>> When you push the cyclic forward, you move the swashplate in the
rotor to
>> increase the angle of attach of the blades in the back half of the
rotor
>> and
>> decrease it in the forward half. This causes the blades at the back
to
>> climb and the blades at the front to dive, creating a pitch down
moment
>> and
>> forward thrust.
>
> That's not correct. See:
>
> http://www.tpub.com/content/aviation/14018/css/14018_394.htm
<http://www.tpub.com/content/aviation/14018/css/14018_394.htm>
>
> "The spinning main rotor of a helicopter acts like a gyroscope.." [..]
"A
> downward force to the right of the disc area will cause the rotor to
tilt
> down in front." [..] "To simplify directional control, helicopters use
a
> mechanical linkage that places cyclic pitch change 90 degrees
> ahead
> of the applied force. Moving the cyclic control forward will
> cause
> high pitch on the blades to the pilot's left. At the same time, low
> pitch
> occurs on the blades to his/her right. This combination of forces
results
> in the rotor tilting down in front."
>
> or:
>
> http://www.repairfaq.org/filipg/RC/F_RC3.html#RC_010
<http://www.repairfaq.org/filipg/RC/F_RC3.html#RC_010>
>
> "In order to get the helicopter's rotor disk to tilt (for example)
> downward
> at the front, you increase the lift on the right side of the rotor
disk
> and
> decrease the lift on the left side of the rotor disk. (This is
assuming
> the
> standard clockwise main rotor rotation.)"
I've often seen these assetions, but as far as I'm aware it's not quite
true. The degree of "lead" required between the swashplate and the rotor
is pro****tional to the teeter stiffness of the rotor system. Thus for
helicopters with teetering rotors (good old Bell 47 etc) the lead is as
near 90 degrees as makes no difference, but for modern "Rigid Rotor"
helicopters the lead is much less. On the Westland Lynx the lead is only
10 degrees. A theoretical "ideal" rigid rotor need have zero lead on the
swashplate connections.
If you look around the toy/model shops you'll see a number of cheap RC
helicopters with twin two-blade, fixed pitch, co-axial, contra-rotating
rotors. These machines are interesting from a number of points of view.
Their control system has a direct connection from the swash plate to the
lower rotor for cyclic pitch, whilst the upper rotor is controlled
solely by a Bell-type flybar stabiliser. The rotors are usually "rigid"
(no teeter hinges) and so the only teeter freedom is the the flex in the
rotor blades themselves. Both the flybar (upper rotor) and the swashplate
(lower rotor) have 45-degree lead angles due to the relatively high teeter
stiffness.
PDR
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