Ralph Jones
(Top Gun)
09/20/15 09:00 PM
Re: Helicopter with robotic legs


Is that because of ground effect?

No, swashplate geometry. Moving the cyclic stick tilts the swashplate, and that varies blade pitch in the correct sequence to make the rotor disk parallel to the plate.

Say the fuselage makes an uncommanded pitch-down movement. The swashplate tilts along with it, tilting the rotor disk forward and producing a forward force component at the top of the mast -- which pitches the fuselage further forward.

Now put the rotor on the bottom. A pitch-down movement still rotates the swashplate forward, the rotor still tilts forward, and still produces a forward force component -- but it's acting below the CG and tends to pitch the fuselage up.

It would also demand a different control technique. With the rotor on top, to get into forward flight, you make a forward stick movement and then relax most of it; with the rotor on the bottom, you'd have to move the stick aft first, then move it slightly forward of center. Much like riding a unicycle.

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