juggler-3310

I've attached two photographs of a sailplane that I purchased locally. It was not in great shape when I bought it, so I decided to restore it and in the process converted it over to electric flight. I removed the single front wheel landing gear, and extended the motor off of the firewall with a motor mount and built a cowling for it, which it did not originally have. In other ways I attempted to keep the design as original as possible. My question is this: does anyone recognize this model? I have no idea what it is. It looks similar to a Carl Goldberg Cumulus from the 1940s, but it isn't one. I am scratching my head trying to figure out what I have. Any help would be appreciated.

The photographs were taken just before a successful launch. It flies beautifully, and its flight characteristics are very forgiving. But I was troubled by not knowing where the CG should be - fortunately I got it very close to the right location. Knowing what model this is would probably allow me to find a set of building plans so that I could see where the CG is designed to be, as well as other useful information. If anyone has an idea of this model's identity, please let me know. Thanks.

Bad_Daddy

It looks like a converted Free Flight model to me - I used to see many like this in the '70's when I'd go to the Free Flight competitions. I think it's similar to the G.Y.S.O.B., but I don't remember that model having the two sub-verticals.

da Rock

It is a freeflight model. I think that type was called a pylon model. That one appears to have a smallish wing for the size of the fuselage. They were designed to be pulled to altitude by an engine of a certain size that would run for a very short time, and then to transition into a circling glide. They were designed and trimmed to turn into lift on their own. It had to be on their own, because they were free flight. They usually had some means of dethermalization. That is, there was some sort of timer that triggered and radical trim change that would bring them back down.

The timer could be a very small, lightweight clockwork timer. It might only be a simple fuse that slowly burned down to a rubber band or cord trigger.

Which one could it be? Sorry, but I was flying back then, but not freeflight.

Bad_Daddy

I've been searching through all the freeflight plans I can find, including the RCU plans site, but haven't spotted it yet.

juggler-3310

One additional bit of information that might help: the wingspan is 62".

juggler

BMatthews

It's definetly an older free flight model. It may also be a kludge of a wing from one and the tail from another on a new fuselage. I'm fairly familiar with a lot of the old timer and nostalgia power models but this one doesn't really hit a note in my brain.

The stabilizer and fin are somewhat like that of a Sattelite power model but the wing looks older other than it looks like it's a flat bottom section so it may well be a more modern design. The fuselage has a lot of the Sattelite look to it other than the pylon is far too high. The pylon being that high puts it into the late prewar old timer category or the early to mid 50 Nostalgia era before the "sticks with wings" took over.

The light weight of the wing construction definetly suggests a later design from the 50's or later.

So all in all I'm guessing it's a kludge of various bits joined together.

NormF

I still say it's/was a Goldberg Interceptor! See: http://www.jimoreillymodelplans.com/OTG1-OTG7.htm

- Norm

da Rock

What NormF says. It does look very much like the CG Interceptor.

But with quite a bit more structure in the fin to create a rudder. And quite a bit more structure in the horizontal stab to create elevators.

The shapes are there. And the subfins. And the lifting stab. And the square to triangular fuselage.

So, could anyone tell where the CG was on the Interceptor plans? It would have been pretty far back, wouldn't it? They did that to have the tailplane carry some load, right.

BMatthews

NORM WINS THE E-BEER! ! ! Well done!

I'll bet the right CG will be back at the 50 to 60% point. The best way to tell and tune it is to use dive testing. Trim for a hands off slow glide using the elevator trim. Then push into a dive until the nose is pointed down a good 45 degrees. Release the stick and watch. If it's still nose heavy the CG and matching elevator trim will let it come up quickly. So move the CG back and retrim for a slow hands off glide and push into a new dive.

You're looking for the model to pull out with a definite but slight nose up rotation. The model should take a good 100 to 120 feet of altitude to pull back to level. The nose may continue to rise backup but it's the height used to just come back to level at the bottom of the first dive. If it shows ANY sign of tucking to a steeper dive at any point then you went too far back with the CG.

Now you may well find that this is a little too close to the edge and makes it hard on the elevator control to fly the model well. By all means reduce the throw as needed. You've got a LOT of elevator area and it's really too much. Use the furthest in hole on the servo and furthes out on the control horn. You could even get a longer control horn to help out. Use low rates if needed. But in the end you'll likely still want to move it a skinch forward from that critical most rearward position. It truly does make the model easier to fly in all aspects.

But similarly if you leave it at the classical 30% point you'll be robbing yourself of some excellent performance.

juggler-3310

I would like to thank everyone who took the time to respond. Your input has been appreciated. Norm, I believe that you are right. It looks very much like a Carl Goldberg Comet Interceptor. The only distinctive differences that I can see between the Interceptor and this model is that the wingspan on this model is 62", and the largest Interceptor wingspan that I can find is 48". The stabilizer also has a slightly different construction that distinguishes it from the Interceptor. But in just about every other aspect of the design it does look like the Carl Goldberg model.

BMatthews, thank you for the suggestion about how to properly locate the CG. It's good advice. I assume that the description you provided for the way the model would respond in a dive test is correct for a modern sailplane that is not designed to generate lift at the stabilizer. But I believe that this model, which was clearly designed to generate upward lift at the stabilizer, might behave differently in a dive test. Am I correct about this? If I am, please tell me how you would expect the behavior of the plane to change in that test based on this model design. If I'm not right, then your directions should work as stated. Thanks!

juggler