Winglet aerodynamics

[stuckoutthere]

Hi all

I am new here and have just done a search to see if I could find any posts on winglets. Although I did find and read the post they did not really answer my question.

My question is how has fitting blended winglets affected the basic flight performance of an aircraft in the glide and turn..

Any answers would be gratefully received as this was a question I was asked and could not fully answer and it is annoying me now.

Thanks

 

[rb1957]

as i understand it, winglets increase a wing's aspect ratio. so i'd expect they'd reasonably improve a plane's glide performance (higher L/D) but the affect on turn performance would be small.

 

[KENAT]

If canted wouldn't they act as anhedral (negative dihedral)

http://en.wikipedia.org/wiki/Dihedral_(aircraft)

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[tmoritz]

Winglets are designed to be effective for a very narrow range of angles of attack, usually to optimize cruise. Since your angle of attack will be higher for glide and turn you will probably find winglets are detrimental to performance.

Tom Moritz
Mechanical Engineer
US Bureau of Reclamation

 

[btrueblood]

Dunno, sounds right Tom, for winglets optimized for cruise. But other people state the winglets (for various planes) were done to improve climb rates, and/or had secondary benefits of improving lateral (and/or longitudinal) stability. The right answer may be (as usual) -- "it depends".

see

http://en.wikipedia.org/wiki/Wingtip_device

 

[tmoritz]

The winglet angle of attack is a function of the wing tip vorticies. Change the loading on the wing and you change the circulation around the tip of the wing which will change the angle of attack on the winglet.

I haven't heard of adding winglets to improve climb rates. This is usually such a small portion of a mission. Maybe for something like Scaled Composite's Space Ship One where half of it's mission was climb profile and the other half you want high drag anyway.

As for lateral stability, only if it's located well behind the COG like on the Rutan canard designs.

Most winglets are found on long distance commercial jet aircraft and gliders. In both cases they are optimized for cruise, or in the case of gliders the best L^2/D.

Tom Moritz
Mechanical Engineer
US Bureau of Reclamation

 

[btrueblood]

Dunno, see the stuff about gliders, 737 and other short-haulers, and bizjets. Climbout is a big fuel hog, and on short hops can be a pretty significant part of the fuel budget (heaviest load, highest engine power settings).

The lateral stability bit comes up on the 747 Space Shuttle comments, but also in the glider section, and yes, regarding Rutan canard designs.

 

[tmoritz]

Your right btrueblood. They probably design multiple winglets so the customer can select the optimum for their flight profiles. I do recall on glider pilot at Harris Hill that had three wing tips, one straight and two winglets. He said something about selecting the tip based on forecast instability. Higher atmospheric instability means higher speeds and a different flight profile. This is my supposition though.

Tom Moritz
Mechanical Engineer
US Bureau of Reclamation

 

[Compositepro]

To my understanding primary purpose of wingtip winglets is energy savings by reducing wingtip vortex. Lift is created by horizontal wing surface. The vertical winglets simply block airflow from the wing tip toward the lower pressure over the wing, which creates the vortex.

 

[tmoritz]

Actually winglets serve many purposes, it extends the effective span of the wing without impacting the physical span of the wing. This means you don't have cut notches in your hanger doors when you buy the extended range 737.

They can be pitched slightly outward to reduce both bending moment on the wing and counter the wing tip vorticies.

The fact is, winglets have a sweetspot in regards to air speed and flight profile. If I recall correctly there was some attempt to construct a trimmable winglet so you could expand that sweetspot.



Tom Moritz
Mechanical Engineer
US Bureau of Reclamation

 

[Compositepro]

"it extends the effective span of the wing without impacting the physical span of the wing"

It does this by reducing wing tip vortex which causes loss of lift at tips of straight wings.

 

[rb1957]

"It does this by reducing wing tip vortex ..." yes,

"... which causes loss of lift at tips of straight wings." no; the wing tip vortex causes drag (= induced drag).

IMHO

 

[KENAT]

Oops, for most configurations I've seen the wiglets would add dihedral, not anhedral, with it's implications on Dutch Roll (from memory) and general stability. Sorry for the mix up.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[tmoritz]

Remember, the winglets are set at an angle to the wing. It's lift vector is more horizontal. You can have a winglet with an angle of attack that will generate a force outward which will counter the vortex generated by the circulation around the wing. An outward force generated above the wing axis will also generate a bending moment counter to wings normal bending moment. I suspect it's minor though.

I don't think roll has much effect. Side slip though...

Tom Moritz
Mechanical Engineer
US Bureau of Reclamation

 

[WKTaylor]

Guys...

Excellent engineering references on winglets (free download):

http://www.aiaa.org/content.cfm?pageid=2

search: winglet

>>MANY winglet design reports pop-up including theory, structures; dsgn for heavy acft, for GA acft, for sailplanes, etc...

http://www.dtic.mil/dtic/search/tr/index.html

search: winglet

Several good reports are available for public access....

Regards, Wil Taylor