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Exhaust System Clearance to Nose Gear Mount & Cowl

twcroy

Active Member
We had a heat wave in MI today, upper 40s, sunshine, no wind. So I went out to check the exhaust system adjustment prior to putting the cowl back on. Black tape on one strut showed some bumping on the strut, so I dropped the exhaust pipe a bit to stop that. Put lower cowl on and no clearance to the pipe.

It seems I can either have the exhaust pipe hit the strut or cowl. I don't think either is good, though the strut shows where the paint from both exhaust pipes have rubbed against it in the past. My A&P in DC adjusted it a year or two ago and I believe it was probably doing it even then, but I'm not sure.

Finally, the question is do most exhaust systems bump the struts, maybe only at lower RPMs on taxi when it may run a bit rougher on idle jets? The tape showed bumping on it but I don't know if it's at lower RPMs or a broader range. I'll be getting it dynamically balanced soon so maybe that will help? If I had a little more clearance on the cowl for one pipe, I'd probably be ok.

The guy I bought it from five years ago said it's always important to minimize vibration, stress cracks can occur, etc.. So I'm trying to see how to do that.

I haven't had a chance to fly more than 2 hours the past four months other than warming her up on the taxiway, and today sure gave me the itch. My workload is easing a bit so I'm trying to get back in the air.

Thanks,
Tim
 
Sounds like you will need to remove some material from the lower cowl for clearance.
 
Pipe Bumping

Not sure exactly which airplane you have but there is more adjustment to the exhaust pipe clearance than you would think. If you didn't build it, you might not know that where the pipes connect, there is a kind of ball joint. The pipe coming from there back is usually slightly offset.

That offset allows you to adjust the side to side clearance as well by twisting the pipe and rotating it 90 degrees. What typically happens is that the pipe is set for the builders perception of straight and nice looks, etc. That could mean they like the more extreme angles where the pipe exits the cowl and set it for a more vertical but parallel orientation. If they do that, then it limits the up and down movement. Break it loose and you may be surprised how much difference a few degrees will make at the cowl exit area.

I know that's not very clear but I suspect it you go to Vettermans web site, it will show you some picture of the ball joint connection and how it works.

If you know all this already .... just grin and go on ! :)
 
Thanks Bill, and Raymo,

I'll check that out. I have a 9A. Original problem was the rubber dampening tube slipping on one of the stainless steel tubes due to a slight oil leak eventually wetting the connection (I'm talking about the exhaust pipe holder near the cowl here). I think that holder may still allow me to turn the pipe a little hopefully for more clearance.

This helps,
Tim
 
Do you have muff cabin heaters on the pipes? If so, it is very difficult to find a happy middle ground. My muff heaters have been frequently rubbing the lower cowl for 1500+ hours. I suppose one could beat a dent into the heaters to help...

As others have written, a combination of rotating the pipes and the heaters can help.
 
I do have one muff heater, but no rubbing on the cowl. There is some from the FAB though. I'm watching that. I'll see if I can rotate the pipes a little, if not I may "dremel" off some of the fiberglass cowl. I've never cut fiberglass before so I need to look into it beforehand. Thanks.
 
saggy Vetterman (sorry, that might come off wrong)

Before you start grinding away at that cowling... have a go at moving the hangars (small stainless tubing) back upwards where they were before.
If they got slippery inside from oil or other contaminants... gravity and vibration probably just let the pipes sag. I spent a fair bit of time under my 9A recently doing just that. I tried fancy silicone tubing because it was pretty blue... but it turned out to be inherently slippery inside as well as outside. Live and learn. Now I am back to the plain black rubber hose. You can either rinse it out with a solvent or take the time to install some new pieces. They do age in the heat down there, so it might be worth thinking about.
 
... Now I am back to the plain black rubber hose. You can either rinse it out with a solvent or take the time to install some new pieces.

I had the oily/slippery rubber hose connector issue on the port-side pipe, but was able to clean the oil out and (apparently) solve the problem. It's holding well now. It is true there's not a lot of "roaming room" for that pipe if it is to clear the aft edge of the bottom cowl. Mine clears OK, but does not have the recommended slope angle per Vetterman to minimize noise and resonance in the cabin. The only was to achieve it is to grind away at the aft cowl edge, which I'm heretofore unwilling to do.
 
Roughing up the steel inserts on a grinding wheel seems to let the hoses "grip" them better. No slippage for me. And, as Alex said, it's a tight fit with muffs.
 
The hose slipping is kind of what started this little adventure. I got that fixed, but then noticed the pipes had been hitting the nose wheel braces close by. I tried to adjust for that and they ended up on the cowl :) So I will need to take a look at rotating the pipes or trimming the cowl, which I'd rather not do. Do your pipes hit the nose wheel braces by leaving rub marks there. I'm also trying to get rid of a vibration, but think that'll be taken care of when I have the prop dynamically re-balanced after putting some leading edge protectors on it.
 
The hose slipping is kind of what started this little adventure. I got that fixed, but then noticed the pipes had been hitting the nose wheel braces close by. I tried to adjust for that and they ended up on the cowl :) So I will need to take a look at rotating the pipes or trimming the cowl, which I'd rather not do. Do your pipes hit the nose wheel braces by leaving rub marks there. I'm also trying to get rid of a vibration, but think that'll be taken care of when I have the prop dynamically re-balanced after putting some leading edge protectors on it.

They have, only during shutdown during the "wet dog shake". I think I found the sweet spot, at least temporarily, where nothing touches anything. It took a few iterations to get there however...
 
Here's how I fixed it.

Lots of gloom and doom in that thread, but mine is still working after a couple hundred hours. I was constantly fooling with the tubes and hose stuff. Don't miss them. I'll check with Larry and see how his is doing.
 
Thanks Alex. I think I may just end up trimming a little off the cowl. I think the marks I saw on the electrical tape were probably from the "wet dog shake" at shutdown, but I may trim a bit anyways. Now I need to see how to do that. I appreciate the help.

Tim
 
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