Fretting on lower cowl and snorkel

[Lkitson]

I have an issue with contact between my lower cowl and the intake snorkel on my -8 that’s causing wear on both parts. The photo on the left below is the snorkel and the photo on the right is the cowl. I first noticed signs of it after the aircraft’s first flight last year but it appears to be getting worse and it’s time to do something about it.

I’ve seen two other -8’s with the same issue and both of them had some kind of anti-chafe tape/pad on the surfaces to stop the fretting. I’d like to do something more permanent than that so I’m thinking about reworking either the cowl or the snorkel to provide more clearance.

My options:
1) Cut out the wear area on the snorkel and layup a repair to create a new surface that’s more concave than the current surface. I did this during my build to provide clearance for the mixture lever and alternator. My concern with this option is that it could potentially affect the airflow through the snorkel.

2) Grind down the wear area on the cowl, including the inner skin and some of the core. This seems like a better option to me as long as I don’t remove too much of the core.

3) Just put some anti-chaff tape over the contract areas of both parts and don’t modify either the snorkel or the cowl. If I go with option 1 or 2 I’ll probably still put some anti chaff tape on one of the parts in case they still make contact.

Looking for opinions on the best option especially from anyone who has dealt with this issue before.

 

[lr172]

That is more than just light wear. Something needs to be re-shaped IMHO. It looks to me that it is beyond the type of light contact that will respond to tape. If it were me, I would reshape the snorkel. I would also reinforce the wear area in the cowl with a small layer of glass, as it appears that you have worn through a layer. As long as you use smooth shapes, it should make no difference to flow.

Larry

 

[riseric]

I also have a contact point that's more or less the same as yours altough not as much/no significant wear between the 2 parts.
No touching when the cowl is on and the airplane is static.
I presume that the cowl moves of flexes slightly rearward when flying.

I put some UHMW tape on the snorkel and monitoring frequently.
80 hours now and the tape is still looking good...

If tape is not your solution, I would suggest to flaten the snorkel instead of triming/weakening the cowl.
A slight snorkel re-work should have an insignificant effect in the airflow.
I had to trim my snorkel so it would fit around the larger B&C starter.
Can't say if the flow is disturbed as I didn't measure before and after.
As far as I can see, it doesn't seem to affect the engine's performance from idle to 2700 RPM...

 

[fixnflyguy]

What I did

Likely, many others are in the same boat from what I've seen doing inspections and pre-buys. My RV-4 (I built) has a close fit in the snorkel/airbox area and the alternator mounting. Static, there is clearance, but start-up and highpower G loads will cause some contact. I have some red silicone based rubber sheeting (bulk stock thick baffle/gasket material) that is about 3/16" thick, and I made "bumpers" by cutting small circular pieces which I bonded to the inside of the cowling with high temp contact cement. These have worked perfectly for 400 plus hours, and eliminated the wear spots. The material I used is heat resistant and impervious to fuel and oil , but I'm sure any good rubber or baffle material will do the same job. Simple process, and no need to rework parts for better clearance.

 

[riseric]

Thin rubber bumpers !!!


Good idea Bill, I will remember that and use that solution.

 

[KiloFoxtrot]

I would tackle that problem with a heat gun and slowly heat in a circular pattern around the worn area on the snorkel. Only enough heat to where you can push the high spot in a little. Hold it into the new shape until it fully cools.

On the cowling I would smear some epoxy on the worn area to seal it up.

I used this technique on my tail fairing to make them seal up perfectly to the vertical and horizontal. Works great.

Note: Don't overheat and don't use this technique to make large shape changes.

 

[BoydBirchler]

You might look at the interface of the cowl with the spinner to make sure the engine is lined up left to right and has not sagged (may need washers under the lower mounts).

 

[Southern Pete]

My view is you must cut something away to increase the clearance.
Heating up and bending won't really work.
Grind out the honeycomb on the cowl over an area that is much larger than the wear patch (don't go through the outer skin) and lay up a couple of layers on the inside to seal.
If that doesn't provide enough clearance a flat will have to be moulded on the snorkel.
Adding anything as a bumper is bad news as it will only reduce the clearance and make your situation worse.

 

[Toobuilder]

I agree with Pete above. Sand a "bowl" shape in the honeycomb core right down to (but not through) the outer layer. Place a couple of patches of glass over the raw core and call it done. I did the same thing on my -8 to clear the alternator. NBD!

 

[Nihon_Ni]

Heat gun

I used the heat and bend method described above to shape my snorkel a bit to provide the clearance required in the plans. It worked like a charm. Personally, I didn't and wouldn't consider cutting into the honeycomb structure of the cowling because I'm not smart enough to figure out how to restore the strength of such a modification.

 

[Toobuilder]

Caution and knowing ones own limitations are a good thing, but the cowlings we use on these airplanes is not structural - its an aerodynamic fairing. The honeycomb core is a great way to add a lot of stiffness over a large diaphragm stucture with minimal weight. The early cowls achieved this stiffness with a thick (relatively) matrix of glass and resin. Cutting out a small section of core as suggested above is not going to degrade the performance of the cowl.

That said, repair techniques of honeycomb composite structures is readily available on the web or local library. Homebuilding is about recreation and education.

As an aside, the lower part of my lower cowl on my Rocket is made up of a similar sandwich matrix, of which the core is recycled foam board wall calendars I took out of the dumpsters from work. When people say "the month just flew by", that may have been me - at 235 knots.

 

[toc pilot]

Hello Lkitson,

I have an RV7 and have experienced the same issue you are having. IMO, the issue is with your engine sagging over time and the airflow of pushing on your cowl during flight. My fix for this after trying to make more clearance was to simply put a piece of neoprene/foam on the snorkel and have not had to worry about it since. That is all it took and no more issues.

 

[Richard Connell]

 

[Lkitson]

Thanks for all the suggestions. It's too cold here to do composite work right now so my plan is to put some anti-chafe tape on both parts and continue to monitor for any signs of wear through the tape.

When the weather warms up I'll probably do some reshaping of the cowl and possibly the snorkel as well.

The rubber bumper idea is interesting. If the cowl is flexing in flight the only thing stopping it might be when it contacts the snorkel. If I remove material for added clearance the cowl may just flex further and still make contact. So I might add a bumper in addition to reworking one or both parts. I don't see how it would hurt.

 

[abuura]

View attachment 35651

Nicely done mate!!

 

[Carl Froehlich]

Hello Lkitson,

SNIP.. IMO, the issue is with your engine sagging over time and the airflow of pushing on your cowl during flight. SNIP

Look to see if the spinner is low on the cowl. If so, the simple fix is to add a single engine mount washer between the engine and engine mount on the two, lower mounts. One washer will move the spinner up ~1/4”.

You may or may not need to use a longer engine bolt on the two lower mounts.

Carl

 

[Lkitson]

Look to see if the spinner is low on the cowl. If so, the simple fix is to add a single engine mount washer between the engine and engine mount on the two, lower mounts. One washer will move the spinner up ~1/4”.

You may or may not need to use a longer engine bolt on the two lower mounts.

Carl

It's actually a little high. I built it that way to allow for engine sag in the future.