Say you are taking off on a hot day from Flagstaff, AZ (7014 ft, 2138 m), is it not wise to use High supercharger setting on takeoff in steam gauge planes such as the Constellation or DC-6? Please give reason(s), thank you.

 

 

Why would you?  Those engines had to be operated by the numbers - including time limits for the higher power settings - if they were to survive the flight.  So if you can get take-off manifold pressure (55" IIRC) with low blower, there's just no point in switching to high.  You'd only have to throttle back so as not to exceed the specified MAP, and high blower absorbs more power from the engine and also heats the air more, with possible detonation issues.

There might be a disconnect about the purpose of superchargers in airplanes vs cars. In a car, you generally install a supercharger to accelerate faster so you can have more fun or outrun other cars in races. Especially with modern computer-adjusted fuel/air mixtures, you don't need boost to keep the engine running when you go from sea level to the top of a mountain.

In a prop-liner, you put a supercharger on so the engine keeps running decently at high altitudes. There isn't enough air to burn the fuel once you get up high, so you need a way to stuff more air into the engine.

In other words, the DC-6 supercharger is to keep the engine from quitting from air starvation, not to make it go faster.

In both cases, the supercharger is only as effective as the ability of the fuel pump to keep up. If you stick a big supercharger on your old Honda and forget to upgrade the fuel delivery system, you will run very lean because you'll be jamming a lot of air into the engine without enough fuel. Running a little lean is good. Running a lot lean slows you down and hurts the engine.

In the plane, when you're up high the supercharger is just stuffing more air into the engine to make up for the air it isn't getting because of the altitude - in other words, the engine really isn't getting any more air into it than it gets at low altitudes on low boost, so the fuel delivery system can keep up.

If you wanted to run high boost on takeoff to accelerate faster, you'd have to shove more fuel into the engine too (which means upgrading the fuel delivery system) or it wouldn't work and you'd do damage to the engines - an especially bad thing in a time when airplanes had 4 engines in part because odds were pretty decent that even if you babied the plane, you were going to land with at least one fewer working engine than you had at takeoff.  But we're already taking off with the mixture set to full rich so adding even more fuel flow than that isn't an option.

Basically, if it were advantageous to do it, the plane would be designed for it, but there's no need for it, especially since if you *do* need extra power on takeoff, you use the water injection system which cools the combustion chamber and allows you to run a little lean (for more power) without risking engine damage as you would by running on high boost. And the PMDG plane does model this.

 

Edited July 23, 20241 yr by eslader

Interesting to know, seems longevity of the engines was paramount back then. Good info.

I can say I've already tried several DC-6 wet takeoffs with high supercharger netting the takeoffs quicker and faster in all cases. I have yet to notice any additional plane wear and tear but the maintenance panel is not exactly highly defined (nor probably very accurate) with only basic dot color changes.

If you can get full MP on low super-charger why would you want the high selection. Don't forget the wet takeoff selection.