Draker
Well Known Member
Finally got around to doing a max static RPM test prior to first flight of my -7A.
First of all, side topic: Tying down an -A model by the gear legs. Since the gear legs sweep back on the -A, you can't just tie strap around the gear leg near the fuselage mount because the strap would slip down the leg when engine power was applied. I was also not comfortable strapping near the wheels because that results in a very long torque arm to the center of force. So, I called up an absolute genius and he suggested since the legs also sweep outward, wrap a third strap tightly around both legs, below the main straps. That third strap will keep the main straps from slipping down. Perfect! Strapped down, chocked and full-brakes, the plane didn't go anywhere (or buck around or even move) during the test.
Anyway, results:
New Lycoming IO-360-M1B and Hartzell C2YF-1BFP constant speed prop. Prop knob full forward, mixture full rich. Slowly advanced throttle. RPM smoothly rose to 2790, then after around 2 seconds, abruptly dropped to 2330, and stayed there for the remainder of the test. MP took 14 seconds to reach 28.7 and remained between 28.7 and 28.9 through duration of test (LVK altitude 400ft). I did not try adjusting the prop cable or leaning the mixture. Ended the test at about 60 seconds to keep CHT from exceeding 350F.
[Visual plot attached to this post]
So, that's a bummer. I'm not going to take off on my first flight barely making cruise RPM. Here are some ideas I have:
Anything else to try that I'm missing? I'm really hesitant to start messing around with factory-set stop screws.
First of all, side topic: Tying down an -A model by the gear legs. Since the gear legs sweep back on the -A, you can't just tie strap around the gear leg near the fuselage mount because the strap would slip down the leg when engine power was applied. I was also not comfortable strapping near the wheels because that results in a very long torque arm to the center of force. So, I called up an absolute genius and he suggested since the legs also sweep outward, wrap a third strap tightly around both legs, below the main straps. That third strap will keep the main straps from slipping down. Perfect! Strapped down, chocked and full-brakes, the plane didn't go anywhere (or buck around or even move) during the test.
Anyway, results:
New Lycoming IO-360-M1B and Hartzell C2YF-1BFP constant speed prop. Prop knob full forward, mixture full rich. Slowly advanced throttle. RPM smoothly rose to 2790, then after around 2 seconds, abruptly dropped to 2330, and stayed there for the remainder of the test. MP took 14 seconds to reach 28.7 and remained between 28.7 and 28.9 through duration of test (LVK altitude 400ft). I did not try adjusting the prop cable or leaning the mixture. Ended the test at about 60 seconds to keep CHT from exceeding 350F.
[Visual plot attached to this post]
So, that's a bummer. I'm not going to take off on my first flight barely making cruise RPM. Here are some ideas I have:
- Tachometer error
I'm using the screw-in mag reader that comes with the Garmin stuff. I wonder if mine has any error. Might have to verify with an independent measurement.
- Throttle control not hitting stop
Pretty sure I checked this many times during the build and condition inspection. I did adjust the idle RPM, but that should not affect max RPM. MP is also ~29 so I don't think the throttle is the problem.
- Prop control not hitting stop on governor
Same as above but it wouldn't hurt to check.
- Is the prop or the governor limiting RPM?
I should have slowly pulled back the Prop knob during the test. That would have told me if the prop's stop was limiting RPM or if the governor was in charge. I'll do that next time.
- Governor high RPM screw (per MT's manual) or prop low pitch stop (Hartzell)
This is where I'm starting to get cautious. These are supposed to come correctly adjusted from the factory, no? Hartzell also lists "stop adjustment" as corrective for overspeed but not underspeed.
- Governor oil pressure/passage problem
This might be beyond my ability to troubleshoot.
- Incorrect test procedure
Hartzell recommends: a. backing out the RPM stop on the governor one turn during the test, b. going full power with the prop control about 1" out, and then c. advancing the prop control in until RPM stabilizes. I didn't want to mess with the RPM stop on a brand new install (in case it was set correctly) so I did not use this procedure.
Anything else to try that I'm missing? I'm really hesitant to start messing around with factory-set stop screws.