Air cooled inlines: any advantages vs. the 'classics'?

[wuzak]

http://www.flightglobal.com/blogs/aircraft-pictures/FA_16484slarge.jpg

 

[Shortround6]

Thanks for the info. Too bad Italians did not started with 30-32 liters for the Delta, instead of 27.

The bore and stroke of the Delta was 132mm X1 60mm. Larger diameter cylinders are harder to cool, more volume per unit of wall area. larger diameter also means a bigger distance between cylinder bore centers making for a longer engine. The stroke was already the same as a DB 601 and long strokes affect the rpm limit.
The Allison company in the 1920s had made some aircooled conversions of the WW I Liberty aircraft engine. They kept the original crankcase and crankshaft and in order to get enough room for the fins they shrank the bore and turned a 1650cu in engine into a 1410 cu in engine. Power slightly exceeded the original engine (about 425hp?) but being almost 10 years newer it should have.

Could someone shed some light about the DH Gipsy Twelve it's supercharger - Wiki states the take off power with 0 lbs boost (I presume +0 lbs boost?). Was the supercharger only in the function of making the air-fuel mixture a more uniform there?

WIki (at least the English version) is less than clear about the power rating. In this case they are saying Max power and not take-off.

Take off power was 525hp at 2600rpm and using 37inchs (3 1/2lbs) boost.

Max power of 425hp was at 2450rpm at 7500ft with "zero" boost. which actually means the supercharger was supplying sea level pressure at 7500ft.

The DH Gipsy Twelve used the same bore and stroke and quite possibly a number of cylinder parts from the Gipsy Major 130/140 hp four cylinder engine used in the Tiger Moth and the 205 hp Gipsy 6.

 

[tomo pauk]

Thanks for both, people, makes a lot of sense.

 

[Shortround6]

Lumsden has the Gipsy Twelve as takeoff power as 505hp @ 2600rpm, +3.5psi boost. Max power is given as 425hp @ 2450rpm, 7250ft, +0psi boost. That is the supercharger is providing enough boost to compensate for the altitude, and thus is essentially unsupercharged.

In the terms of the day the engine would be referred to as "moderately" supercharged. Using a supercharger to improve low altitude performance or take of was usually called "ground boosted." Hitting peak power at 3,000to 8,000ft or so was moderately boosted or supercharged and having critical altitudes of 8,000ft to 14,000ft or higher was "fully" supercharged or boosted. Terms were not universal and varied from country to country and perhaps even from company to company.
Since 87 octane was "HOT" stuff and and had only been around a few years many engines in the 1930s were running 2-4lbs boost at maximum.

 

[wuzak]

With the Albatross it has often been suggested that Merlins could have been fitted. But maybe Merlins were a bit big/too powerful.

Let's compare the Kestrel XXX with the Gipsy Twelve (both from 1938 ).

Gipsy Twelve
L: 82.6"
W: 31.5"
H: 37.4"

Max power: 505hp takeoff
425hp @ 7,750ft
Weight: 480kg/1058lb

Kestrel XXX
L: 69.82"
W: 24.4"
H: 37.53"

Max power: 720hp takeoff
585hp @ 12,000ft
Weight: 449kg/990lb

The Kestrel was slightly lighter without the radiator and coolant, and would be slightly heavier than the Gipsy Twelve with them. But it would deliver at least 50% more power at the higher altitudes, and nearly 50% more at takeoff. Size wise the Kestrel is in fact smaller, except being slightly taller (by 0.13"). The updraft carby may break the clean nacelle lines. As for the radiator(s) I could envision the intakes for the air cooling system housing small round liquid coold radiators - maybe the holes would need to be increased slightly. Or use the nearly 13" of length savings of the Kestrel to mount a radiator behind the engine, fed by the same air ducts that fed the Gipsys, using a similar exhaust flap system.

 

[wuzak]

Teh Gipsy Twelve had an updraft carby too

http://www.flightglobal.com/airspac.../500x400/de-havilland-dh-gypsy-12-cutaway.jpg