Quick cowling check before I begin trimming

UnPossible · #1 12-29-2010, 07:58 PM

Hey - I think I am ready to start trimming my cowling, but want to check with those that have gone before, to make sure that I'm not making a time consuming mistake.

I first started by rough fitting the two cowling halves together.... they fit fairly well and result in a spinner circle with a 13.2" diameter and the two nostrils are the same size.

As you can see in the pictures below, I have the top cowling centered on the spinner backplate (popsicle sticks worked great to get them to be the same)
The cowling is also level. What I am not 100% confident in is, is the cowling centered at the firewall? The sides of the cowling each come down an equal amount down the sides of the firewall... assuming that the cowling is symmetrical, it should be centered on the firewall... (why can't they mold in a slightly raised centerline that you could sand flush when finished?)
Is there anything else I need to do before I begin trimming the cowling to fit the firewall?

Thanks again,
Jason

pierre smith · #2 12-30-2010, 05:39 AM

...you're looking really good and don't be overly concerned about it being perfectly centered on the firewall.

One consideration a lot of guys have is for future engine sag as the rubber mounts age. You may want to lower the cowl an 1/8" or so at the spinner and after a 100 hours or so, it will be more centered on the spinner.

Best,

Jaypratt · #3 12-31-2010, 07:05 AM

I cut the bottom first. Makes more since to me.
You are coming along well.

dougweil · #4 12-31-2010, 08:31 AM

I agree with Pierre on setting the cowling low to allow for sag. In fact on my RV-4, I set it 1/4 inch low and it was perfect by around 50 hours.

ambrosesquadronca · #5 12-31-2010, 07:05 PM

Jason,

I started with the upr cowl and tried to level it with the inlet walls thinking it was the best feature to use and an important feature to have level for appearance. This turned out to be a mistake. The upr cowl doesn't "key" like the lwr and slides around.

After speaking with Vans they hand crafting their cowls on the prototypes (at least on the -8) and made molds from them so basically there's no good reference feature to use on the upr cowl and they said start with the lower. The lower "keys" into position on can't slide around so trimming the sides of the lwr and not shorting yourself on the upr is relatively easy.

Here's the advice Vans gave after I made a bad cut on the upr I had to patch...

I would not use the inlet walls as a level reference for the cowl. The cowls
were originally laid up and carved by hand, and will not be perfectly
symmetrical. A better way is to use the recessed mold line in the lower cowl
behind the spinner, and lay a long straight edge across that to project that
line out to the front corners of the cowl. If the upper cowl is trimmed to
match the lower cowl after the lower cowl has been trimmed that way, the
two forward corners of that can be levelled, and that should be reasonably
symmetrical back at the firewall.

Do not trim the lower edges of the upper cowl until you know where the
overlap area is with the lower cowl on the firewall (ie you must first offer the
lower cowl up to the firewall and see how high the sides can be). The join
line may end up a little different on one side that the other. This cannot be
seen, and does not matter.

UnPossible · #6 01-10-2011, 11:38 AM

Hey - thought I'd provide an update to the cowling saga.
Last week, a local RV-9A bulider dropped by and we spent 4 hours sanding and fitting and sanding and fitting, etc....

I think the cowling came off 50+ times that night.

However, we got it very close. With a bit of additional fitting, I am pretty close to having a decent fitting cowling. I will have some minor fiberglass building up to do on the front, but that can wait until warmer weather when I can sand outside and not cover the garage in that lovely white powder.