G-loads - RC Groups
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Dec 30, 2012, 06:38 AM
Registered User


Hi Guys,

I was directed here by another kind member, as I posted this question under Fuel Plane Talk to begin with.

Is there someone who can tell me how to fix a problem with my OS 40FX,

I have installed the engine in a home construction of an airplane built for speed, the plane flies very well, but I have a problem getting my engine to run smoothly under G-loads, as long as i fly straight and level it spins like a dream, but when I make abrupt pitch changes or tight turns (High G-loads) the engine hesitates until I ease the stick and letting the G-load drop, then it is back in the zone in a flash.

The engine swings a 8x8 or 9x8,

Why is this ??. Wrong level of the fuel tank in relation to the carb is my guess but I don't know


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Dec 31, 2012, 11:05 AM
ScorpionRacing's Avatar
Hi Ulrich,
If the engine looses fuel in a quick turn, I would say it is running too lean for the setup you have or loosing fuel delivery. Are you using a bladder type tank? A bladder tank (Tetra or Jett) virtually eliminates this by placing the fuel in a bladder inside the plastic tank and using the muffler pressure to press the air between the plastic tank and the bladder (full of fuel) so that you have constant fuel delivery. That is why we use these tanks in racing, to save our engines! I would still try to keep the centerline of the carburetor and the tank in line with each other.

I am not sure what the OS40 can turn, but we use the Thunder Tiger Pro 40 in 424 (USA) sport racing with the APC 9x6 prop. It will turn this prop @ +/- 17K with 15% nitro fuel (80/20 synthetic/castor mix). Make sure you have bearings in the engine that will handle the higher rpm's, or it will not last long. I would try a few different props and try to get the rpm over 16K and see if that helps any. Depending on the shape and drag of the design, you may benefit from a longer lower pitch prop or a shorter higher pitch prop. If it is all out speed you want, than the most pitch you can run and still get the plan to fly the better. However, this will destroy the ability to turn and maintain speed. That is why we will sacrifice overall speed in order to maintain a higher overall speed in pylon racing.

Best wishes,

Scott Smith - 86t
NMPRA District 7 VP
SEMPRA President
Jan 01, 2013, 12:51 PM
Registered User
vic welland's Avatar
Second on Scott's bubbless fuel tank comment.
Most of the time this will eliminate the problem.
We also had a similar problem with the .25FX that was caused by the needle valve. This was corrected by wrapping some sewing thread around the O-ring groove to tighten up the fit.
Jan 05, 2013, 04:00 AM
Registered User
Hi Scott and Vic,

I am running the OS40 with a normal rectangular plastic tank, the design of the plane is a knock-off of a PC-21 made of balsa, foam and fiberglass, it flies fantastic not a pylon racer though, just to enjoy the speed on a long downwind low pass followed by a pull up and turn for another one :-) I have a gps sensor in it and it clocks 192 kph or 120 mph on a calm day, I have not had a tachometer on it yet so I donīt know how fast it spins but, it sounds like it puts out quite a bit of revs.

I will try to not lean the engine out quite so much and see if it helps before i change the fuel tank.

Thank you very much for your replies
Jan 12, 2013, 12:13 AM
Registered User
I was taught by a racing engine guru to think of it in another way when setting my race engines: Don't see how lean and fast you can go, but rather see how rich and fast you can go. Look for a healthy exhaust trail.
Jan 12, 2013, 07:49 AM
Registered User
vic welland's Avatar
That is very good advice. Hardly taken by guys when they are beginning in the world of fast though.

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