprit for your observations?

Flying into a wingtip vortex not only disrupts the lifting surfaces, but is like wind shear for the prop--sudden dramatic change in aoa for the prop.

I have noticed the same kind of difference on a small, twister sized profile--same engine, same speed, once flying with one of Leonards' 12.5 x 5.5 Bolly's on a TT36 v. 11 x 4 APC.

>Perhaps its the way the propellor reacts to the turbelence
>vice what it CONTRIBUTES to the turb. that is the culprit
>for your observations?
>
>Flying into a wingtip vortex not only disrupts the lifting
>surfaces, but is like wind shear for the prop--sudden
>dramatic change in aoa for the prop.

I don't know. I might guess (based on some observations of hanging exhaust - not quite as good as a smoke tunnel, but same idea) that the downwash isn't in the right place to induce problems for the airplane. Pretty much certain that the downwash travels, well, "down" relative to the airplane's track. Therefore, maybe you don't fly back through it. But the propwash *does* hang around in the track - it goes along the track instead of at right angles.

"Pretty much certain that the downwash travels, well, "down" relative to the airplane's track. Therefore, maybe you don't fly back through it. "

Well, I suppose downwash is not a problem for you guys who put all your consecutive maneuvers in the same place.

The prop would have to pull on the airplane to overcome the energy lost to wing induced drag, and to other drag increase during maneuvers. So prop induced losses would also cause wake turbulence. I would think that the prop would put less energy into the wake than the wing: 30% as much for a 70%-efficient prop.

Where does the prop wake go? I'd reckon that during corners, it would be displaced downward with the wing wake. Much of the airspeed recovery-- hence prop wake-- from turning a corner would come after the corner when the airplane is flying straight. In this case, the prop wake would blow straight back for you to enjoy when you come to the next corner in the same place. I'd think it would be worst when the model is going uphill. I always thought of the 6th corner of the triangles as the Corner of Death. Maybe it's the 4th.

If an engine is offset so that it points to the outside of the circle, would the prop wake blow toward the center of the circle such that the airplane would hit it when the flyer is walking backward? There is induced loss due do side force from the prop bending the airflow through it, either due to sideslip from engine offset or due to maneuvering downwind. This would cause more wake. This is getting complicated. I sure don't see wingtip doodads helping, though, which is the original question.

Sixth corner of the Triangles was the last turb.induced crash I had. My Hurricane made the sixth corner at about four and a half feet, flew 10-15 feet level then dropped flat to the ground. I had forgotten to make that last backstep. All the turb. crashes I've seen have been in the square tricks especially the sq. eights.

Perhaps the propeller is a contributing factor when combined with the inboard wingtip vortex.

Slipstream effect from the propeller winds around the fuselage over the left wing root, across the top of the fuselage, down behind the right trailing edge, and around the bottom of the fuselage ahead of the left horizontal to strike the left side of the fin/rudder. This is part of the P-factor that causes the airplane to yaw left with a clockwise rotating prop (from the cockpit).

So if a large diameter prop was to have moved a large amount of corkscrewing air with a smaller amount of dispersing effect from the fuselage as compared to a smaller prop, this combined with the inboard wingtip vortex, that is rotating the same direction could substantially increase the power of the vortex.

I've seen near instantaneous reaction of a model after flying into it's wake. One I remember very well was Whitely practicing for the 84 China trip with a 46 Derringer in calm conditions. On the inside squares, the model made it's fouth corner of the first square and then came level for an instant before being literally pushed/thrown into the ground with such force it reminded me of a strong down draft. It made a visualization in my mind of the vortex pushing it into the ground.

Fly higher and back up in practice with very calm conditions. I have to,I build slowly.

This is interesting. The prop vortex would add to the left wing's vortex in inside maneuvers and the right wing's vortex in outsides. Are inside squares worse about wake encounters than outside squares?

Good point. Completes the inverted portion of the theory.
Chris...

Howard,

That's where I lost my Magnum was the Vortex on the down leg of outside squares. I have hit turbulence in outsides with other planes as well. It seems that my Intrepid has handeled turbulence the best. I guess the swept tips do actually do something in that regard.

Interesting reading for full size aircraft

http://www.asy.faa.gov/safety_products/WakeAC.html

http://www.acflyer.com/learn/train15.htm

And some pics

http://www.1000pictures.com/aircraft/F-18/

Dick,

THANK YOU!!! == Your links are clear, authoritative and to the point we've been kicking around here.

I'll study the first one more closely, later. Good stuff!

From Chris:

"I've seen near instantaneous reaction of a model after flying into it's wake. One I remember very well was Whitely practicing for the 84 China trip with a 46 Derringer in calm conditions. On the inside squares, the model made it's fouth corner of the first square and then came level for an instant before being literally pushed/thrown into the ground with such force it reminded me of a strong down draft. It made a visualization in my mind of the vortex pushing it into the ground."

My only contribution here is to emphasize above comment. While, like all of us, I have had times when various models went goofy on me, evidently from flying through their own wake, such has never been much more than a minor annoyance.

However, I did (once) see exactly what Chris describes, although it was at first lower turn, on outsides and during square eights. Second time through, of course. This has been documented/discussed at length, so I will not mention it was good friend Will Reeb flying Keith Varley's beautiful Saturn just about a year ago.

Will was flying *high,* not pretty low as Whitely is assumed to have been doing. The model made full rotation of ninety degrees, settled to level flight momentarily, and then literally fell, *flat,* to the asphalt. Ooops, Canadian site, Canadian friends, make that "tarmac."

I was stunned. Only maybe 5 or 6 of you on this site have enough credibility with me to describe flight path of that model and then have me actually believe it. I mean, *I saw it,* and it is still difficult to believe. To the point where when I tell someone about it and their eyes glaze over, it bothers me not at all...

Still, some of us--me included--really ought to be a little more conservative when flying in calm conditions, regardless of source of turbulence.

(And my money is on PW's objective testing having nailed it as being related to big props. You guys run those large-diameter props all ya want, but now you know at least one reason why I try to stay away from them, thanks to Paul's input.)

I know this is probably silly to bring up at this late date, but...

If it is the prop wash that's doing the dirty, why aren't we crashing in level laps in calm air the way we do in the last corner of triangles? I might not fly the most concentric consecutive loops but I'm pretty darned good at concentricity in level laps! Is everyone backin' up in level flight 'cept me?

Ted

p.s. Chris makes an interesting observation I've never thought about before. I can't recall nearly as much of a problem with self induced turbulence in outsides as I have in insides. Do I just have a bad memory or have you guys experienced the same difference.

I've seen several airplanes crash from wake turbulence in my 50 or so years of stunt ... more than three but certainly less than a half dozen ... and all of the ones I remember were in the third corner of the second triangle. Now, even if the "Gs" are the same in the triangle corner as in a square, whatever angle of attack/vortex producer there is in the wing at that G loading it is maintained for a greater length of time/distance. I've always felt that was the reason for the greater incidence of ooopsies in triangles.

Hmmmm.

Ted,

Yes, I have experience turbulence in outsides. In fact that's what caused me to plant (literally) my Magnum two weeks before the 2002 Nats. It was during the down leg of the second outside square loop.

>Ted,
>
>Yes, I have experience turbulence in outsides. In fact
>that's what caused me to plant (literally) my Magnum two
>weeks before the 2002 Nats. It was during the down leg of
>the second outside square loop.
>
>Jim Pollock

But my question had to do with frequency. I know it can and does happen but my experience is I don't worry nearly as much about it as on insides. My guess is this is because I've had many more problems with insides.

Heck Ted, that's how I know that I actually hit the last corner of the triangle in the same spot as the first time around and didn't walk all over the circle.

I did experience this last night. Nice evening with near stunt Nirvanna conditions. About 75 degrees, a slight wind blowing (may a mile or two an hours... would have preferred a bit more breeze). Came down to the last corner on the second triangle and the plane does a little dance. I hate reading the printing on the top of the outboard wing while flying and then seeing the gear mount. But it leveled out as I was backing up pretty hard and on we went without further incident. Woke me right up.

Ted, model in level flight has very little drag, thus the prop has very little thrust and thus does not make too much vorticles. As Howard wrote already it is the lift (thrust in this case) what makes it. So no surprise that we do not feel it in the level flight. But the same goes on with wing too. It also does not make any signifficant lift.

Anyway, it is good and eyeopening question. In vertical segment of wingover we fly without wing lift but with strong pull of prop. So I will little changne your question. Do we feel it in vertical flight of wingover or square loops or legs of hourglass (means it comes from prop) or we feel it only in maneuvers like loops and corners (it is wing related).

>I've seen several airplanes crash from wake turbulence in my
>50 or so years of stunt ... more than three but certainly
>less than a half dozen ... and all of the ones I remember
>were in the third corner of the second triangle. Now, even
>if the "Gs" are the same in the triangle corner as in a
>square, whatever angle of attack/vortex producer there is in
>the wing at that G loading it is maintained for a greater
>length of time/distance. I've always felt that was the
>reason for the greater incidence of ooopsies in triangles.
>

Of course, there is also far less tolerance for error or anything going wrong in the third corner of the triangle as well. That's why you don't see a lot of crashes in level flight - it bumps around, but you're not counting on it making a 15G corner for 120 degrees and coming out within 4 feet of a very unforgiving celestial body. I get bumps *a lot* in level flight - but it doesn't take much control, lift, or line tension to fly around level.

Dick,

Yes, and I have had the misfortune of watching large jet turbulence in action on a light aircraft at low altitude twice. Not a pretty sight seeing a Cessna 172 wrenched from the sky killing all on board when they foolishly tried an intersection take-off too close behind a
C-141 at Elmendorf AFB. I was in another Cessna 182 that was rolled upside down by a Boeing 707 doing practice approaches at the Stockton CA Airport. Fortunately, we were at 2,000 feet and I was experienced at aerobatics. Not a problem there, but at 100 feet would it have been a serious problem.

>Dick,
>
>Yes, and I have had the misfortune of watching large jet
>turbulence in action on a light aircraft at low altitude
>twice. Not a pretty sight seeing a Cessna 172 wrenched from
>the sky killing all on board when they foolishly tried an
>intersection take-off too close behind a
>C-141 at Elmendorf AFB. I was in another Cessna 182 that
>was rolled upside down by a Boeing 707 doing practice
>approaches at the Stockton CA Airport. Fortunately, we were
>at 2,000 feet and I was experienced at aerobatics. Not a
>problem there, but at 100 feet would it have been a serious
>problem.
>
>Jim Pollock

I can add one to this scenario as well.

I was once making an approach in a B720 straight in at Dulles International Airport behind a landing DC-8. The winds were calm and the approach stabilized and we were anticipating just another nice landing. A couple hundred feet in the air shortly before the threshold we got caught in a sinker that required firewall thrust to keep from striking the ground before we reached the runway.

The vortices from the wingtips twirl aft and slightly below the aircraft that is making them. The air circulates from the bottom high pressure area of each wing and tries to work around the tip to the top low pressure area. Of course the airplane is moving forward when this happens and the air simply turn into this miniature horizontal tornado off of each tip.

In calm air the vortices turn toward each other and in the space between them the air mass is moving downward ... a form of vertical windshear, if you will. Thus our 720 flew from a stable airmass into one that was descending and the airplane wanted to go with it.

The normal vortex problem is more as Jim described where a smaller airplane (or a big one which can generally handle the problem because of mass and inertial)flies into one of these horizontal tornadoes and simply finds itself rotating with the mass of air. If the rate of rotation of the air is greater than the roll capability of the airplane it is impossible to keep the airplane from rolling with it. Disasters as described by Jim are often the result.

Ted,

Exactly as you describe, and when you are doing high G triangles you are creating two of these counter-rotating tornadoes and since the time of the maneuver is shorter the sinking downwash probably doesn't have time to dissipate enough. The resulting vortex's are approximately equal to 1/2 the wingspan very shortly after generation and then expand - but of course you know all this stuff anyway since you used to fly one of the biggest vortex generators except for a C-5.

Ted,

I seem to get the outside turb just as bad as the inside. However that is if I just stand there.

So let's say I am flying around at hobby park and it is a nice 99 degrees out in the mid august, that means zero air. I go into some loops. The insides just jerk around and it bangs around pretty good. Then I fly all those life force eating inverted level laps, crash free just like you do I might add. Then when I get to the outsides I know I am going to have to move or get toasted by the 140 degree BLACK tarmac. So I move and get very little if any bouncies. I think that is why we dont get it as often outside as we do inside. The inside is kind of a gauge and you go to the outsides ready and you dont get the boucies.

Just an idea. Just thinking outloud.

A great thread, in the best SSWF tradition!

I'm really enjoying this, I've observed the turbulence, but have nothing intelligent to contribute.

Thanks to all.

[photo not recovered: 3e69bb3870f12ad5.jpg]

"My wife said she wanted to have sex in the backseat of the car. With ME drivin'!" -Rodney Dangerfield

YOU GUYS HAD ME ALL CONVINCED UNTIL....

I have often watched jets land during snow and all is as you Pros describe it. The problem is that those vortices are moving, and initially rather rapidly. Snow is great for seeing these things clearly.

That brings up the question " shouldn't they be rapidly moving away to the outside of a maneuver" and be gone before the next one?

This is probably a dumb amateur's question but humor me fella's.

Yeah, the 747 develops lots of wake turbulence.

What is also interesting, however, is the fact that the B757, a much smaller airplane is noted for being a producer of vortices apparently much greater than was initially expected. It is treated as a "heavy" aircraft for air traffic control separation on approaches for that reason even though its weight is well under the "arbitrary 300,000#" maximum take off weight generally used to determine "heavy" status.

A likely reason why is interesting in terms of this discussion. (and I apologize in advance for not having the "numbers" in hand, my flight manual from nearly 10 years ago is at my NASA office so I'm going to have to talk in general terms)

One of the reasons the 757 was attractive to the airlines was its fuel efficiency (what they call specific fuel consumption ... basically miles per gallon). One of the primary reasons for this efficiency was the comparatively high aspect ratio wing.

As we've discussed numerous times over the years, long wings make less drag for a given amount of lift than the same area surface that is short spanned. They produce that lift at a lower angle of attack, produce a lot less drag and take a lot less energy to move forward (assuming the form drag of the long wing and structural necessities don't militate against the long wing).

An unintended consequence (or perhaps just not a high priority consideration) was that such a long winged airplane can also fly a lot slower. On approach to land this meant a significantly lower approach speed and consequently a lot less runway for safe, legal operations. This was also attractive to the airlines because it meant that a pretty large airplane could fly into runways that were unuseable by other airplanes of similar gross weight (and payload, the real silver lining for the airlines).

What was later determined was that the 757 through some combination of the lower speed, higher angle of attack and wing configuration produced greater wingtip vortices than expected and, consequently, Air Traffic Control rules were changed to allow safe operation behind it.

Ted,

Could be that what you're outlining here contributed to the sensitivity to wake turbulence that I experienced with high aspect stunt designs. The longer wing has less drag and had some real advantages in my mind, but generates more tip vortex and so, more turbulence. Coupled with the greater possibility for disturbance you noted earlier, made for some interesting flights in what were turbulent conditions anyway. (love the wind coming off of those hangers at Albany).

>>>and I apologize in advance for not having the "numbers" in hand, my flight manual from nearly 10 years ago is at my NASA office so I'm going to have to talk in general terms<<<

I see it is going to be more design oriented thread, so I will add some relevant info.

The aspect ratio just says how much air leaks at tips sidewise and thus wastes some energy to vortices.

The induced drag is something on top of airfoil drag and its value depends on wing lift and aspect ratio. Following chart shows it:

[photo not recovered: 4128fbeb4ce5ddf9.jpg]

It just means that if we have aspect ratio (lambda on picture) AR = 6 and the wing airfoil makes lift (coefficient cy) = 1 then we have to add value cx=0.05 to airfoil drag. Original airfoil drag could be say 0.01 at that lift. So it is really very important part of wing drag.

But that all is true only for elliptic wing. We use trapezoid plan form. And it means following:

[photo not recovered: 4128fc2d4cfb1851.jpg]

It shows that if tip chord is 0.4 of root chord, then the induced drag is very close to that elliptic, only 1% higher. But the situation for constant chord wing is 5% worse (for AR=6) and for zero chord tips 12% worse.

But again, there are “buts”. First is, that flaps at tips make it worse (less flaps is better) and also sweep back makes it worse (up to 10 deg OK, but 20deg make 10% and 30 deg 40% worse).

And now what it means in reality. The aspect ratio lowers the drag in maneuvers. The induced drag at low AR say 4 could be 1.5 higher then at AR=6. That could make difference in engine size .40 to .60 with everything else equivalent.

The tip chord is optimal at 0.4 of root chord (from this point of view). But also 0.6 is still not bad. However large flaps at tips are not very clever idea. I do not vote for “French” approach, but anyway, I thing the flap chord at root and at tip should be maximally similar to wing root chord and tip chord, or better to be smaller at tip.

No.

I have one plane that seems to experience the turbulence more than others. It's a scratch built stunter( built up body) and I decided to design a rather large rudder, sort of over-doing-it in the sleek rudder dept. The rudder is 4-1/2" tall and 10" long. It reacts most violently when running through its own turbulence. Much worse than any other plane I've flown.( and that's quite a few).
The wing may cause the turbulence but does the rudder size create the most reaction to the turbulence?
It's still in one piece!

Aha.....

Now I know why the Genesis is like it is..

Interesting post !
I have nothing intelligent to add tho I did wonder how much all this affects the fast combat guys, who are regularly closely following their opponents plane ?

The discussion is centered around disturbances in the air caused by vortices produced by wings generating lift and propellers and the effect they have on lift when flying back into these disturbances.

Combat ships with their relatively large thrust to weight ratios and low drag, will power through most of this junk with a little bump at the handle. I'm sure Phil could add to this comment.

Combat planes get hit by turbulence all the time. The classis one is chasing through a hard outside corner. A common combat tactic is a defensive eight- a low, hard outside corner off to your left, into the wind, followed by a shallow climb to your right, and then a hard low inside turn, back and forth. Very hard to get cuts against.

Due to the way the pilots generally stand, the chaser is on the right. If he tries to follow the lead plane exactly the hard turns put him right in the middle of the lead plane's vortex, so his plane does a ninety degree roll into the circle. Tends to curl the toes a bit and makes him back off a bit from chasing. Takes a very stiff wind to clear the air when you are only 20 feet behind.

This is an interesting question. I haven't had much trouble with wakes when following people. Maybe it's been so long since I was able to do that that I don't remember. Combat planes certainly have a problem with wake turbulence from consecutive maneuvers in calm air. It's sufficiently bad that you can't trim an airplane unless it's pretty windy. Fortunately, except for trimming, we don't have to put our tricks in the same place twice. Flying combat, one intersects the other guy's wake, but probably doesn't stay in it very long. Now and then, an airplane does get upset and come inside the circle. That's another plane-saving advantage of fuel shutoffs.

>Now and then, an airplane does get upset
>and come inside the circle. That's another plane-saving
>advantage of fuel shutoffs.

Not to mention pilots' underwear saving.