Zero Force Member in Engine Mount?

[Toobuilder]

Originally Posted by BillL

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...

Thanks for that Bill - I am seeing that too. However, there is some hefty triangulation sends that "spreading" force into the cross bar at the firewall. A free body diagram of this load case resolves to equilibrium through the firewall cross piece. And as my structural engineering skills are limited, I'm seeking to fill any holes in my assumptions.

I know there is a lot going on with this structure, but once you eliminate known influences, anything left over is redundant.

For example, based on discussions I've had with people, there seems to be some back and forth between trying to determine if the cross piece in question satisfies landing gear loads or flight loads. It seems to me that if you look ONLY at the trapezoidal truss formed by the gear sockets and the "Z" bar that connects them (imagine cutting the rest of the mount away with a sawzall), then all landing loads are satisfied. There are four (4) 3/8 diameter bolts to handle shear loads, the beefy truss itself EASILY contains "splay" loading of the gear legs, and the fore/aft loads for braking or potholes are through the 4 bolts in tension or compression into the lower longerons and center keel structure of the fuselage.

If the above is a valid assumption, then we can ignore gear loads from here on out, right?

[BJohnson]

Originally Posted by Toobuilder

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.

This is a bad assumption. Statically indeterminant does not mean that it carries no load. The loads are shared based on their stiffness. Taking load off the bolts with the cross bar is a good thing, particularly when considering landing loads. Redundant structures are much safer than single point of failure ones.

[F1Boss]

I have seen the investigation report of the forces that affect the F1 mount. It had a page with colors showing stress levels - there was plenty of red on that page. I can't lay my hands on that report just yet, but that tube is a necessity. We added one across the top attach points later on...

We also changed the firewall bottom/fwd cross-angle to 1x1x.125 to absorb the heavy loads from the gear.

Best to keep that tube - but you can move it up so the bottom of the tube is level with the fwd belly skin to allow for a cowling outlet fairing to be placed there.

Carry on!
Mark

[Toobuilder]

Originally Posted by F1Boss

I have seen the investigation report of the forces that affect the F1 mount. It had a page with colors showing stress levels - there was plenty of red on that page. I can't lay my hands on that report just yet, but that tube is a necessity. We added one across the top attach points later on...

We also changed the firewall bottom/fwd cross-angle to 1x1x.125 to absorb the heavy loads from the gear.

Best to keep that tube - but you can move it up so the bottom of the tube is level with the fwd belly skin to allow for a cowling outlet fairing to be placed there.

Carry on!
Mark

Thanks Mark.

I would love to see that report. Are you suggesting that the cross tube in question satisfies landing loads? Because it appears that the "Z" bar trapezoid truss does that all by itself, so I'd like to see what I'm missing.

[SMO]

Here is an example of what happens to this tube when excessive side loads are applied - enough to put a permanent bend in the titanium gear leg. Not sure if this adds anything to the discussion, but another data point.

[Toobuilder]

Originally Posted by BJohnson

This is a bad assumption. Statically indeterminant does not mean that it carries no load. The loads are shared based on their stiffness. Taking load off the bolts with the cross bar is a good thing, particularly when considering landing loads. Redundant structures are much safer than single point of failure ones.

I'm assuming the "classic" frictionless pin or socket joints.

I'm also seeing the cross bar there... But it's at the firewall. I do have a hard time imagining a scenario where a designer would use two members to share the same load case when a single, properly sized member will work.

Redundant structures do have their place (combat, for instance), but I'm not seeing the need here.

[Toobuilder]

Originally Posted by SMO

Here is an example of what happens to this tube when excessive side loads are applied...

Wow. Did you shear one of the lower longeron bolts? If not, then I can assume that you have some deformation in the fuselage as those two lower bolts got squeezed toward each other.

[Toobuilder]

Originally Posted by F1Boss

...Best to keep that tube - but you can move it up so the bottom of the tube is level with the fwd belly skin to allow for a cowling outlet fairing to be placed there...

Just reread this in a new light...

Just for clarification, the structural tube I'm talking about is NOT the lower tube at the firewall (i.e. the kinked tube in Mr. Olson's example above). It's the lower tube between the engine mount (engine side) ears. It is the one tube that is NOT used on the RV-4 mount.

[BillL]

Michael you are not going to be happy until you do it yourself. Make a structure of coat hangar wire and solder it together, then bend it to satisfaction. You might be surprised at how much can be learned so easily.

I think you may have underestimated the spreading forces of that engine mount due to vertical loading. Overhung moment, assume frictionless wedges and do the free body diagram.

With a 300 lb engine, 18" moment arm (less for the spread from upper to lower mounts) and 6 g's of loading the loads would have to be transferred to the lower structure in bending, not tension/compression. There are NO center bolts, only those at the corners of the fuse. So any load that does not triangulate at those points will be bending.

Good luck.

Edit:I should add, it might not fail outright but due to fatigue, but if these spread the engine "droop" and prop drop with deflection could be significant with g loading. Hopefully there would not be a vertical resonant vibration due to prop forces/balance.

[Toobuilder]

Originally Posted by BillL

...There are NO center bolts, only those at the corners of the fuse. So any load that does not triangulate at those points will be bending...

Thanks Bill, I will take your comments to heart and build a model.

However, I'm unclear on your comment above... There clearly are "center" bolts on the cross bar between the upper gear sockets, and it is exactly this tie to the fuselage structure that my entire hypothesis hangs.

Are you suggesting that these bolts are irrelevant to the load case, or something else?

Just looking for clarity, not argument.