Zero Force Member in Engine Mount?
 

Just doing some pondering, and the lower horizontal member between the two lower engine mount ears is in the way of something I want to do. In considering a redesign of this member I cracked open some text on truss analysis and quickly came to the possibility that once the mount is attached to the fuselage it becomes a "statically indeterminate" structure, and this particular member is actually under no load. The mount ears and the tops of the gear leg sockets seem to resolve to the structure without this tube - therefore the "redesign" is irrelevant - removal is the prudent action.





I have not done the math on this yet, but before I do (or break out the sawzall), has anyone done the numbers on this mount? The RV-4 mount does not feature this cross tube and the load paths are the same (different magnitude, of course).

Is this tube a holdover from fabrication, perhaps required to keep the dynafocal ring true during welding?

Any thoughts on the purpose of this single tube and the load path it satisfies?

It appears to me that the cross member will go into tension with landing gear loads, although I'd need another view (from front) to be sure. As the gear sockets attempt to "pivot" under load, they put the members between their tops and the lower engine mount into compression, which in turn try to move the lower engine mounts laterally apart.

If the joints are considered to be spherical joints (meaning no bending moments on any of the members, all pure tension of compression), the thing will not be statically indeterminate. That may be a reasonable assumption, as the small amount of bending of the members is probably a very small portion of the system stiffness.

Perhaps build a somewhat to scale model with small popsicle sticks connected by blobs of rtv?

Mount
 

The Wittman gear/gear mount as originally designed has 7/8 diameter tubes from the top of the landing gear socket to the upper firewall/engine mount bushing.
The gear in the OP does not have this so the landing loads are transferred thru the short diagonal tubes from the sockets to the engine mount and then back to the firewall. This places significant tension loads on the crossmember across the bottom. Very bad idea to remove this.
if we were discussing pure engine mount loads it may or may not be ok to remove this tube. Many of the four cylinder aerobatic airplanes do not have a tube across the bottom. There is no room for a straight bottom tube on a four cylinder Lycoming. I don't remember if the six cylinder aerobatic airplanes have the crossmember.
Also back to the Wittman gear on aircraft such as the Wolf Pitts, the shallow angle of the gear truss to the firewall does not allow the bracing as Wittman intended it, so it is braced similar to the OP picture.

Thanks for that response. I'm still trying to picture the condition you describe.

I think wheel loads induced by "splay" are reacted by the "Z" member at the firewall. Lower member in tension, upper in compression.

Loads induced by braking are reacted by the tie to the lower longerons in compression with the lower bolts, and to the center fuselage keel in tension by the "middle" pair of bolts.

Truss vs Frame
 

Zero Force Members are characteristic to trusses, where each member terminates at a joint and there is no load on a member between joints. This structure looks a lot more like a frame (where each member does not necessarily terminate at a joint but can have other members attached to a longer continuous member) than a truss, where the tube running between the upper end of each gear socket is one continuous member with loads at both ends (provided by the gear) and at each ear. This would make the member in question a two-force member (a simplification), but not necessarily a zero force member.

The point at which each ear attaches to the fuselage could be thought of as a fixed point. Fixed points characteristically resist forces in all axes as well as moments about all axes. If the member is question is a two force member, then the points where the ears attach to the firewall would be resisting a moment equal to the axial load from the tops of the gear sockets multiplied by the distance from the ear to that member. So, the horizontal member between the ears provides a reaction moment for each ear, keeping each ear from having to resist the moment acting on it by the landing gear loads.

I'd be hesitant to remove this member because each ear attach point would have to resist a much higher moment, potentially causing buckling or bending of the firewall and surrounding material.

Don't think of splay, think what happens if one gear leg is pushed back due to a rabbit hole...:)

This is a question that is best answered by the person/organisation that initially did the design engineering on the specific engine mount. Asking the question on VansAirforce is fraught with danger and relying on the advice here for making such a major structural change could potentially be disastrous for you.

A while back there was a video circulating here that showed the substantial movement in dynafocal mounts when an aircraft is manoeuvring. That is likely to generate significant asymmetric forces in the engine mount, so aside from any landing gear loads, I'd be willing to bet that tube takes its fair share of the bending, torsion and axial loads in the space frame, and is not a redundant member. I suggest this is a question for Vans.

Edit: Found it: https://www.youtube.com/watch?v=B0gQrDnG4oA

Asking a question is rarely dangerous in itself, nor is seeking diverse opinion - even if completely unqualified. That said, I think VAF IS a good place to ask if "...anyone has run the numbers..." or "...why the -4 mount does NOT have this bar..." because not only are there plenty of engineering types here, theres a good chance that the actual designers of this mount are here.

Discussion of this type is good because unlike many found on this forum, there is only one right answer here. Black or white. It just might take a bit of digging to find it.

In any case I can assure you all that there will be no removal or redesign of this member until I have THE answer to the question. This is not subject to "experiment"... my actions will be governed by hard facts.

If you think about positive g's then the lower mounts are in compression. While the actual action from the mounts may be along the bolt axis, the actual displacement of the lower engine mounts are parallel, thus spreading the lower mount points on the frame. The horizontal tie bar accepts that separating force.

You can draw your own conclusion from that. While analyses may be perfect, 95+% of the issues are with the loading cases. You should go back and look at your assumed loads on the structure in this light.