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Top Cowl - Engine Sag Question

L

lrfrey

Well Known Member
I have reviewed several builders sites regarding fitting the top cowl and how to allow for engine sag after a few engine runs... excellent documentation I might add and I appreciate the work.

What is not clear to me is how (where) to measure the 1/8" inch (spinner)above the cowl measurement. Anyone care to comment?

Maybe an easier to answer question would be how much did the engine (as measured at the spinner) "fall" after run up?

Thanks, Larry
 
I made sure my spinner was 3/16" higher than the cowl when measured from the top center of the cowl forward edge to the top center of the spinner at the aft edge. After 75 hrs the two are now in alignment. I believe the drop of 3/16th of an inch occured within the first 10 hrs.

Cheers,

db
 
lrfrey said:
I have reviewed several builders sites regarding fitting the top cowl and how to allow for engine sag after a few engine runs... excellent documentation I might add and I appreciate the work.

What is not clear to me is how (where) to measure the 1/8" inch (spinner)above the cowl measurement. Anyone care to comment?

Maybe an easier to answer question would be how much did the engine (as measured at the spinner) "fall" after run up?

Thanks, Larry
Click to expand...


Oddly enough, I spoke with a -7 builder about this very subject at the Reklaw, TX fly-in recently. His nugget of advice, albeit possibly too late if you're mounting things now, was to hang your engine as early as possible.

He said by allowing the engine to settle over time, as they worked on everything else, let them fit the spinner / cowl exactly to one another, and had no sag after running the engine.

Now they have a few hundred hours on the plane, and I can attest that their cowl and spinner match perfectly.
 
Of the two RVs that I built, neither of them ended up having the engine sag problem. The engines were both hung on the plane about one year before they flew and the cowl was fitted about six months after the engines were hung. So, just one more data point for you.
In the event that the engine does sag, you can put an extra washer between the lower motor mounts and the motor mount itself, in order to lift the engine back up. Or, place an extra washer on the two top mounts now, and if it sags, take them out to raise the spinner back up. As usual, there is more than one way to skin this cat. (By the way, I believe there was an article in an old RVator about using the extra washer method and Van approved of it.)
 
Thanks Guy's, very helpful. My engine has been hung for about a month now so I will plan on slight sag.

I had forgotten about the washer trick, I remember reading about it somewhere, might have even been in the instructions! :eek:

Larry
 
We also installed 1/8th higher than the cowl.After 20 hours I cannot see any difference yet. Maybe it will still sag but I doubt.It might depend on the type of engine mounts used.Good luck. Deon.RV7A project. South Africa
 
while we are on the subject...

Yes, I want my plane to look nice setting on the ground, but what is the effect of engine sagging in the motor mounts while flying?
Does the prop pull the engine back to the original alignment?
What is the best aerodynamic alignment for the engine and cowl?

Kent
 
Thrust line orientation

Don't worry about the small differences in thrust line orientation during flight. This thing is pretty solid and the performance is not sensitive to small variations in thrust line angle. How it looks on the ground is probably the most important consideration given the amount of angular variability permitted by the design. I mounted my engine early as well and after 2.5 years of flying I notice no sag. The Spinner on my plane is smaller in diameter at the interface than the diameter of the cowl flat plate disc area behind the spinner. If I align my spinner at the top with the upper cowl and it did sag it would still be contained within the projected cylinder from cowl flat plate disc area behind the spinner.

Bob Axsom
 
Engine Sag Question

I was planning my build process (QB) to be in this order; 1) Emp, 2) wings 3) Fuse and then avionics and panel with the engine last. seeing the comments on engine sag would builders recommend I install the engine before I do my avionics/panel electrics therefore giving the engine time to settle into the mounts and create its "sag" ?

Many thanks


Martin (newbie) :)
 
Last edited:
OK. Mine's sagged.

Well, after installing my engine 3 months ago, I finally reinstalled the cowling after painting, and I've got about 3/16" sag. :(

Anyone remember how much shim equals how much the height difference at the spinner?

Thanks,
Heinrich Gerhardt
 
Engine mounts,,, Sagging good or bad.

This is what I found on my -6. After 470 hours of flight time, the spinner is level at the top and bottom of the cowl while sitting on the ground not running. What I see in flight at cruise speed is the spinner is maybe 3/16" to 1/4" above the cowl on top. I'm thinking that in flight when the engine is pulling the plane through the air that the front of the engine is pulled UP and lifting it upward. For speed and less drag in cruise, I'd think that if the spinner was in line with the cowl would be better in cruise for less drag reasons. So if the spinner is lower than the cowl while not running and on the ground should be cleaner in flight and maybe more speed. Mine is not perfect BTW. Just a thought OK. If one wants to make it look nice while sitting on the ground then make provisions to get the spinner and cowl matched on the ground. Yep the mounts do settle in time depending on climant, temperature and age. The spinner to cowl will never be perfect in all conditions, best thing in my opinion is to get it in the middle and not worry about it. AJ
 
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