Reducing drag through wing tip vortices ?
 

Hi,

Based on your experience, what is the most efficient way to limit these vortices ?

I am currently thinking of wing tip extensions... I have an idea of the way I could design them, but I want your opinion !


thx a lot !

RE: Best way to avoid those ****** wing tip vortices ?
 

Keep the model stationary on the ground... that will pretty well eliminate the vortices. ;)

-David

RE: Best way to avoid those ****** wing tip vortices ?
 

as far as I know, the design of efficient "winglets" is something very dificult and the results are somethime marginal.
On full scale airplanes, they makes several prototypes ans detrmine the perfos in wind tunel.

RE: Best way to avoid those ****** wing tip vortices ?
 

What are you trying to do to them? As I understand it, you really can't do much to reduce the vortex, as it's a conservation of energy thing, the vortex is a direct and unavoidable result of creating lift.

But you can move the vortex around to help with things like drag, lift, flutter and so on.

Personally, I use winglets to reduce tip stall in high alpha/high G turns, and to reduce aileron flutter, and they work amazingly well at both. In my case, the shape isn't that critical, but the winglet should extend at least 1" below the wing, and 2" above the wing at the TE. It should start no later than the high point of the airfoil. Here's a picture of the ones I used last year. I'm altering the design slightly to extend it the full chord of the wing (actally, these were salvaged from ealier plane with smaller tip chords, but they wor just fine, but I'm making new ones now)

Now, these are low tech, and I've never been too worried about drag reduction. But these winglet absolutly allow for a tighter turn before stalling, and when I test flew this plane with out the winglets, it almost imeditally stripped out the aileron servo due to nasty flutter. Adding the winglets fixed the flutter totally.

http://www.wheek.org/macombat/gfx/rapier/main.jpg

If you're looking for drag reduction, I haven't a clue.

RE: Best way to avoid those ****** wing tip vortices ?
 

increase the wingspan !!!!!!!!!!!!!!!!! but keep the same wing aera :D

as you know the coefficient of induced drag is function of CL (coefficient of lift) divided by the aerodynamic aspect ratio...........

RE: Best way to avoid those ****** wing tip vortices ?
 

I have no idea how much this cuts down the vortex, it just looks cool...I use it on a lot of my planes.

Rough cut

http://www.rcuniverse.com/forum/upfi...54/Ki18528.jpg

Top View

http://www.rcuniverse.com/forum/upfi...54/Gb89243.jpg

Side View

http://www.rcuniverse.com/forum/upfi...54/Ch96295.jpg

Finished bottom view

http://www.rcuniverse.com/forum/upfi...54/Ga76606.jpg

Don

RE: Best way to avoid those ****** wing tip vortices ?
 

Yes, Salmon, I agree, but actually design has been finalized... I only have the sorta 'winglet' improvement....

BTW these big airliners, b747-400, a340, etc..., use winglets which do not necessarily reduce vortices, but which actually create forward "thrust" (also "side" thrust, but this thrust is not used, this energy is absorbed through mechanical flexibilty) ! Indeed, these winglets are just like 'wings' placed within the airflow coming from under the wing towards the upper part of the wing.

I think this is too hard to implement on models without any wind tunnel, indeed.

I know some people have tried infinite ascpect ratio [sm=bananahead.gif] through annular wings, but that does not help much [&o] ...

To get back to your picture, Montague, that is what I was thinking of... My understanding is that these bits help reducing tip vortices, thus increasing 'useful' upper wing area, and calming down airflow near the aileron tip... It might be just that :eek:.

RE: Best way to avoid those ****** wing tip vortices ?
 

Martin Simons recommends spending effort elsewhere. The gains are too small to be noticed.
Planes flying at high angles of attack slowly would benefit more than a fast airplane.
A 6% change in sink-rate for a full-scale with tiplets has been claimed.. That's almost too small to see on a model.
A straight trailing edge and slightly upturned tips -might- be better than a smoothly rounded tip.
As winglets extend the span in essence, extend the span without the upturn. (rules permitting for a contest airplane.)

RE: Best way to avoid those ****** wing tip vortices ?
 

My best results have been with the same tip as in hattend's pics...

The way I make them:

Tack-glue a thick sheet balsa tip plate, then triangle stock to the wingtip such that the top of the tringle stock is a 45 deg corner and it as close as possible parallels the aft 2/3 of the top of the rib.

End view... carve the top to match the upper surface of the wing. the lower part, do the same for the front appx 30%.. and from there back its a straight line to the bottom rear corner of the triangle stock.

Front view you carve to a pleasing rounded outer shape. The inside is holowed out and sanded with a dowel wrapped in sandpaper.

Pull the tips off and hollow them out to reduce weight. then reattach.

Results have been amazing to people that take one look and say it can't work... (many say it can't have much effect if any... but its HUGE!)

I stuck this type tip on a Sig Kobra (.25 size 1970's Pattern design) Max speed was noticeably improved and the plane suddenly was able to float with planes having much lower wing loading (same wing area) and flat bottom airfoils.

I put these tips on a 2 CH slope racer and was suddenly outrunning the kit designer's model... Same weight... no other mods. (and he was a MUCH better pilot...) And the little thing was able to thermal with a Gentle Lady in light lift...

RE: Best way to avoid those ****** wing tip vortices ?
 

Strykaas,

If the design is final, there are limits to what you can do. Are you interested in reducing induced drag? If so, longer span is kind of the big hammer. You don't need to keep the area the same. Actually, since induced drag increases as the coefficient of lift increases, increasing the area helps as well, since the wing will operate at a lower CL. Beyond increasing span, it is ideal to make the distribution of lift along the span as close to elliptical as possible. A geometrically simple way to get a good approximation of an elliptical lift distribution is to build your wing with a taper ratio of around 0.4 to 0.5, and no twist. Some kind of tip plate or 'winglet' probably would reduce induced drag, but, as has been pointed out, they are hard to design optimally.

banktoturn

RE: Best way to avoid those ****** wing tip vortices ?
 

Check out kwong2001's thread (Check out my dad's SPAD) on the SPAD Aircraft-Coroplast design site. Simple bolt on plates are shown in the detail pics. You can easily make tip plates and tips matching your wing airfoil to experiment on the effects with and without tip plates.

I believe effects of vortices would vary depending on the wing tip shape. For a large tip on a low aspect ratio/constant chord wing (i.e. wide tip), the vortices would be exacerbated over that of a tapered wing with a small/narrow tip. Tip plates would then be more advantageous for the larger tip over the smaller tip. Just my guess though.

On the other hand, tip plates would aid in stall resistance which in turn may inhibit aerobatic maneuvers which require the wing to stall (spins and snaps). Then again, thats just my guess.

RE: Best way to avoid those ****** wing tip vortices ?
 

Well, I look at it like this:
1) the vortex is always out there, and pretty much proportional to the lift created, so my winglets don't get rid of the vortex, they just move it somewhere else.
2) the reduction in tip stall is very real and very measureable, allowing higher elevator throws on the radio setup, and visibly tighter turns.
3) the reduction in aileron flutter is very real, where I've fixed flutter problems just by installing the winglets and NO OTHER changes to the plane.

I don't have a wind tunnel to actually see what is going on, but my guess is that the vortex is moving to the top and/or bottom of the winglet, away from the aileron, hence less aileron flutter. And away from the lift-providing portions of the wing, hence a later stall at the tip.

I suspect (but have no proof) that the over all drag is higher with these winglets. Also, installing the winglets does not result in any pitch trim changes, so I assume they don't add much in the way of lift either (though the large horizontal stab might be taking care of that for me).

Getting back to the shape of my winglets, be sure to extend the winglet above and below the wing. I tried a few other shapes, and the best results were had when the winglet was above and below the wing. The winglets also extend all the way to the aileron TE, or maybe 1/16" past the TE of the aileron. If the winglets don't go back far enough, you get aileron flutter anyway. (of course, I'm using rather large ailerons)

However, I never tried a pretty carved balsa tip like in the pictures above. It wouldn't surprise me if it worked better, but for my application, it's not practical. (My winglets are made from coro, and held on with tape. I keep spare winglets in my box, since they are high-casualty items, and quite expendable)

So I think it all depends on your application.

RE: Best way to avoid those ****** wing tip vortices ?
 

Montague,

There was a loooong discussion on this a while back that you might like to check out:

http://www.rcuniverse.com/forum/m_11...tm.htm#1138176

http://www.rcuniverse.com/forum/m_11...tm.htm#1152352

You might want to check out all that information.

RE: Best way to avoid those ****** wing tip vortices ?
 

if somebody can give me a method to quantify the effect of winglets on a RC aircraft, I am ready to buy.

I don't talk here about an effect on stability or flight behaviours, but the effect on drag and airplane consumption.

I thruly believe that it's so small that it can't be mesure on a rc bird fit with our conventional equipement

RE: Best way to avoid those ****** wing tip vortices ?
 

.
And THAT'S the way it is!
Any improvements are subjective at best.

RE: Best way to avoid those ****** wing tip vortices ?
 

Ken,
I read those threads, a while back and then went back and went over them again just now. Lots of stuff in there

But I didn't see anything in there that contridicted what I said :D. (my comment about the vortex being related to the lift could be taken out of context. I was assuming a fixed wing, already designed and built. Certainly, differnet wings, differnet chord, planform, etc blah blah blah will have differnet vortexes. But for a given wing shape in the real world, messing with the tip can do a lot, but, as I understand it, you aren't getting rid of, or even reducing, the vortex, you're "just" moving it around).

Anyway, Strykaas asked how to reduce the vortex, and my question in reply is "why?" If Strykaas is trying to reduce drag, I haven't a clue for suggestions, I don't spend much time worrying about drag. (just look at the picture I posted, uncowled engine, two servos and control rods hanging out in the breeze, a veratable drag farm). Heck, the mounting of the winglets on my plane creates a 90degree corner, and that's gotta be increasing drag right there.

However, if Strykaas wants better stall handling, that's something I've spent a LOT of time messing with, as getting even a small improvement in turning radius gives me a competitive edge in competition. Though I've been dealing mostly with "high" speed, high-G turns, not low speed flight. I don't know if that makes much difference.

RE: Best way to avoid those ****** wing tip vortices ?
 

There's more to flying than drag reduction :D.

RE: Best way to avoid those ****** wing tip vortices ?
 

(sorry, I forgot to click that "notify" check box)

RE: Best way to avoid those ****** wing tip vortices ?
 

.
Depends.
On a TD glider or pylon racer drag is BAD!
Get it as small as possible, then go fly. :)
The original purpose hasn't been stated... it would be nice to know that from the beginning..

RE: Best way to avoid those ****** wing tip vortices ?
 

Agreed :D. but in acrobatics, it helps keep the downlines looking right. And in RC Combat (what I do), it's not much of a factor at all. (speed is actually not a great thing in RC combat, believe it or not. it's better to turn really really REALLY tight than to go fast. If a guy is a lot faster, that's ok, he has to turn around and come back eventually :D)

RE: Best way to avoid those ****** wing tip vortices ?
 

Well... at low speed, induced drag dominates. Parasite drag is small. So, your best option for reducing total drag at low speeds is to decrease C_L (which is a factor in induced drag).

You can reduce C_L by either increasing your wing area (which will result in a smaller C_L to stay aloft), or by increasing your speed. This will of course alter the flight performance of your craft drastically.

Adding an engineered tip will reduce induced drag by effectively increasing the span of the wing. But, you are adding parasite drag by adding the tip. It is nonsensical to use a winglet (in the true sense of the term) on an RC craft unless you are going for distance/endurance. Winglets are designed for a single flight condition, and are not as effective at others and can actually detract from performance. You can use a modified tip (such as a Hoerner, drooped tip, or other variation) to reduce induced drag. A simple cutoff tip is a better bet than a smooth tip for reducing induced drag. Your best bet is to go with a Hoerner tip... it's simple and proven. I'll let whoever is interested to go find the old NACA memos on Hoerner tips et al.

The whole point of an engineered tip is to make spanwise flow more difficult... spanwise flow results from lift on a real wing.

Why would you want to reduce drag on an RC aircraft? Good question... i dont have a clear cut answer for that.

RE: Best way to avoid those ****** wing tip vortices ?
 

he wants to reduce the total drag to go faster with a given engine, he will have a race during holidays and he his preparing his fatal weapon.

RE: Best way to avoid those ****** wing tip vortices ?
 

I'd say that reducing the wing area is a good option. Not much though... i'll let you figure that out. You can also increase the span and keep the total area the same (basically, increasing the AR).

When you decrease your wing area you need to make sure that your elevator is not adding the drag that you just lost due to the decreased area. In other words, your balance may have changed, especially so if your CG is not too close to your neutral point on the craft.

RE: Best way to avoid those ****** wing tip vortices ?
 

impossible to reduce the wing aera, for two main reasons
1) the airplane is determine (shrike 10 with .30 size engine), we may improve it but not alterate it)
2) clip the wing of the shrike 10 (already overloaded by a double sized engine) will make the hand launch very marginal

needless to say that the guy he is going to race is me :D

RE: Best way to avoid those ****** wing tip vortices ?
 

Working with that tip is the best (only) option then!