Studying the P47's dive performance in DCS, I noticed how it's easy to damage the engine bearings if you windmill the prop at high speed. Doing some research, I discovered that all radial engines are prone to this type of damage.

If, because of insufficient gas load (manifold pressure), the propeller is allowed to drive the engine, the resultant force on the crankshaft is applied to the opposite side of the crankpin, where the oil supply is not optimised, and may quickly damage the bearing. This damage worsens over time and eventually the bearing will fail. Failure may occur several hours after the initiating event, and therefore a pilot may inadvertently damage the bearing without seeing any immediate symptoms requiring maintenance intervention. The engine is designed to cope with some reverse loading for brief periods, for example when the propeller is driven at lower airspeed when landing, but critical damage may occur quickly at higher speeds

Source: AAIB Bulletin: 10/2021 / G-INVN / AAIB-26839 (PDF)

I can't understand at 100% why V, Inline , boxer ecc.. engines don't suffer of this problem too. Probably the reason is due the load distribution that's completely different, but not sure.

enter image description here

Other source: https://forums.eagle.ru/topic/260841-engine-bearings-and-how-you-can-keep-them-happy/

  • 1
    $\begingroup$ Why do you say non-radials don't suffer the same? Share your research please. $\endgroup$
    – user14897
    Sep 30, 2021 at 8:16
  • 3
    $\begingroup$ If you read the second link carefully, it describes everything quite precisely, so I'm not sure why you're posting this question.The master rod is getting loaded on the opposite side of the oil feed hole when the prop is back driving, and the pressure on that side is insufficient to keep the metal apart under high backdriving loads,which are all concentrated on just two master rod bearings for all cylinders.Other engines don't suffer from this because those loads are distributed across individual connecting rods and the pressures trying to squeeze out the oil at the rod big end are much lower. $\endgroup$
    – John K
    Sep 30, 2021 at 14:18
  • 1
    $\begingroup$ I agree with @John but I'm not convinced by the explanation in the article, as this reverse load also occurs when the engine is decelerated, not only when windmilling. $\endgroup$
    – mins
    Sep 30, 2021 at 17:20
  • 1
    $\begingroup$ I think I understand the concept of that is going on.The master rod's bearing is being pushed on by the articulating rods in sequence, which tends to make them push on the same point on the crank pin, so if you clock the oil hole in the crank pin at spot X, that spot is always facing the articulating rod of the cylinder being fired as they go around, and therefore when back driving, the crank pin is now pushing on the master rod bearing at a point 180 deg away opposite the crank pin's oil hole.Normally the loads have declined where this doesn't matter, except in an extreme high speed windmill. $\endgroup$
    – John K
    Sep 30, 2021 at 22:02
  • 2
    $\begingroup$ This all could be condensed into an answer, but oil deprivation seems to be key. A straight crankshaft may have oil splashing equally on all sides, whereas the radial is famous for oil tending towards the bottom. I'm also seeing all pistons turning one wheel on the radial. With the inline, the heavy crankshaft can be absorbing differences in piston pressure better. $\endgroup$ Oct 2, 2021 at 13:48


You must log in to answer this question.

Browse other questions tagged .