winglets

[courtney]

Does anyone know anyone that has developed winglets for the RV line of aircraft.

Courtney.

[rv6ejguy]

I've contemplated building and testing some figuring that the small aspect ratio wing on most RVs would benefit quite a bit however I won't have time until my -10 is done. Priorities!

I've got quite a selection of photos compiled from several different aircraft using them and there are some good NASA papers online, outlining design considerations.

[Kevin Horton]

A well designed set of winglets could help reduce induced drag slightly, but they would increase the profile drag. The total drag could be slightly lower at low speed, but would be higher at high speed.

A well designed set of winglets might offer a slight increase in climb rate and climb gradient, and they might provide a small drag reduction for very slow speed cruise (e.g. speed for absolute max range).

But, the extra frontal area will lead to an increase in profile drag, which will increase drag at normal cruise speeds. I believe the vast majority of builders would not be prepared to sacrifice normal cruise speed.

I don't see any point to adding winglets unless:

• you need to get the maximum range, and you are prepared to cruise at very slow speed, or
• you have added a turbo-charger, and plan to cruise at very high altitude (e.g. 25,000 ft).

Also keep in mind that it is not easy to design a good set of winglets. You could easily find that you have increased the profile drag, but not realized any reduction in induced drag.

[CraigC]

Kevin,


Yes drag is higher at higher speeds and lower at lower. But if they did not work nobody would use them. Simple endplates were tried first on slower aircraft. Then winglets. They also reduce drag just by being there. With the right design and angled properly, winglets reduce vortice drag. If using anything other than a flat plate design, such as an airfoil where the lift is directed inward, the lift vector is directed at the center of the aircraft. In the proplerly designed winglet, profile drag is negated by decreasing vortice drag on the end of the wing.

[Kevin Horton]

If you look at the production aircraft using winglets, I think you will find that most of them are aircraft that cruise at high altitude. High altitude cruise means the CAS is low, even though the TAS is high. Thus they are operating in a speed range where induced drag is a significant portion of the total drag. RVs typically cruise in a speed range where profile drag is much greater than induced drag. Look at the RV-6 CAFE Aircraft Performance Report. The curve on page 4 shows how induced drag and profile drag vary with calibrated airspeed. Based on Van's performance numbers, a 180 hp RV-6 will cruise at 198 mph TAS at 75% power at 8,000 ft. This means the CAS on a std day will be 176 mph. The CAFE data shows that the profile drag will be 7 times greater than the induced drag. Adding winglets will almost certainly increase the total drag.

Adding winglets might make sense if we were prepared to cruise at around 105 mph CAS. This implies that either:

• we need to achieve maximum possible range, and are prepared to cruise at very slow speed to achieve it, or
• we have a turbo-charger and oxygen, and we plan to cruise at high altitude (105 mph CAS at 25,000 ft = 156 mph TAS).

Or, we want to have something different, and we don't care if it hurts performance.

A flight test engineer who was with one of the major business aircraft manufacturers told me that the winglets on one of their aircraft actually hurt the cruise speed and range on most missions. But they had to leave them on for marketing reasons.

[CraigC]

Kevin,

You are going to make me drag out my college notes!! If solving for effectiveness and less drag we could use a simple endplate. Sized correctly an endplate would not drastically increase induced drag or profile drag. The first endplates were tested by NACA on light aircraft and they did help with cruise. NACA tests proved that to be effective from ground to 7-8,000 ft and less than 150 mph. You need a airfoil winglet above those speeds. Winglets need not be the same airfoil as the main wing. Sized and positioned correctly to the relative wind, these would have the effect of reducing the induced drag.

You said " The CAFE data shows that the profile drag will be 7 times greater than the induced drag. Adding winglets will almost certainly increase the total drag." Profile drag will always be higher on RVs because they are designed well. The reduction of drag off the wingtips from a properly designed and placed winglet or endplate might offset that increase. And the lift vector gained off of that winglet would also add to that offset. It would take tunnel or real world tests to prove either way. On paper at least I believe it possible with these caveats. Proper design, proper placement. To me, I would not want to add winglets to an RV for structural reasons. They just have not been designed for it.

Thanks Kevin. This is a fun topic for me. And I am enjoying re-starting forgotten regions of thought for under used brain cells

[jcoloccia]

I don't know much about winglet design other than it requires a lot of trial and error and is still somewhat of a black art (unless something's changed recently). That said, aside from the usual structural concerns (moments, blah blah etc..), something tells me that adding lots of additional mass in funny places and fiddling with the aerodynamic loads like this can do strange things to a structure's flutter characteristics.

On the other hand, they'd look really cool.

[chuck]

They would look cool and serve as a place to hide your Bob Archer COM antenna...

[pierre smith]

Guys,
I fly an Airtractor 502 with a PT6/680HP engine and there are winglets available for them, have been for years. Mr. Leland Snow, designer of the airplanes and owner of Air Tractor, says NO. If you decide to add them anyway, your wing life is REDUCED by a third (or thereabouts) of the calculated service life. Mine is 8000 hours without winglets and I now stand at 7620 TTAF.

Apparently the added torsional load at only 140 MPH working speed for that many hours has a real impact on wing life. In our case, ag pilots/operators are always trying to find ways to reduce/eliminate vortices which entrap fine chemical particles and increase unwanted chemical drift.

Another downside is the reduced visibility at three and nine o'clock, not to mention the expense for what little is gained,

[Alex]

Originally Posted by CraigC

Profile drag will always be higher on RVs because they are designed well. The reduction of drag off the wingtips from a properly designed and placed winglet or endplate might offset that increase.

Profile drag is higher on an RV because they cruise at such a low lift coefficient. This is because the stall speed is very low (low wing loading).

So I did the logical thing, and wrote a script in Matlab. Here are my inputs:

alt = 10000; % ft > higher is better
V = 180; % speed, mph > slower is better
S=120; % wing area, ft^2
L=1750; % weight, lb > higher is better
E=0.80; % wing elliptical coefficient > lower is better
AR=5; % wing aspect ratio
HP=180; % engine horsepower > lower is better
PP=0.65; % percent power > lower is better
EP=0.85; % propeller efficiency factor, typical
Sw=4; % winglet planform area > lower is better
ARw=6; % effective aspect ratio with winglets >
% higher is better, this is very generous


and here is the output:


Density, sl/ft^3 = 0.001755
Cruise speed, fps = 260
Dynamic pressure, psf = 59.319
Lift coefficient = 0.24585
Propulsive power, lbs*fps = 54697.5

Without winglets
Zero-lift drag coefficient = 0.024745
Induced drag coefficient = 0.0048097
Cruise total drag coefficient = 0.029554
Zero-lift drag, lbs = 176.1384
Induced drag, lbs = 34.2366
Cruise drag, lbs = 210.375

With winglets
Zero-lift drag coefficient = 0.025569
Induced drag coefficient = 0.0040081
Cruise total drag coefficient = 0.029577
Zero-lift drag, lbs = 182.0096
Induced drag, lbs = 28.5305
Cruise drag, lbs = 210.5402

Drag change due to winglets, lbs = 0.16517


So the net result is that even with a heavy plane, medium altitude, slow cruise, and very optimistic induced drag reduction, the drag increases by 0.17 lbs when you add the winglets. I'll try to post my *.m file if anyone is interested.

Alex