Author Topic: swept back wing tips (Read 20608 times)

Matt Colan · « **on:** April 01, 2010, 08:03:42 PM »

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

Dennis Adamisin · « **Reply #1 on:** April 01, 2010, 09:40:28 PM »

Here's the proverbial 1000 words on the subject....

Serge_Krauss · « **Reply #2 on:** April 01, 2010, 09:50:53 PM »

Matt-

"Looking good" is of course a matter of personal preference. I like your "Ares" or the "Argus" or "Cavalier" tips!

The advantage is that raked tips tend to reduce tip vortices, which reduces induced drag. Extending further spanwise as they go back, they get in the way of the air that wants to curl around the tip. There are two things you should consider when choosing configurations for this reason:

1) The small swept tips, as on the Flight Streak" have a smaller effect than the larger tips (like 45^o) that show a significant drag reduction on full-sized wings (larger, on heavier man-carrying planes; shown in the second figure comparing lift coefficient squared divided by drag coefficients at higher Reynolds Numbers than our models fly at). I've posted these wing shapes here and elsewhere, with explanations. For now, I'll just re-post the figures showing relative advantages, since I don't have time now to re-read the stuff myself. You might want to do a search on this to find the material discussed earlier, when some of us did the research the first time. The figures below show that the raked tip does have an advantage over the squared tip. However the efficiencies shown for the so-called "crescent wings" (Xt = 1.00 and 1.5 in the first figure) are greater, the 1.09 figure meaning that it is 109% as efficient as an elliptical wing, configured like a Spitfire's (Xt = .25).

******2) Except for lessening the chance of wake turbulence accidents in still air, I really doubt that the raked tip has as significant an advantage on stunt-model performance as on larger aircraft, since the pressures above and below the wing are not nearly as different as on a heavy large airplane. Just compare the wing loadings of a model stunter and a full-sized aircraft. Also, a stunt model's tip chords are so small that much less air has can spill around the tip. Essentially the little tip tornado is more than proportionally less intense. The power available now sort of dwarfs losses in climb due to tip losses.

SO...I think that the "cool" factor - whatever rattles your personal cage - is what should determine your choice. 'guess I should add that split tips are more efficient than any of these. Despite what I just typed, I do have a pet project where I think they may make a sizeable difference. You can find some nice NACA ans NASA reports on this stuff - down loadable for free on the NASA Report Server site. The first figure comes from NASA 1996001 5887, p. 46 (really p.37).

SK

Serge_Krauss · « **Reply #3 on:** April 01, 2010, 10:00:50 PM »

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

Howard Rush · « **Reply #4 on:** April 01, 2010, 11:22:39 PM »

Gary James explained that tip shape effect to me on his last visit here. I have heard professional airplane designers refer to aft-swept tips as "Flite Streak wingtips". Picture is of the latest product of my homeboys.

This tip shape is good for gliders and airliners-- it reduces drag for a given span loading-- but has some effects for stunt that you may not want. To wit, it probably increases (negative, I think) rolling moment due to sideslip, which makes the airplane more sensitive to where you place your tricks relative to the wind. That said, the effect is probably small, and guys with some 20 Walker Trophy wins among them are currently using Flite Streak wingtips.

Dennis Adamisin · « **Reply #5 on:** April 02, 2010, 06:09:50 AM »

Quote from: Serge_Krauss on April 01, 2010, 10:00:50 PM

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'm sure the effect is small but I think it is a positive - I'll take it! I think its easier to be 1% better in 10 areas than it is to be 10% better in any 1 area..

Serge_Krauss · « **Reply #6 on:** April 02, 2010, 07:59:25 AM »

Quote from: Dennis Adamisin on April 02, 2010, 06:09:50 AM

I think its easier to be 1% better in 10 areas than it is to be 10% better in any 1 area..

Absolutely! Well said.

SK

RandySmith · « **Reply #7 on:** April 02, 2010, 08:04:33 PM »

Quote from: Matt Colan on April 01, 2010, 08:03:42 PM

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

Hi Matt

There some advantages that the tip shape has that I like most. What I have found is the wingtip shape that is shaped like a Flite Streak-SV11-TP-etc has the effect of making the wing act like it is..on my ships..., 5 to 6 inches longer span, the wing carries more weight at the same sq inches, and acts like a higher aspect ratio wing, It also ..to me... goes thru the corners cleaner using less power.
I have tested this many times and I have even flown the exact same plane with the two differant tips..and the same tips reversed.
The Chipmunk style tips had the effect of a lower A/R wing...did NOT carry as much weight..required more power, and more flap.
The opposite was true of the swept back tips..
It seems like the span is the lenght of the TE of the wing and not the LE or Middle

try them both, see how it feels to you.

Randy

Scott B. Riese · « **Reply #8 on:** April 02, 2010, 09:33:42 PM »

Are these wing tip called "LOW VORTEX"??

Scott B. Riese · « **Reply #9 on:** April 07, 2010, 10:04:07 AM »

Bump

L0U CRANE · « **Reply #10 on:** April 07, 2010, 02:25:05 PM »

Denny and Serge, re: Replies #1 and #2...

MOST of the time, our stunters' wings carry little load, so the comment about slight pressure differences is good - for those conditions.

However, we also do very rapid changes from low lift required, to -relatively- high lift required. Tip vortices vary with the pressure difference, and high lift means 'high' pressure differences. As Randy mentions, sliding the vortex further out towards the extreme tip may act as if there were more working area available, just when we need it most.

I seem to recall that Dornier modified the wing on a WW2 Do-26 or Do-27 (high wing flying boats or amphibs) to a stepped-taper planform. Straight inner section, moderately aft-raked next section, and sharply aft-raked tip zone section, to approximate the elliptically curved leading edge in Denny's photo 3. I think they referred to this planform as a Schleunemann type... I read of this over 20 years ago, so memory may not be accurate....

The article was on initial testing, which ASIR indicated "promise."

Some military light tactical transport designs have used something like it. They wouldn't if there were no advantage - it's a bit more work than a straight or straight-tapered planform.

Four our purposes, if it does limit the width of the wingspan so obviously affected by vortex conditions rolling over onto the lifting side, as in Denny's pix #1 and #2, it may bring two good things: the apparent "area increase" Randy mentioned, and, since the width of the vortex is tighter, residual wake problems may be less. ...Or may be worse, as the same energy is packed into a tighter vortex trail...

Howard Rush · « **Reply #11 on:** April 07, 2010, 04:26:28 PM »

For an electric stunter, with high fuel weight and good speed regulation, reducing induced drag might help. For a stunter with an Otto cycle engine in the wind, I'd rather not have any less than I have, nor would I want any more rolling moment due to sideslip.

Dean Pappas · « **Reply #12 on:** April 07, 2010, 04:56:37 PM »

Ah yes ...
Angle of attack dependant yaw into roll couple: the enemy of finding the right tip weight. Anyone notice how strong runway wind wants less tip-weight?

Regards,
Dean P.

Barry · « **Reply #13 on:** April 07, 2010, 05:11:43 PM »

FAI pylon racer, probably best in world, note wing design. These things go 225mph plus on 40 engine using 80/20 fuel. High AR wing is to reduce induced drag in turns (its turning 40% of the time). Swept tips are to reduce overall drag by reducing tip vortex.

Barry

Howard Rush · « **Reply #14 on:** April 08, 2010, 12:21:10 AM »

Cool.

Shultzie · « **Reply #15 on:** April 20, 2010, 11:54:07 AM »

Quote from: Howard Rush on April 08, 2010, 12:21:10 AM

Cool.

AMEN...HOWARD...YES! WAAAYCOOL!

FLOYD CARTER · « **Reply #16 on:** May 14, 2010, 12:57:33 PM »

Some of the most successful stunters have wingtips that just "stop"
at the tip rib! Like, no wing tip shape at all!

What am I missing?

Floyd in OR

steven yampolsky · « **Reply #17 on:** May 14, 2010, 02:51:20 PM »

Quote from: Matt Colan on April 01, 2010, 08:03:42 PM

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

Stunt models don't fly fast enough or are large enough to take advantage of tip shapes. Just make things pretty.

minnesotamodeler · « **Reply #18 on:** May 15, 2010, 01:25:39 AM »

According to the photos in 1st reply, square wingtips are the worst of all???

FLOYD CARTER · « **Reply #19 on:** May 17, 2010, 10:03:21 AM »

Quote from: Steven Yampolsky on May 14, 2010, 02:51:20 PM

Stunt models don't fly fast enough or are large enough to take advantage of tip shapes. Just make things pretty.

Now, that I believe. Seems like much of the heated aerodynamics discussion we hear really doesn't apply to our planes.
If you were to design a passenger airliner, some of these issues would become important. But not here.

Floyd in OR

RandySmith · « **Reply #20 on:** May 17, 2010, 11:47:44 AM »

Quote from: Steven Yampolsky on May 14, 2010, 02:51:20 PM

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

Dennis Adamisin · « **Reply #21 on:** May 17, 2010, 07:55:35 PM »

Quote from: RandySmith on May 17, 2010, 11:47:44 AM

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

What Randy said.

The AR part of that discussion is can be seen in the smoke flow exiting the tips back in post #2 of this thread.

Chris Wilson · « **Reply #22 on:** May 17, 2010, 10:17:09 PM »

There is obviously a lot more going on in regards to tips than pure aerodynamics here and considering that stunt models must house adjustable leadouts and wieghts in them, sometimes have full span flaps that share that wing section and must be built to a fine taper if you want to take advantage of that wonderful swept back look.

It's no wonder that the simple, functional and easy to build square hollow block has found favour over more purist ideals and on a thick wingtip section I doubt that any advantage would be apparent.

Now is that smoke diagram showing a full size wing or a models? And at what speed, angle of attack? What effect does the lines have around this area?
We have to ask 'model' type questions here to really get a suitable answer surely.

Cheers.

RandySmith · « **Reply #23 on:** May 17, 2010, 10:39:19 PM »

"It's no wonder that the simple, functional and easy to build square hollow block has found favour over more purist ideals and on a thick wingtip section I doubt that any advantage would be apparent"

You may want to fly the same airplane with differant tips, I have, I saw huge gains in performance and the opposite with other shapes, I don't really know why you would doubt that.
When you change the effective AR of a wing and also make the same wing's weight carrying ability substantially differant it is simple to see why you may have such a differant flying ship. It was as simple for me as putting the SV tips on backwards. I have also flown the same plane with square tips, and tips like you would see on a Piper. I also may mention that I did similar to my full scale airplane with the results making the plane more efficient, lowering stall speed and increasing top speed.

Randy

Chris Wilson · « **Reply #24 on:** May 17, 2010, 11:42:57 PM »

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.

Matt Colan · « **Reply #25 on:** May 18, 2010, 07:20:17 AM »

Quote from: Chris Wilson on May 17, 2010, 11:42:57 PM

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.

many many many designs now use those types of wingtips. Obviously all of Randy's designs, and there are others, a couple of Windy's recent ships (excluding the Spitfire) has had the raked tips...

RandySmith · « **Reply #26 on:** May 18, 2010, 08:59:47 AM »

"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."

Hi Chris

I have seen more and more over the years, and I see planes everywhere with this type of tips more and more. They may or may not have as much of a differance on all planes but I will keep using them. I took one of the Noblers years back and installed SV-11 style tips after cutting off the round tips, it made the plane fly easier thru corners and it had better weight carrying abilty, I don't know exactly why but it glided and landed better too.
One of the SV ships I got to test had rounded type Chipmonk style tips on it, I replaced those with stock Intrepid tips and it makes a very big differance, The plane flew..or acted.. just like the AR had gone up, cleaner corners and the ship would sink in the hard corners, after the tip replacement the sinking went completely away. many have done the same on the sinking by adding about 1/4 inch of flap chord, But in my opinion I believe I like the tip fix better

Regards
Randy

Serge_Krauss · « **Reply #27 on:** May 18, 2010, 09:10:33 AM »

Quote from: Chris Wilson on May 17, 2010, 11:42:57 PM

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.

Since aspect ratio equals span²/area, the aspect ratio will increase regardless of whether the tips are raked by removing area or adding it at the tips. The effect is generally greater, when adding the tips, since the span increases by a greater percent than the area and gets squared. I wasn't sure that the relative effects of increased aspect ratio and tip shape could be distinguished on our sized wings, but if Randy says they can, then I can't argue with his experience. It certainly far exceeds my own. I would have felt that aspect-ratio effects would dominate.

Anyway, as previously posted, research shows that even when wings of the same area and span (therefore same aspect ratio) are compared, the raked-back tips on otherwise rectangular wings give higher efficiency than rectangular wings. Applying a similar idea to elliptical wings, those with chords alligned near their trailing edges, giving a swept back leading edge with greatest sweep curvature at the tip, can give several percent greater efficiency at the tested Reynolds numbers. These ideas are illustrated in my post above.

SK

Hi Serge

I think your statement about the sweptback tips being more effective maybe at the root of what I am seeing in the stuntship wing shape test.
I don't think there is any doubt that they are cleaner going thru manouvers

Regards
Randy

RandySmith · « **Reply #28 on:** May 18, 2010, 12:23:03 PM »

"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."

Chris let me try to explain it this way"
I took the exact span wing and had tips put on front ways similar to the way a chipmonk wingtip sweeps back and get smaller span at the TE, and then changed the tips so the LE was short and the TE was greater span, this was exact same span on both setups, The one with the tips flared back to increase the TE span was far more effective than the shorter TE and longer LE. It carries more weight and "acts" like it has a Higher AR and goes thru the manouvers cleaner with what seem like less power used.

Randy

Howard Rush · « **Reply #29 on:** May 18, 2010, 03:30:37 PM »

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.

RandySmith · « **Reply #30 on:** May 18, 2010, 03:37:57 PM »

Quote from: Howard Rush on May 18, 2010, 03:30:37 PM

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.

Hi Howard

Which is why I have found ,for me 5.5 to one is about the best AR for Stuntships for all conditions.

Regards
Randy

Chris Wilson · « **Reply #31 on:** May 18, 2010, 05:10:10 PM »

Ok, you got me!

I have tried the curved back tips like you have described on combat wings and probably never really looked for the improvement but I will have to try the same on a stunt model.

Thanks for your time here.

Howard Rush · « **Reply #32 on:** May 18, 2010, 06:57:19 PM »

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.

Kim Mortimore · « **Reply #33 on:** May 19, 2010, 11:46:25 AM »

Quote from: Howard Rush on May 18, 2010, 06:57:19 PM

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.

I have a couple of unflown older Yuvenko (sp?) fast combat ARFs (with square wood boom) that have swept back tips.

Howard Rush · « **Reply #34 on:** May 19, 2010, 05:18:50 PM »

"Wind up" is a virtue in combat planes.

RandySmith · « **Reply #35 on:** May 19, 2010, 07:58:55 PM »

Quote from: Howard Rush on May 19, 2010, 05:18:50 PM

"Wind up" is a virtue in combat planes.

Build a Winder Howard !!!

Randy

Will Hinton · « **Reply #36 on:** May 21, 2010, 02:02:20 PM »

I feel the need to address the comment that "as slow as our stunt ships fly there's not enough difference to mean anything", or something like that. (Not a direct quote, but darn close.)
Keep in mind that the slower and heavier an airplane flies, the more tip votex generated. While we try to build as light as possible, we still have some weight, and we fly SLOOOOOOOOOOW.
Tip vortex makes a difference in any airplane, regardless of size/speed.
Will

Howard Rush · « **Reply #37 on:** May 24, 2010, 05:32:38 PM »

I feel the need to address that comment. Yes, tip vortex is a function of speed, and it applies to any airplane making lift. For an airplane flying level, as Will said, the slower it flies, the more tip vortex generated. (Ask me to tell you the story of my boss and the Thunderbirds). I was curious to see what happens for an airplane flying through a turn, so I did the cypering while waiting for the weather to abate yesterday at Arlington. I figure that vortex energy per unit length along the airplane's path is equal to induced drag. Energy is force x distance, and drag is a force. Does that sound right? I calculated induced drag in straight, level flight, and in a manuever (a loop corner, for example) disregarding gravity. I used formulas I had in my head for "centrifugal" force and induced drag coefficient and definitions of lift and drag coefficients. I can type them here if anybody is interested, but it's kinda a waste of effort, because probably nobody will read this. The upchuck was that induced drag (vortex energy per unit airplane path length) is inversely proportional to airspeed squared, as Will said, for straight, level flight. In a maneuver, though, induced drag is directly proportional to airspeed squared.

The formula I calculated for induced drag in a maneuver is kinda interesting. It's (2 m^2 V^2) / (pi A e S R^2 rho), where m is airplane mass, V is airspeed, A is aspect ratio, e is the efficiency factor in Serge's plot above (higher for Flite Streak wingtips), S is wing area, R is loop radius, and rho is air density. So fly slow and go easy on the corners on a calm day, particularly if it's hot or high.

You guys check those calculations.

steven yampolsky · « **Reply #38 on:** May 27, 2010, 02:21:09 PM »

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!

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.

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).

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.

Geoff Goodworth · « **Reply #39 on:** May 27, 2010, 11:43:55 PM »

'The fact that the first loop is always clean means that there is no vortexes for the wing to contend with.'

Steve, on the first loop, you are correct. However, if you stand still in completely still air, you can expect the last loop of three to be a figure 9. Top competitors take a step back with each successive loop to avoid the turbulence.

John Kilsdonk's original article mentions multiple tests at 35, 50 and 65 mph and one of the photos shows two of John's rat racers that were used for the tests so the wings were fairly small. John also says that the wind speed used for the photos was 20 mph because the smoke stream broke up too much to be photographed satisfactorily at higher speed.

Matt, John talks at some length about Hoerner and his work but does not say that the tips are Hoerner tips. He did say that the airfoil on the wings was Clark Y.

Will Hinton · « **Reply #40 on:** May 28, 2010, 07:49:14 PM »

Quote from: Steven Yampolsky on May 27, 2010, 02:21:09 PM

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

steven yampolsky · « **Reply #41 on:** May 29, 2010, 09:20:24 PM »

Will,

I think I better use the words from someone more knowledgeable on this topic: Martin Simons. Martin Simons wrote a book called "Model aircraft aerodynamics". Here's an excerpt:

"In terms of practical experience in flight, it is very difficult if not impossible to show that any particular type of wing has a large advantage. This is the case with full-sized aircraft - it is even more so with models. Mere fashion sometimes seems to be quite influential. There is a tendency for manufacturers and designers to introduce new shapes as soon as there is the slightest hing of experimental evidence to support them - the changes draw attention to their products. The promoters stress the latest research findings, hoping thereby to make more sales. Further changes, again with some slender scientific support, may follow after a few years.

Compared with aspect ration and the general planform and twist of the wing, wing tips are of small importance although, if a wing has a very bad tip, the resulting disturbance of airflow may promote tip stalling
.....
In general, however, the difference in performance between a model with a good tip shape and a poor one will be barely detectable in flight. There may be something to be gained by trial and experiment, but probably no very much. Practical aircraft wings must end in tips of some kind, and wherever there is a difference of air pressure above and below a wing a vortex will form at or near the tip. There will be a drag penalty - the greater the relative difference of pressure the more severe the penalty will be".

This proves my point that tip shapes don't matter in two ways.

Martin also goes on to say that the point where trailing edge(NOT the leading edge) begins to curve forward to form a wingtip is where effective wingspan ends and the wingtip begins. A lot of people make a mistake of assuming that the wingtip begins where they build it and curve flaps to give "the elliptical shape". Bad idea.

steven yampolsky · « **Reply #42 on:** May 29, 2010, 09:22:35 PM »

Quote from: Matt Colan on April 01, 2010, 08:03:42 PM

Awhile back, I remember reading that there was some sort of aerodynamic advantage to having swept back wingtips, but can't remember it.

Matt, I just re-read your statement and realized that the simplest answer is: "those are not wingtips! That area is still developing lift!".

RandySmith · « **Reply #43 on:** May 30, 2010, 11:24:17 AM »

Quote from: RandySmith on May 30, 2010, 11:23:04 AM

"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..

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

Will Hinton · « **Reply #44 on:** May 30, 2010, 03:39:06 PM »

Hi again, Steve,
Let's remember that writing a book does not make one an expert - experimentation and experience does much more. I used to take my flight students to an overpass over the Ohio turnpike that was directly on the end of runway25 at Toledo Express when it was too foggy for them to have a lesson, but the ILS was still open to the airlines.
We would set there and watch the big boys land. The sight is absolutely phenominal! The smaller business jets would show the vortex in the fog and that was impressive, then a 737 would land. Wow, what a difference! The thing I wanted to impress on my students was the way the vortex would lay in place for some time in a no-wind condition! "Don't you be taking my 150 in behind those boys until it's clear, ya got it?"
We can bander this issue all over the place and likely none of us will change our minds until we all do as Randy says and experiment ourselves.
I, for one, as a result of the several full scale experiments, believe tips and wing shapes make a difference on ALL airplanes.

Quote-
"In terms of practical experience in flight, it is very difficult if not impossible to show that any particular type of wing has a large advantage. This is the case with full-sized aircraft - it is even more so with models"

This is a really, really bad statement. If any type of wing has no advantage over another, why did the Piper Cherokee Six gain nearly 30 knots when Piper went to the tapered wing over the old barn door? I used to flight plan our retractible gear '76 Lance with the barn-door wing at 150 knots, (180mph) then when they put tapered wings on the 'Six, the fixed gear Six was as fast as our high-maintenence retract Lance!! Tends to tick a guy off, I'll tell you that.

I think I should just rest my case with that example of the inaccurate staements the gentlman made in his book.
Let's fly,
Will

Matt Colan · « **Reply #45 on:** May 30, 2010, 06:36:58 PM »

This might not mean anything but for the past few days as I watched birds fly by, I looked at their wingtips and they all were swept back. It kinda made me think...maybe mother nature knows what she's doing.

steven yampolsky · « **Reply #46 on:** May 31, 2010, 09:14:43 AM »

Quote from: Will Hinton on May 30, 2010, 03:39:06 PM

I think I should just rest my case with that example of the inaccurate staements the gentlman made in his book.

Hmm, his book has been translated It's been published four times, translated to German and Chinese, a must read by the UAV community but you are correct: he doesn't know what he's talking about.

Hershey bar example is absolutely nonsense for three reasons:
1) you are still talking full size airplanes and at 2-3 times the speeds our models fly at.
2) The new wing had 2+ feet of extra wingspan resulting in higher aspect ratio and lower drag.
3) I flew PA-28-160 with and without hershey bar. No speed difference whatsoever! Explain that one.

Mark Scarborough · « **Reply #47 on:** May 31, 2010, 03:56:28 PM »

Steven,
Quote-
"In terms of practical experience in flight, it is very difficult if not impossible to show that any particular type of wing has a large advantage. This is the case with full-sized aircraft - it is even more so with models"

.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.

There is CLEARLY a difference between airfoils, tip shapes and Aspect ratios. To ignore it because they are only models, and the effect is minute, well my friend in comparison to a 737, yeah your right, but compared to another model, well I must say that it is real and relevant.

Will Hinton · « **Reply #48 on:** May 31, 2010, 04:37:52 PM »

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

steven yampolsky · « **Reply #49 on:** May 31, 2010, 06:04:13 PM »

Quote from: Will Hinton on May 31, 2010, 04:37:52 PM

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

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!!!

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