Urban myths
Posted: Wed Sep 04, 2019 12:55 am
We have all heard about the “ the shakey four banger [lycoming engines] “ and how they allegedly crack windscreens and cowls and other important fixtures on a 170. It doesn’t matter what engine that you have fitted up front ,vibration is a destroyer of good parts . So let’s have the engine and prop, and associated parts dynamically balanced from the start.
So do we assume that 6 bangs are better than 4 bangs . It all depends on the engine design [counter weights on the crankshaft {fixed or floating} ,propellor and it’s design and materials of construction and speed of the engine [RPM] , mounting systems of the engine to the airframe. Pilot operation has some influence also.
Then we look at the present , Cessna 172,s are now lycoming fitted. Is there a increase in cracking of components or extra maintenance. While Cessna,s choice in materials has also changed in the later models. While 2024T3 was the flavour in past years , a newer alloy 6013T4 has been substituted that is not so prone to cracking or corrosion and is 10%to 15% stronger [ plus it’s weldable ] . In the STC for fitment of the lycoming into the 170 it is mimicking the later Cessna 172,s in construction and materials. Sound harmonics also play their part in vibrating airframes and can be easily blamed on the engine for the vibration. Plus some airframe parts not supported correctly can vibrate and mimics engine vibration. Eg, had a radio rack in a Cessna 152 not supported and as engine RPM increased the vibration increased to the point it was effecting the instruments . It felt the engine was about to tear it self out of the airframe. The rotating mass was balanced but this vibration was still there but it’s frequency was different to a engine vibrating frequency . Then it was though to be a airframe vibration as it was being felt in the rudder pedals also. Then a eagled eyed mechanic saw the radio rack trying to tear itself away.
So is the reality of all this the shakey four banger does it really exists or is it lack of maintenance or incorrect operation.?
So do we assume that 6 bangs are better than 4 bangs . It all depends on the engine design [counter weights on the crankshaft {fixed or floating} ,propellor and it’s design and materials of construction and speed of the engine [RPM] , mounting systems of the engine to the airframe. Pilot operation has some influence also.
Then we look at the present , Cessna 172,s are now lycoming fitted. Is there a increase in cracking of components or extra maintenance. While Cessna,s choice in materials has also changed in the later models. While 2024T3 was the flavour in past years , a newer alloy 6013T4 has been substituted that is not so prone to cracking or corrosion and is 10%to 15% stronger [ plus it’s weldable ] . In the STC for fitment of the lycoming into the 170 it is mimicking the later Cessna 172,s in construction and materials. Sound harmonics also play their part in vibrating airframes and can be easily blamed on the engine for the vibration. Plus some airframe parts not supported correctly can vibrate and mimics engine vibration. Eg, had a radio rack in a Cessna 152 not supported and as engine RPM increased the vibration increased to the point it was effecting the instruments . It felt the engine was about to tear it self out of the airframe. The rotating mass was balanced but this vibration was still there but it’s frequency was different to a engine vibrating frequency . Then it was though to be a airframe vibration as it was being felt in the rudder pedals also. Then a eagled eyed mechanic saw the radio rack trying to tear itself away.
So is the reality of all this the shakey four banger does it really exists or is it lack of maintenance or incorrect operation.?