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V-Bat is a VTOL aircraft built by a company called Martin UAV.

It seems to have a single pusher propeller. But then why does it not counter-rotate to balance the rotation of the propeller?

Any clear images of the mechanism or report would be great.

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    $\begingroup$ From what I see there are moving blades downstream which are possibly used to control the aircraft and offset the torque of the rotor. $\endgroup$
    – sophit
    Mar 1 at 5:20

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Right underneath the prop (highlighted in red) there is a set of control surfaces (highlighted in blue).

enter image description here

it's very reasonable that these get set to counteract the induced rotation of the prop in addition to being a discount vectored thrust mechanism.

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Looking at the video of it, the V-BAT appears to have a fixed pitch propeller for simplicity, directly driven by the a small 2-stroke drone engine.

Two issues with counter-rotating props:

  • You now have to have a gearbox for the propellers to drive two concentric propeller shafts in opposite directions. Quite a bit of added weight and complexity. Which means you'll need a bigger engine to boot. And away you go on the weight/complexity design merry-go-round (added weight > more power needed > more fuel because of the thirstier engine > oops more power needed because of extra fuel > oops need to make the prop gearbox heavier > oops need more power > oops need more fuel... you get the idea).
  • Stacking two propellers close together is very inefficient, if the propellers are fixed pitch, because the downwind propeller has to work in its sister's wake all the time, with the flow conditions always changing. It'll have to be set at a different pitch, and even then it'll never make as much thrust as the one above it. And you'll STILL have to use anti-torque vanes for precise control, so you're not really eliminating that mechanism.

So much easier, simpler, lighter, to use anti-torque vanes large enough to counter the torque of the single propeller, for a machine that's kind of quasi-disposable anyway.

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  • $\begingroup$ Don't forget that contra-rotating rotors do have also some advantages like they are more compact (which might be a must-have in some design) and the vanes would be used basically only to control the aircraft instead of antitorque+control so they can be made more compact too. $\endgroup$
    – sophit
    Mar 1 at 15:48
  • $\begingroup$ Of course, but the weight and complexity of a gearbox and the need to use variable pitch negates most of that. The anti-torque demands aren't that high with a prop that small anyway. $\endgroup$
    – John K
    Mar 1 at 19:59

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