Crush Plate Question

[Dick Pacini]

When adding crush plates to an existing design or kit, it is advisable to relieve the motor mounts the amount of the plate thickness to keep the thrust line the same or is it better to leave the mounts full size and carve the nose/shim the tank to suit and so forth?  On a profile, it wouldn't be as much of a problem.

[Mike Haverly]

Dealer's choice IMO.  I've done it both ways, what important is that things are flat and accurate when you are done.  If you have a milling machine you can mill the notches, drill the holes and  mill a little down thrust into the plate while it is set up.  Another way is to sand everything flat after the plate are installed and before you put the crutch in the fuselage.  That crutch is where the alignment starts and if it is not correct it will cause problems down the road.  The same goes for profiles or full fuselage.  Gotta be right!

[Dave_Trible]

Hi Dick.  Crush plates aren't going to effect the thrust line enough to matter either way.  It might require shimming a ready-made tank a bit to get even lap times upright and inverted.  You can do it either way.

Dave

[Tom Niebuhr]

I move the motor mount up to compensate for 1/8 aluminum plates. start with a 1/8" shim between the mounts and the tank. This has the added advantage of letting you shim the tank up or down as require.

[Tim Wescott]

What everyone else said, with the observations that:

If you put in the crush plate and move the motor down, then you're not building exactly to plans.  Unless the plane already has vertical CG problems it's not a technical issue, but it may be an aesthetic one if the motor is already low, or create stickiness with a stickler in OT or classic (assuming that they notice).

If you mill a pocket for the crush plate in the motor mount then you'll weaken the mount a bit.  My gut feel is that it won't make a difference, particularly because Mike's done that and he didn't say "but I've broken planes that way".  The mount will lose and absolutely imperceptible bit of stiffness, and will be substantively weaker at the back of the notch -- but I suspect that those mounts are plenty strong there in most cases.

If you have room to move the whole crutch up by the thickness of the crush plates, then your motor will stay in the same place and you won't lose any strength.  I know this won't work in a blue-box Nobler, but maybe there's room in your plane's nose?

[Randy Cuberly]

I've built fuselages both ways and never had a problem with either.
I now use 3/32 thick 6061 aluminum relieved into 1/2 X 3/8 Maple or 1/2 X 1/2 Maple.  I do make sure that the end of the relieved notch has a radius rather than an abrupt corner to relieve some of the stress there.  The aluminum plates are epoxied as well as screwed (with small wood screws) to the maple at both front and back (uses pilot holes for the screws again to avoid stress).
I've built several airframes this way and never had an issue with mounts...even with ST60's.

Randy Cuberly

[Brett Buck]

What everyone else said, with the observations that:

If you put in the crush plate and move the motor down, then you're not building exactly to plans.  Unless the plane already has vertical CG problems it's not a technical issue, but it may be an aesthetic one if the motor is already low, or create stickiness with a stickler in OT or classic (assuming that they notice).

If you mill a pocket for the crush plate in the motor mount then you'll weaken the mount a bit.  My gut feel is that it won't make a difference, particularly because Mike's done that and he didn't say "but I've broken planes that way".  The mount will lose and absolutely imperceptible bit of stiffness, and will be substantively weaker at the back of the notch -- but I suspect that those mounts are plenty strong there in most cases.

If you have room to move the whole crutch up by the thickness of the crush plates, then your motor will stay in the same place and you won't lose any strength.  I know this won't work in a blue-box Nobler, but maybe there's room in your plane's nose?

   Moving the engine 3/32" up or down will have no discernible effect on the vertical CG. Most airplanes are so far off in trim for other reasons that the tiny variations in the vertical CG are utterly invisible. I have managed to design several very good airplanes without once considering the vertical CG or doing anything to adjust it (except as an experiment that show nearly no effect).

   Cutting away the wood to recess the plate concerns me much more, but people have been doing that for years with no real problems. I don't do that, either, which makes my tank line up better. Recessing makes it slightly easier to fair in the spinner without hitting the blind mount screws.

   Brett

[Dave_Trible]

I agree with Brett on the vertical CG stuff.  It became popular to talk about a few years ago but I've never really considered it in design or trim.  Given other more nessessary design parameters there isn't much you can do with it anyway.

Dave