The International Cessna^® 170 Association

First off I am an A&P that is willing to learn outside of the box by simply listening to others and their experience.....unlike some others that I know. Here is the situation - I have a 1950 Cessna 170A - purchased it last year. I am loving the plane - and have been flying and fixing for the past year. Ever since I have owned the plane - it displays a weird if you will behavior during every flight. The plane runs fine - with an updated cockpit displaying fuel flow and all engine parameters nothing appears abnormal. As I level off after a climb - I get almost like a engine vibration and as the engine accels rpm wise either through leaning or just level flight - the momentary vib goes away. The flight test engineer in me wonders of a possible air flow/airframe anomoly - the mechanic in me questions the overall unknown health of the carb - as all other indications in the cockpit are status quote. Are there any thoughts or experience with this out there?

How accurate is your tach?
And exactly which RPM does this occur?
And exactly which model propeller do you run? (McCauley? Sensenich?...which sometimes has transient harmonics.)
Can you put the engine at that EXACT RPM and duplicate it for longer periods?

Have you seen any chafeing of the nose-cowl against the engine? (Air pressure can press the nose cowl to touch the engine and cause a momentary vibration, and also place pressures against the carb-air-box, until things settle down for cruise.)

Lastly, have you had your prop dynamically balanced?

I receive rpm through the engine analyzer - the sensor for rpm is installed on the right mag. The problem occurs around 2400-2450 and only if I am aggressive with pushing the nose over. I have never experienced this anywhere else in the flight profile. The prop is a Sensenich and has been balanced. I do not see any issues with cowling making engine contact. Any other thoughts?

I'm thinking this could be lots of things like windshield, wing fairings or anything that can vibrate.

How was the prop balanced? statically or dynamic balance. If it was dynamically balanced, they usually balance it for the least vibration at cruise and may not even run it at full power. Have you inspected the engine and firewall mount bushings and the mount itself?

Sensinich props are notorious for blade flexing. Since you are pushing hard over probably flexing blade angles.

kidalways wrote:... As I level off after a climb - I get almost like a engine vibration and as the engine accels rpm wise either through leaning or just level flight - the momentary vib goes away. ..?

kidalways wrote:... The problem occurs ... only if I am aggressive with pushing the nose over. I have never experienced this anywhere else in the flight profile. ...

Stop doing that.

When you abruptly change angle-of-attack by "pushing over" aggressively, the gyroscopic acton is hard on your crankshaft and propeller. (Can you imagine the stresses placed on that crankshaft when that large gyroscope disc (propeller) insists on remaining in-its-plane, yet you insist of sudden changes? Each time you do that you are requiring the crank to flex within it's bearings, and are stressing the main bearing supports.) You've adequately tested the crank and prop-flange for cracks, that's for sure....not to mention your engine/firewall mount.

Momentary vibrations which occur during normal RPM changes (such as leaning, throttle-changes, or normal changes of attitude which result in RPM changes) are not abnormal, as transient harmonics of all that machinery and attached sheet metal sound-boxes.

Keep in mind also, that your propeller creates whirlwind streamers which impact against the fueselage if the fuselage is suddenly manuevered into the propeller slipstream. Once things align again in the new position.... that impact disappears.