Engine type: RAF 3a...

While reading Colin Owers two new volumes on DeHavilland Aircraft of WWI I came across photos of the version of the D.H.4 with the RAF 3a engine. The cowling is very neat and tidy with only the tall single angled exhaust stack coming up in the top center of the top decking in front of the pilot's cockpit just in front of the top wing leading edge. The cowling panels are so neat and tidy that the only clues about the configuration are that:

1. It is water cooled
2. That because of the exhaust stack, it must be in either inline-6 configuration, or inline-8 (but that, in my opinion, would be too long).
3. Was it geared? The propeller shaft/thrust line is higher than other engines, about center, instead of the lower placement in the B.H.P., Rolls Royce, or FIAT variants.

Any details of this engine, it's appearance, and what other aircraft used it would be welcome.

Av

[Dan_San_Abbott]

Aviatik:
The R.A.F.3A was a 60° V 12 cylinder water cooled geared engine, rated @ 200hp. The bore was 4.5" and stroke 5.5" with a displacement of 1049.64 cu.in. The compression ratio of 4.75 :1. The later engines had the compression ratio increased to 5.5:1 and the horse power increased to 266 Hp.
Blue skies,
Dan-San

... Oh, I might add that no details of the engine are visible except for the exhaust stack, and the propeller shaft position extruding from the radiator. It is an rectangular opening with rounded top and bottom surfaces, but not tapered narrower at the top like the B.H.P. powered Westland variant.

Thanks Dan-San,

I wouldn't have guessed vee-twelve. It was confusing because there are no cylinder head or exhaust openings on the side like the Rolls. The central cylinder stack led me to believe that it was an inline rather than a vee-configuration. So much for guessing, eh?

How did the RAF 3a differ from the RAF 4a engine in the RE-8?

Av

RAF.4 was an air-cooled 60-degree V-12 first run in December 1914. First version had RAF.1a cylinders, 4.5 to 1 compression ratio, generating only 90 hp from a swept volume of 13.19 litres. Lubrication was by flywheel pickup. It employed two carburettors and two 6-cylinder BTH magnetos. Weight was 605lbs.

The improved RAF.4a weighed 637 lbs. and was rated at 140 hp at 1600 r.p.m. (actually 160 hp at the full-out 1800 r.p.m.).

Simba.

[Dan_San_Abbott]

Aviatik:
The RAF 4A had "F" head cylinders, the RAF 3A had overhead valves, the intake was to the outside of the cylinder and the exhaust to the inside of the cylinder. The displacement of the RAF 3A was 1049.6 cu.in. where the RAF 4A was 806.12 cu.in. The RAF 3A was rated @ 266 hp @ 1800 rpm. The RAF 4A was @ 140 hp @ 1800 rpm.
Blue skies,
Dan-San

Sincere thanks, Dan-San and Simba.

Another question. Where were the exhaust ports for the banks of cylinder-heads. Did they exit between the vees and up through a central collector and out the near vertical stack, instead of to the outside of the vee like on the Rolls?

[Dan_San_Abbott]

Aviatik:
On the RAF3A the exhaust was on the inside of the cylinders, so the exhaust ports faced each other. The individual exhaust pipes led to a central collector and the central collector became the vertical exhaust manifold. The dual carburrettors were mounted low on the outside, and on both sides of the lower crankcase. the intake pipes led vertically upwards the a horizontal log style manifold, from which the individual intake pipes led upwards to the intake port on the outside of the cylinder head, both sides of the engine.
Blue skies,
Dan-San