Thanks robo, that clarifies the story considerably. As I don't have the book I was going on what text was available in this thread and online. It makes sense that the two situations (below FTH and above FTH) are different.

Increasing rpm and supercharger output at high alt will give access to higher boost, in fact the FTH will increase for any particular boost level. But it still seems odd to me that a useful strategy would be to attempt a cycle of rpm pulsing in between maximal power and thrust settings (with the conversion inefficiencies inherent in this). Once you give yourself permission for higher rpm (and boost/power) at height, then it seems more likely that an intermediate constant rpm than the extremes of your pulsing would deliver better performance.

I'm never sure whether doubting a historical pilot assertion is really appropriate from behind a computer in 2012, and Steinhilper could well be right that there was real edge doing the pulsing. However there are many examples of pilots using procedures that the engineers would have frowned upon to give a perceived edge. The edge may have been good for morale but vanishingly small or even a placebo.

My favorite example is the RAF bomber pilots who always turned on their IFF sets over Germany, in the belief that it confused radar operated searchlights. The brass encouraged it in the belief that it improved morale, the scientist RV Jones thought this was totally unacceptable as the IFF sets generated radiations that Germans could exploit for detection sooner or later.

There is another possible explanation as to why pilots were doing this:

http://i91.photobucket.com/albums/k3...e/R-28002a.jpg
http://i91.photobucket.com/albums/k3...e/R-28001a.jpg

http://www.amazon.com/2800-Pratt-Whi.../dp/0768002729

If the hydraulic coupling of the supercharger was generating too much heat then the pilots had to take steps to cool the supercharger down

Pstyle's first post the hydraulically coupled supercharger was being "rested" and cooled.

I am being a nit picker here (and thread drifter) Camber :)

"My favorite example is the RAF bomber pilots who always turned on their IFF sets over Germany, in the belief that it confused radar operated searchlights. The brass encouraged it in the belief that it improved morale, the scientist RV Jones thought this was totally unacceptable as the IFF sets generated radiations that Germans could exploit for detection sooner or later."

I think you are referring to "Monica" an active Tail warning radar. The Hun were quick to exploit it and home passively on it using devices like Flensburg. Using the basis of the radar equation they were capable of homing on it from twice the range it was capable of actually detecting them. This info was withheld from the crews (to their detriment) based on the good morale that Monica was a good defensive system.

Drift away Ivan! You had me worried for a second there... :)

"Most Secret War" by RV Jones, p 275

"Bombers were frequently being caught in German searchlights, and the idea had grown up that the searchlight control could be upset if a bomber switched on its IFF radar recognition set, and so the bomber could then escape. The proffered explanation was that the searchlights were directed by radar which was somehow jammed by British IFF"

So I remembered a bit wrong, the IFF was only (pointlessly) switched on when a searchlight found the bomber or was nearby. Jones worried that the Germans would find a way to interrogate the IFF set to the bomber's detriment.

The Monica system problem sounds a bit similar, although Monica was an initially useful countermeasure which became a disadvantage, rather than being a placebo countermeasure from the beginning.

Thanks for posting your findings.

At high altitudes, the airplane is essentially in slow flight for most of the envelope. That makes cooling harder and overboost conditions will heat the motor up faster.

If they wanted to use a limited overboost condition, they would be constantly changing rpm between maximum continious and higher limited overboost to cool the motor.

Not the same thing as maintaining constant rpm at the overboost condition to realize the speed gain. In otherwords, when you set the engine to say, 1.35ata @ 2400U/min, you will adjust pitch to maintain a constant 2400U/min rpm to achieve best performance.



You must change pitch, rpm, or airspeed. If you increase rpm and airspeed, you must coarsen the pitch to keep rpm steady and airspeed increasing.....

That is how it works.

Same as a CSP. Reduce pitch to increase rpm and then coarsen the pitch as the speed increases to take advantage of the new rpm setting.

Same as it ever was and what I have repeated for several pages....

RPM stays constant....

They overloaded the super by increasing rpm at high altitude, the engine will have a cooling problem in a short time period.

A complete sidetrack as to how they are using the propeller and rpm to gain speed.

See?

See what?

Look at the Operating Notes for any aircraft.

If I am in cruise flight and want to achieve maximum level speed, then I must change the settings from cruise manifold pressure and rpm to a higher limited overboost manifold pressure and rpm.

Performance occurs at the specific manifold pressure and rpm setting.

You must maintain that rpm setting at a constant rate.

In a selectable pitch propeller, this is done manually by coarseing the pitch to maintain a constant rpm.

Is that hard to understand or something? It must be as we have multiple pages on this simple concept.

so RPM doesnt stay constant if pilots increase RPM at higher alts for more speed?

Of course it does. Best thing of keeping something unchanged is to change it, obviously.

I am trying hard to figure out why is this happening - reading this:

...I reckon Crumpp is not aware of the fact that by coarseing the pitch in the 109 the rpm drops accordingly.

Crumpp wake up!