Terminology Question

[OzzyJo]

I am sorry for asking such an elementary question but I have been somewhat confused by the different terminology used to describe WWII aircraft engine power ratings. Terms that I frequently see are:

Normal power
Military power
Combat power
Boost Power
War emergency power
Take off power

These are generally not clearly defined and some of the differences appear to reflect the different terminology used by the USAAF and RAF. Can anybody properly explain the differences?

 

[krieghund]

here's some definitions from 1946

 

[Shortround6]

I am sorry for asking such an elementary question but I have been somewhat confused by the different terminology used to describe WWII aircraft engine power ratings. Terms that I frequently see are:

Normal power
Military power
Combat power
Boost Power
War emergency power
Take off power

These are generally not clearly defined and some of the differences appear to reflect the different terminology used by the USAAF and RAF. Can anybody properly explain the differences?

I will take a shot at it. But please remember that in some cases the terminology also changed with time even in the same service.

Normal power= The normal maximum continuous power an engine was rated for. Continuous being 30 minutes or 1 hour or until the fuel runs out depending on the engine, the time in question or the service.

Military power= Usually a US term, the maximum power an engine could give for 5 minutes (occasional engines 15min) without any special maintenance procedures or notes made in the log book. It was usually equal in rpm and manifold pressure to the take-off rating.

Combat power= Usually a British term, the maximum power an engine could give for 5 minutes but, depending on the engine and amount of boost (manifold pressure) used would require notes made in the log book for each use for accelerated maintenance procedures (change of spark plugs, oil inspections, shorter times between overhauls, etc).

Boost Power= Another British term (?) pretty much like combat power. Many planes used an automatic boost or pressure limiter on the engine but gave the pilot an override button or switch. Hurricanes and Spitfires in the BoB were normally limited to 6lbs of boost but by hitting the override could use 12lbs. The use of the override switch would have to be noted in the log book.
I will admit I am mostly guessing on this one.

War emergency power= A US term that is about the same as combat power. Usually a 5 minute time limit and again the use of which would be noted in log books. in fact on some US planes the WEP setting could only be used by pushing the throttle lever through a tell-tale wire across the throttle and breaking it so the mechanics would know if it was used when the plane landed.

Take off power= The maximum power the engine was rated to give at sea level for, usually, 5 min. The use of this setting required no special notes or maintenance procedures. Depending on the engine this could be lower power than the engine was rated for at a higher altitude.

All of these power settings and time limits were subject to the engine staying within normal temperature limits and oil temperature limits.

Combat power-Boost Power-War emergency power, should be pretty close in how they are rated. this should be an engines highest rating but not all engines had this rating.

Military power-Take off power should be close, at least as far as rpm and manifold pressure. some engines were throttle back for take-off so that even given the same rpm and pressure they made less power for take-off than at high altitudes.

Normal power is the lowest of the group and in some cases is either maximum continuous climb or maximum cruise speed.

Hope this helps and that I have not steered you wrong.

 

[OzzyJo]

Thanks to both responses. They clear a few things up. A number of documents also refer to maximum power. Presumably this refers to Combat or Military power.