Post by Bill Von Sennet on Aug 22, 2008 22:24:54 GMT -5
Tom Goodrick
Turbine Aircraft
« on: Jul 6th, 2007, 10:59pm »
What does "Turbo" mean?
You have to be careful with this term because it means many different things with regard to aircraft engines. In reality it is the same thing: a turbo is a mechanically driven set of fans that compresses air. In the section on flying piston-engine aircraft, we have already discussed the use of the turbo. We'll just hit the main points here. The turbo is a "turbocharger" for piston aircraft, also called a "turbonormalizer" if you want to get fancy. Its fan adds pressure to the air being fed to the engine so the engine behaves as though at a lower altitude. In an aircraft with a "normally-aspirated engine" (meaning without a turbocharger), the power starts to diminish at 6,000 ft where the most power you can get is about 75% of the maximum power you can get at sea level. Above 4000 ft you have to adjust the mixture to increase the fuel flow to get more power. You can continue to do this to about 10,000 ft where you will only have about 30% of full power. At that altitude the climb rate is slow. You can horse the aircraft up to 12,000 ft with patience. You don't gain much at these altitudes unless it becomes possible to fly OVER mountains instead of through them. You will fly at a higher speed at 30% power at 10,000 ft than you would at the same percent power at a low altitude. The reduced density makes this possible. But 30% power is still rather slow. With a turbocharged engine, you just leave the mixture set at full rich and climb the aircraft. You will have 80% power, which is normally used for extended climbs, until you reach about 16,000 ft in most aircraft. You can crusie at 75% power at up to 21,000 ft where you will find a true airspeed as much as 40% higher than you would find at 6,000 ft without a turbocharger.
We must note that many "turbocharged" aircraft are not set up correctly for engine operation in FS9. This includes the default Mooney. You'd think they would have gotten that one right. Look in the [piston_engine] section of the aircraft.cfg file. If you see the line:
turbocharged= 1
you should also see the line
fuel_air_auto_mixture= 1
This will assure that you can follow proper operating procedure of setting mixture to full rich for all climbs. indeed, in FS9 you have no need to make further mixture adjustments in a turbocharged aircraft. If fuel_air_auto_mixture=0, then you must use mixture adjustment as you climb and the adjustment and power settings will not be correct. They get far from reality.
The way you fly a turbocharged aircraft, and its basic utility for most people, depends on whether its cabin is pressurized or not. The basic performance does not change but you must use it differently if not pressurized. Without pressurization, you must breath oxygen from a canister through a mask when above 12,000 ft. This applies to passengers and pets too. There is a danger here because, if something happens to the oxygen supply while cruising at 22,000 ft or higher, it will be a real trick to get the airplane down below 12,000 ft fast enough without breaking it or losing consciousness. Also, you must trust a line boy to fill your oxygen tank from the right gas canister. Some have used nitrogen or ordinary shop air. The fact is that few people who own turbocharged, non-pressurised aircraft such as the Mooney fly them at high altitude very often. Most find value in the turbo for good climb performance on hot days from high elevation fields and use 10,000 ft as a common cruise altitude. Also, a non-pressurized aircraft must descend no faster than 500 ft/min to keep the passengers comfortable. Many people will report ear pain if you descend faster than that. Getting down from 22,000 ft at 500 ft/min will take about 40 minutes and something like 100 to 120 nm distance.
The Cessna 340, Cessna 414, Piper Aerostar, Beech Duke, Piper Mirage and several other piston planes are turbocharged and pressurized. The Baron 58 was made in a pressurized version until 1982. These aircraft use their turbochargers to pressurize the air in the cabin as well as the air going into the engine intakes. A common measure of the pressurization is the altitude at which the cabin can maintain pressure equivalent to 8,000 ft altitude. This would be described as "the 8,000 ft cabin altitude" which is 20,000 ft for the Cessna 340. That means while it flies at 20,000 ft, the passengers sitting comfortably in the cabin (with no oxygen masks) will feel as if they were at 8,000 ft. (Actually, all pressurized aircraft must carry oxygen bottles and masks for all people on board in case there is a leak in the cabin. But this is stashed under the seats. Only the pilots must wear their masks so they are ready to put them on and get the plane to a lower altitude quickly in a loss-of-pressure emergency. I have made cabin altitude gauges for all of my pressurized aircraft that will show the cabin altitude anytime the aircraft is flying. You can use this for interest. But it also has a use in piloting techniue. The pressurization system depends on the engine power. If you suddenly lose power, you will lose pressure as well. If you slow down rapidly by cutting power to idel while at cruise, the cabin altitude will shoot up. You might have felt this in real planes as they prepare to descend. By reducing power in increments of about 20% spaced over several minutes, you can avoid severe spikes in the cabin pressure. Many current aircraft have automatic ways of regulating pressure reduction so this does not happen in most cases. But I feel you should learn to fly this type of aircraft with attention to the pressure so that you know what you are dealing with. Real pilots have been dealing with this complexity for decades.
Turboprops, turbojets and fan jets are called "turbine aircraft" because their engines use turbines as the main motive force instead of pistons. The engines suck in the air with a forward turbine stage and compress it before it is ignited. The hot gas from the ignition serves to turn secondary turbines which power the front turbines. In the case of turbojets, a large mass of air exits the exhaust. Throwing this mass continually out the back end makes a thrust force toward the front end. The turboprop has the same internal structure except the secondary turbine, driven by the hot gasses, has a geared connection to a propeller on the front of the engine that acts like the prop on a piston aircraft and provides forward thrust. Almost all "jet engines" in use today are fan jets, not turbo jets because turbo jets are too noisy and too inefficient. The fanjet engine has a turbo inlet area about 40% larger than the turbojet. The air from the outer periphery goes into a bypass chamber that bypasses the ignition chamber. It rejoins the central flow just aft of the secondary turbine and gets sucked out with the central hot air. This has two effects. It reduces the noise considerably. It also adds more mass to the exhast given more thrust. There are more pounds of thrust per pound of fuel expended.
Normal piston planes fly no higher than 10,000 ft most the time as do unpressurized turbocharged aircraft. The turbo just gets them there faster. In the section on mixture control, I have shown a comparison between the normal and turbocharged versions of the Skylane RG. There is no difference (on a standard day) up to 6,000 ft. few fly faster than 200 knots. Pressurized aircraft can come down at rates of 1000 to 1500 ft/min. They have efficiency values of 5 to 10 nmpg.
Turboprop planes fly between 15,000 ft and 28,000 ft at speeds of 220 to 300 knots. They burn a little more fuel than a comparable piston aircraft. They can come down at rates of 1500 to 2500 ft/min. Smaller aircraft can have efficiency values of 2 to 4 nmpg.
Fanjet aircraft fly between 28,000 ft and 51,000 ft at speeds of 370 to to 480 knots. They come down at rates of 2,000 to 5,000 ft/min. They burn quite a bit more fuel than the other types of aircraft. Their efficiency can be as high as 2.5 nmpg at the lower speeds.
« Last Edit: Oct 13th, 2007, 8:20pm by Tom Goodrick »
Turbine Aircraft
« on: Jul 6th, 2007, 10:59pm »
What does "Turbo" mean?
You have to be careful with this term because it means many different things with regard to aircraft engines. In reality it is the same thing: a turbo is a mechanically driven set of fans that compresses air. In the section on flying piston-engine aircraft, we have already discussed the use of the turbo. We'll just hit the main points here. The turbo is a "turbocharger" for piston aircraft, also called a "turbonormalizer" if you want to get fancy. Its fan adds pressure to the air being fed to the engine so the engine behaves as though at a lower altitude. In an aircraft with a "normally-aspirated engine" (meaning without a turbocharger), the power starts to diminish at 6,000 ft where the most power you can get is about 75% of the maximum power you can get at sea level. Above 4000 ft you have to adjust the mixture to increase the fuel flow to get more power. You can continue to do this to about 10,000 ft where you will only have about 30% of full power. At that altitude the climb rate is slow. You can horse the aircraft up to 12,000 ft with patience. You don't gain much at these altitudes unless it becomes possible to fly OVER mountains instead of through them. You will fly at a higher speed at 30% power at 10,000 ft than you would at the same percent power at a low altitude. The reduced density makes this possible. But 30% power is still rather slow. With a turbocharged engine, you just leave the mixture set at full rich and climb the aircraft. You will have 80% power, which is normally used for extended climbs, until you reach about 16,000 ft in most aircraft. You can crusie at 75% power at up to 21,000 ft where you will find a true airspeed as much as 40% higher than you would find at 6,000 ft without a turbocharger.
We must note that many "turbocharged" aircraft are not set up correctly for engine operation in FS9. This includes the default Mooney. You'd think they would have gotten that one right. Look in the [piston_engine] section of the aircraft.cfg file. If you see the line:
turbocharged= 1
you should also see the line
fuel_air_auto_mixture= 1
This will assure that you can follow proper operating procedure of setting mixture to full rich for all climbs. indeed, in FS9 you have no need to make further mixture adjustments in a turbocharged aircraft. If fuel_air_auto_mixture=0, then you must use mixture adjustment as you climb and the adjustment and power settings will not be correct. They get far from reality.
The way you fly a turbocharged aircraft, and its basic utility for most people, depends on whether its cabin is pressurized or not. The basic performance does not change but you must use it differently if not pressurized. Without pressurization, you must breath oxygen from a canister through a mask when above 12,000 ft. This applies to passengers and pets too. There is a danger here because, if something happens to the oxygen supply while cruising at 22,000 ft or higher, it will be a real trick to get the airplane down below 12,000 ft fast enough without breaking it or losing consciousness. Also, you must trust a line boy to fill your oxygen tank from the right gas canister. Some have used nitrogen or ordinary shop air. The fact is that few people who own turbocharged, non-pressurised aircraft such as the Mooney fly them at high altitude very often. Most find value in the turbo for good climb performance on hot days from high elevation fields and use 10,000 ft as a common cruise altitude. Also, a non-pressurized aircraft must descend no faster than 500 ft/min to keep the passengers comfortable. Many people will report ear pain if you descend faster than that. Getting down from 22,000 ft at 500 ft/min will take about 40 minutes and something like 100 to 120 nm distance.
The Cessna 340, Cessna 414, Piper Aerostar, Beech Duke, Piper Mirage and several other piston planes are turbocharged and pressurized. The Baron 58 was made in a pressurized version until 1982. These aircraft use their turbochargers to pressurize the air in the cabin as well as the air going into the engine intakes. A common measure of the pressurization is the altitude at which the cabin can maintain pressure equivalent to 8,000 ft altitude. This would be described as "the 8,000 ft cabin altitude" which is 20,000 ft for the Cessna 340. That means while it flies at 20,000 ft, the passengers sitting comfortably in the cabin (with no oxygen masks) will feel as if they were at 8,000 ft. (Actually, all pressurized aircraft must carry oxygen bottles and masks for all people on board in case there is a leak in the cabin. But this is stashed under the seats. Only the pilots must wear their masks so they are ready to put them on and get the plane to a lower altitude quickly in a loss-of-pressure emergency. I have made cabin altitude gauges for all of my pressurized aircraft that will show the cabin altitude anytime the aircraft is flying. You can use this for interest. But it also has a use in piloting techniue. The pressurization system depends on the engine power. If you suddenly lose power, you will lose pressure as well. If you slow down rapidly by cutting power to idel while at cruise, the cabin altitude will shoot up. You might have felt this in real planes as they prepare to descend. By reducing power in increments of about 20% spaced over several minutes, you can avoid severe spikes in the cabin pressure. Many current aircraft have automatic ways of regulating pressure reduction so this does not happen in most cases. But I feel you should learn to fly this type of aircraft with attention to the pressure so that you know what you are dealing with. Real pilots have been dealing with this complexity for decades.
Turboprops, turbojets and fan jets are called "turbine aircraft" because their engines use turbines as the main motive force instead of pistons. The engines suck in the air with a forward turbine stage and compress it before it is ignited. The hot gas from the ignition serves to turn secondary turbines which power the front turbines. In the case of turbojets, a large mass of air exits the exhaust. Throwing this mass continually out the back end makes a thrust force toward the front end. The turboprop has the same internal structure except the secondary turbine, driven by the hot gasses, has a geared connection to a propeller on the front of the engine that acts like the prop on a piston aircraft and provides forward thrust. Almost all "jet engines" in use today are fan jets, not turbo jets because turbo jets are too noisy and too inefficient. The fanjet engine has a turbo inlet area about 40% larger than the turbojet. The air from the outer periphery goes into a bypass chamber that bypasses the ignition chamber. It rejoins the central flow just aft of the secondary turbine and gets sucked out with the central hot air. This has two effects. It reduces the noise considerably. It also adds more mass to the exhast given more thrust. There are more pounds of thrust per pound of fuel expended.
Normal piston planes fly no higher than 10,000 ft most the time as do unpressurized turbocharged aircraft. The turbo just gets them there faster. In the section on mixture control, I have shown a comparison between the normal and turbocharged versions of the Skylane RG. There is no difference (on a standard day) up to 6,000 ft. few fly faster than 200 knots. Pressurized aircraft can come down at rates of 1000 to 1500 ft/min. They have efficiency values of 5 to 10 nmpg.
Turboprop planes fly between 15,000 ft and 28,000 ft at speeds of 220 to 300 knots. They burn a little more fuel than a comparable piston aircraft. They can come down at rates of 1500 to 2500 ft/min. Smaller aircraft can have efficiency values of 2 to 4 nmpg.
Fanjet aircraft fly between 28,000 ft and 51,000 ft at speeds of 370 to to 480 knots. They come down at rates of 2,000 to 5,000 ft/min. They burn quite a bit more fuel than the other types of aircraft. Their efficiency can be as high as 2.5 nmpg at the lower speeds.
« Last Edit: Oct 13th, 2007, 8:20pm by Tom Goodrick »