Engine Cowl/Spinner Question

[stickandrudder1]

I'm installing my engine cowl on my 7A, and at present have a 1/8" gap between the spinner and the cowl on the top, and a 1/2" gap on the bottom, with a gradual increase in the gap from top to bottom. This is with the top half of the cowl trimmed at the rear, and the bottom yet to be trmimed. Once the bottom is trimmed, the gap will only get larger, as the aft end is rotated up into position!! I'm using a Whirlwind 200RV prop. I could minimize the problem by adding a spacer between the top engine mount attach points and the firewall, or add spacers to the top engine mounts (Lord) to lower the front of the engine, but I'm not sure either if these would be an acceptable practice. The only other alternative I can think of is to build up the front of the cowl around the spinner once I get the cowl fit and finished. Any insite in how to deal with this problem would be greatly appreciated. Thanks

 

[Kyle Boatright]

Don't worry about the different gap at the top and bottom. It's made that way to make the lower cowl easier to remove. Once you get the baffles, gear fairings, etc. installed, the lower cowl becomes tough to remove and reinstall. I think I have a 3/8" - 1/2" gap there and wish it was more like 1/2" - 5/8".

 

[Captain Avgas]

I'm installing my engine cowl, and at present have a 1/8" gap between the spinner and the cowl on the top, and a 1/2" gap on the bottom, with a gradual increase in the gap from top to bottom. This is with the top half of the cowl trimmed at the rear, and the bottom yet to be trmimed. Once the bottom is trimmed, the gap will only get larger, as the aft end is rotated up into position!! I'm using a Whirlwind 200RV prop. I could minimize the problem by adding a spacer between the top engine mount attach points and the firewall, or add spacers to the top engine mounts (Lord) to lower the front of the engine, but I'm not sure either if these would be an acceptable practice. The only other alternative I can think of is to build up the front of the cowl around the spinner once I get the cowl fit and finished. Any insite in how to deal with this problem would be greatly appreciated. Thanks

I had the same problem with my RV7A supplied cowls and I couldn't tolerate the gap differences. The cowls vans supplies vary enormously and some just need some remedial work (mine certainly did) . Aircraft judges will tell you that it is not the size of the a gap that aesthetically matters (within reason) but the consistency of the gap. Aircraft judges always look for significant gap differences as a sign of less than optimum workmanship.

My solution was to build up the cowl on the INSIDE with layers of glass where required and then sand the cowl nose back from the OUTSIDE to gain a consistent gap all around between the cowl and spinner. It's not a big job.

The question of how MUCH gap you need in this area crops up from time to time (see archives). Different people will tell you different dimensions.

However one thing that is almost always overlooked in this regard is the method you use to attach the top and bottom cowls to the firewall. If you use standard Vans piano hinge then you will need a bigger spinner gap because both the top and bottom cowls will need to slide forward for the teeth on the hinges to disengage at the vertical hinge (down the sides of the fuselage) before the cowls can be removed.

If on the other hand you are using 1/4 turn camlocks around the firewall then the cowls do not need to slide forward to remove them (so the gap can be smaller...say 3/16").

Having a grossly oversized gap between the cowls and the spinner not only looks unpleasant but it allows more air to flow from the outside into the lower cowl in flight, thus reducing engine cooling efficiency.

 

[Rick6a]

My solution was to build up the cowl on the INSIDE with layers of glass where required and then sand the cowl nose back from the OUTSIDE to gain a consistent gap all around between the cowl and spinner.......

There is no one-size-fits-all answer. The spinner to cowl gap is largely dependent upon what engine you have fitted. I quoted Bob because (I'm told) this is the fix that Van's commonly recommends to adjust the gap. I have found that since my IO cowl lacks an air snorkel to deal with, I was able to build in very little spinner-to-cowl gap on the -8 relative to the wider gap on the -6A with its carbureted engine. This is because the lower cowl on the -8 simply lifts straight up into place without any jostling around an air snorkle. In practice, even though the cowl to spinner distance is narrower on the -8, I find its cowl is significantly easier to remove and reinstall compared to the machinations I have to deal with on the air scoop equipped cowl fitted to my -6A.

 

[erich weaver]

Had a similar problem with mine. I was able to fix by cutting some vertical slices near the front of the cowl in the appropriate places, wedging them open, and then filling the gap with an epoxy and flox mixture. Much faster than laboriously building up the front of the cowl and sanding to shape.

erich

 

[robertahegy]

On mine I made the fit tight on the top so the gap on the bottom was minimized. I then layed up several layers of 9 0z. glass in the inside. Now I was able to reshape the front of the cowl to be more vertical with the spinner. A small layup of glass on the outside bottom added that little amount needed to get the nice fit I wanted. Very easily done and well worth the effort for a nice looking cowl.

Roberta