Trimming the Fiberglass Cowl

Hello all,

I'm considering trimming a larger opening from the lower cowl to open more clearance for the exhaust pipes. Is there a recommended tool for this? I'm considering a dremel tool and putting masking tape over the painted surface to avoid any chipping. Will this work or am I making it too easy? I also need to figure out a way to smooth and seal the exposed, cut edge.

Thanks for any thoughts, and your time,
Tim

I used a thin cut off wheel.

Be sure to go slow. The cut off wheels can produce a lot of heat on F/G parts and could bubble the paint if you let the heat build up.

The best tool I have found to cut straight lines in fiberglass is a cutting disc on a detail sander. Feinstein makes the best.

If all you want is to enlarge the cutouts, have you considered a carbide burr in a die grinder? I've used one to do that.

edit: What I did was to cut semicircles into the trailing edge of the cowl at the points where the two pipes exited the cowl.

Charlie

twcroy said:

Hello all,

I'm considering trimming a larger opening from the lower cowl to open more clearance for the exhaust pipes. Is there a recommended tool for this? I'm considering a dremel tool and putting masking tape over the painted surface to avoid any chipping. Will this work or am I making it too easy? I also need to figure out a way to smooth and seal the exposed, cut edge.

Thanks for any thoughts, and your time,
Tim

Click to expand...

A HF small diamond grit disc will work well. Dremel is a little hard to make a precisely straight cut, but taking care not to cross the line and then sanding up to it will work just fine. Cut in 3-4 passes as it will heat, as mentioned. 80 grit sandpaper on a wood backer works good to sand and finish. Progress to finer grit for desired finish.

On green epoxy (less than fully cured) a more aggressive tooth is needed.

How much are you cutting off?

Hey all,

Thank you. I think I might try the carbide circular saw bit on a drill. Hopefully if I use a water pool around the cut it won't heat up too much. I used a 3/4" I think for cutting holes in porcelain tiles this summer for pipes. It may just work here too, or maybe a bigger one, and then sand it down a bit as Bill mentioned. I'm not sure what I'll do for sealing the edge, maybe put a little bit of paint on it.

Bill, I think I'll only need to cut less than a 1/2 inch off in a close to semi-circle. It probably won't look perfect (cowl opening isn't symmetric to the pipes), but it shouldn't bad either.

Thanks as always for the help, God bless,
Tim

You might think I'm crazy, but try a fine tooth hacksaw blade, handheld. Clamp a guide to the fiberglass and saw along it with light pressure.

Hacksaw

DanH said:

You might think I'm crazy, but try a fine tooth hacksaw blade, handheld. Clamp a guide to the fiberglass and saw along it with light pressure.

Click to expand...

Plus one. Any hardware store will have a lightweight hacksaw blade handle for one handed operation. The blade slips into a slot in the handle and is held by a single screw.

Use some electrical tape to determine where to cut, then be very careful.

I used a dremel, cut wide, and then used a straight four by four with a belt sander belt glued to it to sand it to the final line.

You're overthinking it. You could use practically anything to make that cut. A cut off wheel (same as recommended for the plexi canopy) will make quick work of it. Probably lose some speed thou opening up the lower cowl. My -9A ran 161-163 KTAS before and about 157-158 KTAS after trimming back the bottom cowl exit about 1-1/2". Improved cooling though.

Resurrecting....

I am considering trimming my cowl opening for better airflow / cooling performance. Does anyone have photos pre- and post- op on a nose dragger? I have a RV-8A with an O-360 / carb, so that version would be the best reference, but any photos of any version be be of great help.

*side note: the "low" pressure side of my cowling seems to be significantly charged with air trying to escape. You can see the oil door arching in flight, and the piano hinge cowl to fuse margin *seems* to expand a smidge from what I assume is air trying to escape. I've rebaffled, sealed up what I can, timing is spot on, cleaned up the exit area as best I can - all the usual suggestions here. I have an EZ-Cool flap ready to install, but it seems to me that it may only be part of the solution (useful only for climb out.) I've got some pressure numbers around somewhere, but they seem to suggest a less than ideal pressure ratio top-to-bottom (I need to find them.)

maus92 said:

*side note: the "low" pressure side of my cowling seems to be significantly charged with air trying to escape. You can see the oil door arching in flight, and the piano hinge cowl to fuse margin *seems* to expand a smidge from what I assume is air trying to escape.

Click to expand...

All normal. What matters is (1) the pressure difference between the upper plenum and the lower plenum, and (2) how much heat is transferred to the air.

This chart maps out required deltaP at various altitudes and OAT's. The heat transfer is a built-in empirical assumption. Do better than the assumption and you'll need less deltaP.



Regarding pressure, a builder may (1) increase upper plenum pressure, and/or (2) decrease lower plenum pressure, either of which results in more deltaP. Increasing upper pressure is more desirable than decreasing lower pressure, as more lower cowl pressure results in higher exit velocity. Less velocity loss equals less cooling drag. For example, assume a deltaP of 8" H20 would result in the desired mass flow for a low CHT. In fast cruise you might have 18 upper and 10 lower, which will definitely make your cowl bulge. Or (given very poor conversion of available dynamic pressure, Q) you might have 10 upper and 2 lower. Cooling will be the same (both 8" deltaP), but the second case will be higher drag. The bulging cowl indicates a good thing.

It's a double whammy. A cowl with poor conversion of Q won't generate enough upper plenum pressure at climb speeds, even if the lower cowl pressure was at freestream static. Can't get a deltaP of 8 when you start with 5.

Obviously another way to run hot is to have a bunch of leakage between the upper and lower volumes, as (1) it tends to equalize pressures, and (2) it lowers heat transfer (it won't meet the empirical assumptions in the cooling chart).

I've rebaffled, sealed up what I can, timing is spot on, cleaned up the exit area as best I can - all the usual suggestions here. I have an EZ-Cool flap ready to install, but it seems to me that it may only be part of the solution (useful only for climb out.) I've got some pressure numbers around somewhere, but they seem to suggest a less than ideal pressure ratio top-to-bottom (I need to find them.)

Click to expand...

Glad to look at those numbers. And tell us the whole package; engine, compression, ignition type, test altitude, OAT, etc. Got pictures?

I'm not allowed to post attachments for some reason....

Here are my raw numbers:

RAW DATA Altitude = 3000 MSL
11/28/16 CABIN Pressure
OAT = 6C

IAS kts DIRECTION UPPER LOWER RATIO

100 N 245 125 0.510204082
E 247 125 0.506072874
S 250 130 0.52
W 250 125 0.5
AVG 248 126.25 0.509069239

120 N 345 166 0.48115942
E 345 164 0.475362319
S 347 164 0.472622478
W 342 164 0.479532164
AVG 344.75 164.5 0.477169095

140 N 451 222 0.492239468
E 455 222 0.487912088
S 452 222 0.491150442
W 459 222 0.483660131
AVG 454.25 222 0.488740532

AVERAGE 349 170.9166667 0.491659622

Sorry that the characters don't line up. I used a Rise manometer with piccolo tubes, but had the meter set on "mystery" units (meaning I forgot to ensure it was on inH20.) I tried experimenting with converting these numbers to inH20, but the results didn't make sense (at least to me.) So I can only derive a pressure ratio.

I don't have the logs at my house, but it's an O-360-A1A, the timing is set to 25BDC, cyl #3 is always the hottest, and cyl #1 is the outlier and runs significantly (10-20%) cooler than the other 3. Any comments are appreciated.