Supercharger Question

[fsjoe 0]

The supercharger has high and low blower. The POH describes that one can shift to high blower when the critical blower altitude is reached. 

I however was not able to find out what the critical blower altitude actually is. Is there a chart indicating this or did I miss this in the POH?

 

thanks

Nik Schild

[Pat Mussotte 92]

Hi fsjoe!

In expanded checklists, page 202, it is stated"(5) High blower climbing powers should be in accordance with the Take-off, Climb, and Landing Charts in Section VIII". If you refer to those charts, you will see the blower setting according to altitude pressure,  and manifold pressure at carb temp!

Regards

Pat Mussotte

[fsjoe 0]

Thank you for pointing out the reference. On my POH it is page 208. Do I have an outdated version?

Let me go though an example to check if I got it right:

I chose METO power, and am climbing from 8000 ft to 10'000 ft pressure altitude. My carb temperature is at -10 deg and therefore I can switch to high blower. Once I go to cruise power, the pressure altitude for high blower seems to rise. 

This leads me to the next question. I somewhere read in this forum that with an DC-6 one will typically not fly often over 8000ft. This means that high blower is rarely used which I find is a bit strange. Is this maybe because the 8000 ft cruise altitude refers to how the aircraft is used as of today (medium range flights)? In contrast, during the good old times, higher cruise altitudes were used for long haul flights (i.e. crossing the atlantic) and there of course the high blower had its purpose?

thanks

Nik Schild

 

[Pat Mussotte 92]

7 hours ago, fsjoe said:

Thank you for pointing out the reference. On my POH it is page 208. Do I have an outdated version?

My mistake, yes it's page 208!

7 hours ago, fsjoe said:

 

This leads me to the next question. I somewhere read in this forum that with an DC-6 one will typically not fly often over 8000ft. This means that high blower is rarely used which I find is a bit strange. Is this maybe because the 8000 ft cruise altitude refers to how the aircraft is used as of today (medium range flights)? In contrast, during the good old times, higher cruise altitudes were used for long haul flights (i.e. crossing the atlantic) and there of course the high blower had its purpose?

thanks

Nik Schild

 

Nowadays, I guess they don't fly high in order to avoid wearing engines and the fuselage(Use of the pressurization).

Best regards

Pat Mussotte 

 

[downscc 1,327]

9 hours ago, fsjoe said:

This leads me to the next question. I somewhere read in this forum that with an DC-6 one will typically not fly often over 8000ft. This means that high blower is rarely used which I find is a bit strange. Is this maybe because the 8000 ft cruise altitude refers to how the aircraft is used as of today (medium range flights)? In contrast, during the good old times, higher cruise altitudes were used for long haul flights (i.e. crossing the atlantic) and there of course the high blower had its purpose?

Correct, but the typical mission profile today is cargo on short to medium distances.  This aircraft was quite busy during and after the Berlin Airlift era and the altitude selection would depend extensively on wind, icing, cloud tops (to shoot the stars), etc. The mission profile was much more complex in those days, and the engines were newer.  The change from low to high is simply shifting the gears that drive the supercharger, the only reason to do this is if the supercharger is not providing enough pressure due to altitude..., the impeller needs to spin faster at a given engine RPM.  It's not like you are increasing the MP rather you are tying to sustain it.