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Making small fit adjustment: Cowling

cduster

Active Member
Rookie fiberglass question here...In an attempt to help combat higher than desired CHTs on my rocket, we faired in the lip on the upper cowling to try and make a smooth transition and eliminate the possible eddy current caused by the abruptly concave lip. We used three layers of cloth with West epoxy that spans a gap from the trailing edge of the upper lip back to the interior of the upper cowling covering a distance of about 8-10". Well, I'm sure you have already guessed that this greatly strengthened the upper cowling, and it is more difficult than it should be to insert the piano hinge wires since the cowl previously compressed just a bit when the wires were inserted.

My question is, for a minor adjustment like this (probably need to compress the width of the upper cowling by about 1/4" to make things fit well), would heat and a little persuasive pressure have any possible positive effect? I was thinking an insulating blanket and heat lamps for the better part of a day with the pins inserted and maybe even a rigged up clamp of some sort.

If that is a useless method, whats next, cutting lengthwise slits in the new glass and then fixing those slits with the cowling either installed or compressed to proper dimensions?
 
I have had great success adjusting fibreglass using a heat gun and clamping the part. I’ve straightened leg fairings, adjusted intersection fairings and other non structural parts. I don’t know if it would create any strength issues in parts though.
 
My question is, for a minor adjustment like this (probably need to compress the width of the upper cowling by about 1/4" to make things fit well), would heat and a little persuasive pressure have any possible positive effect?

I wouldn't fool around with a heat gun. The upper cowl is going to reach around 200F every time you shut down after a summertime flight.
 
I wouldn't fool around with a heat gun. The upper cowl is going to reach around 200F every time you shut down after a summertime flight.

Reading into this... Are you saying that once the cowl goes through a few heat cycles it'll "give" enough to fit a little better, or are you just saying that a heat gun won't work well in this application?

My personal experience was that the cowl fit well once installed, but fit even better after a few heat cycles.
 
Reading into this... Are you saying that once the cowl goes through a few heat cycles it'll "give" enough to fit a little better, or are you just saying that a heat gun won't work well in this application?

High probability of both. Likely one heat cycle. I doubt the cowl itself will budge, but the added troublesome patch, being low Tg West 501, may reshape, in particular if not well cured (like slow hardener and low temperature to date).
 
Well, a couple of heat lamps, a blanket over the top of the cowl and a little 110v ceramic heater shooting up through the cowl exit heated up the top cowl enough to get the pins in without any undue force. After a couple of test flights, I think things are going to relax enough that cutting relief slots won't be necessary. If I had it to do over again, I'd probably try and put the cowls together as soon as the resin wasn't sticky anymore. Thanks guys.
 
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