Cowl Fitment

[rockitdoc]

I've almost got the top cowl fitted thanks to all the help here.

Now, I have another issue.

The top cowl fits pretty well all the way round the firewall except on the port side near the top where it makes the curve down the side of the firewall. I can push the cowl down against the Skybolts so the elevations of both cowl and firewall match, but it takes quite a bit of effort, ie, the cowl does not fit nicely by gravity. If I construct it this way there will be upward tension on the Skybolts in this location. I am concerned the stresses will cause trouble down the line especially considering the pressures inside the cowl as DanH has pointed out.

Two solutions I can think of:
1. Change the shape of the firewall to match the cowl
2. Add material to the underside of the cowl in a thickness sufficient to match the elevation difference then grind the top down to match the firewall elevation.

I hope there is a third option because neither of these, especially #1, seem questionable.

The starboard side fits pretty well with little effort required


The port side, not so much

[DanH]

Originally Posted by rockitdoc

The top cowl fits pretty well all the way round the firewall except on the port side near the top where it makes the curve down the side of the firewall. I can push the cowl down against the Skybolts so the elevations of both cowl and firewall match, but it takes quite a bit of effort, ie, the cowl does not fit nicely by gravity.

Glass/epoxy will creep given warm temperatures and time. Before making any big moves, just put a cinch strap around the cowl and pull it tight. Check for good fit at the seams with the gap pulled down, adjust as required. Cinch it tight again, and put a heat source in the cowl, something to push air temperature up around 200F. Hold it there overnight, then let it cool before removing the strap.

[wirejock]

Originally Posted by DanH

Glass/epoxy will creep given warm temperatures and time. Before making any big moves, just put a cinch strap around the cowl and pull it tight. Check for good fit at the seams with the gap pulled down, adjust as required. Cinch it tight again, and put a heat source in the cowl, something to push air temperature up around 200F. Hold it there overnight, then let it cool before removing the strap.

I've had good results heating and reshaping like this. Just don't go too hot.

[rockitdoc]

Originally Posted by wirejock

I've had good results heating and reshaping like this. Just don't go too hot.

I put a 25w bulb in one of my lead lights and hung it inside cowl port side where cowl fit was marginal. I will let you know what the temp gets to.

Fitment is pretty good thanks to you guys. Once the heat treating is complete I will trim the bottom cowl to fit and start the process of getting the spinner clearance spot on.

Is bodywork recommended between top and bottom on the longitudinal seam so there is no 'step'? Or, do the two halves fit better when Skylocks/hinges are in place? I know there is a recess in the bottom cowl that should accommodate the top.

[rockitdoc]

Originally Posted by rockitdoc

Is bodywork recommended between top and bottom on the longitudinal seam so there is no 'step'? Or, do the two halves fit better when Skylocks/hinges are in place? I know there is a recess in the bottom cowl that should accommodate the top.

Duh. The bottom is trimmed to butt up against the top. The 'step' only occurs because the top overlaps the bottom until trimmed. I'm catching on slowwwwly

[Larry DeCamp]

Don’t forget the Dave Anders approach to capturing lower cowl airflow with a felt seal on the crank extension. This gives some flexibility to the spinner/cowl gap options. Your sense of aesthetics becomes the defining dimension and it could make it a lot easier to get the lower cowl half off .

[rockitdoc]

Originally Posted by Larry DeCamp

Don’t forget the Dave Anders approach to capturing lower cowl airflow with a felt seal on the crank extension. This gives some flexibility to the spinner/cowl gap options. Your sense of aesthetics becomes the defining dimension and it could make it a lot easier to get the lower cowl half off .

Hmm. Not familiar with this approach. Where can I find the writeup?

[rockitdoc]

The cowl on the port side upper did not fit quite right. At DanH and Wirejock's suggestions I put a 25w bulb in a trouble light and hung it under the cowl on the port side to heat up the cowl. Temperature of the upper cowl got to 80F, just 3F over ambient. I replaced the 25w with a 100w and left it over night with a blanket on top the cowl to hold temp in. This morning I measured 105F on the top cowl at 74F. Not sure if I should try to go to higher temps.

[wirejock]

Originally Posted by rockitdoc

The cowl on the port side upper did not fit quite right. At DanH and Wirejock's suggestions I put a 25w bulb in a trouble light and hung it under the cowl on the port side to heat up the cowl. Temperature of the upper cowl got to 80F, just 3F over ambient. I replaced the 25w with a 100w and left it over night with a blanket on top the cowl to hold temp in. This morning I measured 105F on the top cowl at 74F. Not sure if I should try to go to higher temps.

Use a heat gun and temp gun. Heat it to 200 and let cool.

[DanH]

Originally Posted by rockitdoc

This morning I measured 105F on the top cowl at 74F. Not sure if I should try to go to higher temps.

Yes, higher temperature, held for some period of time. No guarantee, but it should work.

The experience basis is the behavior of cowlings subjected to normal engine compartment heating. I have removed hot cowls and set them on floors and benches, then during reinstallation days later, found they had changed shape slightly...they had cooled while slightly deformed, although not enough to prevent installation. Come back and R&R them later, after a few flights (i.e. heat and cool cycles in place), and they fit just as they did originally.

200F is roughly the air temp in the cowl after shutdown on a hot day. You won't hurt it with the same in the shop.

Heat gun is fine if the plan is to aim it into an inlet as a space heater. Aiming it at a surface tends to result in uneven heating, with little dwell time.