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Cox Engine of The Month
February-2024
1/2A Nut's

"RC Speed Boat Power Drive TD .020"

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Carb setting getting rich in the air? Empty Carb setting getting rich in the air?

Post  balogh Sun Feb 04, 2024 1:23 pm

In an early springtime but exceptionally windy and sunny Sunday I had a terrific 6 flight session in the field with my Roaring 20..the plan was to find the best setting for the NV of the
COX TeeDee020 in the plane's nose. While tweeking the carb setting
I kept wondering why the static carb setting on the ground may become richer in the air, and the only reason I can imagine is, that the flight speed added to the propeller air blast over the carb air intake increases the depression in the carb, but will reduce the air inflow, while the depression boosts the fuel inflow...the two combines into a richer fuel-air mixture..but I may be wrong...

Another reason could be the air becoming thinner with altitude, but the few 10-s of meters altitude I fly may not mean significant reduction of specific weight..any thoughts or theories that you may have?
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Post  rdw777 Mon Feb 05, 2024 5:41 pm

Hi Andras, Glad your getting some flights in Thumbs Up ….. I don’t know the answer to your question but interesting to think about……Typically I set the needle slightly rich on the ground and hope that it will lean out just about right in the air….. Checking throttle response is good indicator for me too….That was the habit before balloon tanks (which are nice by the way Very Happy ) and still treat the balloons the same way…. Works out OK most of the time….There seems to be a “window “ where the engines will tolerate a little bit of a not quite perfect setting and go ahead and do their thing…. Some of my engines are more tolerant than others ….

Going toward rich in the air is different…. Is there any way extra heat or air could be getting to the balloon in flight compared to on the ground causing a slight increase in pressure?….
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Post  944_Jim Mon Feb 05, 2024 6:32 pm

Andras,
Is this a CL plane, or RC?
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Post  balogh Tue Feb 06, 2024 12:22 am

Robert, Jim,

Thank you for trying to solve this puzzle with me.

Robert, my balloon tanks are in a closed space in the plane's nose, they are not exposed to air blast so the fuel pressure and temperature are not impacted.

Jim, this is RC...after I land, I always check the NV position and this is unchanged, so the riching  phenomenon must be related to aerodynamics I.e. kind if air starvation, in my guess. This may not be the same with reedies that inhale air through their backplate, which is shaded from the propeller and flight air blast..but I observed it with - at least this particular - 020 TeeDee, having the front rotary carb.
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Post  KariFS Tue Feb 06, 2024 4:34 am

How do you conclude that it’s running rich in the air? Losing rpm, or four-stroking, or something else?

It’s already practically spring in Budapest, but I have had carburettors freeze in humid air in temperatures way above the freeze. Mostly with mopeds, lawn mower and once even with a car when I had not turned the intake snorkel to winter position.

Usually the symptoms felt like fuel starvation, but those ”real” carburetors are quite a bit different compared to the one on Tee Dee Smile Just wondering if the venturi could build up just enough freeze to reduce the air flow, which would cause a greater vacuum and suck in more fuel?

When you reduce the rpms to bring the plane down, or when the engine shuts off, the heat of the engine quickly evaporates the frost. Also, on the ground when you adjust and test things, the air is warmer, the air flow past the venturi is slower, hence no frost.

Just supposin’ Wink
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Post  balogh Tue Feb 06, 2024 4:53 am

Hi Kari, thanks for your thoughts.  This is a simple TeeDee020 carb with no throttling. The engine peaks when the plane is hand launched from the ground, then is 4 stroking in the air in an indication of getting rich mixture. It revs up to peak again a few seconds before the tank is slurped empty and fuel supply gets reduced.
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Post  1/2A Nut Tue Feb 06, 2024 8:43 am

The TD .020 cools off in the air quickly, the fuel density improves and rich flight is the result.

The tricks for more power: Pray

Cold engine start, Wink
Peak the NV with the plane level on the ground before it reaches it max thermal state and toss.
If you allow the engine to heat up too much on the ground it will force a richer setting before the toss.

Cold weather flying, Cold
Prop down to help keep rpm up enough to keep it warm. Best results with a hot turbo plug in the air.
Reduce to l5% nitro in cold weather as a higher % of methanol burns hotter than nitro keeping the
engines thermal state at balance while zipping around in the cooler air.

Hot weather, Hot
Can up the nitro to 35% to keep the rpm's frisky in the summer. When you add in nitro's high heat
of vaporization it yields a significant cooling effect which is nearly twice that of methanol.


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Post  balogh Tue Feb 06, 2024 9:00 am

Thanks, Brad, these points you suggested are all in line with my field experience, and all make sense:

Indeed, I do not wait too long after the engine starts and I peaked it, before I hand launch the plane, and maybe it failed to reach its operating temperature ..the very strong winds and cool temperatures Cold last Sunday may have cooled the engine mid-air way too much...and I have 30% nitro in my fuel that I originally mixed for my 010 TeeDee-s, but next time around I will go with the 20% nitro fuel of my 049-s...I have  hot plug in the Kamtechnik head, and a APC 4.2x2 prop allowing a static ~22k rpm, though, ..


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Thanks a lot, again..I can't wait to check these remedies when next time in the field..  Clapping  Flying cheers This Site Rocks!
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