Alternative motor mount material.

I was in our local hardware store the other day and saw some three foot lengths of 1/2" sq. aluminium tubing and was wondering if this would work as an alternative engine mounting material. Has anyone out there tried anything like this?

Since no one else jumped in here, I'll offer my two cents worth.

First, the material you find in the local hardware store, and especially the extruded shapes, are very soft. They won't cut and hold threads very well and will bend pretty easily. This may or may not be a big issue. THe softness may indeed help with issue two.

Second, the vibration damping characteristics of the wood mounts are better in my opinion. You might get by with an aluminum tube if it were filled with some material (like wood).

From a weight standpoint, I would say you might be a little better off than with maple but the engine mount area is not a place to scrimp.

One thing I do to my maple bearers is to channel them so they become a "c" shape just aft of the engine mounting area. I also drill them in the same area. The C is open to the inside of the plane. THe holes, about the same size ast the slot, are horizontal and spaced about 2.0 diameters apart. In the top view, the top and bottom of the C channel are tapered for about 3-4 inches from the aft end.

The balsa cross grain pieces are keyed into the open faces of the C channel.

THis assembly is very stiff, and pretty light. I'm not sure there would be any weight savings, if any, for an aluminum tube. And then there is the attachment problem. Special techniques are required to bond aluminum since it oxidizes to a clearcoat almost instantaneously on air contact.

JMHO

Wood is a good motor mount material because it offers some damping to the vibration. Sometimes the solid alum R/C mounts work good as well because they have plenty of mass to soak up the vibration. But mass is weight so there is a penalty when you use those heavy solid alum mounts.
The composite glass filled nylon R/C mounts offer a good option. A few years ago Paul Walker told the Golden State Stunt Champs group about changing to R/C filled nylon mounts from some solid carbon fibre mounts on the B-17. Said it solved his vibration problem nicely.
I guess the hardware store alum might be worth a try. Let us know how it works if you try it.

Hi Glenn

A few points.

1 Price. Is the extruded Al cheaper?

2 Springiness. I don't have much experience of al in this type of application but generally the extrudedd material is quite soft.

3 Deflection. Soft Al deflects. So does wood but the internal damping available from good wood cannot be underestimated.

One of the measures used in structural steel design—different material, I know—is a deflection limit of span divided by a number. For domestic floor framing span/250 or span/300 is fine. For steel, span/650—a much smaller number—is too springy. Although it is currently hard to justify the figure by theory, span/750 works well.

I would not bother.