Question:
Winglets have become quite popular as of late for smoother flying and increased (?) fuel economy. Why do fighter aircraft not have them? Increased costs of retrofit? Or are they simply not a good idea for fighters? And why are winglets at an approximate 45 degree angle?
Answers:
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First of all the "winglets" do not nescesarily make for smoother flying. They do however increase effieiency of lift on wings (and make the flight more stable). The economy part (if at all) is in the fact that it doesn't take as much power to "keep" the aircraft flying", - they affect the "turbulence" at the wingtips to some degree, - which I suppose might drop resistance co-efficient a small amount! You will aslo see that a lot of aircraft,-especially homebuilt (experimental) have "vertical fins"that are "vertical" at wingtips. There are wingtips that "droop down" also, - these are especially useful in "short takeoff and landing aircraft" (STOL). These "trap" air under the wing when the aircraft is close to the ground,- this is called "ground effect". When airplane is close to ground - air is "jammed" in between the wing and the ground, - this makes the aircraft able to stay airborne at a lower speed (or lift off ground)! Naturally the shorter the takeoff run, the "smaller" the runway can be, - and airplane "free of ground friction" will accelerate faster, - so that the airplane can pick up flying speed and climb out! On landing the reverse happens, it can stop shorter, if it is going slower at touchdown! By the way these are very expensive, as the FAA's approval for such things can cost millions of dollars (because of testing required for "certification"), - therefore the expense for buying and fitting these "special parts" is very high! In aviation, you can't just "throw" anything that "looks good" on an airplane and fly it!B-52 stratofortress motivation?
As for "retrofitting", - there are a number of factors here, --not all wings will accept the extra "drag, and stress" that the winglets will cause! Not all wingtips are "terminated" with sufficient "structural integrity" to be suitable for securing same! The wingtips are the "least strong" part of the wing, - so lighter material, and less internal structure is used there! Also winglets do not lend themselves to all types of aircraft perormance, - meaning that to gain something in one area, you might loose something in another area! The stress on a fighter plane would be many times greater than a "standard category" aircraft, becacuse of "friction " caused by speed, combined with stress of combat maneuvers, - combined with "G" forces that occour in combat flight!..... Note: "general and commercial aircraft" crew and passengers are not required to wear "pressure suits" to keep them from blacking out when flying!! Strucrurally - fighter airplanes are built stronger, -but there is a limit to how much "structure" you can build in, -- as the airplane can get so heavy it won't perform properly (if it can get off the ground at all)!
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................. NOT A GOOD IDEA .............. its useful on a straight level flight of a longer duration ..................... where economy matters ...... whereas fighter has not much of an endurance and trajectory of flight is unpredictable .............. it is on a mission ........ and not on economy on a mission ...................... therefore under high G factors no use ......................What airliner had 4 engines on the tail?
Winglets work by changing the spanwise load distribution on a wing, which yields a virtual aspect ratio increase. Higher aspect ratio is usually better, but comes at a cost as it makes the structure heavier (a thinner wing is not as structurally efficient as a thick one), so very large aspect ratio will be found in gliders for instance.At the other end of the spectrum are fighters aircraft with typically very low aspect ratio, which is better from an agility point of view, and that is what fighter aircraft are after: agility is preferred over long range endurance.
The relatively modest gain in long range that winglet allow on airliners makes sense, but on a fighter even a 10% gain in range is not as important as being able to outmaneuver an opponent.
As for the angle, it is to go along with the general sweepback of the wing, to delay the onset of shockwave as airliners typically fly at a speed that makes the air get to the speed of sound at some point on the wing; having a proper sweepback reduces the drag that comes with the shockwaves.
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Winglets only do their work on the back-side of the power curve (where induced drag is most prevalent). This includes take-off, climb, and cruise at high altitudes.What is the requirement to take the ATPL written in the US?
High speed fighters operate on the front-side of the power-curve where winglets would slow them down.
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