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.
 
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.
 
Winglets

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.
 
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.
 
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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 ;)
 
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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. :D
 
They would look cool and serve as a place to hide your Bob Archer COM antenna...
 
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Bad idea....

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, :rolleyes:
 
According to my calculations...

CraigC said:
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.
Click to expand...

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
 
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mlw450802 said:
Post away! :)
Click to expand...

Ok, pretty simple calculations. Only hitch is that I didn't include the standard atmosphere alt to density function (rhoalt). As they say in the textbooks, that excercise is left to the student.


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

% Density, sl/ft^3
rho = rhoalt(alt);

% Cruise speed, fps
U=V*5200/3600;

% Dynamic pressure, psf
q=1/2*rho*U.^2;

% Lift coefficient
Cl=L/(q*S);

% Propulsive power
P=HP*550*PP*EP;

% Cruise drag
D=P/U;

% Cruise total drag coefficient
Cd=D/(q*S);

% Drag due to lift
Cdl=Cl.^2/(pi*E*AR);

% Cruise zero-lift Cd
Cd0=Cd-Cdl;

% Induced drag coefficient with winglets
Cdlw = Cl.^2/(pi*E*ARw);

% Zero-lift drag coefficient with winglets
Cd0w = Cd0*(S+Sw)/S;

% Cruise drag coefficient with winglets
Cdw = Cd0w + Cdlw;

disp(['Density, sl/ft^3 = ', num2str(rho)]);
disp(['Cruise speed, fps = ', num2str(U)]);
disp(['Dynamic pressure, psf = ', num2str(q)]);
disp(['Lift coefficient = ', num2str(Cl)]);
disp(['Propulsive power, lbs*fps = ', num2str(P)]);
disp([' '])
disp(['Without winglets'])
disp([' Zero-lift drag coefficient = ', num2str(Cd0)]);
disp([' Induced drag coefficient = ', num2str(Cdl)]);
disp([' Cruise total drag coefficient = ', num2str(Cd)]);
disp([' Zero-lift drag, lbs = ', num2str(Cd0*q*S)]);
disp([' Induced drag, lbs = ', num2str(Cdl*q*S)]);
disp([' Cruise drag, lbs = ', num2str(Cd*q*S)]);
disp([' '])
disp(['With winglets'])
disp([' Zero-lift drag coefficient = ', num2str(Cd0w)]);
disp([' Induced drag coefficient = ', num2str(Cdlw)]);
disp([' Cruise total drag coefficient = ', num2str(Cdw)]);
disp([' Zero-lift drag, lbs = ', num2str(Cd0w*q*S)]);
disp([' Induced drag, lbs = ', num2str(Cdlw*q*S)]);
disp([' Cruise drag, lbs = ', num2str(Cdw*q*S)]);
disp([' '])
disp(['Drag change due to winglets, lbs = ', num2str((Cdw-Cd)*q*S)]);
 
Alex said:
So I did the logical thing, and wrote a script in Matlab.
Click to expand...
My first job writing software after dropping out of school was working for The MathWorks. Hope my old code isn't still running when you're doing your winglet drag calculations. They'd probably tell you to put your winglets on the gas caps. Just kidding. :cool:
 
I use Matlab and Simulink for probably 50% of my work. I couldn't imagine doing engineering without it.
 
CraigC said:
This is a fun topic for me. And I am enjoying re-starting forgotten regions of thought for under used brain cells ;)
Click to expand...
This is very interesting to me also but I must admit I lack the engineering gene necessary to crunch all of those numbers you engineer types crunch. Because of that I would like to ask for help from all you engineers in understanding this issue of winglets and induced drag.

I have read that: induced drag (ID) plus parasitic drag (PD) = total drag (TD).
ID + PD = TD

I have further read that:
any increase or decrease in one form of drag will be offset by the oppositie increase or decrease in the other form of drag because whatever the numbers are for either form of drag, when added together, they must always equal 100% of TD. So attempts to decrease ID will have an effect of increasing PD.

I think I have a grasp on what I have read but if my thinkin is 'stinkin thinkin' someone please enlighten me.

Here is my question(s) concerning winglets and induced drag.

Again from readings I have done:
Induced drag is highest during slow speeds at low altitudes, or, during high speeds at high altitudes, but parasitic drag is highest at cruise speeds anywhere in between those two extremes. (I am aware that these terms "low and slow" and "high speeds and high altitudes" are poorly defined here. I do not know any of the specifics on the key boundary airspeeds or altitudes to inject specific numbers here.)

It appears to me that, if you want to fly low and slow or you want to cruise as high as possible at as high an airspeed as possible, you are best served by reducing induced drag. If you want to cruise as fast and efficiently as possible in the 5,000 - 10,000 altitude you should concern yourself more with parasitic drag. So I ask anyone in the know, who would be willing to do so, to enlighten an amatuer aeronautical engineer on whether this is indeed the case with winglets and induced drag or not?

Most of the posts concerning winglets and induced drag have discussed the issue of reducing ID for faster cruise. If the above mentioned premise is true, would not winglets to decrease ID only cause an increase in parasitic drag in the cruise airspeed and altitude range that RV's fly in? It seems to me that since most RV's do not fly in the 25,000 + altitudes where winglets might benefit the cruise speed they would better serve our needs in the reduced ID experienced in low and slow flying such as coming into a landing?

My reasons for exploring the answer to these questions are really anchored on flying a non RV. I currently fly a "low and slow" airplane that I built prior to the RV. Its top cruise speed is in the 70 - 80 MPH range with a Vne of 110 and a stall speed of 38 MPH. In examining the winglet design and its impact on induced drag, it appeared to me that winglets on my "low and slow" flying airplane might reduce induced drag with only a minimal impact on parasitic drag enough at the slow speeds that by placing winglets on the plane I could see an increase in its performance characteristics down low and slow.

So these are the issues associated with winglets and induced drag that are sketchy in my mind. I would appreciate it if anyone with some expertise could shed light onto whether I am thinking on the correct lines or not.
 
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RVbySDI said:
I do not know any of the specifics on the key boundary airspeeds or altitudes to inject specific numbers here.
Click to expand...

The key airspeed is the minimum drag airspeed, which also happens to be the best glide angle airspeed. Assuming a quadratic drag polar, which is a good approximation for most GA aircraft, at that airspeed the induced drag is equal to the zero-lift (or parasitic, or profile, or whatever you want to call it) drag.

If your indicated speed is faster than the minimum drag speed, there's more drag coming from the zero-lift drag coefficient. Below minimum drag, there's more induced drag.

For an aircraft with a low wing loading such as the RVs, the minimum drag speed is low, around 100 kts. By the time you get up to a reasonable speed, induced drag is a really small player. With a higher wing loading the minimum drag speed would be higher. This is why winglets would make sense to a high flying, high wing loading Lancair IV-P, but not to a typical RV.

BTW, why would you want less drag on approach? IMO drag is usually a good thing in the pattern.
 
winglet

Thank you for the reply to the question Winglets on the RV line of aircraft.

When I read through the responces, there was alot of good insight into the question of if it would be a venture worth looking into. If a person looks at the cost of fuel these days and has the insight about what the fuel costs could be next year. There is no effective aerodynamic solution in the market at this time to make up for rising opperating costs. We normally look at how the powerplant could be upgraded to meet the chalange at a expence of ?? but in my mind, we should be looking at the airframe inwhich we are installing the powerplant into.

My background is 19 years in the aviation industry Canada and the last four were in the field of aerodynamic modifications to existing commercial airframes. I was the designer of the Cessna Caravan Wing Modification program in Kelowna British Columbia Canada. The goal of the program was to have the aircrafts flight envelope expanded in the stall range and to still keep the top end speed. There were two aircraft modified a Cessna 208 and a Cessna 208B. Both aircraft experanced a reduction in stall speed and a reduction in fuel burn at cruise speed.

Courtney Hunter :cool:
 
Alex said:
The key airspeed is the minimum drag airspeed, which also happens to be the best glide angle airspeed. Assuming a quadratic drag polar, which is a good approximation for most GA aircraft, at that airspeed the induced drag is equal to the zero-lift (or parasitic, or profile, or whatever you want to call it) drag.
Click to expand...
so if the minimum drag airspeed = best glide angle airspeed, can I make the assumption from the stall speed that best glide angle = 1.5 X stall speed (a conservative calculation I use for slow flight)? So the minimum drag airspeed would be 1.5 X stall speed?

If your indicated speed is faster than the minimum drag speed, there's more drag coming from the zero-lift drag coefficient. Below minimum drag, there's more induced drag.

For an aircraft with a low wing loading such as the RVs, the minimum drag speed is low, around 100 kts. By the time you get up to a reasonable speed, induced drag is a really small player. With a higher wing loading the minimum drag speed would be higher. This is why winglets would make sense to a high flying, high wing loading Lancair IV-P, but not to a typical RV.

BTW, why would you want less drag on approach? IMO drag is usually a good thing in the pattern.
Click to expand...
Well, really my thoughts were concerning slow speed at low altitudes rather than "on approach". Again, refering to my low and slow airplane, I am trying to determine if benefits can be gained by flying with winglets on a slow flying airplane. Would winglets decrease the induced drag at a slower speed enough to warrent using them or would they just increase the zero lift drag more than they are worth?

Using the example of my slow flying airplane, it has a wingloading of 5.07 lb/sf, and a stall speed of 38 MPH. If minimum drag airspeed = best glide angle airspeed and I calculate best glide angle airspeed to be 1.5 X 38, I get 57 MPH. So, if I fly slower than 57 MPH can I see any benefit in using winglets? Will the added zero lift drag be a greater detriment to flying faster than 57 MPH and therefore wipe out any benefits to adding winglets?

I know I am discussing these aerodynamics in airspeed performance examples well below those seen in an RV but it does help me to evaluate this idea in terms of something I have experience in. This in turn can help me understand its effect on the RV I hope to fly someday.

Alex, thanks for answering my question. If you have any other comments on what I am thinking I would welcome them along with thoughts anyone else may have.
 
With the typical low and slow airplane, you're going to lose twice by adding winglets. You're going to add weight that is going to increase induced drag, and you are going to add parasite drag.

Most of the winglets you see today are there because of marketing departments. I have heard that the ones for the MD-11 were successful, and I'm sure that some of the biz jets that get up high benefit. Still, unless you have some wingspan limitation, it'd be better for both performance and structure to just add span.
 
c hunter said:
I was the designer of the Cessna Caravan Wing Modification program in Kelowna British Columbia Canada. The goal of the program was to have the aircrafts flight envelope expanded in the stall range and to still keep the top end speed. There were two aircraft modified a Cessna 208 and a Cessna 208B. Both aircraft experanced a reduction in stall speed and a reduction in fuel burn at cruise speed.
Click to expand...
Correct me if I'm wrong, but I think this mod was a lot more than just a set of winglets. I think there was some sort of wing LE mod (glove?) that changed the airfoil shape. Was there also a wing area increase? One of the main design goals (based on info from IAR-NRC) was to increase the allowable take-off weight from shorter runways.

The published take-off distance for newer design FAR 23 aircraft is the ground roll plus the distance from lift-off to 35 ft altitude. Thus an improvement in low speed climb gradient can reduce the distance from lift off to 35 ft, and can increase the allowable take-off weight from a short runway. Winglets could help here.

Was any cruise perf testing done with the wing mod, but without the winglets?
 
don't forget the vortices

Someone mentioned in an earlier post that winglets are as much cosmetic as they are functional. This is absolutely true. I spent the last 2 years as a member of a winglet design team for a major a/c manufacturer and ramp appeal was one of main drivers in deciding to add winglets to our airplane. This was reinforced one day during the flight test phase of the program. We had our testbed aircraft fitted with winglets sitting out on the runway and a prospective customer on a tour spotted it. He turned to the sales person and said "I want my jet to have fins on it". Hey...for $20 million I'd want my jet to look cooler than the other guys too.

One of the key concepts that has been missing from this discussion is how a winglet interacts with the complex flow that occurs at the tip of the wing. A PROPERLY DESIGNED winglet actually takes advantage of the vortex energy at the wingtip and generates a forward lift component. This lift component helps to offset the small increase in parasite drag and results in a lower overall drag situation. I emphasize the word PROPER because many winglets are simply a fin of some size, and shape that someone (most likely in the marketing department) came up with by holding their thumb out in front of them. A well designed winglet requires the use of very powerful (expensive) fluid analysis software and someone very specialized in the field of aerodynamics.

I would be very hesitant to bolt anything on to the tip of my airplane wing unless I was extremely confident in my structural and aerodynamic analysis skills. There are two major issues that would concern me:

1. Adding winglets will change the lift profile of the wing. This will result in the wing structure seeing different loads than it was analyzed for. Those loads would need to be determined and the proper beef-ups (if required) would need to be made to the structure.

2. The airflow over some outboard portion of the aileron will, without a doubt, be influenced by the addition of winglets. This change in flow can potentially have effects on flutter characteristics, control authority, control feel, stall characteristics, spin recovery, etc.

Number 2 is probably the one that would scare me the most. The ONLY way to confirm things like flutter resistance and stall characteristics is thru actual flight testing. You can mitigate risk thru analysis but it all must be verified by actual flight testing.

As mentioned before, the RV series does not really operate in a flight regime that allows it to take full advantage of winglets. You might (just might) see a one knot decrease in stall speed and maybe (just maybe) a small increase in range at some reduced cruise speed. But does anyone really want to fly slower to save a few bucks in gas?
 
Wow... great info in this thread... Thanks guys.

I've been thinking a lot about winglets lately...

One thing my Aerodynamics professor drove home (back in my undergrad years) was that winglets are only really useful on wings whose lift distribution wasn't optimized to begin with... Hense while on CAD designed/analyzed Turbine-ship wings they're often aesthetic, on simpler, constant chord wings there is actually a real benefit to be gained.

He also gave me the best description of winglets I've ever heard (at least for my conceptual understanding of how they work)... Really, winglets are little more than 'sails' operating on the same principle as in boating, capitalizing on the predictable 'wind' of the wing vortex to provide a forward lift component which offsets some of the vortex's drag.

Thats why, on the surface, it seemed like the non-laminar constant chord RV wings would be great canidates... Unfortunately I didn't put 2+2 together than winglets derive their primary benefit below Vg (which makes perfect sense now that I think about it... unfortunately my two semesters of aerodynamics in college are getting a bit rusty).

Thanks for the great post!
 
In response...Yes, the Caravan project included significant modifications. Also, in testing, it WAS flown without winglets for comparison and yes, it was designed to fly with increased gross weight. I think you must have read the published NRC report, written in Canada, but this report was based on an old mission statement. This statement changed after the report was published. I welcome your comments in relation to this and will get back to you more on the modifications soon....have to go to work.
c hunter :cool:
 
thread # 23 was meant to include this quote. sorry
Kevin Horton said:
Correct me if I'm wrong, but I think this mod was a lot more than just a set of winglets. I think there was some sort of wing LE mod (glove?) that changed the airfoil shape. Was there also a wing area increase? One of the main design goals (based on info from IAR-NRC) was to increase the allowable take-off weight from shorter runways.

The published take-off distance for newer design FAR 23 aircraft is the ground roll plus the distance from lift-off to 35 ft altitude. Thus an improvement in low speed climb gradient can reduce the distance from lift off to 35 ft, and can increase the allowable take-off weight from a short runway. Winglets could help here.

Was any cruise perf testing done with the wing mod, but without the winglets?
Click to expand...
 
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