As most of you have figured out, I am coming down to the nitty-gritty on my Airbike prior to the inspector's visit.
I just completed weighing my Airbike with me in it with full fuel and no fuel. My question to the group is this: the CG with me and full fuel came in at 11.7" (21.66% of chord). when I weighed it with no fuel and me sitting in it, the CG was 11.8" or 21.85% of chord. I don't think the difference is statistically significant. I am surprised that burning off the 10 gallons of fuel have practically no effect on the CG's. My other observation is that the forward-most limit of CG (11.3" per manual) is really close to where I came out both with fuel and without. Does this concern any of you? I was concerned about the CG's being too far back but this borders on too far to the front? I promise to get smarter about airplane building so I can help those farther back on the trail than me..
In airplane stability problems Cg forward is good, if there is enough elevator power. My experience has always been to start with the nominal Cg at 25% MAC, or chord for a rectangular wing. This will size the horizontal tail. Then check everything at the forward and aft limits of of Cg travel. Usually works out to fwd Cg of 20% and an aft of 28%. These values are only rule of thumb, the actual no's will very a little depending on configuration and speed range. These are for regular wing tail type layout, canards can be very different. I think your no's are good given the info you supplied. Is your chord length 54 inches?
You've got the best problem to have. Too many guys discover that their planes are aft CG heavy, then they have a hard time bringing it foward with limited moment arm forward to affect it. It won't take much ballast back towards the tail to have a major nose up CG effect. That's neat that you are perfectly within limits first try! ---David.
I didn't deviate from the plans at all. Everything seems to work really well when you stick with the design that is laid out in front of you. So far, so good.
My Airbike with a 447 was also near the forward limit of the CG range. With the forward CG, you will find that you have to hold lots of back pressure on the stick at cruise and that your landing flair will be more difficult to control.
I installed a plastic tool box under the horizontal stabilizer and carried a small bag of tools and a five lb block of lead there. This put the CG at about 29% which made the Airbike fly much nicer.
A/B Plans #112 has 1008 hours as of 02-09-2015 Ace
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My CG with the 503 engine (moved to the fwd-most position on the engine tube which I left 2"longer than the plans stated), my seat location moved 2" aft of the plans position and 20# parachute located two bays behind the pilot seat comes out at 14.9" and 27.9% of chord when measured at a gross weight of 650#. I have adjusted the elevator spring to provide me with level flight at gross weight with an engine RPM of about 5800-6000.
Since I carry 1-1/2 gallons of fuel (emergency reserve) in the fwd storage compartment plus 1/2 gallon of smoke oil plus a maxium of 10 pints of oil in there also, my CG moves forward as fuel burns off. The amount of pitch down isn't significant and slight aft stick pressure during the final half hour of flight keeps things leveled out nicely.
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