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Cowl to spinner fit due to engine sag?

swisseagle

Well Known Member
Hi

In the process of aligning the cowl to the spinner, engine sag came in mind. so a few questions around this are open for me. Any input helps, thanks!

Basic info: IO-320, Vans LORD mounts, engine hangs since 16 month (without prop), Senenich metall prop.

- Do the engine still sag, also with the new LORD mounts?

- By how much?

- Is it a "cosmetic" problem or is it important that the trust-line gets corrected after a engine sag occured?


I think there are two possibilities to go on:
1. Build on and make a straight fit, wait until engine sag occure, then if neccesairy shim the lower engine mount. This would give a nice fit and a correct trust line.
or
2. Build in a positive engine sag of a certain amount, wait until the engine sag and cosmetic wise all is fine and live with the trust line angle.

Or you tell me, that I do anyway not recognise a trust line change by such a small amount :eek:
 
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in our case:

cosmetic only

initially installed it on the "up" side to possibly avoid inserting the washers later (which was a PITA)
however, once it had set, when we put in the quite thick extra washer, it now sits slightly high. but less than it was too low before. nobody notices except for the builders prying eyes ;-)
next time i would probably install it neutral or even lowish in full anticipation of fitting the washers later.

no change in flying qualities noted.
 
I used Lord mounts and anticipated for a slight sag about 1/8" which proved to be too small. I wound up shimming the lower motor mount after about 100 hours. 1 washer made about 3/16" difference at the spinner. My cowling was completed about two months after hanging the engine. I imagine all mounts will sag a bit after weight, vibration, heat and prop loading do their thing. Just one more data point. I never noticed any difference in performance or handling either way so I believe also it is just cosmetics.
 
I installed straight and was expecting to have to change it after some times. It?s been over 400 hours and it has sagged but not nearly as much as I had expected nor worth putting the shim. It is probably about 1/16 to 1/8 lower than it was from spinner to cowl alignment. You would not know it if don?t look very closely and had a previous reference.
 
Thanks!

So, I put it to cosmetic only and build in a 1/8" and will see what happen with time. Only other builder will notice it ;)

Would be interesting to know what Piper or Cessna do on there planes.

Looks like everyone is getting a bit different results.

I know the safety pin are a real pain to install on the lower mount.
 
Where to shim?

Stupid, I know but, having read lots of threads on this subject, I am still not absolutely clear whether the washers go between the rubber isolation mounts and the engine mount or between the engine mount and the firewall. Can some patient soul clear that one up please?

Surely, the firewall option has to set up stress in the mount?
 
U shaped washers

I did it the hard way by pulling the engine mount bolts, inserting the thick washers and reinserting the bolts and torquing the nuts. On my Superior IO360 it was a 4 hr job due to the lack of clearance between the engine case and the nuts. I hear some motors have a bit more clearance there.
Would have been a bit quicker if I had known the trick of cutting the washers like a U shape and slipping them in without pulling the bolts right out.
 
Cowl high

Somehow my cowl turned out a little high. Either that or the 0-320 engine with Cato 3 blade prop already sagged 3/16 or so since it was mounted six months ago on Lords mounts supplied by Vans.

At only 3/16, I could have left it but any (further) sag will make it worse so I set about leveling it.

I tried some washers similar to the ones that came with the Lord mounts but not quite as thick. However, now the spinner was too high.

Being a Sunday, I had to work with what I had on hands and I found that a scrap of 063 aluminum give me a perfect result.

Question is: can I make permanent shims out of aluminum? Presumably they have to be steel.
 
I installed my cowl five months after the engine was hung. I set the cowl a little over 1/4" low to allow for engine sag. It's now 100 flying hours and 3 yrs later, and my cowl is still 1/4" low. :(
 
Somewhere I read a 1/16" washer will raise the aft end of the spinner 1/8". That measurement seems to be very accurate. Thanks to who ever figured that out, you saved me a ton of guess worth. Took care of 3/8" engine sag (after 300 hours) with 3 each of a 1/16" think x 2" washer this afternoon. Don't forget, you will need longer bolts. AN7-46 is what I ended up with using 3/16" thick of 2" spaces. ;)
 
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Not all engines sag with time. Most of the ones I have dealt with have not. It is easy to slide a washer under the lower mounts if required.
Every one seems to worry about how the spinner and cowling line up on the ground. I am more concerned with how they line up in flight. I want a smooth line, in flight between the cowling and the spinner.
On the ground the engine is hanging from the motor mount. In the air the engine is pulling the airplane, thus the spinner goes UP in flight, I have noted 1/4" on one of my rockets. Thus if your engine does happen to be a bit low you can say that you planned it that way!
 
Cut in the washer acceptable?

Assuming you do want to shim the engine up to match the spinner with the cowl is it acceptable to cut a U in the washer to slip the washer in place without having to pull the bolts? It seems to me that what could slide in could also slide out given heat and vibration.

Don't like the idea of having to pull out the bolts to put in uncut washers but definitely want to be on the safe side.

Jim
RV9a flying
 
Somehow my cowl turned out a little high. Either that or the 0-320 engine with Cato 3 blade prop already sagged 3/16 or so since it was mounted six months ago on Lords mounts supplied by Vans.

At only 3/16, I could have left it but any (further) sag will make it worse so I set about leveling it.

I tried some washers similar to the ones that came with the Lord mounts but not quite as thick. However, now the spinner was too high.

Being a Sunday, I had to work with what I had on hands and I found that a scrap of 063 aluminum give me a perfect result.

Question is: can I make permanent shims out of aluminum? Presumably they have to be steel.

It is not structural or at least not performing any holding force, so aluminum is fine for a shim. At least I would be comfortable with an aluminum shim there.

Larry
 
RV-12 Sag?

Hi all. Does anybody have any experience or thoughts regarding sag with the RV-12, standard mounts from VANS, standard Rotax 912ULS? My issue is that the engine/spinner is high by about 3/16" on the ground. Everything else looks good with a nice 3/16" gap between the spinner and cowling. Personally, I am hoping for a bit of sag but am concerned after reading Tom's comments. Any thoughts would be appreciated.

Kyle
 
Assuming you do want to shim the engine up to match the spinner with the cowl is it acceptable to cut a U in the washer to slip the washer in place without having to pull the bolts? It seems to me that what could slide in could also slide out given heat and vibration.

Don't like the idea of having to pull out the bolts to put in uncut washers but definitely want to be on the safe side.

Jim
RV9a flying

I'm curious about this as well... :confused:

Anybody?
 
Cutting the washer to allow installation without pulling the bolt is a great time saver. But, (IMHO) a really bad idea. There is no way that one could expect the engine not to move reducing the pressure holding the washer. I am sure that there will be replies stating how many hours of inverted flight at 9G's they have with the slotted washers. But, as I said it is only my humble opinion...and not my plane!
It is not that hard of a job to level the plane as needed, and rig your engine hoist to provide the necessary lifting geometry. As has already been stated, chances are you are going to need longer bolts anyway!
 
I fitted the cowl low with anticipation of the engine sagging but I later wanted the cowl and spinner level just because it looks right. After researching on this forum, I made slotted aluminum washers out of various thicknesses and simply loosened the two top engine mount bolts between the engine mount and firewall. Slipped the washer between the mount and firewall and after a couple of tries, found the right thickness. In about an hour, I had my spinner perfect and the nuts torqued back up and cotter pins installed! I have 50 hours on the plane now and when the engine sags a little, I will just replace the slotted washer with a thinner one or remove it. I have checked the washers and they haven't moved at all and I'm confident they will not with all the torque on the bolts.
 
Sag

This seems hard to predict. In anticipation I put my cowl about 1/8 inch low. After many years following the hanging of the engine and about 40 hours of flying it hasn't changed one bit. This was with the mounts obtained from Vans, not the aerobatic versions. I have a friend in Oregon however whose engine sagged dramatically, over a half inch. I wish the sag were readily predictable. Does anybody have a magic formula for this? Surely would be nice!
 
IO360 engine sag from cowl on 7A

I'm interested in knowing if I should set my cowl low in anticipation of engine sag. Plans say so. This is IO360 on a dynafocal mount on 7A. Engine has been on for about 2 months and I'm fitting cowl now.
 
Still may sag

I had mine mounted for 4 months before setting the cowl. Given the time on the mounts I only shifted the cowl half the recommended amount.
It was a mistake. I doubt these mounts will shift much until they are hit with the heat and vibration of operation.
 
Or it may not sag at all...mine didn't (400+ hours). I think it's a coin toss as to whether it will or won't, and if so, how much.

ETA: First, I'm knocking on wood that mine won't now decide to sag since I've said it didn't :)

That said...I think if you search around here, you'll find threads where people have had theirs sag and they "unsagged" it by adding a washer to the stackup on the mounts...not sure, but check around.
 
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I fitted the cowl low with anticipation of the engine sagging but I later wanted the cowl and spinner level just because it looks right. After researching on this forum, I made slotted aluminum washers out of various thicknesses and simply loosened the two top engine mount bolts between the engine mount and firewall. Slipped the washer between the mount and firewall and after a couple of tries, found the right thickness. In about an hour, I had my spinner perfect and the nuts torqued back up and cotter pins installed! I have 50 hours on the plane now and when the engine sags a little, I will just replace the slotted washer with a thinner one or remove it. I have checked the washers and they haven't moved at all and I'm confident they will not with all the torque on the bolts.

Well its been about 120 tack hours since I posted this and I have removed my washers from the top engine mounts as my engine sagged about 3/8" over the last year. Worked out perfect for me. :p
 
The engine on my RV7A with 1300hrs on the clock shows no detectable sag. I used Barry Controls mounts on this one. On my 1991 RV6 I used Lord mounts and had no sag at 1700 hrs when I replaced the engine. I used new Lord mounts with the replacement engine and no sag was evident when I sold the plane 800 hrs later. I fit both cowlings lined up with the spinner during the build before the engine ran the first time. While hanging the engine I was very careful to seat the mounts properly in the dynafocal ring by repeatedly raising and lowering the engine a little with the hoist while torqueing the mounting bolts.

My advise would be to set the cowl perfectly with the spinner or no more than a small amount lower than you could live with in case no sag develops. If you set the cowl lower and sag does develop you will be better of to shim the lower mounts to bring it back to normal alignment. If you set the cowl low to allow for sag, your baffeling and your air intake will no longer be in the right place if the engine sags to the "desired" alignment.

Martin Sutter
Building and flying RV's since 1988
EAA Technical Councelor
 
So there you go.......

A place (Van's) that has built nearly a dozen airplanes in the past 20 years and seen some level of sag on all of them (I know because I installed the cowl on the majority of them and compensated for an expected amount of sag which did occur) and recommends that builders plan for at least about 1/8" after 100-200 hrs....

Some builders that say they had about 1/8".....
Some that say they had 1/2" (likely exaggerated).....
Some that say the had somewhere between those two......
And some that say they never had any sag......

What is a builder to do......:rolleyes:
 
Scott,

The mounts that Van's sell are "Barry" mounts, "equiv" to Lord mounts?

Is there a possibility that the Lord brand don't sag as much?

Bevan
 
Scott,

The mounts that Van's sell are "Barry" mounts, "equiv" to Lord mounts?

Is there a possibility that the Lord brand don't sag as much?

Bevan

Entirely possible that there is a difference between the two but Van's has sold both over the years (depending on cost and availability) and I have seen it occur with both brands.
 
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