Hot Temps on 1969 Twinkie (non turbo)

[Zach Grant L1011jock]

First, hot EGT's are not indicative of anything. Hot relative to peak is. EGT readings are very dependent upon many things, including where the probe is situated, RPM, Throttle position, and lastly, mixture. Comparing raw EGT temperatures from side to side is not a real good test for these reasons, unless everything is exactly the same...and it never is!

Fuel flow- the only way to get the fuel flows exactly matched is to send both servos out and have them bench flow checked and matched. Otherwise you are limited to adjusting the idle mixture for below 1100 rpm fuel flows, but that will have some effect on the higher end, but not much. Specs for the IO-320 engine are takeoff FF of 13.6-16.8 GPH at full power, sea level, std day according to Lyc. That is the spec, you can choose to have the engine set up anywhere in the spec.

CHT- BAFFLES, BAFFLES, BAFFLES!!!!!!! Ok, lets start with the flexible baffle on the nose bowl, that should be visible in the inlet. It should run from under the lip of the nose bowl, and should be OVER the top of the front cyl baffle. If it is under the lip, all the high pressure air is going to the lower deck, and decreasing cooling efficiency significantly. Next, look at the baffle material along the cyl. It should be on the pressure side (inside) of the hard baffle if it is attached by rivits. If it is on the outside, and in good shape, use copious amounts of RTV on the inside where the soft baffle meets the hard baffle, so no air escapes through the gaps. Next, look at the soft baffle on the rear baffle. It should be compressed and pointed forward so the pressure makes the seal tighter, not allowing air to shelf over the baffle. Fill any holes, and all areas between the baffle and the case with RTV to maximise the efficiency. Think like the air coming in the front. Try to maximise the pressure on top of the engine (uper deck) and minimise the pressure below the engine (lower deck) and the greater the delta P, the more efficient the cooling will be.

-Zach

[steen]

Zach has it right!
The Twinkie has (still today) one of the best cooling nacelles ever put on an airplane IF the baffles are maintained to a high standard. The airflow design is very low-drag and very efficient but only if the baffles are "air tight".
I have seen folks get rid of high temps and gain TAS increases of 3-5mpn just by repairing engine baffles and RTVing leaks.
To say the Twinkie is a critical airplane is perhaps an overstatement but it sure gets a lot out of very little horsepower (fuel flow) only if engine baffling and airframe rigging are correct. A slight slip from optimal in either causes a noticable effect on performance.
Steen

[Scott Ducey]

Thanks Zach and Steen. My mechanic is a Comanche savy, and he tells me my baffles are fine. However, I will print out what you both indicate and see if he has any response. Since my last post I did take the airplane out and did a number of take offs and landings. I brought up a bunch of Young Eagles, so the airplane got a lot of exercise. I am not sure that this was the best venue for looking at the temp problem, but she still ran hot. I want to try to replicate what happened down in Sun N Fun, and the only way to do that is to climb up to 9,000 and look at the temps there. It wasn't until I got up to that alt and leveled off, did the temps back off, and the right engine would only stay cool by leaving the cowl flap all the way open. Because of this problem, and because I am a relatively new Twinco owner, I am really looking forward to the CPPP to see if any of this has to do with me or if it is something else. Again, thanks for the input, and will provide an update when i have flow the plane a bit more.

Scott

[Scott Ducey]

I thought i would let everyone know that I took the twinco out for a steep climb up to 10K feet yesterday to give the new oil cooler / vernatherm a test. I brought my mechanic along for the ride. I left the cowl doors open on the climb and there were no temperature issues, as I experienced going down to Sun N Fun. We even closed the cowl doors on descent, which normally would result in the right engine getting hot - fortunately, the engine stayed cool. I am not sure if was the new oil cooler or the new vernitherm, but all is well that ends well!!

Thanks for everyone's input and assistance.