DESIGN: AeroVantage - Transition Mechanism - Proposed Method - 1x2 - Extendable Tubes
Opposing forces, which may be good. During transition from cruse to hover, the front PropRotor will want to assist in extending the tube whereas the moments of both the PropRotor/motor assemblies will oppose this transition. The situation will also apply in the other direction. This might mean that the power required to transition might be less.
The front PropRotor is mounted on two ball-screw / tubular beams.
Alternatively, hydraulic, or even pneumatic (see below), cylinders.
Sikorsky's Variable Diameter Tiltrotor (VDT). "The design uses a jackscrew-actuated telescopic blade that extends in the hover to reduce disk loading and retracts in forward flight to reduce tip speed."
Cruise to Hover:
The ball-screw / tubes extend the front Proprotor forward.
Then the front and rear PropRotors rotate from their cruise orientations to their hover orientations. These rotations might, or might not, be in exact unison; depending on what is required to eliminate any pitching moment.
As with other 1x2 configurations, all flight control will be by full cyclic and collective control on both PropRotors, which are extremely rigid. Fuselage flight controls might also be includes to supplement the PropRotor controls and to serve as redundancy, during forward flight.
The forward portion of the transition mechanism is part of the leading edge of the wings, at their roots.
The extension of the front Proprotor may be, or may not be, assisted by increasing the thrust of the front PropRotor and decreasing the thrust of the rear PropRotor. Conversely, The retraction of the front Proprotor may be, or probably may not be, assisted by increasing the thrust of the rear PropRotor and decreasing the thrust of the front PropRotor.
The rotation of the PropRotor/Motor assemblies will probably be done by four hydraulic 3-vane radial actuators. See Item 1696.
The ball screws, or simply threaded screws, may be hydraulically powered, as might the flight control actuators in the rotor heads.
Alternatively. A hydraulic cylinder in both transition beams may be used to extend and retract the front PropRotor.
Alternatively, all the actuators might be electrical.
The pair of extendable beams will be located under the pair of wing fences
At 150 psi the extension cylinders should have an inner diameter of roughly 3"
Model: Bimba makes 7/16" bore cylinders. This is 3/0.4375 = 1/7th the full size. Therefore, could make 1/4 scale model c/w pneumatic transitioning.
Wrist: Consider using a coil spring inside the wrist to counter the gravitational caused torque on the PropRotor assembly. The coil spring must have little torque at one angle and half the counterbalancing torque 90º away.
Front Streamtube Location:
See; DESIGN: AeroVantage ~ Trim, Stability & Control - Longitudinal During Transition - Transition & Longitudinal Balance
The upper sketch shows all tipping taking place when the front PropRotor assembly is fully extended.
This sketch shows the relation of the streamtube to the fuselage and wing at 90º, 60º, 30º and 0º if the change of the tipping angle has a direct 1:1 correlation with the extension of the forward PropRotor assembly.
Driving of Transition Mechanism: below
It may (or may not) be aerodynamically better to have the entire tipping take place when the unit is fully extended. Or have the tipping take place during the outer portion of the extension. Because this will delay the downwash on the fuselage until the craft has achieved a faster forward velocity and the wings are providing more of the lift. Note that the downwash from the front PropRotor on the fuselage probably means that the craft will want to pitch down, however the rear PropRotor may also be reducing the lift of the wings as it approaches the cruise position and thereby offset the downward pitch.
Driving of Transition Mechanism:
Note that if a threaded rod (tube) is used for extension then this 1:1 ratio allows for the a worm wheel (with a 60:1 ratio plus 4:1 spur gears) will allow the threaded rod to tip the PropRotor. The advantage of this is that the changing torque that is required to rotate the PropRotor (due to gravity and the angle (horizontal length of the moment arm)) will counter the changing force of the PropRotor (due to horizontal component of thrust) attempting to pull the threaded rod forward. Note that the pivot point of the tipping can be relocated above or below the centerline of the threaded rod due to the spur gearing.
It may be possible to couple the tipping of the rear PropRotor to the threaded rod. This will result in more countering of the pull on the threaded rod and even less power being needed. Plus, one centrally located motor (behind the seat of the pilot) will drive the whole transition mechanism.
Pros & Cons(erns):
This may be the cleanest and lightest transition method.
The craft may be able to do running takeoffs and landings with the PropRotors at 45º of tilt. But why?
The arch portion of the mechanism of the front rotor could be made deeper (i.e. more of the wing's planform area) so that the pilot has a clearer view to the side. Unfortunately, this deeper arch will block more of his forward view during hover. Perhaps this root leading portion of the wing and arch should be radiused.
- How to get double, or triple, redundancy, for safety
- Re Extendable tubes: From a paper on the Variable-Diameter Rotor system."The jackscrew is a heavy mechanism and the associated retention nuts will experience wear due to the high tensile loads they must bear. Mechanical tests on the jackscrew mechanism in 1976 indicated that the retention nuts could withstand 1000 retention and extension cycles with the materials available at the time , but a long system life may be difficult to achieve. The advantages of the jackscrew mechanism are that it is self locking and friction between the nuts and screw are available to dissipate rotor kinetic energy during retraction. Ballscrew mechanisms would eliminate wear, however, they are not self-locking."
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Initially displayed: January 10, 2009 ~ Displayed on PPRuNe; January 12, 2009 ~ Displayed on Rotary Wing; January 12, 2009 ~ Last Revised: September 3, 2009
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