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Engine sag question

cjensen

cjensen

Well Known Member
My engine has been hanging for almost a year now, and I started the cowl work tonight...

I've heard that the engine sags on the mounts the most in the first year. So my question would be...do I allow for any more significant sag when fitting up the cowl, or is the year's worth of sag about as bad as it will get for several years?

:confused:
 
cjensen said:
My engine has been hanging for almost a year now, and I started the cowl work tonight...

I've heard that the engine sags on the mounts the most in the first year. So my question would be...do I allow for any more significant sag when fitting up the cowl, or is the year's worth of sag about as bad as it will get for several years?

:confused:
Click to expand...

IMHO, I would not allow much.
I did & did not see the sag as others have mentioned.
Maybe others can chime in, but it's should not be too much. 1/4"?
 
My engine hung on the mount for four years. It's now been flying for nearly a year. The spinner is almost level to the top of the cowl for sag. So far, it's still where I mounted it..............at least as well as the eye can tell.

L.Adamson --- RV6A
 
DanLandry said:
IMHO, I would not allow much.
I did & did not see the sag as others have mentioned.
Maybe others can chime in, but it's should not be too much. 1/4"?
Click to expand...

I usually plan for (and see) about an 1/8" sag, but that is on cowl installations that are done right after the engine is mounted. I have never seen a change occur with it just sitting static. I think the engine running and the airplane flying is what makes it occur during the first 100 hours or so.
 
No sag yet

At 74 hours I have yet to notice any appreciable sag in the installation. Needless to say, I am really not expecting any to show up at this point. That being said, I do no aerobatics (9A), so I would expect that aircraft exposed to more stress on the mounts may be more likely to experience a little sag. Best to plan for a little in your installation, as it would be much more difficult to allow for clearance later.

Just a thought,
Chris
 
I have 350 hours on my -8. I initially installed mine with the spinner about 1/4- 1/2" above the cowl. Why so aggressive?.... I used to fly the AF T-6A. When we picked them up at the factory the spinner was even or just slightly above the cowl, after 1000 hrs, the spinners were about a solid inch or more below! I fly a lot of aerobatics in mine, I started 1/4-1/2" above and now I'm at least a 1/4" below!

Jerry
RV-8 N84JE IO-360

350 hrs 2 1/2 years




cjensen said:
My engine has been hanging for almost a year now, and I started the cowl work tonight...

I've heard that the engine sags on the mounts the most in the first year. So my question would be...do I allow for any more significant sag when fitting up the cowl, or is the year's worth of sag about as bad as it will get for several years?

:confused:
Click to expand...
 
Engine sag, don't yell at me but.

Hi gang:

One thing I've always wondered about in regard to mounting the cowling to allow for sag, is this not just cosmetic for us looking at it on the ground?

What I mean is won't the engine pull itself forward on its mounts when its under power and center itself in it's dynafocal mount? Therefore would you not want to mount your cowl for this and not worry about what it looks like on the ground?

If this is true, I believe the best approach would be to hang the engine with a fresh set of mounts and mount the cowl as soon as possible before they have a chance to sag down.

Like I said, just something I've wondered about so don't yell at me.


tailwinds, terry
 
Alignment on the ground vs. in flight?

So here's a question for y'all. I assume that the amount of sag that everyone is describing is as observed on the ground, with the engine off. I wonder then, with the engine on, in flight, if the engine assumes a slightly different position? After all, the rubber mounts still have a lot of give, and in the first scenario we only have the force of gravity, whereas in the second scenario we have additional forces created by the engine/prop (forward thrust, as well as various turning effects).

If there is a significant difference in the engine's position, then it seems to me that one would want to set up the cowl so it lines up with the spinner as best as possible under normal cruise conditions, to minimize drag. This may not necessarily be the same position that makes it line up to look pretty on the ramp with the engine off.

I have no experience with this (yet). Just thinking out loud.
 
With 927 Hours, 3.75 years of flying.....if I have any sag, it sure hasn't been noticeable to me! Lots of Acro, lots of flying with a couple of "G's" on the airplane, and no one has ever come and say "boy, that engine is sagging". I think folks worry about this one too much - but then, I have never intended to enter my airplane in the judging at Oshkosh either...;)
 
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Thanks all...I'm just gonna set the cowl on the spinner and call it good then...not gonna worry about this one anymore...

:)
 
I have an O360 with CS prop, 6 years and 500 hours. Fortunately, I had forgotten about engine sag when I did my cowl. Everything is still in perfect alignment.
 
I know nothing about mounting engines or resulting sag...

But it might be worth checking/retorquing the mounts in case they settled?
 
I agree with Scott McDaniels about engine sag v.s. time mounted before cowling is done. On my RV-6, my engine was hanging for quite a while before I did the cowling. On my RV-8, I went right from engine to cowling. I had no sag on my -6 (400 hours), and 1/4" or more at the prop on my -8 after 80 hours. I put spacers under the airbox when building, per plans, to plan for some sag, but it wasn't enough (the washers are removed after cowl fitting). To correct the sag I put large steel washers under the bottom mounts (on the firewall) and it brought it right back into alignment. I loosened the top bolts first, and then re-torqued everything back to specs together.
As far as the difference in engine sag in flight v.s. on the ground, I don't understand why there would be a difference. You still experience 1 G most of the time. I don't think that after the engine sags, it is moving up and down after you take off. My noticeable engine sag - noticeable behind the spinner on top - took about 60 -70 flight hours to develop. I think everything just sort of settles after some stresses are put on the airframe/engine and assumes a stabilized state. I don't know if my thinking is correct and would be interested in knowing from the engineer types in the forum, what happens in flight that would affect engine placement in 1 G unaccelerated flight.

Scott Hersha
RV-8 FB
 
Scott Hersha said:
....... I think everything just sort of settles after some stresses are put on the airframe/engine and assumes a stabilized state......Scott Hersha RV-8 FB
Click to expand...
My -6A fitted with the kit supplied engine mounts developed a slight but noticeable sag over time. Note the spinner to cowl gap has remained consistent.

2z9mh4j.jpg


For that reason, to account for future (potential) sag, I fitted the cowl on the -8 so the spinner is more or less flush with the upper cowl surface.
 
Scott Hersha said:
To correct the sag I put large steel washers under the bottom mounts (on the firewall) and it brought it right back into alignment. I loosened the top bolts first, and then re-torqued everything back to specs together.
Click to expand...

Scott,
What was the approximate thickness of the washers that you used to take up the 1/4" droop at the spinner?
Thanks!
 
jetjok said:
Scott,
What was the approximate thickness of the washers that you used to take up the 1/4" droop at the spinner?
Thanks!
Click to expand...

I can't speak for (the other) Scott, but it is a basic ratio of geometry. The distance between the top and bottom lord mounts is approx. 1/3 the distance from the lord mounts to the back of the spinner. Since a line between the spinner and the top lord mounts, and then top lord mounts to the bottom lord mounts (approximately) forms a right triangle, if you wanted to raise the spinner 3/16", a 1/16" spacer between the bottom lord mounts and engine would get you pretty close.
 
Rick6a said:
My -6A fitted with the kit supplied engine mounts developed a slight but noticeable sag over time. Note the spinner to cowl gap has remained consistent.

2z9mh4j.jpg


For that reason, to account for future (potential) sag, I fitted the cowl on the -8 so the spinner is more or less flush with the upper cowl surface.
Click to expand...

Looks like the sag on Rick's airplane was about 1/8"...That is what I typically see.
 
I fit my cowl soon after mounting the engine and aligned the cowl 3/16th" below the spinner in anticipation of sag. After 25 hrs of flying the two line up. However, my brother did the same and his has not shown any sag in over 100 hrs. What makes this even more interesting is that he has a relatively heavy BA Hartzell and I have a 17 lb Catto 3 blade--both on parrallel valve 360s. Go figure!!!

Cheers,

db
 
I have read numerous posts over the years on VansAirforce from builders reporting on the amount of their engine sag (or lack thereof). In my opinion those reports are quite useless in terms of providing useable information to others unless the builders specify the type of engine mounts they installed.

For example, the Lord mounts so favoured by many RV builders are of completely different construction to the standard mounts provided by Vans in their kit. Other builders opt for dedicated aerobatic mounts.

Engine sag is primarily a function of permanent deformation of the mounts with load over time and it is logical that not all brands and styles of mounts will deform at the same rate.

Congratulations to Rick Galati who was on the ball in post #16 when he reported that he had "noticeable sag" and was using the "standard Vans mounts" (and included a couple of excellent photos).

Perhaps if all builders who report on engine sag matters could follow Rick's lead and include details on the mounts used we could start to interpret some meaninful trends. :eek:
 
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You're right Bob - not having all the data does no good.

I have the Van's standard mounts supplied in the FWF kit, and no noticeable sag after 930 hours and a fair mount of Acro.
 
I have vans standard mounts and do have sag of about 3/16" in 400hrs. Mine is noticeable, and I'm not going to do anything about it. No aerobatics :).
 
I've reshimmed mine twice and I started with the spinner about 1/8" above the cowl. After 260 hours, it's about 3/16" below the cowl again. I have about .062" worth of shims in there now. And these are real Lord mounts (softer non-aerobatic that Van's used to sell).

Heinrich Gerhardt
RV-6
 
Type of engine mount also needed

One more thing to add to Bob's suggestion that the type of isolator mounts be listed. Conical vs. dynafocal amounts also have a reputation for sagging different amounts. Don't quote me, but if I remember correctly, a general rule of thumb is that Van's isolators sag more than Lord isolators and conical mounts sag more than dynafocals. Thus, at the most likely to sag end of the spectrum would be Van's isolators on a conical mount and at the other end of the spectrum, the least likely to sag would be Lord isolators on a dynafocal mount. However, the forum archives are full of threads talking about sag, though, so it might be worth going back through them.
 
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Reference post #20, both aircraft used the standard Vans FWF motor mounts.

I say again---go figure!!!???

Cheers,

db
 
My first post suggested a possible 1/4" to allow for sag. I did and after 350hours, I've seen no sag. Still waiting. I will not adjust at this point.
Very light acro, only rolls.
 
The Mounts I received from Vans came in a box marked 'Barry Controls' from Burbank CA with a FAA-PMA approval. Others have stated that Vans supplied them Lord mounts.

So I suspect Vans has used various suppliers along the way so it would be necessary to identify the actual Mount manufacturer if at all possible.

My mounts have the words 'BARRY ENGINE VIBRATION ISOLATOR' (~0.1" high) moulded into the rubber close to the edge that fits into the enging mount ring. It may not be easy to see on an installed engine.

I will soon install my engine so I am very interested in any definitive data on the issue.

Doug
 
Esthetics?? What about the spinner touching the cowl??

I have the Van?s standard engine mounts. I fitted the cowl maybe a year after I first hung the engine. The cowl ended up a little bit lower than I wanted, but since the engine had already about a year on the mounts, I thought it would not matter that much. However, I noticed that there was only very little clearance between the spinner cone (Whirlwind 200RV) and the inner flange of the cowling.

When I pulled the cowl after first flight, I noticed that the spinner cone had been rubbing the internal flange of the bottom cowl (probably during a tight turn or during landing 'touch down"). I grinded away as much as I could from the cowl flange, to get some more clearance. Now, after about 10 Hrs., engine sagging is noticeable and the spinner has contacted the inner flange of the bottom cowl again.

I am having some washers made to put under the bottom engine mount points, which should get the spinner in line with the top of the cowling. I hope to end up with enough clearance so the spinner no longer touches the cowl.

Regards, Tonny.
 
engine sag database

Just to add more data,

I have Van's standard mounts, and the heavy IO-360 angle valve, but a light whirl wind 200RV prop. Very occasional aerobatics.

My engine had hung on the mounts for almost a year with no sag, until it was run.
I mounted it with 1/16" shim washers on the top mounts initially, with the spinner perfectly centered on the cowl. At about 30 hrs, it had sagged enough (1/8" - 3/16") that I removed the upper shim washers, and it was again pretty near perfect. At about 50 hrs, it had sagged another 1/8" +, so I put the shim washers under the lower mounts. At 120 hrs now, it has not changed any more.

For those that are currious about the flight position vs ground position, the cruise thrust is around 150 lbs, but engine-prop weight is still 350 lbs or so pulling down. I doubt the cruise position of the spinner is significantly different than the static positon.
 
Where should shims be added???

With 200 hours on my RV-7 / IO-360 /CS prop, I've got about 1/4" of sag at my spinner. I regularly fly aerobatics, pulling as much as 5 g's at times. My aerobatics aren't always quite that extreme - most flights top out at about 3.5 g's.

Anyways, the sag is starting to show, and I'd like to get rid of it. I've read in various threads/posts that shims can be added on the lower firewall mounts of the engine mount, or else washers can be added between the engine and the vibration isolation mounts on the engine mount.

Any suggestions about which location is better? Does anyone know the part number and source to get the vibration isolation mount washers?

Thanks!
 
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