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Engine runtime?

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Engine runtime? Empty Engine runtime?

Post  Cox International on Tue Sep 24, 2019 1:38 pm

How long should a Cox .049 high-performance reed-valve engine, such as this one, run on an 8cc tank, with 25% nitro? With 15% Nitro?

https://coxengines.ca/cox-.049-engine-venom-ii.html

I guess one could substitute a Golden Bee or Black Widow, as performance is almost the same.

A buyer claims that he is only getting 1.5 minutes at 15% nitro; which seems low.
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Post  Marleysky on Tue Sep 24, 2019 2:55 pm

1.5 minutes? That does seem low. Does he say the tank empty’s completely, or is fuel still left when it quits? My first assumption is the fuel tube inside the tank is off the nipple or not at the bottom of the tank. 2nd guess would be the venturi gasket is not sealing and the engine quits when it drops below the centerline of the tank.
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Post  Cox International on Tue Sep 24, 2019 3:08 pm

This is the mail thread (fluff removed):

Hallo my friend. I make all that you told me. I check the fuel tank the fuel line and everything works perfect. The engine still run for 1.30 minute. Keep your mind the engine is new.

The Venom is a higher-perfoming engine than the Babe Bee but 1.5 minutes is still short. You could check if the fuel line is properly attached to the nipple inside and not leaking air. Another test would be to see if all fuel is used but that would entail filling a syringe with 88 of fuel.

Lastly, if the prop is not well-balanced, you would get excessive vibration which, in turn, creates foaming fuel; which then escapes though the filler / vent tubes.


thanks for your informatiom i use the cox in rc plane not cl.and the fuel line is in the bottom. my fuel is 15% nitro and 10% synthetic oil and 10%castrol oil
with 5x3 and 6x3 prop i have 1.5 minutes run time realy i can't understand wy. the baby bee i have it run about 3.5 minutes with 8cc fuel tank and the same prop and fuel.
i will check it and i wil, inform you.
thanks a lot for your time.


Runtime with 25% Nitro is between 2 and 3 minutes, depending on propeller, elevation, etc. 1.5 Minutes seems to be on the short side. Not sure if you are flying control line but, if you are, you would need to reposition the fuel pick up line to the 7 or 8 o’clock position. The pick up line is always positioned back to the backplate as per attached photo.
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Post  aspeed on Tue Sep 24, 2019 3:43 pm

On this video, it runs for almost exactly 1 1/2 minutes. It's a Black Widow. Fairly similar engine. It can be leaned out and run at a slower rpm with more pitch to last longer. https://www.youtube.com/watch?v=FQjSROfiKQk I know at the one busy rc field, by the time someone else would land, and I had the motor running, I would run out of gas.
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Post  Cox International on Tue Sep 24, 2019 4:23 pm

Thanks for the replies Smile
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Post  Cribbs74 on Tue Sep 24, 2019 9:59 pm

Yeah, 1.5 min sounds about right. The more RPM’s the faster the fuel is consumed. That’s why he gets double out of the Babe Bee.
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Post  Tee Bee on Wed Sep 25, 2019 3:42 am

I haven't timed it but I figure I'm getting about 2 mins on my Venom II .049 with a 5x3 prop and 25% nitro. It runs longer in the air(leaner) than when run static with the same needle setting.
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Post  aspeed on Wed Sep 25, 2019 8:15 am

Smiles per gallon.
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Post  RCH2016 on Sun Oct 13, 2019 1:08 pm

Recently bench running an old 0.049 from a PT-19 owned long ago, planning to try 1/2A again after long absence. The engine runs ok, using a 1 oz Sullivan tank am getting about 2 minutes from 1/2 oz ( 14 cc ) with 5x3 prop on 25% Brodak. I am running is on rich side of maximum RPM. From the previous discussion, this is about twice expected fuel consumption rate.

I am expecting that in flight the engine would unload and lean out, so maybe fuel economy would improve?

What factors cause excessive fuel consumption besides a worn-out piston/liner? What would be other signs the engine is worn out? This engine seems to have good compression and starts easily.
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Post  aspeed on Sun Oct 13, 2019 1:34 pm

RPM and how far open the needle is will affect the mileage.  Higher nitro will require the needle to be open more, as will a twin port cylinder and a smaller prop among other things like compression and running slobberry rich.  Higher compression normally give better mileage until it detonates.  A reedy will flop back and forth when overcompressed and the prop is too big. If the motor starts and runs well, then it is not worn out IMHO.
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Post  batjac on Mon Oct 14, 2019 12:44 am

Also, does the motor start right up? Does he flip and flip for a couple of minutes? That wastes fuel also.

The Guzzler Mark
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Post  roddie on Mon Oct 14, 2019 12:40 pm

Bernie's customer states having a Babe Bee with 8cc tank which runs much longer on the same fuel and prop. His Babe Bee likely has a single-port cylinder and the .0625" diameter air-intake. The high-performance variants usually have dual intake ports and a larger diameter (.082") air-intake which allows more fuel/air to pass thru the engine. Some high-performance engines may use a Tee Dee style cylinder and high-compression glowhead. These will naturally burn fuel at a higher rate..
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Post  coxaddict on Mon Oct 14, 2019 2:42 pm

The top vent on my stunt tanked Bees tend to siphon fuel out from the tank when running. I use a short piece of fuel tubing on the vent to minimize the negative pressure created from prop wash and the forward motion of the plane. Babe Bee engines with the vents on the back plate don't have this problem as the vents are not submerged in fuel.
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Post  jmendoza on Mon Oct 14, 2019 5:34 pm

Higher nitro, smaller prop, bigger venturi, and twin bypass/free-porting will all reduce run time. Baby Bee with #2 cylinder, 10%, 6x3 prop you can get over two minutes. My RR-1 runs for about 1.5 minutes. Sandblaster cylinder is good for a longer runtime.
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Post  Cox International on Tue Oct 15, 2019 11:33 am

Thanks for all the great replies Smile

We don't know how much time he spends on starting, etc but our customer is now informed and seems to have accepted the runtimes as being normal.
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