spinner rubs on cowl

Hi folks,

Looking for something obvious that I'm probably overlooking. I did my first trial fitting of the prop hub last night and it's rubbing up against the cowls. There's also about a 1/4" vertical mis-alignment. I hung my engine first, then fit the cowl after letting it "settle" for about a month. I searched through the forum last night and saw that there's a little bit of room to trim the aft end of the backing plate, but since I have two misalignments I suspect that I've done something wrong.

thanks in advance,
-dbh

Wondering - -

been a long time, but did you trim the rear of the cowls to the scribe lines ? We early builders did not have an engine to install, so we cut exactly to the scribe lines, then fit it as required. Not a perfect fit, but have enough clearance for sure. Could you clearly see you were cutting exactly to the scribe lines ?

Another thought - -

take 2 pictures from the same angle, but the rear of the cowls. If you see any scribe lines, that is the answer.

It sounds as though you fitted the cowls without the backplate in place. With the spinner backplate in position I found I had to do quite a bit of trimming to the rear of the cowls to get them to fit properly. If you just cut to the trim lines marked on the cowl, that may be the problem. You may have to remove the hinges from the rear of the top and bottom cowls, fill in the rivet holes, re-trim the cowls and re-fit the hinges. Not a pleasant job. Hopefully someone may come up with a simpler explanation. A few more photos may be helpful.

David, I had to trim a past the scribe lines also to get a proper fit. Unfortunateley, you will have to remove the hinges and try again. I don't see another way to correct it.

A cowl installation that mimics the design model will have a spinner to cowl clearance of 1/8"

I have seen as little as 1/16" work just fine with no spinner rub issues.

Nominally, a cowl fitted with the initial trim made to the scribe lines, drilled to the hinges while the cowl is butted against the fuselage, and then the cowl edges sanded to provide the needed gap from the fuselage skins will result in a gap of about 1/8" from the back of the spinner. I say about, because there are other things that can influence the actual fit. If during the fitting/drilling process, force has to be applied to the cowl to get the two halves to align at any location, a distortion of the cowl is likely occurring elsewhere (I.E., the front can be bulging forward, etc). This situation will also result in a cowl that will have a high probability of broken hinge sections or failed rivets in the future.
Strive for a fit that results in the two cowl halves not requiring them to be forced into position for them to align properly.

Builders that have installed their cowl with the engine already mounted, and have purposely made a gap bigger than 1/8", have contributed to reducing the clearance between the oil cooler and muffler, and possibly having an influence on the duct interface to the radiator.

The nutplates on the rear spinner bulkhead are specified to be located on the fwd portion of the flange so that the aft edge can be shortened if spinner interference occurs. I think there is also a spacer available that can be used to position the propeller/spinner an additional 1/16" fwd if this is not enough.

As already mentioned, the only way to resolve the vertical alignment would be to get some more hinge and try drilling it on again (this would also allow for adjusting the longitudinal interference).

two silver linings

thanks all, for the input. I was hoping for a "duh!" type of oversight, but it appears that I simply need to drill off the hinges, re-fit the cowls and redo the hinges. I've dealt with worse, so this isn't a big deal overall.

The silver lining? I apparently followed the long standing advice of cutting too long rather than too short . The price of a new set of cowls & duct is almost 1 Aviation Money Unit ($1000)!

The other silver lining? The metallic Heat Shield of course!

Shipping would probably be a killer, but I have a new never touched set of cowls surplus for far less than $1000..

dbhill916 said:

thanks all, for the input. I was hoping for a "duh!" type of oversight, but it appears that I simply need to drill off the hinges, re-fit the cowls and redo the hinges. I've dealt with worse, so this isn't a big deal overall.

The silver lining? I apparently followed the long standing advice of cutting too long rather than too short . The price of a new set of cowls & duct is almost 1 Aviation Money Unit ($1000)!

The other silver lining? The metallic Heat Shield of course!

Click to expand...

Fabricated spinner spacer

I meant to post this when I saw your picture. I fabricated/fitted the cowls before ordering the engine and when I later installed my propeller I had very little space between the spinner and cowl. (If I was building another -12 I would wait until after mounting the engine.)

http://www.mykitlog.com/users/displ...5&project=1975&category=8853&log=175046&row=6

After exchanging emails with Van's, I fabricated a spacer which gave me a larger gap which is much closer to the recommended clearance. Needed to use longer AN bolts as well. Your fit looks tighter than what I was facing so this may not be an option, but worth considering before you re-position all those cowl hinges.

http://www.mykitlog.com/users/displ...5&project=1975&category=8853&log=175040&row=3

Nice job on the spacer. How did you fabricate it?

This was an issue my TC picked up on last month. In my case, I used a pair of washers under the upper mount, but the long tem solution is to get a slightly longer spacer from Saber Manufacturing and remove the washers.

Spacer fabrication

rgmwa said:

Nice job on the spacer. How did you fabricate it?

Click to expand...

Thanks. If you look through the other entries in my KitLog you'll see some fairly obvious pictures. I ordered aluminum from Aircraft spruce and clamped to the propeller plate on the aircraft. Marked locations for all the holes and used a drill press to punch them out. Used jigsaw to cut out outer edge. Some filing, priming and all done.

PilotBrent said:

Thanks. If you look through the other entries in my KitLog you'll see some fairly obvious pictures. I ordered aluminum from Aircraft spruce and clamped to the propeller plate on the aircraft. Marked locations for all the holes and used a drill press to punch them out. Used jigsaw to cut out outer edge. Some filing, priming and all done.

Click to expand...

Ah, so you used the traditional homebuilder's technique requiring patience, skill and craftsmanship. I thought it looked so nice it must have been laser cut or some such pay-someone-else-to-do it method.