Bristol Pegasus

[Stealer]

Here are some photos of the Bristol Pegasus 2 valve air cooled 9 cylinder radial engine. Of course, these engines powered many aircaft including the Short S23 C-Class Empire 'boats and the Mk III Sunderlands. What's interesting about the design is the fact that the valve pushrods are behind one another:

[Obanboats]

I have currently a Pegasus XVIII at Oban recovered from the sea bed in excellent condition complete with prop. It is to be sent on to RAF Cosford for stabilisation.

[seawings]

Hi Obanboats,

Gosh, I would love to see some pictures of that! Know anything about its history?

[dogsbody]

Hi Obanboats,

Gosh, I would love to see some pictures of that! Know anything about its history?

Me, too.

[sunderlandmr5]

Hi Obanboats,

Gosh, I would love to see some pictures of that! Know anything about its history?

Yes please, I would too

Thanks

Alan

[Philthy]

An interesting feature of the Pegasus was specially designed valve gear that ensured correct valve opening when cold or hot, allowing the engine to be run up to full power without the need for a lengthy warm-up which, in a flying boat, could be a bit of a problem.

[Philthy]

Here's a question for the technical experts:

Most sources seem to quote the Pegasus Xc as fitted in the Short Empire flying boats (S.23 & S.30 at least) as having a power output of 920 hp.

However, both the British and Australian CofAs for these aircraft give the Pegasus Xc as 740 hp at 2,250 rpm at rated height of 4,500 ft and 815 hp at 2,475 rpm.

Any ideas why?

[MrWidgeon]

In piston engines the rated HP drops with altitude, hence the need for superchargers or turbo-superchargers.
It can also vary slightly with temperature, humidity and barometric pressure.

[Philthy]

Indeed they do Mr Widgeon, but one would hardly expect a loss of more than 10% of rated power in 4,500 ft, especially since the Pegasus was supercharged. Density altitude shouldn't come into it as these are standard figures.

[SimonThomas]

The "international rating" for the Xc is 710/740 hp at 2475 rpm at 3,500 rpm. (From Flight magazine Dec 12, 1940 p 509)
The same reference also states the maximum altitude rating as 830 hp at 2600 rpm at 5250 ft; the take off power as 920 hp and the dry weight as 1030 lb.

The Flight magazine dated Nov 12, 1936 states the normal output at rated altitude as 785/815 hp (rated altitude nominated as 4,500 ft). This edition also nominates 920 hp as the takeoff rating and the dry weight as 1015 lb.

I guess in modern terminology, 920 hp is for takeoff and 740 hp is maximum continuous.