I reckon that provided the bearers are glued to some good ply doublers which go  back beyond the first former, you should be o.k. But that is for normal flying stresses only.
I did build such a model, (picture in recent CLAPTRAP), and it was quite o.k. until it landed nose first, upon which it burst rather like a paper bag!

Tapering the engine bearers starting right behind the first former and going back to where they end sounds like a good idea to me, as does drilling lightening holes in them behind the same former.  My knee-jerk reaction to the tapering is that it'll work better, because the formers will tend to flex in the back, instead of snapping, or snapping the fuselage wood.  But I don't see that done much -- so I don't know if it's a good idea or not!

   You want to be careful about "lightening up" anything around the front end, particularly when you are using old 4-2 motors. But it still matters even with modern engines. Even small amounts of excess vibration can have dramatic effects. For just about any engine above a 19, I would use 3/8x1/2 maple all the way to at least the leading edge of the wing. On my airplane I typically use 18" motor mount stock, full dimensions all the way to the wing LE, then taper it down to about 1/8" wide to about the wing high point, and then glue it to the wing sheeting for maximum rigidity. I also use 1/64 ply doublers from the nose to well behind the wing cut-out.

   95% of the vaunted "Big Jim" run on the ST60 was, in my opinion, that they built the front end of the airplane sturdy enough to handle the shaking. That was even tougher than mine, at some cost in weight. The engine run was enough better that the weight gain was easily offset.

   My airplanes have tended to have generally better engine runs that some others, and I think 99% of the reason is that I build the nose strong - the one thing I actually *did* learn from Big Jim, et al. It definitely matters, even with modern engines, with some interesting examples that I will choose to keep to myself for now.

   Brett

All of my Snaggletooth series Mustangs had short motor mounts exactly as you suggest and compensated for strength by gluing the tank in place.  The reason for the short mounts was that I canted the engines to keep the mufflers in and one of the long engine bearers would have interfered with the tank located behind the engine compartment bulkhead.

Snaggletooth is still hanging on my workshop ceiling with 2200 flights, a NATs and Walker Trophy win and 2nd at World.  No cracks.  I'd show photos of the internal structure but don't know how to do photos on stunthangar.

Al

Those maple engine bearers come at 12".  I normally cut them to 9" or 10".  To save weight, I drill holes in them aft of the firewall.  Some builders taper the mounts to save weight.

Floyd

You reckon this is too short?

You reckon this is too short?

Hmm, looks  like a set of swollen tonsils in someones throat more than a set of engine bearers there mate!

But in all seriousness, that has got to be the absolute minimum I have ever seen in a model.

Thanks.