Source: Flickr courtesy of: U.S. Air Forces Central Public Affairs, photo by Master Sgt. William Greer
Notice at the side edges, the wing peels down and back around towards the fuselage. (The airplane is upside-down in this photo.)
Never noticed this before. Is this a wing-tip device producing more lift? If so, what kind is it? I would like to read more about it.
The wingtip devices used in A-10 Warthogs are called drooped wingtips (also called Hoerner wingtips in some cases), which essentially increase the aspect ratio of the wing by forcing the vortices further out.
Source: zenithair.com
There are a few reasons for having this wingtip device:
The drooped wingtips act in a manner similar to the winglets and reduce the induced drag. As a result, the loiter capability of the aicraft, an important one for Close Air Support (CAS) aircraft, is improved. The A-X competition, that lead to the A-10 develpment, required the aircraft to have Two hour mission loiter time at max mission radius with 9,500 lbs payload
The drooped wingtip increases the local span loading near the wingtip, due to which the aileron response is improved, increasing the manuverability at low altitudes. The A-X competition required the aircraft to be Highly maneuverable below 1,000 ft
The drooped wingtips also improve the takeoff performance, an advantage as the aircraft is expected to be operated from short, forward fields. Infact, drooped wingtips are found in STOL kit of some aircraft, like DC-2 Beaver. One of the requiremens of the A-X competition was that the aircraft should have 4,000 ft takeoff distance at MTOW.
As can be seen, the drooped (down) wingtips wingtips serves these purposes.
There seems to be some confusion regarding the reason for these wingtip devices. One theory (@Peter Kämpf) is that they are there to protect the ailerons from ground contact. However, I think this is unlikely, given the ailerons, when extended, go well beyond the wingtips, as shown below:
Source: scienceforums.net
Dropped down wingtips do serve that purpose (sometimes) in gliders. However, there is no reason to believe that is the case here, considering the height above the ground and the aspect ratio. The drooped wingtips, on the other hand serve to hold the countermeasure system.
Source: rcgroups.net
It's called a drooped wingtip.
The general idea is to move the vortex away from the wing, reducing it's influence. As with all the winglet variants, opinions vary about whether or not it generates more lift than other variants.
There are more aircraft that employ this type of wingtip device:
If I add raised wingtips to one of my RC planes they significantly decrease the aileron authority, meanwhile improving the dihedral effect and the self-stabilizing tendency. But for maneuverability, it's not as good. If I add an autopilot, like Ardupilot, the raised wingtips will make harder for the autopilot to instantly correct the leveling when passing through turbulence, because the effect of the ailerons is dampened by the self-leveling tendency. Basically, the ailerons have to fight not only to change the attitude of the wing, but against it's tendency to rotate by itself, that might not act in the right direction when flying through turbulent air.
If I add dropped wingtips, they improve lift (the same wing is capable to carry more weight) and they increase greatly aileron authority, while decreasing passive auto-leveling.
The plane becomes harder to fly manually, but for the autopilot the effect is good, because the smallest deflection of the ailerons will have a big effect. The autopilot is capable to correct instantly the horizontal attitude of the wing, by moving the ailerons very fast, and in minute increments. The plane will look like it's flying on rails, even when it passes through turbulence. And the lower the altitude, the greater the turbulence, because the wind is moving over ground obstacles and it's not flowing laminarly. When the wind is strong and there are bushes and trees on the terrain, the turbulence can be so great, that a RC model can become impossible to fly low if it doesn't have some sort of electronic self-leveling device.
The conclusion is that, for a plane that has to fly low, dropped wingtips and an active self-leveling device are the best combination. If the plane was low wing - high CG, I would have used raised wingtips.
Oh boy, you guys are obsessed with that wingtip vortex, seemingly the source of all drag. No, the reason for this wingtip shape is much simpler.
It's about the protection of the ailerons from ground contact at low level flight. This is especially important for gliders with their central wheel, but also for aircraft which maneuver a lot at low altitude. They will benefit from something that takes the impact forces and keeps the control surface intact and working, especially when this surface will be deflected downwards at the time of ground contact.
Honestly, this is the reason, not some esoteric vortex shifting.
I thought of another one; if you're installing wingtips and it doesn't matter whether they point up or down, the latter will be less of an obstruction to the view from the cockpit, which is convenient for an aircraft also used for forward observation.
It's an interesting question! I think my answer wasn't mentioned yet?
Aeroalias, ROIMaison, et al all provide excellent explanations and I believe they're correct. However, I think there are two additional possibilities for this wing-tip design.
I have no specific knowledge that indicates the A-10 design considered these issues, but I suspect (educated guess by an Aero & Astro Engineer currently working with the A-10C) one, the other, or both may have been part of the design decision.
Note: the wing tip is very delicate and does not extend far enough to provide support during gear-up landings.
The wing tip devices aren't large enough to protect the decelerons
Or even from more modest aileron deflections
Even if the wing tip devices were larger, the wing dihedral keeps those wing tip devices farther above the ground, even during a gear-up landing, than the ailerons (as shown in the first picture).
Ground Effect
There's another reason for the wingtip "droop" (Hoerner Wing Tip) (note: other references for the Hoerner Wing Tip provide photos or diagrams of several different possible configurations) besides the improved wing efficiency from the winglet effect, namely ground-effect.
For fixed-wing aircraft, ground effect is the reduced aerodynamic drag that an aircraft's wings generate when they are close to a fixed surface. During takeoff, ground effect can cause the aircraft to "float" while below the recommended climb speed. The pilot can then fly just above the runway while the aircraft accelerates in ground effect until a safe climb speed is reached.
The A-10 was designed for short takeoff and landing operations. For this type of operation, landing in a short distance is harder than taking off (especially from primitive fields). Ground effect allows a plane to fly slower and remain airborne by providing a region of increased high pressure below the wing when it is close to the ground (low-winged aircraft benefit more from this effect than high-winged aircraft). This allows for lower landing and take-off velocities, which in turn allow for shorter take-off and landing distances.
The Hoerner Wing Tip helps trap this region of high-pressure air under the wing.
Improved Stability
Another possible reason for this configuration, which I discovered while linking references, was apparently the Hoerner Wing Tip helps reduce control instability in dutch roll for wings with dihedral.
This addition was done in order to counteract the dutch roll characteristic present in the original He 162 design, related to its wings having a marked dihedral angle.
I wasn't aware of dutch roll instability concerns in the A-10 design before the addition of the Hoerner wing tips but its wing does possess a dihedral. All images that I've seen of the YA-10A wing include the Hoerner wing tip.
