109 prop pitch (rpm) and the supercharger

[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??



This community is toxic.

[Robo.]

Originally Posted by 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??



This community is toxic.

No. You were saying that they were changing the pitch to keep it constant. That is something completely different.

The rpm won't stay up when you coarsen up, it will drop when you touch the rpm lever again. That's all I am saying.

[Crumpp]

You were saying that they were changing the pitch to keep it constant.

Go back and read what I wrote. You did not understand it and keep misinterpreting something.


Second thought, don't...

Just keep believing you have the concept and are correct. Welcome to the ignore list.

[Robo.]

Originally Posted by Crumpp

Go back and read what I wrote. You did not understand it and keep misinterpreting something.

So how exactly do you change the rpm to keep it constant again?

[kohmelo]

About the supercharger...

At "normal flying alttitude" I really don't believe that pulsing engine rpm to add small boost to supercharger rpm would be efficient. As it takes its RPM directly from engine RPM there would be no (turbo)lag and it would not transfer the "kick" when coarsening the pitch.
Centrifugal Supercharger (as in DB601) gives more boost the higher the rpm is and because the rpm relevant of the Engine RPM, the extra boost is lost when engine RPM is dropped.

Still if there is some changes on superchargers own pitch on level flight this is incorrect but I would believe that Chargers pitch changes only by alltidute Untill it hits its top performance pitch. --> After this point the extra oxygen was given by water-Methanol injection? because german belived that extra stage or added turpo would be heavier for added high alt performance than MV-50

[ACE-OF-ACES]

Originally Posted by Robo.

So how exactly do you change the rpm to keep it constant again?

Only one way..

He has to redefine the meaning of 'maintain'

http://www.youtube.com/watch?v=j4XT-l-_3y0

Clinton would be proud!

[pstyle]

My understanding from the text is that rpm would oscillate with this technique.

Also that there would be a corresponding oscillation in engine sound. Steinhilper footnotes this.

[pstyle]

Here is an illustration of what I understand Steinhilper to be talking about;
http://s18.postimage.org/4wlow6b2v/s..._technique.jpg