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In a quad setup like the Air One where back propellers are at different ground clearance than the front propellers, what are the effects of such setup vs if all props had the same ground clearance.

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For the purpose of this question let's asume the back props are at double the front props clearance as shown in the image. Also let's asume the aircraft weight is perfectly balanced.

For a straight vertical takeoff, will back and front props have to spin at different speeds? Will all props generate the same lift at the same RPM during takeoff? Does the RPM needs to be adjusted on the front/back propellers to keep the aircraft balanced as the aircraft increases altitude(no ground effect)?

Yes, I know the Air One example technically is not a quad but a Octacopter, just ignore that.

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Propellers and rotors are subject to ground effect: lift increases when the rotor approaches approximately within 1 diameter distance. This is the reason why a landing helicopter manages its own touchdown to a degree: the closer the rotor approaches the ground, the more vertical speed deceleration.

enter image description here

Pic above is from this answer which describes the rotor ground effect mechanism. The figure is from Principles of Helicopter Aerodynamics by j. Gordon Leishman, the next figure 5.36 shows a quantification of increase in lift in ground effect at constant power, showing about a 10% increase in lift at a distance of 1 rotor diameter.

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This aircraft sits at a nose down attitude on the ground. It's propellers are clearly tilted forward.

For a straight vertical takeoff, thrust has to be vertical. This means before taking off, it has to rotate while still on the ground, around main landing gear wheels as a pivot point. A reaction force with the ground is needed to prevent it from moving forward. Therefore it needs brakes applied during this phase.

Once at an attitude where propellers' discs are parallel to the ground, it can takeoff vertically like any other quadcopter, adjusting rpm like any other quadcopter, and there is no noticeable ground clearance difference between front and aft propellers anymore.

This video shows how it's performed, and attitude during stationary flight. One can visually notice front propellers increasing rpm relative to aft propellers for that rotation purpose.

(rotation is at 12 seconds, takeoff at 20)

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  • $\begingroup$ Oh, thanks for the video, so basically it starts by adding more power to the front props, then once the clearance is equal (@ 00:16) all props run at the same power. Interesting. $\endgroup$
    – Gabe
    Commented Oct 1 at 13:01
  • $\begingroup$ +1 but there is likely more to explain, as the neutral attitude is not with the propellers at the same height (compare with the horizon)., perhaps more efficient for forward velocity, and the wing providing some lift. $\endgroup$
    – mins
    Commented Oct 1 at 13:16
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    $\begingroup$ @mins OP is about ground clearance, ground effect (Koyovis answer) and vertical takeoff with no horizontal velocity. Probably the attitude it sits on the ground is for rolling takeoffs with wings creating lift, and wing incidence w.r.t prop disc angle is optimized for lift and drag during cruise . Difference in height while hovering is small, and there's probably smth interesting too regarding that angle between each pair of contrarotating props (those are not exactly coaxial) $\endgroup$
    – user721108
    Commented Oct 1 at 15:23

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