Need to start a discussion about engine mounts

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wsimpso1

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I think Wittman bolted his V8 solid with no rubber.
That would make for a strong buzz through the airframe. A lot of racecars and motorcycles are done that way, and you sure can feel the engine. Might have had to bump tube size to stand up to the increased vibration. One other thing about solid engine mounts is that the airframe must then have its lowest natural frequency at least 1-1/2 octaves above max firing frequency or the airframe will resonate with the engine... I do not know if that will require any beefing up of the airframe or not...

Billski
 

BBerson

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I had the V-twin rigid base mounted. It shook back and forth with each power pulse. I bolted another additional rigid mount near the top rear and that eliminated the shake. Yes, probably a high frequency buzz now, but the alternative was shake it apart. Different parts resonated at different rpm.
 

wsimpso1

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I had the V-twin rigid base mounted. It shook back and forth with each power pulse. I bolted another additional rigid mount near the top rear and that eliminated the shake. Yes, probably a high frequency buzz now, but the alternative was shake it apart. Different parts resonated at different rpm.
Somebody else thinks so too.

Initial system was too soft, and it shook. Lucky for you it was not close enough to resonance to tear the system apart on first run. Your response was to stiffen the system until it did not shake. If that was stiff enough to drive resonance off scale high, it worked fine. If you had guessed stiffer, but not stiff enough, you would have known it with an rpm band that resonated. You could then stiffen it up some more until you drive the resonance out of range high.

Attempts to strengthen without knowing you need big stiffness increases are likely to fail. "It breaks here, make that stronger." Now resonance breaks the new weakest spot. This can be a long chain of rebuilds with fixes. But if you know up front that the goal is stiffness improvement, you can get good much faster.

The process of doing this with trial and error requires some luck. If the engine tears up your install, that really makes the process expensive and time consuming. If in the process of stiffening the system, it gains too much weight for flight, the process fails. If you start driving m up nearly as fast as k you stop increasing the resonant frequency and the process fails.

On the good side, we know a lot of engines have been rigid mounted and made to work on flying machines, so odds are decent it will work. With known engines and installations, we have solutions already known. Cool too.

Billski
 
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BBerson

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Many of the 75 year old airplanes are still flying with the original rubber mounts tightened occasionally. They are tight and almost rock hard. I guess it works.
 

Pops

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On the motor mount on the SSSC I used rubber donuts from auto rear shocks from NAPA. Worked great. The VW engine was balance and when flying it was almost as smooth as an electric motor. Even a blind hog can get a acron once in a while.
 

pfarber

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Got a rough sketch of the firewall (from the plans) and measurements of the motor.

I'm planning to pretty much follow the ideas I've seen for other conversions, that being the bellhousing for the PSRU will incorporate feet for isolators as wide as I can fit. This will also become a mount point for an under engine radiator and duct. Initial plans are to get it flying, then work on efficiency of cooling and aerodynamic clean up.
The BD-4 originally was designed for 'swing out' engine access, and short of replacing some major structural parts, I have to use what's already there. I want to keep the cooling system on the mount, the calculated radiator size will definitely fit in the cowl

I'll post a few sketches to see if there are any glaring errors, then start doing some math to size the tubes. Initially the engine will have no offset. I think it will be easy enough to adjust with washers, or trim tabs.

Been googling and looking at engine installs all week, so far nothing looks to be to complicated. Found places that do CNC tube cutting fairly cheap.

Once I get a rough sketch I'll measure up the engine and find the CG. The PSRU is not here yet, so I will use the drawings to estimate the moment and then once it arrives I'll recheck the numbers.

To cold to be in the garage, so this is the perfect time to get this part hashed out.
 

BJC

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Why go with CNC tube cutting for such a small job. 10 minutes with a cheap HF electric grinder with a 3" for 4" cut-off wheel and a file will do the job for each tube.
Back in the 1970's, I knew a technician from Denmark who had a master craftsman's card. On his first day of metal working in the apprenticeship program, he was given an irregular shaped chunk of steel and a file. He had to form a cube to some really precise accuracy for a grade, using only the file to shape it.

He said that he worked on it every free moment, and did not get very much sleep.

I'm with you, Pops; often it is better to grab a hand tool and form a part than to diddle around with machine tools.


BJC
 

Pops

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Back in the 1970's, I knew a technician from Denmark who had a master craftsman's card. On his first day of metal working in the apprenticeship program, he was given an irregular shaped chunk of steel and a file. He had to form a cube to some really precise accuracy for a grade, using only the file to shape it.

He said that he worked on it every free moment, and did not get very much sleep.

I'm with you, Pops; often it is better to grab a hand tool and form a part than to diddle around with machine tools.


BJC
Same for an apprentice in Germany.
 

Rockiedog2

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I like a bench grinder with a 6”x1/2” wheel with the corners shaped in a half circle.
 

Pops

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And I have half round files in 3/8", 1/2", 3/4" and a tapered 1/4"to 3/4" file in mine and coarse.
 

pfarber

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Why go with CNC tube cutting for such a small job. 10 minutes with a cheap HF electric grinder with a 3" for 4" cut-off wheel and a file will do the job for each tube.
After a wood dowel mock up I'll have the basics for starting a drawing.

With the model in Solidworks for FEA then why not use that invested time so I can simply email a shop and get parts. The cost is not significant, and its done right the first time, or at least to the plans.

Some things are just better/easier when you give your CC number and a week later things show up in the mail.
 

pfarber

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Back in the 1970's, I knew a technician from Denmark who had a master craftsman's card. On his first day of metal working in the apprenticeship program, he was given an irregular shaped chunk of steel and a file. He had to form a cube to some really precise accuracy for a grade, using only the file to shape it.

He said that he worked on it every free moment, and did not get very much sleep.

I'm with you, Pops; often it is better to grab a hand tool and form a part than to diddle around with machine tools.


BJC
We had a similar project in A&P class. To familiarize students with hand tools and measurements, you have to make a small ruler/thread gauge/square about 6x3in starting from a random 1/8th thick cut off they purposely did on angles. One corner had to be 90deg and another 45deg. The gauges were 1/8thNF and NC through 1/2in in various pitches. Then mark off 16ths for a ruler.

It was a week long project. I don't think I kept it, but it might be somewhere. Was it a good learning experience? yes. Do I want to do it again? no.
Can I use a die grinder/bandsaw/hand files? Sure. Do I want to? No.
 

Dan Thomas

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Die grinder (straight, not angled) and carbide burrs are the ticket for shaping the tubing ends. Do the longest ones first to get the hang of it, and a ruined end gets turned into one of the shorter pieces. It's art, and you use your eyeballs instead of CAD.

When I was your age...
 

karmarepair

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I had the V-twin rigid base mounted. It shook back and forth with each power pulse. I bolted another additional rigid mount near the top rear and that eliminated the shake. Yes, probably a high frequency buzz now, but the alternative was shake it apart. Different parts resonated at different rpm.
I'm also working on mounting a V-Twin, on a test stand, but aiming at possibly hanging it on a firewall eventually. I cribbed some ideas for soft mounting from the way Generac mounts these engines in gensets. They "float" the Engine/generator assembly as a unit in soft mounts, which are loaded in shear, kind of along a radius from the crank. The engine is on a "tray", and the "tray" is soft mounted into the chassis of the genset. Part #17 in this view is the isolator Generac 4582-2 Parts Diagram for Generator (jackssmallengines.com)

I've attached a picture of a really rough physical model of my current thinking. My proposed "tray" is stiffened at the back with a plate, and the lower struts are extensions of the lower longerons. The diagonal brace runs up to the upper longerons. The isolators that Generac uses have studs top and bottom, and a "rubber biscuit" vulcanized between them. The Barry Mounts used on a lot of ultralights have a bushing through the center of a two-piece "rubber biscuit" and constraining washers at either end - I feel like the failure mode is a little less frightening, although they are a little more trouble to install. dim_barrymt_2.jpg (150×150) (leadingedgeairfoils.com)

In the attached picture, the little red cubes are modeling the Generac isolators for this size engine. The red "tray" bolts to the bottom of the engine. The engine in this model is a Kohler Command. And it still has all the shrouding on it, and there is enough of a gap to the firewall to let air flow to the flywheel mounted fan. The plan is to pull the flywheel, the fan, and the shrouding OFF after the initial baseline runs, and devise baffling that will flow air the opposite direction, off the prop, which will initially be direct driven off the PTO end.

Constructive feedback is welcome. I don't love the way I've triangulated the diagonal -it should run out to the end of the extended longerons, but the "tray" is sort of in the way. I haven't really thought hard enough about gyroscopic loads trying to twist this out of the airframe, and the Generac mounts were never intended to resist the thrust loads of a prop trying to PULL the engine forward either. I think it may need a brace(s) to the cylinder heads, also resiliently mounted. Those two lower longeron extensions should also be tied together. Etc.
 

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BBerson

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The Briggs engines flying in Europe (Sd-1 and Luciole) use firewall mount, not base mount.
The vertical V-twins have numerous threaded holes for firewall attach on the engine PTO end. Sort of like the VW firewall mount on a Volkplane.
 
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