BMW801 ATA drop off V Altitude

[Bert_Foster]

Why in the Sim does the ATA wuth fixed throttle drop during the climb from Sea Level to the Full throttle height (1.35ATA is about 2500m in the Sim) ? To maintain the 1.35 ATA in the climb you have to keep pushing the throttle up to hold the ATA. Does not KG have its own Automatic Boost Controller to hold ATA constant with throttle position constant ?

[JtD]

The BMW810 is a two speed supercharged engine. The first full throttle altitude at 1.32ata/2400rpm is about 0.9km. The second gear will automatically kick in at about 2.6km. Between that, the boost goes down because you're above FTH for that setting.

If you move the power control forward between 0.9 and 2.6km, you will not open up the throttle, but increase engine rpm and thereby supercharger rpm, effectively increasing boost above fth. This way, you can maintain 1.32ata up to about 1.6km at 2700rpm.

 

The BMW does not have the hydraulic clutch of the DB, so the behaviour is normal for an engine with a two speed supercharger and ABC.

[Bert_Foster]

Ok makes sense I was thinking 1.32/2400 FTH was 2600m rather than 900m

[Crump]

The BMW810 is a two speed supercharged engine. The first full throttle altitude at 1.32ata/2400rpm is about 0.9km. The second gear will automatically kick in at about 2.6km. Between that, the boost goes down because you're above FTH for that setting.

If you move the power control forward between 0.9 and 2.6km, you will not open up the throttle, but increase engine rpm and thereby supercharger rpm, effectively increasing boost above fth. This way, you can maintain 1.32ata up to about 1.6km at 2700rpm.

 

The BMW does not have the hydraulic clutch of the DB, so the behaviour is normal for an engine with a two speed supercharger and ABC.

 

 

What you describe is how SOME engines with pure mechanical linkage operate.  Not all of them because you have to be careful as you can overboost the engine and damage it.  The Operating Instructions will detail how it should be operated and will specifically say if the pilot is move the throttle to maintain power in the climb.  Those fixed distance linkage throttle systems would work the way you describe but you simply leave the throttle alone so you do not damage the engine unless the instructions tell you differently.

 

 

The BMW801 Kommandogerat works as a hydraulic-electric-mechanical FADEC system.

 

The throttle lever has detented positions which the pilot simply moves the lever to the correct detented position.  The KG then manages the engine including the propeller rpm.  When the supercharger reaches FTH and cannot maintain manifold pressure at the selected detent position, the propeller acts as a Constant Speed propeller maintaining engine rpm until it reaches the propeller stops resulting in a change in engine speed.

 

That change in engine speed triggers the supercharger gear change. 

 

That is the most efficient way to operate the engine and is exactly what a pilot having to do it manually would do.

 

An FW-190 Pilot moves the lever to the detented position to achieve the power he wants....

 

 

That is why the FW-190 has labeled detent positions on the throttle lever.  You can do that because the pressure capsules in the linkage downstream of the main Kommandogerat control valve compensate for atmospheric conditions and maintain the correct linkage calibration point in the heat of summer or the dead of winter.  

 

 

 

You CANNOT have specific throttle positions on any engine that has a fixed distance linkage because the manifold pressure at that distance will change with density altitude.

 

 

So the correct operation of the BMW801 is the pilot moves the lever to the detented position he wants to maintain and leaves it alone.  In Steig u Kampfleistung detented position, the Kommandogerat maintains 1.32ata at 2700U/min all the way to FTH and when it cannot maintain 2700U/min rpm, it changes supercharger gear.

 

The pilot does not jocky the throttle in the climb.

 

Notice that cruise settings have no detent.  In cruise flight, the pilot sets the manifold pressure he wants to maintain and the KG maintains it for him as he maneuvers or changes atmospheric conditions in flight.

Edited October 2, 2016 by Crump

[Crump]

There is no physical connection between the throttle lever and the propeller governor.  

 

 

The throttle lever is only connected to the Kommandogerat control lever.  The KG control lever is a hydraulic valve which moves as required to maintain the pilot selected detented throttle lever power settings.