Piston Engine versus Jet Engine Power

[saddlestone-man]

TL;DR Summary

How can I compare a piston engine powered airplane versus a jet engine powered one?

Hello All

The power output of piston engines fitted to aircraft were (and still are?) specified in terms of their HP rating, whereas jet engines are typically rated in terms of their trust. For example, a Mosquito two engined fighter/bomber was equipped with two Merlins, each producing something like 1600HP. The Gloster Meteor was fitted with two RR Derwents, each producing about 3,600lbf of thrust.

Ignoring the different aerodynamics of the two planes, how can I compare their relative performances based purely on their engines?

best regards ... Stef

 

[jbriggs444]

A quick trip to Google asking for exhaust velocity of the Gloster Meteor turned up an article which you may find helpful:

https://history.nasa.gov/SP-468/ch11-2.htm

 

[Lnewqban]

Your question is very interesting.
The productive result of both types of engines is thrust.
I don’t know why power has been relevant for propeller engines.

Copied from
https://en.m.wikipedia.org/wiki/Turboprop

“Unlike the small diameter fans used in turbofan jet engines, the propeller has a large diameter that let's it accelerate a large volume of air. This permits a lower airstream velocity for a given amount of thrust. As it is more efficient at low speeds to accelerate a large amount of air by a small degree than a small amount of air by a large degree, a low disc loading (thrust per disc area) increases the aircraft's energy efficiency, and this reduces the fuel use.

Propellers lose efficiency as aircraft speed increases, so turboprops are normally not used on high-speed aircraft above 0.6–0.7 Mach.
However, propfan engines, which are very similar to turboprop engines, can cruise at flight speeds approaching 0.75 Mach. To increase propeller efficiency across a wider range of airspeeds, turboprops are typically equipped with constant-speed (variable-pitch) propellers. The blades of a constant-speed propeller increase pitch as aircraft speed increases, allowing for a wider range of airspeeds than a fixed-pitch propeller. Another benefit of this type of propeller is that it can also be used to generate negative thrust while decelerating on the runway. Additionally, in the event of an engine failure, the propeller can be feathered, thus minimizing the drag of the non-functioning propeller.”

 

[jbriggs444]

I don’t know why power has been relevant for propeller engines.

Because, to a somewhat reasonable approximation, a propeller driven craft is pulling the craft forward by pushing on stationary air. The rearward air velocity imparted by the propeller is low compared to the aircraft's forward speed. For fixed power input, you get high thrust at low air speed and low thrust at high air speed.

Your variable pitch prop will be more nearly feathered and the lift generated by the prop blades will more nearly opposed to the rotation imposed by the engine as the airspeed increases.

To a somewhat reasonable approximation, a jet craft is driven by imparting a large rearward velocity to a small quantity of air. The rearward air velocity imparted by the jet is high compared to the aircraft's forward speed. For fixed (and high) power input you get the same thrust at all speeds. You are expending a large amount of your engine power putting energy into the exhaust stream.

 

[anorlunda]

Don't forget high bypass fanjet engines which are so prominent in modern airliners, and mentioned above. @Lnewqban called them propfan. A fanjet can be reasonably be described as a hybrid between propeller and turbojet.

 

[Lnewqban]

Another reason could be the practical different ways in which those two different types of engines were tested by manufacturers.

First biplanes with fixed-pitch wood propellers used piston engines, which supplied torque and brake horse power could be easily measured by manufacturers in static test devices such as Prony brakes.
The static thrust of any engine-propeller combination could also be measured, but that value was always much lower than the effective thrust at flying speeds, due to the stalled condition of that propeller’s blades in static air.

Measuring the brake horse power of a jet engine is not less difficult than coupling a dynamometer to its shaft.
A more practical approach is to anchor the jet engine to a stand or structure equiped with force sensors that indicate the static thrust that the engine can achieve at normally working rpm’s.

Please, see:
http://www.aerospaceweb.org/question/propulsion/q0195.shtml

http://www.aviationchief.com/turbine-engines.html

https://en.m.wikipedia.org/wiki/Prony_brake

https://simple.m.wikipedia.org/wiki/Thrust

 

[saddlestone-man]

Many thanks for the replies to what I thought was a simple question which might have a simple answer - ie x HP from a piston engine is equivalent to y lbs of thrust from a jet engine. I realize now that the piston engine relies on the the propeller to generate thrust efficiently, and this varies greatly depending on speed, pitch. etc.

The combination of a jet engine and a propeller is still an effective combination. I live close to RAF Brize Norton and the Hercules C-130s and Airbus A400Ms flying around are a testimony to that. And what is a high-bypass turbofan engine other that a jet engine with an enclosed propeller?

There are of course many jet engines (or should I say gas turbines?) which are rated in HP, for example the engine in the Abrams M1 tank, APUs in aircraft, gas turbines in quick-start power stations, gas turbines in railway locos, etc.

 

[russ_watters]

Many thanks for the replies to what I thought was a simple question which might have a simple answer - ie x HP from a piston engine is equivalent to y lbs of thrust from a jet engine. I realize now that the piston engine relies on the the propeller to generate thrust efficiently, and this varies greatly depending on speed, pitch. etc.

Right, but with enough information about a specific enough scenario you can calculate one from the other, or either from flight performance. Do you have one in mind?

The combination of a jet engine and a propeller is still an effective combination. I live close to RAF Brize Norton and the Hercules C-130s and Airbus A400Ms flying around are a testimony to that. And what is a high-bypass turbofan engine other that a jet engine with an enclosed propeller?

There are of course many jet engines (or should I say gas turbines?) which are rated in HP, for example the engine in the Abrams M1 tank, APUs in aircraft, gas turbines in quick-start power stations, gas turbines in railway locos, etc.

Well, if you look up a few spec sheets, I think you'll find that if all of the jet exhaust output is used for shaft power it is rated for shaft power, but if any of the output is used for propulsive thrust it is rated for propulsive thrust.

It's also worth noting that in some cases a propulsion jet and a power plant are basically the same engine, but with an extra section to turn the rest of the jet exhaust thrust into shaft power. For example, the GE LM2500 turbine (33,600 hp) powers ships and power plants and is derived from the CF6 (41,500 lb, static), which propels the DC-10 aircraft.

[edit] And I think you'll find that the regime of flight (situation) determines which metric is more relevant. For example, if you know speed and distance or time for a takeoff roll, and airplane weight, you can calculate average/constant thrust. If you know rate of climb and weight, that easily converts to power (albeit power delivered to the air, not the shaft).

 

[saddlestone-man]

The example I had in mind was a 1945-ish scenario of a state of the art two piston engined fighter: ie a Mosquito fighter/bomber equipped with two Merlins, each producing something like 1600HP. and its up and coming replacement, the Gloster Meteor fitted with two RR Derwents, each producing about 3,600lbf of thrust.

Can it be said that the Meteor had x-times the performance of the Mosquito, purely by comparing the engine outputs? One complication is that the Meteor was considerably lighter, presumably because a pair of Merlins were much heavier than a pair of Derwents.

 

[hmmm27]

Even though - according to Wikipedia - the Meteor replaced the Mosquito as a night-fighter, they aren't really comparable aircraft : one's a fighter, the other a fighter-bomber.

Per the original question, you can stick any propeller onto a piston engine, depending on the use, which changes the thrust.

 

[russ_watters]

The example I had in mind was a 1945-ish scenario of a state of the art two piston engined fighter: ie a Mosquito fighter/bomber equipped with two Merlins, each producing something like 1600HP. and its up and coming replacement, the Gloster Meteor fitted with two RR Derwents, each producing about 3,600lbf of thrust.

Can it be said that the Meteor had x-times the performance of the Mosquito, purely by comparing the engine outputs? One complication is that the Meteor was considerably lighter, presumably because a pair of Merlins were much heavier than a pair of Derwents.

It's going to be more complicated than just "x-times the performance" because "performance" is undefined: it's a broad category, not a specific characteristic/attribute. But we should be able to figure out a difference in takeoff or climb thrust, for example. I don't have time right now, but I'm sure we can dig into it...

[edit]
Well, here's an easy one: the wiki for the Meteor says it had a 7,000 fpm rate of climb and a gross weight of 15,700 lb. If those values are coincident, that's 3,300 hp. If we guess the propeller efficiency of the Mosquito was 50%, that would make the Mosquito's power delivered to the air 1,600 hp, or about half that of the Meteor.

 

[russ_watters]

The example I had in mind was a 1945-ish scenario of a state of the art two piston engined fighter: ie a Mosquito fighter/bomber equipped with two Merlins, each producing something like 1600HP. and its up and coming replacement, the Gloster Meteor fitted with two RR Derwents, each producing about 3,600lbf of thrust.

Even though - according to Wikipedia - the Meteor replaced the Mosquito as a night-fighter, they aren't really comparable aircraft : one's a fighter, the other a fighter-bomber.

Agree with @hmmm27 -- it also says the Meteor first replaced the Spitfire VII, which was a pure fighter/interceptor. It doesn't mean they can't be compared, but it may mean that the comparisons may be more multi-faceted.