I was recently asked about balancing a gyro in Germany. I did not want the owner to suffer the expense of my trip only to find that he was experiencing 2/rev vibration and I could do nothing for him.
I have said some of this before but here we go again.
Balancing and tracking are only good for reducing 1/rev or synchronous vibrations and cannot help reduce 2/rev vibrations.
I find that when I talk to people about frequency analysis and vibrations at different frequencies, their eyes glaze over and I can see that I've lost them. I know this feeling, its like when somebody explains to me how simple it is to update my computer, my eyes glaze over and they might as well be talking Chinese.
I will try to explain.
In my experience with eurogyros, mostly tandems, there are three vibrations that are usually dominant.
- 1/rev or synchronous at about 6 hz or 360 rpm. This vibration is mainly due to unbalance or tracking or both. These vibrations can be reduced by balancing and tracking either statically or dynamically.
- 2/rev vibration at about 12 hz or 720 rpm. These vibrations are usually due to four potential causes.
a) Rotating drag, or the difference in drag between when the rotor is at 3-9 o’clock and 12–6 o’clock. This is inherent in all 2 bladed Bensen type rotors. Solutions (or rather methods to reduce) include slider roll pivots and “limber” masts.
b) In-plane resonance. Explained better by CB but basically the in-plane (or xylophone) natural frequency of the rotor is about 6 hz which is the same as the rotor rpm and this creates a 2/rev (yes surprisingly) vibration. In my opinion this probably isn’t applicable to all of our gyros because while we experience high (in my opinion) 2/rev vibrations I don’t think they are high enough to be true resonance. The published examples I know of are CB’s gyro and the video of the early Bell helicopters. In both cases the pilots reported that the machines were unflyable. While most eurogyros have high 2/rev vibrations I wouldn’t say they were unflyable. I would guess that we are near the natural frequency but probably not on it. Solutions are stiffer in plane rotors, slider roll pivots and “limber” masts.
c) Inertia around the spanwise blade to blade C of G axis, better explained by CB or J.C.Debrayer. Due to the fact that the blade tip plane is not perpendicular to the rotor bearing axis there is a 2/rev moment at the teeter bolt acting in the bearing shaft. With an optimum undersling the 2/rev vibration caused by this moment is minimised but there is always some. Incorrect undersling (either too large or too small) increases the moment of inertia around the spanwise axis and increases the resulting 2/rev vibration. The best solution is to optimise undersling.
d) High friction in teeter bearings. This is seldom a problem on Eurogyros because they usually use needle bearings. This could be a problem if these bearings were badly corroded and started to seize up. This problem seems to be more common with smaller gyros with teeter bushings rather than bearings.
- Prop unbalance. This is usually about 33 hz or 2000 rpm.
Typically the 2/rev is dominant, the 1/rev is more uncomfortable and the prop needs to really bad (it sometimes happens) before it is worse than the other two.
I wanted to find a simple method so that owners (like my German owner) could carry out a trial to see if a tracking/balance would be valuable.
1/rev is pretty easy to simulate, I’ve explained before my “hands on” (thanks Juergan) method. Take a bottle of ketchup and shake it as fast as you can (make sure the top is well screwed on). That’s 5 to 6 hz. If you feel that sort of vibration then a track and balance is probably interesting. I’ve noticed that the 1/rev vibration feels more like a 1/rev pulse than a smooth backwards and forwards cyclic motion. High 1/rev vertical vibration is usually a sign of tracking error rather than unbalance.
2/rev is a bit more difficult to find a simple example so I propose the following.
Measure the height of the centre of your car wheel from the ground.
Get some tyre balance weights, you’ll need a lot. I had to put 130 grams (4.5 ounces) on mine to get a decent vibration. Stick them on the inside of the rim so that centrifugal force will help keep them in place.
Find a smooth bit of straight road and drive at the speed from the graph that corresponds to 2 times your rotor rpm.
The vibration you feel will be your 2/rev vibration. Get used to what it feels like and then go fly after shaking a bottle of ketchup.
This should help you decide if you need to find someone with a balancer or not.
Another solution is to download an app onto your smartphone that gives you a frequency spectrum.
I hope this helps.
Mike G