Item 1466
DESIGN: UniCopter ~
Pusher Prop - Variable Speed Rotors and Prop - General Arraignment
Sketches:
A/ General Arraignment of 2-way Differential:
B/ Arraignment Specifically for the UniCopter:
The red connection between the differential and the jackshaft may be a belt, for rotor disengagement and soft-start. When disengaged, the drive pulley will rotate however no rotation will get to the propeller due to the speed controller. This will probably require a Sprag clutch between the propeller and the speed controller so that the prop can freewheel if the speed controller fails.
C/ Planetary Gear Arraignment for the UniCopter ~ with Planetary Gears: 
This replaces the spider gear differential with a planetary gear system. The ring gear would have V-grooves on its OD and then drive the other sheave to the rotors by V-belts. As above, the adjustment of the sheave spacing will be the soft-start for the rotors.
Option:
It may be better to replace the two sheaves, V-belts and soft-start mechanism with a pair of spur gears and centrifugal clutch.
- This will reduce the friction on the rotor portion of the powertrain from 2.5% to 1%.
- This will reduce the drag since the driven gear will have a smaller diameter then a driven sheave and be closed to the center of the propeller's disk.
Description of Operation:
Power Train:
The power from the engine is transmitted to the planet holder of a planetary gear set [functioning as a differential], (a)). The ring gear of the planetary transmits power to the rotors via speed reducers. The sun gear of the planetary transmits power to the propeller(s) via a speed controller.
If the speed controller is allowing its shaft to 'freewheel' then 50% of the power (torque?) will be going to the rotors and 50% to the propeller(s) [cruise]. If the speed controller is stopping the shaft from any rotation then 100% of the power (torque?) will be going to the rotors [hover].
The 2:1 power (torque?) change that the rotors will experience will manifest itself as a change in the rotational speed of the rotors.
The speed controller's function is to set the rotational velocity at which the power-train can drive the propeller. Therefore, consider locating an overrunning clutch between the speed controller and the propeller shaft. This should be a safety feature that will allow the propeller to freewheel should the power-train stop during forward flight. In addition, there should probably be a friction coupling between the differential's sun gear and the speed controller to allow the speed of the rotors to slowly increase, should the speed controller lockup.
Components:
Engine:
UniCopter ~ Engine
Speed Controller:
- To control the rotational speed of the propeller and thereby the rotational speed of the rotors; based on fixed motor speed.
DESIGN: UniCopter ~ Pusher Prop - Variable Speed Rotors and Prop - Speed Controller
Soft Start:
- Sheave grooves on outer surface of ring gear, V-belts, and sheave on shaft to Secondary reducer.
- The sheave end of the above shaft moves up and down to engage the rotors; similar to the Robinson.
- Alternative:
Consider replacing this with a more compact spur gear set and use the Friction Coupling mentioned below. Or see alternative mentioned in section on Friction Coupling.
Overrunning Clutch
:
- An overrunning clutch between the speed controller and the propeller shaft. This should be a safety feature that will allow the propeller to freewheel should the power-train stop during forward flight. It will also allow the propeller to freewheel at the lower airspeeds and thereby decrease the propeller's drag.
Friction Coupling:
- A friction coupling clutch between the differential's sun gear and the speed controller to allow the speed of the rotors to slowly increase, should the speed controller lockup. There may have to be an indicator to the pilot to slow the craft down, should the loss of thrust from the propeller not be a sufficient means of reducing the forward speed.
- Option:
This might not be required if the belts between the ring gear and the sheave are allowed to slip under excessive loading.
Propeller:
DESIGN: UniCopter ~ Pusher Prop - Variable Speed Rotors and Prop - Propeller
Possible Concerns:
What happens if the Speed Controller (servomotor etc.) should fail?
See [Overrunning Clutch] and [Friction Coupling] above.
Perhaps, consider two servomotors, one on each end of the worm. The motors would have independent electrical supply and control systems.
Can a servomotor rotate a 20,000 rpm?
Check that the spread between static friction and sliding friction (approx. 5:1) is not excessive for a small servomotor.
Rotor induced vibration as the RRPM passes through the harmonics of the blade's modes of vibration. The extreme rigidity of the blade should minimize this concern.
Will thermal changes cause the viscosity of the worm & gear's lubricating oil to change, thereby changing the dynamic and static friction? Will the servomotor be large enough to handle this change?
Consider a rotationally flexible coupling between the servomotor and the worm. This might allow the motor to 'index' (albeit at a very fast rate) while the rotational speed of the worm is constant and thereby not subjected to static friction during operation.
The 'spider gears' must be considerably larger than those use in road vehicle. This is because the deferential in the helicopter will be spending a lot of its time transmitting unequal RPM's to the two outputs. I.e. the spider gears are rotating a lot more. Use a planetary gear arraignment.
The additional gears will result in a power loss of approximately 1%; at all times.
Will the difference in value between static friction and sliding friction in the speed controller's worm and wheel create a problem?
The upper sheave will restrict and create turbulent airflow to the upper quadrant of the propeller.
Improved Airflow to Prop: The jackshaft from the soft start to the rotor's mid-gearbox could have universal joints at both ends to get the sheave closer to the prop shaft. Similar to the drive shaft in a car. If the jackshaft had a sliding spline then the vibration of the engine would not effect the rotor drive train.
Crazy Idea:
Could the servo(s) be eliminated and drive the worm from a 'fan' where the fan is receiving air from a high pressure area and discharging air into a low pressure area during forward flight.
Notes:
Vibration and Slow RRPM:
The blades cross 16 times per revolution with 4-blade rotors. This will be an 8 times per revolution opposed lateral crossing. At the slow speed of 275 RPM the opposed lateral cyclic will be 36.5 cycles/sec. The worst frequency is 16 cycles/sec.
Propeller to have a greater pitch than normal because it is intended for cruise, the rotors contribute to the forward velocity and the propellers' variable is rpm.
Does all this stuff depend upon the variable pitch of the rotors and the fixed pitch (or variable) pitch of the propeller?
More Information:
Nick L. mentioned that the Sikorsky X2 is to have an infinitely variable Rotor and/or propeller rpm.
The thread 'Topic: Variable Speed Rotors and Prop(s)' started on PPRuNe 9th/July/2005 discusses this concept.
Related Web Pages at This Site:
OTHER: Miscellaneous - Thoughtless Ideas - Multiple Speed Rotor
DESIGN: UniCopter ~ Rotor - Disk - Large Chord & Low Tip Speed
Information related to a similar electric drive, for reference; Inrunner+Outrunner_Motor
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Initially displayed: July 7, 2005 ~ Posted to PPRuNe: July 9, 2005 ~ Last Revised: May 29, 2007
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