OS 65LA Fluctuating RPM
04-18-2009, 08:28 AM
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OS 65LA Fluctuating RPM
Bill, thank you for your help in my last thread.
I've another problem. I'm trying to measure static thrust with different propeller sizes and I'm finding quite a bit of fluctuation with the RPM. An example:
Wuth the throttle connections locked down to give around 12,500 RPM, I get fluctuations of up to 300 RPM either side of 12500, i.e. from 12,200 to 12,800. The fluctuations appear to be random in size within this range and occur with a frequency of about half a second or a bit more
The effect is real as it shows up both with my Hobbico Mini-Tach and with the spring balance built into the test stand.
I am using the glow plug supplied with the motor, said in the manual to be an OS A3. The height of the tank appears to be correct in relation to the spray bar. I enclose two photos of the motor mounted in the test stand. I am using an APC 11x6 prop, following David Gierke's advice. The fuel is 5% nitro. I have seen some foaming in the fuel tank; but it is well away from the feed tube and the clunk.
Can I reduce or eliminate this fluctuation?
Another possible problem which is just beginning to worry me is the resonance at certain RPM bands with the test stand.
I'm working on my own well away from any club; so I will tend to ask lots of stupid questions, forgive me.
Best wishes
Mike
I've another problem. I'm trying to measure static thrust with different propeller sizes and I'm finding quite a bit of fluctuation with the RPM. An example:
Wuth the throttle connections locked down to give around 12,500 RPM, I get fluctuations of up to 300 RPM either side of 12500, i.e. from 12,200 to 12,800. The fluctuations appear to be random in size within this range and occur with a frequency of about half a second or a bit more
The effect is real as it shows up both with my Hobbico Mini-Tach and with the spring balance built into the test stand.
I am using the glow plug supplied with the motor, said in the manual to be an OS A3. The height of the tank appears to be correct in relation to the spray bar. I enclose two photos of the motor mounted in the test stand. I am using an APC 11x6 prop, following David Gierke's advice. The fuel is 5% nitro. I have seen some foaming in the fuel tank; but it is well away from the feed tube and the clunk.
Can I reduce or eliminate this fluctuation?
Another possible problem which is just beginning to worry me is the resonance at certain RPM bands with the test stand.
I'm working on my own well away from any club; so I will tend to ask lots of stupid questions, forgive me.
Best wishes
Mike
04-20-2009, 09:45 AM
#3
My Feedback: (11)
RE: OS 65LA Fluctuating RPM
1) Your test stand looks like it can move around a bit. That will cause some variations because it will allow vibration to affect the engine and fuel system.
2) To properly test RPM, you open the throttle to full, lean the engine to peak and check the RPM. You don't try to "lock" the engine to a given RPM. That's not reliable, and the engine won't really do a good job of holding the RPM there because them mixture won't be "dead on". If you want to test different propellers, you run at full throttle and lean to peak RPM and tach them...it's that simple.
3) It would be normal to find that some RPM levels will cause resonance in your test setup. That's most likely because it's not robust enough to require a much higher RPM to achieve resonance. The test stand we use has a custom-machined engine mount that involves over 50 pounds of steel. It can hold engines from .10-size up to the 86 cc gasoline engines. None of those engines can develop enough vibration to cause resonance on our test stand.
2) To properly test RPM, you open the throttle to full, lean the engine to peak and check the RPM. You don't try to "lock" the engine to a given RPM. That's not reliable, and the engine won't really do a good job of holding the RPM there because them mixture won't be "dead on". If you want to test different propellers, you run at full throttle and lean to peak RPM and tach them...it's that simple.
3) It would be normal to find that some RPM levels will cause resonance in your test setup. That's most likely because it's not robust enough to require a much higher RPM to achieve resonance. The test stand we use has a custom-machined engine mount that involves over 50 pounds of steel. It can hold engines from .10-size up to the 86 cc gasoline engines. None of those engines can develop enough vibration to cause resonance on our test stand.
04-20-2009, 01:50 PM
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RE: OS 65LA Fluctuating RPM
Thank you again, Bill. Very good answer.
The idea of measuring static thrust with different props at different RPM means my test stand must have some movement. So I have to accept both the vibration and the resonance.
You have told me in effect that these problems are inherent in my test setup and are not related to my lack of knowledge or an accidental arrangement of my test stand.
I enclose a graph show real, but unreliable, values for the APC 11x6 and imaginary values for an APC XxY. I'm sure I won't get my imaginary propeller; but it would be very nice to find a high thrust in midrange RPM like that.
The APC 11x6 values are real; but unreliable as I am still learning the idiosyncracies of my static dynomometer test stand. I can't blame anyone else: I designed and built it.
These posts should give a pretty good idea of what I'm on about. I welcome any comments or brickbats. Particularly, any suggestions to make my tests more useful.
Best wishes
Mike
The idea of measuring static thrust with different props at different RPM means my test stand must have some movement. So I have to accept both the vibration and the resonance.
You have told me in effect that these problems are inherent in my test setup and are not related to my lack of knowledge or an accidental arrangement of my test stand.
I enclose a graph show real, but unreliable, values for the APC 11x6 and imaginary values for an APC XxY. I'm sure I won't get my imaginary propeller; but it would be very nice to find a high thrust in midrange RPM like that.
The APC 11x6 values are real; but unreliable as I am still learning the idiosyncracies of my static dynomometer test stand. I can't blame anyone else: I designed and built it.
These posts should give a pretty good idea of what I'm on about. I welcome any comments or brickbats. Particularly, any suggestions to make my tests more useful.
Best wishes
Mike
