How dissimetry of lift is managed in Bensen type gyroplanes?

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oriol

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Hi all!


I am very intrigued with the rotor found in many light gyros like the Bensen type. I understand that the rotor is windmilling all the time and the pilot can only adjust the orientation of the rotor, without being able to modify the blades pitch in flight.

One can tell from pictures that the system to change the orientation of the rotor is pretty simple like that found on weight and balance aircrafts: two perpendicular bolts that allow for motion on all directions.

The thing is that Bensen seems to do well without having to displace the controls affected by precession? Perhaps he does not have to move the controls and the pilot bears with it moving the stick to the right to go forward etc...

However what intrigues me the most is, how Bensen does to avoid dissimetry of lift given that the blades can not flap, since they are bolted together in a single piece, and can not change pitch indistinctively either to overcome dissimetry of lift?


-)In the first picture below one can see clearly that the control goes straight to the rotor without any linkage or anything that can serve to avoid precession.
-)The second picture is curious, it shows a gyro with dual controls one that goes straight to the rotor and another one that goes below the pilot seat with some linkages that might(?) serve to deal with precession.
-)The third picture shows a standard rotor found in gyros. Although the blades can flap they can not do so individually to avoid dissimetry of lift.


Any input would be greatly apreciated!


Oriol


Bensen gyroglider on tow.JPGbensen_b-8m with dual controls.jpgGyrobee Rotor Head.jpg
 
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D Hillberg

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Cyclic feathering / flapping hinges

If you notice as you increase speed with forward cyclic ( adding throttle with out climbing by adding forward cyclic or just diving) the advancing blade has less pitch & retreating blade will more pitch by use of the gimbaled rotor shaft (pilots actions)

the rest is taken up by blade flapping for the smaller differences in air speed. (AoA)
 

Dan Thomas

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The advancing blade has less AoA because its airspeed is higher and because it is rising with respect to the flight path, and the retreating blade has a higher AoA because it is slower and is falling.
 

oriol

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Look again, the blades do flap.
Yes I did noticed, however from what I red on heli literature in some cases flapping helped overcome dissimetry of lift by flapping the advancing blade independently from the retreating blade, wich caused the advancing blade to produce less lift. I did not expected what Dan Thomas and Daniel Hillberg pointed that flapping together as a unit would result in different AoA.

I supposed that both blades flapping at the same time worked in the Jet Ranger helicopter, wich has both blades pivoting/flapping together like in the Bensen gyro, because the collective remains fixed wich automatically reduces/increases the pitch of the blades indepently to compensate the dissimetry of lift.


Oriol
 
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BBerson

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The two bladed semi-rigid rotor flaps automatically as a unit. One up and the other down to balance dissimetry.
 

Aviator168

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Oriol, when the gyro moves forward, the air speed on the advancing blade is greater than that on the retreating blade, so the lift on the advancing blade is higher than on the retreating blade. Since the both blades are tied to a single piece and can be flap freely, the advancing blade moves up (higher lift) and forces the retreating blade moves down. While this action is taking place, the AoA of advancing blade is reduced and increased for the retreating blade naturally. When the lift of both blades is balanced, that's what the tilt angle is.
 

oriol

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Aviator168,


I understand that dissimetry of lift is overcome in helicopter by changing the AoA of the blades indepently. The point was how could gyros overcome dissimetry of lift if the blades have their pitch fixed? Luckily the mistery is now answered:by flapping.


However what I still can not figure out is how Bensen manages to deal with precession without any intermediate linkage between the pilot stick and the rotor? Does the pilot moves the stick to the right to go forward?

Same for the hanglider/gyro seen on the russian video. The pilot controls the gyro without any intermediate linkage, just like a hanglider, although it is dificult to say from the video if he is compensating precession with his weight.

https://www.youtube.com/watch?v=1JrT8bKQB0A


Thanks so much for the helpful and interesting responses!

I never payed much atention to gyros but recently I saw some picture of earlier sikorsky, de la cierva wood fabric covered blades and they seem very interesting from a homebuilt perspective. In overall a gyro seems a friendly build because of the small quantity of parts if compared to any 3 axis conventional aircraft.


Oriol
 

D Hillberg

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cyclic feathering is used just like a helicopter move the stick forward and watch the blades at 9 o'clock & 3 0'clock positions, as the stick is moved forward the advancing blade gets pitch removed by mast tilt the retreating blade being solidly attached to the hub bar get the same pitch removed in the same instance. a pilots action = forward stick faster air speed rotor disk forward position, add aft stick (precession from cyclic feathering at 3 & 9 0'clock) tilts rotor disk aft causing a climb.

AoA changes automatically as air flows over the blades with flapping action with no pilot actions.
 

Dan Thomas

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With the typical gyrocopter, the AoA changes aren't just due to flapping. The rotor disc is tilted back so that the airflow is upward through it. Therefore, the advancing blade is already moving upward simply due to the tilt, and that decreases its AoA and therefore the lift at the higher airspeed. The retreating blade is descending and its AoA is higher, increasing its lift at the lower airspeed.

The same effect is seen in the need to hold right rudder in the fixed-wing airplane when climbing. One of the major factors in the nose wanting to go left has to do with the downgoing blade, on the right, being at a higher AoA than the upgoing blade, so that more thrust is generated on the right side, pulling the nose to the left.
 

oriol

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Thanks for the responses!


I am doing some good reading on gyros and rotors and I need some time to digest all the info. However it would be definetively great after the study phase to get some hours with an instructor to become familiar with this definetively interesting machines.

Despite that gyros are not mainstream aviation machines, this is the country of de la Cierva and to my luck it is not very difficult to find an instructor nearby.


Oriol
 

BBerson

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Aviator168,


I understand that dissimetry of lift is overcome in helicopter by changing the AoA of the blades indepently. The point was how could gyros overcome dissimetry of lift if the blades have their pitch fixed? Luckily the mistery is now answered:by flapping.


However what I still can not figure out is how Bensen manages to deal with precession without any intermediate linkage between the pilot stick and the rotor? Does the pilot moves the stick to the right to go forward?

Same for the hanglider/gyro seen on the russian video. The pilot controls the gyro without any intermediate linkage, just like a hanglider, although it is dificult to say from the video if he is compensating precession with his weight.

https://www.youtube.com/watch?v=1JrT8bKQB0A


Thanks so much for the helpful and interesting responses!

I never payed much atention to gyros but recently I saw some picture of earlier sikorsky, de la cierva wood fabric covered blades and they seem very interesting from a homebuilt perspective. In overall a gyro seems a friendly build because of the small quantity of parts if compared to any 3 axis conventional aircraft.


Oriol
I don't think there is any 90° offset on the Bensen, since it is weight shift, not cyclic pitch change.
I look forward to your conclusion.
 

D Hillberg

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weight shift is completely wrong. the cyclic inputs control the blades, not weight shift. the precession of cyclic feathering controls the blades and the machine follows. the rotor head transfers control through tilting affecting cyclic feathering.

take your bicycle tire & spin it up to 400 rpm & try to 'weight shift it'

with rotorcraft cyclic feathering is controlling the direction of flight.
 

D Hillberg

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The Bensen has no cyclic. Blades do not feather.
TILTING THE MAST FEATHERS THE BLADES - if you take the stick with the blades at 9 o'clock & 3 o'clock and put a blade protractor on them and move the stick fore and aft
what pitch reading will you have as the blades attached to the mast move (feathering is what you get added pitch to one and subtracted pitch to it's sister) about 7 degrees difference of cyclic feathering.
 

BBerson

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TILTING THE MAST FEATHERS THE BLADES - if you take the stick with the blades at 9 o'clock & 3 o'clock and put a blade protractor on them and move the stick fore and aft
what pitch reading will you have as the blades attached to the mast move (feathering is what you get added pitch to one and subtracted pitch to it's sister) about 7 degrees difference of cyclic feathering.
Ok, I thought when you said feathering, that you meant individual feathering bearings on each blade like a controllable prop or three bladed autogyro or helicopter.
So the whole blade tilts on a "gimbal"?
When you move the control stick aft, which makes the blades feather at 9 o'clock and 3 o'clock, does the rotor react 90°later to make the disc tilt aft?
 

D Hillberg

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Ok, I thought when you said feathering, that you meant individual feathering bearings on each blade like a controllable prop or three bladed autogyro or helicopter.
So the whole blade tilts on a "gimbal"?
When you move the control stick aft, which makes the blades feather at 9 o'clock and 3 o'clock, does the rotor react 90°later to make the disc tilt aft?
exactly! a gimbal! you got it! 90 deg later through procession the maximum deflection of the rotor disc reacts to pilots inputs
 
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