Vibration problem
Posted: Mon Aug 08, 2022 1:00 pm
Hi All,
I have a problem with vibration in flight. I’ve tried various things to rule out possible problems as described below and I would appreciate any feedback or ideas. I have not flown any 170 but this one, nor any early C172s with the O-300 so I have no feel for what it’s performance should be so again, please feedback how yours compares.
Thanks,
Gareth.
----------------------
1948 Ragwing with O-300-BCC (B converted to a C using a replacement crank) McCauley 1C172/EM7653
Run-up is perfect, the engine sounds and feels great. Appropriate drops on mag checks and carb heat. Oil pressure and temperature consistently green. No vibration felt on the ground.
Static power check yields about 2220RPM on the electronic tach – the needle just passes 2200 on the mechanical tach, so I would call it just shy of the low end of the range. Field is at 300ft, OAT 23C. Range from the C172A TCDS (O-300 C with this prop):
McCauley 1C172/EM 7652, 53, or 55
Static rpm at maximum permissible throttle setting: Not over 2350, not under 2230
Take-off power is smooth and she’s off comfortably in 300m (<1000ft) with 250lbs of overweight pilot and half tanks. At full power in level flight the engine never gets over 2450 RPM – balls to the wall 2500ft MSL and on take-off (70mph IAS) never exceeds 2350.
I ran a rich test – set 800 RPM (on the ground) with the engine nicely warm after a short flight and then pulled the mixture quickly. I would expect at 300ft MSL field height to see about 50RPM increase in RPM before it starts to die which would show the carb to be set rich of peak at full rich. However, there is no such increase, it just dies. Thus the carb could be set a little richer, but I don’t think it’s running lean enough to cause the vibration (which is only perceptible in flight).
The vibration problem did not occur on the first couple of test flights. It did then occur on the third flight when we let the fuel get low while operating on the L tank (we wanted to drain it). Thinking it was related to that, on the next flight I filled both tanks to the brim. Flew 2.5 hours of circuit training in 2 sessions without vibration. I also flew a 100mile trip without problem. Then …
It has happened as follows quite consistently (all performed flying orbits over the field):
From about 300ft AGL (or 1 min into the flight) there is a noticeable vibration in the aircraft – as in shaking the water in the compass and making me uncomfortable! At first I thought it might be a resonance at a specific RPM so changed RPM to max or pulling back to 1900 for slow flight, the vibration remains.
Leaning has no effect on the vibration.
Running at max power – added carb heat: power dropped as expected and the vibration seemed worse. However, repeating this yesterday at 2200RPM the carb heat does not change the vibration.
Mag check – climbed to 2000 ft throttled back. Cylinders 2 gets hotter than the rest (445F) at full power on the climb and inspecting the plugs it is running a bit leaner than the others (Number 3 runs hotter than the rest but not as much as 2). I've read George's description of the induction system's inconsistencies, so this does not appear unusual. Pulling back to cruise power of 2200 and leveling off, all are back to the green in <30 seconds. Max allowed per TCM = 525F. As cylinder 2 is consistently hotter than the rest I replaced the intake gasket and ensured the jubilee clips on the rubber sleeve on the inlet for 2 are snugged up nicely. No change.
At 2000 ft I let all EGT and CHT even out – and they are very even and consistent. Switched off L mag and flew 2 mins on R mag only. Dropped 60ish RPM as expected and no engine roughness. No cylinder readings (EGT CHT) any different from the others. Back to both, then R off, flew on L mag only … same. No change in vibration.
Swapped the plugs from the hot cylinder (2) with the top plugs from 4 and 6. No change.
I’m stuck and hoping someone here can help:
1. I can’t see how it could be an ignition problem – or is it?
2. While the engine is running slightly lean on the ground – it’s nothing to write home about and I can’t see how it could cause the vibration.
3. Fuel flow at take-off is about 11gph and in cruise at 2400RPM is 8.5gph. Seems about right?
4. I have considered a prop imbalance, but would I not feel that on the ground, especially at max (static) power? Also, this vibration didn't happen for about 3.5 hours of operation.
5. There is nothing loose on the airframe. All bolts have been replaced and torque striped and everything remains attached correctly.
I have a problem with vibration in flight. I’ve tried various things to rule out possible problems as described below and I would appreciate any feedback or ideas. I have not flown any 170 but this one, nor any early C172s with the O-300 so I have no feel for what it’s performance should be so again, please feedback how yours compares.
Thanks,
Gareth.
----------------------
1948 Ragwing with O-300-BCC (B converted to a C using a replacement crank) McCauley 1C172/EM7653
Run-up is perfect, the engine sounds and feels great. Appropriate drops on mag checks and carb heat. Oil pressure and temperature consistently green. No vibration felt on the ground.
Static power check yields about 2220RPM on the electronic tach – the needle just passes 2200 on the mechanical tach, so I would call it just shy of the low end of the range. Field is at 300ft, OAT 23C. Range from the C172A TCDS (O-300 C with this prop):
McCauley 1C172/EM 7652, 53, or 55
Static rpm at maximum permissible throttle setting: Not over 2350, not under 2230
Take-off power is smooth and she’s off comfortably in 300m (<1000ft) with 250lbs of overweight pilot and half tanks. At full power in level flight the engine never gets over 2450 RPM – balls to the wall 2500ft MSL and on take-off (70mph IAS) never exceeds 2350.
I ran a rich test – set 800 RPM (on the ground) with the engine nicely warm after a short flight and then pulled the mixture quickly. I would expect at 300ft MSL field height to see about 50RPM increase in RPM before it starts to die which would show the carb to be set rich of peak at full rich. However, there is no such increase, it just dies. Thus the carb could be set a little richer, but I don’t think it’s running lean enough to cause the vibration (which is only perceptible in flight).
The vibration problem did not occur on the first couple of test flights. It did then occur on the third flight when we let the fuel get low while operating on the L tank (we wanted to drain it). Thinking it was related to that, on the next flight I filled both tanks to the brim. Flew 2.5 hours of circuit training in 2 sessions without vibration. I also flew a 100mile trip without problem. Then …
It has happened as follows quite consistently (all performed flying orbits over the field):
From about 300ft AGL (or 1 min into the flight) there is a noticeable vibration in the aircraft – as in shaking the water in the compass and making me uncomfortable! At first I thought it might be a resonance at a specific RPM so changed RPM to max or pulling back to 1900 for slow flight, the vibration remains.
Leaning has no effect on the vibration.
Running at max power – added carb heat: power dropped as expected and the vibration seemed worse. However, repeating this yesterday at 2200RPM the carb heat does not change the vibration.
Mag check – climbed to 2000 ft throttled back. Cylinders 2 gets hotter than the rest (445F) at full power on the climb and inspecting the plugs it is running a bit leaner than the others (Number 3 runs hotter than the rest but not as much as 2). I've read George's description of the induction system's inconsistencies, so this does not appear unusual. Pulling back to cruise power of 2200 and leveling off, all are back to the green in <30 seconds. Max allowed per TCM = 525F. As cylinder 2 is consistently hotter than the rest I replaced the intake gasket and ensured the jubilee clips on the rubber sleeve on the inlet for 2 are snugged up nicely. No change.
At 2000 ft I let all EGT and CHT even out – and they are very even and consistent. Switched off L mag and flew 2 mins on R mag only. Dropped 60ish RPM as expected and no engine roughness. No cylinder readings (EGT CHT) any different from the others. Back to both, then R off, flew on L mag only … same. No change in vibration.
Swapped the plugs from the hot cylinder (2) with the top plugs from 4 and 6. No change.
I’m stuck and hoping someone here can help:
1. I can’t see how it could be an ignition problem – or is it?
2. While the engine is running slightly lean on the ground – it’s nothing to write home about and I can’t see how it could cause the vibration.
3. Fuel flow at take-off is about 11gph and in cruise at 2400RPM is 8.5gph. Seems about right?
4. I have considered a prop imbalance, but would I not feel that on the ground, especially at max (static) power? Also, this vibration didn't happen for about 3.5 hours of operation.
5. There is nothing loose on the airframe. All bolts have been replaced and torque striped and everything remains attached correctly.