Thorp Air Command - T18.net

So, there I was at the end of the runway - 4606 MSL......OAT was 18 deg F - no wind, runnup complete, feeling very pretty and off I go - my mixture was set for takeoff - normal (actually impressive with 10 gallons in the tank) climbout at 95 mph and was at pattern altitude 900 AGL about 5000 feet down the runway - leveling off and turning crosswind, I glanced at my CHT/EGT gauge and the EGT was at 1400 plus - WHOA!! - I enriched the mixture a few turns and it still was not going down - backed off the throttle a bit and went full rich down it went, but it took me a few miles to get it to stabilize around 1300, with the prop turning 2350 with the mixture much richer than I would normaly run - makes sense by and large as the temps were chilly and altimeter 30.45, thereby some cool thick air with lots of oxygen, rendering a leaner fuel air mixture. After a few minutes of admiring my surrounding and all being as it should be, I was about to do some wingovers and other accelerated fun stuff, so I set the prop at 2600......and up went the EGT..... - so I put in the mixture a bit and got it stable again - after getting my yank and bank fix, I transitioned to straight and level and started messing with the prop/throttle settings a bit and, shornuff, I could vary the EGT without changing the throttle and mixture, but by merely increasing or decreasing the RPM - at 2350 RPM WOT and the mixture set to 1300 degrees, if I dialed the prop out to 2600 RPM, the EGT would read 1350 degrees - an increase of 50 degrees - going back to 2350 RPM, same throttle and mixture settings, back down it went......go figure?!?!.........
Now, having said all that, I have noticed in the past, during cooler temps, that when descending local into the pattern at speed, the EGT would climb a bit and I have to dial in the mixture, but I always thought that normal during decent, so did not pay much attention to it from an RPM/EGT relationship. The CHT were nominal with no noticeable change throughout.
I suppose drilling out the jet to a slightly larger size might help with keeping the EGTs down (slightly richer mixture)during winter flying, but that pesky Law of Unintended Consequences then comes into play, and the carb was recently rebuilt (last June) with all of 40 hours on it, so I'm inclined to leave be.

Single Probe on #3 cylinder is 1.5 years old, as is the gauge.

Thoughts and feelings?

Fraser,
I think what is happening is that you are still getting combustion of fuel in the exhaust pipe. The fuel will only burn so fast and at higher RPM's the time in the cylinder is less. That's my thought about this. I usually pass when my wife wants to talk about feelings so I'll pass with you also.

Jim ;-)

This sounds kind of familiar. Drilling the jet worked for me, but this is the first winter for me with this set up so the change was a little anticipated.
My question is has anything else changed? Have you flown in similar OAT

_________________
http://www.dixiefriedfabrication.com/

Lou - I usually stay indoors when the high for the day is under 20 deg - the only advantage of that cold is the underwear fits better - under normal spring/summer/fall conditions, I peak at 1280 to 1320 or so in cruise before engine stumble and after a 1/2 turn settles around 1280 to 1300 - I try to stay just under the stumble setting, as I have bought into the idea of 50 deg ROP (per most POHs of Pipers, Cessnas, etc.) is putting the worst internal cylinder pressures situation for motor longevity - maybe 1400 is normal for climb, but I don't recall seeing that high of a EGT temp before and I puckered - I'm turning 2700 at take off and generally start dialing back the prop as I turn crosswind, or at a safe altitude depending on the situation. I just read a Mike Busch article where he states not to worry at all about EGT and that he regularly sees 1500 and that the CHTs are far more important (which I think we all knew anyway), so maybe I'm up in the night about nuthin.

Jim - that sounds logical - thanks ferdat.

Hey Frase,

The relationship between rpm & EGT were good observations. But first of all, if the engine is running smoothly & cylinder temps are OK, why worry?

At your altitude however, if you had to go to full rich for it to go down, you might want to start thinking about how it will run when you're at sea-level. slightly enlarging the main nozzle may indeed be needed. I did that on mine, after changing the original nozzle out with a "pepper-box" style one. Before you do that though, be sure to check all the basic stuff - induction system leaks, float adjustment & good needle/seat, fuel supply issues (partially plugged filters, etc.), and proper alignment of the mixture arm/ mixture valve. Make sure you have full throw on the arm in both directions. The mixture arm needs to be correctly aligned with the hollow half shaft, & that can be checked by taking the top off the carburetor. The supply hole (that feeds the main jet) has to be fully open at full rich & fully shut at idle cut-off. Sometimes poorly cast single-piece venturi's can cause real distribution problems - just something to consider if you're seeing real unequal EGT's between cylinders. This is kind of a continuation of the conversation with Lou.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

John - yeah, the thought did occur to me about ops at sea level - having said that, it ran like Brazell to a brothel down in Texas not long after installation, but it's prolly time for an inspection of just as you describe.

John has good points, as usual. My thoughts are these... First and foremost, your engine is an air cycle machine. Higher RPM means more air through the cylinders. Simple. Regardless of what you do with the mixture and throttle, more RPM equals more air, which requires more fuel to maintain the correct mixture. You may need a bigger jet. I think most of our applications do. But as you pointed out from Mike Busch, don't read too much into EGTs alone. If everything else is smooth, and your cylinder heads are cool enough, don't worry about it. Same with Lou's discussion earlier. Low temps, and lower baro pressure mean more oxygen and therefore, higher EGTs.
Cubes

A couple of years ago when Bob and I built the RV7, we bought an 0-360 from Mattituck. When we applied power on takeoff it would pop and fart for the first 100 or so feet down the runway. Also you never had to lean it. We talked to Mattituck and there comment was "it's the right carb and it must be something with the fuel flow. Called Vans and was told it was the carb. Back and forth between Mattituck and Vans I ordered a carb from Mattituck with an enrichment kit (which means they drill out the jet) It solved the problem even to the point of leaning. Talking to Superior , said you could put the problem carb on a half dozen engines and it would work, install it on the next and it will run lean, install it on another it it would work. Seems each engine can have a slight difference in castings and create this condition.

1. JR , I see you like the Link (Mod Squad) logo also ! 2. Regarding Texas...what the hay Fraser were you following me ! I saw you going into room 402 !

Regarding my carb (MA-4-5) , it seems I need to lean right after T/O . I aggressively lean on the ground during taxi . Sea level here in San Diego . CHT/EGT in normal range . If I leave it full rich after T/O you can feel it run a bit rough . I don't see a big difference in CHT/EGT with the mixture rich , only a slight roughness . Leaning to smooth the CHT/EGT remains in the normal range...according to the Lycoming manual .

RB

I always wondered what CHT/EGT stood for. I have one in my Thorp but never used it. Just kidding. We tend to get wrapped up with EGT. The Lycoming rep told me long time ago, "if it isn't under full power lean it" I take off (sea level) full rich, when power comes back on climb out I lean , don't know what the EGT reads and don't care. In cruise I lean to 1075. Why, because my probes are located different than yours.

Bill : Pretty much sounds like what I do . I only use the CHT/EGT to show a "trend" ...min/max . Keep it in the "green." Lycoming rep has the right idea .

RB

The EGT indicated that I had a lean scenario, I did ever so slightly open the main jet up. Get a set of number drills for this, Just go at it a hair at a time just a couple thousands at a pop. In my case the first little bit I took out of it done it. I lean an extreme amount on the taxi way, and I lean right after lift off and as I continue to climb. 1400 deg EGT with cool cilinder heads won't hurt anything( chime in here Steve) I can have a smooth engine at 1400 EGT and cilinder heads that are cool as a cucumber. If your cilinder heads are cool then the valves will cool when they are seated and your valves will receive no punishment. fat running engines really dirty up the oil, the excess fuel will wash the cilinder walls and puke the plugs, it will rob you of some performance and some gas money also.Its hard Somtimes to find the sweet spot with changing temps and Barametric pressure, but practiced and tried a few times you will hit the sweet spot on a fairly good amount of the time. Dan

Thanks for the comments all - good discussion and I learned sumpin....airplane enjuns are intrestin.

How did Lindberg do it crossing the Atlantic ? What amazing engine ! CHT/EGT/RPM ? The sound of the engine was telling him what is wrong after years of experience ? I think sometimes we get caught up in all the techno stuff and forget about basic flying and listening to the engine ! Not even sure if he had CHT/EGT ?

RB

Sure do enjoy reading all the notes about engine, cylinder, and exhaust temps. When I first flew my T-18 I had an egt and cht. I was a migrant worker for 26 years and flew in just about every physical environment there is in the US. Leadville CO, Mississsippi, Tuscon etc. I NEVER had any temperature problems except getting the oil temp up to where I thought it should be (190 degrees F). When I restored my plane back in 1992 I took all of that unneccesry stuff out of my plane and have never been sorry. I lean by engine rpm as I was taught back in 1953 and it seems to be working well. John Thorp sure did design a winner!!!
Steve Hawley