Cessna 172C Performance

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CpnCrunch
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Re: Cessna 172C Performance

#26 Post by CpnCrunch » Tue Jan 30, 2018 12:17 pm

PilotDAR wrote:
Tue Jan 30, 2018 12:12 pm

If you believe that your engine will not meet the manual standards, then you have made the determination that the engine is not airworthy, and you should not fly the plane. It could not be more simple.
Well, first of all you need to run-up and then (perhaps) fly the plane to see how it flies. It's probably best not just jumping into a plane you don't know, filling it up to full gross, and blasting off from a short runway on a hot day :)

Is there any way a pilot can determine whether the engine meets the standards (other than reaching the required static rpm)?
Will the static rpm guarantee that the engine is delivering the certified power if you don't have a MP gauge?
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photofly
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Re: Cessna 172C Performance

#27 Post by photofly » Tue Jan 30, 2018 3:39 pm

Upon discussion, someone was kind enough to point out to me that prop condition can make a difference: a beat up prop can lose some (a lot?) of efficiency in turning torque into thrust.

Also: for a fixed pitch prop you can get a good power check by noting the full-throttle RPM at the start of the takeoff run. (That's why it's noted in the POH). If you get the proper RPM, your engine is making near enough the rated power.
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Re: Cessna 172C Performance

#28 Post by CpnCrunch » Tue Jan 30, 2018 4:36 pm

photofly wrote:
Tue Jan 30, 2018 3:39 pm
Upon discussion, someone was kind enough to point out to me that prop condition can make a difference: a beat up prop can lose some (a lot?) of efficiency in turning torque into thrust.

Also: for a fixed pitch prop you can get a good power check by noting the full-throttle RPM at the start of the takeoff run. (That's why it's noted in the POH). If you get the proper RPM, your engine is making near enough the rated power.
Given that an engine attached to a fixed pitch prop never develops anywhere near full power on takeoff, I'm somewhat skeptical that the full throttle static runup is a reliable indicator of engine health.
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Re: Cessna 172C Performance

#29 Post by photofly » Tue Jan 30, 2018 5:19 pm

A full throttle run up reaching (say) 2300rpm requires more power than the same engine develops in cruise flight at 2300rpm (perhaps 65% power) because the prop is harder to turn when the aircraft isn't moving, and shaft power is torque x rpm. So it's not 100% rated power, but it's a significant proportion of 100% power.

I'm trying to think of some kind of engine defect that *wouldn't* affect the static rpm but would prevent the engine reaching full power. Some very specific magneto fault that only shows up at >2300rpm perhaps? But that would be a very specific fault, wouldn't it? Anything like a mis-timed mag, blocked induction system or fuel flow blockage should show up on a static check, no?

If we're talking only about takeoff roll distance, the static rpm is a very good proxy for engine health during that manoeuvre, since the rpm never reaches red-line (and the engine never develops 100% power) until the aircraft is settled in a fast cruise climb, at the earliest.
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Re: Cessna 172C Performance

#30 Post by PilotDAR » Tue Jan 30, 2018 5:45 pm

Yup, Photofly has it. The full throttle static RPM is the required demonstration of engine performance for a fixed pitch prop aircraft, when that RPM is stated as the requirement.

For example, have a look at page 2 of the 172TCDS here: http://rgl.faa.gov/Regulatory_and_Guida ... Rev_84.pdf

You'll see here, and throughout the TCDS, references to static RPM requirements. This is the pilot's way of knowing that a fixed pitch prop plane they are flying is making rater power at full throttle. As Photofly states, you're not going to get full engine red line RPM still on the ground, as the prop blades may be somewhat aerodynamically stalled, and creating lots of drag. That drag prevents maximum engine speed. If the correct prop for the plane will exceed the maximum stated RPM, I'd be looking at the prop condition, has it been reworked to be toothpick blades? If the engine won't make the minimum of the stated RPM, I'd be looking at engine condition - low compression cylinders. If an interested pilot wants to check for that, while employing all cautions about turning propellers of stopped engines, listen at the exhaust and crankcase breather for blowby - poor exhaust valve seating or rings. If you've gotten that far, again, great caution turning props, you may just feel a flat cylinder, it takes no muscle to pull the engine through that compression stroke.

The reason that the maximum RPM is limited to a lower value still on the ground is that there is also a prop overspeed requirement in a power off dive, and that maximum RPM assures compliance with that too. I have flown 172's in which a full throttle takeoff was around 2350 RPM at the beginning, and the throttle had to be pulled back a little in cruise flight to prevent an overspeed.

No, for a fixed pitch prop equipped engine, the MP gauge is of no value, as long as the prop conforms as demonstrated above. If you did have an MP gauge on your fixed pitch propeller engine, it would rise and fall nicely with throttle setting - so what's the point? And, What would it's limitations be anyway?

Not all TCDS for fixed pitch prop planes state static RPM limits, but if in doubt, its the first and best place to look for an uncertain pilot. The flight manual may not present this information, as it is not limiting in the pilot operating sense, it's really a maintenance activity.

The only factor which could affect engine power at altitude, while not on [lower altitude] ground could be bad mag leads. On the ground, the dense air is too much for the spark to jump across broken mag lead insulation. But, at altitude, the less dense air allows the spark to jump to ground before it gets to the spark plug business end. I had this in my 150 at about 9000 feet, when I first bought it. New mag leads, and never a problem since. That's rarely a factor for takeoff of a 172 in North America, but, because someone asked...
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