A080

DESIGN: UniCopter ~ Weight & Balance

Outside Helicopter

Robinson R22:

Ultrasport 254 Center of Gravity Limits:

The center of gravity on the Ultrasport 254 must be between 4.5" and 1.5" in front of the mast.

UniCopter

Center of Gravity:

It is appearing that the optimal position for the center of mass is on the centerline of the mast (or perhaps slightly in front of the centerline of the mast; but not as far forwards as found in conventional helicopters). See [Source ~ RWP2 p.36] for best coverage of this subject.

I think that the moment arm between the center of thrust and the center of mass is only relevant during acceleration. The exception to this is gravity, which is being constantly applied. During forward flight, with the center of gravity ahead of the center of thrust, both parasitic drag and CG are attempting to cause a nose down attitude. Even if the CG was directly under the center of thrust during hover, because it is below the center of thrust, it will move on the X and Y axis when the attitude of the helicopter changes. Having a high CG will reduce this.

Correction on above ~ For stability during gusts the center of mass should be ahead of the center of thrust, so that an upward gust will cause a corrective nose down pitch of the craft.

Weight Penalty:

Higher Empty Weight to Payload Ratio: The following is a comparison of the older UniCopter I with the SynchroLite.

	Item:	Increase x SynchroLite Wt.:
	Blades	* 3.0
	Hub (to handle extra moment)	* 1.5
	Pitch bearings (to handle extra moment)	* 2.5
	Mast bearings (to handle extra moment)	* 2.0
	Frame (to handle extra moment)	* 1.2
	Fuselage (to handle extra moment)	* 1.1
	Total Empty Weight	* 1.33
	Gross weight ( assuming EW/Payload ratio of 50/50)	* 1.165
	Increase in Empty weight (because of need for stronger bigger components) to handle the above increase in weight	If the GW is now 16.5% greater then the original then Total Empty weight will have to be increased by 1.165 also. There for the Total Empty weight must be increased by 1.165 * 1.33 = 1.54 (roughly) 254lbs. * 1.54 = 393 lbs. (550 - 254 = 296) (393 + 296) / 550 = 25% increase in Gross Weight for no increase in Payload.

Possible solutions:

Use lightweight composite construction as much as possible. (rotor hubs?, drive frame?)

If accelerometers were incorporated to limit the amount of 'torque' (and not so snappy response) then the Gross Weight may only have to be increased by 10 - 15 % since the rotor and drive train could be built lighter.

The UniCopter's mast angle is closer to the vertical than the SynchroLite's and others. This difference give a 1% increase in lift for the same thrust, - no big deal.

Note: The above is a comparison to the ultralight SynchroLite. When the UniCopter is compared to a conventional single rotor helicopter, the results are not so bad. The elimination of the tail rotor results in an 8% saving in power for the coaxial. If the dry weight to payload ration is 50/50 then the first (8% x 2) = 16% increase in the craft's net weight is a freebie. The intermeshing configuration has a better lift to weight ratio than the coaxial, therefore it should be even better than the 16%

Datum:

Origin (0,0,0) is the point midway between the centers of the two rotor hubs.

Trim:

To move the center of gravity ahead, the radiator(s) could be moved forward to the entry openings.

In fact, move all movable items forward.

Same Page ~ Different Craft: ~ SynchroLite ~ Dragonfly

Last Revised: February 5, 2004