dan(at)syz.com
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 Posted: Fri Sep 30, 2016 4:01 pm Post subject: Accurate arm measurements for W&B |
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Hi everyone. I have something of a "heads up" for people (unless it's more of a "heads up" for me, if I've done something wrong 
In comparing my W&B numbers to others who have published theirs, I realized that many people seem to be using the arm measurement for the wheels as given by Van's in their "Sample W&B" information (right: 124.44", left: 124.31", front: 50.44"). Since it's unlikely that everyone's measurements match Van's exactly to the hundredth of an inch, I assume most people are just assuming that since the RV-10 is designed so that a person can't help but build it right, Van's numbers should be close enough to theirs.
But out of curiosity, I decided to measure mine, as accurately as possible, with the door frame/fuselage longeron (Van's level datum line) perfectly level.... and I ended up with somewhat different measurements. On my RV-10, I measured right: 123.07", left: 123.19", front: 50.32". The front wheel is pretty close to the same place as Van's, but the rear mains are slightly more than an inch different. I'm not sure why the difference is as big as it is - maybe Van's measured theirs after the wheel struts had flexed a bit over time.... maybe they measured their arms when the aircraft was not empty (which would also cause the struts to flex and move the mains aft).... or maybe there is some tolerance in parts.
But whatever the reason, I think it's probably important that when doing a W&B, the weights and arms are measured for your particular aircraft - don't use the weights for your aircraft but the arms from Van's example data. And it does make a difference - using my weights and arms, I end up with an empty CG at 108.2". Using my weights and Van's sample arms, I'd have an empty CG at 109.2". An inch off in calculations of empty CG might be notable considering there is only a 8.4" window in CG in the first place.
If anyone else has measured the arms to their mains and nose gear, I'd be interested in hearing what numbers you came up with.
Dan
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Dan Charrois
President, Syzygy Research & Technology
Phone: 780-961-2213
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