Wingtip Device Questions for aero folks..

jwp_145

jwp_145

GhostRider in the Sky
Hey folks I have a question concerning wingtip devices, fences, winglets, etc...

I lurk over in the Tech FOrums at a.nutters and one of the more respected members in the tech section is a flight test engineer at Boeing. He is very knowledgeable about aerodynamics, aero-engineering, and flight test and aircraft in general...

Occasionally he makes comments to the effect that certain wingtip devices on certain models of a/c are just there for aesthetics and have negligible positive effects on the aerodynamics of the wing.

A few models he has mentioned when talking about this is the 747-400, the Airbus A-32X series with the older style fences and some others that I cannot remember.
He justified in his explanation why this was true.

SO my first question is why would they even bother putting it on if only for aesthetic value in this world of high fuel prices?

My second question is, can anyone give me a good layman explanation concerning the thrust produced by a blended winglet? I've read in a few different places that due to the angles at which a blended winglet is installed with respect to the wing, due to pressure distribution around the winglet and the angle at which it is mounted that a certain component of thrust is produced in the rearward direction, contributing to the forward thrust of the a/c.
Can anyone clarify that for me please? I can understand technical talk, but it's easier to understand on the first passover if it is in layman's english!

Thanks for any help.
 
In layman's terms you can't make thrust unless you're burning dead dinosaurs. So to answer your question blended winglets don't make thrust, they mitigate drag.

Wingtip fences are a debate in if they are effective or not, but they do do something. The idea behind all winglets is to reduce/eliminate induced drag that is created by wingtip vortices. In basic terms the most efficient wings are wings that have no end/inifinite/in-a-windtunnel-go-wall-to-wall/not-possible-in-real-life. Winglets try and simulate that by reducing the area that the high pressure underneath a wing can "spill" over onto the low pressure on top of the wing. The idea behind fences was that it would simply creat a barrier in which that air couldn't go around. Well they kind of do their job, but they're not very big and as it turns out the bottom half of the fence really doesn't do anything at all because it's all high pressure on the bottom of the wing. So some engineers say fences eliminate more drag than they create, whereas other engineers think they create more drag than they eliminate. I was also told in one of my areo classes that on the guppies the fences help with stability somehow, but nonethless they're kinda break even.

The reason why blended winglets have caught on so well though is that they manage to take that higher pressure below the wing and seperate it from the low pressure on the high part of wing and create a pressure difference that creates a force (think lift) that is parallel in the direction that the aircraft is travelling. By using those fancy angles that people much smarter than myself came up with, they can take a force that would have been drag and now move it in a direction where it is assisting the aircraft.

So you can think of it like making thrust, but it's really not. Thrust only comes from the kerosene corn holes. However it is making the entire wing much more efficient.

P.S. Don't believe everything you read on a.net. Anybody can go online and say they're god's gift to airplanes and yet somehow a.net seems to bring out all the wackos. Just MHO
 
Interesting, but the use of the term "thrust" is actually not inaccurate. In actual fact, as was pointed out in a thread on this topic at http://flightwiseforum.webplus.net/forum.html (go to the aerodynamics section) you can see that the "The person who invented winglets, Richard Whitcomb, described their effect as producing forward thrust". I don't know about you, but I would be very hesitant to say that Whitcomb was wrong about anything!
 
I'll readily concede that my understanding of how wingtip devices offer an advantage is small. But I still have trouble with the statement that winglets produce thrust. Maybe it comes from how we're defining the boundaries of a system.

My understanding goes like this:
  1. In order to produce thrust you have to exchange momentum with the surrounding airmass.
  2. When the momentum exchange accelerates the freestream, that's thrust.
  3. Because the ideal "surrounding airmass" is homogeneous and equipotential, the energy to accelerate an airmass has to come from somewhere else; usually either by burning dinosaurs or by incurring an equal or greater momentum extraction from another part of the airmass (i.e., drag).
 
seagull said:
Interesting, but the use of the term "thrust" is actually not inaccurate. In actual fact, as was pointed out in a thread on this topic at http://flightwiseforum.webplus.net/forum.html (go to the aerodynamics section) you can see that the "The person who invented winglets, Richard Whitcomb, described their effect as producing forward thrust". I don't know about you, but I would be very hesitant to say that Whitcomb was wrong about anything!
Click to expand...

Mr Whitcomb is no doubt much smarter than myself, however to say a man single handedly invented winglets is saying a lot. Regardless if you invented something wouldn't you try selling it as best you could by saying that it can make something out of thin air?

Winglets are great, but saying they produce thrust all on their own is def. a spin to sell the idea to aero people.
 
crazyjaydawg said:
Mr Whitcomb is no doubt much smarter than myself, however to say a man single handedly invented winglets is saying a lot. Regardless if you invented something wouldn't you try selling it as best you could by saying that it can make something out of thin air?

Winglets are great, but saying they produce thrust all on their own is def. a spin to sell the idea to aero people.
Click to expand...

Whitcomb wasn't trying to "sell" anything. He was employed by NASA. Richard_T._Whitcomb
 
If you would like to think of it as lift in the aft direction instead of thrust, that would be fair. They do, however, produce a force that is in the same axis as thrust, and as such, are considered thrust when we do the calculations. While it might be semantics, to an aero guy, a force in the x axis is thrust, regardless of how it is produced

The "going online and say they're god's gift to airplanes" phenomenon is hardly limited to Anet. I see it here quite a bit too. If your answer starts with "I think" or "I heard" you are probably not the expert. There are experts on this board in just about every area. They just dont feel the need to answer every single post.

Less anyone ask, and without trying to blow my horn too loud, I am one of the test pilots for Aviation Partners and have a MS in Aero. I also know the FTE on the ANet site, and he does not post stuff that he is not sure of.

Jim Young
(dont feel the need to hide behind screen names)

crazyjaydawg said:
In layman's terms you can't make thrust unless you're burning dead dinosaurs. So to answer your question blended winglets don't make thrust, they mitigate drag.

P.S. Don't believe everything you read on a.net. Anybody can go online and say they're god's gift to airplanes and yet somehow a.net seems to bring out all the wackos. Just MHO
Click to expand...
 
I tried to go back and edit this, but cant seem to. Just to be clear crazyjaydog, the below part of my post is not directed at you, but is a general comment.

Sorry for any confusion

Jimflyfast said:
The "going online and say they're god's gift to airplanes" phenomenon is hardly limited to Anet. I see it here quite a bit too. If your answer starts with "I think" or "I heard" you are probably not the expert. There are experts on this board in just about every area. They just dont feel the need to answer every single post.
Click to expand...
 
Once again, Google:

winglet.jpg


It's a wing with the relative wind being the vortex. As it proceeds from the bottom of the wing to the top of the wing, it produces a relative wind that creates a nice AOA on the winglet. Since lift is produced perpendicular to the relative wind, it gives an inward and slightly forward force. Hence a slight amount of thrust.

Also:

Winglet%20Diagram%202.jpg
 
What I don't like is that the bottom diagram states "large vortex, higher drag", "smaller vortex, less drag". That leads some to think that the drag reduction is a result of the vortex reduction, rather than the fact that the vortex reduction is a result of the drag reduction. In other words, it's putting it backwards.
 
Jimflyfast said:
I tried to go back and edit this, but cant seem to. Just to be clear crazyjaydog, the below part of my post is not directed at you, but is a general comment.

Sorry for any confusion
Click to expand...

No offense taken. It's partly my fault as this topic isn't something easily communicated over the interwebs.

To summarize my points:

-Mr. Whitcomb was a genius, I just feel like he liked to toot his own horn a lot (some rightfully so, but sometimes a litttle over the top).

-Winglets don't *independently* create thrust. As I tried to explain before, and obviously earned a major fail, was that it takes the force of the vortices and directs it in an axis parallel with the fuselage. I feel like in layman terms that kinda bush league to call it thrust because if the airplane is sitting on the ground it won't just start moving forward. Likewise if you turn off the blow driers under the wing the airplane will still become an aluminum lawn dart.

-I am not much of an aero guy (did it for undergrad but certainly not my area of expertise) so take everything I say with a big grain of salt.

-Finally yes this website has some dbags, but I think other websites can be much worse. Not knocking a.net as a site because I would rather look at that than porn, but the forums there seem to draw out a lot of ppl that take a few pictures and all of sudden understand everything. I don't personally know this engineer from boeing and I'm def not knocking him either. Just trying to explain winglets in a simple way that makes sense to me FWIW.
 
Jimflyfast said:
If you would like to think of it as lift in the aft direction instead of thrust, that would be fair. They do, however, produce a force that is in the same axis as thrust, and as such, are considered thrust when we do the calculations. While it might be semantics, to an aero guy, a force in the x axis is thrust, regardless of how it is produced

The "going online and say they're god's gift to airplanes" phenomenon is hardly limited to Anet. I see it here quite a bit too. If your answer starts with "I think" or "I heard" you are probably not the expert. There are experts on this board in just about every area. They just dont feel the need to answer every single post.

Less anyone ask, and without trying to blow my horn too loud, I am one of the test pilots for Aviation Partners and have a MS in Aero. I also know the FTE on the ANet site, and he does not post stuff that he is not sure of.

Jim Young
(dont feel the need to hide behind screen names)
Click to expand...

Good statement there... I have understood all of the basic concepts for winglets and other wingtip devices for a long while now, and understand some of the more abstract concepts about them as well, and I know about the infinite wing schtuff and all of that, so I dind't want to sound like I dind't know about any of it.
THe main bit that was needing clarification was the x-axis force production part.
 
ANd I understand that this force isn't just being created out of thin air... I know that it is just an already-existent force that is simply being redirected.

So the one bit that hans't been answered... if the 744 and 32X wintip devices don't do much, why are they there?
 
NJA_Capt said:
One word for you guys......SAILBOAT.

http://yachtpals.com/wing-sails-7066
Click to expand...

Well, I don't know if I would compare a winglet to a sailboat... A sailboat requires wind for power, sure, but the interaction of the water on the keel in combination with the wind on the sail is what produces the thrust. In other words it takes two very different fluids acting on to very different "wings" (the sail and the keel or hull) to make a sailboat move. Take away either one and the sailboat won't work.

With winglets, or airplanes in general, there is only one fluid. The air.
 
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