RR vs Bristol

[PMN1]

From what I've read, in the very late 1930's the 1,550 cubic inch Taurus was developing around 1,100hp and the 2,360 cubic inch Hercules was developing around 1,300hp.

However the 1,348 cubic inch RR Exe was developing about 1,200hp.

What was the reason for this difference in HP per cubic capacity of the RR and Bristol engines - number of cylinders?

(I'm not sure about what octane fuel was being used)

[simon]

I'm not an engine man so I'm really taking a stab in the dark here, but I have read of engines being de-rated (The reference actually was in regard to the Blackburn Botha, shudder...), my guess is that engines designed to run at a lower horsepower than their theoretical maximum capacity would last longer and broadly be more reliable especially for long distance flights where speed was perhaps not the critical factor, for example a bomber's engines.

Also Liquid cooled engines would be able to run at a slightly higher level since the liquid coolant should prevent overheating, so generating more horsepower.

I'm hoping this is right, it seems a fairly sensible explantation, however like I say I'm not an engine man (Until recently I didn't even know what a supercharger was!!!), so I'm probably hugely off the mark.

Over to you Greg... :D

[GregP]

Many inline engines have internal gearing. That means the engine can turn, say, 3500 rpm, while the propeller turns whatever rpm it takes to keep the tips from going supersonic. Many WWII propellers turned at 2000 rpm or less.

Most, but not all, radials turn at crank speed with no internal gearing. It is much harder to make serious horsepower at 2000 rpm than it is at 3000 rpm.

One notable excepiton to this is the current Russian M14P radial. It is a small engine and turns out 360 to 400 hp, and is internally geared.

Many WWII engines were required to run on 80 Octane gasoline. That also didn't help because there is only so much cylinder pressure you can generate until the mixture detonates ... a bad thing.

I don't KNOW the reason behind the horsepower differences, but I DO know that the inlines with internal gearing had an advnatage on non-geared radials when it came to horespower.

The "old reliable" R-2800 started life at about 1800 hp and was quickly improved to 2000 hp. This engine was used in this form in many US aircraft including the Hellcat and Corsair (same engine and propeller for most models). Late versions of the R-2800 made 2800 hp!

To put that into perspective, Merlins didn't make horsepower equivalent to cubic inches displacement until they got over 1600 hp. That wasn't for awhile.

A last consideration is that many WWII engines had built-in boost. That is, built in supercharging, turbocharging, or turbosupercharging. Boosted engines will always outpower unboosted engines of otherwise equivalent configuration.

There is also the matter of turbonormalizing versus boost. A turbonormalized engine makes a certain horsepower at sea level and uses the turbocharger or supercharger to maintain the same power level as altitude increases until the boost device reaches an altitude at which it can no longer maintain the desired power level. This is called the "critical altitude." The engine system gradually loses pwoer as you go up from critical altitude.

Standard boost is used to increase the power output at ALL altitudes including sea level, so the engine is in the boost regime all the time.

By way of example, if a plane has a critical altitude of 18,500 feet .. then that is very probably close to where the maximum speed is to be had. High altitude fighters and bombers had compound boost devices to increase the critical altitude to over 30,000 feet, so they were fastest and most efficient at very high altitudes. The P-39 was not allocated a boost device (for which it had been designed) and was very effective at low altitudes, but was a dog above 15,000 feet. This is because the "critical altitude" for a normally aspirated engine is sea level.

That leaves the question: Of the engines you are speaking about, which were boosted and which were normally aspirated? Were they geared or not? Were they boosted or normalized?

The answer probably lies in there and not with the gasoline octane since the gas supply was probably not segregated for use by engine type.

[Wuzak]

quote:Originally posted by GregP

The "old reliable" R=2800 started life at about 1800 hp and was quickly improved to 2000 hp. This enmgine was used in this form in many US aircraft including the Hellcat and Corsair (same engine and propeller for most models). Late versions of the R-2800 made 2800 hp!

To put that into perspective, Merlins didn't make horsepower equivalent to cubic inches displacement until they got over 1600 hp. That wasn't for awhile.


The 60 series Merlins made around 1hp per cubic inch, depending on the fuel.

http://www.pratt-whitney.com/about_history_classic_r2800.asp
The 61 had a maximum rating of 1565hp, slightly lower than 1hp/cid. The 66 had a maximum rating of 1705hp with 100 octane fuel, 2000hp with 150 octane fuel.

The 66 was the same as the V1650-7, as used in the Mustang.

http://www.spitfireart.com/merlin_engines.html

According to Pratt and Whitney, the R2800 produced "up to 2500hp".

It was my impression that the R2800 could produce 2800hp when using water/methanol injection.

[Mark J]

Interesting! I know the watercooled inline engines were thermostat controlled to a certain temperature, generally running cooler which enabled a leaner mixture, a more effecient burn cycle and more power (and of course, better economy).
Unlike the Bristol radials, the RR Exe was an air cooled inline which was geared, enabling higher engine revolutions without the prop blades 'cavitating', thus producing more power per cubic inch although the Exe's fuel economy couldn't have been very good (compared to other inlines) as the rear cylinders would have run very rich to keep the temp down.

cheers

[Wuzak]

quote:Originally posted by Mark J

Interesting! I know the watercooled inline engines were thermostat controlled to a certain temperature, generally running cooler which enabled a leaner mixture, a more effecient burn cycle and more power (and of course, better economy).

Part of the reason why liquid cooled engines can outperform air cooled engines is that the heat is more effectively taken away from the combustion chamber, which means that the engine block, head and pistons experience less thermal expansion and hence can use tighter clearences between piston and bore.

[GregP]

If liquid-cooled engines can outperform air-cooled engines, then why didn't they make a large-displacement, liquid-cooled engine to be the ultimate powerplant?

The answer is that only so much horsepower can be produced by an engine in HP per pound of fuel. Much of the pwoer goes out the exhaust as heat.

The radial was bigger in displacement but made good, reliable power. The inline, liquid-cooled V-12 was smaller but probably boosted and more efficient.

Of the two, the radial was more reliable and protected the pilot much better from front fire ... and if I were flying in combat, I'd opt for a radial thank you in most circumstances, but I MUST state that the best piston fighters of WWII, unarguably the Bf 109, Spitfire, and Mustang (in no particular order ... tom avoid THAT fight) were ALL liquid-cooled V-12's. So, the case for the radial is nowhere NEAR complete and final.

In a final decision, I'd say that the pilot made the difference, but I'd REALLY rather have a reliable, tuned, good-perfroming fighter than a particular ordinary one with any particular engine.

The crew chief was probably THE really important factor in fighter versus fighter combat since tuning and rigging made the palne much better or worse.

I salute the crew chiefs of the world and want the best damned crew chief for myself. [:I] To hell with everyone else. Give me a guy (or gal) who KNOWS the engine, airframe, and weapons better than anyone else.

A great combination of the three was probably responsible for Erich Hartmann, Gerhard Barkhorn, and Gunther Rall, the three greates combat pilots of all time. If they had ordinary maintenance crews, they would have been average or slightly better-than-average pilots due to average aircraft reliability.

In the actual event, these three aces mostly had dead solid perfect aircraft in which to fight. The pilot skills could, therefore, be exploited to the maximum, and were.

[Tony Williams]

quote:Originally posted by GregP

If liquid-cooled engines can outperform air-cooled engines, then why didn't they make a large-displacement, liquid-cooled engine to be the ultimate powerplant?
Actually they were planning to, but the end of WW2 and the development of the turbine engine caused the final series of big piston engines to be cancelled.

These included the following:

Pratt & Whitney H-3130 (24 cyls in H formation, 61 litres, 2,650 hp)

Allison V-3420 (24 cylinders in W formation, 56 litres, 3,000 hp)

R-R Eagle II (24 cyl in H formation, 46 litres, 3,500 hp)

Lycoming XR 7755 (36 cyl in radial formation, 126 litres. 5,000 hp: 7,000 hp planned)

Studebaker H-9350 (24 cyl in H formation, 153 litres, 5,000 hp - probably never completed)

You wll note that the Lycoming was the only radial, and that was liquid-cooled as the cylinders were in four rows.

Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk

[Wuzak]

quote:Originally posted by Tony Williams

quote:Originally posted by GregP

If liquid-cooled engines can outperform air-cooled engines, then why didn't they make a large-displacement, liquid-cooled engine to be the ultimate powerplant?

Pratt & Whitney H-3130 (24 cyls in H formation, 61 litres, 2,650 hp)

Allison V-3420 (24 cylinders in W formation, 56 litres, 3,000 hp)

R-R Eagle II (24 cyl in H formation, 46 litres, 3,500 hp)


The RR Eagle 22 was the same size as the R2800 (2800cid) but early in development was already making more power than the R2800. It was pointed out in an earlier discussion that there were some problems with the RR design.

The H-3130 seems a little underpowered for its size, while the V-3420 was basically two V-1710s joined together, so was about twice the power of them.

The Napier Sabre was around 2200cid (37l), the same as a Griffon, but was, by the end of the war, matching the power outputs of the R2800.

There has to be a point that running a larger engine gains no benefit over running two smaller engines. For instance, two Merlins are a similar capacity together as one Centaurus.

[Tony Williams]

quote:Originally posted by Wuzak
There has to be a point that running a larger engine gains no benefit over running two smaller engines. For instance, two Merlins are a similar capacity together as one Centaurus.
There are pros and cons, of course. Other things being equal, a plane with one big engine is going to be smaller and lighter overall than one with two small engines, cheaper to buy and easier to maintain. It will also be faster because of less drag (unless you're taking the Do 335 route).

OTOH the twin may be more survivable, and if the engines are in the wings that leaves the nose clear for armament or radar.

The Douglas Skyraider was the last of the big, radial, single-engined warplanes, and I doubt it would have been any better with two small engines - in fact, the extra size would have a problem on board a carrier.

Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk

[Wuzak]

quote:Originally posted by Tony Williams

quote:Originally posted by Wuzak
There has to be a point that running a larger engine gains no benefit over running two smaller engines. For instance, two Merlins are a similar capacity together as one Centaurus.
There are pros and cons, of course. Other things being equal, a plane with one big engine is going to be smaller and lighter overall than one with two small engines, cheaper to buy and easier to maintain. It will also be faster because of less drag (unless you're taking the Do 335 route).

OTOH the twin may be more survivable, and if the engines are in the wings that leaves the nose clear for armament or radar.

The Douglas Skyraider was the last of the big, radial, single-engined warplanes, and I doubt it would have been any better with two small engines - in fact, the extra size would have a problem on board a carrier.

Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk



Sure, drag might be a little more for a twin installation, but that wouldn't always be the case.

For example, the Lycoming XR 7755 was ridiculously large. For the initial power levels they could have had two Griffons or twin R2800s, or for the developed power levels a pair of similarly developed Sabres would have given the same result.

Then there is also the possibility of running twin engines in the fuselage (for a single seater) in the manner that was used for the Dornier Do335 and the Kawasaki Ki-64.

The Centaurus was about 3/4s the weight of twin Merlins, so it wins on that count. As for drag I would think the two options would be closer than that.

The other advantage of a twin is the possibility of having counter rotating props without a contra-prop mechanism.

Of course to get the super huge motors they either have to make the individual cylinders bigger, which isn't necessarily the best for combustion, or run more cylinders, which is risky in terms of complexity and reliability.

[Lightning]

Hi Greg,

Quoting you:
quote:...the best piston fighters of WWII, unarguably the Bf 109, Spitfire, and Mustang (in no particular order...)

Now now, Greg. Of all the words you could have used here, unarguably was probably the worst choice! Remember where you are! :D

Regards,
Lightning

[Lightning]

Hi Wuzak,

Quoting you:
quote:Sure, drag might be a little more for a twin installation, but that wouldn't always be the case.

I tend to agree with you here. Just because a plane is bigger does not necessarily mean that it has more drag. A lot depends on shape, location and mounting of components, surface texture of skin, and--very important--how the engines are cowled. Tony qualified his statement by saying, "other things being equal," but that is a hard situation to achieve when comparing planes of such different configurations.

Regards,
Lightning

[Wuzak]

quote:Originally posted by Lightning

Hi Greg,

Quoting you:
quote:...the best piston fighters of WWII, unarguably the Bf 109, Spitfire, and Mustang (in no particular order...)

Now now, Greg. Of all the words you could have used here, unarguably was probably the worst choice! Remember where you are! :D

Regards,
Lightning


I agree! Arguably was probably the best word to use there....

[Wuzak]

quote:Originally posted by Lightning

Hi Wuzak,

Quoting you:
quote:Sure, drag might be a little more for a twin installation, but that wouldn't always be the case.

I tend to agree with you here. Just because a plane is bigger does not necessarily mean that it has more drag. A lot depends on shape, location and mounting of components, surface texture of skin, and--very important--how the engines are cowled. Tony qualified his statement by saying, "other things being equal," but that is a hard situation to achieve when comparing planes of such different configurations.

Regards,
Lightning



Not to mention that a twin engined aircraft always has to be bigger than a single....

Remember that Supermarine's proposed twins Types 324/325/327 were very much the same overall dimensions as their Hawker Tornado/Typhoon competitors.

[GregP]

I think we HAD the "best fighter" argument and I think the Bf-109, Mustang, and Spitfire accounted for most of the votes. AM I wrong?

You COULD argue that the FW 190 was right in there during one time period, but it was eclipsed by later varianet of at least two of the above-mentioned three planes later in its career.

Without data, it is tough to include Yaks and Lavochkins, though they were a HUGE factor on the Russian Front.

I dismiss the Me-262 out of hand since its contribution to WWII was insignificant. Its contribution to Post WWII aviation is undeniable, but during WWII, it was nothing and Germany would not have missed it had it not been produced.

Sorry folks, I really SHOULD have remembered where I was posting!

OK, arguably ... :)

[Lightning]

Hi Greg,
quote:OK, arguably ...
Now that's more like it! I certainly won't argue with the use of that word. :D

Regards,
Lightning

[Shortround6]

Resurrecting an old thread.


From what I've read, in the very late 1930's the 1,550 cubic inch Taurus was developing around 1,100hp and the 2,360 cubic inch Hercules was developing around 1,300hp.

However the 1,348 cubic inch RR Exe was developing about 1,200hp.

What was the reason for this difference in HP per cubic capacity of the RR and Bristol engines - number of cylinders?

(I'm not sure about what octane fuel was being used)

"number of cylinders" is exactly right.

Smaller cylinders are easier to cool, there is more wall area per unit of volume and the walls are closer to the center of the piston making for better cooling of the piston.

Shorter strokes allow for higher rpm at similar piston speeds. The Exe had a 4in stroke for a piston speed of 2800fpm at 4200 rpm compared to the the Taurus's 2906fpm at 3100rpm or the Hercules's 2925 fpm at 2700 rpm.

The approximately 50% increase in rpm for the Exe helped equalize the displacement difference. But it did not come at no cost. The Exe was about 200lbs heaver than the Taurus, it's higher efficiency due to high compression might be partial canceled by it's higher internal friction. The manufacture of 24 sleeve valve cylinders is not likely to be cheaper than 14 cylinders even if they are larger.
We also don't know if the Exe would have worked out in service. The Napair Dagger, a 24 cylinder 'H' style air-cooled engine had cooling problems.