Track and balance on the ArrowCopter

ckurz7000

Senior Member
Joined
Nov 7, 2006
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3,438
Location
Vienna
Aircraft
ArrowCopter
A couple of weeks ago we did three concentrated days of tracking and balancing of the rotor system on the ArrowCopter. Our equipment was two accelerometers, one photo-electric rpm pickup, a strobe and a Vibrex 2000+.

We mounted one accelerometer pointing left-right up near the rotor head and the second one oriented vertically near the pilot's right leg in the cabin. For rpm pickup we used reflective tape on the big ring gear on the rotor head with the sensor looking on from the bottom. We also attached two reflective targets near the tips of both blades to give us in-flight tracking information with the strobe lamp.

Starting out we had 0,42 IPS laterally and 0,26 IPS vertical shake.

We used essentially three ways to adjust the rotor: add a weight to either blade at the tip, shift the hub bar laterally between the cheek plates and adjust the angle of incidence of the blades. The blades and hub were standard Stella Averso, 8.6 m long and 20 cm wide. We also enlisted the help of a professional helicopter maintenance mechanic who regularly balances rotors on helis.

Before you start any sophisticated tracking and balancing you have to make sure that the blades are statically balanced and strung well. Unless you have that squared away you won't get anywhere and readings are just pure luck and happenstance.

The next thing you learn is that there is a theory and then there is what really happens...and the twain never meet. Ever. Period.

Subjectively, the machine we used for track and balance had about 3 inches of elliptical stick motion when let go of the stick in flight. In general this felt like a soft vibration but at speeds in excess of 150 km/h there was a definite hard knock coming in. So there was ample room for improvement.

Fo all data points we recorded measurements during straight and level flight at 130 km/h when rotor rpm was stable within 1-2 rpm.

Making adjustments in each of the three parameters one at a time we established the influence on each separately on the vibration pattern (amplitude and phase). With this information it became less of a guess work what adjustments to make in order to make an improvement. Since each adjustment affects both lateral and vertical vibrations, it is a bit like playing chess and anticipating the "opponents" reaction to your move. You can never get both vibrations down to zero. In helicopters they stop messing and call it a day when both measurements are below 0.20 IPS.

At the end of the day we got the lateral value down to 0,01 IPS and the vertical down just below 0,10 IPS. Subjectively, this felt like a very quiet stick that had minor residual soft circular motion left when let go during flight. There was no hard knocking anymore at any airspeed. No cabin hop was discernable anymore.

We also learned that any slop in the control linkage at all will result in vibrations of the stick. These are soft vibrations that are readily damped by putting your hand on the stick.

It seems that 0.10 IPS laterally and 0.15 IPS vertically are good limits below which most any pilot would be satisfied. By playing with the system longer, it is possible in most instance to go well below that limit.

Using the strobe lamp we measured that one blade tip flies about 3/4 of an inch higher than the other. Adjustments to get the two blades on track result in higher vibrations, however. In the experience of the professional, you use the strobe lamp to get into the right ball park but let yourself be guided by vibration measurements for the final adjustments.

Greetings, -- Chris.
 
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