1935-45: a 'helping hand' for the aircraft powerplants?

[tomo pauk]

... in the shape of a small engine added to the 'main' engine.
Basically, the air-cooled engine installed behind the cockpit or in the aft part of the nacelle, connected with the 'main' engine, so the total power can be, say, 1250 HP = 1000 HP + 250. Reason why air-cooled is to avoid one system to be designed and installed. Reason why no full-blown 2- or 4-engined aircraft is that, for some aircraft types, a 2-engined aircraft can be more of a problem than it is worth the investment, like the naval aircraft, or the land-based fighters that need to be manufactured in big quantities; making 4-engined bombers was also a tall order for most of the countries.
The resulting aircraft will need to be a bit bigger and with a bit more fuel than an aircraft without the aux engines.

Some other legit questions/remarks:
- Why not just crank up the boost on the main engine? Not easy for a country that has next to no access to the hi-oct fuel. Also, bebefits of the hi-oct fuel diminish at the higher altitudes.
- But, I have hi oct fuel! Great, that means your Fulmar down low has 1600 HP, not just 1300.
- Will I be needing an at least half-decent S/C on the aux engine? Yes.
- I'll just attach the turbo and be on my merry way! Not if you are not American.
- The next shiny and amazing engines are just behind the corner! Yes, we know how that panned out for many engine types.

 

[red admiral]

I'm not sure on a mechanical gearing onto the main engine crankshaft and packaging this into the airframe. Instead, how about no supercharger on the main engine = power boost from reduced load, and reduced size/mass. Then use the auxiliary engine as variable speed/power to drive a supercharger and then duct the air forwards into the main engine. Still going to need a decent power secondary engine though I think e.g. dH Gipsy 6 for 200hp.

Or go for two common engines side by side through a common gearbox to drive one propeller each as per MC.72? Avoiding the complexity of a high power gearbox.

 

[tomo pauk]

Instead, how about no supercharger on the main engine = power boost from reduced load, and reduced size/mass. Then use the auxiliary engine as variable speed/power to drive a supercharger and then duct the air forwards into the main engine. Still going to need a decent power secondary engine though I think e.g. dH Gipsy 6 for 200hp.

(hmm, how I didn't thought of this...)
The engine-driven supercharger on the V-1710 was sucking a lot of power, 100-300 HP depending on the drive ratio and airflow achieved (up to 350 G for the post-war -G with the bigger S/C). Having a 250-300 HP engine driving the impeller of the S/C frees up to 250-300 HP for the propeller, so our V-1710 makes 1300+ HP at ~14000 ft, instead of 1040-1090. And that is already for 1940-42. for the pretty small impeller that is turning at sedate speed.
(reason I'm talking about the V-1710 is a simple one - it is the only engine that I have the good data readily available)
Similar math is probably worth for the Merlin III, DB 601A and Jumo 211B-D of the day.

Another thing might that the aux engine drives the impeller of second S/C, that in the case of P-47 was driven by exhausts via the turbine. Or to turn two impellers = 2-stage S/C driven by the aux engine? Already on the small V-1710 2-stage versions, driving both impellers was sucking ~700 HP total at high altitudes - making the engine, capable for 1800+ HP at S/L, doing just about 1100 HP at 25000 ft, depending on the version.

Or go for two common engines side by side through a common gearbox to drive one propeller each as per MC.72? Avoiding the complexity of a high power gearbox.

Food for the new thread