In a story I am working on, the characters need a fixed-wing drone that can rendezvous with a blimp, where by "rendezvous" I mean "come within a very short distance at low relative airspeed". The challenge is that the blimp will typically be flying through powerful gusts and abrupt wind shear—ideally hurricane-like conditions or worse. The characters have the time and resources to design and build a custom drone for this purpose, and while I realize that I will probably have to stretch credulity at some point, I'd like the drone's design to at least seem plausible. Hence my question: What would a drone custom-built for such missions look like?

Here's what I found so far (and which I hope is accurate):

  • Wing loading is the single largest factor in gust response. If the only concern were dealing with the wind, the drone would have as much wing loading as could be managed. However, I also need the rendezvous to happen at low (blimp-relative) airspeed.

  • A long wingspan is a liability in wind shear, increasing the chance for an asymmetric stall and upping the rolling moment even if there is no stall; the drone should probably have its wings tucked tight to its body.

  • Given the risk for sudden stalls, the wings should be designed with substantial washout. They will probably not be swept (in either direction), both because drag reduction is not a driving consideration and because swept wings would tend to push the drone nose-up in a stall.

  • Anhedral wings, while less good for stability, would cause the drone to bank into crosswinds, which could be useful given the feedback delay the drone pilot has to deal with, and some of the stability could be made up by placing the wings high, creating a pendulum effect. But maybe stability is the more important factor here.

  • An inverted tail will cause the drone to both roll and yaw into crosswinds. I am not sure whether an inverted T or inverted V would make more sense.

I have also found some planes that are intentionally flown into hurricanes: the Lockheed WC-130, the Lockheed WP-3D Orion, the Gulfstream IV, the Douglas DC-8, NASA's ER-2 (a Lockheed U-2 derivative), and the AAI Aerosonde, the only drone on my list. The last, at least, backs up some of the ideas I have above.

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    $\begingroup$ If you remove requirement for fixed-wing i would suggest you to try blimp drone. $\endgroup$ – vasin1987 Mar 22 '18 at 12:34
  • $\begingroup$ @vasin1987 I agree that that would make a lot more sense, but it would also require reworking the plot, so I wanted to at least see if I had any chance of hand-waving something fixed-wing. $\endgroup$ – Edward Mar 22 '18 at 12:56
  • $\begingroup$ The blimp would have far more problems under those conditions. But a great source of design would be seagulls, as they have a much lower weight to surface area ratio than a/c. A seagulls in 30 mph winds would be an excellent model. They have anhedralled wing tips to reduce roll in side gusts, and finely "brain computer" controlled adjustments to hold them steady in flight. $\endgroup$ – Robert DiGiovanni Nov 13 '18 at 17:05

First, kudos for the well-researched question! You certainly did your homework before coming here.

As you realized yourself, your requirement is quite contradictory. I also wonder what blimp would be flown into hurricanes, but I leave that aside for the moment.

In order to reduce gust susceptibility, a variable configuration would be one option. However, just when you want steady flight in gusty conditions, you also will fly slowly, so variable wing sweep or elaborate wing flaps will not help. The next option which combines low flight speed and high wing loading would be some direct lift, like in the VTOL-designs of the 1960s. Since the direct lift is needed for a short time and when propulsive demands are low, an engine like the Rolls-Royce Pegasus would allow to add lift that is less affected by gusts than the wing.

Make sure your design will draw ample bleed air in order to feed the reaction control system, and maybe a gyro platform to control this system, and the contradiction can be resolved.

Stalls should now be less of a concern, too. Much of how much lift is lost in a stall depends on the choice of airfoils, and if you use one with benign stall characteristics, stalls can be tolerated.

Anhedral is a poor idea, however, especially if you use unswept wings. Only if you have a high wing on a large fuselage can anhedral be justified. Examples are the Harrier jump jet and the F-104 Starfighter, where the large T-tail made wing anhedral necessary. You want moderate dihedral in order to make the aircraft roll in a sideslip. This rolling motion will tilt the lift vector sideways and pull the aircraft away from the sideslip condition.

Also, please reconsider whether there really is such a thing as a pendulum effect.

Regarding the tail: The best one is as symmetrical as practical.

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  • $\begingroup$ Dear Peter: big difference between anhedralled wing tips and net anhedral. Also when one has much larger surface area to weight as in bird vs 747, obviously aerodynamic effects will be more dominant. Look at a seagull. Wings are dihedral/anhedral. Very effective in controlling roll with side gust. Similar to "Lippisch Ears". As far as pendulum (when NOT turning), yes high wings have more self righting tendency. But as this is enjoyably academic and interesting, much is learned from all opinions. $\endgroup$ – Robert DiGiovanni Nov 14 '18 at 3:20
  • $\begingroup$ Do agree with symmetrical tail. Perhaps a modern Taube might not be so bad. $\endgroup$ – Robert DiGiovanni Nov 14 '18 at 5:17
  • $\begingroup$ @RobertDiGiovanni: Great, you know what Lippisch Ears are! But then you must know that they were added to the He-162 as a kludge when it was too late to reduce wing anhedral. So I wonder what the big difference between them and wing anhedral is. And you are sure about the better self-righting tendencies of high wings? I don't think they make much of a difference. $\endgroup$ – Peter Kämpf Nov 14 '18 at 18:05
  • $\begingroup$ Witnessed a 10 page discussion of this on another thread, so, I will say look at seagulls. As far as high wings, it is actually a "double" pendulum. Any displacement of Clift and CG from sky/ground orientation creates a righting moment to restore it not unlike the ballast of a sailing ship. Lift goes up, weight goes down. Combine that with balance of area above and below CG from side forces (to prevent rolling from side force) you have stable high wing trainer. Anhedralling is also seen in horizontal stabilizers to improve turn performance. $\endgroup$ – Robert DiGiovanni Nov 14 '18 at 18:34
  • $\begingroup$ No big difference between kludge and anhedral. We know dipping wing helps in cross wind. $\endgroup$ – Robert DiGiovanni Nov 14 '18 at 18:35

Consider the "freewing" concept -- each wing is free to pivot, pitch-wise -- as described here https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700014919.pdf -- google "freewing gust alleviation" for much more-- unfortunately "freewing" is now also the name of a popular manufacturer of radio-controlled model airplanes, so that will complicate your search.

The flexibla nature of the wing structure used in hang gliders and "trikes" (powered hang gliders with wheels) also has some natural load-shedding characteristics. In addition to the properties created by flexibility, these aircraft can also be considered "freewings" in a way, although the left and right wing panels do not move independently in all but a very few cases.

(A little late now-- is the book finished yet?)

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