From: nlapposNOSPAM@miami.gdi.net (Nick Lappos)
Subject: Re: Rotor Question
Date: 24 Nov 1999
John Eacott wrote:
>Back to Brett's theory about a blade flexing at the tip, and
>reducing/eliminating the need for lead/lag damping, if I understand your
>proposal Brett, I can't see it working. The mechanics of a flexing tip
>would be horrendous.
>Since this is cross posted to rec.aviation.rotorcraft, someone like Nick
>from Sikorsky may be able to pick up the thread and offer a better
I missed the original question, but perhaps can offer some insights into the
general discussion of blade lag motion, and dampers.
Blades want to lead or lag for two reasons, one is the conservation of
angular momentum (already well discussed in another post above - ice skater
brings arms in to whirl faster) and the other is the natural reduction in drag
that occurs as the blade changes in airspeed and angle of attack as it rotates
around the mast. We articulate the blade and allow it to lag so that the
stresses at the hub are lower, since a strong lag force (the force can be a
lead force, too, of course) can't be transmitted through the lag hinge. Lower
stresses save weight and drop the vibration level.
This lag force is caused by the individual changes in blade drag as speed and
feathering angle are changed. A teetering rotor ties the blades together to
achieve simplicity of head design, at the expense of vibration (in plane
forces that are not releaved by lagging) and weight (beefier grips needed to
absorb the force).
Why doesn't the blade just lag back against the rear stops when we increase
pitch (and therefore drag)? Because the centrifugal force (physics jocks cut
me some centripital slack, OK?) tries to make the blade stay at 90 degrees to
the hub, and that force is awesome. Typical CF for an S-76 is 33,000 pounds.
To get the blade to lag 3 degrees back, the CF will fight with a restoring
lead force of 1700 pounds.
A lag motion has very little natural damping, unlike flapping motions.
Flapping pushes a fat, flat blade up or down, so the air provides some natural
resistance, and flap motions don't tend to oscillate at high frequencies.
Also, the natural 1 per revolution resonant flapping is a nice thing, since we
actually control the rotor with it - cyclic pitch excites this resonance
(remember the famous post about resonant cyclic versus gyro precession!!) Lag
motions slide a skinny blade into or against the wind, with little damping as
a result. This means that an articulated blade will "hunt" or oscillate back
and forth and produce low frequency vibrations that the pilot will feel as a
lateral or longitudinal vibration, typically at about 2/3 per revolution
frequency. We put a damper on the blade, a hydraulic cylinder that absorbs
energy to damp the oscillations like a screen door closer. We can also use
rubber laminates to absorb the energy, like the Eurocopter heads or the
Any rotor can show some lag frequencies, even without lag dampers, because the
blade bends (flexes) in all directions. The wigglier the blade, the more
likely the motions. That's why the rigid rotor blades are so stiff, to push
these lag and flap frequencies well above the natural rotor frequency of 1 per
revolution. We actually tune the blades in flight test to drive these
frequencies away from natural excitement, using strips of stiff carbon fiber
laid up on the blade. Exciting stuff to put the aircraft into an
extreme load factor maneuver during shakedown and feel the rotor start to
vibrate! We can usually pick out the frequencies before the
engineers in telemetry see them on their squiggly charts. If the vibrations
are high enough, our dentist can tell, too.
Sometimes, the BO rigid family can show some low frequency lateral vibe,
especially if the blade grips are slightly worn. This is a weakly damped lag
oscillation that is someimes called "air resonance." Modern bearingless
rotors are not rigid, but rather "soft inplane" rotors with natural lag
frequencies below 1 per revolution. These rotors need dampers, which are
usually buried in the blade root cuff, like the EC-135 and the Comanche.
One strong factor for the need for dampers is the natural coupling of the
rotor blades angle with the lag and flap motions. If the blade is hinged so
that as it lags there is a strong change in feathering angle, the lag
oscillations can be supressed or made worse. We call that Alpha One coupling,
and some increase in pitch as the blade lags aft is nice and desirable.
Some other time I'll blather about BERP tips and swept tips. Gotta go to work
(is it work?)