CG location
 

Does anyone know the proper CG location on the Top Gun Interceptor. A friend worked all Winter on one but was not sure of the exact location. Today he tried to maiden the plane and it would not rotate even at 50+ mph.
Thanks
Glenn

Hopefully some will respond that has that specific plane figured out. But, ...

How heavy is the nose wheel? All my tricycle geared planes are not very heavy on the nose wheel. ( Just kind of a gross indicator of a CG problem.)

Also, is the length of the nose strut correct? It could be too short thus raking the plane with the nose down.

Did he have the right battery in it? (e.g. 5S vs. 6S?)

The reason I even responded is; We had a pilot with a similar situation. He ended up with a crashed plane. His plane was simply underpowered. They frogged around with moving the CG back to get it to rotate. Then the plane rotated. But then, ... it almost immediately stalled and crashed. His plane simply did not have the power to climb at a reasonable rate given the configuration of the wing. It was an F-105. The Interceptor kind of appears to have a relatively similar sized and shaped wing.

We also had an F-14 that was underpowered and it took a very long run to get it off the ground. It eventually flew. But it too stalled and crashed. And we know that its GC was spot on. We figure it to was just under-powered.

A couple of instruction manuals I have read specifically address the nose strut length having to be correct to allow easier rotation. Think the Sprint may even be one of them. ( I read a lot of Manuals.)

Good luck. Be careful if you are flying your bud's plane.

You may contact Terry Holston. He hangs out on rcu..
Also may try to locate Marty Snell. He designed the Interceptor
Did a search, both Terry and Marty are listed using their respective names

I may still have the plans for one. I will look and if I do I will post here.

Contact Brian at Dynamic Balsa he has the Top Gun molds.

I had one do the same thing years ago. Had negative wing incidence due to short nose strut.

Nose strut length
 

Thank you to everyone for the responses so far. We do not believe that it it is a nose strut problem. This is because at approximately 50 mph the plane hit a bump in our runway. The bump launch the plane about 12 to 18 inches into the air with the nose pointed upwards. The plane still did not take off and fly it settled back to the runway.

Wing image
 

Attachment 2212399

Since I have seen at least two different wings on these planes, I wanted to show which one Thunderchief and I have on this plane.

With the plane sitting on its tricycle gear put a protractor on the main axle. follow a line up and forward 10 to 15 degrees. Mark where it hits the wing. Check if your CG is somewhere near the mark......... PS this is not a good rule of thumb for a space shuttle.

Go to this site, I've used it forever . It will calculate your c/g . Read the instructions and understand what " static margin " means . That's important .
Good luck .

Thanks again to everyone for the responses. So I would like to share some results. Jofunk, I checked with the protractor and wow, the 15 degree angle points about 1.50" behind where I had the CG. In contrast Jetmaven, the online CG calculator indicated 0.25" ahead of where I had the CG. :(

I think I know why no one makes this plane anymore.... :(

I would like to simply change the CG to the 15 degree pointer mark on the protractor. The problem is, the fuel tanks cannot be moved any further aft. This CG location would be completely behind the tanks...

What number did you enter as the static margin ?

Static margin
 

I entered 10% as the static margin.

Some of the early ,and I mean very early d/f models got " stuck " to the ground due to angle of attack
issues . I suspect that's what's going on .

Changing the CG
 

So Thunderchief and I made several checks on the plane. We found that the wing, stab and motor thrust is all at a very nice +1.0 degrees. This practically leaves only the CG to blame.

If I move the CG back as I stated above, it will be behind the fuel tanks. This effectively generates a different CG for dry (no fuel) vs wet (full fuel load).

Those numbers are as follows, if I target half a fuel load as the center-line:
Dry, the CG would comes out to 33% MAC and wet would be 23% MAC. This is too large a swing in my opinion. But I am not an Interceptor expert (yet)...

Would some of you please chime in? Is this too much of a swing for this plane?