Winglet effect on wing lift and drag?
Winglet effect on wing lift and drag?
(OP)
OK, anyone want to take a stab at the effect on wing lift and drag with the addition of winglets. I am looking for an equation or rule of thumb. I have read Kroo's papers and others. I had seen something at one time that said the winglet could be considered to add half its height to the wing span (or something similar). Any and all help appreciated. If you know of a particular NACA or NASA report please cite it.
Thanks, Tim
Thanks, Tim





RE: Winglet effect on wing lift and drag?
RE: Winglet effect on wing lift and drag?
By the way, we did start out with the same theory you did... adding a winglet is the same as adding a proportional amount to the wingspan. I can't recall the specific resources because they all applied to high Reynold's number aircraft designs, not the low Reynold's number designs we were developing.
This probably doesn't help you much since you are most likely dealing with high Reynold's number designs, but I thought the info may be beneficial in general.
You can check out my alma mater www.msoe.edu and search from SAE Aero Design information, but I couldn't find much that is technically useful.
Scott Wertel
RE: Winglet effect on wing lift and drag?
If I remember my university aerodynamics, small winglets on the underside of the wing, approximately 2/3 of the chord back from the leading edge, are the most effecient use of winglets. These work best in preventing the vortices moving from the upper surface to the lower surface. I do believe that this property is independent of Reynolds Number.
It may be with larger aircraft that the winglets cannot be placed below the wing, for ground clearance reasons. The 737 NG winglets are something like 8 feet high... mount that under a wing, and you'll have to give it its own landing gear!
RE: Winglet effect on wing lift and drag?
I never really noticed them before. Very interesting. Wish I could have found out more about winglet use when I was still designing the airplane.
--Scott Wertel
RE: Winglet effect on wing lift and drag?
As Swertel stated above, the winglet design that he used for the SAE Aerodesign competition was for a low Reynolds number. If I remember the SAE Aerodesign East competition correctly, MSOE's aircraft was an extemely high aspect ratio aircraft. Another stipulation that the aircraft had was it was designed for heavy lift with a low reynolds number for the competition.
The winglet design is highly dependant on the mission of the aircraft. The winglets almost have to be designed solely on aircraft at hand. The winglets help alleviate some of the induced drag by affecting the wing tip vortices. They help to increase the efficiency which was lost by a 2D airfoil becoming a 3D planform. The winglets either help or adversely affect performance during various stages of the flight which are highly dependant on lift required & reynolds number and almost every aspect of planform shape & chord sizing.
RE: Winglet effect on wing lift and drag?
We had one of our design members researching winglets, only to find limitted information on high Reynolds number designs. After minor testing, these winglets obviously did not work for our application. Because of this, our final design was not based on winglet theory at all.
By having such a large wingspan and short wheel base ("wider is better" -- except for frontal drag) the plane had a tendency to tip on rough landings, dragging the wingtips. Our winglets were meant to be only skid plates with a small interference and low weight. Of course, after our first couple of tests we found that these skid plates increased our efficiency, so we researched a bit more to try to determine a reasonable size for the winglets. Sadly, our design time was over before discovering any concrete evidence of winglet design. Subsequent year's designs may have expanded on our research (www.msoe.edu).
To get more info on the Selig airfoils, check our the University of Illinois Urbana-Champaign
http://www-aero.aae.uiuc.edu/~m-selig/ads.html
It has many other links on it.
--Scott
RE: Winglet effect on wing lift and drag?
RE: Winglet effect on wing lift and drag?
I believe Boeing's approach to winglet design is different from AirBus's approach.
AirBus acft GENERALLY have the double winglets that span up and down for genuine drag reduction [as described above].
Boeing primarily uses a tall/curved winglet for the purpose of adding a substantinal negative bending moment to the outer wing. This concepts improves wing load distribution and enhances fatigue durability, for heavier flight weights. NOTE: I believe the wind-tunnel tests also showed a very minor drag reduction in certain flight regimes for the 747-400 and the 737NG versions. If it weren't for the structural improvements, Boeing would probably avoid winglets altogether.
regards, WKT
regards,
RE: Winglet effect on wing lift and drag?
RE: Winglet effect on wing lift and drag?
For small winglet height to wingspan ratio, try adding their height to the span of the aircraft and recalculate the aspect ratio to give a new induced drag. The winglet also increases the profile drag due to the increase in wetted area. Here is where the Re number has a big impact.
To be effective you need to balance the reduction in induced drag with the increase in form drag.
A good source of infor for sailplane / glider winglets is a paper written by Peter Masak. A quick internet search with his name and winglets should turn it up easily.
RE: Winglet effect on wing lift and drag?
The basic summary is that Winglets increase the circulation around the wing and reduce induced drag. Winglets will help to create more of constant downwash, like what is seen in an elliptical wing.
Optimization in Applied Aerodynamics Jean-Jacques Chattot
it was in the CFD Journal Vol 9 No3 in October of 2000.
RE: Winglet effect on wing lift and drag?
I have to say thanks for all of the input. This Thread is almost 2 years old, I had almost forgotten about it. Obviously winglets are still under investigation. Especilly in the low Reynolds number region.
Tim
RE: Winglet effect on wing lift and drag?
RE: Winglet effect on wing lift and drag?
My degree is in Aerospace but I am working in Automotive. Unfortunately not in an aero application. But the post was/is for a personal project I am working on.
Thanks,
Tim
RE: Winglet effect on wing lift and drag?
I red a book that says that you cannot eliminate induced drag because it generates the lift. The aspect ratio does not change, it`s the lift coefficient CL which changes. CDi=0 means that CL=0 not AR=infinite (CDi=CL2/(pi*AR)). The author Stepen Dalton received a medal from the Royal Photographic Society. "Induced drag exist where there is lift". "The induced drag reaches its maximum when there is stalling (CLmax), it is not proportional to V2". "When the angle of attack is maximum, the tip vortex is more violent which results in more induced drag".
Bye!
RE: Winglet effect on wing lift and drag?
Just my two cents worth....
RE: Winglet effect on wing lift and drag?
Unfortunately a vortex can not just end at the wingtips, it must drip off (vortices don’t just end, they are or closed in themselves or ending on an infinite wall or ending at infinity). If it would be possible to end the vortex at the wingtip there would be no induced drag.
All vorticity of the wing is also kept behind the wing. Some of it is left behind along the trailing edge of the wing, but most of it is dripping off the tip-region of the wing. This vorticity behind the wing is inducing velocities onto the wing (law of Biot and Savart) which reduce the effective angle of attack of the wing. The delta angle of attack (the induced angle = CL/(Pi A e)) has the effect of tilting the lift backwards.
In other words, Lift being defined as the force perpendicular to the direction of incoming airflow, is now not perpendicular to the direction of flight any more, but slightly tilted backwards by the induced angle.
This creates a drag force, the induced drag ~equal to Lift * induced angle
= Lift * CL/(Pi A e)) = CL* .5 * airdensity*V^2 * S * CL/(Pi A e))
V is the airspeed, S is the area of the wing, Pi = 3,141592…, A is b^2 / A, b is wing span, CL is lift coefficient of the wing.
e is a factor depending on the wing shape. When the wing is elliptical e=1, else e < 1.
For a given wing span b, an elliptical wing has minimal induced drag. It is useless to put a winglet on such a wing if the only intention is to further reduce the induced drag of that wing. Else, if the wing is not elliptical (e<1) a winglet can further reduce drag, but you need to design the winglet together with the wing.
Every wing needs its own special winglet!.
Regards,
Onemorechance
RE: Winglet effect on wing lift and drag?
RE: Winglet effect on wing lift and drag?
Don’t exactly know what you mean.
I more or less agree if you mean that a bird doesn’t need winglets to reduce its induced drag. But, I would say that all birds have their own solution to the induced drag problem if its important to them.
Birds that need to soar efficiently either have very slender (slenderness A = b^2 / S is high) and more or less elliptical wing shapes (for example seagulls), or they have wings that end in several curling up (more or less winglet-shaped) feathers (for example eagles).
But both these solutions serve the same purpose, to reduce the induced drag by finding the most efficient distribution of wing lift (or vorticity) over the wingspan.
By the way, in relation to this there is lots of interesting stuff to discuss about how birds create thrust when they fly.
Regards,
Onemorechance
RE: Winglet effect on wing lift and drag?
RE: Winglet effect on wing lift and drag?
Our experience with winglets showed that there are many factors involved. At high subsonic speeds care must be taken to avoid local shock waves; the twist, camber and tickness distribution must be carefully designed for maximum efficiency. And it must be optimum at the condition where the performance improvement is needed. If the intention is to increase range, then the optimum condition should be the long range speed; if the intention is to improve takeoff climbout, then the target speed must be something not far from V2.
The most effective and time-saving way of dimensioning winglets is by using a good CFD tool.
And last but not least: winglets do generate a lot of bending moment, not much different from the extra bending moment due to increasing the wingspan by about the same dimension.
We suspect that:
1 - The 747-400 winglet is just cosmetic;
2 - The A330/340 winglets have very little effect;
3 - The A318/319/320/321 winglets are slightly more effective;
4 - The MD-11 has classical winglets that are fairly effective;
5 - The blended winglets on the 737BBJ are the most effective.
RE: Winglet effect on wing lift and drag?