UniCopter ~ Trim, Stability & Control - Musings on Trim

Overview:

Some random thoughts related to Trim, Stability & Control and the use of 'Absolutely' Rigid Rotors.

Longitudinal Trim Solutions: ??

Hover:

I think that in a flapping rotor helicopter, the axis of the rotor disk, as viewed from the tip path plane, is the lift vector. In hover this disk will be horizontal and the fuselage will have a nose down attitude [Θ] because the center of mass, which is slightly infront of the mast on the body's X-axis, wants to be located directly below the center of lift.

.I think that in a absolutely rigid rotor helicopter, the thrust vector will be normal to the rotor disk but its location on the disk will vary depending on the cyclic that is being applied. In other words, the collective will determine the length of the vector and the cyclic will determine the location of the vector on the rotor disk.

The drawing assumes that the axis of parasitic drag and the C.G. are coincident. If the axis of drag is aft of the C.G. then the Thrust will be located between the two.

The lower of the above sketches is also in;- Trim - Longitudinal Solution

 

Forward Flight:

I think that in a rigid rotor helicopter,-----

The drag of the tail (including the boom) should exceed the drag of the fuselage so that forward cyclic is required to hold orientation [Speed Stability].

The lower of the above sketches is also in;- Stability - Static - Longitudinal

Without drag the cyclic could be advanced to initiate forward flight and then returned to the neutral position to maintain the speed. the thrust would have to be increased of course.

H-force will probably be neutral in respect to pitching but the fuselage's parasitic drag will cause a pitch down and the horizontal stabilizer's parasitic drag will cause a pitch up. The thrust, weight and drag should all be on the X-Z plane (symmetrical). The horizontal stabilizer's parasitic drag should increase slightly faster than the fuselage's parasitic drag as the downward pitch is increased. This will give positive stability since the cyclic must be advanced to give more downward pitch.

Alternatively, a precone of 4º to 5º of the 'absolutely' rigid rotor may eliminate or significantly reduce the size of the horizontal stabilizer.

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Last Revised: March 5, 2007