Engine/Prop "Sag"

Deems Davis · #1 02-03-2007, 11:41 AM

For those who have finished your plane or previous planes. I'm interested in your real life experience regarding whether you've experienced or noticed any sag in your engine since initially mounting it. I've heard that after a hundred or two hundred hours that the engine will settle on the mounts which results in the prop dropping ( 1/8", 3/16", 1/4" , ?????). Is this a real phenomena or an urban airport myth? If its real, how much 'sag' have you experienced? I'm attempting to install my cowl and I'm leery of adding a 'fudge factor' not knowing exactly what it should be, and 1/8" tolerance isn't what I'd like to achieve. Am I striving for the holy grail? It seems like there ought to be a better way to ensure things fit after flying for a while I'm in the process of installing my cowl and would like to have it and the spinner mate evenly when it's all said and done.

Thanks for any information received

RV6_flyer · #2 02-03-2007, 01:42 PM

My RV-6 has been flying since September 1997. I replaced the CHEAP conical rubber engine mounts at around 18 months and 307 hours with new CHEAP conical rubber engine mounts after my airplane was hit by a car and engine tore down for inspection along with installing a new firewall and engine mount.

There is now 1,977 hobbs hour on the airplane and cowl. Subtract 307 from that number for what there are on the CHEAP RUBBER CONICAL engine mounts.

There is NO noticable SAG on my prop spinner to cowl. There may be some sag but if there is it is very small. (IMHO, less than 1/8" but have NO hard measurements of before and after.)

I have one of the early polyester resin cowls.

JoeBlank · #3 02-03-2007, 03:05 PM

I think with the dynafocal mounts you can expect about 1/8" or so of sag after about 100-200 hours. It's not real noticable to the eye, but if you look for it... it's there. It is more of an esthetic thing though. If it were me, I'd plan on compensating for it. Although you'll probably never reach the 'perfection' stage with this aspect.

Joe Blank
RV-6
N6810B

db1yg · #4 02-03-2007, 03:25 PM

When fitting my cowl I intentionally made the spinner 3/16th of an inch higher than the top of the cowl. After about 15 hrs they were even and at 105 hrs they are still even. BTW, the mount is Dynafocal.

Cheers,
db
RV9a/ECi0360/James Cowl/Catto prop--105 hrs and a permanent smile!!

szicree · #5 02-03-2007, 03:30 PM

No personal experience here, but how bout a washer or two between lower engine mount and firewall to fix sag if it occurs?

Mel · #6 02-03-2007, 04:16 PM

The washer trick will work fine. Dynafocal mounts will sag from 1/8" to 1/4" from new depending on the mounts. Lord mounts seem to sag less. Once the mounts "settle in" they stop sagging. A few people have added washers to the top of the mount to start out and removed them after the sag.

fodrv7 · #7 02-03-2007, 04:28 PM

Quote:

Originally Posted by db1yg

When fitting my cowl I intentionally made the spinner 3/16th of an inch higher than the top of the cowl. After about 15 hrs they were even and at 105 hrs they are still even. BTW, the mount is Dynafocal.

Cheers,
db
RV9a/ECi0360/James Cowl/Catto prop--105 hrs and a permanent smile!!

Mine worked out the same.
Pete.

Deems Davis · #8 02-03-2007, 05:18 PM

Wow, This list is great! I also posted the same question on Matronics. While there's some difference of opinion, the majority of the real life responses indicated little or no noticeable sag after several hunderd (1600) hours and many years. The other piece of useful information was that if some sag does occur and you want to take it out. a couple of properly sized washers at the annual condition inspection would take care of the problem. So. Rather than make a guess at what the 'offset should be now and wait for it to 'come-in', I've decided to make it fit flush now, and adjust if and when it gets noticable and annoying.
THANKS again for all who have replied.

RV8N · #9 02-03-2007, 06:26 PM

I had my engine in advance and it was just sitting around so I went ahead and installed it. Turns out it was about 1 year before I got around to doing the cowling. I centered the spinner and it never moved. No sag. I guess that's one big oops that worked out in my favor.

Karl

gmcjetpilot · #10 02-03-2007, 08:25 PM

A quick calculation, looks like 0.040t washer would be plenty to get the 1/8" sag/raise at the front prop spinner. It does not take much. I think the thin washers are 0.032t, thick washers 0.063t, so it might take some combo or grinding to get it just right.

I guess if you know it will drop 1/8" to 3/16" might as well do the spinner high method, that way you will with a little luck, you'll not have to mess with extra washers.