Need to start a discussion about engine mounts

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

pfarber

Well-Known Member
Joined
Feb 21, 2019
Messages
749
Location
Pennsylvania
There used to be a great site called rotaryeng.net. Dude by the name of Paul Lamar posted an article (you have to use the Internet Time machine, and when you do, donate $5 you cheap b*****rds) called 'Suggestions for building a rotarty motor mount'. it was a PDF and he goes over some pretty straightforward methods for building a motor mount.

Since at some point you need to hang that car motor on the front of the airplane, this seems like the a good starting point to fab a sled type mount.

CNC cut tubeing is a thing, so once the maths are done, having the lengths and angles cut is not the issue.

Has anyone else read this article, and what are your thoughts?
 

mcrae0104

Well-Known Member
HBA Supporter
Log Member
Joined
Oct 27, 2009
Messages
3,685
There used to be a great site called rotaryeng.net. Dude by the name of Paul Lamar posted an article (you have to use the Internet Time machine, and when you do, donate $5 you cheap b*****rds) called 'Suggestions for building a rotarty motor mount'. it was a PDF and he goes over some pretty straightforward methods for building a motor mount.

Since at some point you need to hang that car motor on the front of the airplane, this seems like the a good starting point to fab a sled type mount.

CNC cut tubeing is a thing, so once the maths are done, having the lengths and angles cut is not the issue.

Has anyone else read this article, and what are your thoughts?
That website looks like the worst of the web circa 1996.

Here is the article: SUGGESTIONS FOR BUILDING A ROTARY ENGINE MOTOR MOUNT

It appears to have some good practical information, though not particularly specific to rotary engines. It would not be my first choice as a resource for the basic principles of statics.
 
Last edited:

pfarber

Well-Known Member
Joined
Feb 21, 2019
Messages
749
Location
Pennsylvania
I did download a copy of the referenced book at the end, and its pretty much the same type of analysis.

If there is a flaw to the article, what is it? It seems to be pretty straightforward on the maths and offers good tips re: spacing the mount points to reduce vibration.
 

pfarber

Well-Known Member
Joined
Feb 21, 2019
Messages
749
Location
Pennsylvania
I asked for and received permission from Mr. Lamar's son to repost the article. So I will add as much as I can that is applicable from the rotaryeng.net web site.

I'll be adding it to my auto conversion site in a few days.

A copy of the E.F. Bruhn book (like 600 pages) is available via scribd.com. If you search for it you can download it. I'll leave it up to you if the copyright is still valid. Just because it's online doesn't make it legit. Print copies currently sell for about $400
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
7,792
Location
Saline Michigan
That website looks like the worst of the web circa 1996.

Here is the article: SUGGESTIONS FOR BUILDING A ROTARY ENGINE MOTOR MOUNT

It appears to have some good practical information, though not particularly specific to rotary engines. It would not be my first choice as a resource for the basic principles of statics.
I would take everything in that article with a grain of salt. I saw his isolators on the firewall - probably not a good idea. Most mounts are rigidly bolted to the firewall (if wood or composite) or fuselage tubeset (steel tube fuselage).

This is not a place for a weak structure. I am designing mine in EAA member benefit SolidWorks, and will run it for stresses and resonance.

If anyone is really interested, I will read it much more critically, and report back on tech errors, etc.

Billski
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
5,750
I would take everything in that article with a grain of salt. I saw his isolators on the firewall - probably not a good idea. Most mounts are rigidly bolted to the firewall (if wood or composite) or fuselage tubeset (steel tube fuselage).
Back in the '50s Cessna used small rubber isolators at the mount-to-firewall bolts. They discontinued that. The isolators were too small to absorb much vibration, and they needed aluminum ground straps around them to properly ground the mount to the airframe. Starter current is pretty big. Cessna started using engines that had provision for much larger engine isolators and bolted the mount solidly to the firewall structure.

Old:

1609284049439.png

Later:

1609283750372.png
 

pfarber

Well-Known Member
Joined
Feb 21, 2019
Messages
749
Location
Pennsylvania
I am not saying the article is perfect, but its enough to get you started and into a protoype.

I am also thinking of of using Solidworks to validate, but Solidworks has some major issues (it can do gussets). I've watched a few FEA videos online and while it would seem to work, there also appear to be quite a few 'gotchas'.

You would think the EAA would have a book out on this. Oh, wait, its not Oshkosh, so no, we shouldn't expect the EAA to have anything available.
 

Lendo

Well-Known Member
Joined
Feb 6, 2013
Messages
625
Location
Brisbane
Engine Mounts, if memory serves me most Aero engines mounts have Conical Mounts and the angle is set to the CG of the engine package.
Mazda engines the best is the Bed Mount.
George
 

JRC

Member
Joined
Dec 28, 2020
Messages
16
I am working on the design and construction of the " Beby Benoist ". We plan to use an electric motor.
Q - What are the design-safety differences between engine mounts and electric motor mounts ?
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
7,792
Location
Saline Michigan
I am also thinking of of using Solidworks to validate, but Solidworks has some major issues (it can do gussets). I've watched a few FEA videos online and while it would seem to work, there also appear to be quite a few 'gotchas.
SolidWorks is widely used and well thought of by pros. That being said, all FEA has GIGO issues. Model it poorly or apply boundary conditions inappropriately, and the analysis can give results that take you way off course. Even working with pros, I have had to wrestle them back to reality on load cases, restraints, and even material moduli. Always wise to do laugh checks and comparisons to existing stuff....
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
5,750
Engine Mounts, if memory serves me most Aero engines mounts have Conical Mounts and the angle is set to the CG of the engine package.
Only the Lycomings, and not nearly all of them, have the Dynafocal mounts that are angled like that, and there are at least two versions of them. One has the focal point at or near the CG, and the other has the focal point ahead of it. Continentals have a variety of mounts, none of which are focused on the CG. I have never seen focused mounts on any other aircraft engine, whether Franklin or deHavilland or any radial. There may be some I haven't seen, of course.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
7,792
Location
Saline Michigan
I did download a copy of the referenced book at the end, and its pretty much the same type of analysis.

If there is a flaw to the article, what is it? It seems to be pretty straightforward on the maths and offers good tips re: spacing the mount points to reduce vibration.
The single biggest issue is that if you do not already understand truss design, this document will not help much. The authors attempt to cover Free Body Diagrams and Truss Analysis, but if you do not know how to do these things before reading his article, you still won't after. It takes doing exercises and checking your answers afterwards, then having someone help you understand what ever you miss... How do you check your answers? The authors did not tell us. Sum of moments in each plane must equal zero, sum of forces in each of three mutually perpendicular directions must sum to zero. He never covers how to do this and it is essential to the process.

He failed on some basic isolation stuff too. If you are designing a new engine mount, scheming out new locations for isolators, and trying to make existing isolators work, you have some potential pitfalls...

The reason to use isolators is to put an appropriately soft spring between a vibration source and the rest of the system, while it must also carry all of the static loads of keeping the engine attached to the airframe. In mounting a two rotor Wankel, the vibe source is the engine, with it speeding up and slowing down twice per rev of the output shaft being a big one, plus whatever pulses we get from the prop.

We will talk about the rest of it later, but Paul tries to tell us that standard Barry mounts have some natural frequency. Barry mounts are rubber springs. They do not have a fundamental frequency. Natural frequency of an engine on the mount is related to the square root of the spring rate (k) divided by the mass (m). In systems oscillating under torsional excitation, like engines on mounts, k would be a torsional rate and m would be mass moment of inertia, both centered on the roll axis of the engine. The natural frequency of the system is thus dependent upon spring rate of the isolator, isolator radius from roll axis of the engine, how many isolators, and how spread the engine weight is radially. Mount them too far away from the roll axis of the engine, and the spring rate is too high, too close to the roll axis, the rate is too low. Then there is strength...

Isolation of engine from airframe is achieved by making the natural frequency of the engine on the mounts 1-1/2 octaves or more below the lowest firing frequency you use, and isolation gets better and better the more octaves you get above the resonant frequency. That is why we usually feel and see engine vibe at idle, but at flight speeds it feels and looks far smother. More octaves from resonance... If the natural frequency coincides with firing, the system vibration will amplify, so we really do want want this frequency a minimum of 1-1/2 octaves above firing frequency during cranking as well as a minimum of 1-1/2 octaves below firing frequency at idle. This way you do not excite vibrations while cranking or idling, and your resonance is crossed very rapidly during starts and shut downs.

Just an example that is not too far off of any real engine: Let's say your Wankel engine cold cranks at 100 rpm and ground idles at 1200 rpm. That is 3.33 Hz and 40 Hz. So your mount torsional natural frequency band is bounded by:

3.33Hz * (2^1.5) = 9.4 Hz.
40Hz / (2^1.5) = 14.1 Hz.

To be securely not amplifying both during cranking and during idling, you have to hit that window of 10 to 14 Hz. A little too low and it will be unpleasant while cranking, but it might not break stuff. Too high and it is both unpleasant and likely to break something - this can be solved by raising ground idle speed, but that has other bad sides. The authors did not talk about this at all.

Paul was a passionate advocate for Wankel type engines, and really wanted folks to be successful with them. The fact that some folks had been flying these engines successfully with Barry isolators is strong hint that we would want to emulate their details, like positions of the isolators for each part number used. Then other designers can just copy the successful isolator setup when coming up with a new install, like we currently do with Lycoming and Continental engines and their fixed positions for their isolators, whether conical or Dynafocal.

Ugh. Between getting the right spring rate for your powerplant MMOI and making it strong enough, the isolators must be carefully designed or selected and placed to give the right range of natural frequencies. But if we already have a successful recipe, why re-invent?

Billski
 
Last edited:

slociviccoupe

Well-Known Member
Joined
Apr 1, 2011
Messages
366
Location
Palm Bay Fl.
Interested in the direction mount bushings are placed. Ive seen all sorts of combinations. There is mount bushings with their through bolts mounted paralell to the crankshaft, bed mounts with bolts at an angle leaning inward to crankshaft centerline with bolts perpendicular to the crank. Even the conical and dynafocal mounts orientate the bushings and mounting bolts differently.

Also in mount design ive seen 2 piece mounts. One piece solidly bolted to firewall, other piece soludly bolted to engine with bushings between the two. I like this aproach as you can leave engine, gearbox, exhaust mostly assembled and remove engine from the plane. But what orientation of the bushings and mounting holts is optimal?
 

Rockiedog2

Well-Known Member
HBA Supporter
Joined
Dec 11, 2012
Messages
2,588
The last mount I built has vertical bolts at the firewall. It's a swing out. Worth considering. Beats removing the motor to change a mag or accessory section.
 
Top