Here is my theory, but remember I know nothing and am making a lot of assumptions.
I understnad that winglets help by forcing potential airflow that might be lost by going off the sides of the wings to go over the wing, giving it more lift.
The raked wingtips don't go up, so I don't think that they have that purpose, which is weird that it's compared to winglets on the 737-800.
But here's what I figure. You have a longer plane (209 feet), which is obviously heavier and you need all the help you can get to compensate. Extending the wings makes sense, but I guess you want to keep any extensions as light as you can, so instead of jsut extending the wing, you reshape the extension, which would probably be lighter, and you still get that extra lift.
I'm sure I'm wrong and stupid, but oh well.
Any input from you smart guys? Tell me about the raked wingtips!
Gigneil From United States of America, joined Nov 2002, 16215 posts, RR: 88 Reply 1, posted (9 years 10 months 2 weeks 5 days 16 hours ago) and read 5794 times:
The high angle of the wingtips give you the at least some of the same effect as an increased wingspan without making the plane incompatible with existing 767, DC-10, or L-1011 sized gates.
Delta made it very clear that its 764s needed to fit in tight spaces, which is a large part of why the 764 isn't the world's most effective aircraft... its a bit big for its wing. The raked wingtips mitigate that problem.
They're also lighter than the 747 or 330/340 wingtips.
The 330 doesn't fit in 767 sized gates, which severely limits its possible markets or causes logistical nightmares at some crowded airports with insufficient facilities.
Goboeing From United States of America, joined Jun 2000, 2635 posts, RR: 12 Reply 3, posted (9 years 10 months 2 weeks 5 days 13 hours ago) and read 5741 times:
Winglets are designed to reduce the negative effects of vortices that are found behind the tips of an airfoil when air passes over it. If a never-ending wing was made (an infinite wingspan), there would be no vortices. But at the end of a wing, it creates drag. In ground effect, which occurs when the plane is about one wingspan high or less, these vortices don't complete their entire "circle" and are reduced, thus reducing total drag and making the plane more effecient, and perhaps making it "float" and cause a prolonged flare. In flight at higher altitudes, these vortices are not reduced by anything, and that's what the winglet tries to do.
I'm not exactly sure if the raked wingtip has a different goal, but I'm sure part of it is to reduce vortices like winglets do. They sure look good though!
Goboeing From United States of America, joined Jun 2000, 2635 posts, RR: 12 Reply 5, posted (9 years 10 months 2 weeks 5 days 12 hours ago) and read 5704 times:
Ground effect is a phenomena that occurs when a plane of any size descends to within approximately a wingspan above the ground. So a 747 will enter ground effect before a J-3 Cub will. When the plane's in ground effect, it's more effecient. This is due to the lessening of "Wingtip Vortices."
Wingtip vortices are what cause "Wake Turbulence." Like wake from a boat in the water, the air behind the wingtips of a plane gets disrupted and turbulent. Air flows over a wing the same until you look towards the tip, when it's different. I can't really explain why it's different, but it is. And if you were to look at a wind tunnel that can show the flow of air over a wing, you'd see that, behind the tips, it's like a cyclone moving backwards and parallel to the body of the plane. It's like tipping a tornado on it's side. This cyclone effect happens as long as air is moving over the wing. As the wing (plane) comes in to land, for example, the cyclone itself is disrupted by the ground. Where the vortex would normally spin back and down from the wingtip, it's now hitting pavement. So it's reducing the entire thing. That means less drag, since vortices create drag.
Flaring is what planes do when they are in the last stages of flying down the approach. It's what you see after the expressway turn to 31 is complete, they're past your dock, and over the threshold. The nose comes up even farther and the main gear is about to touch. That maneuver is called the flare.
A "floated" landing is the opposite of a quick, abrupt touchdown. In a floated landing, the plane probably came in too fast. If you have flight sim on your PC, try flying down to the runway in a 172 at 130kts. Cut the power, and the plane won't want to land, because it's going way too fast. So a floated landing is basically a prolonged touchdown. This is bad on a short runway for obvious reasons. In fact, planes decelerate about 3 times faster on the ground than they do floating down the runway (I forget what plane was used in that example I read somewhere, but it's the same idea for any plane).
When the reduced wingtip vortices take away drag, the plane is flying more effeciently. Remember, thrust has to overcome drag for the plane to accelerate. When thrust is low on landing, any reduction in drag will make a difference. Less drag will keep the plane in the air longer, causing it to "float" down the runway.
Back to wingtip vortices, the winglet reduces them. So you can see how all this comes together. The less drag, the faster a plane can go, or if it flies the same speed, it can fly at it with less thrust because the winglet has taken away that little bit of drag that trailed from the wingtip.
I assume raked wingtips are designed to do the same thing, although I am not certain. But reducing drag and thus reducing fuel burn as well has got to be one of their primary reasons for being put on the end of a wing.
Sims747 From Canada, joined Aug 2003, 19 posts, RR: 0 Reply 8, posted (9 years 10 months 2 weeks 5 days 2 hours ago) and read 5651 times:
Air flows over a wing the same until you look towards the tip, when it's different. I can't really explain why it's different, but it is
As some of you may (or may not) know, a wing produces lift by creating an area lower pressure over the wing's curved surface than the area under the wing. Think of the atmosphere as being composed of an infinitesimal amount of minuscule air particles. An air particle from a region of high pressure always wants to go to an area of lower pressure. This same rule applies to the aircraft wing, an air particle in the higher pressure region under the wing, wants to go the lower pressure region over the wing. Over the whole wingspan, it is impossible because there is a wing in the way!! However, a the tip of the wing, the air particle has a path to join the lower pressure region at the top of the wing (it can "curve" around the wing tip and join the area of lower pressure). This will induce a rotating motion at the wingtips (clockwise on the left wing and counter-clockwise on the right wing when looking from the back of the aircraft) which combined with the forward speed of the aircraft will create a "vortice" which was described accurately by Goboeing. Winglets help in the sense that they provide an obstacle to the particle of air trying to reach the lower pressure region, therefore reducing "vortices" which are a direct contributor to induced drag (drag caused by lift, due to the fact that the vortices have drag component).
MITaero From United States of America, joined Jul 2003, 497 posts, RR: 9 Reply 9, posted (9 years 10 months 2 weeks 4 days 23 hours ago) and read 5600 times:
This is from http://cf.alpa.org/internet/alp/2001/feb01p22.htm:
Walk-around inspection revealed some interesting aerodynamic changes. First are the raked wingtips (see photo). Boeing considered installing winglets on this model, but found several competing considerations. The objective was to recover the performance lost by increasing the maximum gross weight of the airplane. Increasing the wing area (gaining lift) would enhance takeoff and climb performance, while decreasing drag (using winglets) would recover performance over a long period of time at cruise.
The existing wing could not support the loads that winglets would impose, so Boeing chose the raked wingtip design. Set at 57 degrees of sweep, the raked wingtips add 7 feet 8 inches to each wing and account for 74 square feet, or 2.4 percent, of the total wing area. The raked wingtips reduce balanced field length, make possible a higher initial cruise altitude, and still reduce fuel burn by 1 percent versus the basic wing design. Now, if the world’s tallest catering truck manages to wrinkle one of your wingtip extensions, you can still operate by removing both wingtips. They are designed to be removed easily, and the flight can continue to a repair destination with the wingtips MEL’d.
Vikkyvik From United States of America, joined Jul 2003, 8316 posts, RR: 28 Reply 11, posted (9 years 10 months 2 weeks 4 days 7 hours ago) and read 5512 times:
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not to get too scientific here, but i remember some of this stuff from a basic flight mechanics class....
increasing the wingspan of any aircraft has a couple of purposes. first of all, obviously, the lift is increased due to the increased surface area of the wing. granted, the weight is also increased, but that depends on materials, shape of the airfoil, etc.
secondly, increasing the wingspan decreases the induced (or vortex) drag, which is caused by the vortices which have been explained in previous posts. to get a bit more scientific, the coefficient of induced drag (a non-dimensional measure of the induced drag), is as follows:
coeff. of induced drag = ((lift coefficient)^2)/(pi*e*AR)
where e is the span efficiency factor, which is a constant, and AR is the aspect ratio, which is defined as the wingspan (b) squared over the wing area (S)....that is....AR=b^2/S
basically, if you work it out, as the wingspan gets larger, the induced drag decreases, which reduces the overall drag. i don't remember the actual derivations, but you can see that increasing the wingspan would have a double purpose in increasing lift and decreasing drag. as a side note, planes with a large aspect ratio can generally attain higher altitudes with equal power(ex. the U-2).
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Lehpron From United States of America, joined Jul 2001, 7028 posts, RR: 22 Reply 12, posted (9 years 10 months 2 weeks 3 days 21 hours ago) and read 5457 times:
This is what I figure, they aren't just raked (a word I see as offensive -- i think of a garden rake for God's sake), they are little delta-wings that use their steep leading edge to produce extra lift with the induced drag from the wing, i.e. vortex lift. You know what they say about Concorde, apply that to this wingtip and we have a similar effect.
I think the use of deltas at the tip manipulates the strength and size of the induced flow by getting something out of it while loosing it.
If winglets were CDR's then delta-tips are the CDRW's -- that's how they relate.
I could be somewhat off but this has been what I thought, in fact i took it several steps further than this, no prob.
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MD-90 From United States of America, joined Jan 2000, 8422 posts, RR: 13 Reply 13, posted (9 years 10 months 2 weeks 3 days 16 hours ago) and read 5404 times:
Here's the thing about wingtips:
Rounded tips (think of a British Spitfire) are draggy. For optimal airflow around the tip, you need a sharp, straight edge. It doesn't really matter if there's a slight curve up or down (think lightplanes, like the Cirrus SR20 or Lancair 300).
However, the edge should trail backwards, not forwards. Imagine if the leading edge was farther outboard of the trailing edge. That would be bad, bad, bad. There's probably some kind of factor that I wouldn't know without seeing the numbers, but that's why the 764's wingtip extensions look like they do (I'm not sure why they're that swept, I'd think Boeing couldn't gotten off with less sweep [think 777] but hey...).
MITaero From United States of America, joined Jul 2003, 497 posts, RR: 9 Reply 14, posted (9 years 10 months 2 weeks 3 days 16 hours ago) and read 5412 times:
Do you have any theoretical explanation/data to back up the edge argument (just wondering)? Also, the Spitfire wing was actually ideal in terms of planform (elliptical lift distribution) but too hard/expensive to mass manufacture.
Vikkyvik From United States of America, joined Jul 2003, 8316 posts, RR: 28 Reply 15, posted (9 years 10 months 2 weeks 3 days 14 hours ago) and read 5401 times:
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i'm not sure how one would use raked wingtips to increase lift using the vortices. the vortices in general create a downwash on the upper surface of the outer wing, decreasing the apparent angle of attack, and hence lift. also, i'm not sure if this is what MD-90 was saying or not, but swept wings are useful due to the apparent velocity that the airfoil sees. if one were to fly straight wings at airliner speeds, mach shock waves would develop at much lower speeds, increasing drag significantly, and thus preventing the airplane from flying as fast. swept wings (whether they're swept forwards or backwards) reduce the apparent velocity seen by the airfoil, and thus increase the airplane velocity at which the local air velocity above the wing will be supersonic. thus, airplanes can fly up to around mach 0.85 without seeing a significant mach drag increase. however, there are probably stability aspects to the swept-forward wing that prevent it from being widely used.
"Two and a Half Men" was filmed in front of a live ostrich.
PPVRA From Brazil, joined Nov 2004, 8497 posts, RR: 42 Reply 16, posted (6 years 9 months 3 weeks 2 days 19 hours ago) and read 4039 times:
Sorry to bring this thread back from years ago, but it answered most of a question I was about to post. . . but not all of it.
So I see how it made sense for Boeing to use raked wingtips for the B764. It was the best solution given the restrictions on the wing size and extra re-enforcement (=extra weight) that would have been needed.
However, tell me about the B777- why not go with winglets? Was MTOW a problem--performance not quite what was promised to the airlines, therefore the choice for raked wingtips instead? Had the B773 anything to do with the choice (MTOW)?
Or maybe just an experiment? Did raked wingtips possibly offer better economics than non-blended winglets, which maybe were not technologically available or efficient at the time of design?
The reason I thought of this is because the B787/A350 don't make use of it and I was wondering why.
Thanks for any insight.
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LTU932 From Germany, joined Jan 2006, 13864 posts, RR: 51 Reply 17, posted (6 years 9 months 3 weeks 2 days 11 hours ago) and read 3962 times:
Quoting PPVRA (Reply 16): However, tell me about the B777- why not go with winglets? Was MTOW a problem--performance not quite what was promised to the airlines, therefore the choice for raked wingtips instead? Had the B773 anything to do with the choice (MTOW)?
I don't think performance had anything to do. The 777 family is known to be overperforming, meeting and even exceeding performance promises made by Boeing during development.
I believe the 764 also served the purpose of being a bit of a testbed for the 773ER, by it using raked wingtips and not regular or blended winglets.
For a first-order analysis of the benefit from a winglet, lay it down flat as if it was a span extension. Hence, taller is better. Then, look at whether the winglet joins the wing in a sharp break or a smooth flow. Smooth is better.
Using this criteria, I'll rank the winglets (and raked tip):
1) The A3100/310/318/319/320/321/380 "winglets" are the least effective, but also increase wing bending moments the least. I have never seen a published number for how much they reduce drag (even Jupp's patent US4,714,215 says little), but I'd guess they are worth 1-1.5% drag improvement.
2) The MD-11 winglets come next. They are a direct outgrowth of Whitcomb's original designs. I have some NASA reports on the flight testing of a similar winglet on a DC-10. According to CR-3704, these winglets are produce a 2-2.5% drag reduction in cruise.
3) The Valsan/Quietwing 727 winglets are a latter Whitcomb influenced design. Coupled with a flap droop on the 727, they are reputed to be worth about 5%. The flap droop reduces wing bending moments and helps reduce wave drag, so the 5% is a mixture of effects. The winglet by itself is probably worth about 3%.
4) The 747-400 and A330/340 winglets are rather similar. They fall on the wing span extension line. I've read that the 747-400 winglets produce about a 3.5% drag reduction.
5) The 776-400ER/777-300ER/777-200LR raked tips also fall on the span extension line. They produce varying amounts of drag reduction, depending on their span. They also increase wing bending moments quite a bit.
6) The API/APB blended winglets produce better results than the span extension line indicates. The 737NG winglets are first generation and I've read that they produce about a 4.5% drag reduction. Later API/APB winglets improve on this.
Dakota123 From United States of America, joined Aug 2006, 104 posts, RR: 0 Reply 21, posted (6 years 9 months 3 weeks 1 day 19 hours ago) and read 3809 times:
Quoting BigPhilNYC (Thread starter): 5) The 776-400ER/777-300ER/777-200LR raked tips also fall on the span extension line. They produce varying amounts of drag reduction, depending on their span. They also increase wing bending moments quite a bit.
Is the increase in wing bending moment due to added structure or aerodynamic loading?
Lehpron From United States of America, joined Jul 2001, 7028 posts, RR: 22 Reply 22, posted (6 years 9 months 3 weeks 1 day 18 hours ago) and read 3801 times:
God, my last response from three years ago made no sense to me at all, lol.
Quoting AA777 (Reply 20): Its pretty simple, they create lift (winglets dont), while reducing drag- like winglets.
Careful, for most airplanes with a swept back wing, as it bends up to lift, it also twists downwards because of the backward sweep angle. As a result, the wingtip device lifts inwards, upwards and forwards due to the twist and bend of the wing. The drag reduction comes from the forward lift component.
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