That was probably EAA Ch 105 here in Portland, OR.  Four of us were trained up by the DSS factory guy back in November using the DSS Micro unit.  It's a basic unit (forget the model) but all we are going to use it on is an airplane.  I forget exactly what we paid for it but it was about $5600.  This chapter has upwards of 100 active members and half of them are building RVs. 

If you're serious about the investment I can get the details for you.  A person could make it back balancing experimentals.  Our rules are that the owner is responsible for adding any weights, we just tell them where to put it.  Any certified bird has to come with it's own A&P to do the wrenching and make the log entries.  (Our chapter president is a lawyer and Das Fed is a member of the chapter.  We (ahem) know the rules rather well.)

So far we have balanced a couple of dozen planes and it's making a lot of difference here on the local fleet.  The challenges are as you have discovered, gear reductions, getting a good trigger signal, and realizing that you can't get a good reading because a plane has a bad engine mount or some such thing.  These computer based units are great because you can even show bad cylinders, frozen harmonic balance weights and similar problems by looking at the spectrum.  On a gear reduction unit like ours you can look at the spectrum and see the prop side vibes as well as the engine side vibes.  I suspect on a rotary, with some practice, you would be able to see a bad apex seal.  Not to mention on a rotary you will have a spectrum on the shaft side (6000 rpm) and the rotor side (2000 rpm).  If you put the transducer on the accessory end you can see the vibes from your alternator and water pump (adjusted for pulley ratio).

My current struggle is with a guy's Thunder Gull sporting a Hirth 2 banger and a 2.53 gear.  Spent an afternoon with him only to discover a broken motor mount...(grrrrr..).  Since I'm the alternative engine guy in the group I get to deal with all the "Renaissance Men".  The Lycomings are a 20 minute job once the cowling is off -- too easy.

Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode, RD-1C, EC-2

 

Thanks for the detailed info, Mike.

I am going to bring a copy of this e mail when I get my prop dynamically balanced, just so the guy can have a better ideal of what to expect.

 

Ed A