View Full Version : Delta 3 & drag hinge questions
09-07-2005, 09:04 PM
Would Delta 3 reduce or increase the drag hinges load?
Wouldn’t the Conservation of energy principal dictate that when the blade flaps up, it must also accelerate?
09-07-2005, 09:06 PM
Drag hinge location?
Where is the best place to put drag hinges? Up close to the center of the rotor, or out maybe 10% of the radius?
Would it make any difference whether the hub spines the blades (helicopter) or the blades spin the hub (gyroplane)?
09-08-2005, 06:06 AM
David: Yes, conservation of energy would require that... if the blade in fact "flapped up." But it doesn't.
The blades follow a flat, circular orbit, not a roller-coaster up-and-down path. The effect of the flap hinges is not to let the blades "rise and fall" into and out of the plane of the rotor disk. Rather, the hinges cause the blades to experience a cyclic pitch change as they travel around their simple flat-plane path.
Lag hinges accomodate the U-joint effect caused by the fact that the rotor disk's plane is not square to the mechanical axis of the spindle. You don't need them with a 2-blade gyro rotor because it's OK for the spindle itself to speed up and slow down 2/rev -- just as happens to the driveshaft connected to a U-joint in a rear-wheel drive truck.
09-09-2005, 11:21 PM
To read your post, one might think that the Cierva or flap hinges aren’t necessary.
In fact in relationship to the spindle the blades do flap up and down.
I don’t want to belabor the point, but in the “AERODYNAMICS OF THE HELICOPTER” BY Alfered Glassow & Garry C. Myers, Jr; as well as, Jukka Tervamaki, who said. “But remember, there are higher harmonics as well, a2 and b2 giving the blade tips a wavy motion.”
Both show a roller-coaster up-and-down path of the blade tips.
But all this is beside the point of my question.
My question regarded delta-3 flap hinges like are on the Pitcaren that is in the Pitcaren Hanger at Pioneer Airport in Oshkosh WI.
This hub has a delta-3 angle of 30 degrees.
So the question becomes, are the blades accelerating when they flap up or decelerating?
Does the fact that as the blade swings up on delta-3 hinges it also accelerates in relationship to the spindle, increase or decrease the work of the drag hinge?
The first picture is of the patent model of this hub. (I should have turned the flash off when shooting in a display case).
The second is the gas cap for the belly tank. Man you can’t buy that low an octane any more, can you?
09-10-2005, 06:33 PM
Delta-3 coupling suppresses cyclic flapping; an upflapping blade is depitched, imposing, in effect, an aerodynamic spring between blade and rotorhead. This raises the flap frequency somewhat above its natural frequency of 1/rev so that the phase shift between cyclic input and rotor is less than 90º.
Harmonics of the periodic aerodynamic loads do slightly distort the rotor plane, causing some flexing of the rotorblades as they change from advancing to retreating. Offset flap hinges also cause some distortion of the rotor plane.
If a rotor is to be controlled by rotorhead tilt, then flap/drag hinges are unavoidable. The rotorhead couldn’t otherwise be tilted; serving the same function as a swash plate.
There have been several floating hub rotors where it is possible to dispense with flap/drag hinges.
Probably the first were the Doblhoff machines built in Austria during WW II. One found its way to this country shortly after the war where Dr. Bensen participated in its evaluation at the General Electric Flight Research Center at Schenectady, NY.
All of the McDonnell/St. Louis helicopters used rotors based on Doblhoff’s concepts without flap or drag hinges.
The Doman helicopter also used a floating hub rotor, conceived independently of the Doblhoff machines.
09-11-2005, 12:38 PM
Your question. "So the question becomes, are the blades accelerating when they flap up or decelerating?" As an addition to what C. Beaty said;
The blade tip is accelerating visa vie the hub when the blade IS FLAPPING up or down from the rotor hub plane, if the flapping hinge has delta3. The blade tip is decelerating visa vie the hub when the blade IS FLAPPING down or up toward the rotor hub plane, if the flapping hinge has delta3.
Cyclical Corollas results in the blade tip having a faster velocity visa vie the hub WHEN the blade HAS a flap angle.
Unfortunately, the faster tip velocity due to delta3 and faster tip velocity due to cyclical Corollas are out of phase with each other by approximately 45-degrees. In other words, they cannot be made to cancel each other.
You may find to following boring information of value;
Info. on Pitch-Flap coupling. (http://www.unicopter.com/0941.html#delta3)
Info. on Lead-Flap coupling. (http://www.unicopter.com/0433.html#Teetering)
A 'theoretical' advancement on Doman's rotor, (http://www.unicopter.com/S_Hub_CVJ.html) which was mentioned by Beaty.
09-11-2005, 01:53 PM
Yes indeed. Twirl a rock on a string and using your forearm as the reference axis, it flaps, leads, lags and we have to call upon Mr. Coriolus to explain it all.
But if for some strange reason we choose to be rational and use the axis of the rock’s orbit as our reference, it does none of the above and happily twirls in a circle.
Hitting the rock on one side of its orbit with a perpendicular air jet causes it to follow the rules of gyroscopic precession but it still doesn’t flap; the plane of the rock’s orbit tilts in accordance.
The concept of flapping and Coriolus forces needlessly obscures what is in reality a simple subject.
09-11-2005, 04:05 PM
"....it does none of the above and happily twirls in a circle."
Assuming that the flapping hinges have offset, won't the rotor be a lot happier if it is twirling in an ellipse? http://www.unicopter.com/wink.gif
09-11-2005, 04:46 PM
It depends upon the amount of offset. Usual offsets are in the range of 2 or 3% of rotor radius. A straight line from hub center to the blade C of M isn’t much shorter than the distance via the offset flapping hinge in the normal range of flapping. Technically an ellipse but undetectable except on paper.
The photo of a Pitcairn hub above shows the flap hinges to be on or very nearly on the center of rotation. That must be a model of a direct control Giro since there is no such need in a machine controlled by ailerons and elevator.
I wouldn’t be surprised if the delta-3 coupling was more a matter of convenience in accommodating central flap hinges than anything else
09-11-2005, 05:54 PM
Thanks one and all
C. Beaty, I have seen the hub you show as well as the pictures of the one you built. My only squawk is the need for swash plate or spider cyclic pitch control. I wish to use direct control.
It is very easy to design a 3 or 4 blade rotor with the flap hinge centerline going through the spindle center line. I have modeled such a hub. The delta-3 does complete it some but not much.
Dave Jackson, I have read most of the UNICOPTER web sight and have found it very helpful. I just wish my math was up to it.
Now if we can figure the optimum location for the drag hinges? :rolleyes:
Most helicopters have them tucked in as close to the spindle as practical. I wonder if a rotor that is driving the hub should maybe move them farther out such as Dick Degraw is using?
Does it make any difference if the blades are being driven by the hub or are driving the hub?
09-11-2005, 08:00 PM
Drag hinges closer in is better from the standpoint of minimizing control force and vibration with a tilt head rotor but close in drag hinges makes ground resonance more of a problem and requires stronger dampers.
The Brantley B-2, with drag hinges and extra flap hinges at about 1/3 radius didn’t require drag dampers at all, just rubber cushions. But the landing gear struts had to be in good condition. That also imposed a tremendous load on the inner blade spars, which looked like 50 cal. machine gun barrels.
09-11-2005, 09:07 PM
Maybe that explains Dick Degraw’s comment to me at Mentone. When discussing his drag hinges, he said something to the effect that now if you can just figure out the correct place to put them.
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