5-minute limit at Take-off Power and Combat Merlin 65,66,67

[ofp85]

I am sorry, I badly speak in English.
I live in Russia.
Tell please, whether it was possible, in air battle to use Take-off Power longer than 5-minutes.
Whether there are facts of use of mode "Take-off Power" or "Combat" is longer than 5 minutes at fighters the Mustang and Spitfire?
Why such restriction of time of use of the maximal capacity is established?
My friends speak pilots of fighter Spitfire could use in battle in air +12 lbs boost, and +18 lbs boost could use only 5 minutes.
I think it not so.
Tell please as was actually.

 

[FLYBOYJ]

You didn't want to run the engine longer then 5 minutes Take off Power because you could rapidly heat up the engine and cause internal damage as well as warp the cyliner heads and place the engine in a situation where the cooling system couldn't handle the excess heat (in-line engines). You also had a supercharger that could pump too much air into the engine's intake manifold during these periods as well.

+12 and +18 sounds kind of low. I would suspect between +30 and +60

 

[ofp85]

In America pressure measure in millimeters of a mercury column
For example for the engine v-1650-7 67 " hg
In England add to existing, for example +18lb
I have one certificate of an opportunity of work of the engine v-1650 in mode Take-off Power longer than 5 minutes
From the Mustang pilot's notes:

It is often asked what the consequences will be if the 5-minute limit at Take-off Power is exceeded. Another frequent inquiry is how long a period must be allowed after the specified time limit has elapsed until Take-off Power can be used. These questions are difficult to answer, since the time limit specified does not mean that engine damage will occur if the limit is exceeded. Instead, the limit means that the total operating time at high power should be kept to a reasonable minimum in the interest of prolonging engine life.

It is generally accepted that high-power operation of an engine results in increased wear and necessitates more frequent overhaul than low-power operation. However, it is apparent that a certain percentage of operating time must be at full power. The engine manufacturer allows for this in qualification tests in which much of the running is done at Take-off Power to prove ability to withstand the resulting loads. It is established in these runs that the engine will handle sustained high power without damage. Nevertheless, it is still the aim of the manufacturer and to the best interest of the pilot to keep within reasonable values the amount of high-power time accumulated in the field. The most satisfactory method for accomplishing this is to establish time limits that will keep pilots constantly aware of the desire to hold high-power periods to the shortest period that the flight plan will allow, so that the total accumulated time and resulting wear can be kept to a minimum. How the time at high power is accumulated is of secondary importance; i.e., it is no worse from the standpoint of engine wear to operate at Take-off Power for one hour straight than it is to operate in twelve 5-minute stretches, provided engine temperatures and pressures are within limits. In fact, the former procedure may even be preferable, as it eliminates temperature cycles which also promote engine wear. Thus if flight conditions occasionally require exceeding time limits, this should not cause concern so long as constant effort is made to keep the over-all time at Take-off Power to the minimum practicable.

There are still similar facts or stories of veterans?

 

[FLYBOYJ]

I've worked on large in-line and recips, this sounds about right

 

[ofp85]

Five minutes or more than five minutes. This question important for a fair estimation of opportunities of fighters Mustang and Spitfire. I do not know why but nominal and maximal capacity differed at these fighters considerably. Such big difference is not present at Russian engines for example M-82 and M-105
One more certificate of an opportunity to work on high-power for a long time and reliably for Marylin engine.
From the air ministry instructions to Spit V pilots, August 1942 (Prune's guide for living)

When being briefed, always ask at what revs and boost you should fly. This will naturally depend on the length of the sweep, but don't forget that;

a) when hard pressed you can fly 16 lbs boost 3000 rpm without any danger of blowing up, but

your consumption will be 150 gallons per hour.


Finally, when unlikely to be engaged, fly minimum revs and under 4 lbs boost, but when in the vicinity of Huns, fly maximum everything and in good time.

 

[FLYBOYJ]

It might of varied based on the size and routing of coolant lines and supercharger duct routing. Engineers at the factory would determine this requirment, that probably explains the difference

 

[ofp85]

Our pilots flying in WW2 on P-39 and P-40 c engines Allison V-1710, told that did not observe the instruction and used high-power the engine so much how many it was necessary in battle.
Some pilots at all did not know about what existence or time limits
Many our pilots spoke " Or you observe maintenance instructions of the engine or you knock out fascists. "
In result V-1710 in 50 hours of such extreme operation became unusable.
Later the American engineers have improved V-1710 and the resource became a little bit higher.
How pilots of Mustangs and Spitfire used the engine?

 

[FLYBOYJ]

There a two-sided issue here; In the heat of combat you do what you need to do to fight, survive and win, but when you get back to your base you have to deal with your maintenance officer. When you're burning up engines every 25 or 50 hours I'm sure someone is going to want an explination!

 

[DaveB.inVa]

Its all relative, you've got to do what you've got to do. Your supposed to keep takeoff or War Emergency power down as doing so will reduce engine life. However if it means running on WE power to get you home or out of harms way for more than the alloted time then why not do it? If not running on WE meant a lost crew and aircraft, whereas running all the way home on WE only means engine replacement.

There are plenty of examples of just this thing. I've read of a B-17 crew losing one engine while heading on a Shuttle mission over Germany through Russia. The engine was lost fairly early in the mission and to keep up with the bomber group they had to run on WE power the rest of the way. The engines handled it just fine but we're goners in the end.

I know of another case where a B-29 ran on WE for about 5 hours until it reached Iwo Jima. The pilot said he thought the engines might not make it but it was their last ditch effort. If he cut power they wouldnt make Iwo anyway, so why not try.

 

[FLYBOYJ]

Its all relative, you've got to do what you've got to do. Your supposed to keep takeoff or War Emergency power down as doing so will reduce engine life. However if it means running on WE power to get you home or out of harms way for more than the alloted time then why not do it? If not running on WE meant a lost crew and aircraft, whereas running all the way home on WE only means engine replacement.

There are plenty of examples of just this thing. I've read of a B-17 crew losing one engine while heading on a Shuttle mission over Germany through Russia. The engine was lost fairly early in the mission and to keep up with the bomber group they had to run on WE power the rest of the way. The engines handled it just fine but we're goners in the end.

I know of another case where a B-29 ran on WE for about 5 hours until it reached Iwo Jima. The pilot said he thought the engines might not make it but it was their last ditch effort. If he cut power they wouldnt make Iwo anyway, so why not try.

Yep! and also know for a fact that once an engine is properly broken in, running it on the high side of the power curves (high RPMs, high power) is actually GOOD for it, providing its properly maintained (frequent oil changes, spark plug cleaning, replacement etc.).

 

[ofp85]

Look, what restrictions are specified in instructions for pilots of Mustangs.
This implies that the engine used the basic part of battle boost 61 " 1490 PS.
But it is insufficient Power for the plane in weight more than 4000 kg.
If in battle basically used boost 61 " (Military Power 15-minute limit) then P-51D a little than differed P-51H.
Then it turns out that all improvements of the engine only from for five minutes.
I do not think that it was so.
But there are no documents speaking about the opposite.

 

[ofp85]

The American variant

 

[ofp85]

The English variant