Minimum Manifold Pressure?

[VulcanB2 5]

Hi,

I understand that there is a risk when coming from cruise that you could shock-cool the engine, but if the engine hasn't been running at a particularly high temperature, is there a minimum throttle limit?

I know you shouldn't let it drop below 1,000 RPM on the ground, but I'm interested about in-flight. It appears there is a possibility to get the engine wind-milling and damaging the engine (detonation occurs I believe)?

Best regards,

Robin.

[Finn 873]

Hi,

I understand that there is a risk when coming from cruise that you could shock-cool the engine, but if the engine hasn't been running at a particularly high temperature, is there a minimum throttle limit?

I know you shouldn't let it drop below 1,000 RPM on the ground, but I'm interested about in-flight. It appears there is a possibility to get the engine wind-milling and damaging the engine (detonation occurs I believe)?

Best regards,

Robin.

I´m not sure about the Catalina, but on the DC-3 there was a rule never to go below square - meaning that with prop RPM at 2050, MAP was not reduced below 20.5 inHg before retarding the the throttle for landing, cause the bearings didn´t like the excessive backpressure when windmilling.

Finn

[William G 11]

You are referring to what is know as "reciprocating load" also known as under-boosting. where the prop is said to drive the engine. When the DC-3 was in airline service it was a big deal because it could cost the airline a lot of money in overhauls.

With the radial engine each cylinder in a row attaches to a master rod so every time a cylinder fires that load impacts the same spot on the crankshaft. So much so that there is a hole designed specifically to supply oil to that one spot. If you should pull the power back and the slip stream puts back pressure on the engine that impact is on the reverse side of the crank shaft where there is no extra oil and it causes a lot of wear over time. It may not fail the engine at that moment if at all but it dramatically reduces time between overhauls.

There are many radial engine operators even today that don't understand the concept that well. The whole issue was greatly complicated by World War 2 where the military made manuals for aircraft based on the fact that the crew were likely to be young low time pilots and the engines were expected to be run hard without concern for engine life. Time between overhauls were often 50 hours or less.

An example would be rpm on downwind, A major airline at the times SOP would be cruise rpm with MAP over square on downwind and not advancing rpm until short final or even after touchdown. The military would have that same engine advanced to go around rpm settings on downwind so the inexperienced crew would have one less thing to worry about if they have to go around. After the war most DC-3s were actually C-47s with the military manuals still out there and those type operations were carried on by many.

Ideally an airline of the time when radial engines were state of the art would only reduce the manifold pressure two inches if that much when descending enroute and not drop below square until short final unless absolutely necessary. Warbird operators that are well informed also practice this technique. That's on the DC-3 with 1830s how much of an issue it is with Cat drivers I am not familiar with but I would bet they don't pull that much power off on descent if they don't have to.