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Is 1/8" really sufficient for cowl-spinner gap?

alpinelakespilot2000

Well Known Member
I have my cowl all fitting pretty well. The top cowl may actually fit too well. Once drilled to the firewall, I ended up with a strong 1/8" (somewhere between 4/32" and 5/32") gap between spinner and cowl at the top and probably 1/4" at the bottom. Between a little bit of future sag, and a little bit of filler on the bottom half, I'm assuming this will eventually even out to something around 3/16" all around. However, if I end up with no sag, is 1/8" between cowl and spinner really enough over the long term? Van's says anything 1/8" to 1/4" is OK, but I'm just checking for confirmation from those with experience that being on the smaller side of that range will work out. Definitely don't want the spinner ever catching on the cowl on startup or any other time the engine might shake a little.

Side note: From the archives, I see that many nosewheel folks and those with 3-blade props prefer larger gaps (1/4" to 3/8") for ease in removing and installing cowl. However, I have no nosewheel and only a 2-blade prop, so I'm pretty happy with the smaller gap assuming it's conservative enough already.

Thanks for any tips.
 
Hi Steve

I also wound up with a cowl-spinner gap similar to what you're seeing. After 60 hours now on my 8A there is no evidence that the spinner plate has ever touched the cowl. The engine has sagged a bit but the gap remains about 1/8" all around.

When you run the engine with the cowl off, watch the engine closely on start and shutdown. Most of the "shaking" is rotational with very little up/down or left right movement.

Hope this helps

Paul Danclovic
 
Thrust...

Hi Steve


When you run the engine with the cowl off, watch the engine closely on start and shutdown. Most of the "shaking" is rotational with very little up/down or left right movement.

Hope this helps

Paul Danclovic

.....will help increase the gap in flight as well, as the rubber mounts allow. With a C/S prop, the braking action at closed throttle might decrease the gap a bit, but not 1/8".

Regards,
 
I'm one of the nosewheel/three blade types and I needed about 3/8 of an inch. One thing about rotational movement is making sure the metal portion of the baffle has adequate clearance from the underside. IF it's too close the metal may contact the cowl with bad results.
 
1/8 with nose gear and a 3 blade prop WILL make for a miserable cowl removal experience. 2 blade and TW you are ok.
 
I've got a Catto three blade and 1/8" spacing and do not have any issues with install or removal of the cowling. I think it really depends on how you final trim and fit the cowling.
 
Steve,

I have about a 3/8 to 1/4" gap and it works fine. I also trimmed the lower cowling, per one of the RVator's to help with the removal of the lower cowling.

Which engine mount do you have? The conical mount on my engine sagged much more than I ever thought it would, about 3/8" or so. :(
 
Ross,

Thanks for the heads up on the spacing with a 3-blade prop!

A three blade prop doesn't require more gap from the spinner...it requires a deeper slot (cut further forward) for the nose gear so that the cowl can be lowered down much further to clear the prop blades. I had probably the first RV "A" model with a 3 blade prop back in 1993. I just made an aluminum cover to screw over the slot in the cowl bottom.

BTW a 1/8" gap works fine (in fact an even 1/16" works with no rubbing on most installations) but the closer the gap, the higher the difficulty of removing and installing the cowl. Especially if there is an improper interface between the FAB and the induction air inlet.
 
A three blade prop doesn't require more gap from the spinner...it requires a deeper slot (cut further forward) for the nose gear so that the cowl can be lowered down much further to clear the prop blades. I had probably the first RV "A" model with a 3 blade prop back in 1993. I just made an aluminum cover to screw over the slot in the cowl bottom.

BTW a 1/8" gap works fine (in fact an even 1/16" works with no rubbing on most installations) but the closer the gap, the higher the difficulty of removing and installing the cowl. Especially if there is an improper interface between the FAB and the induction air inlet.


Yep... what Scott said! :D
 
Steve,

I have about a 3/8 to 1/4" gap and it works fine. I also trimmed the lower cowling, per one of the RVator's to help with the removal of the lower cowling.

Which engine mount do you have? The conical mount on my engine sagged much more than I ever thought it would, about 3/8" or so. :(
Thanks for the confirmation guys. I'll stick with the 1/8".

Yes, Bill, I'm trying to keep in mind engine mount style and isolator type. I have a dynafocal mount with good Lord isolators so I'm anticipating very little, if any, sag. However, it's still tough to know whether I should lay up the front of the bottom cowl to decrease the current 1/4" to match the 1/8" current at the top or whether I should leave a little room for a little sag. I suspect I'll split the difference and add a couple layers to the bottom to make that gap about 3/16". The variation from top to bottom would thus be less noticeable.

Thanks again to all.

BTW Bill: What RVator tip are you following re: the trimming of the lower cowl? I have 23 Years of the RVator, so I'll take a look through that before proceeding.
 
BTW Bill: What RVator tip are you following re: the trimming of the lower cowl? I have 23 Years of the RVator, so I'll take a look through that before proceeding.
Steve,

Sometime last year there was a picture of the lower cowling in the RVator where one of Van's Wizards had trimmed the propeller opening in the lower cowling to make it easier to remove and install. It was trimmed in such a way that you couldn't see it when installed. Unfortunately I couldn't find the RVator with that article.
 
Thanks for the confirmation guys. I'll stick with the 1/8".

Yes, Bill, I'm trying to keep in mind engine mount style and isolator type. I have a dynafocal mount with good Lord isolators so I'm anticipating very little, if any, sag. However, it's still tough to know whether I should lay up the front of the bottom cowl to decrease the current 1/4" to match the 1/8" current at the top or whether I should leave a little room for a little sag. I suspect I'll split the difference and add a couple layers to the bottom to make that gap about 3/16". The variation from top to bottom would thus be less noticeable.

Thanks again to all.

BTW Bill: What RVator tip are you following re: the trimming of the lower cowl? I have 23 Years of the RVator, so I'll take a look through that before proceeding.

Steve,

The article is in the 4th issue of 2007. page 15
 
Cool, seems there are a couple of options and/or workarounds. Guess I'll cross that bridge when I get there. ;)

Thanks guys,
 
I have my cowl all fitting pretty well. The top cowl may actually fit too well. Once drilled to the firewall, I ended up with a strong 1/8" (somewhere between 4/32" and 5/32") gap between spinner and cowl at the top and probably 1/4" at the bottom. Between a little bit of future sag, and a little bit of filler on the bottom half, I'm assuming this will eventually even out to something around 3/16" all around. However, if I end up with no sag, is 1/8" between cowl and spinner really enough over the long term? Van's says anything 1/8" to 1/4" is OK, but I'm just checking for confirmation from those with experience that being on the smaller side of that range will work out. Definitely don't want the spinner ever catching on the cowl on startup or any other time the engine might shake a little.

Side note: From the archives, I see that many nosewheel folks and those with 3-blade props prefer larger gaps (1/4" to 3/8") for ease in removing and installing cowl. However, I have no nosewheel and only a 2-blade prop, so I'm pretty happy with the smaller gap assuming it's conservative enough already.

Thanks for any tips.
For me that's way too much! Mine is about 0.025 with the prop pulled forward. Initially it was 0.010, and when the FAA inspector asked me if there would be rubbing when I pulled power I told him that it wouldn't be for very long as the spinner would abrade into the cowling. You have air that flows forward around the crankshaft, unless you have a seal there, and it will be centrifugally pumped out that gap perpendicular to the airflow, creating turbulence in the air flowing past the spinner and onto the cowling. One way to correct that is to first of all decrease the gap, then round the edge of the cowling all around behind the spinner with at least a 1/4" radius. That will form a Coanda surface which will cause the flow through the gap to turn and follow the flow over the cowling. In your case you have a sufficient gap which you can close with Bondo or micro-balloons and form the rounded edge.
 
I've done extensive tuft, oil and manometer measurements on my RV and air flows from the outside of my 3/8 spinner gap to the inside of the cowling. That is how we cool our PSRU and my OAT thermistor is attached to the prop brush holder- it reads accurately in flight.

Spinner gaps create drag so smaller is better in that respect.
 
I don't think 1/8 is enough. When you fly air loads will push the lower cowl up and when yo pull a few g's the engine will sag a bit. I have about 3/16 on my RV3, acro mounts, 0320, light wood prop, and after a couple of hundred hours the spinner started to wear agroove in the lower cowl.
Your milage may vary
tm
 
I don't think 1/8 is enough. When you fly air loads will push the lower cowl up and when yo pull a few g's the engine will sag a bit. I have about 3/16 on my RV3, acro mounts, 0320, light wood prop, and after a couple of hundred hours the spinner started to wear agroove in the lower cowl.
Your milage may vary
tm

Your milage may vary

Your right...because there are so many variables.
That's why a general statement such as this is not always valid.
Your RV-3 probably has a polyester/white gelcoat cowl, and probably a conical mount engine? The polyester cowls are much more flexible than the epoxy prepreg ones that are used now, and conical mounts are much more subseptable to sag under G load compared to dynafocal. Even then I am surprised you had contact with 3/16".
Do you pull 6 G's regularly?
 
Your milage may vary

Your right...because there are so many variables.
That's why a general statement such as this is not always valid.
Your RV-3 probably has a polyester/white gelcoat cowl, and probably a conical mount engine? The polyester cowls are much more flexible than the epoxy prepreg ones that are used now, and conical mounts are much more subseptable to sag under G load compared to dynafocal. Even then I am surprised you had contact with 3/16".
Do you pull 6 G's regularly?

It is a polyester cowl 1996 vintage, dynafocal mount, really don't pull many g's at all. Like i said YMMV. Scott, why did you have to rebuild your RV6?
cheers :)
tin man
 
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I don't think 1/8 is enough. When you fly air loads will push the lower cowl up and when yo pull a few g's the engine will sag a bit. I have about 3/16 on my RV3, acro mounts, 0320, light wood prop, and after a couple of hundred hours the spinner started to wear agroove in the lower cowl.
Your milage may vary
tm
Actually, if this is true, then 1/8" at the top with 3/16" at the bottom would seem to work fine. If bottom cowl gets pushed up by air loads, the top cowl gets pushed back, away from the spinner. Likewise, if the engine sags, the 1/8" at the top will increase while the 3/16" at the bottom will decrease.

The numbers above are where I'm at right now. Van's says anywhere 1/8" - 3/8" is fine, so I should be OK where I'm at--especially if I get a little sag. If anyone has any other thoughts with this, I'm interested in hearing them.
 
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Can someone post the picture?

Steve,

The article is in the 4th issue of 2007. page 15

Unfortunately RVator 2007 and later issues are not included in 27 Years of the RVator and this link to past issues on Van's website is currently broken: https://shop.vansaircraft.com/public/RVator.htm

Does anyone have the PDF file of that issue or can post the picture of how to cut the lower cowl to make it easier to remove?

Finn
 
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As requested from this other thread...

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RV-4

On the RV-4 it looks like I may be able to cut a larger opening for the gear sockets/legs, pull the rear side hinges out to clear the fuselage sides, slide the cowl back and then down and out. So, between the diagonal cut at the front and bigger gear leg openings at the rear, it looks like the lower cowl can be fit pretty tight to the spinner too.

The top cowl should lift straight up after bending the lower rear part of the cowl out to the sides (clearing lower sections of firewall hinges). Of course this requires that hinges are offset enough that they will clear the lower cowling side and top forward skin (Section P-P on Drawing #43).

Finn
 
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