Item 1074
DESIGN: UniCopter ~
Vibration Analysis - Rotor Induced - Downwash on Fuselage
Overview:
Several prototype helicopters have been designed with low rotor-fuselage spacing, and these have encountered insurmountable vibration problems that could only be solved by increasing the rotor-fuselage spacing ~
[Source ~ PHA p.477]I think that downwash on the fuselage may not be a significant problem with the UniCopter. This is because the fuselage is very small in plan view and its location is ideal for the two rotors, which have large blade cutouts. This may not be the case for a large transport intermeshing helicopter, such as
Stepniewski's concept. Perhaps this is why he has the advancing blades on the outside.
The frequency is twice that of the Robinson R-22, if the horizontal component due to counterrotating swirls is taken into consideration. The frequency will be four times that of the R-22 if swirl is not a factor. For additional information on frequency see:
OTHER: Dynamics - General - Vibration Analysis
Amplitude:
The amplitude should be about a third that of the Robinson, because the thrust is distributed among six blades versus two.
The amplitude on the tail may be even less (compared to the R-22) because only the boom is in the downwash. The tail feathers are above it in relatively clean air.
Drawing:
On Body - during Hover and Forward Flight:
Inner Portion of Rotor Disk(s):
The inner circles on the above drawing represent the end of the blades' cutout. The majority of the body is located within these circles and it is vertically very close to the rotor disks. During hover, it should not contribute to vibration.
During forward flight, the hubs, the blade cutouts and the interactions between the two may contribute slightly to vibration. The shortness of the blades' moment arms combined with streamlining in this area should keep vibration very low. See:DESIGN: UniCopter ~ Rotor - Blade - General - Cutout
Outer Portion of Rotor Disk(s):
The downwash from the mid to outer portion of the blades will be striking a very aerodynamic body. As mentioned above the high frequency and low amplitudes should result in minimal vibration.
On Tail Boom - during Hover and Forward Flight:
The blade tip is over the tail end of the boom at 8º azimuth. The blade - boom crossing continues in a forward direction until 22º azimuth. This means that in
22 - 8 = 14º the downwash will have traveled (96-36) / 96 = 62.5% of the distance to the craft's CG. The theoretical downwash wave will reach the CG in 14º * (100 / 62.5) = 22.4º.This action takes place every 60º, alternating between the port and the starboard rotor blades. the 22.5 to 60 is close to a 1:3 ratio. So what?? The rotor frequency is 70 Hz. Does this mean that the boom should not have a resonate frequency of
3 * 70 = 210 Hz???The strongest downwash on the boom will be experienced at the tail end. The horizontal and vertical stabilizers will dampen some of this vibration. The lay-up of the composite plies in the boom must be oriented so that the boom is not excited at the operational rotor rpm.
Higher Harmonic Control: Hughes Helicopter reduced the pitch by 0.33º at 330º-azimuth and there by reduced the lateral and vertical vibration levels on the tail boom.
[Source ~ RWP$ p.87]See:
DESIGN: UniCopter ~ Fuselage - Tail - Boom
Other Web Pages:
For general information see:
OTHER: Aerodynamic - Vibration - Rotor InducedLangley Tech Report:
John C. Wilson and Henry L. Kelly , Aerodynamic Characteristics of Several Current Helicopter Tail Boom Cross Sections Including the Effect of Spoilers , NASA TP-2506, AVSCOM TR-85-B-3, January 1986, pp. 73, (3.9MB).
Format(s):
PDFJohn D. Berry, Victor B. Letnikov, Irena Bavykina and Mark S. Chaffin, A Comparison of Interactional Aerodynamics Methods for a Helicopter in Low Speed Flight , 23rd European Rotorcraft Conference, Dresden, Germany, September 22-23, 1997, pp. 10, (2MB).
Format(s):
Postscript, or PDFJohn Berry and Nicolas Bettschart, Rotor-Fuselage Interaction: Analysis and Validation With Experiment , 53rd Annual Forum of the American Helicopter Society, Virginia Beach, Virginia , April 29--May 1, 1997, pp. 25, Also published as NASA TM-112859. (2MB).
Format(s):
Postscript, or PDFJohn D. Berry, Victor Letnikov, Irena Bavykina and Mark S. Chaffin, A Comparison of Interactional Aerodynamics Methods for a Helicopter in Low Speed Flight , TM-1998-208420, June 1998, pp. 17, (1MB).
Format(s):
Postscript, or PDFSusan Althoff Gorton, John D. Berry, W. Todd Hodges and Deane G. Reis, Flow Environment Study Near the Empennage of a 15-Percent Scale Helicopter Model , NASA/TP-2000-210085; AFDD/TR-00-A-004, March 2000, pp. 30, (2MB).
Format(s):
Postscript, or PDFRaymond E. Mineck and Susan Althoff Gorton, Steady and Periodic Pressure Measurements on a Generic Helicopter Fuselage Model in the Presence of a Rotor , NASA/TM-2000-210286, June 2000, pp. 188, (2MB).
Format(s):
Postscript, or PDF
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Last Revised: August 8, 2002