Wing tip shape

[Steve Bakac]

G/day all
           Just wondering your opinion on wing tip shapes.Does it make much difference in the trimming n flying of a model?Eg square tips compared to tapered or sweep back or sweep forward tips?What are the differences if any ? Thanks
           regards  Bucky

[Russell Shaffer]

I'll be very glad to see the responses to this, I've been building some Flite Streaks with their complicated tips.  I do like the looks, but it is a pain. 

[minnesotamodeler]

Supposedly a swept-back tip minimizes the tip vortex. I doubt it makes enough difference to matter.

[Bob Reeves]

From what I have picked up from those that know. The Flight Streak is the best wing tip shape for all types of conditions. It doesn't make allot of difference except when it's dead calm and you are flying through your own wake.

[Brian Hampton]

When I designed my latest stunter I wanted to take advantage of anything that might help (real or imagined and I knew that airflow near the tips begins to flow outwards instead of straight across so I figured why not put some wing and flap to catch that angled flow instead of letting it spill off the wing? Did it work? Beats me but I like the look of swept back tips and you get a larger flap as well .

Adelaide, South Australia (and how do I add that to my profile?)

[Mike Palko]

From my experience's flat tips are the worst case scenario and elliptical tips create the least amount of drag. It would be interesting to test tip shapes in a wind tunnel using colored smoke to see the vortices produced or lack there of.

Mike

[RandySmith]

From my experience's flat tips are the worst case scenario and elliptical tips create the least amount of drag. It would be interesting to test tip shapes in a wind tunnel using colored smoke to see the vortices produced or lack there of.

Mike

Hi Mike

I have seen these in a wind tunnel before with many of the shapes we use in CL stunt, the tips I have on my SV-22  were the lowest drag-highest lift and produced the smallest tip vorticies both straight and when deflected, This was less than the elipticals which were good too but hasn't the lift of the  swept back tips. These were close to the same  shape I have used for decades, I copied the tips they tested for my SV-22  and Katana design

By the way the exact same SV-11  wing with the tips reversed and put on like a Nobler will  loose about 10 ounces of lift,  or in other words  the tips swept back will carry more than 10 ounces more than the reverse.

Randy

[Ron King]

From my experience's flat tips are the worst case scenario and elliptical tips create the least amount of drag. It would be interesting to test tip shapes in a wind tunnel using colored smoke to see the vortices produced or lack there of.
Mike

We did this in the mid-1980s and the tests verified some of the reports I had read earlier in my undergrad days. The swept back tip (as Randy mentions) does give the best performance unless you want to go with tiplets or Hoerner tips. We are flying at very low speeds and low Reynolds Numbers anyway, so all of our "normal" tip designs were within acceptable parameters.

We did not try reversing tips as Randy mentions, so I don't know how that would have affected our tests. It was a long time ago in a galaxy far away and I'm afraid many of my colleagues have passed away.

Ron

[Shultzie]

From what I have picked up from those that know. The Flight Streak is the best wing tip shape for all types of conditions. It doesn't make allot of difference except when it's dead calm and you are flying through your own wake.

Even kids love those wing tips...Check out the grin on my son's first model

[Mike Palko]

Hi Mike

I have seen these in a wind tunnel before with many of the shapes we use in CL stunt, the tips I have on my SV-22  were the lowest drag-highest lift and produced the smallest tip vorticies both straight and when deflected, This was less than the elipticals which were good too but hasn't the lift of the  swept back tips. These were close to the same  shape I have used for decades, I copied the tips they tested for my SV-22  and Katana design

By the way the exact same SV-11  wing with the tips reversed and put on like a Nobler will  loose about 10 ounces of lift,  or in other words  the tips swept back will carry more than 10 ounces more than the reverse.

Randy

Now there is some good info based on facts!!! Thanks Randy and Ron. 10oz is a substantial amount, that could really make or break how successful an airframe will be.

Randy, did you find the swept tip shape worked well and stopped there or did you refine the shape to maximize the performance? Am I correct in saying you are splitting hairs at that point?

Also, has anyone done any wind tunnel testing to prove how effective vortex tips really are?

Mike

[Ron King]

Also, has anyone done any wind tunnel testing to prove how effective vortex tips really are?

Mike,

There has been a lot of testing done on different tips, but most of it relates to higher speeds and much higher Reynolds Numbers than we see in CLPA. The Hoerner tips work very well, no matter if you roll them up or down. They work great on sailplanes, which sometimes fly at our speed (80 - 100 fps, approx.), but with higher Reynolds Numbers. I think they would also work in our arena, but don't know if the additional work is worth it. My old Cessna Skylane actually had a Hoerner tip, but I learned that the reason was driven more by the Marketing department than by any serious increase in performance. There are STCs available which use Hoerner tips as part of some STOL packages.

Just like the thread on full span versus partial span flaps, we are talking ounces of difference. As I mentioned, we didn't turn any of our tips around to see how much better or worse they would work, but the difference between the best tip (swept) and an elliptical tip was mere percentage points in the tests I saw.

My original CLPA Alouette had swept tips. Not only was it a great plane, it also looked nice. But my newer planes have more "normal" style tips and I really cannot tell any difference. I personally feel we have plenty of power to move these things through the pattern, so it's a moot point for me as I now have to design my planes to fit into a smaller car. 

Take care,

Ron

[RandySmith]

"""Now there is some good info based on facts!!! Thanks Randy and Ron. 10oz is a substantial amount, that could really make or break how successful an airframe will be.

Randy, did you find the swept tip shape worked well and stopped there or did you refine the shape to maximize the performance? Am I correct in saying you are splitting hairs at that point? ""


Hi Mike

Yes I have flown the same plane with the tips reversed, the lift and load carrying ability is very real...to the tune of at least 10 ounces.

You are correct, this can make or break an airplane, so much so one of my planes built this way dropped in hard corners and was not a good airplane.  cutting off the rounded tips and putting the stock ones on completely cured this problem, plus it tracked better in manouvers.

As far as a  Katana tip  compared to a SV-11 , or even Trivial Pursuit  tip, it is  a very tiny differance

Randy

[L0U CRANE]

Randy, this not specifically to you, just to keep it at the end of the thread...

The swept tips seem to work for us too, RN regardless, for a fairly simple reason. But better tip planforms and profiles can exist, as you've determined.

When our wings produce lift, they reduce pressure in the air above them and increase pressure in the air below them. Roy Clough aside, I'd heard that the pressure reduction above was around 2 to 3 times as much as the pressure increase below.

Like the bow wave from a speedboat, the air passing along the wing chord tries to go from away higher pressure to lower pressure as it moves from LE to TE. But the wing is in the way... So, higher pressure air under the wing slides outboard toward the tips, where it CAN join the lower pressure region. Similarly, the air to the sides of the lower pressure region above the wing tends to slide in over the tips.

So the tips are where this condition allow the different pressure bunches of air to join. The high pressure air rolls over the tips into the lower pressure air above. Recall, tho, with a FliteStreak type tip planform, the air sliding out is also sliding back along the chord. Having the TE of the tips wider than the LE delays the point where that rollover occurs.

For low lift loads, it doesn't matter much. For high-g wing loading conditions - like in a corner - the pressure difference between upper and lower surfaces is much greater, because there's more lift being generater. The swirl of hi-pressure into lo-pressure forms the tip vortex - a twisting funnel of disturbed air trailing behing the wing. When the pressure differences are great, that vortex can do nasty things, like breaking one of Ty's models.  If you've ever stood withiin 10 to 20 feet of the edge of the circle where a model has done square conrers, you can feel how hard the wing has punched the air. It can blow your hat off.

So, the TF tip is generally good. And, careful, scientific testing for our conditions can improve on it... Another little thought: sharp-corner flat tips are not too bad for straight line flying airplanes. The sharp corner helps create a smaller vortex AT that location. A big. round, radiused tip almost invites a broader vortex to form, which masks off more of the upper area from being lifting surface.

[Jim Treace]

Great discussion on this line.
I like the swept back tips, but have also flown both in models and full size. Haven't noticed much difference, if the same speeds are compared. And also, wing tip vortex, since it is mostly compressed air, is denser than the surrounding air and tends to spiral down (fall) away from the wing tip. In full size aircraft, we time our TO and TD to avoid the preceding aircraft landing or take off vortex. Wing tip vortex only develops if the wing is "flying" and the heaver the aircraft, the more dense and dangerous the vortex. This falling vortex is what Ty mentioned that caught him in the bottom of the loop. He flew into the falling vortex. Close to ground, no room to recover.
Jim

[phil c]

Hmmm,  lemme see-  60 oz. plane pulling 30 g's in a square corner,  about 1800 ounces of lift.  Good wing tips generate 10 more ounces.  I'm not going to worry.

Try taking a look at something else.  The efficiency of a tapered wing increases dramatically as the tip chord is reduced to the range of 25-50% of the root chord.  It is commonly accepted that a 50% wing tip fairly closely approximates an elliptical wing.  So most stunt wings, with tips 80-90% of the root chord, are throwing away 10-20% or more of their efficiency.  They generate a lot more drag in turns than a more sharply tapered wing, and consequently there is more speed variation in maneuvers.

[Garf]

How about tip plates?

[Mike Palko]

Hmmm,  lemme see-  60 oz. plane pulling 30 g's in a square corner,  about 1800 ounces of lift.  Good wing tips generate 10 more ounces.  I'm not going to worry.

Try taking a look at something else.  The efficiency of a tapered wing increases dramatically as the tip chord is reduced to the range of 25-50% of the root chord.  It is commonly accepted that a 50% wing tip fairly closely approximates an elliptical wing.  So most stunt wings, with tips 80-90% of the root chord, are throwing away 10-20% or more of their efficiency.  They generate a lot more drag in turns than a more sharply tapered wing, and consequently there is more speed variation in maneuvers.

Phil,
   To compare the two you need to multiply the 10oz also, making it 300oz of lift you are losing (a little more substantial). The fact that Randy has done realworld testing makes me a believer. Think of what just a few ounces of payload or excess weight can do to a good flying model. Why couldn't 10oz more lift help just as much as a heavier airframe hurts?

   I also agree with your more efficient wing planform. There's no reason to throw away any advantage. Of course the advantage has to be worth the extra effort, but both tip shape and airfoil cost very little if any extra effort.

Mike

[Steve Bakac]

Many thanks for your input guys . Interesting reading about your tests Randy
     Thanks again Regards Steve

[EddyR]

Twenty years ago I had a typical foam wing stunter that I added small "Flap" tip plates on the end and the center. It was a well known plane and had well known flying qualities.I had noticed that many balsa covered wings had a sideways slip in the wingover and in some corners that the I/beams did not have. I may have explained it incorrect but that is not important. The experiment worked well but the plane crashed when a down leadout broke at Atlanta with only 40 flights on it. Soon after that My flying buddy Bill Rich started flying Randy designs and I didn't see the problem any more. It would appear the swept back wing tip corrected the problem I saw in many planes at that time. I never tried that experiment again but it might be worth the effort to retrofit a old plane. I found this picture after I wrote the above and I see I had the FLAP tip plates on both ends of the flaps and the center.I was going to take this plane to the 88 Nats but the crash forced the completion of another plane.
Ed

[don Burke]

I read a NACA report (no idea when or report number) that mentioned tip plates.  The least acceptable design to see any improvement over just a plain squared off tip was to make the plates one airfoil max thickness all the way around.  For example a 1" thick symmetrical wing would have 3" high plates that extended 1" ahead of the LE and behind the TE.  I believe the swept back tips as mentioned are really the best way to go.

There's an interesting thread on SSW about the swept tip subject, with graphical comparison data.