B226
DESIGN: SynchroLite ~
Vibration Analysis - Rotor Induced
Outside Helicopters
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Name |
Item |
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UniCopter ~ Vibration - Rotor Induced - Analysis - Re: Sikorsky |
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UniCopter ~ Vibration - Rotor Induced - Analysis - Re: Flettner FL-282 |
Model:
A model synchropter was entered in the 1995 World RC Championships. It had vibration problems and tipped over. The web site w/ picture are no longer on the Web.
Gyrocopter:
The lead-lag of the rotor disk; see DESIGN: Power Train - Frame [
B132]
SynchroLite
Random Notes:
Vertical Vibration:
Consider the idea of a vertical motion detector in the tip of each blade, where the tip weight goes. This would record the amount, and with shaft azimuth detector, the location of all excessive blade action. Mold 2 wires into composite blades, just incase?
To minimize vibrations consider;
Frequency:
The SynchroLite's rotors turn at a faster speed than those of a conventional single rotor helicopter. This means the the rotor induced vibrations will be at a higher frequency, which is not so objectionable.
Cyclic Stick:
Not only will the vibration be in the frame of the craft, and eventually into the body of the pilot but it will also show up in the cyclic stick. Probably the more that the blades' center of lift and center of mass is off the center of feathering, the greater the vibration. Note what Flettner had to do to reduce vibration to the cyclic stick.
Instruments for Testing
:All instruments, sensors and wiring etc. that are use for testing and product development might very likely be exempt from the 254 lbs. weight limitation.
Web Sites:
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Flight and analytical methods for determining the coupled vibration response of tandem helicopters. |
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Flight measurements of the vibration experienced by a tandem helicopter in transition, vortex-ring state, landing, approach, and yawed flight. |
Vertical Vibration:
The above report NACA 4409 is a measurement of vertical vibration only. The vertical vibration is 4.3 in/sec at the rear rotor and 1.6 in/sec at the front rotor in hover (see item 2. below). In forward flight at 40-knots the vibration is reduced by a factor of about 3 at the rear rotor and is only slightly reduced at the front rotor when the helicopter is yawed to an angle of 23 degrees (see item 1. below).
Assume that the flow area 'x' feet above a rotor disk is twice the size of the flow area 'x' feet below the rotor disk. This will result in a velocity in the upper flow area that is 1/2 the velocity in the lower flow area. Pass paddles of the same size and traveling at the same horizontal speed through both the upper and lower flow areas. The upper paddle will experience half the force as the lower paddle but for twice the duration.
The following equations (using arbitrary values) show that the upper paddle will be moved twice the vertical distance as the lower paddle is moved. This may be the reason why the upper rotor-disk is subjected to greater vertical oscillation.
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a |
Acceleration |
Upper |
Lower |
Equation |
Upper Paddle (Blade) |
Lower Paddle (Blade) |
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f |
Force |
20 |
40 |
a = f/m |
20/10 = 2 |
40/10 = 4 |
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m |
Mass |
10 |
10 |
v = v0 + a*t |
0 + 2*2 = 4 |
0 + 4*1 = 4 |
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d |
Distance |
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d = d0 + v*t + .5*a*t^2 |
0 + 4*2 + .5*2*2^2 = 12 |
0 + 4*1 + .5*4*1^2 = 6 |
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v |
Velocity |
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t |
Time |
2 |
1 |
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If the above proposition is true then the majority of 2-P and/or 4-P vibration experienced by an intermeshing helicopter might be caused by the upper blade activity as it passes through the "downwash" of the lower blade.
The timing of this interaction, in hover and forward flight, has yet to be calculated. It also has to be associated with the 4 locations on the rotor(s)-disk.
Flettner Vibration as it relates to the SynchroLite:
The summary of a report from an interview with Hohenemser state that the vibration about the vertical axis (3.) is the objectionable one not (1.) If this is the case then vibration should not be a problem.
The pilot of the SynchroLite is sitting very close to the center of gravity. This should mean that the 3 rotational motions should not be of much concern. Of the 3 linear motions the only one of concern could be the vertical, see (1.)
Re; Vibration:
The following was on PRA. It discusses 2P vibration caused by the drag of the blades at 0-180 degree azimuth vs. 90-270 degree.
Rotorhead ~ Author: CA BEATY ~ Date: February 05, 1999
A good example is the Young (see-saw) rotor. No engineering book to my knowledge covers the dynamics in enough detail to be useful. What is the correct under-sling? Should the teeter bolt lie in the plane of rotor CG or center of percussion? A plausible argument can be made either way. I have chosen CG but could be wrong.
During translational (forward) flight, there is a periodic, nearly sinusoidal drag variation, 2/rev with a Young rotor. Drag is greatest when the blades are broadsiding (90 degrees) to the airstream and least at 0 and 360 degrees.
There are 2 in-plane rotor resonances of greatest concern. The lowest resonance is the 'tuning fork' mode, with the rotorhead clamped rigidly to a fixed object, say a bulldozer. This mode is usually lower in frequency than the rotational speed.
The other mode, the 'free mode' has nodal points at about 25% of diameter from the tips. This mode can be excited if the rotor is suspended by cords at the nodes and struck in the center. Its frequency is normally higher than the rotational speed of the rotor.
With the rotor mounted on a mast or pylon that is really a spring, the two modes coalesce into a single resonance that often is within the operating range of the rotor.
When Arthur Young designed the Bell-47, the greatest problem was 2 per rev vibration. Even though Young's education was in mathematics, the solution was found by cut and try methods. A Young rotor must be as stiff as possible in-plane and suspended from rubber biscuits. In the case of the Bell-47, the engine-transmission-rotor shaft is a single unit, mounted in rubber.
With a 'soft' mast, only the 'free' resonance can exist and this can be kept above the rotational speed of the rotor. The mast need be soft only in the fore and aft direction - The aerodynamic exciting force is striking a rubber anvil.
My gyro with tail boom on top by nature has a rigid pylon. With standard Bensen type rotor, 2/rev shake was intolerable above 40 MPH. I first tried an articulated rotor- no undersling but with a coning angle hinge like a door hinge at the teeter bolt and drag hinges outboard. There was no improvement. Cierva conducted many experiments with 2-blade rotors without success.
When I gave the rotor compliance in the fore and aft direction all 2/rev shake disappeared. The rotor hub was concurrently stiffened in-plane.
The wear pattern indicates that movement is very slight, 1 or 2 mm so that it is not so much isolation as it is the change of resonance that eliminates the vibration.
Have you ever searched the French literature for Autogiro information? Georges Lepere was a famous French aeronautical engineer who collaborated closely with Cierva and is credited with deriving the relationship for tail volume.
Dragonfly
A pusher prop should unload much of the rotors and thereby significantly reduce rotor to rotor induced vibration.
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Last Revised: November 11, 2004