Question about variable-pitch propeller aircraft & slow flight
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190 kts g/s
Jab,
Even Qantaslink plan slow going out , fast coming home and their blokes have got Class 1s !!!!!
It's not always there but there is usually a tailwind [westerly ,that is] in the FLs....tonight Area 41 14000 230/30 area 40 240/35.......
Flopt again ....now a CASA DAEE !!!!!!!!!!!!
PS special rates for prunes...
Even Qantaslink plan slow going out , fast coming home and their blokes have got Class 1s !!!!!
It's not always there but there is usually a tailwind [westerly ,that is] in the FLs....tonight Area 41 14000 230/30 area 40 240/35.......
Flopt again ....now a CASA DAEE !!!!!!!!!!!!
PS special rates for prunes...
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Have you got any references for your comments. I have been in this game for a long time and I am sure I have never heard any of this before
baron_beeza, the reference behind this assertion of mine would have to be the Val Dyson-Holland textbook for the CPL syllabus subject "Engines and Systems".
This is by far the most comprehensive book on the theoreticals of piston engines that I have ever seen. I still have the book but it is in storage in a different state to that where I am living now.
It was 20 years ago that I did the CPL subjects so the significance of the friction, in my mind, may have changed over that time. I would really need to go back into that book and check that.
It was also from this reference that I made the statement that "heat loss is the source of greatest efficiency loss" in the reciprocating engine.
I thank you and ZEEBEE for raising the alternative of vacuum being the main driver of engine braking. I was unaware of this and have learned something new.
2500 rpm 22" 66% 124 KTAS 8.8 GPH
2200 rpm 24" 66% 124 KTAS 8.9 GPH
The engine is producing more MP at a lower RPM for the same percentage of power output. I would assume the engine is working a little harder to turn the prop via the governor at a higher RPM loosing about 2" in the process.
2200 rpm 24" 66% 124 KTAS 8.9 GPH
The engine is producing more MP at a lower RPM for the same percentage of power output. I would assume the engine is working a little harder to turn the prop via the governor at a higher RPM loosing about 2" in the process.
Engines do not "produce" MP. The higher fuel flow at the lower RPM (for the same power) is because with aircraft piston engines, the lower the RPM, the less fuel efficient is the engine.
Petrol burning engines get the most work from the fuel at RPMs around the 3,000 mark. Diesel engines are thus more efficient for aircraft as they are far more fuel efficient in the RPM range that is required for aerodynamically efficient propellors.
The "solution" for petrol burning aircraft engines is the gearbox, which allows the engine to run at the higher speeds it likes (in terms of fuel efficiency) whilst allowing the prop to turn at the lower speeds it prefers (around 1,700 RPM - depending on typical cruise airspeed).
The gearbox brings weight and complexity penalties however.
Petrol burning engines get the most work from the fuel at RPMs around the 3,000 mark. Diesel engines are thus more efficient for aircraft as they are far more fuel efficient in the RPM range that is required for aerodynamically efficient propellors
Fuel - Diesel fuels contain more energy per unit and burns slower so lower consumption can be achieved for the same power output. The fuel is also generally cheaper than gasoline (dependant on goverment tax policy). Due to the high flashpoint they are also safer to handle and less likely to ignite in an accident.
Simple and Robust - Diesel engines have less componants (no ignition source required). The fuel also has better lubricating properties and the nature of construction and operation allows longer time between overhaul.
Why are they not in all aircraft?
Because diesels are inherently heavier than gasoline engines and the fuel itself is heavier. Weight has been the enemy of aircraft designers since day one. The cost of producing diesels is generally higher as well and once you start getting past about 500hp the jet based engines easily win the weight/power/reliability/consumption/cost argument.
The "solution" for petrol burning aircraft engines is the gearbox, which allows the engine to run at the higher speeds it likes (in terms of fuel efficiency) whilst allowing the prop to turn at the lower speeds it prefers (around 1,700 RPM - depending on typical cruise airspeed).
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Lowest fuel consumption?
Fgd
I suggest you have a look at some Lycoming engine manuals. You will find that lower RPM settings DO result in lower fuel consumption for the same horsepower.
A long time ago I was told that Rolls Royce advised that maximun endurance is achieved with "high boost and low RPM." The Lycoming manuals and my own experience definitely confirm this.
I suggest you have a look at some Lycoming engine manuals. You will find that lower RPM settings DO result in lower fuel consumption for the same horsepower.
A long time ago I was told that Rolls Royce advised that maximun endurance is achieved with "high boost and low RPM." The Lycoming manuals and my own experience definitely confirm this.
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das Uber Soldat the good majority of the posts here are rubbish or off-topic. There is only a handful of decent posts that help to answer my original question.
Since the time I made the thread I've scanned the flight manual & got my books open on constant speed propellers so I'm going back to basics & clearing up the gaps in my knowledge that way. At least these two resources were definitely written by adults so can be trusted.
Having left aviation for 4 years & only just getting current again & rated in a Cessna 172RG with constant speed prop I have proven to my instructor I am a capable pilot. Surely I can be forgiven for having some gaps in my knowledge but I am making every effort to fix that before I next take to the skies.
Since the time I made the thread I've scanned the flight manual & got my books open on constant speed propellers so I'm going back to basics & clearing up the gaps in my knowledge that way. At least these two resources were definitely written by adults so can be trusted.
Having left aviation for 4 years & only just getting current again & rated in a Cessna 172RG with constant speed prop I have proven to my instructor I am a capable pilot. Surely I can be forgiven for having some gaps in my knowledge but I am making every effort to fix that before I next take to the skies.
Last edited by steve181; 13th Sep 2009 at 06:06.
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As Bushy says, high MP and low RPM win the fuel consumption stakes. It took Charles Lindbergh to drive home this point to WWII P-38 pilots so that they could extend their area of operations in the Pacific.
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43Inches,
You disagreed with my statement about diesel engines being a better match for aircraft than petrol burning engines. You said:
I was actually referring to diesel engines burning diesel fuel. The Thielert engines burn jet fuel so this is nothing like the situation I was alluding to.
As I said in my previous post, avgas burning engines get peak work from the fuel (maximum torque) at an RPM value somewhere between 3,000 and 4,000. This is because of the (relatively rapid) rate at which the fuel/air charge burns.
At issue is the relative geometry of the con rod and crankshaft during the period of maximum pressure in the cylinder (on the power stroke). The arrangement is well less than ideal at RPMs in the 2,000 range as the peak pressure occurs too early. Get the RPM up to around 3,500 and the peak pressure now occurs across the period where the con rod and crankshaft "arms" are now at 90 degrees to each other.
Car engines are virtually identical to aircraft engines in this respect. If you have a tachometer in your car, you will notice that you get peak torque at somewhere around the 3,500 RPM mark (allowing for some variations between engine makes/models of course). (Cars have the luxury of a gearbox so they can afford to run the engine at these speeds).
This is all due to the rate at which the fuel/air charge burns. If that rate could be controlled then this issue would not exist.
Run your engine at 2,000 odd RPM and you won't be getting anywhere near the same bang out of your fuel.
Diesel engines, burning diesel fuel, are an entirely different proposition. They get their maximum torque at much lower RPMs - the RPM range that is well suited to driving an ungeared prop on an aircraft (memory limitations prevent me from quoting any rough figures on this).
Another reason you don't see aircraft with diesel engines that burn diesel fuel: there are no aerodromes with bowsers that supply diesel fuel!
bushy,
Until now, I have been talking only about what is going on inside the engine. If you now start talking about the performance of the aircraft as a whole (e.g range, endurance), then you are talking about the powerplant (which is the engine/prop combination) and the airframe.
We all know well that aircraft design is about choosing compromises. The powerplant is a collection of compromises where the optimum performance may be at a point well removed from where any individual efficiencies may peak.
The two most significant efficiencies on the reciprocating aircraft
powerplant are:
1. Engine specific fuel consumption (what I have talking about above), which is best somewhere around 3,500 RPM but drops off as RPM reduces, and
2. Propellor efficiency, which is best somewhere around 1,800 RPM but drops off as RPM increases.
At some point of compromise - somewhere in the middle - you get the most economical performance for the powerplant as a whole. For the typical powerplant at a typical cruising speed, that could well be, for example, an RPM of 2250.
There are limits to the general statement "the lower the RPM the better". When people espouse this (e.g. Rolls Royce, Charles Lindbergh) they are referring only to the RPMs in the "useable" (or permissible) range. I would wager that an RPM value of 1,000 for example, would be hopelessly inefficient.
You disagreed with my statement about diesel engines being a better match for aircraft than petrol burning engines. You said:
Thielert diesel engines are geared from 3900 engine rpm to 2300 prop rpm so not sure that argument works. Diesel, petrol and turbine engines running props can have gearbox reduction and still be very efficient.
As I said in my previous post, avgas burning engines get peak work from the fuel (maximum torque) at an RPM value somewhere between 3,000 and 4,000. This is because of the (relatively rapid) rate at which the fuel/air charge burns.
At issue is the relative geometry of the con rod and crankshaft during the period of maximum pressure in the cylinder (on the power stroke). The arrangement is well less than ideal at RPMs in the 2,000 range as the peak pressure occurs too early. Get the RPM up to around 3,500 and the peak pressure now occurs across the period where the con rod and crankshaft "arms" are now at 90 degrees to each other.
Car engines are virtually identical to aircraft engines in this respect. If you have a tachometer in your car, you will notice that you get peak torque at somewhere around the 3,500 RPM mark (allowing for some variations between engine makes/models of course). (Cars have the luxury of a gearbox so they can afford to run the engine at these speeds).
This is all due to the rate at which the fuel/air charge burns. If that rate could be controlled then this issue would not exist.
Run your engine at 2,000 odd RPM and you won't be getting anywhere near the same bang out of your fuel.
Diesel engines, burning diesel fuel, are an entirely different proposition. They get their maximum torque at much lower RPMs - the RPM range that is well suited to driving an ungeared prop on an aircraft (memory limitations prevent me from quoting any rough figures on this).
Another reason you don't see aircraft with diesel engines that burn diesel fuel: there are no aerodromes with bowsers that supply diesel fuel!
bushy,
Until now, I have been talking only about what is going on inside the engine. If you now start talking about the performance of the aircraft as a whole (e.g range, endurance), then you are talking about the powerplant (which is the engine/prop combination) and the airframe.
We all know well that aircraft design is about choosing compromises. The powerplant is a collection of compromises where the optimum performance may be at a point well removed from where any individual efficiencies may peak.
The two most significant efficiencies on the reciprocating aircraft
powerplant are:
1. Engine specific fuel consumption (what I have talking about above), which is best somewhere around 3,500 RPM but drops off as RPM reduces, and
2. Propellor efficiency, which is best somewhere around 1,800 RPM but drops off as RPM increases.
At some point of compromise - somewhere in the middle - you get the most economical performance for the powerplant as a whole. For the typical powerplant at a typical cruising speed, that could well be, for example, an RPM of 2250.
There are limits to the general statement "the lower the RPM the better". When people espouse this (e.g. Rolls Royce, Charles Lindbergh) they are referring only to the RPMs in the "useable" (or permissible) range. I would wager that an RPM value of 1,000 for example, would be hopelessly inefficient.
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???
FGD
Have a look at Brian Abraham's Lycoming graph which shows that Lycoming also believe that their engines are more economical at low RPM.
If you want there are other charts which tell us the same thing.
These engines are designed to run at low RPM.
Have a look at Brian Abraham's Lycoming graph which shows that Lycoming also believe that their engines are more economical at low RPM.
If you want there are other charts which tell us the same thing.
These engines are designed to run at low RPM.
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FGD135, to take up some points, whether a diesel engine uses diesel or jet fuel (kerosene) is immaterial. The original diesel engine was in fact to use the oil coming straight out of the oil well. Ship diesels use bunker oil which has to be heated in order to get the viscosity low enough in order to pump the stuff. You can not compare aircraft piston engines to motor vehicles. Aircraft engines are big bore and one of the reasons for dual plugs is so that the charge in the cylinder all gets burnt. This is because of the slow speed of the flame front. With only one plug not all the charge would be burnt. The DB engine as fitted to the Me 109, although it had dual plugs, both were installed on the same side of the head, rather than on opposite sides, and the engine suffered some thing like a 10% loss of power as a result. The RPM of an aero engine is limited by the prop aerodynamics (Mach number of the tip), extra horse power can be extracted by increasing RPM, but that then necessitates a gear box to reduce the prop RPM. Added complexity. The Bear bomber has turbo props putting out some 15,000 horse power each with a prop RPM of only 750 and is capable of near jet speeds. Peak pressure does not occur at 90° crank on any internal combustion engine.
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These engines are designed to run at low RPM.
To illustrate the point I was making in my previous post about RPM that is TOO low, I will again quote these figures that The Green Goblin originally extracted and posted in this thread. Note that the fuel economy is starting to go to mud as the RPM gets below 2200:
2500 rpm 22" 66% 124 KTAS 8.8 GPH
2200 rpm 24" 66% 124 KTAS 8.9 GPH
2200 rpm 24" 66% 124 KTAS 8.9 GPH
Diesel is a process, the generally available fuel happens to be named as such.
Jet A fuel is virtually identical to diesel and can run in normal diesel engines provided a lubricant is added (check with your manufacturer first).
Obviously the forum here deals with aircraft engines and any mention of diesel refers to those that may be run on JET A. These engines however may be fueled by either Jet A or straight diesel (Diesel #1, Diesel #2, bio-diesel EN 590) as they are diesel engines.
If a direct drive diesel in th 1500 - 2000rpm range was suited to aviation then why have geared higher reving diesels been developed and produced?
Jet A fuel is virtually identical to diesel and can run in normal diesel engines provided a lubricant is added (check with your manufacturer first).
Obviously the forum here deals with aircraft engines and any mention of diesel refers to those that may be run on JET A. These engines however may be fueled by either Jet A or straight diesel (Diesel #1, Diesel #2, bio-diesel EN 590) as they are diesel engines.
If a direct drive diesel in th 1500 - 2000rpm range was suited to aviation then why have geared higher reving diesels been developed and produced?
Last edited by 43Inches; 13th Sep 2009 at 13:22.
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The Bear bomber has turbo props putting out some 15,000 horse power each with a prop RPM of only 750 and is capable of near jet speeds.
Peak pressure does not occur at 90° crank on any internal combustion engine.
