BLADE FLAPPING 1

[jplot]

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

 

[MikeHalloran]

I have been under the impression that dissymmetry of lift was taken care of by changes in the angle of attack as a blade rotates, as effected by the swashplate tilting under control of the cyclic stick.

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

 

[jplot]

Mike,

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

 

[MikeHalloran]

Your question got me to thinking. I think the core idea is that in order to have directional control of the rotorcraft, you must be able to direct the thrust vector produced by the rotor in a direction other than straight up the mast. Flap hinges and teeter hinges allow that to happen.

I can't explain how rigid rotors with more than two blades work.



Mike Halloran
Pembroke Pines, FL, USA

 

[WPAK23]

Jerry,
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.

 

[karlbamforth]

The expalation of flapping hinges etc is quite a complex subject to explain here. Probably easier to get a book.

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

 

[jplot]

Nice to have a few people show an interest in this very interesting phenomena of rotor flight. Karl, your explanation was very good although you did not mention that the same condition would hold true with the helicopter sitting still and perhaps a ten mph wind blowing.
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

 

[jjrl]

A somewhat brief answer to your question. The advancing rotor blade increases in forward velocity. Known as the leading blade. The retreating blade decreases its forward velocity. The lagging blade. So the advancing blade speeds up and the retreating blade slows down. As the advancing blade rises due to lift (and forward airspeed across the leading edge)then Ciervas theory was that the weight of the blade moves inwards towards the centre of the disc thus increasing blade speed (just like an ice skater speeds up her spinning action as she/he moves her arms or legs inwards), the flapping hinge allows the blade through the pitch varying housing (Feathering hinge) attached to the horizontal hinge (flapping hinge) to move relieving many stress loads on the blade (which would lead to root end failure).The coning angle is obviously affected. The retreating blade slows down due to a number of reasons but one of the reasons would be forwrd airspeed now acting from the trailing edge to the leading edge. As lift decreases the blade drops slightly and as it drops the weight moves away from the disc further slowing the blade. The forward and rearward, leading and lagging action is absorbed by the lead/lag shock absorbers/dampers. Yes the horizontal hinge or flapping hinge allows for the dysemetry of rotor flight, (without which the rotor blade would eventually fail). Lift between the advancing and retreating blades reaches an equality via the amount of lift being produced at different rotor blade airspeeds and relative angles of attack. Modern rotor disc dynamically unstable helicopters use automated flying control sytems which through vert giros and computers constantly monitor the aircrafts attitude and compensate via small flight control inputs for any rotor disc instabilities, the aircraft trying to fly where it shouldnt.
Hope this helps slightly, but it is a very in depth concept. I apologise for any spurious entries.
Good luck with trying to understand

 

[jjrl]

I forgot to mention the amount of flex in the rotor blade and amount of flapping is all dependent upon forward airspeed and the load being transmitted to the disc. How much does the aircraft have to lift , fuel load , cargo load and external freight, it will all have an effect. So for each aircraft the coning angle will be different for each flight profile.Its a design aspect. As a slight insight for you during rotor track and vibration analysis the track of the rotor blades for a 30 foot blade is usually a 1 inch split at the tips. (For a particular aircraft a 4 inch split would lead to harsh vibration probably felt at the pilots station and if he was paying attention he would see and tell you that one of the blades was flying higher or lower than the other). That is the total difference between the lowest flying blade and the highest flying blade. This can be corrected by pitch change adjustments weight adjustments and usually for forward airspeed out of track adjust the trim tabs. All depends on the aircraft.
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