Blade Flap

[MichaelBurton]

Michael, so would you say that a gyro with a slightly smaller blade length than should be, say all up weight 500lb with 22 ft Bensen blade, could flap, after a high speed decent (say 85mph) followed by a hard bank?

The blades always flap. What you want to know is if this will cause a problem for your gyro. I can not say. The thing is every "Benson" is different. different blade length, different weight, different head design. Each of these things are important when we consider the dangers of the big blade flap at high speed.

The hard turn will probably decrease the flapping motion as it will add G force to the gyro increasing the load that the rotor is under. This will increase the lift generated and the speed that the rotor turns.

I don't think that 85mph is fast enough to cause the problem. But again it is dependant on the design.

 

[ferranrosello]

Thanks for your comment, Tim. I understand your point about pitch. You are talking about control inputs, and I was explaining flapping due to asimetrical airspeed. But because of your explanation, I think we are trying talking about the same thing: what you call pitch is what I call AOA.

Michael, what you have quoted in your post 96 is a quote of Tim's post 89. I entirely agree with your explanations. In my post 92 I've tried to explain the blades flapping movement.

Ferràn

 

[mceagle]

G’day Michael. I can now appreciate why you subscribe to the line of thought that you do regarding “cyclic flapping”.
Personally I follow the line of thought that Chuck B explained in his article (post 21 under Rotor head geometry, quoted below):-

To further explain, would you not agree that any force optimized at the 3 o’clock position manifests itself at the 12 o’clock position – ie a lifting force at the 3 o’clock position causes the blades to rise at the front (and not at the right-hand side)? In forward motion this extra “force” on the advancing blade creates what is called here ‘the blow-back’ angle. The force is constant so it needs a constant counter force to stop the disc from diverging further. The counter force is supplied by the joystick, by moving it further forward as speed increases. Thus in forward motion there is a slight misalignment between the rotor-head axis and the rotor disc axis. Note that the joystick is moved forward, not to the right, as the tendency is to climb, not to roll left.
In any case, with blow-back happening in the fore and aft plane rather that the lateral plane, the blades still do not flap but rather scribe a perfect circle, albeit slightly misaligned with the rotor-head axis.

Another question :- If the advancing blade flaps up and flies higher, how is it then that on a gyro with LH propeller rotation, the tendency is for the retreating blade to fly higher???

“Viewed from the rotorhead axis, the blades appear to flap and with a coned rotor, the CG of the upward flapping blade moves nearer to the center of rotation and must speed up to conform to conservation of energy law. The retreating blade must slow down since its CG moves away from the center of rotation. The same law that governs pirouetting ice skaters as they tuck in or spread out.
Cierva’s rotor analyses always used the rotorhead frame of reference, perhaps to bedazzle and befuddle his competitors. When the rotorhead reference frame is used, to make the math work, Coriolius theory must be applied to explain the need for drag hinges on rotors with three or more blades.
The NACA as well as textbook authors picked up Cierva’s analysis and ran with it. Gessow and Meyers (“Aerodynamics of the Helicopter”), to their credit, point out that Coriolius forces are in the eye of the beholder. An imaginary force.
In forward flight, the axis of the rotor plane tips rearward with respect to the rotorhead axis; the amount dependant upon airspeed.
Blade pitch is fixed relative to the teeter bolt. The blade on the near side (advancing side) has less pitch than the blade on the retreating side. Lift is thus equalized without jumping through vectorial hoops. It is important to understand the equality of cyclic “flapping” and cyclic pitch.”

 

[C. Beaty]

From Gessow and Myers; “Aerodynamics of the Helicopter” Chapter 7:

“….An observer riding on the control axis* and rotating with the blades observes that the blades flap up and down each revolution but they are fixed in pitch. At the same time an observer who sits on the plane of the tips, rotating with the blades, observes that the blades do not flap at all but do change their pitch—high, then low—once each revolution. The pitch is low on the advancing side of the rotor and high on the retreating side.

Examining Fig-33, it is seen that the amount of blade feathering with respect to the plane of the tips is equal in degrees to the amount of blade flapping with respect to the control axis……

….The control axis is the axis of no feathering; the axis perpendicular to the plane of the tips is the axis of no flapping….”

*The control axis is the swashplate axis in a helicopter and is the rotorhead axis in a tilt head gyroplane.

 

[MichaelBurton]

I may need to rethink a few things

I may need to rethink a few things

G’day Michael. I can now appreciate why you subscribe to the line of thought that you do regarding “cyclic flapping”.
Personally I follow the line of thought that Chuck B explained in his article (post 21 under Rotor head geometry, quoted below):-

To further explain, would you not agree that any force optimized at the 3 o’clock position manifests itself at the 12 o’clock position – ie a lifting force at the 3 o’clock position causes the blades to rise at the front (and not at the right-hand side)? In forward motion this extra “force” on the advancing blade creates what is called here ‘the blow-back’ angle. The force is constant so it needs a constant counter force to stop the disc from diverging further. The counter force is supplied by the joystick, by moving it further forward as speed increases. Thus in forward motion there is a slight misalignment between the rotor-head axis and the rotor disc axis. Note that the joystick is moved forward, not to the right, as the tendency is to climb, not to roll left.
In any case, with blow-back happening in the fore and aft plane rather that the lateral plane, the blades still do not flap but rather scribe a perfect circle, albeit slightly misaligned with the rotor-head axis.

Another question :- If the advancing blade flaps up and flies higher, how is it then that on a gyro with LH propeller rotation, the tendency is for the retreating blade to fly higher???

I am going to do a bit of thinking and typing at the same time.

I see e no problems with the description. A force applied to the rotor will be realized 90 degrees ahead in the plane of rotation. This is why the left or right motion of the cyclic causes the most pitch change fore and aft and no pitch change when the blades are aligned with the lateral axis.

Ok so if we follow this reasoning what we would see as we increase speed is the force being reduced on retreating side causing the blade to fall 90 degrees ahead in the rotation. This would then cause the pitch angle to be reduced further reducing the lift on the retreating blade. Hmm, I think I need to look at this a bit more as I can not see a way to make the rotor fly using this reasoning. I agree that it sounds reasonable I just cant see where I made my mistake.

This could be the key that I missed earlier in this line of reasoning. Lets say we want to fly faster. We do this by pitching the aircraft forward. This increases the pitch on the retreating blade and decreases the lift on the advancing blade keeping the lift equal across the rotor and preventing the blades from flapping. Now that makes better sense. Using this reasoning there would be no blade flap in flight.

Blade flap on the ground would happen due to the fact that the speed is not coupled to the stick allowing things to be unbalanced. However the flap would be fore and aft and not left and right as described in the handbook.

I still have some unresolved questions.
Why does the gyro roll over with excess speed as compared to the rotor rpm? There are many examples of this type of accident.

 

[birdy]

I've experienced stalls and spins in FW and I've never heard this kind of loud sound.
FWs lift isnt cyclicl, and i dont recon the FW you stalled was do'n 600 odd mph just before it stalled Ferran.

 

[mceagle]

Blade flap on the ground would happen due to the fact that the speed is not coupled to the stick allowing things to be unbalanced. However the flap would be fore and aft and not left and right as described in the handbook.
I still have some unresolved questions.
Why does the gyro roll over with excess speed as compared to the rotor rpm? There are many examples of this type of accident.

I couldn't agree more Michael. I have not witnessed a ground roll over to the left but a saw a gyro come very close to it one day. I put it down to the fact that too much forward speed caused the advancing blade to start to 'fly' or 'sail' while the retreating blade was completely stalled, and the blades were too slow to allow gyroscopic precessive forces to overcome the aerodynamic forces.

 

[ferranrosello]

Michael, Tim and Chuck: I don’t know why we are making things so difficult to understand. To understand a rotating rotor is much easier. Things can be looked from different points of view, but the final result is always the same.

I’ll try to explain, Michael: the only thing that is affecting blades is aerodynamic AOA. Pitch, feathering and flapping don’t mean anything real until you look at the blades AOA.

When you increase your air speed, the advancing blade “sees” more relative wind, and the retreating blade “sees” less relative wind. This is an easy thing to understand. Isn’t it? Consequently the advancing blade would produce more lift and the retreating blade less lift.

If there was not a teetering hinge (or a flapping hinge), the result would be a hard roll tendency towards the retreating side.
One of the main Physics principles is that every thing does the easiest thing to do. If you introduce the teetering or flapping hinge in the rotor then the advancing blade which is seeing more airspeed, will develop more lift and will…climb. And because of the climb the advancing blade will “see” less AOA and will reduce its lift. Te opposite will happen in the retreating side.

As you can see the relevant thing is AOA no pitch.

This should answer your question:

“….we would see as we increase speed is the force being reduced on retreating side causing the blade to fall 90 degrees ahead in the rotation. This would then cause the pitch angle to be reduced further reducing the lift on the retreating blade..”

The mistake in your reasoning is about introducing pitch… Once more, pitch is an artificial thing and doesn`t mean anything… If the force is reduced in the retreating side (because a reduction of blade airspeed) the effect will be to lower the blade 90º after: just tilting the rotor disc rearwards. And by doing so: The AOA of the retreating blade will be increased, just at the same time that the lift in the advancing side will be decreased.

The real truly result is the same amount of lift, but perfectly equalized between both sides. This is the key point in the understanding of rotors.

Of course, and this is sometimes a confusing point, the dissymmetry of lift can be equalized by “mechanic” or artificially forced pitch variations. The result in this case will be a variation of AOA’s in the same sense that I’ve explained before…

but:

The control system will have to reduce the pitch in the advancing side, decreasing blade AOA’s in this side of the rotor. And the control system will have to increase pitch in the retreating side which will increase its AOA’s. Certainly this “forced” way of equalizing lift will produce a forward tilt in the rotor disc. This way is partially used in helicopters because they need forward tilted rotor discs for better performance in translational flight.

Ferràn

 

[ferranrosello]

“FWs lift isnt cyclicl, and i dont recon the FW you stalled was do'n 600 odd mph just before it stalled Ferran. ”

Birdy Why not? I think that a stall is a stall in a FW or in a rotor.

You can be sure that if the blade is seeing 600 mph is not stalled.

And I’ve never stalled a jet flying at 600 mph.

Anyway I’m not saying that this was not a stalled rotor, what I say is that I can’t believe it was… By now. I will change my mind if some one is able to explain this thing in an understandable way for me.

Ferràn

 

[ferranrosello]

Aerodynamic forces acting on the blades are really hard. They can bend and twist the blades originating more and harder aerodynamic forces when centrifugal force is reduced (because of too low rpm). An immediate result is that both blades are completely out of tracking and the vibration is so hard…

Centrifugal force is needed to put and maintain blades in its place in the rotating rotor. When this happens the rotor is smooth enough to be controlled by a man.

ferran

 

[C. Beaty]

Ferran, the rotor is connected to the rotorhead by a universal joint (Cardan joint in the case of a seesaw rotor) and is free to rotate about its own axis, the axis of the tip plane.

Viewed along the rotorhead axis or the swashplate axis in the case of a helicopter, we can imagine the blades to flap.

We can also imagine the front wheels of a front wheel drive automobile to flap but most everyone will agree that they don’t.

Jack up the car, run the front wheels, put in a steering angle and view a wheel along its driveshaft axis and the valve stem (or a spot of paint on a tire sidewall) moves nearer and farther from the viewer and the wheel flaps; doesn’t it?

Of course it doesn’t flap; we picked the wrong axis from which to view the wheel. The wheel’s real axis of rotation isn’t the drive shaft axis; it is the axis of the rotational plane of the wheel, its hub.

So it is with rotors; viewed along the rotorhead axis, they sure look like they’re flapping and the vector sum of upward flapping velocity, rotational velocity and airstream velocity is a convenient fiction. As is Coriolis force.

Viewed along the axis of the tip plane, the blades don’t flap, don’t speed up and slow down, don’t move nearer and farther from the axis of rotation.

As the textbook says, viewed along the axis of the rotational plane, the only strange thing is a cyclical variation of pitch if the two axes are not in alignment.

The only thing that tilting the rotorhead or swashplate can do is to cyclically vary the pitch of the rotorblades. The perceived flapping angle and the cyclical pitch variation are identical.

 

[karlbamforth]

Mr Beaty does it again.

Your front wheel drive car analogy is the best I have seen yet to explain this difficult to understand issue. :first:

 

[C. Beaty]

Thanks, Karl. Engineers like you don’t need coaching from me.

I’m trying to convey to individuals without technical training, comprehension of this very confusing subject.

Scale models made from welding rod―a miniature barbell with teeter hinge, for instance, make the point quickly.

The military knows the value of training aids; a working mockup of hydraulic and electrical systems laid out on plywood sheets is a good example.

 

[MichaelBurton]

I couldn't agree more Michael. I have not witnessed a ground roll over to the left but a saw a gyro come very close to it one day. I put it down to the fact that too much forward speed caused the advancing blade to start to 'fly' or 'sail' while the retreating blade was completely stalled, and the blades were too slow to allow gyroscopic precessive forces to overcome the aerodynamic forces.

So what you imply with this is that due to the slow speed of the rotor. The blades produce force that is realized at the point applied and not 90degrees ahead. Otherwise the action would still be 90 degrees ahead in the plane of rotation and the gyro would not roll. It seems then that the faster the blades rotate the more force would be moved to the 90 degree position. Or as the blades increase in speed the position approached the 90 degree position. The results would be the same in either case.

I found the equation for the forward flight and it does indeed place the highest point of flap at the 0 degree position and the lowest at the 180 degree position with some variation for cone angle.
The reference I found is in rotary-wing aerodynamics by Steiewski & Keys vol. 1 page 24. I don’t have an electronic copy.

What I would like to see is an equation that looks at blade speed going from 0 rpm to steady flight rpm. Is there an equation that describes this?
At what blade RPM would the force be nearly at the 90 degree position?

 

[C. Beaty]

A gyroplane differs from a helicopter inasmuch as the gyroplane rotor stalls first at the root end and the stall spreads outward as forward speed is increased. This serves as a speed governor.

At the ratio of forward speed to rotor peripheral speed reaches a value of ~0.35, the stick will be near or on the forward stop and a farther increase of power will simply cause the machine to climb without a corresponding increase of forward speed.

We used to crank Bensen metal and Rotordyne blades up against the pitch adjustment stops and the machines wouldn’t go any faster than 20-30 mph with the stick hard against the forward stop. More power and it would simply climb, producing the feeling of screwing itself up and down with throttle.

Here is the result of NACA blade stall testing of a Kellett KD-1A (military YG-1B).

Observe that as the ratio of forward speed to rotor peripheral reaches 0.35, nearly 70% of the retreating blade is stalled, a little over 100 mph (40’ rotor turning 210 rpm).

http://ntrs.nasa.gov/search.jsp?R=2...ll&Ntx=mode%20matchall&N=0&Ns=HarvestDate%7c1

 

[mceagle]

Jack up the car, run the front wheels, put in a steering angle and view a wheel along its driveshaft axis and the valve stem (or a spot of paint on a tire sidewall) moves nearer and farther from the viewer and the wheel flaps; doesn’t it?
Of course it doesn’t flap;

An excellent example for us mere mortals. I wish I had thought of that!!

 

[MichaelBurton]

I guess that old Benson had more of a limitation on forward stick. I have never reached the max forward stick in flight. Andy says he has taken the gyro to 120 mph. I prefer a more sedate speed of 60-70mph most of the time.

Do you have a spread sheet that can give a prediction for stall speed based on rotor rpm and blade size? I would love to be able to create plots for blade size and rpm. I don't want to do the work if it is already done.

 

[C. Beaty]

No, it was not due to a limitation of forward stick travel. By cranking the blades up against their adjustment stops, the blades were slowed down enough, -perhaps mu = to 0.25 in this low speed case- that cyclic “flapping”* angle was great enough that rotor disc angle prevented any farther airspeed increase. No one had useful air speed indicators or rotor tachs in those days.

There is no magic to any of this; at a forward speed of approximately 35% of rotor peripheral speed, all gyros will run out of forward stick.

Some of the old winged Autogiros were commonly flown as high as 50% of rotor peripheral speed because the wings served to unload the rotor a la Cartercopter.

*Flapping angle is an unfortunate choice of terminology. Blowback angle would have been more appropriate.

 

[mceagle]

Chuck, how would you reconcile with Carter Avaitation's statement:-

"As the aircraft speed increases, the average air velocity increases over the advancing rotor blade but decreases over the retreating blade. In order to keep the blade from creating unequal lift and causing a rolling moment, a teetering hinge allows the advancing blade to flap up, decreasing its angle of attack while the retreating blade flaps down and increases its angle of attack."

Unfortunate choice of wording or terminology perhaps?

 

[ferranrosello]

Chuck, this is a very good explanation of pitch versus flap. I agree with you, thanks, but this is not my point.

I'm trying to explain that the relevant thing in the aerodynamic force created by a blade is AOA, angle of attack against relative wind. When talking about flapping I'm not referring to a control input, I'm referring to flapping motion due to airspeed, that’s to say the blowback. Flapping is a much easier way to see AOA than pitch. It explains the blowback effect and the equalized lift between advancing and retreating sides easily.

Ferràn