With respect to the tip-path plane...

XXavier

Member
Joined
Nov 13, 2006
Messages
1,481
Location
Madrid, Spain
Aircraft
ELA R-100 and Magni M24 autogyros
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913 gyro (June 2023)
I hope that someone here may help me understand the feathering motion of a semi-rigid rotor...

[IMG2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/i.imgur.com\/gXHuitJ.jpg?1"}[/IMG2]

In the 'blown-back' rotor, when a blade moves from A to B, the feathering changes from zero to 'max. down'. In other words, and provided I'm getting this right, the blade 'pitches down' with respect to the tip path plane.

How can this be...? The blade isn't free to rotate around its longitudinal axis...
 
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Perhaps easier with this sketch:
Sans titre.png
 
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Thanks, JC, it really helps, but my powers of spatial visualization have always been poor. Perhaps I should build a model in wood...
 
TeeteringRotor.jpg

The blade isn't free to rotate around its longitudinal axis
This is true, XXavier, the blade is not free, it is forced into that position by your tilting the rotor head. In the for and aft position (psi=180°) both the advancing and the retreating blade see the same angle of attack equal to the preset blade angle (2° - 2.5°). If you tilt the rotor head backwards then in the position psi=90° the advancing blade is forced upward by the teeter bolt and the retreating blade is downward. In my very extrem example the angle of attack of the retreating blade is negative, which is not true for a real rotor, but it might help to understand blade motion about the longitudinal axis of the blade. The two (short) parallel blue lines in the lower picture indicate that these lines are parallel.
 
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A scale model of a see saw rotor is an excellent way of visualizing its operation. I used to build them from spruce yardsticks, at one time a give away advertisement by lumber suppliers and building material suppliers. There used to be a barrel of these things near the entrance doors of such businesses with a sign saying; take one. But those days are gone forever.

But even without a yard stick, a strip of wood ~ 2 ft long with a Clark Y sort of airfoil will spin nicely in front of a box fan and might even flutter if the fan is strong enough.

Such rotors can also be spun with a portable electric drill and will provide an excellent illustration of flapping, feathering, etc.

A strip of aluminum 10 inches long x 1 inch wide can also be used as a model rotor; if it’s to spin in front of a fan, it will have to be given some camber; -curvature like a venetian blind slat. It will also flutter without nose weights; a short machine screw with nut near the leading edge at 70% radius will make it stable.
 
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Many thanks to all that tried to help, but I 'cant see' it... However, I believe that I've got a good idea for a plywood working model. I hope to be back with photos in a couple of weeks...
 
No need to get too fancy; a slice from a wood dowel 8-10 inches long will tell you everything you need to know about autorotation and cyclic pitch control.
As a windmill, hold this thing out of the window of a moving car, give it a flip and it will rotate in whichever direction it is started. Fitted with teeter hinge, it will behave exactly like a see saw rotor. It should spin in front of a large fan but in a rotating slipstream, it will rotate better in one direction than the other.

dowel.JPG
 
Thanks again... The trouble with a real, wind-operated model is that it may move too fast to observe it well. Instead, I plan to make a simple teeter articulation for a balsa strip, 'the rotor', that will turn around a 'control axis'. Then, I'll force the blades with the fingers in order to turn them in a different plane, a new orbit, the 'blown-back orbit', and observe the periodic feathering of the blades against a fixed wooden ring, the 'reference tip path plane'...
 
The trouble with a real, wind-operated model is that it may move too fast to observe it well

Try a 200+ feet diameter rotor, will be slow enough to observe....;-)

Sorry, couldn't resist that one...... :-(
 
Jean-Claude: "Feathering up" indicates an increased angle of attack, and "feathering down" the opposite, right?
 
Tyger;n1139265 said:
Jean-Claude: "Feathering up" indicates an increased angle of attack, and "feathering down" the opposite, right?

Just a 'sideways comment'... Yes, but –as I see it–, that 'feathering' is a mechanical effect. In a model (or in the real thing) if the rotor is blown back by a force different from the relative wind (the fingers, or an electrostatic or gravitational attraction...) and the tip path axis is at an angle with the control axis, that feathering will always take place, also in a vacuum, where we couldn't speak about an 'angle of attack'. It is of course true that, with a blown back disk, in the presence of a relative wind, that feathering will indeed cause a change in the AoA, but there are also other variables influencing that AoA...
 
In the vacuum, or simply with a round section without lift (just the drag), changing the orientation of the hub bearing does not change the tip path plane. It is only the cyclic aerodynamic lift due to the cyclic angle of the blades that produce the plane change. Sans titre.png
 
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Jean Claude;n1139294 said:
In the vacuum, or simply with a round section without lift (just the drag), changing the orientation of the hub bearing does not change the tip path plane. It is only the cyclic aerodynamic lift due to the cyclic angle of the blades that produce the plane change.

Of course, but –in the absence of air– you can change the tip-path axis and induce a cyclic feathering by forcing an angle between the tip-path axis and the control axis... That's what I wanted to say... In a real rotor, it's the relative wind that provides the necessary force, but other forces could work the same effect in the absence of air...
 
In the absence of air, to force the tilt of a tip path plane, like MTO rotor, at the rate of only 60 degrees per second, it would be necessary to produce on his rigid plate a huge torque of 6000 Nm
Nose up torque for right tilt
Nose down torque for left tilt
This is what the blades produces in air from tilting with no forcing the swashplate at an angle of 6 degrees relatively to tip path plane
 
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Jean Claude;n1139300 said:
In the absence of air, to force the tilt of a tip path plane, like MTO rotor, at the rate of only 60 degrees per second, it would be necessary to produce on his rigid plate a huge torque of 6000 Nm
Nose up torque for right tilt
Nose down torque for left tilt
This is what the blades produces in air from tilting with no forcing the swashplate at an angle of 6 degrees relatively to tip path plane

No swashplate at all... Imagine a seesaw rotor. I can force the blades, simply with my fingers in the case of a model, to follow an orbit that defines a plane (the tip-path plane) whose perpendicular forms an angle a with the control axis; under those conditions, and for purely mechanical reasons, the blades will feather with respect to that plane, the angular amplitude of that feathering between extreme values being double of the value of that angle a...
 
At the end or at the root, you will need to cyclically press the blades to get the change of plane of rotation. This pressure corresponds to applying a cyclic torque around the flapping axis. As I said, in the vaccum you will need about 6000 Nm to change of the tip path plan at the rate of 60 degrees per second.
6000 Nm is like 1000N at 3m from the center, opposed on each blade:

Sans titre.png
 
I throw in the towel, JC... Either I'm not properly explaining what I wish to convey, or you aren't understanding me, or both. As soon as my model is ready, I'll come again to the thread and explain it in images...
 
Sorry, Xavier. I think it's because of my very bad English
 
With tilt head cyclic control, whether Cierva or Bensen, the rotorhead is the swash plate. Prof. JAJ Bennett, chief engineer of Cierva Autogiros at the time, explains it as follows:

The tilting hub and reflex methods of control are fundamentally Identical because whenever the control lever is moved in any given direction the cyclic variation of the blade incidence is the same in both.

Inertia prevents a sudden displacement of the tip-path plane, so that when hub is tilted the periodic displacement about the flapping hinges gives the same variation of blade angle with respect to the tip-path plane as is obtained about feathering control hinges in the reflex method.

It is wrong to imagine that to tilt the hub the pilot must impose a load on the control column sufficient to displace the blades against their own inertia.

The control mechanism is a relay, in which the moments are relatively small and when the pilot operates this relay he only causes the blades to feather cyclically.

The result is a cyclic variation of lift in phase with the cyclic variation of blade incidence. The cyclic variation of lift displaces blades from their normal path, but owing to the natural frequency of motion about the flapping hinge being equal to the rotors angular speed, the displacement of the blade from its normal path occurs 90 deg. later in azimuth, thus effecting the required tilt of the tip-path plane, and therefore the lift vector which is normal to this plane.”

Prof. Bennett’s use of the term, “reflex” means via feathering bearings and swash plate.
 
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