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I am working on disaster relief drones for a college project. I was thinking about having multiple set of drones launched from an aerial vehicle over the disaster prone area. That's when I came across a big challenge.

How would you effectively decelerate a UAV that is air launched from an aircraft which is flying at a much greater speed than the maximum allowable speed of the UAV?

The stall speed of the aircraft is much larger than the maximum speed of the UAV (assuming, it has to be a very low cost UAV and hence the idea of having a parachute for deceleration is not very effective).

I would be glad if anyone could suggest me simple and robust ways to do achieve this.

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    $\begingroup$ What kind of UAV are we talking about? A multicopter or a fixed wing UAV. Are you afraid the UAV could take structural damage because of the speed or that the UAV might be uncontrollable at this kind of speeds? $\endgroup$
    – user33651
    Commented May 14, 2019 at 6:31
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    $\begingroup$ Use a helicopter. $\endgroup$
    – 60levelchange
    Commented May 14, 2019 at 6:37
  • $\begingroup$ What kind of drones are you thinking about? Commercially available off-the-shelf types, or custom self-built? $\endgroup$
    – bjelleklang
    Commented May 14, 2019 at 7:51
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    $\begingroup$ You should specify what you mean by "much greater speed than the max. speed of the UAV". This can mean greater than the max self-attainable speed of the UAS, which is not necessarily an issue, or greater than the $V_{NE}$ of the UAS, which would risk causing structural damage, and has no easy solution besides slowing down the mothership for this mission phase. $\endgroup$
    – AEhere supports Monica
    Commented May 14, 2019 at 11:01
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    $\begingroup$ "the stall speed of the carrier aircraft is about 0.25 Mach" in cruise configuration, I assume? I mean, it has to take off somehow, unless that too is launched from another mothership. $\endgroup$
    – AEhere supports Monica
    Commented May 14, 2019 at 14:49

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......the same way you decelerate any other kind of aircraft. Once released from the Mothership at idle power, it slows down until reaching an appropriate cruise speed thence power is increased to maintain speed.

If the release speed of too high for the UAV, one might use some kind of a frangible capsule equipped with a drag chute or other high drag device of some sort to enclose the drone during launch, thence the capsule fractures once the speed is slow enough, releasing the drone to begin flight under its own power.

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  • $\begingroup$ Based on the feedback I have obtained from this forum it seems to me that a parachute system for such an application is inevitable. Is there simply no other possible mechanical method to slow it down? $\endgroup$
    – CuthillMckee
    Commented May 14, 2019 at 11:51
  • $\begingroup$ What's wrong with a chute @CuthillMckee? They are inexpensive, reliable and easy to deploy using a static line system. Relief supply chute material tends to get repurposed by those on the ground into all sorts of life saving purposes like shelters, clothing and bags. $\endgroup$
    – GdD
    Commented May 14, 2019 at 13:02
  • $\begingroup$ @CuthillMckee airbrakes are a thing, but they are heavier and more complex, so for a one-time-per sortie application, a parachute makes more sense. $\endgroup$
    – AEhere supports Monica
    Commented May 14, 2019 at 14:50
  • $\begingroup$ Yeah basically a parachute or some other kind of detachable or retractable high drag kit which can slow the drone down to operating speeds prior to detaching or retracting. It need not take the form of a chute per se, but a chute is a simple and reliable option. $\endgroup$
    – Romeo_4808N
    Commented May 14, 2019 at 14:54
  • $\begingroup$ I acknowledge your suggestion for using parachutes. But I had the idea of utilizing the propeller (Propeller used for propelling the UAV) and let it windMill to generate drag and decelerate the platform. I am aware of some of the major drawbacks of this idea but I'm trying to soak in as many suggestions as possible to take a very informed decision. $\endgroup$
    – CuthillMckee
    Commented May 15, 2019 at 8:05
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One way might be to design the UAV to be deployed with flight control surfaces folded, use a parachute to decelerate, unfold the flight control surfaces at a safe speed and release the parachute.

Another would be to deploy the UAV inside some sort of canister that only falls away at a safe airspeed, letting the drone then fly away.

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    $\begingroup$ Thankyou for the reply. But letting the cannister freely fall would increase its speed due to the Earths Gravity pull right? It would end up accelerating the speeds probably even higher. $\endgroup$
    – CuthillMckee
    Commented May 14, 2019 at 11:49
  • $\begingroup$ A properly designed canister will slow down aerodynamically. You can also install a small drogue chute that only deploys a larger chute at the correct airspeed. Parachute deployment can also be controlled with deployment rings that prevent the chute from fully opening at too high a speed. $\endgroup$
    – Juan Jimenez
    Commented May 14, 2019 at 11:57
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    $\begingroup$ Thank you I'll keep that in mind. So a properly designed cannister might also decelerate the UAV! $\endgroup$
    – CuthillMckee
    Commented May 15, 2019 at 8:07
  • $\begingroup$ A canister that does not have its own propulsion will have no choice but to decelerate once deployed from your "aerial vehicle". What you have to consider is how to design the canister so it decelerates and allows safe deployment before it makes a hole in the ground. :) $\endgroup$
    – Juan Jimenez
    Commented May 15, 2019 at 10:15
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You can just let drag take care of it.

Granted this means that you need to have the drone sturdy enough to handle that headwind.

Too high speed is only a real problem when you go to maneuver the drone. Straight and level flight and gently adjustments is less of a problem.

The next option is to launch upwards in a climb and wait until the drone has bled enough speed that it can safely maneuver it's way down.

A set of speedbrakes in the tail can help keep the drone stable until the speed drops enough for normal flight.

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  • $\begingroup$ Being dropped includes going through the wake of the dropping aircraft. Gust loads and flutter will be unavoidable - there is no option to just fly straight and level when going through the wake. $\endgroup$
    – Peter Kämpf
    Commented May 14, 2019 at 21:08

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