Pit-Viper 1

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I am ready to maiden an old, think it's a 1963, Goldberg Skylark twin. All I need now is the CG range so I can finish setup. Anyone know the proper CG for this plane? I was going to just start at the spar and go from there.

Thanks,
Shane

BobHH

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About 3-1/2 to 4 inches back from the leading edge is what is shown on the plans.

Bob Harris

gittarpikk

Seen a person do a build on here for one...in silk and dope.. The problem was it required a LOT of noseweight to bring CG inline. This is due to engines close to wing LE and short nose...so if you can lighten tail empennage any, it sure helps keep it lighter

Pit-Viper 1

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Ok great, thanks Bob..

Shane

buzzard bait

My Skylark 56 single engine had Dutch roll with CG at the far end of the range. The vertical tail is pretty skimpy. With the extra engine nacelles ahead of the CG it might be even worse, so I would balance at the forward end of the range, maybe even a little forward of that. The chord is a little under 10 inches, so the range shown is more than 35 to 40%. That's pretty far back by most standards. It works for pitch stability because the stab is pretty large and the tail arm is pretty long.

I normally put my CG back as far as I can so the plane has neutral stability in pitch. But with this plane, there just isn't enough SIDE area aft of the CG to keep it stable in the yaw axis. By moving the CG forward you get a longer tail with more side area and a shorter nose. You could go closer to 30% and it would fly fine. Of course, it will also be quite stable in pitch, but that's not necessarily so bad.

But yes, it can be hard to achieve on this plane. Try to provide for putting the battery and receiver as far forward as possible, and I agree with keeping the tail light.

Jim

Pit-Viper 1

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Thanks Jim,

That's how I have it set up now with the battery and receiver all the way forward. Was going to maiden it today but while doing a little tuning at home prior to going to the field I noticed there was a lot of movement and a different sound pitch coming from the right motor. When I checked it out I discovered that the nacelle had broke away from the wing from the LE to about mid point on the wing. That could've been scary and disastrous. I think my son got into my plane room and was pushing the wing around on the floor, not sure though. Could just be that the glue is over 30 years old to. It was the only place I didn't go over the wing with glue. We'll see tomorrow, supposed to be 70 and sunny. Using a 9x4 prop I have about 1/8" clearance between the prop/fuse and the same for the prop/ground. I used a 1/2" bigger tire on the front raise it a little and it should be good now, may end up getting an 8x6 prop just to be sure.

Shane

buzzard bait

You're welcome Shane.

Another idea on the props...cut a quarter inch off each tip and re-balance. Not hard to do. I've done it on MA props and wood props. What engines?

If the nacelle came loose it was probably marginal to begin with. The big danger on these old planes is oil-soaking.

Good luck...should be a great sight and sound if you can pull it off.

Jim

Pit-Viper 1

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Jim

The motors are OS .20FP's made back in the early 80's, one of them seems to be brand new and the other was seized from sitting for years. I used Marvel mystery oil and it loosened right up. I will try to get a little vid of them running today while I'm finishing tuning. The guy did a really good job of protecting it from fuel and it is ok in that area but man is there a ton of epoxy on the inside of the nacelles. I was going to grind away some of it but thought it may weaken it, guess it wouldn't have mattered now. It was solidly built, a lot of overkill IMO but that's probably why it's lasted this long.

Shane

maxpower1954

I've been flying my Skylark 56 twin for two years now, it's a great airplane. Two OS .15s are plenty, even on one engine I can loop and roll it. No issues with it being tail heavy, in fact I moved the CG back a little and increased the size of the elevator to improve pitch authority. No Dutch roll issues at all. I've posted this Youtube link before but some of you may not have seen it.

Always tune the stronger running engine to the weaker one. Do the nose high test for 30 seconds before takeoff. Run them both slightly rich

Make the take-off at a shallow climb, in case you lose an engine. When you do lose one, USE THE RUDDER, not the ailerons to control heading (make sure your rudder has the maximum amount of throw.) "Don't turn into the dead engine" is old incorrect information - with the airspeed up it doesn't make any difference. If you get into problems single engine, just reduce the power until you have it back under control. On landing, carry some power but bring the nose up to fly slow - the accelerated airflow over the wing from the props contributes quite a bit of lift. If you land with the engines at idle, the slow spinning prop discs will kill lift behind them. One thing you will notice is a lag in roll because of the weight of the engines out on the wing. Good Luck with the maiden! Russ Farris

http://www.youtube.com/watch?v=bkG75JeJgfk

http://www.rcuniverse.com/forum/m_91...tm.htm#9124682

Pit-Viper 1

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I've been thinking of switching to the 15's due to the fact that one of the 20's was seized when I got it and doesn't run well enough for me to trust putting it in the air. Tell you one thing though, with those 20's I can hold the tail and it will pull itself straight up with quite a bit of force. Could probably launch it that way but I'm not that daring... Actually been considering going electric as well just due to the fact that it's so old and seemingly brittle. It would be a lot less stress on the frame with out the vibration.