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01-17-2006 | 03:08 PM

#24

William Robison

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From: Mary Esther, Florida, FL

RE: OS FF-320 pegasus

Mongoose:

Quote:

ORIGINAL: RSL_Mongoose
...basically the same as a Heli... heli in hover you will notice that it always tilts toward the right...

Not really.

Let's assume, as you said, the rotor turns clockwise looking from the top. This makes the torque reaction tend to turn the fuselage to the left, and the tail rotor has to push to the right to keep the fuse from turning. Then, since this side thrust is at the tail end and there's no matching thrust in front, the net effect pushes the entire heli to the left. Tipping the rotor cone to the right allows a stable hover, without going either way.

This effect is also why some heli instructors advise putting a block under the left skid while learning; with the bird slightly tipped to the right it's felt that taking off into a hover is more easily learned.

In forward flight another little nasty comes into effect, adding to the uneven aerodynamic forces noted in a hover. To an extent it somewhat cancels the hover conditions.

In forward flight there is a difference in the airspeed of the leading and trailing blades. Let's posit a 38: rotor diameter, and a head speed of 1800 rpm. This gives a rotor tip speed a hair over 200 mph. In a hover, all blades "See" the same air speed, but then we transition to forward flight. At a forward speed of 50 mph, the leading blades have an effective tip speed of 250 mph, while the trailing blades have slowed to 150 mph. Naturally, this transfers the center of lift to the left side of the rotor cone, and there's your right turn tendency. An adjustment of the cyclic compensates of course, just the opposite of the correction needed for a hover.

Way off topic for this thread, but I hope it helps.

Bill.

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