109 prop pitch (rpm) and the supercharger

[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?

[Crumpp]

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.

[macro]

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

[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.

[Kurfürst]

Originally Posted by NZtyphoon

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.

Nope, it's incorrect. Hydraulic couplings generate more heat to the oil during slip (friction generating heat), but it doesn't mean the oil was overheating, because

a) the fact that increased oil cooling requirements could be and were compensated by increasing oil cooling capacity (note the rather sizeable oil radiator on the 109)

b) the fact that at and above FTH the hydraulic coupling has minimum slip (in the order of about 3%) and therefore, the heat load is only marginally different from a fixed ratio mechanically geared supercharger. If there's no extra friction, there is no extra heat, simple as that.

This is evident from DB heat charts, i.e. the DB 605A lubricant heat transfer was 65 000 kcal/hour at sea level, when the hyd. supercharger was operating at maximum slip, but only 43 000 kcal, or roughly 2/3s at FTH, where the hyd. supercharger was operating at minimum slip.

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.

Bull on all accounts.. the supercharger itself does not "overheat", its just a piece of magnesium alloy and does not operate at extreme temperatures, neither does it have any sort of forced cooling.

Secondly, increasing revs by about 200 rpm _was_ a sanctioned tactic that increased the supercharger capacity and altitude output of the engine, as noted in the November 1940 LWHQ notice that has been already posted, and led to some noteworthy speed increase above rated altitude, as noted by the 109F manual. And if the speed increases, the pitch angle does need to be changed of course, just as at any rpm and at any altitude, when the speed increases.

All they did was manipulating the pitch to let rpm increase, and then - by when the rpm has increased - manipulating pitch to compensate for increase airspeed AND maintain increased rpm.

[Crumpp]

All they did was manipulating the pitch to let rpm increase, and then - by when the rpm has increased - manipulating pitch to compensate for increase airspeed AND maintain increased rpm.

Exactly.

I have been saying this for how many pages now??



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