Winglet effect on wing lift and drag? 4

[TurboTag]

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

 

[Aerosc]

A broad rule covering a general performance gain associated with using winglets is not possible as the design and integration of the winglet to the wing is key to its potential performance gain (if it was really that easy, all planes would have them). A poorly designed winglet can even reduce the total performance of the aircraft and even theoretical improvements may not translate to real-world success. Anecdotal stories of the winglets on the 747-400 suggest no overall performance gain, but allowed the sales staff a differentiation between previous models. Extensive research on this topic was performed here at NASA Langley by one Richard T. Whitcomb (developer of the supercritial airfoil and the transonic area rule).

 

[swertel]

To add another anecdote, the Reynold's number does have some affect on winglets. For example, when designing a low Reynold's number model plane (for the SAE Aero Design competition), we noticed that a typical winglet (one that is upswept from the wing) actually decreased our effiency (lift to drag). On the other hand, a small winglet below the wing, rather than above, increased our efficiency -- something you'll never see on a 747. The resulting analysis yielded less wingtip effect on airflow (seperation of boundary layer from airflow under the wing being "thrown" out over the wing at the wingtip).

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

http://www.msoe.edu

and search from SAE Aero Design information, but I couldn't find much that is technically useful.

Scott Wertel

 

[Anthony]

On the other hand, a small winglet below the wing, rather than above, increased our efficiency -- something you'll never see on a 747.

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!

 

[swertel]

So kind of the like the U2 Spy plane. It has small winglets on the underside of the wing and also it own landing gear (but for different reasons).

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

 

[WhiteRabbit]

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.

 

[swertel]

You are correct on our aircraft design. In my year (1998 -- although the designs are all variations on a theme), we had a 20 foot wingspan with an 12 or 16 inch chord. (I can't remember exactly.) The airfoil, I think, is still confidential; but I can say that it was a Selig with a very difficult to manufacture trailing edge.

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 (

http://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

 

[Earhart]

The only reason winglets were put on the BBJ was so as to let corporate owners think they had a differnt aircraft to a B737. It was only after flight tests that they discovered the winglets could reduce fuel consumption by up to 5percent. Check out Aviation Partners website. They do say its the equivilant of adding half the lengh of the wing on to the end of it. I did a report on winglets but never found a formula to consistently depect the effect of the winglet!

 

[WKTaylor]

Folks... all winglets are not alike...
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,

 

[Kenneth]

Interesting article, which quotes ex-head of BBJ, in support of Earhart's statement (above) regarding winglets on the 737:

http://seattlepi.nwsource.com/business/81596_air07.shtml

 

[AeroEdn]

Reynolds numbers play a big role in effectiveness of winglets. ESDU 98013 suggests that winglets reduce the induced drag proportional to their height.

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.

 

[jonaggie]

My Aerodynamics professor published a paper on performance optimzation and analysis, in regrad to how to teach it.

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.

 

[TurboTag]

WOW,

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

 

[ornerynorsk]

This is a little outside of my field, but I did notice that your designation in this forum is automotive. Is the winglet in question for automotive use? If so, an inverted winglet may be used to create wing-in-ground-effect, if it is actual lift that you are trying to generate. Just my 2 cents. Good luck!

 

[TurboTag]

Ornerynorsk

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

 

[Rollerblades]

TurboTag,

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!

 

[chasart]

Take a look at a birds wing or a feather. They don't have winglets up or down. There can't be anything more efficient than what nature provides, so for a slightly longer wing span with pointed tips, you can have the most efficient wing. Birds flap their wings, which demonstrated a good variation in angle of attack and wing load.

Just my two cents worth....

 

[Onemorechance]

A wing is like a vortex-holder, it holds a vortex. The combination of the vortex along the wing and the parallel stream perpendicular to the wing creates lift (the wing flying in the air).

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

 

[chasart]

last chance, or one more chance to see if a bird will fly with winglets!

 

[Onemorechance]

chasart,

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