Zero Force Member in Engine Mount?

[BillL]

Originally Posted by Toobuilder

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.

Nah - No need for argument, we are all good here. Hmmm,Yes I was not seeing the bolts on the top gear socket cross bar, I was seeing across the bottom bar, sorry. Nevertheless - I don't know what structure is behind those would guess they are for stability or vibration stops, not major structural load carrier. That is a pure guess, though as I am ignorant with the aft FW structure of the 4. Does it form a channel/tunnel back to the spar?

Off topic. When my son was a sophomore in HS, there was a toothpick bridge contest. His older sister held the record for weight. He used an online statics program (with very little guidance) and made a design. He tested scarf joints and compressive stability. The bridge, with 1/8" assembled beams held 318 lbs over a 14" distance. (~3.5" tall X 3" wide)

A tension/compression triangulated structure can produce a very high load/weigh structure. Maybe, the cross tube could be greatly reduced (as it is in tension) to reduce the weight. Need numbers to verify precisely.

[Toobuilder]

Originally Posted by BillL

Nah - No need for argument, we are all good here. Hmmm,Yes I was not seeing the bolts on the top gear socket cross bar, I was seeing across the bottom bar, sorry. Nevertheless - I don't know what structure is behind those would guess they are for stability or vibration stops, not major structural load carrier. That is a pure guess, though as I am ignorant with the aft FW structure of the 4. Does it form a channel/tunnel back to the spar?...

Those center 3/8 dia bolts tie to a substantial "V' shaped keel which terminates at the elevator bellcrank. It is a very significant structural element of the system, and one can consider those attach points fixed in every plane, but particularly in longitudinal loading.

[David Paule]

As far as I can determine, looking at the photo, you're wanting to remove tube "C." If so, then....

Tubes marked "A" are all in a plane.

Tube "B" is the tube that takes forward/aft loads from the top of the landing gear socket and reacts them.

Tube "C" is the only tube that is out of the plane of the "A" tubes, and is therefore necessary.

At least that's how it appears to me from this photo.

Dave

[Toobuilder]

[quote=David Paule;1136120]

Originally Posted by David Paule

[As far as I can determine, looking at the photo, you're wanting to remove tube "C."

I'm not seeing your demarcation of tube "C" in the above picture, but...

Originally Posted by David Paule

[Tubes marked "A" are all in a plane.

Agreed

Originally Posted by David Paule

[Tube "B" is the tube that takes forward/aft loads from the top of the landing gear socket and reacts them.

I can sure see that, yes, but I have an alternative perspective. I think "B" is redundant (or plays a minor role) to landing gear socket loads because the top of the socket reacts into the fuselage keel structure behind the firewall through that cross tube and bolts.

That said, "B" is a primary load path (in compression) to keep the lower engine mount pads from closing together (working in concert with middle "A" and lower "A", both in tension). If the lower mount ears move inboard, compression force drives from ear to "B" to upper socket, to cross tube to bolt to "keel" (and resolves further to spar, skins, etc).

In short, what I "see" is a classic pyramid structure formed from the firewall attachments of "lower A", "Middle A" and "B" all fixing an engine mount ear right at the apex. If the engine mount ear is fixed by those three members, then the cross tube spanning between the ears is not required, right? After all, a tie between two pyramid structures does not keep either one from falling over.

[David Paule]

I just replaced the photo, sorry.

Dave

[David Paule]

Originally Posted by Toobuilder

....In short, what I "see" is a classic triangulated structure formed from the firewall attachments of "lower A", "Middle A" and "B" all fixing an engine mount ear right at the apex. If the engine mount ear is fixed by those three members, then the cross tube spanning between the ears is not required, right?....

It's not the stability of the lower engine mount that's at risk, it's the strength of the landing gear mount.

You could download the trial version of the LISA finite element analysis program and verify that.

Dave

[F1R]

The easiest / best way to reduce drag to have a look at Tom Martins cowl exit arrangement.

The Rocket 540 mount is just barely strong enough as it is. Just imagine standing with a leg in two canoes and 2 feet space between them on water. Then add some waves. IMHO You are just begging expensive disaster by reducing anything from what you have.

[Toobuilder]

Originally Posted by David Paule

It's not the stability of the lower engine mount that's at risk, it's the strength of the landing gear mount...

Copy that Dave, yes, "C" is the member in question.

And you bring up a good point which needs to be resolved. If we assume the upper socket support is not adequate with only that trapezoidal truss and its 4 attachment points to the fuselage, then I agree that "C" is required.

That said, that trapezoid should be very easy to analyze.

Clearly the four 3/8 attachment bolts are overkill in shear with a hefty safety margin, so no worries there.

landing gear splay should easily be reacted through the hefty cross members of that structure, so TLAR indicates a bunch of margin there...

So in my feeble brain all that remains is the reaction to fore/aft movement of the tires. Braking, gopher holes, etc. Lower attachment bolts tie the compression loads into the lower longerons so we know we're safe there, so we are left with the tension loads into the keel:

Cross tube, bolt bosses, and the bolts themselves.

If this truss is "good" as a stand alone structure, then "C" is redundant to landing loads, right?

[F1Boss]

...basic assembly of the part. How else might you keep those points secure while building the mount? I would think a shallow X below and behind these two lower points - welded to the lower area of the sockets - might add some rigidity, but a small tube would almost need to be tacked in place to hold the 4 points in a square while the X is welded in..but that X would use more assy hours, and add some weight.

If I could remember how to add pics - I have two mounts with this X structure in place here in my shop...but no gear leg sockets are used on those two.

The assy fixture holds that square setup in place; the firewall area is welded 1st, then the welded square is set into the fixture, then the fore-aft tubes are welded in place.

Note the short tubes welded on the lower corners in SMO's picture - those are critical to the F1 mount.

Yep - was thinking you were referring to the cross-tube between the two lower firewall attachments.

This is an interesting thread indeed. Merry Christmas to all participants and readers!

Carry on!
Mark

[Toobuilder]

Originally Posted by F1R

...IMHO You are just begging expensive disaster by reducing anything from what you have.

I'm "begging" to develop an understanding why things are done the way they are. Many homebuilts are "designed" through direct copy, TLAR or other informal means. I'm not suggesting that the F-1 or Rocket did not receive the appropriate attention, but this kind of thing happens often enough to warrant a closer look.

Fortunately, we don't have to deal with opinion in this case. There can be only one right answer, and nothing is going to change on my airplane until that answer is found.