Polyester vs Epoxy RV-6 cowling

I am thinking of replacing my very old RV6 Polyester cowling (never fitted) with a new style epoxy cowling ? Question I have is this.

As I intend to fit a IO360M1B ? With an internal vertical plenum - My old Polyester cowling has a smooth bottom half ?

Now the thing is, I can get a good deal on a new epoxy cowling ? but it has the bottom air intake scoop.

Any reason why I cannot just cut of the bottom air intake and patch in a section ?

Is the general shape of the cowling the same ? or will I run into problems with the front section of the epoxy cowling with the bottom air scoop fitted, being to shallow to allow the internal plenum to be fitted ..

Hence further hacking into the bottom cowling would be required ?.

Any thoughts or advice on this would be appreciated ?

I suppose it depends on how happy you are with epoxy work.

No problem hacking the scoop off. If it is then just a matter of filing the defect and making good you could use some mat and epoxy to cover the inside plus a margin. Then use some divinyl cell foam as the meat of the sandwich, and the use some more mat and epoxy outside, use a bit of low density filler to make it easier tossed smooth/flat and the job is done. If you need to make some sort of bulgeto fit over the plenum, then its probably easiest to do it in situ, use lots of packing tape to protect the relevant bit of the plenum and then use a bit of modelling clay or similar to provide the necessary contour and again build your sandwich.

The divinyl cell foam isn't I the k absolutely necessary but the cowl will have something of a foam sandwich and filling up a big hole just with glass mat and epoxy isn't a neat way to go. The foam is easily sanded and worked.

Hope that helps. Hopefully Dan Horton will be along to advise asI think he is the absolute master of this kind of work.

Cheers

Chris

PS from memory you are at Woodvale?

I did it. It was a pretty extensive project! Lots of filling and sanding for sure. Not sure what a vertical plenum is, but one thing to consider, if you have horizontal induction, you will wind up doing MAJOR surgery on your snorkel! The smooth cowl Vans sells has some extra room for snorkel clearance. The cowl would have to be free for me to do it again. You can see more pictures on my blog.

Chris,

Thanks for you comments - looks like you memory is just as bad as mine ;-) ... I am just outside Durham in the North East ...

Mike,

I am having a good look at your blog - Per say the work does not bother me .. soon I may have more time than money ;-) .. on the other hand ... I see you are flying now .. Hence my question would be .. Having cut down the vertical snorkel ... making it 16%smaller .. any issues with that ??

I tested my engine sitting still (plane tied down) running full throttle 2670 RPM and had .7 inch of manifold PSI loss. Current Barometric PSI less manifold PSI. I had researched then and believe the AFP criteria was 1.5" and below was ok. My engine has been performing very well so far. My elevation is 2700' and and I climb out at 115 kts and 1700' per minute at gross. I started with shims in the top of the motor mount and have removed them as my engine settled. Too much engine sag will reduce the clearance between cowl and snorkel. I maintain 3/8"......It worked for me but that snorkel surgery took a lot of patience!

Edit; If you do surgery on that snorkel, keep in mind that it must be built back as strong as the original.

Thanks for the that Mike ... I think my good deal is starting to look like not such a good deal any more .. ;-) ...

I discovered my problem after I had the cowl fitted, snorkel and baffles and everything finished. I had fitted the cowl so the spinner was about 3/8" high to allow for engine sag. I read about installing shims in the engine mount to lower or raise the spinner and I wanted my spinner to match the cowl perfectly. I installed the shims in the top engine mount and it lowered the spinner perfectly! I was very pleased until I realized I didn't have enough clearance between the cowl and snorkel after lowering the nose of the engine! I absolutely panicked! All is well now but I can tell you at that point I felt helpless! All of this because my project came with a vertical induction cowl and I wanted horizontal induction! If I had known the cowls were different other than the snout, I would have went the vertical induction route in a heartbeat!

JanRV6UK said:

Thanks for the that Mike ... I think my good deal is starting to look like not such a good deal any more .. ;-) ...

Click to expand...

Maybe not, having fitted both on my RV-6A, the epoxy cowl is so much stiffer it is easier to handle and to fit in place.

The old white cowl can best be described as "floppy" and never went on in the same position every time it was test fitted. It was also quite distorted around the inlets needing a lot of heat massaging and filling.

If you cut the lower portion of the epoxy cowl away you should be able to make a quick mould off the white cowl to get that smooth lower surface, and the easier fitting would probably make up for this extra work.

az_gila said:

Maybe not, having fitted both on my RV-6A, the epoxy cowl is so much stiffer it is easier to handle and to fit in place.

The old white cowl can best be described as "floppy" and never went on in the same position every time it was test fitted. It was also quite distorted around the inlets needing a lot of heat massaging and filling.

If you cut the lower portion of the epoxy cowl away you should be able to make a quick mould off the white cowl to get that smooth lower surface, and the easier fitting would probably make up for this extra work.

Click to expand...

Jan's point is that he is now aware that the overall profile of the smooth cowl (no molded on inlet) intended for use with the induction snorkel is sightly larger in the lower front. It was specifically developed for using that induction system and is not simply one of the other cowls with the lower scoop removed.

I do agree that the epoxy cowls are a bit easier to install because of the additional stiffness.

rvbuilder2002 said:

Jan's point is that he is now aware that the overall profile of the smooth cowl (no molded on inlet) intended for use with the induction snorkel is sightly larger in the lower front. It was specifically developed for using that induction system and is not simply one of the other cowls with the lower scoop removed.

I do agree that the epoxy cowls are a bit easier to install because of the additional stiffness.

Click to expand...

Thanks for the clarification.

Is the snorkel version more efficient? ... or does it just look better?

az_gila said:

Thanks for the clarification.

Is the snorkel version more efficient? ... or does it just look better?

Click to expand...

I'm not sure what you mean by more efficient?

It was designed to be fit the slightly larger angle valve IO-360 so it is slightly larger to clear the tops of the front cyl, and the induction snorkel.

As far as look better... I guess that is in the eye of the beholder, but it is the cowl Van's supplied for any RV-7 that has no induction scoop.
My guess is that most people probably can't tell the difference (other than the missing induction scoop).