Wingtip designs

Andy Hill · #1 08-25-2010, 02:12 PM

It is intriguing that Van's have produced 3 wingtip styles (415, 715, 715-1), yet there is little debate about their relative merits.

I am clearly aware of the visual differences, but no more. Any advice, ideas or knowledge about the differences, including "why" Van's chose to redesign them at each stage would be appreciated.

To put it in context, would there be any performance (or other?) reason for someone with 415 wingtips on, say, an RV-8, to consider fitting 715-1 tips? I am assuming not

Or even v-v

Thanks in advance... trust I am not starting a primer wars / NW v TW debate!

Andy

scsmith · #2 08-25-2010, 02:32 PM

Hi Andy,
There is quite a lot I could say about wingtips, in general and specific to RV's.

To answer your question, one HUGE difference between the 415 and 715 tips is that the 715 tips increase the wingspan by a foot. This is a pretty big deal.
It will significantly improve climb rate, and high-altitude cruise, as well as low-speed peformance. They probably have a slight disadvantage at top speed at low altitude. (induced drag is very small anyway, and the added wetted area hurts)

These tips are shaped with the highly swept side edge primarily so that the span increase does not come with a large increase in wetted area. There are some other interesting implications of this tip shape for moderate and high angle of attack performance and handling. But I don't want to write a thesis here. The 715-1 tip reduced wetted area further by removing the silly crescent trailing edge. (analysis in the literature that showed benefit of the crescent shape was largely the result of numerical artifacts, later debunked)

Given the very low aspect ratio, it is not really possible to do much about the tip shapes that is going to have much difference, other than the trade-off of wing span and wetted area.

To a large extent, you can think of wing tips on general aviation airplanes as a form of genetic drift. Take a walk at a busy airport and look at all the miriad different shape tips. If a shape were really substantially better, there would be some natural selection that would move all the designs toward that. The presence of so many successful airplanes with such a wide range of tip shapes suggests that it doesn't matter very much, and the design 'drifts' without any evolutionary pressure in any particular direction. Even on high performance sailplanes, there has really only recently been a noticeable convergence of tip shape. (Most nowadays have winglets, so I'm talking here more about the tip of the winglet.)

One feature that has been shown to be important is that the point of maximum wing span should coincide with the intersection of the trailing edge and tip side edge. All of Van's tips meet that criterion.

Mel · #3 08-25-2010, 02:41 PM

Quote:

Originally Posted by Andy Hill

It is intriguing that Van's have produced 3 wingtip styles (415, 715, 715-1), yet there is little debate about their relative merits.

I just changed from the original 415 tips to the 715 "bat-wing" style on my RV-6.
The only flight characteristics I noticed changing is that the high sink rate seems to start at a higher airspeed.
With the original tips the high sink rate started about 75-80 mph. With the new tips, that high sink rate starts at around 80-85 mph.
In my opinion, the original 415 tips are the most efficient of all tips offered.
Even though the 715 tips increase wing span slightly, the original "hoerner" style tips increase "effective" wingspan.

Stall and top speeds did not change at all.

I was told by an "un-named" person of authority at Vans that they changed because people thought the "Rocket" style tips are faster. Remember in the "good old days" when chrome valve covers and air cleaners were worth 10 mph on your hot rod?

GAHco · #4 08-25-2010, 08:21 PM

Quote:

Originally Posted by Mel

I just changed from the original 415 tips to the 715 "bat-wing" style on my RV-6.
The only flight characteristics I noticed changing is that the high sink rate seems to start at a higher airspeed.
With the original tips the high sink rate started about 75-80 mph. With the new tips, that high sink rate starts at around 80-85 mph.
In my opinion, the original 415 tips are the most efficient of all tips offered.
Even though the 715 tips increase wing span slightly, the original "hoerner" style tips increase "effective" wingspan.

Stall and top speeds did not change at all.

I was told by an "un-named" person of authority at Vans that they changed because people thought the "Rocket" style tips are faster. Remember in the "good old days" when chrome valve covers and air cleaners were worth 10 mph on your hot rod?

Your actual findings confirm my suspicions, I have always preferred the Hoerner tips appearance and that's what my 6 kit came with.
They are prepped and primed, just waiting for the day I get to put them on.

scsmith · #5 08-25-2010, 09:04 PM

Mel's observations are expected. Although the 715 has more span, the highly swept side edge separates the tip vortex forward along the side edge at moderate to high angle of attack, which both reduces the effective span and creates a lot of pressure drag. So I would expect the sink rate to be increased below 75 kt. But, I would still expect the maximum rate of climb to be higher with the 715 tips - at moderate angles of attack, the flow should stay attached and get greater effective span. The 415 tip shape is quite good for what we have to work with on such a low aspect-ratio wing. If I were going to dog-fight, I would expect that the 415 tip would bleed less energy at corner speed, even though it has less actual wingspan.

Quote:

Originally Posted by Mel

I just changed from the original 415 tips to the 715 "bat-wing" style on my RV-6.
The only flight characteristics I noticed changing is that the high sink rate seems to start at a higher airspeed.
With the original tips the high sink rate started about 75-80 mph. With the new tips, that high sink rate starts at around 80-85 mph.
In my opinion, the original 415 tips are the most efficient of all tips offered.
Even though the 715 tips increase wing span slightly, the original "hoerner" style tips increase "effective" wingspan.

Mel · #6 08-25-2010, 09:12 PM

I saw no measurable change in rate of climb or glide ratio.

WingsOnWheels · #7 08-25-2010, 09:31 PM

I'm glad this topic came up. I'm still quite a ways away from the fiberglass work (wings are built though). The tips that came with my 20 year-old kit are a bit warped. If I can't get them back in shape, I was going to buy a new set. I like the look of the older tips, but assumed the new tips were faster (why change them if they are not...). If there is no advantage, i'll stick with old tips.

PropMan · #8 08-25-2010, 09:38 PM

Several years ago my son built a wind tunnel per plans from a fellow in the NASA building at Airventure. He used the two different wingtips as test subjects. What he found was the old style had less drag and more lift. Now this was a model of just the tip not the complete wing.

PCHunt · #9 08-25-2010, 11:29 PM

Does anyone have photos or drawings of the 3 different wing tip designs?

scsmith · #10 08-26-2010, 12:54 AM

Quote:

Originally Posted by PCHunt

Does anyone have photos or drawings of the 3 different wing tip designs?

Sure, go to the Van's accessory catalog on-line, and look up the RV 7/8 wingtip lighting kit. That page shows the shape of the 715 and 715-1 tips. Then, go back to Van's home page and select the RV-6 model information. (or RV-4) Scroll down to the 3-view drawings. They show the 415 tip.

Note the 415 tip has a 90 degree swept side edge in plan view. In front view it has a flat top, and the bottom surface curves up to meet the top surface (one of the two classic Hoerner tip designs).

The 715 and 715-1 tips are 6" wider each, adding a foot to the physical wingspan.