Denny-
Thanks for posting those; they really do illustrate the effect! I remember that John Kilsdonk RR article (re-read it a couple years back). Based on what he said to my friend Dean and me at a summer contest in Kokomo during our college years (after writing the article and completing his project), he pretty much privately agreed with my assessment above. Of course, his wing is a really good shape, but at 140 mph, I'd guess that the effect is much less than shown. That may be the root of his private assessment.
As usual though, even though I don't think the effect is very significant at our scales, I do use it. Even at my "skill" level, any advantage...
Go figure!
SK
I think its easier to be 1% better in 10 areas than it is to be 10% better in any 1 area..
Awhile back, I remember reading that there was some sort of aerodynamic advantage to having swept back wingtips, but can't remember it.
What is that advantage, besides looking good H^^
Cool.AMEN...HOWARD...YES! WAAAYCOOL!
Awhile back, I remember reading that there was some sort of aerodynamic advantage to having swept back wingtips, but can't remember it.
What is that advantage, besides looking good H^^
Stunt models don't fly fast enough or are large enough to take advantage of tip shapes. Just make things pretty.
Stunt models don't fly fast enough or are large enough to take advantage of tip shapes. Just make things pretty.
Doesn't work that way on my models, they are vastly differant with the tips reshaped., The tips affect the lift and AR on my airplanes, also affect corner and how the plane goes thru the manouvers, they are way better with the right tip shape and nowwhere near as good with the tip put on backwards.
Regards
Randy
Randy, are we talking Hoerner Tips here or a simple vertically viewed profile change?
Because I don't see how simply changing the shape of the tip could increase the effective AR, if you do increase the AR by increasing the actual span measurement AND change the tip shape also then some improvement must be attributed to the former and this clouds the experiment somewhat.
But look, I shouldn't doubt your real world experiences on this but you must admit that if they were that good then a heck of a lot more designs would feature them.
Cheers mate.
Because I don't see how simply changing the shape of the tip could increase the effective AR, if you do increase the AR by increasing the actual span measurement AND change the tip shape also then some improvement must be attributed to the former and this clouds the experiment somewhat.
Stunt planes probably have an optimal effective aspect ratio. More is not always better, as Randy Powell can tell you. If you were building an airplane from scratch, would you be better off keeping the same aspect ratio using Flite Streak tips or using a higher aspect ratio with Chipmunk tips and the wing moved aft a little to duplicate the center of pressure of the wing with Flite Streak tips? We go through a similar process at work with span vs. winglets.
I think Flite Streak tips would be good for combat planes. I'm going to put some on my Sons of Snort if I get them finished before the AMA combat event disappears. Carved swept tips like those on Steve Hills's Arrowplanes keep your wing tips from getting beat up when you hit the ground, too.
"Wind up" is a virtue in combat planes.
I think we are missing a big "BUT" here: All research on effects of wing shapes is based on FULL SIZE AIRCRAFT. Let me offer several counter points to consider:
Our models just don't have that much mass or size. Even small GA airplanes barely generate enough vortex to matter much. For example, until mid-70's, all Mooneys had square wing tips. When they added pretty wingtips to 20 G models, the performance improvement was barely noticeable. We are talking improvement of 2-3 knots top speed on an aircraft that moved at 160knots!
reply - I added the new Horneor tips to my '56 Tri-pacer and they took a 116 true airspeed ship to 121. That's a significant difference when you consider how dirty a Tri-Pacer is.
Boeing 747 can take off immediately behind a fully loaded Cessna 172 but switch them around in the order and we're talking 5 minutes before 172 can take of. Why? Because 172 wing does not generate much of tip vortex. What it means is that the smaller the airplane, the less tip vortex is generated, the less it matters.
reply - Not to be disrespectful, but this is a ridiculous comparison, Steve. It's like saying my Honda CRX will be buffeted more by a semi than the semi will be bufeted by the CRX! Of course it will! Has less to do with vortex than with mass in this case.
According to FAA AIM Section 7-3-3, Vortex strength is mainly proportional to aircraft weight( http://www.faa.gov/air_traffic/publications/atpubs/aim/Chap7/aim0703.html )
If you read the 7-3 section, it says that the most turbulence is generated when aircraft is heavy, clean and slow. But all of this is besides the point. The vortexes are generated near the back of the wing tip while the lift is generated by the front of the wing(that's why the shape of the wing behind CL does not matter).
reply - So does that mean I won't hit my bump at all? I fail to see the relevance of this comparison as far as vortex generation. It happens, we have to deal with it, and the fact that we DO hit our bumps if we don't back-pedal in calm air says it all as far as I'm concerned. Vortex generation is a fact of life, even with our little tiny stunters. Any decrease of this phenomenum is helpful.
We've all flown in calm, warm weather. The first loop always looks so nice, giving hope of high points for the maneuver. All over sudden, BAM!!! The model hits an invisible gravel road and the hope is mercilessly squashed by the model's own wake turbulence. The fact that the first loop is always clean means that there is no vortexes for the wing to contend with.
reply - Very true, because, as you pointed out, they are generated at the back of the wing and the ship has already passed by the point. Second loop, the vortex is still there so the front of the aircraft now sees it.
Please don't take any of these points as contentious, Steve, they are not meant to be, but we simply differ in our opinions. You are welcome to yours and I to mine. We can still fly together in calm or not-so-calm air and enjoy the comraderie!
Blessings,
Will
Awhile back, I remember reading that there was some sort of aerodynamic advantage to having swept back wingtips, but can't remember it.
"This proves my point that tip shapes don't matter in two ways. "
No Steve It does not prove anything, Fly the exact plane with 2 differant shape tips, You "can" see a huge differance, not a subtle one. I have actually done this. It also decreased the stall point and added a lot of extra weight carrying ability, and the "feel"
of the plane was like it had more AR. So I think the only thing that "proves" since a lot of what he wrote as "fact" is not , is that he is not an expert on CL stunter design.. ;D
Simple exercise , take a 65 ounce SV-11, put the tips on backwards then fly it both ways, do the same to a 65 ounce TP or Vector, you will feel and see a big differance
Regards
Randy
I think I should just rest my case with that example of the inaccurate staements the gentlman made in his book.
Steve, if you'll notice, the last quote I did refering to the fellow's book states, "This is the case with full size aircraft." End quote.
The guy claims the wing shape for full size aircraft makes no difference! Yet you keep going back to models. Makes no difference, the man is just plain wrong. What is he drawing his information from?! What are his credentials? As far as the book being translated, I've checked into having my own published novels translated into other languages, all it takes is money! That still doesn't make the guy an expert. I'm sorry, but as far as I'm concerned, what Mark just posted is accurate, as are all the other comments above from Randy.
Will
.So with a straight face, you can tell me that a Nobler airfoil is not at a disadvantage to an SV-22 or Impact airfoil? come on,, PLEASE, making broad sweeping generic statements like that is patently a bad idea especially ones that are so easily disputed.
I think the reason that an elliptical wing generally works so well is that it has an even longer trailing edge for a given span. And, as the air tends to flow towards the tip, it still exits the wing more or less at a right angle to the trailing edge and pushes the tip vortex even further out than a swept tip.
I can't imagine building a wing with split tips. The fancy modified ellipses may be a few more percent efficient, but besides being hard to build, using a swept back wing on a stunter introduces trimming problems. The slightly swept forward quarter chord, angled stab, and other tricks on F2D combat ships really seem to give the most forgiving, best handling of any wing shape for CL maneuvering.
While on the subject of wingtips, what difference does it make if the wing was eliptical, rather than having the conventional leading edge with a raked wing tip?If I am doing the appearance judging, you might score higher. You might talk your math teacher into giving you extra credit for calculating the rib shapes for the elliptical wing, particularly if you use zigzag ribs and flaps. Other than that, not much.
The T-rex tip is kinda simi-eliptical is that what you are advocating as the best shaped wing tip???Here's what I think: Taper is good for lateral gust response. Forward sweep is good for minimizing rolling moment due to sideslip. A straight flap hinge line is mechanically good and maybe aerodynamically good. Discontinuities at the end of control surfaces are bad. Any efficiency you get from wingtip shape can be compensated for by a tiny change in aspect ratio. You don't want to maximize efficiency (minimize induced drag) on a stunt plane.
I don't really know, but my T-Bird did fly exceptionally well back in '69
You don't want to maximize efficiency (minimize induced drag) on a stunt plane.
I'm inclined to think that this may be true of more massive, highly powered ships with good thrust to weight ratios and line tension, but I'm not so sure I'd be happy with a lighter, lower powered plane losing speed in it's maneuvers and then having to maintain line tension and control efficiency in climb at reduced speed. I've always thought that parasite drag was valuable in preventing windup and excessive speed buildup in descents, but induced drag doesn't seem so desirable to me.
Could you say a bit more about what these consist of: "Discontinuities at the end of control surfaces are bad." ?
I would avoid stuff like a piece of fixed flap adjacent to a piece of movable flap. Not only are you opening and closing a gap while you are trying to fly level accurately, but you are changing the sign of swirly air at the TE. These discontinuities are common on stunt planes, but I've only seen them cause a problem for sure once.
I guess you could increase the aspect ratio to what you think it ought to be. I don't see little airplanes (or classic airplanes, which were powered by wimpy engines) with appreciably more aspect ratio than modern big ones, but the little or old ones often don't have as much speed regulation from engines, so a high aspect ratio would tend to make them more prone to windup. I would reckon that the optimal aspect ratio of an electric stunter is greater than that of an Otto engine powered stunter, both because the electric can regulate speed better and because its fuel weighs more.
I just thought of another aspect of aspect ratio: to wit, higher aspect ratios are more sensitive to turbulence because they have higher lift curve slopes (hence my enthusiasm for super-low aspect ratio canard surfaces, but that's another story). Maybe your fancy wing tips can give you the requisite efficiency with less turbulence sensitivity. Was David Fitz onto something?
Tweaking just seems like such a crude way to adjust roll, though. Personally, I have a terrible problem getting a tweak just right. There needs to be a better way to adjust roll on a stunt plane. If it can't be a roll tab for the above mentioned issues, then maybe some sort of spoiler further up on the chord?Dorin has some really cool adjustable flap control horns that allow precise flap tweaks. See his piece on the Stunthangar electric page.
So my last post was directed only at the statement about induced drag. All else being equal, I think lowering induced drag can be beneficial, while lowering parasite drag is generally not helpful at all in stunt models.I reckon the relative importance varies from model to model. I was thinking of my 67-oz. VF-powered airplane on semifinals day at the 2003 Nats and a couple other such experiences. I found myself wishing for a little more speed on the straightaways and a whole lot less in round loops. I wouldn't have minded a little more induced drag in the square corners, too. Those square eights sure went fast.
Talk about taking things out of context. Get the book, read it, then comment. The excerpt was from the section on SHAPES OF WING TIPS, just like the topic of this thread. ALL comments are about wingtip shapes!!!