Urban myths

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Urban myths

Postby ghostflyer » 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.?
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Re: Urban myths

Postby IA DPE » Wed Sep 04, 2019 1:21 am

I routinely do Checkrides in a 1977 C-172N that has 12,000+ hrs on it. It looks and runs better than most rental airplanes with 1/3 the total time on them. I attribute this to the careful routine maintenance it receives (vs. most rentals), but also to the fact Cessna built/ builds good airplanes. Far more 172s have been built with four cylinder Lycomings than were built with six cylinder Continentals. If there truly was a problem, we’d know. If anything I’d say the Continental powered ones require more maintenance (due to their age) than the newer ones. I like the smoothness of my Continental but if I had a 180 hp Lycoming I’d be happier. :wink:
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Re: Urban myths

Postby GAHorn » Thu Sep 05, 2019 11:06 am

There are some sensitivities regarding this subject which naturally exist and there are indeed some myths being perpetuated. The STCs which install Lycoming O360 engines in 170s are not the same installation which Cessna uses on later 172s nor is the cowling the same. Considerable modifications were undertaken by Cessna to use vibration isolators in order to achieve the perceived smoothness in modern 172s. I don’t believe this is identical to the STC conversions. There are no vibration isolators between the cowling and the cabin on any of the converted 170s I’ve seen.

I’ll add that the TCM cowls certainly do suffer cracks, mostly due to the thin material used in the nose bowl and the fact the cowl is not isolated from the fuselage but is screwed directly to it. This is worsened when the Lycoming conversions are made. In fact the nose bowl is replaced with a heavier-duty fiberglass bowl which is possibly worse as far as cracking goes but might be easier repaired since it’s fiberglass. The ones I’ve seen repaired have not been successful. I’ve flown several of the Lycoming conversions and they are fun tho’.
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