BLADE FLAPPING
BLADE FLAPPING
(OP)
I have had a troubling question ever since I became interested in rotary flight and I know I must be careful with words so as not to be miss interpreted here. My understanding is that the major secret that Cierva uncovered was that the movement of the rotor blade in an up and down motion as it spins in the relative wind (flapping) is what allows a spinning rotor blade to overcome what is called dissymmetry of lift.
My question is: In the various stages of flight, hovering, level flight, turning, how much deviation in inches would the tip of lets say a rotor of 20 foot length (10 feet on each side) actually move up and down as it makes each individual revolution? I would also like to know what the average maximum angle of that tilt would be. I would also like to know what these figures would be for a model helicopter with perhaps a 50 inch rotor.
Many thanks to any of you engineers who can supply me with the correct answers to this puzzle.
Jerry Plottner, Canton, Ohio
My question is: In the various stages of flight, hovering, level flight, turning, how much deviation in inches would the tip of lets say a rotor of 20 foot length (10 feet on each side) actually move up and down as it makes each individual revolution? I would also like to know what the average maximum angle of that tilt would be. I would also like to know what these figures would be for a model helicopter with perhaps a 50 inch rotor.
Many thanks to any of you engineers who can supply me with the correct answers to this puzzle.
Jerry Plottner, Canton, Ohio





RE: BLADE FLAPPING
Three and a half decades ago, I spent a few hours watching USCG helicopters maneuver in Mobile. I was paying close attention because I was in the process of designing a simulator for them.
The flap hinges get a little exercise at vertical liftoff, when the collective stick is lifted, the AOA of all blades goes up in unison, and the blur formed by the spinning rotors changes from a disc to a noticeable cone. I'd guess the tips of a ~40' diameter rotor might lift by three or four feet.
As the aircraft stops accelerating vertically and transitions into a hover, the cone flattens out some.
As the aircraft transitions from elevated hover into forward flight, the cone seems to tilt forward, i.e. the blade tips rise maybe five feet above the tail, make a lot of noise as they pass the tail cone, and remain aligned with the horizon at the frontmost portion of their path. The fuselage tilts forward in response, the rotor axis tilts forward, and the aircraft starts accelerating forward. I.e., the fore/aft asymmetry in coning is a result of the blades having more total lift force on the, er, backstroke, i.e. they are applying a couple to the gearbox to pitch the aircraft forward.
As the aircraft gains forward velocity, I know the pilot has to apply (left or right, I forget which) lateral cyclic stick to offset the dissymmetry of lift. I'm not sure that there would be an associated lateral asymmetry in coning, but I don't remember noticing it. Except of course when the pilot is trying to initiate a banked turn, when of course the blades are applying a lateral couple to the mast.
Probably because they were USCG pilots training or being trained, I think the pilots were working the controls pretty hard, making the blade path variations more visible than what you might see on a commercial flight.
If you live near a USCG helicopter base, go and watch.
Oh. Phone ahead and tell the base PR officer you will be hanging around the perimeter fence. They may be a little jittery these days. Try not to look like a terrorist.
Mike Halloran
Pembroke Pines, FL, USA
RE: BLADE FLAPPING
Thanks for the note about my Flapping question, it was nice to see someone is out there even if I do not fully agree with your assesment. Apparently the real Rotorcraft engineers are not out there as I thought they would be on this question of mine.
I am familiar with Coning of a rotorblade and when the machine is in the air the spinning rotor is holding up all that weight. Also I would suppose when it is flying straight and level the spinning rotor probably adds even more lift because of the airodynaminc affect of the spinning rotor now moving through the air at considerable speed.
The simplest explanation of the solving of dysemmetry of lift I think is in the Cierva story (he invented the autogiro) and during his first efforts the units rolled over sideways when the rotor came up to speed. His solution was to let the rotor teeter (seesaw) and my understanding is that in this action the outer tip of the rotor is either moving up or down, depending on which side of the relative wind it is spinning in. This slight teeter (that is what I am searching for the measurment of),causes the proper side of the rotor to teeter down and in the simplest of design, the other end of the rotor to tip up, and in doing so it automatically compensates perfectly for dysemmetry of lift.
In my model Rotorkites (rotorkite.biz) I found I indeed needed that same teetering-seesaw action for the rotors to fly properly.
Maybe you can add yet more to your end. I'll be watching and thanks again for writing.
jvp
RE: BLADE FLAPPING
I can't explain how rigid rotors with more than two blades work.
Mike Halloran
Pembroke Pines, FL, USA
RE: BLADE FLAPPING
What Cierva discovered was that rotors must be free to flap. His rotor system was rigidly fixed to its mast. Think of a propellor pointing up. When the auto gyro was moved forward the air rushing toward the rotor system would blow back the blades at the front of the auto gyro. This would make the rotor system tip left since the system rotated counter clockwise, similar to if a helicopter pilot wanted to pitch left. However Cierva could not control the rotor system of his autogyro and counter-balance this left pitch with right pitch and the autogyro would roll left. He fould that if he let the blades flap freely the rotor system could absorb this blowback and thus remain upright. I have equations that will give the flapping angle given cyclic and collective inputs and I will post them when I find them.
For Mike's question 2 bladed rotors use teetering while multibladed rotors use flapping hinges.
RE: BLADE FLAPPING
When viewed from above the blades on one side are seen to be moving in a forward direction whilst the opposite blades are moving rearwards. As the helicoper starts to move foreward the airspeed must be added to the forward moving blade speed and subtracted from the rearwards moving blades.
The result is that the blades moving forwards have more airspeed and therefore more lift, the opposite is true for the rearwards moving blades.
If the blades were fixed the dissimetry of lift would cause the helicopter to roll over. This is overcome with flapping hinges or a teetering system. By allowing the forwards moving blade to climb we artificially reduce its angle of attack to the relative wind and thus reduce the lift it is producing. The rearwards moving blade descends increasing its angle of attack and consequently increases its lift.
Basically the forewards moving blade has a high airspeed but a low angle of attack, and the rearwards moving blade has a low airspeed but a high angle of attack, this mechanism cancels out the dissimetery of lift and prevents a rollover.
That is quite a simplistic explanation I hope it helps.
Karl
RE: BLADE FLAPPING
The only problem for me is that all this still does not answer my original inquiry which is the actual amount of teeter that happens with a simple 2 blade rotorblade as it teeters in that relative wind? I am interested in knowing what the total rotor tip deviations would be in inches and in degrees for various size rotors (including model helis) and perhaps the various speeds of the rotor would have to be taken into consideration.
jerry
RE: BLADE FLAPPING
Hope this helps slightly, but it is a very in depth concept. I apologise for any spurious entries.
Good luck with trying to understand
RE: BLADE FLAPPING
Should help you a bit more but the coning angle changes you require for ground to transition of hover/flight are a bit more awkward to quantify, you would need design plans and a scientific calculator.
Again hpoe this helps a bit