109 prop pitch (rpm) and the supercharger

[ACE-OF-ACES]

Originally Posted by Robo.

So you're saying (again!) that they were constantly changing the rpm to maintain rpm?

You noticed that too?

Talk about trying to have your cake and eat it too! Or put another way, he is just not able to admit he made a mistake!

Simple fact is the 109 pilot accounts clearly show that they were not trying to 'maintain' the RPMs..

They purposely changed the pitch (flatten) to allow the RPM to increase (rev up), even exceed the recommended RPM limit..

Which in turn allowed the supercharger to rev up!

Than the pilot would change the pitch back (course) to convert the excess RPM into a boost in speed and/or climb as the RPM decreased..

This process was not done once, but done over and over, back to back..

Which is what the 109 pilot noted accounted for that 'wa-wa' sound.

The analogy of the manual transmission clutch fits well here!

Back when I raced dirt bikes we called it 'clutch revving'..

In this analogy, the clutch pressure plate is the prop.. and the flywheel is the air, and the clutch lever is the prop pitch adjustment.. As in pulling in or releasing the clutch lever is analogous to adjusting the prop pitch from flat to course..

The way it worked was you would pull in the clutch to let the engine 'rev up'.. Than release the clutch to convert that revved up energy into a boost in speed or torque..

Here too we did this more than once, not all the time mind you, but when ever you needed a little boost..

Notably, this process also produced a 'wa-wa' sound..

[camber]

Crumpp as others have stated, Steinhilper is clearly not complaining that rookies fell behind because of their inability to manually duplicate a CSP (i.e. continuously changing their variable prop pitch for a constant optimal rpm).

From his account he believed that he could only get optimal performance from pulsing the rpm, i.e duplicating a CSP with the rpm control being moved back and forth. This seems a little odd, and we must consider that it wasn't actually true. Perhaps if the rookie pilot managed to manually control his rpm at an optimum value like a CSP, he could have overtaken Steinhilper busily pulsing his rpms back and forth.

It is hard to state a good technical reason why the pulsing would have helped. Steinhilper believed that the thrust from the rpm boost could only occur if rpm was dropped again, implying that the extra rpm was high enough to not increase thrust. Perhaps 109 pilots decided it was OK to exceed rpm limits if they only did pulses above the limit, they achieved some extra thrust and speed this way but mistook the reason. Or perhaps a quirk of 109 engine/supercharger/prop design did allow a small performance increment doing this over maintaining rpm at a constant optimal value.

Your explanation of CSP function are correct but not relevant to what Steinhilper described.

[Robo.]

Originally Posted by camber

Crumpp as others have stated, Steinhilper is clearly not complaining that rookies fell behind because of their inability to manually duplicate a CSP (i.e. continuously changing their variable prop pitch for a constant optimal rpm).

Yes but mind you this only applies at very high altitudes. Fiddling with the rpm, as he describes it, was because of the supercharger design and function, not because of trying to mimic the CSP. Apparently this practice made a difference in average speed up there.

The chap falling behind on the other hand, that was while they were climbing below fth. He was simple a rookie pilot fresh from the training and due to the lack of experience he could not use the manual rpm lever at all and was struggling to keep up. After he got an order to turn back to France he got the navigation wrong, too, and was heading straight to the UK. At this point Steinhilper left the formation and herded him back to the correct heading.

The two 'rpm quotes' are totally unrelated.

Originally Posted by camber

From his account he believed that he could only get optimal performance from pulsing the rpm, i.e duplicating a CSP with the rpm control being moved back and forth. This seems a little odd, and we must consider that it wasn't actually true. Perhaps if the rookie pilot managed to manually control his rpm at an optimum value like a CSP, he could have overtaken Steinhilper busily pulsing his rpms back and forth.

See explanation above - pulsating the rpm is only applicable at high altitudes way above the FTH.

He got shot at and baled out because he was using his older 109 that was not in use for a while, there was some condensed water in the propellel hub and that water froze up in the altitude so he could not change the prop pitch and overreved his engine quite badly.

Originally Posted by camber

It is hard to state a good technical reason why the pulsing would have helped. Steinhilper believed that the thrust from the rpm boost could only occur if rpm was dropped again, implying that the extra rpm was high enough to not increase thrust.

Good technical reason was the supercharger function at that alt and rpm, just as you said below.

Originally Posted by camber

Perhaps 109 pilots decided it was OK to exceed rpm limits if they only did pulses above the limit, they achieved some extra thrust and speed this way but mistook the reason. Or perhaps a quirk of 109 engine/supercharger/prop design did allow a small performance increment doing this over maintaining rpm at a constant optimal value.

Yes they decided so, because it obviously worked, they knew why it was working that way, even Steinhilper explains that in the book. Overreving the engine at altitudes above FTH was actually approved and recommended by LW authorities shortly after the BoB. There is no doubt the pilots knew what they were doing, at least in my opinion... Again this does not apply for flying below the FTH.

[camber]

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.

[NZtyphoon]

Originally Posted by camber

Crumpp as others have stated, Steinhilper is clearly not complaining that rookies fell behind because of their inability to manually duplicate a CSP (i.e. continuously changing their variable prop pitch for a constant optimal rpm).

From his account he believed that he could only get optimal performance from pulsing the rpm, i.e duplicating a CSP with the rpm control being moved back and forth. This seems a little odd, and we must consider that it wasn't actually true. Perhaps if the rookie pilot managed to manually control his rpm at an optimum value like a CSP, he could have overtaken Steinhilper busily pulsing his rpms back and forth.

It is hard to state a good technical reason why the pulsing would have helped. Steinhilper believed that the thrust from the rpm boost could only occur if rpm was dropped again, implying that the extra rpm was high enough to not increase thrust. Perhaps 109 pilots decided it was OK to exceed rpm limits if they only did pulses above the limit, they achieved some extra thrust and speed this way but mistook the reason. Or perhaps a quirk of 109 engine/supercharger/prop design did allow a small performance increment doing this over maintaining rpm at a constant optimal value.

Your explanation of CSP function are correct but not relevant to what Steinhilper described.

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




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

Originally Posted by pstyle

Ulrich Steinhilper, in his auto-biography (chapter 16) , talks about managing the prop-pitch on the early (E3 and E4 variant) 109s during the Battle of Britain. He states that, in order to achieve max climb rate and airspeed (particularly at higher altitudes) one had to constantly increase and decrease the propeller pitch. Increasing the pitch would engage the supercharger, which would be run for a short period (i.e. a second or less?) to force more air into the cylinders, then the pitch would be dropped back down again to disengage the supercharger and convert the power gained into airspeed, and allowing the engine/ supercharger to rest.

Is this effect modelled in the game? Does this constant prop management to engage and disengage the supercharger, allowing for best continued speed?

My experience is that the prop control is modelled to move quite slowly in the 109 in the game. But perhaps the range over which this manipulation needs to occur is quite narrow, just either side of the supercharger threshold?

the hydraulically coupled supercharger was being "rested" and cooled.

[IvanK]

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.

[camber]

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.

[Crumpp]

Things had started to go wrong as we reached our operational height for the mission. When we flew at that height, the engine only just gave enough pull and we constatnly changed propeller pitch and RPM to improve performance.

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.

Ulrich Steinhilper, in his auto-biography (chapter 16) , talks about managing the prop-pitch on the early (E3 and E4 variant) 109s during the Battle of Britain. He states that, in order to achieve max climb rate and airspeed (particularly at higher altitudes) one had to constantly increase and decrease the propeller pitch.

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.

With a flat pitch we could increase the rpm of the engine and get more pressure from the supercharger.

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.

With a flat pitch we could increase the rpm of the engine and get more pressure from the supercharger. Then, by changing the piitch to a coarser setting, we could make up some speed.

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

RPM stays constant....

Increasing the pitch would engage the supercharger, which would be run for a short period (i.e. a second or less?) to force more air into the cylinders, then the pitch would be dropped back down again to disengage the supercharger and convert the power gained into airspeed, and allowing the engine/ supercharger to rest.

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.

[Robo.]

Originally Posted by Crumpp

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.

See?

[NZtyphoon]

Originally Posted by Crumpp

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.

Interesting how this only now becomes a measure to cool the hydraulic supercharger, according to our resident aviation expert; still it is correct that the hydraulic coupling of the DB supercharger led to problems with overheating that the pilots needed to control by constantly altering rpm - not maintaining it.

Originally Posted by Crumpp

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

Wrong; the pilots were needing to adjust rpm and pitch constantly to periodically rest the supercharger - any gain in speed was a by-product, not a tactic.