B437

DESIGN: UniCopter ~ Control - Flight - Independent Root & Tip

Blade Construction:

Active Blade Twist:

The spar's and skin's strength and resistance will be greater at the root end than at the tip end. Most of the blade's twist would take place at the tip end if the twist was only applied at the two ends of the blade. There may be an advantage to distribute this twist a little more evenly along the full span.

The 'twist' can be off-loaded from the torque tube along its length. Therefor the total torsional load does not have to be transmitted to the its smaller diameter at the tip.
The change in pitch will be more uniform along the span of the blade.
The elastomer, which is transmitting the twist from the torque tube to the to the blade can additionally serve as the tip controls pitch bearing.

In other words; the tube will be elastomerically bonded to the main spar. This will mean that some of the pitch change from the tip controller is directly transmitted to the blade, without requiring the torque to go all the way out to the tip and back. The strength of the elastomer may decrease and/or the elastomer's wall thickness may increase as it gets closer to the blade root, to vary the amount of torque applied along the span of the blade. In addition, this eliminates the need for conventional bearings between the blade tip's torque arm and the blade spar.

Spar:

For information on the spar and pictures of the Bo 104 spar root see; DESIGN: UniCopter ~ Rotor - Blade - General - Root

Manufactured in Twist (mean twist):

The average twist in the table above is 5.67º. ???

Note that built-in twist does not have to be incorporated into the spar or the torque tube. It only needs to be built in to the skin and the mounting holes rotated accordingly on the pitch horns.

Actuator Frequency:

The frequency of both the root controller and the tip controller will effect what the optimal pitch settings will be at each azimuth in each mode of flight.

The speed of cruise will also effect what the optimal pitch settings will be. One reason will be the size of the area of reverse velocity.

Additional Notes:

Pitch Bearings Forces:

Root:

The Absolutely Rigid Rotors will not allow for flapping and lead/lag. This, combined with the centrifugal force, will transmit very large loads from the blade to the hub, via the pitch bearings. The resistance to rotation will be great, but, these bearings are located between the blade-spar and the rotor-hub. They are therefor only handling the pitch change of the root. The root's pitch change is relatively infrequent and is changed by the mechanical actuators.

Tip:

The pilot's cyclic stick controls the pitch at the blade-tips. Since the predominant forces are taken by the root pitch bearings, the force required may, hopefully, not require any power assist. Note: In situations such as fast forward flight there will be a great difference in twist between azimuth 90º and 270º. This will definitely have an effect on the tip cyclic.

Rotor Speed:

The two main rotors rotate at a slower RPM than current conventional rotors. (I.e. a tip speed of 478 ft/min). This will make for a quiet rotor and allow for fast forward flight. See; DESIGN: UniCopter ~ Rotor - Disk - Large Chord & Low Tip Speed

Concern:

The greatest twist is 12º at hover and in autorotation. Note that the amount of twist will be less from the default twist. Will this twist deform the straightness of the feathering axis and how much will this twist decrease the rigidity in-plane and out-of-plane? An asymmetrical blade may be a real problem. A blade can probably be eventually built correctly but the experimenting with varying the ply lay-ups might be phenomenal.

For the blade to have variable twist, it must be designed with a controlled amount of torsional flexibility. This brings with it the disadvantage of making the blades more susceptible to torsional flutter, and divergence instability.

The blade, the tip's tapered composite tube, and the affixing elastomeric will have different frequencies. Perhaps this, in conjunction with dampers between the two composite parts may overcome this concern. A stiffer blade may reduce the flutter but increase the tip's pitch control loads.

The tapered composite tube may not have a large enough cross section near the tip to be able to apply enough torque.

The forces that must be applied by the pilot to control the pitch of the tips of the blades may be excessive for manual power. And/or high friction, high breakout forces and stick vibration.

Thoughts re UniCopter:

Alternative:

If the cutout on the UniCopter's blade is a large percentage of the blade-span then consider locating a separate airfoil 'skin' over the cutout's spar and varying the pitch of this 'skin'. See; OTHER: Rotor Concept - Independent Root & Tip - Floating Root Method

Last Revised: February 12, 2007