DFC Rotor Heads.

Rotorcycle

Junior Member
Regarding Direct Flight Control rotor heads as used on numerous models these days but in particular the larger unit used on Josef Schmirl's R/C "Red Bull" model helicopter.
This model has a Rotor diameter of 131" or 10.9 feet. The main feathering shaft is a loose fit in the main head "T" section and is held in place by a hard rubber damper recessed into each end of this "T" section. The teeter/lead lag movement is minimal however the lead/lag is compensated by a single bolt fixing the blade to each end of the 'feathering shaft".The tension of these 2 bolts is firm and acts as a simplified damper whilst allowing the blades to lead/lag.
Some 3D model pilots replace the rubber dampers with solid copper dampers (absolutely rigid) and run the rotors at higher RPM's.
Why do these models not roll inverted or are the blades flexible enough to offer teetering?
The two blades and "feathering shaft" assembly is free to move in a restricted axial direction which is limited by the hardness of the rubber dampers.
Are the blades forced to be dynamically balanced as appears with Webb Scheutzow's Bee helicopter where the blades are held central again by rubber dampers?
 

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Rotorcycle

Junior Member
Ok! There's been 70 views to date and not one response or explanation of the questions presented!
There are YouTube videos of the Schmirl "Red Bull" model with flight views from attached cameras and these offer no indication of vibrations or rough flight.
Lets forget the models for a moment and consider a larger pitch controlled quad-copter running four 9' diameter rotors.
Is it necessary to incorporate limited teeter or can the rotors be a rigid assembly?
In a typical single rotor helicopter flight with a rigid rotor system is not possible except with zero/low airspeed or the machine simply exhibits uncontrolled roll movement due to dissymmetry of lift.
This is not the case with a Quad-copter as the roll movement is balanced by the opposite rotor direction of the diagonally placed rotors.
It appears a rigid assembly would be most appropriate as the rotors would remain perpendicular to the driveshaft and reduce vibrations?
The bending loads would be transmitted to driveshafts and supporting structures.
Looking for info on a somewhat new area of flying machine.
 
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gyromike

Administrator
Staff member
You probably haven't gotten any responses as this is a forum for full-scale gyroplanes and helicopters, although there may be a few model helicopter pilots here.
 

C. Beaty

Gold Supporter
2-blade rotors without a flapping hinge are impossible at full size; such an arrangement would be worse than riding a jack hammer. 3-blade rotors without flapping hinges are possible if the CG of the machine is at rotor height; otherwise gyroscopic effects would render such a scheme unflyable.
 

hillberg

Newbie
scale up a model as it's constructed you'll have an 18 ton Hughes 500 ???? with 1/2 slop in the rotor head???? and 5,000 horse power???? That only seats one.
 

Rotorcycle

Junior Member
No Title

Thanks Chuck, your reply is appreciated and respected and I can only agree with you regarding a typical single rotor machine relying on cyclic for directional control however a pitch controlled Quad-copter would not require cyclic control. Neither Franz Schofman's or the GEN-H4 have any form of teeter movement and gyroscopic effect is not an issue with this rigid setup.
With two diagonally located rotors rotating in the opposite direction to the other two we have virtually a co-axial machine except the rotors are on different shafts.

Thanks Don for your input as I was after sensible info and you have certainly delivered!
Jukka Tervamaki modified his Helicopter performance program specifically for this application and he supplied the results as in the attached performance graphs.
He divided the Gross weight and available horsepower by 4 and used one set of rotors for his program to calculate.
The small diameter rotors plus the high RPM's were outside the standard program's limits hence the need to modify the program data.
Auto-rotation would be interesting for sure!
 

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C. Beaty

Gold Supporter
The Doman helicopter had a 4-blade hingeless rotor using a “floating hub” that permitted hub alignment with the rotor tip plane. Flap and drag hinges not required.Doman hub.JPG
 
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Rotorcycle

Junior Member
No Title

Thanks Chuck, Just off the cuff how does the Gen-H4 manage with non teetering heads and it is spinning 13' diameter rotors? Dwg. looks as though it has dampened lead lag hinges,
Also the tilt wing is does not appear to have these problems??
 

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C. Beaty

Gold Supporter
Counter rotation cancels both gyroscopic reaction and dysentery of lift vibration. There are high internal stresses between the two rotors but external forces are canceled.
 

hillberg

Newbie
Rotorcycle;n1126769 said:
Thanks Chuck, Just off the cuff how does the Gen-H4 manage with non teetering heads and it is spinning 13' diameter rotors? Dwg. looks as though it has dampened lead lag hinges,
Also the tilt wing is does not appear to have these problems??
the tip path plane doesn't tilt as it moves with the shaft, the mass doesn't change as the tips and shafts share movement as a unit. only coning might add a little but those hub plates are beefy.
 

Steven

Awe Inspiring Human
"Counter rotation cancels both gyroscopic reaction and DYSENTERY of lift vibration".
I really don't think so...unless the pilot's stomach is really upset.
 

C. Beaty

Gold Supporter
Confusing dis-symmetry with dysentery is the reason I flunked 3[SUP]rd[/SUP] grade.

But even worse, I once had a part time employee, a young female college student working toward a degree in education who confused tentacle with testicle. “An octopus has long testicles.”
 
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Rotorcycle

Junior Member
Air-frame for this Quad copter is nearing completion however rotor dynamics is beyond my scope as is with most people.
Calculating the teeter required to cancel or minimize the bending loads on the shafts etc through dissymmetry of lift is in the "too hard basket" for me.
I believe the two sets of contra-rotating rotors balance out the gyroscopic forces and vibration as Chuck mentions and the stresses are contained between the rotors.
Failure/distortion of the booms is not an option.This is why I originally requested info on DFC rotor heads.
If any of our viewers can offer any info/data for this Quad Copter configuration it would certainly be appreciated.
If rotor disks are included it has a huge footprint of 5.25 x 5.25 metres!
 

gyromike

Administrator
Staff member
C. Beaty;n1126807 said:
Confusing dis-symmetry with dysentery is the reason I flunked 3[SUP]rd[/SUP] grade.

But even worse, I once had a part time employee, a young female college student working toward a degree in education who confused tentacle with testicle. “An octopus has long testicles.”
And them kinda octopussies give me dysentery!
 
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