Missing airflow found

A place to relax and discuss flying topics.

Moderators: GAHorn, Karl Towle, Bruce Fenstermacher

Post Reply
User avatar
ghostflyer
Posts: 1390
Joined: Mon Apr 21, 2008 3:06 am

Missing airflow found

Post by ghostflyer »

Since converting to a 4 banger[with a pressure cowl instead of a plenum chamber] I have had doubts on my CHT temps. My single CHT originally was on no3 and EGT on the no 2 cylinder. Cruise CHT,s were around 370 deg [the no 3 cylinder] but in climb they would drop back to 320 to 340 at full power. They were new baffle seals and were sealing well .i even put a camera in the top engine bay and run the engine [idle] play back showed all good . Then fitted 4 probes and checked CHTs ,front cylinder[no 1] was 280,degs . The other 2 cylinders [2 and 4 cylinders] were around 340 degs . So many weeks of research was carried out on airflow , thermodynamics , and a heap of other big words that talk about how a airflow engine is cooled . [ I have had my knee busted and could only move around the house in great pain , so when I say weeks it’s actually months off work.] many suggestions were made by others of great aeronautical fame on my problem with the aircraft. Some said it wasn’t a problem as it was under 400 degs .
No, as there was a spread of 280 degs to 370 degs plus of cylinders bolted to the same crank case . Crank cases like to have a even spread of temperatures . There was talk of the “bradly”mod done to no 3 cylinder . But the new baffling had this incorporated. Talked to the RV guys as I have done some work for them in the past. Sealed off the high pressure areas to the lower pressure areas at the intake area and prop shalf area. No change . The intakes were changed and volutes were formed to direct the airflow more directly into the engine .some temperatures were noted that had dropped and amore even spread.
Then one night having my son ground run the aircraft as I couldn’t fly it. Brakes on and wheels chocked he went to full power . I had a LED torch aimed on the front cowl,and could see in the air intakes and to my horror the new baffle seal at the back of the engine which seals against the top cowl was blowing down leaving a great air gap. Here was a gap of about 1in just above no. 3 cylinder . Access material was cut of the baffle problem solved.
User avatar
ghostflyer
Posts: 1390
Joined: Mon Apr 21, 2008 3:06 am

Re: Missing airflow found

Post by ghostflyer »

After all the trouble and who haa, I have come to the conclusion that back in 1949 when the Cessna 170a was produced the Cessna guys had nailed the most efficient cooling for the engine . The plenum chamber is the best system and how the intakes work is the simplistic but efficient. I have even considered going back to a plenum chamber system . I can’t due to the STC. Even the RV guys have now incorporated the plenum system for engine cooling. I have now a problem of the oil temperature will not go over 165 degs at 95 deg OAT. Due to the high temperatures in this part of the world ,there is fitted a large oil cooler and the vernatherm is open most times .its closed until about 100 degs oil temp.considering going back to standard .
swixtt
Posts: 206
Joined: Wed Dec 10, 2014 6:49 pm

Re: Missing airflow found

Post by swixtt »

Which STC is this? is your oil cooler mounted on the aft side of the baffle? i have similar issues with cylinder cooling on my DelAir. i have not redone my baffles as i think they are too short but after reading this i'm not sure if having them a bit longer may create the 'blow by' you mentioned.
User avatar
ghostflyer
Posts: 1390
Joined: Mon Apr 21, 2008 3:06 am

Re: Missing airflow found

Post by ghostflyer »

My 4 banger has the Del Air STC. The oil cooler sits on the LH side of the engine mounted to the baffle. When I first put this all together ,it was noted that the fiberglass intake wasn’t very efficient . High pressure air [from the intakes] could escape to the exhaust air [lower pressure air ] instead of travelling through the intended path and cooling the engine . The baffling supplied also had a design flaw on the RH intake side as the baffling didn’t met the crankcase correctly . Had many hours on the phone with harry on some of the short falls of the STC. So the interior of the intake cowl,was filled with foam and fiberglassed to stop the air running around in circles . Then flying along one day and a large bird [pelican or large ibis ] came in contact with the cowl. It shattered the fiberglass intake section and sheared some of the stainless screws that held the top cowl on at the firewall. I,contacted Del Air for a new part. This could have been around the time of Harry,s passing but was told that no spare part was available . Spruce couldn’t help either. So the fiberglass intake was repaired and strengthen around all the curved pieces internally with polyester rope soaked in resin and then glassed in and then the foam again and then refiberglassed over. Fate as it is I hit another large bird[pelican] nearly in the same place . Cessna 170A “1” ,bird “0”. Lower cowl had some buckling but easily repaired.

Ps. It took nearly all day to wash the internals of the bird off the aircraft and even now after a long flight the smell of bad KFC. Is still very apparent. Bird bits went through the engine .
Post Reply