Manual supercharger speed change.

[E69_VonPepofen]

Dear Friends:

It is very nice the way you have changed the behavior of the manual changing of the supercharger speed in the La5 family. Early changing now leads to a decrease in manifold pressure as it should.

I would like to encourage you to make the same adjustment in the engines which still do not have it:

 

Klimov M105 family

Napier Sabre family

Merlin XX

Merlin 66-70 with 150 N.O.

 

In all these cases supercharger speed can be changed at 0 m and the engine performs nicely if manifold pressure is adjusted to the maximum allowed by the engine. There is no need to take care of changing supercharger speed.

 

The change will very much improve historical response of the aircraft using these engines. Moreover considering that the rest of the planes now have a realistic behavior.

 

Thank you!

 

[Charon]

I believe you're mistaken. Let me walk through how I understand it to work.

 

In any engine, the second supercharger gear is going to spin the impeller faster and thus generate more pressure at the supercharger outlet. (Edit: And you can observe this in the La-5 itself if you just throttle it back and then upshift) Why then does the La-5 lose manifold pressure at full throttle when the gear is switched?

 

Engines can't tolerate unlimited boost, so they've (typically) got a regulator that throttles the engine to keep manifold pressure within allowable limits. In the case of the La-5, it's rated for a higher manifold pressure (1140mmHg) only in first gear. The regulator is (apparently) linked with the supercharger gear shift so that in high gear, the regulator adopts a lower (950mmHG) limit. But that's enforced by the regulator, not the supercharger itself.

 

Why the engineers rated it for a lower limit in second gear, I don't know. My guess is that radials are more susceptible to detonation (no big cooling reservoir to provide some protection). At higher altitudes cooling will be reduced, and the temperature rise in the incoming air is increased, due to the higher impeller speed.

 

I assume the decision to de-rate the second gear is also why the La-5s have such a high gear-shift altitude. If rated to the same manifold pressure in both gears, I'd expect it to shift much lower.

 

I think the important thing to realize is that usable power output is not determined just by RPM and Manifold pressure. It depends also on carburetor air temp (I think there are gauges for this mostly in the American planes) and on the power lost to the supercharger. If you take those other planes, put them in 2nd gear, and fly them at low altitude, you'll find they're slower. Even though RPM and MP are the same, the engine is working harder to drive the supercharger, and heating the air more in the process, for no real benefit. For example, I just tested the Yak 9s1 at 400m, full power, everything open (not optimal for speed of course, but fine for this test) and found that just shifting to second gear dropped the level speed from 503kph to 481kph.

Edited November 29, 2023 by Charon