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cowl install question

Adam W

Well Known Member
Thank you in advance to anyone who takes the time to reply here
after much research on the cowl install i dove in. filtering through all the recommended measurements for simulated spacers, the inconsistency between VANs manual and the FAQ on their site i determined I'm going with 2-1/8 spacer and 1/4" spacing between cowl and flange on spinner backplate.
Today i got to it, and with the ring gear on, the spacers in and backplate mounted the cowl just makes it to 1/4" space to spinner flange. there is literally nothing to cut off the aft side. This caused me to be concerned I have already gone wrong?
has anyone found the latest cowls require no aft side trimming. im only referring to the top cowl so far I have not fit the bottom yet
im using the Hartzel 2 blade BA constant Speed.
 
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Cowl

Can't speak to the latest cowls but I had about 2" to cut off the back of my cowls (2008 kit) with the standard extension. I would suggest no less than 3/8" clearance from the back of the spinner to the cowl. Any less and it becomes difficult to re and re the cowl. One of the guys who has built 8 or 9 RV's leaves 1/2" for this reason. I split the difference between Vans 1/4" and his 1/2" and did 3/8" which worked out ok. I had a 4 with a 3/16" gap and it was a pain getting the bottom cowl on and off. FWIW.
Al
 
Adam,

I don't think you need/want a spacer with a Hartzell CS prop.

Which engine do you have?

Confirm you have 7 not 7A?

I have RV7A with 0-360 and Hartzell CS prop, no prop spacer. I cut a couple inches off the cowl for a 1/4" space between spinner and cowl. I don't have much trouble getting cowls on off by myself.

Bevan
 
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Adam, I had more to cut off that what you are talking about. I have a 9a with Catto and spacer, but O-320. Something doesn't sound correct. Can you post a picture or a link to one?

One thing to think about is on the horizontal cut, favor cutting more off the bottom that the top. You will have an easier time getting the cowl off. If you are a TD, then not as much problem, but still worth considering.
 
Adam,

I don't think you are alone in your experience. I set up my cowl with 2-1/8" temporary spacers between the starter ring and the spinner backplate, and it worked out just right for a 1/4" gap once everything was installed. I did not have a lot of excess to cut off. Looking at my construction photos before trimming the top cowl, it did not cover all the rivet holes around the firewall, so I'm guessing I trimmed less than 1/4" off the aft side of the top cowl in some areas. Just make sure with starter ring is seated properly, fit the spacers and spinner backplate, remeasure the prop to verify the spacing is correct, and go for it!
 
Just make sure with starter ring is seated properly, fit the spacers and spinner backplate, remeasure the prop to verify the spacing is correct, and go for it!

And make sure the spinner bulkhead is flat on its face (check with a straight edge).

The forming process often leaves a dished shape in the center of the bulkhead which can position the perimeter flange further fwd than nominal.

The RV-14 spinner installation plans (AVAILABLE HERE) are a bit more comprehensive than for the other other kits. Page 44-02 describes checking the aft bulkhead for flatness and gives a nominal dimension between the ring gear and the aft edge of the rear bulkhead flange, that you can use for reference.
 
thank you this has been reassuring and helpful

can anyone comment on the amount they lowered the top cowl to compensate for engine sag?
 
drilling top hinges

Somewhat related, when drilling the top hinges to the cowl, is it best to hold the ?slack? out of the hinge due to the .090 pin?s sloppy fit, or let it ride in the ?middle? of the hinge eyes where it wants to?

Seems like the cowl would end up loose if you don?t hold the hinge separated, but that might make it hard to put on and remove.
 
thank you this has been reassuring and helpful

can anyone comment on the amount they lowered the top cowl to compensate for engine sag?

It is really difficult to determine how much, if any, sag you will get. Some don't sag at all. Some drop a bit.
In the day, folks felt it was a sense of pride to have as little gap as possible. Over time, vanity lost out over ease of use and folks started accepting a larger gap. The larger the gap, the less noticeable the sag will be, if it does sag. It is also much easier to put on and remove.
Others can chime in, but 1/4" gap would be an absolute minimum. I would accept more. I gapped to 1/8" on the top and 5/16" on the bottom hoping for the sag to even out to 1/4" gap. My engine never sagged. I still have the same gap, which frankly isn't noticeable unless you go looking.

As far as top alignment, I think it is important that the transition from the top of the cowl to the spinner makes a nice profile but it is also needs to be centered to the cowl. There is a bit of compromise here and the "gap" plays into how that looks. Sag doesn't come into play as much in regard to the profile unless it really sags a bunch.

I hope others can add to this.
 
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Can't speak to the latest cowls but I had about 2" to cut off the back of my cowls (2008 kit) with the standard extension. I would suggest no less than 3/8" clearance from the back of the spinner to the cowl. Any less and it becomes difficult to re and re the cowl. One of the guys who has built 8 or 9 RV's leaves 1/2" for this reason. I split the difference between Vans 1/4" and his 1/2" and did 3/8" which worked out ok. I had a 4 with a 3/16" gap and it was a pain getting the bottom cowl on and off. FWIW.
Al

20 some years ago, I built my RV-6 with 3/8" space between back of spinner and front of cowl. IMHO, that is the minimum space needed to get my cowl on and off.
 
Somewhat related, when drilling the top hinges to the cowl, is it best to hold the ?slack? out of the hinge due to the .090 pin?s sloppy fit, or let it ride in the ?middle? of the hinge eyes where it wants to?

Seems like the cowl would end up loose if you don?t hold the hinge separated, but that might make it hard to put on and remove.

Another option, use 1/8? SS pins for the top cowl. You need to tapper the last 8? or so to .090? or so to make the curve. I did this and it eliminated the top of the cowl riding up on the hinge slack that you get by using the 0.090? pins.

The hard part is doing the tapper. I chucked up a lenght of 1/8? pin in the drill press and had it run on slow speed. I used a die grinder with a rotary file to make the tapper. Cleaned it up with some sandpaper then trimmed the length. The SS is tough - this took some time.

Carl
 
Another option, use 1/8? SS pins for the top cowl. You need to tapper the last 8? or so to .090? or so to make the curve. I did this and it eliminated the top of the cowl riding up on the hinge slack that you get by using the 0.090? pins.

The hard part is doing the tapper. I chucked up a lenght of 1/8? pin in the drill press and had it run on slow speed. I used a die grinder with a rotary file to make the tapper. Cleaned it up with some sandpaper then trimmed the length. The SS is tough - this took some time.

Carl

My neighbor did this as well Carl. He used a disk grinder and it went pretty quickly. Chucked up the rod in a drill as well. Good suggestion.
I just let mine ride up. Doesn't bother me a bit, but this is an easy "fix" if it does.
 
20 some years ago, I built my RV-6 with 3/8" space between back of spinner and front of cowl. IMHO, that is the minimum space needed to get my cowl on and off.

I wish I had 3/8". Fortunately, the 5/16" I have is on the bottom.
It is a bit tricky but obviously doable. I like your recommendation. 1/4" is really tight.
 
I’m having trouble visualizing how the cowl is ultimately supported in the front. Do the baffles end up supporting the weight of the cowl, with the hinges mainly holding position? Or, should I fit the hinges so that they support 100% of the cowl weight,
prior to installing baffles?

Should I remove my front supports only after all hinges are fitted?

thanks, I’m feeling dense,
dave
 
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I?m having trouble visualizing how the cowl is ultimately supported in the front. Do the baffles end up supporting the weight of the cowl, with the hinges mainly holding position? Or, should I fit the hinges so that they support 100% of the cowl weight,
prior to installing baffles?

Should I remove my front supports only after all hinges are fitted?

thanks, I?m feeling dense,
dave

The cowling is a self supporting structure. Between the hinges which hold it to the firewall, the hinges along the sides of the cowl, and the screws on the inside of the spinner, the cowl is supported all around and quite rigid. Neither the baffles or engine are part of the support structure for the cowl.
 
And

...nor should the cowl be attached to the engine. One look at how much that engine moves will illustrate why...
 
Thanks, makes sense.

Why I ask is that after I drill the top hinge, and remove the front supports, the front sags below where I want it on the spinner. This is prior to trimming the bottom cowl. Should I leave the front cowl supports in until after the cowl bottom and sides are trimmed and drilled?

thanks
 
Thanks, makes sense.

Why I ask is that after I drill the top hinge, and remove the front supports, the front sags below where I want it on the spinner. This is prior to trimming the bottom cowl. Should I leave the front cowl supports in until after the cowl bottom and sides are trimmed and drilled?

thanks

Definitely keep the front in line with where you want it for the lower cowl fitting process. With just the aft hinge holding it, the sides will bulge out under it's own weight. Just make sure nothing is forced into place when final fitting. Taking the whole thing off and on a few dozen times is normal, at least it was for me.
 
Sag

I made a plywood disk the size of the spinner and mounted it. I then set the proper position of the cowl and using spacers tans screw Fromm the cowl to the disk. This screw held the cowl in position while mounting work was completed.
 
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