I'm currently building a GP Skybolt kit, the second one I've done since the first met an untimely end. The kit's an old design, pre-dating 3D and builds heavy, especially tail heavy, and there's not much you can do about making the tail lighter without compromising strength which leaves most of these planes carting useless lead around in the nose.

Currently the fuse is 90% complete, and the engine (saito 1.25) is mounted. The cowl for these is a 2 part affair (front & rear halves) which needs to be cut down to size & glued together. I've done the cowl, but rather than cutting on the supplied "cut here" lines I just trimmed off the flashing from the rear half, leaving the cowl almost 1/2" longer than standard. I was measuring it all up last night, working out how much I needed to trim from the cowl when it occured to me that I could just put a thick ply block behind the engine mount, moving the engine forward about 1/2", and reducing the required lead.

The kit actually omes with 3 such blocks, intended for use with smaller 2 stroke engines, so I can't see any mechanical problem with it My question is will moving the thrust (prop location) forward of it's standard position alter the geometry, dynamics, balance etc of the plane? My suspicion is that it will be fine but theres more that I don't know about aircraft design than there is that I do.

By using the blocks you are essentially creating 'stand offs' the use of which is common to allow for different engines. You are actually changing the length of the moment between the CG and the prop. But if you leave the CG in the recommended position you should be fine. I've done this on 2 planes which are British kits in order to install 4-strokes rather than the designed for 2 stokes and they both fly perfectly.... but I'm not a 3D pilot. So perhaps one of the aerodynamics gurus will chime in. Your other choice would have been to recess the engine into the firewall so as to leave the prop in the same position but I suspect it's too late for that now. And that alternative does not help the tailheavy problem.

Definitely wait for some other responses. But my anecdotal experience on this 1/2" modification says it's ok.

I believe that a WWII fighter wound up with it's engine moved forward during the prototype stage.

"Blocking" the motor mount forward shouldn't cause you any problems aerodynamically. You will be moving the engine farther from the tank, and if the kit design stops the tank a significant distance aft of the firewall as the ARF does, you might consider moving that intefering bulkhead forward at least as far as the engine is going forward.

1/2" on that airplane won't matter at all. Go for it if it helps the C/G and avoids lead.

In the kit building fourm there are a bunch of threads just for the Bolt. When I was in my Bipe years one of the tricks I did on a lot of them was to move the fire wall forward to help the CG, no flight changes noticed at all.

Did the reverse for the same reason. Shortened the nose 3" on an EAA Bipe by Balsa USA in order to install a small gasser. Had flown it with a 120 4-stroke in standard config. Did not notice a difference.

Go for it!

Bedford

PS: Darnit Sandman! I got smudges all over my expensive new monitor because of you!

hehehe sorry about that. You're the second person in a week to try to squish my avatar. It's even caught me once or twice when I first started using it.

When you shortened your bipe, were you doing it to fit the engine into it (thereby leaving the prop in the standard position) or to reduce required tail weight (and moving the prop further back from standard in the process)?

Oh.. for da Rock... the skybolt has its tank pressed up hard against the firewall, and I've actually echo'd a lot of the newer ARFs by cutting a hole in it for the tank neck to protrude through, so a little extra fuel line length shouldn't be a problem.

For Gray Beard... yah I actually posited the same question in the Skybolt clubhouse thread but got no response. I figured I'd get more action from a new thread. It worked.

Thanks to all for your responses. They seem to agree with my guess that it won't matter... the nose of the plane isn't a particularly aerodynamic part of the plane so it shouldn't notice being a little longer. Guess I'd better start shopping for some longer mounting bolts.

Yes, it will alter the balance. This can be used to advantage (Imoved a firewall in 1-1/4" when mounting a G-26 in a Phaeton II). You want to bring the C.G. to the spot indicated on the plans or per the instruction manual.

Will it alter the flight characteristics? As told above several times - no. Some full scale aircraft went from radial to in-line and vice-versa in their development and through various marks (the Navy preferred radials and converted several . As long as the C.G. is kept you'll be fine.

My move was purely for balance. 3" is a lot, but the EAA Bipe has a very long nose and can stand the shortening. In the end, with my Echo 21cc mounted, I was lead free. Nice!

I might have gotten away with using the OEM cowl, but by the time I modified my plane, the cowl was crap. I scratch built my own glass version ... another story that.

Still one of my favorite flyers, but it is currently stored 7000 miles away, so I don't get at it much.

Enjoy,

Bedford

Matter of fact, the P-40 Warhawk was developed from a P-36 Hawk with the radial removed and an inline strapped on.