I just did the initial test flights on my GS540. It flew well enough, but seems to be set up a bit on the tail heavy side. I installed the motor and radio equipment to get the recommended CG location, which is about 7.5" aft of the LE. This seemed to be like it was quite a bit behind the spar at the time (it's about 35% MAC), but I figured the designer would have it set pretty conservatively.

When I roll the airplane inverted, it starts an immediate, pronounced climb and requires up elevator to keep it level.

I've seen quite a few of these models going together here on RCU, where's your sweet spot for the CG?

//Karl

My GS540p exhibited the same instability. I added nose weight till the plane flies inverted with about the same trim as upright. My gas tank is close to the CG, so the effect of fuel weight is negligible(gasoline engine with Walbro Carb). This took about 5or 6 oz of lead.
Plane flies great and is very easy to hover.

Ah, a reply, thanks RossAF.

Whereabouts did your cg end up after adding the weight? I've moved mine about an inch forward by shifting stuff around but haven't had the opportunity to fly it again due to the incessant winds we have here in the flatlands.

Karl

Since this plane is mid-wing, it is very difficult check the CG location accurately. I know that it is forward of the 7 1/2 inch benchmark, but I would guess only about 1/2, i.e. 7 inches. Fly it noseheavy and then start reducing weight until you achieve a neutral trim.

To detect the neutral trim condition, you must assure that your elevator servos center properly. I had to replace my HITEC 605 's with Cirrus Coreless servos to assure centering.

Wonderful, I have 605's, AND they were a b**** to get in there.

I've been grabbing mine in front of the canopy to see roughly where it balances.

Should have a spinner to put on tomorrow and the winds should have died a bit too. Will see how it goes.

Karl

Hang it! With something this big, I suspect it is the easiest method. Get some string and make a sling for it, slipping it over the nose and the tail. Suspend it from a hook. From the same hook you hang a plumb bob. Now you "futz" with it until the plane is hanging level and the plumb bob indicates where the CG is. Note that you might have to put a second wrap of the sling over the hook to get it to stay still in the level position- I had to on mine. This method is much easier than trying to balance the plane on 2 points.

JESTER

Ooops. Forgot, I live in central IL. Winds did NOT die a bit. Still 10-15 mph today (forecast for 5-10). Flew anyway.

With a Tru-Turn 3" spinner, short shaft adapter and the stock landing gear installed, it's just about neutral now. The trickest maneuver is landing, where I still have to constantly fly it to the ground and even add a touch of down elevator now and then. I feel it's going to need just a bit more fwd movement.