On Sunday, September 9, 2018 at 8:02:30 AM UTC-7, wrote:
On Saturday, September 8, 2018 at 6:06:39 PM UTC-5, wrote:
On Saturday, September 8, 2018 at 5:53:12 PM UTC-4, wrote:
On Saturday, September 8, 2018 at 2:42:00 PM UTC-5, wrote:
On Friday, September 7, 2018 at 11:10:08 PM UTC-4, wrote:
My experience bringing power back to idle after glider release in less than 30 seconds and then not adding power all the way to landing has not been a problem. Our previous engine was well over TBO when replaced, with no issues.
Having more sophisticated engine monitoring might help to run lean of peak to save gas.
Keeping your head outside the cockpit may save you more than gas.

What kind of airplane?
Your experience certainly differs from my 40 years of running towplanes.
UH

UH, do you think recording engine parameters and analyzing them post flight might be worth the effort?

I do wonder if our standard procedure is deficient, maybe analyzing engine parameters would reveal a bad procedure (or a tow pilot that doesn't follow a good procedure). And it would gather hard data that might show, as Dan suggests, that shock cooling is a myth....

Our procedure is to speed up to 100-110 mph put a notch of flaps down and throttle back to 2200 RPM immediately after release, descend/spiral down while reducing RPM by 100 every 10 seconds, until we get to 1500, then we are free to go to any speed/RPM. Sometimes we sideslip, but not always. Please critique!

Maybe we ought to start a new thread on cooldown techniques....

Cheers,
Jim

This has been studied and findings published in the past. I think it was in Soaring and was quite some time ago. It may also have been in Free Flight.
Our procedure, used in Super Cub and Pawnee, is to smoothly reduce throttle to 2300 rpm while accelerating to 110 mph. We carry this through the descent, including some of the downwind, followed by gradual reduction as speed is reduced to approach speed. We try to not be less than 2 minutes from release/throttle reduction to throttle closed. We discourage slips because this can make the flow through the cowl unbalanced which can lead to less even cooling.
I know Lycoming does not believe shock cooling is a myth. They have a recommended cooling rate, but I can't quote it from memory.
We have CHT on all 4 cylinders to watch for one getting unusually hot. This tends to be due to a damaged baffle though one time we did find signs of an intake leak that can make a cylinder go lean.
If you want to do the study, go for it and then compare with what your engine manufacturer recommends.
FWIW
UH

Thanks, Hank, good stuff.

I checked out the Lycoming website linked out of the shock cooling myth article posted by Dan:

https://www.lycoming.com/content/how...ng-your-engine

It says keep the rate of cooling under 50 deg F (28 deg C) per minute. Burt Compton says keep it below 30 def F per minute (Soaring Magazine October 2012 Page 14, and I think also in his "Tow Pilot Manual").

Also, I found those articles you referred to, Hank, in the Soaring Magazine archives. Done by some glider and tow pilots with advanced engineering degrees in Canada. December 1992 (for a 150HP Citabria) and April 1993 (for a Pawnee). Briefing looking them over, I am impressed that these tests were done competently, looks like good science (I used to flight test military jets for a major aerospace company, so I have some qualifications to make that judgement).

VERY interesting that the results show a much different letdown procedure is needed for the Citabria than the Pawnee. The Citabria is much more sensitive to letdown procedure, and their original procedure did exceed the Lycoming recommended cooling rate, by up to 200%. And that was the airplane that they had cracking cylinders on, prompting the study. Strong indication that shock cooling is NOT a myth, I would say. They significantly changed their letdown procedure for their Citabria, needless to say.

Their Pawnee did not have cylinder cracking problems, and the data on it showed it to be much more robust to letdown procedure, and that their original procedure (quite similar to yours, Hank) did not exceed the Lycoming cooling rate limits. Hence they ended up sticking with their original procedure for the Pawnee.

They theorized that the differences were due to the Citabria being more tightly cowled than the relatively-wide-open Pawnee cowling. Makes sense to me.

I did find one other reference to letdown procedure mentioned in Soaring Magazine, February 2015, used by Cypress Soaring in Southern California on their Pawnee. "I was advised to keep an eye on the oil temperature gauge and to take the reading (190 degrees (for that particular tow being described - JH)) and multiply it by 10. That would be the rpm that I should use during the descent." Somewhat similar to the procedure determined by the folk in Canada.

I may still proceed with a DIY system for recording data on our towplane, just for fun. And, as raxy noted, when the tow pilots are aware that their letdown procedures are (or can be) checked, they might be more careful to stick to them! Haha!

Keep the comments and information coming, thanks again.

Cheers,
Jim J6

I totally agree that an aerotow engine monitor is money well spent. And I think that data downloads should be done to pillory offending tow pilots, and 'to encourage the others'. In the midwest (airport elevations about 1000msl) aerotow climbs are always (in my experience) best done full rich. If CHTs get too high (which I take as above 400) then the only remedy is to increase speed, reduce throttle a bit, or wave off the glider. Out west (ie above 5000msl airport elevation), and where tows are often much higher, Pawnee engines seem to run rough on the climb at full rich, but leaning must be done VERY carefully, or CHT will soar. A very small decrease in throttle (a movement of the throttle, but hardly enough to reduce rpm at all) seems to help CHT a lot. The hottest cylinders are usually the ones at the rear: baffling is a black art.