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I figure there are limitations to wingspan while on the ground but could we engineer wings that stay aloft or get towed to allow a plane to 'dock' with them in order to fly higher/longer/more efficiently?

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    $\begingroup$ In such a case why not fold them? $\endgroup$ – mins Feb 13 '17 at 23:55
  • $\begingroup$ Interesting question. Rather than docking with wings, I wonder if researchers have given any thought to linking lightweight aircraft together for atmospheric research. Helios is a NASA aircraft that would seem to be a likely candidate. nasa.gov/centers/dryden/history/pastprojects/Helios/index.html Lots of smaller aircraft linked together and the payload launched by a conventional aircraft and then attached to the group. $\endgroup$ – JScarry Feb 14 '17 at 1:44
  • $\begingroup$ @mins, well I remember a couple of F-8s Crusaders' pilots managed to take off with the wings folded without accidents... :-) $\endgroup$ – Marco Sanfilippo Feb 16 '17 at 16:48
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    $\begingroup$ Seems backwards to me. Wouldn't you want the larger wingspan on takeoff & landing, but not for high-speed cruise? As with variable-geometry fighter aircraft? Or the B-70's wingtips that drooped in high-speed flight? $\endgroup$ – jamesqf Feb 16 '17 at 20:29
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    $\begingroup$ @Joe: But my question is why you'd want longer wings in flight. Other than specialized applications like sailplanes. $\endgroup$ – jamesqf Feb 18 '17 at 1:21
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Actually, a similar concept was tested by the USAF in the 50s (like most things in aviation, it seems). Under the name of Project Tom Tom, the military was looking into the possibility of coupling a parasite fighter (an F-84 in this instance) to a B-36 as a long-distance escort as it flew into the Soviet Union by joining the two aircraft together at the wingtips. Flying as such, the F-84 would actually power down its engines during coupled flight and could receive fuel from the B-36. However, the aspect with greater relevance to the question is that the B-36 would actually derive an aerodynamic benefit from this arrangment as well. In theory, the combined aircraft would also benefit from having an artificially elongated wing as well, and, while I remember reading last year that this actually did result in superior fuel efficiency for the combined aircraft during cruise, I am at a loss to find that source right now. At any rate, with respect to this question, the B-36 effectively docked with a pair of "wing extensions" (in this case, a pair of F-84s) while in flight and derived a benefit in cruise. A video of the concept can be found here:

. Also, more reading can be found in this text Flying Aircraft Carriers of the USAF: Wing Tip Coupling by Brian Lockett.

Naturally, there were significant downsides to this idea. While the F-84 actually did manage to engage the coupling mechanism on the B-36 on test flights, logging at least a couple hours of continuous, coupled flight, significant stability and safety issues were uncovered during testing that led to the scrapping of the project. As noted in a quote from the project summary report:

The wing-tip coupling mechanism as developed and flight tested in the Tom-Tom program is not considered satisfactory for tactical use and was barely acceptable for test flying. It appeared that a new difficulty and a now shortcoming were uncovered on virtually every test flight. The extent of rework necessary before the coupling mechanisms as designed were acceptable as flight test articles is covered under the DESIGN section of this report.

Similarly, issues with aeroelasticity turned out to be a much larger issue than originally expected (in part because of the change pitching/heaving forces as well as wing mass distribution and stiffness at the joint). I think that Aurora Flight Sciences ran into similar issues when they were designing an entry for DARPA's Vulture project...the idea was to join multiple vehicles (that were essentially flying wings) to create a super-efficient, high-altitude, solar-powered, surveillance aircraft. Due to conflicting requirements on wing structure and aircraft weight, I think that ultimately didn't fly...but I don't have any of the details for that. Just what I remember from a presentation I saw many years ago.

At any rate, I guess that my answer is: yes, it could. But it is VERY difficult and dangerous to do (at present).

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    $\begingroup$ The question did ask about docking wing extensions to an aircraft while in flight, which this could be considered as a method to do so and illustrates the challenges thereof. In fact, that was one of the motivation for the specific docking methodology used in Project Tom Tom. In both projects mentioned, the wing extension would need to be a controllable vehicle (for the docking concept), making this answer applicable, I'd think. I did not post this to illustrate an efficient solution (if there is one) but to illustrate the difficulties and attempts at doing what the questioner desired. $\endgroup$ – Marius Feb 18 '17 at 18:46
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    $\begingroup$ Oh ok -- that makes sense. But I think we're looking at this from different angles. I'd hoped to express that the B-36 is the aircraft that docks with the wing extensions (e.g., the F-84s) and thereby benefits from increased wing area, higher aspect ratio, etc. (being a practical example of previous attempts related to the OP's question), and not the benefits conferred to the F-84s. It is the B-36, after all, that gets the efficiency bonus by gaining the wing extensions and flies the whole trip. I could make that more clear in an edit. $\endgroup$ – Marius Feb 19 '17 at 2:49
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Why dock in flight? It would be simpler to use telescopic wings, like the fs-29 did. It had a fixed center wing and moveable outer wing gloves which could slide in and out. Wingspan could be varied between 13.3 and 19m this way. The idea was to reduce wing area and drag for fast flight between thermals and extend the wing for slow flight when gaining height in thermals.

Three-side view of the fs-29
Three-side view of the fs-29 (picture source)

fs-29 in flight with wings extended
fs-29 in flight with wings extended (picture source). Note the slightly longer chord of the outer wing panels.

The pilot had to move a lever in the cockpit repeatedly back and forth to move the wing. The concept worked well in theory, but hit a snag in practical life: When moving in, air had to be pressed from the outer wing which would impede fast movement. Much worse, however, was that on the way outward the wing would need to be filled with air and would suck itself stuck on the inner wing. The resulting increase in friction made pilots leave the wing at 17 m span for the rest of the flight after having "pumped" it in and out two or three times.

This experience made Schempp-Hirth abandon their half-built prototype of a glider with a telescopic wing. Somehow, nobody thought of adding winglets with rudders which would had made it trivial to create the side force necessary to move the wing gloves.

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  • $\begingroup$ The refueling is when I though it would take place. I originally thought this would be a neat idea to have an F-35 or F-22 dock into wing extensions to be able to do missions the U2 used to perform. I agree though, this is a dynamic and complex engineering task with a lot to consider. $\endgroup$ – Joe Feb 16 '17 at 15:06
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I suppose you could attempt to design a system that could be towed to altitude by another plane and then the special aircraft could dock with it using procedures similar to in-flight refueling.

Now the question I don't think you asked but should be addressed is whether this would be feasible. I would expect that it isn't for several reasons:

  • Attaching to a moving object while in flight is not easy task. It would require special training by both the pilot of the base aircraft and the operator of the tow mechanism.
  • Adding a complex system to attach the wing extensions would add weight and cost to the specially designed aircraft
  • Attaching wing extensions to the base aircraft would change the flight characteristics of the aircraft while in flight, which could prove costly and difficult/impossible for safety certification
  • Adding wing extensions wouldn't necessarily increase flight ceiling/range/efficiency. You would have to perform the necessary analysis to show that the improvement in flight capabilities are greater than the added weight and cost.
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  • $\begingroup$ Not sure why the downvote, is there something I can do to improve this answer? $\endgroup$ – selectstriker2 Feb 16 '17 at 18:46

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