Gyrocopter Aerodynamics - Page 46 - RC Groups
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Dec 29, 2017, 01:36 PM
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Ran D. St. Clair's Avatar
Quote:
Originally Posted by mnowell129
Let's tackle an easy one for now. Lead Lag hinges. The lead lag hinge is the hinge that lets the blade move backwards and forwards in the plane of rotation. If the blade is running exactly straight out from the hub (viewed from the top) it is not leading or lagging at all. If the blade is running slightly ahead of the rotation it is leading. If the blade is running behind it is lagging.
This doesn't quite make sense because the blade is where it is, so what's to compare to? But consider a 4 bladed head with the blades rigidly fixed to the hub. All the blades are 90 apart. Suppose that one of the blades experiences some mysterious force (to be explained later) that pushes this blade of exact 90 alignment. That is, the other three blades are 90 apart but this last blade is 85 from one blade and 95 from the other. This blade is leading or lagging depending on the direction it is off.
But because the head is rigid, the blade/blade grip is being bent to make the angle. A lead lag hinge is a hinge that allows the blade to lead or lag without producing stress. The pin axis of the led lag hinge is parallel to the shaft so that the blade can swing forward and backwards without stressing the hub. Rotors with more than two blades have lead/lag hinges. In some cases the lead/lag hinge is not an actual hinge but is some material designed to bend, this being called an elastomeric head.
So the question is why do we need this hinge, that is, what mysterious force causes the blade to move forward or backward around the circle.
The answer is called conservation of angular momentum. A mouthfull (handfull when typing?) of words that simply mean that when something is spinning at a certain energy level, inertia tries to maintain the energy level. Linear inertia most people have a feel for, when you shove a heavy object and quit shoving it tends to keep moving. Angular inertia is a bit different in that the inertia of a spinning object depends on how far the mass is from the center of rotation. This is the old ice skater example. An ice skater starts spinning with arms out, as the arms are drawn in the rotation speeds up. But in both cases the momentum is the same. A higher speed of the mass at a shorter radius is equal to a lower speed of the mass at a higher radius.
If you've ever played on a kids merry-go-round (don't know what the international term for this is, but its the big spinning wheel in the kiddie park), you've experienced conservation of angular momentum. If you spin the wheel and stand at the edge you get one speed, but as you move to the center the wheel speeds up. But the angular momentum is constant.
Rotor blades do the same thing. As a blade flaps, say for example up in the front to cancel asymmetric velocity, because it flaps up with respect to the shaft it's radius goes down. As the radius goes down it speeds up. The blade in the back slows down for the opposite reason. So what happens is that the blade in front leads and the one over the tail lags. At the sides they neither lead nor lag. So once per revolution the blades lead and lag around the circle. If you didn't have lead lag hinges the hub would break from the constant bending. Cierva ignored this for a long time and had chronic hub failures until he relented and put lead lag hinges on.
On models we usually don't have lead lag hinges for a couple reasons. One is that our hubs are so much stronger compared to the full sized ones, they can take the stress. Second is that most models, especially helicopters, have the blades attached with one bolt. This allows the blade to lead/lag slightly when necessary.
Finally, two blades systems don't usually have lead lag hinges either. The hub is strong enough generally and the hub simply transfers the lead or lag to the other blade across the hub.

Some side thoughts.

The more the blade flaps, the more it is going to lead lag.

Pinning your blades to prevent lead lag may give you a pretty good shake if your blades are flapping. Besides centrifigul force does a much better job on keeping the blades aligned.

In two bladers, the fore blade is leading, the aft one lagging. So both blades are angled off to the retreating side at the same time and the center of mass of the rotor system isn't in the center of the shaft anymore and you get a 2/rev shake. (The teetering, Underslung head is used to mitigate this and another shake that occurs, more when we get to underslung head).

mick


More topics I though of: Underslung heads and blade balancing.
I know you covered this long ago. I believe I understand what you have said, and it all makes sense. I am thinking there are some other issues though.

The advancing blade will see higher airspeed, and make more lift and drag, except that cyclic pitch will equalize the lift, but not the drag. The induced drag might be similar on the advancing and retreating blade, but not the profile drag. Since drag in general goes up as the square of airspeed, and the difference in airspeed can be quite high with high forward speed, then I assume the increased drag would tend to lag the advancing blade, and lead the retreating blade. Keeping with the general idea that the change in velocity lags the force by 90 degrees, the blade should lag at the front of the disk, and lead at the back. This is exactly opposite of the affect you described due to conservation of rotational inertia.

I am guessing that both effects are happening at the same time and since you spoke to conservation of rotational inertia, I am guessing that is the dominant factor in a typical autogiro situation. I am also guessing that there are about a dozen factors that could change which factor is dominant and by how much. Real math would be required to nail it down.

I ask because I am playing with self driven rotor heads, which admittedly results in something more akin to a helicopter than an Autogiro. If you find my post inappropriate for that reason I respectfully ask that you delete it, or I will if you tell me to.

At any rate, driven rotor heads, with props on the rotor blades, will see a significant change in the effective pitch of the driving props with airspeed, the net effect being less thrust on the advancing blade and more thrust on the retreating blade. I suspect that this is a much bigger factor than the drag of the blades themselves.

Am I in the general vicinity of reality?
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Dec 30, 2017, 09:14 PM
I'm not as bad as they say.
Quote:
Originally Posted by Ran D. St. Clair
I know you covered this long ago. I believe I understand what you have said, and it all makes sense. I am thinking there are some other issues though.

The advancing blade will see higher airspeed, and make more lift and drag, except that cyclic pitch will equalize the lift, but not the drag. The induced drag might be similar on the advancing and retreating blade, but not the profile drag. Since drag in general goes up as the square of airspeed, and the difference in airspeed can be quite high with high forward speed, then I assume the increased drag would tend to lag the advancing blade, and lead the retreating blade. Keeping with the general idea that the change in velocity lags the force by 90 degrees, the blade should lag at the front of the disk, and lead at the back. This is exactly opposite of the affect you described due to conservation of rotational inertia.

I am guessing that both effects are happening at the same time and since you spoke to conservation of rotational inertia, I am guessing that is the dominant factor in a typical autogiro situation. I am also guessing that there are about a dozen factors that could change which factor is dominant and by how much. Real math would be required to nail it down.

I ask because I am playing with self driven rotor heads, which admittedly results in something more akin to a helicopter than an Autogiro. If you find my post inappropriate for that reason I respectfully ask that you delete it, or I will if you tell me to.

At any rate, driven rotor heads, with props on the rotor blades, will see a significant change in the effective pitch of the driving props with airspeed, the net effect being less thrust on the advancing blade and more thrust on the retreating blade. I suspect that this is a much bigger factor than the drag of the blades themselves.

Am I in the general vicinity of reality?
I think you are in the vicinity.
This is pretty advanced stuff, might take some time to refresh my own memory.
I think it gets complicated because multiple factors come in to play, and determining the major component might take some math.....
The lead lag due to drag difference is as you describe IIRC.
I'll give it some more thought, because I think as the blade lags it will also flap down to try and maintain angular momentum, so the blades would tend to orbit around a nominal point as they go around...
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Jan 02, 2018, 09:12 AM
Fly low and slow
TrueBuld's Avatar
An article in the RCM&E November 2014 about the GyrOne states...

"As one turn followed another I couldn't help likening the flying experience to that of piloting a vintage biplane. At times I'd find myself initiating a turn with rudder, whilst using opposite aileron to counter the angle of bank."

From my reading here, the rudder acts in the opposite direction in an autogyro because the mast is angled back. Or have I got it wrong again?

The GyrOne does have vertical mast but the rotor block hinge line (roll) is angled back making to my mind a rear mast angle.
Jan 02, 2018, 02:47 PM
Registered User
No. Rudder works as usual. The opposite roll command keeps the rotor disk horizontally. That makes the gyro turn flat, without banking into the curve. Thus you have no loss of lift. Otherwise there is a big risk to start a spiral dive and crash.
Jan 02, 2018, 03:08 PM
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TrueBuld's Avatar
Quote:
Originally Posted by Ralf W.
No. Rudder works as usual. The opposite roll command keeps the rotor disk horizontally. That makes the gyro turn flat, without banking into the curve. Thus you have no loss of lift. Otherwise there is a big risk to start a spiral dive and crash.
So the section "Turn the page" "What about trying to turn with rudder." is just for fix mast Gyrocopters ?
Jan 03, 2018, 07:07 AM
I'm not as bad as they say.
Quote:
Originally Posted by TrueBuld
So the section "Turn the page" "What about trying to turn with rudder." is just for fix mast Gyrocopters ?
The point is that rudder doesn't cause a roll in a gyrocopter like it does in a dihedral airplane.
The rudder causes the model to yaw.
But the effect is not the same in both directions.
One way the nose pitches up and the other way it pitches down.
Unchecked, a rudder only roll will get out of control, so aileron/elevator is needed to keep a constant turn.
This is also the reason a rudder only single rotor is hard to find without some kind of fin or fuselage to help with the spiral stability.
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Jan 03, 2018, 09:17 AM
Fly low and slow
TrueBuld's Avatar
Quote:
Originally Posted by mnowell129
The point is that rudder doesn't cause a roll in a gyrocopter like it does in a dihedral airplane.
The rudder causes the model to yaw.
But the effect is not the same in both directions.
One way the nose pitches up and the other way it pitches down.
Unchecked, a rudder only roll will get out of control, so aileron/elevator is needed to keep a constant turn.
This is also the reason a rudder only single rotor is hard to find without some kind of fin or fuselage to help with the spiral stability.
From the above it would seem safer to use rudder only for taxing and the takeoff run. Once the wheels leave the ground use only rotor tilt and motor push/pull power for flight control.
Jan 03, 2018, 05:20 PM
I'm not as bad as they say.
Quote:
Originally Posted by TrueBuld
From the above it would seem safer to use rudder only for taxing and the takeoff run. Once the wheels leave the ground use only rotor tilt and motor push/pull power for flight control.
Not true.
An uncoordinated aileron turn is just as bad as it introduces a slip or skid. This is the same as a rudder yaw and produces the same nose up or nose down problem as yawing. Only if the model is perfectly coordinated with ailerons (few, if any, are) can you get away with this.
Best plan is to learn to use the ailerons and rudder correctly and maintain a coordinated turn.
Lack of good rudder skills hampers a lot of beginning gyrocopter pilots whose left thumb is asleep.
Latest blog entry: AIrcraft I've built.
Jan 04, 2018, 02:32 AM
Fly low and slow
TrueBuld's Avatar
Well that clears up my misconception nicely.
Feb 24, 2018, 12:07 PM
FPV really is fun.
Scotth72's Avatar
I always initiate the turn with aileron, then work that stick to keep the rotor angle where I want it. Use the rudder to keep the nose coordinated. It's amazing how much you need to work the aileron stick to keep the rotor angle where you want it. I usually end up with quite a bit of opposite aileron at some point to keep the aircraft from spiraling.


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