engine mount engineering

[akflyer 1427]

I have been looking at various engine mounts and I wonder just what engineering went into the 2 stroke mounts on our planes.  If you look at the jabiru mount it has half the or less tubes in it than the 582 mount.  A supercub mount or one for the 180 is the same (simple and a lot less tubes) and they are holding up a hell of a lot more weight and TQ.  So what gives, is there really a need to have all the bracing and tubes on the 582 mount that is there?  Me thinks the mounts are way overbuilt for what they are holding up

Thoughts from the resident engineers on here?

 

[Allen Sutphin 705]

I agree completely, but I built my mount the same way, overbuilt. Why? Don't have a dang clue! It would probably support a John Deere tractor engine, yet it holds a 100 lb engine. And its held on by 5 1/4" bolts.

[BryceKat 125]

Leni,

which mount are you referring to? The iso mount or the original mount? Bryce

[akflyer 1427]

Leni,

which mount are you referring to? The iso mount or the original mount? Bryce

The original mount.

[Yamma-Fox 403]

From some of the Yamaha mounts Ive seen (that are working just fine) and also my old highly cantilevered sub NSI mount, I would say that the most other mounts must be overbuilt.

Maybe draw something up that is very lightly built and run it through a FEM / FEA program to see what it is calculated to hold up to.

[Yamma-Fox 403]

Oh and it always amazes me how strong steel is. When I was building my mount I kept asking myself if it will be able to hold 500-600lbs of thrust.  Then I realized that it really comes down to merely the THREADS on the AN4 bolts / nuts!

[akflyer 1427]

for comparison, cub mounts, 180 mounts (including sea plane with additional brace) and the avid mounts.  The avid mounts have 2 to 3 times as many bracing tubes for an engine 1/3rd the weight and even less TQ.  

 

[BryceKat 125]

Leni,

Maybe because it is suspended from the mount rather than supporting it from underneath? What does a kit fox 582 mount look like? I much prefer the original mount over the dyna smooth mount which is heavier and more complex. Once we started running saber 100:1 premix and used a 55 idle jet, engine operation is as smooth with the original as with the dyna smooth.

[Allen Sutphin 705]

I don't think I want to be the first one to test a new lightweight Avid/Fox mount. What would be the advantage of a new mount besides a couple pounds weight savings. I agree a couple pounds is a sizable amount of weight to drop, but I assure you, pilots will add something they don't really need and add it right back. If we had all the stuff the catalogs said we needed to fly safe, we couldn't even get in the seat, let alone break ground.

[akflyer 1427]

It is not so much the weight as all the tubes in the way trying to rout cleanly etc.  I just cant imagine that all the extra bracing is truly needed when you look at what holds up MUCH larger and heavier engines.

Might have something to do with the higher RPM and resonance that may have induced cracking??  No clue, just one of the things that roll around in my head and I spit it out and put the thoughts on paper. 

 

 

[BryceKat 125]

Leni,

Thats a great point about the excessiveness of the tubing, tho I don’t know To simplify it. 

[Allen Sutphin 705]

Thing is, somebody with engineering sense came up with the design for a reason. Since kit makers like to boast about lightweight kits, it would seem logical that there is a reason for the excess tubes in the mount. Since I am not a aeronautical engineer, I'll take their design.

[flywise 169]

How about my engine mount, crazy complex

[Allen Sutphin 705]

That's about the most complex one I've ever seen!

[sioux201 84]

 The reason for the additional bracing in a seaplane mount is for negative gs  pounding across the water, Paul S  had a G meter in his kit fox and  saw four gs  negative !!!

[Turbo 162]

FWIW, I took a long look at my Avid's motor mount, and considering the inertia and gyroscopic loads, movement of the thrust center at angles of attack and yaw, in addition to engine weight and G-loads etc. etc. I am glad that it's sturdy-looking, stiff against torques and side loads.  It's just one more thing I don't worry about.  However, I can't believe it really weighs very much.  It does get in the way of properly installing the upper cross-shaft oil line, though!

Edited 28 Jun 2019 by Turbo

[saskavid 47]

It is not so much the weight as all the tubes in the way trying to rout cleanly etc.  I just cant imagine that all the extra bracing is truly needed when you look at what holds up MUCH larger and heavier engines.

Might have something to do with the higher RPM and resonance that may have induced cracking??  No clue, just one of the things that roll around in my head and I spit it out and put the thoughts on paper. 

 

 

Well I will add my ....expertise... to everyone else's that has posted here. It effectively becomes about the horizontal distance between the attachment points of the airframe and the engine and the leverage that can then be generated at/near the weld points to create points of force that effectively exceed the strength of the steel and lead to points of cracking that become lines of cracking and everything then goes for shit and breaks eventually. 

[ChrisBolkan 130]

I haven't looked looked recently, but I think it's all about triangulation and some of the triangles in the engine mount complete structural triangles that begin in the fuselage. I believe it has to be looked at as a whole assembly (fuselage and engine mount) for everything to be completely triangulated.