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Cowl - Spinner Alignment

L

lr172

Well Known Member
I just hung the O-320 on the RV-6A yesterday and am now fitting the cowl. I am wondering if folks make a perfect alignment between the cowl and the spinner at this point or do they leave the spinner up a bit to allow for the engine mounts to compress and drop the front of the engine a bit. I used Lord Mounts from Vans. I have no experience to determine whether or not the mounts will compress over time.

I appreciate any guidance that can be provided here.

Thanks,

Larry
 
I set my spinner higher by 3/16 of an inch. After flying for 5 years, it's just barely below "level" with the cowl...maybe 1/8 inch. I think it's smart to set the spinner higher...but by how much is the question.

BTW, I used the standard mounts that came with the FWF kit.
 
worried

I set mine 3/16 high and now I'm kinda worried because I'm reading that some didn't sag much. May be better to set it perfect and shim the engine up if it sags. But, almost all the RV's I saw at Airventure was sagging some.
 
I believe it depends on engine/prop/acro combo. I built my cowling 1/8 inch high to my spinner, after 65+hours it seems to have dropped 1/8 inch with The Lord mounts/o-360/Catto prop/acro.
 
I lined mine up exactly, and after 22 months/310 hours, it doesn't appear to have moved *at all*. Lots of acro/rat racing, with up to +4.5G on a regular basis.

I think one difference might be that the engine hung on the mount for almost two years before I fit the cowl. I wonder if any sag that would have happened took place over that time.
 
ChiefPilot said:
I think one difference might be that the engine hung on the mount for almost two years before I fit the cowl. I wonder if any sag that would have happened took place over that time.
Click to expand...

I am guessing that the compression is a matter of time and weight. I suspect vibration can accelerate the compression, but time and weight are the key factors. This may explain why some don't report a dropping nose; It happened before mounting the cowl

Larry
 
On the advice of others, I set mine 1/4" high to allow for sag. That was 4 yrs ago, and it's still nearly as high as the day I set it. I used the standard Lord mounts from Vans ... have had almost no sag.
One variable I never considered 4 yrs ago is the type of prop. I think my no-sag installation is partially due to the negligible weight of my Catto prop. I suspect a metal Sensenich would have sagged more.
 
Rv7 O-360 with Catto prop with Lord mounts, I built the cowl to spinner interface 1/8 inch high. After 65ish hours and a decent bit of aero it has moved perfectly inline ( I noticed a little movement after the first several engine runs and the rest after the G testing). Perhaps it will move more after some more time, it's going to have to take a lot of looking to find 1/8 inch difference.....it's going to take more than an 1/8 for me to shim anything.
 
I'm curious about something. I always understood that the engine "lifts" a bit in reaction to the thrust while in flight.

The purpose of the spinner/cowl alignment is to optimize airflow. Not much airflow while it's parked on the ground. Leaving the question open as to where you "set" the engine.

FWIW

mjb
 
Cowl lifts

I was under the impression that the cowl lifted a little during flight, haven't heard about the engine lifting.
 
I've built a few and what I do is set the spinner in line with the top of the cowl
That way it looks good to start and if it sags a bit it will center on the cowling front.
Most sagging I've seen actually sags toward the seven o'clock position, not the six, but ymmv.
You could also put a fender washer behind the top motor mounts at the firewall while building. If it sags just take out the washers. (The change in thrust line from these changes is insignificant, per VAN's.)
If you have sag to take out, add washers behind the bottom motor mounts at the firewall, to raise the spinner.
Just make sure you use washers at least the same diameter as the mount base and the plane won't know the difference.
You could also add washers at The Lord mounts, but I prefer dong it at the firewall location. Much easier.
Good luck
 
Wow!

Wow!, Thanks Steve! I may do that to make my spinner a shade lower now and take them out when it sags.
 
How thick?

sf3543 said:
I've built a few and what I do is set the spinner in line with the top of the cowl
That way it looks good to start and if it sags a bit it will center on the cowling front.
Most sagging I've seen actually sags toward the seven o'clock position, not the six, but ymmv.
You could also put a fender washer behind the top motor mounts at the firewall while building. If it sags just take out the washers. (The change in thrust line from these changes is insignificant, per VAN's.)
If you have sag to take out, add washers behind the bottom motor mounts at the firewall, to raise the spinner.
Just make sure you use washers at least the same diameter as the mount base and the plane won't know the difference.
You could also add washers at The Lord mounts, but I prefer dong it at the firewall location. Much easier.
Good luck
Click to expand...


I just did a few quick measurements to see the relative effects of both shim locations.

Engine mount shims -

A standard 1/16 thick washer will move the spinner approx. 0.105 inches

Lord mount shim location -

A standard 1/16 thick washer will move the spinner approx. 0.150 inches

This can give you an idea of how thick to make the shims...
 
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