Non-aerobatic utility rated RV-6 with Extended Elliptical Wingtips

I would like to replace the standard RV-6 wingtips with some extended length elliptical wingtips for my non-aerobatic utility rated RV-6. The goal would be to achieve a lower stall and lower sink rate at slower speeds. I am not interested in buying or building a RV-9, nor extend the range with wing tip tanks, just have some fun trying something new on my experimental RV-6.

I know that several RV?ers (Bob Axsom) have removable wing tips for their RV-6 for both racing and other uses. I would like to try the extended length elliptical wing tips for improved efficiency.

So with that in mind, I wonder what the maximum length wing tip (properly mounted of course) that could be added to the existing RV-6 wing assuming it will only be operated in the utility category and no aerobatics?

cj

true ellipses?

so CJ, have you gotten out your pencil and french curves? ( only old draughtsmen will know what the h*ll I'm talking aboot!)
I think you're going to need to compromise, and use some kind of truncated ellipses.
A nice, true, spitfire-looking elliptical tip is going to be 8 feet long!....and just a tiny bit of structure in there to support that! ( not to mention, a new hangar!)
..the good news? a glide ratio of 27:1, and a service ceiling of 37,052'

but hey, when you're done, it will look pretty darn cool!

French curve? I know them, I used them, I feel SOOOOO old...

I know the French Curve too... She was 25, sassy, sexy and smart. Then I remembered she was French.

Spank, you bad!

Are you coming up to join Dreamland in the fly-over at the 3rd annual Palatka Open house and Fly-in on 1/26? Great chow, a huge attendance, and lots of planes. The Maverick's from Spruce Creek will be there and we need all the Dreamland pilots we can muster to show them how good Dreamland really is!!

http://www.palatkakaylarkin.com/

1/4 ellipse shape

This is an area of aerodynamic specialty for me.

Some simple guidelines:
1) straight extension of existing trailing edge is best.
2) make a 1/4 ellipse shape, that is, start at the leading edge and draw a 1/4 ellipse that intersects the trailing edge at the maximum span point
3) you can pick just about any amount of extension and this basic shape is still very good - add 3" or 3' or whatever
4) make the tip shape a smooth transition/blend of the existing airfoil. No need to curl up, or curl down, just a nice rounded tip shape as the elliptical leading edge curve comes around to meet the trailing edge at 90 degree angle.
5) pay attention to structural load -- the load-carrying ability of the row of screws around the perimeter of the tip rib on the wing should not be trusted to carry any more area than it does with the stock tips. You can put a spar extension on the wing that connects into the shear web of the wing, and connects to a light spar inside your tip extension.

Trade-offs:
1) a huge reduction in induced drag, both from the wingspan increase, and the improved spanwise lift distribution. This will show up especially as improved climb rate and faster high-altitude cruise.
2) the added wing area will lower your stall speed
3) the added wing area will reduce your low-altitude top speed, where induced drag is not important, but skin friction is.

Other tip shapes may have a better compromise of reduced surface area for the same span extension -- for example the tips that Paul Lipps demonstrated a few years ago were essentially triangular -- minimizing the added wing area for the amount of span increase. This may cause the tip to stall first because of the higher loading on the rapidly diminishing chord, so something in between triangular and 1/4 ellipse may be a better compromise for top speed. But I don't think it is worth worrying about. The 1/4 ellipse looks nice, will fly great.

If you need help with structural loads, I can probably help you off-line.

1/4 Ellipse

Steve,
If I understand your post correctly, the shape doesn't seem too different from the current RV-9(A) tips from Van's. I think these are so called "sheared" tips. I tried to find some research papers on sheared tips without much success. I have Dr. Hoerner's books, but I wanted to find something more recent. Do you have any references you could share?

Besides the severe impact on wing strength, which must be assessed, there are potentially major effects on stability and spin recovery.

Dave

scsmith said:

This is an area of aerodynamic specialty for me...

Click to expand...

I've fabricated three or four sets of Steve Smith tips for various projects. I think that this is about what the RV-6 would look like with 30" quarter-ellipse tips:



Thanks, Bob K.

scsmith said:

Paul Lipps demonstrated a few years ago were essentially triangular -- minimizing the added wing area for the amount of span increase.

Click to expand...

He did but you're missing some details. Paul's tip design maintained the same airfoil all the way to the tip and it was just large enough at the tip to hold the nav lights. He also told me that it didn't matter if it was shaped as a trapezoid or triangular with a straight trailing edge, like Jim Smith's tips. The elliptical tip really does look a lot better though...

ccsmith51 said:

Spank, you bad!

Are you coming up to join Dreamland in the fly-over at the 3rd annual Palatka Open house and Fly-in on 1/26? Great chow, a huge attendance, and lots of planes. The Maverick's from Spruce Creek will be there and we need all the Dreamland pilots we can muster to show them how good Dreamland really is!!

http://www.palatkakaylarkin.com/

Click to expand...

Slide - I do hope to attend. I'll send you guys a not offline.

Spank

Wing efficiency

grubbat said:

Thanks for the drawing Bob.

Steve, how does this design compare to an ellipical such as on a spitfire wing? Does it give the wing the same efficiency.

I wonder how much length could be added to keep the rv-6 wing in the utility catagory?

Click to expand...

The key to wing efficiency is to achieve a nearly elliptical distribution of lift. It is not necessary to have an elliptical planform to do that -- there is an infinite combination of planform + airfoil camber + twist combinations that can achieve that. Although difficult to fabricate, an elliptical wing with no twist is one way to do it. Ironically, my pal R. J. Mitchell kind of missed the point on the Spitfire, which had a beautiful elliptical planform, but then he added 3 degrees of twist. One problem with the elliptical planform is that it tends to stall near the tip first because of the shorter chord lengths operating at lower Reynolds no. (thats beyond the scope of this discussion) and he wanted to prevent that.

It does turn out that an elliptical planform with a straight trailing edge is better than an elliptical planform with a straight 25% chord line like the spitfire - for some subtle reasons, you get a more elliptical loading when the trailing edge is straight. For a good reference on this, see the following article:
"Computation of Induced Drag for Elliptical and Crescent-Shaped Wings" by Stephen C. Smith and Ilan M. Kroo, Journal of Aircraft, Vol. 30, No. 4, July-August 1993.

Putting nice tips on an untwisted rectangular wing is better than one might expect, but it is not going to magically make the whole lift distribution elliptical either. The rectangular wing has a lift distribution that can be approximated by the average between an ellipse and a rectangle (Schrenk's rule). Some of the 'extra' induced drag comes in the last few feet of the wing, where the lift doesn't drop off as quickly as it should. Nice tips help a lot. Triangular tips like Paul Lipps' help drop the lift more quickly, but don't do it in a very smooth way because of the abrupt transition from rectangular to triangular shape. Sheared tips are not too bad, but the primary purpose of sheared tips is to influence the tip flow at high angles of attack, they shed the tip vortex farther forward and it rolls over the upper surface of the wing. This produces some extra lift, especially in the region near the ailerons, maintaining good roll control into the stall. The fact that the sheared tips also help drop the lift distribution more quickly and smoothly is a bonus -- but it they are not as good as an elliptical tip for low induced drag because the effective span is lower. This is evidenced by the newer stock Van's tips that increased the span from 23' to 24' on the RV-8 and had very little influence on the performance.

I can help you with the structural aspects (keeping within Utility catagory loads) but that should be done off-line.