OTHER: Helicopter - Inside - Interleaving - Rubber-Band Powered Model
To test and develop the Interleaving configuration and have fun.
See Optional Arraignment ~ 2-bladed teetering rotors below.
Another option; Set the rotor masts vertical and give the blades a precone of 4 degrees. Then modify the 'frame' so that the rotors can have variable stagger. This will allow the Side-by-side, Interleaving and partial intermeshing to be compared; for at least thrust.
Rotors and X-frame with rubber band. Not shown are the fuselage, plus any pusher prop and its rubber band.
Expanded view of gearing, x-shaft, bushings and rubber-band.
Width 1/16" * 1/8" approximately.
Length 100' approximately.
400 - 500 turns.
Final diameter 3/8" - 1/2".
- A hub is located near each end of the X-shaft. It consists of 3, 4 or more fingers, which are radially located about its centerline. One hub is firmly attached to the X-shaft and the other hub is firmly attached to the X-frame. The rubber band is wound between the fingers of the two hubs.
- Rigid hub. ~ no teetering, flapping or lead/lag hinges.
- Hub drilled to accept 2, 3 and 4 blades per rotor.
- Holes at 0º, 90º,120º,180º,240º, 270º azimuth.
- Precone 3º
- Pitch set before flight by rotating the blade in the friction fit hole in the rotor hub.
- Rotor-to-rotor dihedral 5º
- Consider nylon bushings located along the full length of the x-shaft. The elastic-band is wound on top of these bushings. During flight the bushings will rotate at different rates and thereby reduce any frictional resistance between the elastic strands and the X-shaft.
- The bushings will result in a larger diameter of the rubber wrapping. This will result in more torque but fewer revolutions from the rubber-band.
- Different bushing lengths may vary the torque and speed at which the rubber-bands unwind.
- These bushings can be purchased in packets from electronic parts stores.
- Alternatively, if the x-shaft was oiled it might not provide excessive friction. The elimination of the bushings would reduce the overall diameter of the wrapping and decrease the overall weight of the craft.
Crown and Pinion Ratio:
- The larger the ID and OD of the elastic band wrapping the lower the gear ration. Perhaps a 4:1 (approximate) SPEED UP is required.
- What type of oil does Hans use on his rubber-band? Castor oil?
- A simple overrunning clutch will be required, assuming that the craft can lower the collectives and thereby get into autorotation.
- Consider having a separate propeller and elastic-band unit. Consider having the mechanism such that the propeller is held from being operational until the rotors have operated for some short period of time. Ie. Climb then forward flight.
Advancing Blade Concept (ABC);
- The model's blades and rotor will have a greater rigidity than that of a conventional helicopter. This means that as the forward velocity of the craft increases the advancing (outside forward) blades will assume a greater percentage of the lift, This means a reduction of the downwash in the area between the rotors, through which all four blades pass.
- Consider using a tube for the cross brace and then locate a loose rotateable airfoil over this tube. The intent is that the advantage of streamlining the cross frame during hover and during forward flight is greater than the added weight of the airfoil sleeve.
- An alternative would be to give the actual cross frame an airfoil shape; at some optimal angle.
- Another alternative would be to have three much smaller x-bars, each with the shape of an airfoil. The drag in this situation would be from the 3 x-bars, the x-shaft and the diagonal bracing.
Winding of elastic band:
- Extend both ends of the x-shaft slightly so that a hand-drill can wind up the elastic band. The double ending will maintain lateral balance and allow for possible wear on the extensions.
- Alternatively, extend the mast of one rotor upward or downward so that a hand-drill can wind up the elastic band.
Will the rotors require a forward tilt of approximately 6º so that during initial climb the vertical drag of the HS will not cause the craft to move aft as it climbs?
If the gears are 90º and the disks are laterally horizontal then the gearing etc. will be simpler. Is there any reason why laterally horizontal disks will not work well?
Will the large solidity ratio of the rotor disks plus the dihedral result in a slow and safe descent?
Will the craft require an overrunning clutch?
Will the blades require a 2-position pitch change for authoritative descent?
Would the inclusion of a separate rubber band driven push (puller) prop allow for rotor blades with a single pitch?
Optional Arraignment ~ 2-bladed teetering rotors:
This will give the best hover time of any configuration, with the possible exception of the side-by-side.
The X-shaft should be lowered slightly.
Locate an adjustable hub spring in the hub to restrict the teetering and give a little better control; at the expense of creating a little vibration when the disk is tilted.
The hub spring should give additional resistance to excessive teetering.
The blades are to turn outside forward.
In addition, locate a stationary hub spring such that it resists slightly the downward motion of the blades when they are in the retreating half of the disk. This will cause the disks to rest with the inside blade being slightly higher than the outer blade.
Fortunately, the structure will not have to be as strong since the gyroscopic precession will not apply large forces.
Alternatively or secondarily. But two (or three) small identical electric helicopters w/ tail rotors. They must have symmetrical blades. Then join them together with a X-frame, remove the tail rotor and join the tail rotor shafts or join the tail rotor drive pulleys with one longer belt so that the two main rotors are synchronized.
Later compare the lifting ability of this Interleaving helicopter with the lifting ability of one identical single.
Hans said that he was going to build a model similar to this, and initially without flight controls.
- When hovering, both disks will be horizontal due to the blades operating in higher induced velocity of the overlapping area of the two rotors. The force of the stationary hub spring should help in distributing the thrust a little more evenly over the combined disk area.
- When in forward flight it will also create a slight Advancing Blade Concept.
- Hopefully, this lack of dihedral will not be detrimental to stability.
August 17, 2006 ~ Hans tested his helicopter with the hubs rigid and with them teetering. He said that the flight time was considerably better with the teetering hubs. It is assumed that the teetering allowed for better distribution of the thrust about the two disks. He also reduced the chord of the blade and this might also contributed to the better performance.
- Model R/C helicopters do not have 'teetering' rotors. The rotor hub will have to be 'homemade'.
This might be the first step toward building an electric model, and then enlarging and further developing an electric rotorcraft.
See; OTHER: Helicopter - Inside - Interleaving - Electric UAV
- An alternative to this idea would be to have the axis of the two rotors vertical, and have the crown and pinion gears conventional 90º. Give the rotors a 10º precone. (Sikorsky did on hie early helicopter).
- A centrally located motor could drive the cross-shaft via a small crown and pinion, or a planetary gearbox (pruchased) coupled to the pair of pinion gears.
- The rotors could be spaced for side-by-side, through Interleaving, to widely spaced Intermeshing.
- Absolutely Rigid Rotors.
- See DESIGN: Electrotor-SloMo ~ Rotor - Disk - Zero Obliquity Angle
It should be advantageous to build this so that the rotor stagger can be varied. This way the side-by-side and variation on the interleaving stagger can be compared. See page 1525 for information on the aerodynamic interaction of interleaving rotors.
The current layout maybe laterally unstable, particularly if the rotor disks are horizontal. This might be the result of the induced velocity being higher in the center. ABC during hover may overcome the concern, if it is valid.
Related Web Pages:
OTHER: Helicopter - Inside - Interleaving - Advanced Very-Light Rotorcraft
OTHER: Helicopter - Inside - Interleaving - Simple Very-Light Rotorcraft
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Initially displayed: November 9, 2006 ~ Last Revised: May 8, 2014