DESIGN: Electrotor-SloMo - Rotor - Disk - Location of Gimbal

• Considerations as to the optimum vertical location of the gimbal during changes in flight attitude and during forward flight.

• This sketch shows the coaxial configuration, however it and the following notes are also applicable to all the bilateral configurations.

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• For more information on the gimbal location see; DESIGN ~ Electrotor-SloMo ~ Control Flight - Weight-Shift - Overview

• This locating of the gimbal should be looked into. In addition, the precone angle should be looked into at the same time.
• Speed Stability: I believe that; 1) the precone angle and, 2) the pilot's drag on a 'stand-on' helicopter, will both contribute to speed stability,
• Moments on the rotor due to the vertical location of the gimbal and the weight of the 'fuselage'.
• A force of gravity that is acting above the center of the rotor assemblage will cause a negative (downward) pitching moment. Whereas, a force of gravity that is acting below the center of the rotor assemblage will cause a positive (upward) pitching moment.
• I think that a positive pitching moment (i.e. the gimbal is located below the rotor assembly) will want to right the rotors and thereby provide speed stability. Larger tipping angles will be subjected to larger speed stabilizing.
• However, the force to hold the forward tip might be excessive for the pilot.
• However, an adjustable trim device might rectify this.

• A high gimbal will act somewhat like the Hiller Flying Platform and the de Lackner HZ-1 Aerocycle stand-on helicopter. Here the weight of the pilot assists with the tilting action.

• If the gimbal was located below the rotors and the precone was reduced to 2º to 3º;
• It might make for better performance in regard to Speed Stability.
• It should simplify and lighten the hub, because the diameter of the hub's tube can be reduced.
• The vertical location of the gimbal on the Bensen ~ Little Zipster - B-9 and B-8MH may be too low. See the two drawings on the proceeding link.

• More Thoughts re Location of Gimbal;
• The aerodynamic center of the rotorhead is above the center of the mass of the rotorhead+motor. This might suggest that if the gimbal was located at the center of mass location.
• The arm between the aerodynamic center (center of thrust) and the gimbal will want to restore the craft to a hovering state. This is because the gravitational pull on the gimbal will want to force the arm to become vertical and aligned with the gravitational pull. IE Speed Stability.
• The inertial that is resisting the tilt the rotors, particularly in the pitch direction, will be at the lowest possible amount. This will reduce the effort required from the pilot's arms and minimize the swing of the pilot's body and feet. Note that there will probably be an aerodynamic drag (called profile or parasitic?) due to the moving of the blades flat-wise through the air.

• Possible Advantages:
• Should provide for better response to perturbations. This puts the fuselage's center of gravity slightly ahead of the rotors' center of lift. An upward gust will cause the rotors to tip downward at the front, which will oppose the increase in the lift. A downward gust will cause the rotors to tip upward at the front, which will oppose the decrease in the lift.
• Concern: This may impart sudden jolts to the pilot's arms and his grip on the 'front legs'.
• Allows for synchronization of rotors by pinions on the longitudinal centerline of the craft. Note that the crown & pinions gears must be cut at 84º, not 90º.
• This is the same as a forward center of gravity on a fixedwing aircraft. Consider countering the moment resulting between lift and gravity by a spring. Will allow for longitudinal trim adjustment. Ground adjustable or in-flight adjustable trim-tab?
• This may also want to give the craft a slow forward velocity if the loss of lift was done during a hands-off situation????? The large precone should inhibit the velocity from becoming to great.

• Aerodynamic Center: The mean lift of a blade is at approximately 75% of radius. This implies that the plane of mean lift passes through all blades at their 0.75R. It would appear that having elevation of the gimbal below this plane would contribute to stability. This vertical distance will be sin(a₀) x R. Where a₀ is the coning angle and R is the radius of the rotor disk.
• Perhaps this vertical distance will be comparable to that of the undersling on a comperable teetering rotor, except that the blades on a teetering rotor are not 'absolutely' rigid.
• Center of Mass: The elevation of embedded electric motor will be below the elevation that was calculated. Perhaps the gimbal should be located at the center of the rotors and the frames mass to reduce the rotational inertia to flight control.
• Conclusion?: The gimbal should be located somewhere on the vertical line that joins the above two centers.

• It is appearing that the gimbal should only provide rotation about the X-axis and the Y-axis. It should not allow any rotation about the Z-axis.
• For a potential gimbal with large central passageway for electrical cables, or for ideas see; UniCopter ~ Rotor - Hub - CVJ+HS.
• Gimbal may be billed out on Frame or Fuselage; depending where the disconnect is located.
• Consider an aluminum gimbal with a hollow center for the wiring.
• Could this hub be of any value/ UniCopter ~ Rotor - Hub - CVJ+HS
• Aluminum universal joint? ~or ~ Rod end? Note that the rod end does not prevent rotation about the Z-axis.
• http://www.pegasusautoracing.com/productselection1.asp?Product=1493
• http://www.borgeson.com/xcart/home.php?cat=13
• Is there any reason to give the gimbal a longitudinal offset as is done on the gyrocopter. Perhaps to bypass a synchronization shaft for crown and pinion gears? See DESIGN: Electrotor-Simplex ~ Rotor - Hub - Gyrobee Information
• On the gyrocopter the roll hinge is below the pitch hinge (length of moment arm from center of rotor's lift to hinge). This maybe done so that there is greater stability in roll and greater controllability in pitch.
• A question on this subject has been placed on Rotary Wing Forum. Replies indicate that the vertical separation of the pitch and the roll pins is based on manufacturing and not on flying characteristics.
• Consider locating the pitch hinge to the roll hinge at the narrowest location between the two rotorhubs. One reason for this is because the role's wide swinging lower arm will then be below this narrow restriction between the rotorhubs.

From the drawing on DESIGN: Electrotor-SloMo ~ Frame - Intermeshing - Thin Mast - General

• Plane of mean thrust (0.75 Radius of two rotors) is 6.6999" above plane of the two rotor heads.
• Mass of Principal assemblies is 1.8882" below centerline plane of the two rotorheads.

• Related Pages:
• OTHER ~ Flight Dynamics - Rotor Hub - Cyclic Control for a Simple Ultralight Helicopter

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