winglets?

[spacegrrrl]

Re: winglets?

If I'm not mistaken, this is the "Schumann" wing planform. All the high performance R/C gliders use it nowadays.

Gerry

That is what I was thinking of!

As to the question about wingtips on R/C racers I expect it makes very little difference.

Michele

[Unregistered]

Re: winglets?

If you want to see truly effective uses of winglets and wing planforms, look at the very latest in competition sailplanes. Of course they are are designed for speeds between 60 and about 150 MPH with a very high aspect ratio. Wing loadings are between 8 and 11 lb/sq ft.

The designers wish they could get them to go away at very high speeds, and get bigger in thermals. Generally they are optimized for thermal flight, and as speed increases in cruise, they help less and less until they are a loss at the high end. The skill in designing a glider winglet is in having the least loss at high speeds with the most gain in low.

In the sports class, the only ship that might gain with some short winglets is the Venture. That has a reasonably high A/R and might benefit with winglets to at least straighten out the flow at 400 MPH.

Any comments??

Bruce Patton
Glider pilot and part time venture assistor.

[Blue Foam]

Re: winglets?

If you want to see truly effective uses of winglets and wing planforms, look at the very latest in competition sailplanes. ... The skill in designing a glider winglet is in having the least loss at high speeds with the most gain in low.

In the sports class, the only ship that might gain with some short winglets is the Venture. That has a reasonably high A/R and might benefit with winglets to at least straighten out the flow at 400 MPH.

Referring back to the explaination I wrote earlier, I left out a couple of drivers. First is that increasing either aspect ratio or adding winglets will have the greatest benefit at high lift coefficients. Both are detrimental at low lift. Look at the induced drag equation again.

I personally did drag research and very successful drag reduction on the Venture and the LAST thing that aircraft needs is winglets. It might benefit slightly from a properly contoured wing tip, however the present square tip is close to optimum given the high aspect ratio and low lift coefficient on the race course. For reference, the Venture I modified with less than 10 lb of glass went 10 mph faster in laps at Reno and 18 mph faster cross country. I did research and testing on the tips and they proved to be a non-issue due to the high aspect ratio. 5 other potential drag areas were also investigated that testing showed to be non-issues. I designed and fabricated the fairings based on modern boundary layer stagnation and separation data and the test data taken from the Venture. No one has correctly implemented the same mods and no one has seen the same performance improvement.

A personal note here: my fairings incorporated sharp internal corners at the intersections. This was done for VERY good reasons. A bondo-jockey at a paint shop then filleted all of these intersections and made the aircraft 3 mph SLOWER. These fairings were then poorly copied onto the leading Legacys and achieved about half of the drag reduction they could have. Lots of money has been spent on drag reduction and engine mods in air racing in the last 10 years. I believe that I hold the record for $/mph.

To summarize, low aspect ratio wings operating at high lift coefficients will benefit from a winglet or aspect ratio increase than a high aspect ratio wing operating at low lift coefficients. The sailplane types ought to try optimised wing tips of the same wetted area as their winglets and see which one wins. My money is on the tip.

Winglets became popular in sailplanes because of... (take a guess)

A non-aerodynamic restriction on wing span!

"Standard Class" racing is restricted to 15m wing span. Winglets popped up to improve on this without adding span and there was one hell of an argument in the racing community about whether they should be allowed. The other classes have followed without disciplined trade studies and optimization.

Once aspect ratio has been optimised, an optimum wing tip is ridiculously simple. For a basis, look at any modern airliner horizontal stab or the B737 NG-non-winglet wing tip. If any otehr shape of wing tip yields better performance, then there simply is not enough wing on there.

[Unregistered]

Re: winglets?

"Standard Class" racing is restricted to 15m wing span. Winglets popped up to improve on this without adding span and there was one hell of an argument in the racing community about whether they should be allowed. The other classes have followed without disciplined trade studies and optimization."

Actually, the Open class where wingspans are unlimited, and somewhat out of control, has started sprouting winglets. The ETA, which has a 101 foot wing, has big winglets. The Nimbus series of "small" 86 footers are also growing them. The trouble with them is that the sink rates are so low in real life that meaningful field measurements are not possible when you are talking 1-3% improvements. The real air is never smooth enough. Wind tunnel data does not reflect real life with bugs and wind shear. As a result, contests are the only true gage of performance, and Open is expensive and not getting a lot of competition.

Winglets on the 15 and standard class were a big flap until everyone had them. Glider pilots at that level will trade their children for a small advantage, similar to Air Race types.

"my fairings incorporated sharp internal corners at the intersections. This was done for VERY good reasons."

I understand why at lower R numbers and smoother air flows, but does this still hold at Race speeds in the turbulence of the prop? I will try to convince someone to do some tests this year by flying with and without the very nice 3" radius wing intersection fairings that are bolted on the Venture. I think they might be doing some good beyond 80% chord. Owner will be mighty ticked if it is faster without them!

Recognizing that the spanwise flow is going to be limited at 400 MPH and a high aspect ratio, still isn't there enough energy there to make some sort of flow straightener winglet a positive thing?

Bruce Patton

[Blue Foam]

Re: winglets?

"my fairings incorporated sharp internal corners at the intersections. This was done for VERY good reasons."

I understand why at lower R numbers and smoother air flows, but does this still hold at Race speeds in the turbulence of the prop? I will try to convince someone to do some tests this year by flying with and without the very nice 3" radius wing intersection fairings that are bolted on the Venture. I think they might be doing some good beyond 80% chord. Owner will be mighty ticked if it is faster without them!

Recognizing that the spanwise flow is going to be limited at 400 MPH and a high aspect ratio, still isn't there enough energy there to make some sort of flow straightener winglet a positive thing?

Bruce Patton

Bruce, I can't tell you how interesting it is to discuss this with someone qualified and interested. Next, never underestimate spanwise effects; These are the bane of those unappreciative of stagnation flow and local boundary layer energy and effects. Also, realize that the root chord Re for the Venture at Reno was 7 to 9M. This is a high Rn and with a two blade prop the ittermittant blade wakes are not dominant.

Please understand that winglets do not "straighten" the airflow. They increase the span of the elliptical lift distribution to greater than the non-winglet wing and thereby increase the span efficiency. Energy that exists at the wingtip can be managed by increasing aspect ratio -or- adding winglets. Either solution may be detrimental to overall lap speed due to the penalties of increased wetted area.

I have to disagree with the large radius work. I realize that this was a major drag reduction based on Hoener's classical work. I would point out that the entire science of intersection drag was re-written in 1990-91 by the MD-11 drag reduction crew and well implemented on the B737NG and B777 programs. This data only made itself into generally available knowledge between 1996 and 2002. Anything prior to this is old data; the new data and solutions blatantly contradict a great deal of what we all in that last half century hold dear.

Please contact me on these large radius fairings before you implement them and I will help you with an experimental trade study. No one has gotten the speed improvement out of aerodynamic mods to a Venture that I have. Whichever way the flight test data goes, wins.

[John]

Re: winglets?

In regards to sailplanes;
apparently intersection drag has been an issue for some time.
Currently the designers are utilizing turbulent flow(style) airfoils at
the root intersection to get the flow to remain attached. Custom
designed airfoils are the way to go.

[Unregistered]

Re: winglets?

From what I understand winglets reduce induced drag, or drag that is directly related to the co-efficient of lift. This means that as airspeed increases, induced drag decreases. In fact, it is reduced to such a point that it is almost negligable on the total drag curve. It is in reducing parasite drag that you will see the most noticable effects in regards to speed. So if induced drag varies with CL(Co-effiecient of lift) then we must look at the lift equation.

L=Surface area of wing*1/2Rho(air density)*V(squared)*CL
and
Lift is=Weight

The only directly controllable variables in this equation are Velocity and CL. So if we increase one of these variables the other must be reduced to maintain the same amount of lift and the same amount of lift is maintained. So for an airracer, adding winglets could theoretically add more drag than reduce it. I believe that this is why they haven't been added to an airracer. But that is just my limited knowledge on the subject from my aerodynamics class in college (go UND).

-Nick

[Apteryx]

Re: winglets?

Eric-
Winglets not withstanding, how do you feel about the shape of the wing tips, (other then the Venture), Hoerner, eliptical, fenced, etc....for a racer?

Also curious about what you think of the fences on the alierons & elevators of the F1 Alley Cat.

Paul

[FlyKidChris]

Re: winglets?

[QUOTE=Blue Foam]

A question for you: the winglets only add a couple of feet to each side, is the extra caution taxiing due to a depth perception issue vs. regular tips?

Eric, thanks for the response.
I was referring to operating all types in my carrier's fleet. I'm fortunate enough to operate several versions of the 737. The wingspan of the 737-300/-500 is 94' 9" and a wingletted -700/-800 is 117' 5" (112' 7" for standard 737NGs).
So when "hopping" from version to version, the difference is noticable. And when approaching a gate, that already seems (wingspan) space limited, with a wingletted 737NG, I do exercise additional caution. It's amazing how many vehicles are often obstructing the ramp or gate area. I believe the greatest chance of damage is on the ground - on the ramp near a gate.
Thanks for the recommendation on Raymer's book. I already have that one, so I'll reread it.
Best wishes, Chris

[Blue Foam]

Re: winglets?

Eric-
Winglets not withstanding, how do you feel about the shape of the wing tips, (other then the Venture), Hoerner, eliptical, fenced, etc....for a racer?

Also curious about what you think of the fences on the alierons & elevators of the F1 Alley Cat.

Paul

The optimum wing tip shape based on the combined, multi-decade efforts of Boeing, Airbus, TsAGI (Russia), Bombardier, Embrarer, ATR, etc. is illustrated on all of their horizontal stabilizers and all of their non-winglet wing tips. It is disgustingly simple.

Alley Cat's fences are a trade of increasing control surface efficiency at higher deflection vs. the additional wetted area of the fences themselves. One would have to take them off and do flight tests vs. having them in place to tell if they are good for that one aircraft and its control schedule during a race lap. IM<HO, if they are beneficial, especially on the elevator, it would indicate an issue in another area.