Cowl Fitting

[decathlon737]

OK guys, here we go with cowl questions. Van's manual says to space spinner backplate 2 1/4" inches from flywheel and keep cowl 3/16" to 1/4" from it. Does this allow for the 1/2" flange on the spinner backplate? I'm using Hartzell BA CS prop. I don't have my prop yet. I do have an old AD'd Hartzell hub that places the spinner backplate at 2 1/2" from flywheel so 2 1/4" seems right until you factor in the spinner backplate flange. Am I missing something here? Help would be appreciated.

Bill Woods
RV8 QB

[jsharkey]

Originally Posted by decathlon737

OK guys, here we go with cowl questions. Van's manual says to space spinner backplate 2 1/4" inches from flywheel and keep cowl 3/16" to 1/4" from it. Does this allow for the 1/2" flange on the spinner backplate? I'm using Hartzell BA CS prop. I don't have my prop yet. I do have an old AD'd Hartzell hub that places the spinner backplate at 2 1/2" from flywheel so 2 1/4" seems right until you factor in the spinner backplate flange. Am I missing something here? Help would be appreciated.

Bill Woods
RV8 QB

You should fit the spinner back plate to the engine using the actual propeller spacer that you plan to use - or a same thickness mock-up of it. Use the spinner back plate as a guide to align the circular portion of the cowl and trim it to fit the fuselage so that it clears the aft facing flange of the back plate by 3/16 to 1/4 inch.

[Rick6a]

Originally Posted by decathlon737

OK guys, here we go with cowl questions. Van's manual says to space spinner backplate 2 1/4" inches from flywheel and keep cowl 3/16" to 1/4" from it. Does this allow for the 1/2" flange on the spinner backplate? I'm using Hartzell BA CS prop. I don't have my prop yet. I do have an old AD'd Hartzell hub that places the spinner backplate at 2 1/2" from flywheel so 2 1/4" seems right until you factor in the spinner backplate flange. Am I missing something here? Help would be appreciated.

Bill Woods
RV8 QB

I measured and measured. 2 1/4" is the spacer length I came up with too. As you know, when using a constant speed propeller, you do not install the spinner backplate on the engine. It is installed on the back of the propeller. Although Van's recommends installing the (constant speed) prop before trimming the cowl, many builders get around that by simulating the cowl/spinner distance with a series of spacers. As the photo shows, I was going to do just that but in the end, I just couldn't bring myself around to trimming the cowl to that simulation. I ended up removing the spacers and went ahead and bolted the backplate to the prop and then hung it. It really was not difficult to trim the cowl afterwards and am very pleased with the final gap. I suppose the 2-1/4" spacer trick will satisfy many builders...however it comes out. As always, the builder reserves the option to do it any way they want to.

[andrew phillips]

I followed the manual and used the 2 1/4" spacer method. When the prop arrived and I put it on there was no clearance between the spinner backing plate and the cowl. Vans solution to this (and they didn't seem surprised that it happened) was to have me fill in the recess at the nose of the cowl with epoxy and flox then sand the front of the cowl back 1/8" to get the required clearance between cowl and prop. Before, my spinner was a nice extension of the lines of the cowl, now I have a step. The other alternative was to remove the hinges from the cowl and move it back along the fuselage a little bit but that seemed like a nightmare too. All would have been right if I had just waited for the prop.

[jthocker]

I just finished fitting the cowl to one of the 8's I'm building and thought I'd pass on this tip.

I have the prop (Hartz. BA) and engine (O-360A1A), and I want to fit them to both airplanes. In the interest of speed I wanted to try the spacer method of prop simulation. In the past I've always hung the prop and did the cowl fit with the prop in place. I figured the spacer method should work.

So I went to the local hardware store and bought some 1/2" fine thread bolts, 3 inches long and brought them home.
I then cut some spacers 2 1/4" long from some 3/4" Sched. 40 pvc pipe on a power miter saw so that they would be square cut.

Ready to mount the bulkhead, oops the holes in the bulkhead are 5/8's" bolts are 1/2". Back to the hardware store to look for some bushings.
Ah! Victory! For once the hardware store has not only a 5/8's X 1/2" bushing, but 6 of them!
So here's how it worked. Put 2 washers on each bolt, then a bushing, tape the bushing in place against the washers. Put the bulkhead on the bolt/bushing combo's, put spacer's over bolts.
Put assembly on crankshaft over starter ring gear. Tighten up bolts, making sure bushings haven't slipped down on the bolts.
It sure looked straight and square to me!

Next up, top cowl positioning. I used a piece of 3/16' plywood that was laying around from a kitchen project and positioned it up against the aft flange of the spinner bulkhead. Then I positioned the top cowl tight up against the plywood spacer, marked the cowl from the reference lines and trimmed it. Gee I hope this spacer thing works.
Fast fwd. a month. Cowl finished, baffles finished, carb air box finished, time to do the spinner. I figured I could do that on the bench, but the twisting of the prop blades to check clearances would be difficult. So lets hang that prop and see if that spacer method worked.

Put the bulkhead assembly on the rear of the prop, hang the prop, say a prayer and then put the cowl on.

Guess what! That 3/16's plywood just slips between the cowl and the bulkhead. Ah! Victory again!

The victory is short lived though, as I will now be removing the engine from the first plane and installing it on # 2, starting the whole process over. I will definitely use the spacer method from now on.

Here's some pix of the spacer method.

[decathlon737]

Thanks for the reply, just for a follow up I did the same 2 1/4 spacer deal and when my prop arrived, fit like a glove with 3/16 space. I was worried but pretty much knew I could do most anything with the fiberglass cowl to compensate for bad fit. Glad it worked out for you too.