Helicopter piston engines query

[Weaver]

I've seen it stated several times that when UK manufacturers were designing the first generation of piston-engined helicopters in the late 1940s, the only "suitable" British engine available was the 550hp Alvis Leonides 9-cylinder radial.

Why?

Why couldn't you build a helicopter with a Hercules or Centaurus engine? I appreciate that a helicopter engine needs a cooling fan and a clutch and doesn't need a reduction gearbox, but those are "bolt-on" components to the core engine. The Leonides was made in aeroplane and helicopter versions, so why couldn't more powerful engines be adapted in the same way?

[Captain Canada]

Good question ! Weight vs. lift maybe ?

[PR19_Kit]

I recall something about the Leonides' lubrication system meant it could more easily be installed with the shaft vertical, as used in the Sycamore etc. Maybe that was one reason, but it was also pretty light as well, which the Centaurus certainly wasn't!

[Weaver]

The Leonides' power:weight ratio wasn't anything special:

(from Wikipedia data)

Alvis Leonides : 550 hp, 815 lb, 0.67 hp/lb
Bristol Hercules II : 1272 hp, 1929 lb, 0.66 hp/lb (Wikipedia page claims 0.7, later Hercs were much more powerful)
Bristol Centaurus VII : 2520 hp, 2695 lb, 0.93 hp/lb
P&W R2800-54 (as used in CH-37) : 2100 hp, 2360 lb, 0.89 hp/lb.
Shvetsov ASh-82V (as used in Mil-4) : 1675 hp, 2246 lb, 0.74 hp/lb

What got me thinking about this was looking at the Mil-4 Hound. It looks superficially like a ripped-off Sikorsky S-55, but it was actually much more powerful: no S-55 had more than 800 hp and most only had 600.

I was considering whiffing my Mil-4 model as a British competitor to the Westland Whirlwind (S-55) and wondering what British engine was available for it. I take Kit's point about the lubrication system (I have half a memory of this too...), but then in a Mil-4/S-55 style installation, the engine isn't vertical, just tilted.

[rickshaw]

I think it was a combination of power/weight, size and lubrication.  As they were still stretching their rotors, so to speak they were looking at light helicopters and the Leonides fit the bill.  By the time they started looking at bigger helicopters, the turbine engines had arrived on the scene and they essentially jumped over the big pistons to them or they'd bought American designs, including the piston engines.

[Weaver]

Hang on, I've got half the story so far.... There was also a Leonides Major 14-cyl 2-row engine, as used in later Whirlwinds:

Alvis Leonides Major: 860 hp, 1200 lb, 0.72 hp/lb

Still not earth-shaking in p/w ratio though. The Centaurus has the best p/w ratio of any of the engines I've looked at, and I'd bet that later-model Hercs were in the same ball park.

[jcf]

Your forgetting about another dimension, diameter.
The Centaurus was 55", the 780hp 14-cylinder Leonides Major (Whirlwind Series 2, Mk. 5 - 8 )
was 38.9", and at 1,065 lbs, it also weighed around 140lbs less than the Leonides Major used
on the H.P. Herald, which is what the numbers you've quoted are for.

Hercules and Centaurus lengths, weights and output are all over the map, a specific sub-type
is needed to get anywhere close to figuring out realistic options. Some Hercules variant
would probably be most similar to the Ash-82.

[Dizzyfugu]

Another issue with more powerful radials should be the mounting stability of the technical components. Large machines mean a huge torque leverage, stronger vibrations - all extra weight that eats up more of the theoretical benefit of a stronger radial engine. Turboshaft engines just have a much better power-to-weight ratio, are much more compact and its movable parts should also call for less rigid structures than a comparable radial. The S-56/CH-37 was probably the true end of piston-engined helicopters.

[PR19_Kit]

And any helicopter that used the Centaurus would need an oild tank the size of a small house!

My Dad used to say that a Centaurus had 4 more cylinders, 1.5 times as much displacement, a 1000 more horsepower and a MILLION more oil leaks than a Hercules!

[Weaver]

Your forgetting about another dimension, diameter.
The Centaurus was 55", the 780hp 14-cylinder Leonides Major (Whirlwind Series 2, Mk. 5 -
was 38.9", and at 1,065 lbs, it also weighed around 140lbs less than the Leonides Major used
on the H.P. Herald, which is what the numbers you've quoted are for.

That's still only 0.73 hp/lb for the helo version of the Leonides Major.

The ASh-82V had a diameter of 49.6" and was still a practical engine, so bigger was obviously possible.

Hercules and Centaurus lengths, weights and output are all over the map, a specific sub-type
is needed to get anywhere close to figuring out realistic options. Some Hercules variant
would probably be most similar to the Ash-82.

Agreed on both counts: getting comparable numbers is a minefield!

I think a later Hercules could have been a realistic helicopter engine.

[Weaver]

Another issue with more powerful radials should be the mounting stability of the technical components. Large machines mean a huge torque leverage, stronger vibrations - all extra weight that eats up more of the theoretical benefit of a stronger radial engine. Turboshaft engines just have a much better power-to-weight ratio, are much more compact and its movable parts should also call for less rigid structures than a comparable radial. The S-56/CH-37 was probably the true end of piston-engined helicopters.

No argument there: gas turbines are so overwhelmingly better that the whole world went over to them ASAP and you now only find piston engines in the smallest helos where purchase price is the dominant issue. However the fact remains that there was a generation of piston-engined helicopters, and what I don't understand is why the Americans and Russians could put their bigger air-cooled radials into them, but the British couldn't.

[kitnut617]

A good example is the Westland Westminster for that, it used the rotor and gearbox from the CH-37

[Mossie]

There were plans to put a Wright R-1820 in an S-55 that came to nothing (HRS-4), it eventually went in the S-58 instead.  I don't know the reason why the project didn't go ahead.

[jcf]

Also remember that while the Mil-4 looks like an S-55, due to similar mechanical
and fuselage layout, it is actually larger than the S-58/Wessex. So the choice of the
Ash-82V as power-plant to drive a four-bladed rotor system is unsurprising.

According to the info in Russian Piston Aero Engines the Ash-82V rated power was
1350hp at 1700 rpm with an installed weight of 2400 lbs.

Hercules diameter was 52", a helo engine developed from the 100 Series type would
be most likely. The Hercules was smaller displacement than the Ash-82 and made it's
rated power at higher rpm.

As to the S-55 re-engine not happening, probably because by the time they got done
modifying the airframe and rotor-system to absorb the increased weight and power,
they had a new design. 

[PR19_Kit]

Of course to be REALLY weird you need to look at the engine fitted to the three-rotor Cierva Air Horse.

They used a MERLIN of all things!