enginehistory.org

[jjuutinen]

I was examining a P-51D cruising chart and one thing took my attention: low altitude max range cruising is done at 1600 rpm/30 inHg, i.e. very inefficient combination as the max allowable boost is 46 in. The question is why the Merlin had so high minimum cruising rpm if compared to e.g. American radials whose minimum recommended cruising rpm was usually 1200 rpm so the Merlin´s minimum rpm was quite high, even allowing for its higher max rpm.

The same question about the Centaurus. Its minimum rpm seem to be rpm 1500-1600 leading to max range cruising at -2 lb to -4 lb boost, again very inefficient.

Finally, I am a bit surprised how long it sometimes takes to get questions answered as I have seen far more traffic at forums with far fewer members than we do. This agonizingly slow responsiveness goes against the very idea of having such forums. Sharpen up, folks!!!

[szielinski]

The only things I can GUESS at are prop diameter & coolant flow.

Maybe if the radial has a larger diameter prop (possibly due to larger frontal area of radial engine), chances are it can turn slower for a given pitch and produce the same thrust.

Take into account that the liquid-cooled Merlin has a fixed coolant pump ratio and that at high speeds the designers probably wanted the coolant to be nice and hot to get some heat-pumping effect (ie medium flow rate), hence at low (~1200rpm) speeds and low (ie warmer) altitudes the coolant flow really isn't enough at 1200rpm for sustained operation.

As for the Centaurus which is an air-cooled radial - dunno enough about that engine to totally discredit the above sentences !

Of course the other point is that maximum economy and maximum efficiency are not the same operating points for a given engine.

Just an idea, but since no-one else posted...

[jjuutinen]

Considering the examples on V-1710s cruising at about 1000 rpm (quoted in TM) I find it hard to bilieve that the Merlin would have had so much lower coolant flow.