A question about the big aero engines of the past - capacity
Discussion
I have always wondered why it took such huge capacities to make the horsepower in the Merlin and Griffon engines. Is it simply that in those days the technology had not arrived to run engines at the high revs we see today? I ask because a Griffon, at 37 litres and supercharged to boot could manage about 2400hp which seems enormous overkill in terms of size/capacity per output.
I know they had to be reliable and run for extended periods at near full-power, and I'm not specifically comparing them with a modern dragster's engine which could manage similar power outputs from 1/4 of the capacity (because we all know how long they last) but the huge capacities of those old engines seems SO massive I just feel I've missed the reason for this.
I'm sure it's either obvious (and you'll point it out to me) or I've already hit upon it above.
Thanks.
I know they had to be reliable and run for extended periods at near full-power, and I'm not specifically comparing them with a modern dragster's engine which could manage similar power outputs from 1/4 of the capacity (because we all know how long they last) but the huge capacities of those old engines seems SO massive I just feel I've missed the reason for this.
I'm sure it's either obvious (and you'll point it out to me) or I've already hit upon it above.
Thanks.
I actually wonder if it's more to do with their inability to rev (relative to a modern engine). For example, the aforementioned 8-litre dragster engine, short-life notwithstanding, revs to ~9000rpm (more maybe) which is ~3x that of the Griffon. So, during any fixed period of time it'll be equivalent to a 24-litre (relatively) low-revving aero engine in terms of how much air it scoops up and fuel used.
There's something that supports this in the write-up of the chap who put one into a Rover SD1 - he needed a special transfer case to increase the output shaft revs which correspondingly decreased its torque to a level not inconsistent with a more lower capacity modern engine.
There's something that supports this in the write-up of the chap who put one into a Rover SD1 - he needed a special transfer case to increase the output shaft revs which correspondingly decreased its torque to a level not inconsistent with a more lower capacity modern engine.
Watchman said:
I have always wondered why it took such huge capacities to make the horsepower in the Merlin and Griffon engines. Is it simply that in those days the technology had not arrived to run engines at the high revs we see today? I ask because a Griffon, at 37 litres and supercharged to boot could manage about 2400hp which seems enormous overkill in terms of size/capacity per output.
I know they had to be reliable and run for extended periods at near full-power, and I'm not specifically comparing them with a modern dragster's engine which could manage similar power outputs from 1/4 of the capacity (because we all know how long they last) but the huge capacities of those old engines seems SO massive I just feel I've missed the reason for this.
I'm sure it's either obvious (and you'll point it out to me) or I've already hit upon it above.
Thanks.
To soak up the Griffon's horsepower, even at relatively low rpm you have to use an awful lot of propeller.I know they had to be reliable and run for extended periods at near full-power, and I'm not specifically comparing them with a modern dragster's engine which could manage similar power outputs from 1/4 of the capacity (because we all know how long they last) but the huge capacities of those old engines seems SO massive I just feel I've missed the reason for this.
I'm sure it's either obvious (and you'll point it out to me) or I've already hit upon it above.
Thanks.
If you asked the Griffon to run at high revs it would be useless, as the propeller it drove wouldn't be efficient. As it stands the blade tip speed are near supersonic, and when you take into account the crossing speeds on the contra rotating version - at high revs they already are.
Don't forget the supercharger isn't there purely to make more power as on a dragster either, its there to maintain power at higher altitudes which is why it has more than one speed.
Mmm, I'm not sure I buy that. You increase torque and decrease speed with a gearbox, or if required, the opposite.
I am trying to work out when the first truly high revving engines came about. I recall my father telling me that the Anglia was one of the first of that breed and before then, car engines were low revving plodders. I suspect the tech to provide high revving engines even in aviation wasn't around in the 1930s.
I don't know the calcs for props but in cars people often talk about torque as though it's the holy grail when it's power than dictates a car's speed. The gearbox is there to alter the torque at the wheels.
So, in the 1930s you need power, you increase capacity. In the 21st century you increase the revs (F1 was doing this year-on-year). Effectively you are both getting a similar through-put of fuel and air.
I am trying to work out when the first truly high revving engines came about. I recall my father telling me that the Anglia was one of the first of that breed and before then, car engines were low revving plodders. I suspect the tech to provide high revving engines even in aviation wasn't around in the 1930s.
I don't know the calcs for props but in cars people often talk about torque as though it's the holy grail when it's power than dictates a car's speed. The gearbox is there to alter the torque at the wheels.
So, in the 1930s you need power, you increase capacity. In the 21st century you increase the revs (F1 was doing this year-on-year). Effectively you are both getting a similar through-put of fuel and air.
The development programme for the Merlin was to put one on a dyno and run it at maximum output until it failed. They would then redesign the failed part and do it again.
High revs are a problem because they add stress on the reciprocating parts. For short life racing engines, this isn't too much of a problem because when it goes bang you just pull over and stop. You can't do the same in an aircraft, so the safety factor needs to be higher.
There is no doubt that you could produce a similarly powered aero engine with smaller capacity these days, or you could build a much more powerful one of the same capacity. It is the advances in accurate machining, metallurgy, and stress analysis that would allow it. A lot of engineering was done through rule of thumb because there wasn't the capability to try loads of different possibilities on the chance that it would work. There would be little point in building such an engine these days though, as a turbo prop can produce more power, more efficiently.
High revs are a problem because they add stress on the reciprocating parts. For short life racing engines, this isn't too much of a problem because when it goes bang you just pull over and stop. You can't do the same in an aircraft, so the safety factor needs to be higher.
There is no doubt that you could produce a similarly powered aero engine with smaller capacity these days, or you could build a much more powerful one of the same capacity. It is the advances in accurate machining, metallurgy, and stress analysis that would allow it. A lot of engineering was done through rule of thumb because there wasn't the capability to try loads of different possibilities on the chance that it would work. There would be little point in building such an engine these days though, as a turbo prop can produce more power, more efficiently.
Having said all that, there is a small scale "cottage industry" in keeping these old technology piston engines in good working order or being rebuilt for restoration programmes. It is commonly accepted that a modern, fully rebuilt Merlin is a much better engine than what was coming out of the Rolls Royce and Packard factories during World War 2.
WW2 fighters are not like racing cars, instead spending much of their time cruising to and from combat, and fresh air doesn't work the same as tarmac. So I don't understand this need for crazy revs.
But as a real life test, what engine of today would fit a Spitfire or similar and provide equal performance in terms of max speed at various altitudes and climb?
But as a real life test, what engine of today would fit a Spitfire or similar and provide equal performance in terms of max speed at various altitudes and climb?
Simpo Two said:
WW2 fighters are not like racing cars, instead spending much of their time cruising to and from combat, and fresh air doesn't work the same as tarmac. So I don't understand this need for crazy revs.
But as a real life test, what engine of today would fit a Spitfire or similar and provide equal performance in terms of max speed at various altitudes and climb?
And be happy running for 6-8 hours non-stop in various weather and altitudes?But as a real life test, what engine of today would fit a Spitfire or similar and provide equal performance in terms of max speed at various altitudes and climb?
Watchman said:
Mmm, I'm not sure I buy that. You increase torque and decrease speed with a gearbox, or if required, the opposite.
I don't know the calcs for props but in cars people often talk about torque as though it's the holy grail when it's power than dictates a car's speed. The gearbox is there to alter the torque at the wheels.
So, in the 1930s you need power, you increase capacity. In the 21st century you increase the revs (F1 was doing this year-on-year). Effectively you are both getting a similar through-put of fuel and air.
Torque is the 'holy grail'. Horsepower is a measurement of torque in a set time. Typically one horsepower is xx foot pounds per second. (look it up on wikipedia or somewhere.) As a result horsepower doesn't really dictate a cars speed at all.I don't know the calcs for props but in cars people often talk about torque as though it's the holy grail when it's power than dictates a car's speed. The gearbox is there to alter the torque at the wheels.
So, in the 1930s you need power, you increase capacity. In the 21st century you increase the revs (F1 was doing this year-on-year). Effectively you are both getting a similar through-put of fuel and air.
Some more thoughts on aero engines..
The Griffon ( and Merlin) have a massive reduction gear setup on the front, in the case of the Griffon its 0.44 : 1. Despite this step down the Griffon will swing the two 612lb 13 foot diameter propellers at an engine speed of 2,750 rpm. At sea level, it is using the supercharger's high gear, and water methanol; making 2,435 horsepower at 80" boost to do this. Load the propeller up, by altering the pitch, and you can lose up to 800rpm, through the extra power the propeller then absorbs.
If you should decide you want more performance, you can't just up the rev limit. Its not that simple. As I mentioned earlier once the blade gets near supersonic speeds the efficiency is lost... so its no good making the blades longer either to soak up more power, as the tips will be moving faster. The blade is no longer moving the air as it travels through it. You can add more blades and keep the speed down up to a point - which is where 5 bladed and contra rotating propellers make an appearance.
Its all about efficiency.
You're not just trying to go faster by using power as the ultimate cure all as an F1 car or dragster would, as you're not dependent on traction through a gearbox and tyres. You're trying to keep the propellers doing the most efficient work. This is why you see the Reno racing aircraft with cut down propellers, as with the increase in horspower and revs they can't afford to have the last foot of a heavy propeller blade not doing any work.
There were several attempts to come up with replacements for the Merlin and Griffon (and other aero engines of similar vintage), though with not much point as the propeller is always the limiting factor. That's why after its development the jet engine took over from piston.
Simpo Two said:
WW2 fighters are not like racing cars, instead spending much of their time cruising to and from combat, and fresh air doesn't work the same as tarmac. So I don't understand this need for crazy revs.
But as a real life test, what engine of today would fit a Spitfire or similar and provide equal performance in terms of max speed at various altitudes and climb?
That's a good question.But as a real life test, what engine of today would fit a Spitfire or similar and provide equal performance in terms of max speed at various altitudes and climb?
I would suspect an AMG V12 would be a good starter but the whole power vs reliability might be a problem. AMG V12s can produce enormous power in a car but nowhere near to the sorts of power the Merlin put out.
I concede entirely that those aero engines were reliable but I suspect you could achieve similar power and reliability for a much lower capacity these day. I reckon a 12 litre turbo V12 ought to be somewhere in that sweet spot.
You wouldn't use a piston engine. Turbines are much more suited to high power output. Take a look at the Piper PA-48 Enforcer, which was a development of the P-51 Mustang. It had a Lycoming T55 engine, which produced about 2500HP.
A derivative of that engine is used in the Chinook, producing upwards of 4500HP. It is 1.1m long, 0.6m in diameter and weighs 380Kg. By contrast, the Merlin 66 producing a maximum of 1700HP is 2.2m long, 0.78m wide and 1m high. It weighs 745Kg.
The turbine has a huge advantage.
A derivative of that engine is used in the Chinook, producing upwards of 4500HP. It is 1.1m long, 0.6m in diameter and weighs 380Kg. By contrast, the Merlin 66 producing a maximum of 1700HP is 2.2m long, 0.78m wide and 1m high. It weighs 745Kg.
The turbine has a huge advantage.
Watchman said:
Mmm, I'm not sure I buy that. You increase torque and decrease speed with a gearbox, or if required, the opposite.
I am trying to work out when the first truly high revving engines came about. I recall my father telling me that the Anglia was one of the first of that breed and before then, car engines were low revving plodders. I suspect the tech to provide high revving engines even in aviation wasn't around in the 1930s.
I don't know the calcs for props but in cars people often talk about torque as though it's the holy grail when it's power than dictates a car's speed. The gearbox is there to alter the torque at the wheels.
So, in the 1930s you need power, you increase capacity. In the 21st century you increase the revs (F1 was doing this year-on-year). Effectively you are both getting a similar through-put of fuel and air.
I wouldn't use car engines as a guide. They got built as long stroke low revving lumps to get a lower tax banding. I think it was the diameter of the cylinders that set the tax in the early days.I am trying to work out when the first truly high revving engines came about. I recall my father telling me that the Anglia was one of the first of that breed and before then, car engines were low revving plodders. I suspect the tech to provide high revving engines even in aviation wasn't around in the 1930s.
I don't know the calcs for props but in cars people often talk about torque as though it's the holy grail when it's power than dictates a car's speed. The gearbox is there to alter the torque at the wheels.
So, in the 1930s you need power, you increase capacity. In the 21st century you increase the revs (F1 was doing this year-on-year). Effectively you are both getting a similar through-put of fuel and air.
Gassing Station | Boats, Planes & Trains | Top of Page | What's New | My Stuff