ORIGINAL: ahicks
I think all this is pretty interesting.... would like some thoughts about something I keep thinking about. Maybe we could get away from planes for a second and talk about our automobiles? It uses rubber motor mounts to isolate engine vibration from the rest of the car, right? So take those out and replace them with steel and bolt it direct for a second. You would imagine lot's of vibration, right? Now let's install some plastic mounts. Would you imagine you would have the same vibration level as the solid mounts, more than , or maybe, just a little less? That would be my bet, a tad less, but probably not a lot. Certainly though, not more than the solidly mounted engine?
Bringing this thought back to planes, wondering how the aluminum mount might differ from a solidly mounted automobile engine?
This reminds of a fairly recent discovery when bench running my M38 last year. I didn't have a soft mount ready yet so I installed it on a hard mount and then put that outside on my porch's post where I do all of my bench running. The post is a 6"x6" treated lumber and extends from the ground (it's a foundation post for my porch) to belly height above the deck. It is obviously tied to the deck
Anyway, once I had the engine going I had an intense buzzing sensation (high frequency) on the soles of my feet. There is an awful lot of wood in the way, yet the vibration was quite intense. This is exactly the kind of vibration that is transmitted to an airframe when there is no isolation. Is it harmful? Probably but since I don't hard mount any of these engines on airframes, I will never know
I repeated the run after the isolation mount was installed...... Vibration transmission to the deck was nonexistent.
The type of mount I use and recommend is the Hyde style, rubber isolation mount. The front of the mount accepts the engine while the back is mounted to the firewall. Front and back are connected to one another via a rubber motorcycle inner tube. Over the years I've tried other, simpler mounts available commercially; they produced unsatisfactory results for isolating vibration. I didn't invent the mount; Merle Hyde did. I re-engineered it to accept stand offs