Item 1086
OTHER:
Aerodynamics - Vibration - Rotor Induced - Analysis of Flettner FL-282 ~ Intermeshing
Flettner FL- 282:
It was mentioned in 'Evaluation and Tests of the Flettner-282 German Helicopter' that increasing blade weight on a two-bladed synchropter increases smoothness.
Observed flight vibrations are present even in hovering and do not show any peaking of amplitude but gradually increase in amplitude with increasing speed. This is considered to be a serious defect of the machine. (Synopsis of Air Technical Intelligence Review No, F-IR-133-RE)
An article in American Helicopter, June 1947 says "The two blades rotors, ..., produce vibration of the fuselage which appear to be torsional about the vertical axis.
From 'The Luftwaffe Profile Series N0.9, page 30; "The experimental installation of across-hair sight proved unsatisfactory, moreover the helicopter's high vibration levels presented some difficulties in acquiring the target."
From 'The Luftwaffe Profile Series N0.9, page 5; "A different rotor arrangement - two 3-blade rotors - was tried out on the test bed. It proved to run extraordinarily smoothly, however this was not a consideration for military use."
Comment by Self. ~
The 4-blade rotors on the UniCopter and the their thinness at the tip may reduce this vibration, if it is the result of H-force drag. In addition, The center of the UniCopter's H-force should be closer to the craft's X-axis, because of the greater stagger than the Flettner and the greater blade thickness toward the root.
The Dragonfly and the Unicopter will not be subjected to the vibration caused by the Flettner's 2-blade rotors with their flapping hinge offset.
A pusher prop will reduce the rotor-rotor aerodynamic interaction in forward flight.
Prewitt Report:
Partial Summation:
[Source ~ ETF p.3]The vibration of the ship about an imaginary vertical axis lying between the rotors causes the pilot's seat to vibrate sideways. The angular motion of the vibration is approximately 0.12 degrees, and on the assumption that the pilot is 40-in. forward of the center of this vibration, the total amplitude of this motion would be 0.084-in. sideways. This is approximately twice the threshold of discomfort [see
1059.html].Comment by Self. ~ The pilot in the SynchroLite will be approximately 10-in forward of the center of this potential vibration.
A vertical pickup on the pilot's seat showed total amplitude of 0.085-in. at 30 mph and 0.067-in at 75 mph.
Comment by Self. ~ The horizontal stabilizer may amplify the actual amplitude at the rotor and if this is the case, the SynchroLite's vertical amplitude at the seat will be less. In addition, this vertical pickup [71] is on the starboard side and the motion appears to be coincident with the yaw vibration during forward flight and the moments through the craft's center of mass.
The vibrations of the Flettner ship at the pilot's seat would indicate sufficient amplitude to be objectionable, although the pilot who was flying this machine, and had flown many other helicopters, did not feel that the vibration of the Flettner machine at the pilot's seat to be relatively objectionable.
To my knowledge, rotor induced vibration has not been a concern on the Kaman helicopters and these helicopters use teetering hinges. The Flettner also had 2-blade rotors but it had flapping hinges with an offset of 12.8" (5.4%R). During extreme cyclic stick positions, the 90º-azimuth difference between the two rotors may well have caused vibration. See;
DESIGN: SynchroLite ~ Rotor - Disk - Lift Distribution re: Vibration. This appears to be the same problem that Bell faced with its 'hub spring'It is probably correct to assume that the lower blade will temporally loose some thrust as it passes through the downwash of the upper blade. In addition, it is probably correct to assume that the upper blade rotor will temporally loose some thrust as it passes through the downdraft of the lower blade. It is therefor probably correct to assume that the blades will see a reduction in profile drag as they pass through the downwashes and downdrafts.
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Last Revised: May 25, 2008