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If two or more quadcopters are aligned, would the effect of drag due to the lead quadcopter's slipstream lead to measurable energy savings? And if so, how would one determine the optimal distance between the quadcopters?

My thought are that some energy savings are possible, but that there must be some distance between the quadcopters to avoid the high pressure air behind the lead quadcopter interfering with the quadcopter that's behind. How great this distance is depends on the tilt of the lead quadcopter. Since the drag force is proportional with the square of the velocity this means that energy savings should be larger at high velocity, but unfortunately the tilt of the quadcopters will also be higher causing the minimal distance needed between the quadcopters to increase.

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  • $\begingroup$ My uneducated guess is, no, it would not. Quadcopters are going to produce a lot of turbulence around and behind them (as you have noted), making it more difficult or impractical for those following to fly efficiently. Otherwise, helicopters and airplanes would already be taking advantage of the benefits. $\endgroup$
    – SnakeDoc
    May 30, 2018 at 22:46

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Yes, drag can be reduced by formation flying.

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Geese using a low drag "V" formation. The authors of a 2001 Nature article stated that pelicans that fly alone beat their wings more frequently and have higher heart rates than those that fly in formation. It follows that birds that fly in formation glide more often and reduce energy expenditure (Weimerskirch, 2001).

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Any flying object (plane, bird, helicopter, etc) that creates a down-wash to create lift must also produce an opposite and equal up-wash. A trailing aircraft positioned properly can ride this "wave".

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Research aircraft flying a test point (low drag) for the Autonomous Formation Flight project over California's Mojave Desert. From the NASA Dryden Flight Research Center Web site.

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The goal of the Autonomous Formation Flight project was to demonstrate 10 percent fuel savings of the trailing aircraft. The project extended the symbiotic relationship of migrating formations of birds. In December 2001, an F/A-18 flying in the wingtip vortex behind another F/A-18 exhibited a 14-percent fuel savings. The project ended in late 2001

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    $\begingroup$ But this is all for fixed wing aircraft. I imagine that the dynamics are very different for quadcopters. Perhaps some results for helicopters flying in formation would be more applicable. $\endgroup$
    – ROIMaison
    Jun 4, 2018 at 10:58
  • $\begingroup$ A bird is not a fixed wing... It's thrust is closer to a helicopter than a propeller or jet. $\endgroup$
    – jwzumwalt
    Jun 4, 2018 at 22:28
  • $\begingroup$ Birds and planes use the vortex surfing phenomenon which is not applicable to quadcopters because of the abscence of wings. $\endgroup$
    – GreenIris
    Jun 5, 2018 at 10:45
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    $\begingroup$ Rotating "wings" are affected by vortexes the same as stationary wings. The leading blade is traveling faster than a stationary wing, while the retreating blade is slower. However, It is likely that as much advantage comes from positioning in a low pressure area created by the bow wave, as vortexes; similar to NASCAR drafting. $\endgroup$
    – jwzumwalt
    Jun 6, 2018 at 11:18
  • $\begingroup$ When quadcopters are flying fast they will need a large tilt to provide the forward thrust. Wouldn't you think that the airflow and high pressure this produces causes interference for the follower? $\endgroup$
    – GreenIris
    Jun 10, 2018 at 16:57
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Body of the second quadcopter will experience less drag. Theoretically, energy savings are there for sure.

However: - Slipstream must not interfere with the second drone's propellers, because the stability of flight will be severely hindered. - Since you will want to keep the distance between the leading drone and the following drone as small as possible, additional measures must be taken to prevent collision.

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