ruizmilton
Heliman
Location: Carolina, Puerto Rico, US
My Posts This: Topic Forum | FB vs FBL, phasing, stability ETCDave:
This is the explanation I mentioned on the previous post I gave a fellow Heli Pilot:
"This is what I can tell you from my multiblade experience, in most respects I felt that response around the centering of
the cyclic seems more sensitive compared to a flybarred head, yet that's where it ends, meaning that after you bank or
pitch a few degrees from center the fast response ends, the machine had more of a "float" feeling compared to a flybar head,
and dissimetry of lift (retreating blade stall) is more noticeable, not that it is something that does not happens to the
flybar head, but the flybar will compensate for that making you feel like it does not happen. Same goes for the "ballooning",
some flybar heads still exhibit this tendency, yet in most it is compensated properly by the flybar, so I guess it all goes
back to really understanding how a heli flies and applying this understanding to the way you fly.
Aside from what a flybar stabilization mechanism will do, everything else is the same. A stabilization system provides the
same correction ability of a flybar by reducing dissimetry of lift and balloning, but if you understand how the machine flies
and learn how to adjust there is no need for them. A mixer allows you to change swash timming, below you'll see you can do
that in your computer radio with no need for an external mixer. What a mixer does provide is the ability of mixing a gyro
output to three controls, that is important ONLY if you want to have stabilization on a CCPM system where all comands are
shared by all three head servos as opposed to a single servo collective where elevator is one servo and aileron is one as
well.
Most issues related to multi blade set-ups are related to swashplate phasing, there are two reasons:
1)The more blades you have, the less space is available on the rotorhead and swashplate to accomodate the required 90 degrees
from swash control ball to blade holder control ball, this happens on rotorheads with 5 blades or more. This can be corrected
by swash mixing to compensate for where your pushrod will be located in relation to the machine's centerline on fore/aft or
left/right orientation in order to compensate for the required amount of degrees to reach 90. In a 5 blade head you only have
72 degrees between each blade, so in order to have all pushrods perfectly vertical to avoid binding and non-linear response,
you cannot achieve the 90 degrees required between the swash ball and the blade holder ball, but by properly mixing aileron
to elevator and elevator to aileron you can "rotate" the swash timing so that a command happens 18 degrees later (or earlier)
from the centerline of the fuselage. If it has to happen earlier or later depends on wether you have a leading edge or
trailing edge controled blade holder and your rotor's rotation direction (CW or CC). Any computer radio can do this, there is
no need for an external mixer.
2)Tendency to run head at a lower RPM than required to stabilize, every head/blade combination is different and the minimum
specific RPM required to achieve stability on the system has to be reached before the head will respond to the inputs on the
90 degress due to gyroscopic precession . This RPM/phasing issue does not happen on flybar heads because your swashplate does
not directly control the blades, the flybar does, the flybar is mechanicaly positioned at 90 degrees to the main blades
(up/down travel of a paddle will always be at 90 degress from a main blade) and is controled aerodynamically, meaning that
AIR FLOW controls it, so there is no gyroscopic precession de-phasing, PLUS, there is stability added by the flybar, meaning
that control input will always affect it in the correct phasing, try to picture it and you'll see what I mean. If the rotor
head would have responded to an input with a 45 degree lead due to low RPM(you give forward cyclic but head goes forward and
left) the fly bar would give a "right" correction to the blades, but no correction to the "forward", left and right would
cancel each other and you would only get a forward response.
Makes sense????" |
