Question about CG for vintage planes.
 

I’m getting my Cloud Clipper ready to fly but I need help with the CG.
The book shows the CG behind the main spar.
Do old time RC airplanes with short noses balance differently from normal modern day RC planes?
https://cimg7.ibsrv.net/gimg/www.rcu...9c059cbf64.png
The main spar is in Red. The plan show the CG far behind the main spar. This is a vintage style Cloud Clipper by Value Planes. The manual says NOTHING about balancing this plane. The plans DID NOT show any CG- I had to email the company to get the picture above. I don’t know if I should trust the CG that they show.
https://cimg0.ibsrv.net/gimg/www.rcu...51cddd5ce.jpeg

I would balance between what is shown on the plans and the spar. My bigger concern is the fuel tank being located do far away from the engine ( assuming you are running glow ).

The other way around: this kind of airplane is short-nosed because they balance quite aft. Their wing airfoil is cambered and hence they need some decalage. Apart from that, the pictures of this airplane in the Web show a far forward main wing spar. I would definitely trust the C/G shown in the picture.

Thanks guys.
BTW- I mounted my tank right behind the firewall like it should be.

These older almost glider like designs do balance with a CG well behind what is common for a typical airframe. I'd have no problem balancing the plane as shown on the plans. If you don't trust that CG, add some additional removable nose weight for the first few flights then remove the weight as you feel comfortable.

It might also have a lifting tail. My old VK Cherokee 60 has a fully aerfoiled horizontal stabilizer and a correspondingly aft CG.

UStik has it right. This may be a little confusing, but I'll try to explain it a little more thoroughly.

The 1930's models were flown free-flight. They were designed to be light, and to glide as efficiently as possible. To that end, most were designed not as monoplanes, but as tandem wing biplanes. That is to say, both the wing and the "tail" flew at a quite high, positive angle of attack, both contributing to lift. Because of the high trimmed angle of attack and the rather large horizontal stabilizer (without an elevator), the model was plenty stable in pitch.

A critical element in what the 1930's modelers called the "Force Arrangement" is that the highly cambered wing airfoil had a large nose-down change in pitching moment as the angle of attack approached zero. That meant that the model flew OK at a high angle of attack, but it had to remain at that high AOA for safety.

The upshot of all this is that, when we add radio control to our old time models, it is possible to get into a "death dive" from which there is no recovery. The safest way to prevent the death dive is to keep the elevator fairly small and/or limit the travel of the elevator. Or, you can move the CG forward a bit to alter the "force arrangement" to a more modern setup.

As a side note, having the main spar well forward is more of a structural consideration than aerodynamic. Typically, the spar is placed at the point where the airfoil is thickest. Deeper spars are stronger spars. And coincidentally, the highly cambered airfoils have their maximum thickness point fairly far forward.

As a comment on the Cloud Clipper, that model doesn't appear to conform to the 1930's force arrangement that we're talking about. The horizontal stabilizer is on the small side for a 30's free flight, and the distance from wing to tail is a bit short. If you notice, the airfoil is not of the thin, highly cambered type that we see on a Playboy, Zipper, Ramrod, etc. So one wouldn't expect to see the CC with a super far aft CG like a Zipper or a Playboy. The CC looks more like a conventional monoplane to me. The area of the horizontal is generous, so you will probably do fine with the CG as shown on the plan. If it were me, I might put a little extra nose weight in for the first flight just for added insurance, but would expect to find that it wan't needed.