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O.S. .91 FX

Old 01-28-2005, 10:50 PM
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Default O.S. .91 FX

I have an O.S. .91 FX in my UCanDo 3D. I run 10% nitro, and had some pre-ignition problems (knocking). Noticeably worse on down lines. A friend with the same engine suggested adding a second head gasket which I did. While this reduced the amount of knocking, it still occurs. My UCD hovers quite nicely with the .91, but it doesn't rocket out of a hover like I thought it would. I use a 15x6 APC prop and the plane weighs 8.2 lbs. Did putting in the extra gasket reduce HP? Any suggestions as to how I can maximize power (or ability to rocket out of a hover) without increasing the pre-igniton problem?
Old 01-31-2005, 12:03 PM
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Default RE: O.S. .91 FX

On many large glow engines, you'll get a knocking sound in the middle throttle ranges. We've seen this on many different engines of different manufacture. We've never seen any evidence of damage to the engine. We can only conclude this is an exhaust note, likely caused by unburned fuel/air charge still burning when it exits the exhaust port.

True detonation will usually happen only at full throttle with a too-lean mixture and bad cooling. This adds a "frying egg" sound to the exhaust note.

For more vertical, you will need to go to a propeller with lower pitch. Increase the diameter to keep the RPM within bounds. As long as you're in the high 9,000's at full throttle on the ground, you'll be OK.
Old 01-31-2005, 06:47 PM
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Default RE: O.S. .91 FX

Thanks for the response. So I should remove the extra head gasket? And get say, a 16x4 prop? What about nitro? I run 10%. Should I go up to 15%?

Thanks again!
Old 02-01-2005, 10:38 AM
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Default RE: O.S. .91 FX

You can try a 16 x 4 prop. Get several different brands. They will all perform differently.

10% fuel is fine.

If the engine's running well, you don't have to change the gasket.
Old 02-01-2005, 05:32 PM
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Default RE: O.S. .91 FX

Thanks, I really appreciate your time.
Old 02-24-2005, 12:36 AM
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Default RE: O.S. .91 FX

Sorry to bother you again, but I have another question. I have a new OS 1.60 that I'm putting in a Chip Hyde Cap X. I was thinking about placing the tank close to the CG which would mean about 1 foot of fuel line from tank to engine. Do I need a fuel pump? If so, could you recommend a good one for this engine? Also, I currently have some 15% fuel. Is this too much nitro for the 1.60?

Thanks for your time.
Old 02-24-2005, 11:35 AM
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Default RE: O.S. .91 FX

With the fuel tank far from the engine, you'll need to use a pump or a good pressure/regulator system. There are several on the market and people have been pleased with all of them. Do a search here on RCU for pumps and regulators, and you'll get the best information possible.

15% nitro fuel will be OK in the .91.

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