Cessna 150 full throttle above redline

Dan Thomas said:
The propeller suffers much more than the engine. The Formula 1 racers run those O-200s up to 4000 RPM with a small prop.

The propeller has to withstand some of the highest forces in the entire airplane. Centrifugal forces are huge. A 150 RPM overspeed is a 5.5% overspeed, which translates into a 10% increase in centrifugal stresses. A 10% overspeed is a 21% increase in forces. The propeller makers get nervous about that and want NDI done of the prop. 10% for five seconds, IIRC. It happens sometimes in spin recoveries, when a student gets confused between stall recovery (full throttle) and spin recovery (throttle closed).

Dan
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There isn't a 150 in the fleet that hasn't exceeded redline by that margin in it's lifetime. Few rentals probably go a month without doing it.
 
jesse said:
There isn't a 150 in the fleet that hasn't exceeded redline by that margin in it's lifetime. Few rentals probably go a month without doing it.
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Really? A 10% overspeed is 3025 RPM. In my years of looking after flight school 150s (and 172s and Citabrias) and with instructors and students who weren't afraid to report to me if they'd done something stupid, I ran into one 10% overspeed, and that was for less than 5 seconds.

Dan
 
Dan Thomas said:
Really? A 10% overspeed is 3025 RPM. In my years of looking after flight school 150s (and 172s and Citabrias) and with instructors and students who weren't afraid to report to me if they'd done something stupid, I ran into one 10% overspeed, and that was for less than 5 seconds.

Dan
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We weren't talking about a 10% over speed.
 
jesse said:
If overspending by 150 rpm on a O200 C150 hurt the airplane we wouldn't have any 150s left flying...
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I thought overspending was normal in aviation. :wink2:
 
denverpilot said:
Agreed. That's usually the problem.

Quick and dirty method for double checking before going to the shop...

Go out at night and park where you can see the prop strobing in the airport's sodium vapor lights.

They're usually fed with 60Hz power.

This usually works best with the light behind the aircraft backlighting the prop, and a nice dark area in front of you.

As you approach RPM numbers divisible by 60... 600, 1200, 1800, 2400... You'll see the prop "freeze" and it'll look like the blades are standing still.

The electric power grid is a VERY accurate 60 Hz.

Stop the prop in the light via the strobe effect, which will take some VERY small tweaks on the throttle (I recommend 1200 RPM, since some engines won't run well at 600, and the higher numbers will just suck crap into the prop and airplane) and then note the RPM gauge, trying to lean over and remove any parallax involved by not viewing the gauges dead-on straight.

Works well. My tach was off approximately 50 RPM. We were able to tell our shop which direction also.

Kinda nice to know without investing money in an optical prop tach. :)

If it's off by a significant margin, turn it over to the shop to determine if the tach needs adjustment or what the problem might be.

The only thing that's hard to check, is redline... if it's not divisible by 60. And of course, because of the aforementioned FOD issue. Let the shop deal with that... they ding up the prop, they can always be asked to dress it... ;)
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Just for the hell of it I checked the Flybaby tach last night using the airport lights and it was accurate to within 10 rpm or so. Took me all of about ten seconds to test.
 
jesse said:
Just for the hell of it I checked the Flybaby tach last night using the airport lights and it was accurate to within 10 rpm or so. Took me all of about ten seconds to test.
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What RPM did you check it at? (I think the error needs to be expressed as a percentage if it's going to be extrapolated to other RPMs.)
 
jesse said:
1,200
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Presumably that would extrapolate to a 20 RPM error at 2400 RPM - still doesn't sound a big deal (although I have no expertise on that).

Lately, I've seen a couple of rental planes with variable pitch props that indicate 100 RPM over red line during takeoff, and sometimes more.
 
Palmpilot said:
Presumably that would extrapolate to a 20 RPM error at 2400 RPM - still doesn't sound a big deal (although I have no expertise on that).

Lately, I've seen a couple of rental planes with variable pitch props that indicate 100 RPM over red line during takeoff, and sometimes more.
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The error isn't always linear. I have encountered tachs that were fine at the lower revs and off at cruise RPM. Most tachs will under-read when they get old; the magnet in them weakens and doesn't drag the cup as far around, and the indicator needle is attached to that cup. Last month I encountered a tach that over-reads by 100 RPM at cruise, an unusual condition, caused by ancient and stiff old grease in the little needle steady bearing that the cup shaft uses in the input shaft.

Constant-speed props that over-rev should get the tach checked first, and if it's OK, the prop governor needs resetting.

Dan
 
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