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DrPaul
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Winglets and Raked Wingtips — A Couple of Questions

Fri Aug 30, 2019 2:38 pm

Most airliners these days have either winglets or raked wingtips. I have a couple of questions. Why are they a relatively recent innovation, and why were the benefits of fitting them not realised earlier on, when jet airliners were first introduced in the 1950s?
 
GalaxyFlyer
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Re: Winglets and Raked Wingtips — A Couple of Questions

Fri Aug 30, 2019 3:04 pm

Aviation’s conservatism. The idea dates back to the earliest days of flight. Then, in the 70’s, Richard Whitcomb at NASA started experimenting with them. Fuel efficiency was a driver post-73.

https://en.m.wikipedia.org/wiki/Wingtip_device
 
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DocLightning
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Re: Winglets and Raked Wingtips — A Couple of Questions

Sat Aug 31, 2019 5:28 am

In addition to what GalaxyFlyer said, the benefits of these devices are relatively modest, in the range of 5%. So it took a fair amount of research (time) to develop wingtip devices that would maximize benefit while minimizing weight and additional drag. Keep in mind that these devices increase the lift on the outboard section of the wing, so spar strengthening (ads weight) is necessary. Sharp turns between the wing and a nonplanar winglet (like the 744 winglet) cause interference vortices (drag) and all of these devices increase overall wetted area. So careful design was necessary to minimize the added weight and drag while maximizing the additional lift.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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WIederling
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Re: Winglets and Raked Wingtips — A Couple of Questions

Sat Aug 31, 2019 11:49 am

GalaxyFlyer wrote:
Aviation’s conservatism. The idea dates back to the earliest days of flight. Then, in the 70’s, Richard Whitcomb at NASA started experimenting with them. Fuel efficiency was a driver post-73.

https://en.m.wikipedia.org/wiki/Wingtip_device


Henschelohren :-)
then you have the Let L-13 Blanik and PZL-101 Gawron
Wingtip tanks / shock bodies go in a comparable direction.

Hoerner Tips
Dr. Sighard F. Hoerner, his work straddles WWII and the post WWII time
Murphy is an optimist
 
GalaxyFlyer
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Re: Winglets and Raked Wingtips — A Couple of Questions

Sat Aug 31, 2019 1:56 pm

Yes, Hoerner wingtips were used in Part 23 planes long before winglets were tested and used on jets. The Lear 28 Longhorn was possibly the first Part 25 design.

GF
 
BravoOne
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Re: Winglets and Raked Wingtips — A Couple of Questions

Sat Aug 31, 2019 2:12 pm

It's interesting to note that Delta put winglets on one of their 727-200's and ran it for almost a year as I recall. They decided not to move forward and do the entire fleet as there were concern' that later in the life of the airplane there might be some structural issues passed on the downstream owners and thus legal exposure.
 
GalaxyFlyer
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Re: Winglets and Raked Wingtips — A Couple of Questions

Sat Aug 31, 2019 2:21 pm

Yes, the structure can be a factor. For example, the less vertical winglets on CL350 can’t be used on CL600 series for structure reasons.
 
WIederling
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Re: Winglets and Raked Wingtips — A Couple of Questions

Sat Aug 31, 2019 4:34 pm

GalaxyFlyer wrote:
Aviation’s conservatism.


no strong interest. Design was dominated by keeping the things in the air.
Fuel was a mass/volume limited resource, not money limited. :-)
Murphy is an optimist
 
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DrPaul
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Re: Winglets and Raked Wingtips — A Couple of Questions

Sun Sep 15, 2019 3:47 pm

Thanks for the replies.
 
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trpmb6
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Re: Winglets and Raked Wingtips — A Couple of Questions

Sun Sep 15, 2019 3:51 pm

DocLightning wrote:
In addition to what GalaxyFlyer said, the benefits of these devices are relatively modest, in the range of 5%. So it took a fair amount of research (time) to develop wingtip devices that would maximize benefit while minimizing weight and additional drag. Keep in mind that these devices increase the lift on the outboard section of the wing, so spar strengthening (ads weight) is necessary. Sharp turns between the wing and a nonplanar winglet (like the 744 winglet) cause interference vortices (drag) and all of these devices increase overall wetted area. So careful design was necessary to minimize the added weight and drag while maximizing the additional lift.


Actually, it's not so much maximizing additional lift but reducing induced drag (a function of lift). Induced drag is reduced by artificially increasing aspect ratio.
 
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DocLightning
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Re: Winglets and Raked Wingtips — A Couple of Questions

Tue Sep 17, 2019 2:25 am

trpmb6 wrote:

Actually, it's not so much maximizing additional lift but reducing induced drag (a function of lift). Induced drag is reduced by artificially increasing aspect ratio.


Both planar and nonplanar wingtip devices increase lift on the outboard span. A planar wingtip device, like the Boeing raked wingtip, adds lift outboard of the main span and distributes the overall lift over a larger span, which decreases the overall induced drag. A nonplanar wingtip device (a winglet) increases lift just inboard of the device and helps to move the axis of vorticity above and aft of the main span, which reduces induced drag.

A nonplanar device introduces less overall bending moment on the inboard spar, but it is less structurally efficient. As a rule of the thumb, a three-foot nonplanar winglet will offer the benefit of a two-foot planar wingtip extension, but will only add the bending moment of a one-foot span increase.
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smithhaddon123
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Re: Winglets and Raked Wingtips — A Couple of Questions

Tue Sep 17, 2019 5:42 am

What is the difference bewteen a planar wingtip and simply increasing the span?
 
WIederling
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Re: Winglets and Raked Wingtips — A Couple of Questions

Tue Sep 17, 2019 7:46 am

DocLightning wrote:
trpmb6 wrote:

Actually, it's not so much maximizing additional lift but reducing induced drag (a function of lift). Induced drag is reduced by artificially increasing aspect ratio.


Both planar and nonplanar wingtip devices increase lift on the outboard span.


That is IMU slightly off.
"faking more span" allows to increase lift on the outboard span with less drag penalty.
( see wing twist change on A350 in conjunction with "adjusted" sharklets.)
.. you don't get away from higher center wing box loads though.
Murphy is an optimist
 
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Starlionblue
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Re: Winglets and Raked Wingtips — A Couple of Questions

Tue Sep 17, 2019 7:50 am

smithhaddon123 wrote:
What is the difference bewteen a planar wingtip and simply increasing the span?


The rake affects the wingtip vortex differently.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Armadillo1
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Re: Winglets and Raked Wingtips — A Couple of Questions

Tue Sep 17, 2019 8:10 am

smithhaddon123 wrote:
What is the difference bewteen a planar wingtip and simply increasing the span?

https://en.wikipedia.org/wiki/Wingtip_d ... ed_wingtip
for example

also
https://en.wikipedia.org/wiki/Wingtip_d ... ar_wingtip
if you want to dig into terminology and nomenclature
 
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DocLightning
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Re: Winglets and Raked Wingtips — A Couple of Questions

Tue Sep 17, 2019 7:16 pm

Starlionblue wrote:
smithhaddon123 wrote:
What is the difference bewteen a planar wingtip and simply increasing the span?


The rake affects the wingtip vortex differently.


So here's the deal with raked tips:

A simple span extension becomes an unacceptable design because the leading edge needs leading edge devices. That far out, the wing becomes too thin to realistically install leading edge devices. This means that a non-raked outboard section would be prone to stall at high AOA (landing and takeoff) and that stall could act a lot like flutter and make the crew begin to respond to the situation as such.

Boeing figured out that by increasing the leading edge sweep on that last outboard section, this improved the stall characteristics at high AOA (which is why Concorde didn't have leading-edge devices), but there was still a tendency to stall. Normal high-subsonic wings have a relatively sharp leading edge, but it turns out that a blunter leading edge with a very high sweep doesn't have the same kind of drag problems that the main wing's leading edge would and also fixes the stall problem. So now you have an outboard segment with a thin profile, a blunt leading edge, and a high degree of wing sweep.

The issue is that that high sweep of the leading edge is going to intersect the trailing edge pretty quickly, limiting the length of the planar wingtip device. So the solution is to increase the sweep of the trailing edge so that the leading edge and trailing edge intersect further outboard, allowing for a longer device. This has an additional advantage. Because this device is now on average aft of the main wing, as is its center of lift, if a gust should hit the outboard segment, it will weathervane a little bit to a more negative AOA, which provides gust unloading.

So the advantages of a planar wingtip extension (raked tip) are that it provides more lift increase/drag reduction per unit of length than a nonplanar tip and it provides gust unloading. The disadvantages is that it moves the overall center of lift outboard and thus increases bending moment on the inboard spar, so it makes sense to design a new wing from scratch for these devices, rather than reinforcing the spars to add these devices as an aftermarket option. In addition, the increased span becomes an issue for ground handling. Boeing has gotten around this last issue with the folding wingtips on the 778/9.

A planar wingtip device does not increase the bending moment as much (but it still does) and does not increase the span as much (but it still does) but it needs to be about 50% bigger to achieve the same benefit.

A discussion of most of these points is found here, complete with diagrams: https://patents.google.com/patent/US5039032A/en
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
smithhaddon123
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Re: Winglets and Raked Wingtips — A Couple of Questions

Thu Sep 19, 2019 9:54 am

Thank you very much Doc Lightning, that's an awesome explanation, but did you mean non-planar here?

DocLightning wrote:
...A planar wingtip device does not increase the bending moment as much (but it still does) and does not increase the span as much (but it still does) but it needs to be about 50% bigger to achieve the same benefit...
 
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DocLightning
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Re: Winglets and Raked Wingtips — A Couple of Questions

Thu Sep 19, 2019 2:41 pm

smithhaddon123 wrote:
Thank you very much Doc Lightning, that's an awesome explanation, but did you mean non-planar here?

DocLightning wrote:
...A planar wingtip device does not increase the bending moment as much (but it still does) and does not increase the span as much (but it still does) but it needs to be about 50% bigger to achieve the same benefit...


I did mean nonplanar.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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