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Subject:
Turbofan aircraft engines
Category: Science > Technology Asked by: chip4-ga List Price: $25.00 |
Posted:
05 Jul 2005 20:24 PDT
Expires: 04 Aug 2005 20:24 PDT Question ID: 540399 |
Around 1930 the variable ptch propeller was a very much needed invention, giving aircraft both good low speed (take off and climb) performance and also good high spped (220 mph) performance. The fixed pitch propeller was very bad at low speed if 220 mph was to be achived. Why, then, can the current tubofan engines, which get most of their thrust from a fixed pitch fan, achieve a speed 3 times that of the old fixed pitch propeller aircraft and also have very good low speed (take off and climb) performance? |
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There is no answer at this time. |
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Subject:
Re: Turbofan aircraft engines
From: andrewxmp-ga on 05 Jul 2005 21:01 PDT |
Just more powerful engines, more torque? |
Subject:
Re: Turbofan aircraft engines
From: simon2wright-ga on 06 Jul 2005 14:05 PDT |
If variable pitch fan blades were used on turofan engines it would provide more thrust and reduce the amount of fuel used when the aircraft is flying at low speeds, however the thrust produced using fixed blades is still quite high at low speeds due to the high power of the turbine engine, and to make a turbofan engine with variable pitch blades would cost more to make and weigh more. |
Subject:
Re: Turbofan aircraft engines
From: tpg2114-ga on 23 Aug 2005 14:58 PDT |
Your statement would present a problem if it were accurate; however, turbofan engines do not get much thrust from the fan itself. The fan is merely the first stage of the compressor and only acheives a compression ratio of about 3:1. Most of the work is done by the compression and then combustion of the fuel and the hot exhaust does the work. So the next obvious question is, why use a fan at all? Using a fan allows a bypass air flow to be pulled off before the main compressor which serves multiple purposes. This bypass air can be used to provide heat/pressure in the cabin, cooling for the engine core (although the air is still hot (700 deg. F), it is much cooler than the core air (2000 deg. F)). Also, the bypass air can come out the back of the engine to mix with the "hot" airflow and cool the overall airflow to cut down on emissions and noise pollution, all good things. Why not use variable pitch blades on a fan? In variable pitch propellers, there is a complicated (compared to fixed pitch propellers) mechanical structure in the hub. In a fan spinning at several thousand RPMs, it's not possible to do this. The force on the hub is several tons per blade and any defects or weaknesses would cause the engine to fly apart... decidedly not a good selling point in an engine. As you may know, when a fixed pitch prop or fan is designed, it is created for an ideal operating RPM. In an engine that is just a shaft attached to the turbine, the fan spins as fast, or as slow, as the turbine, and is thus inefficient at any speeds other than it's design speed. However, adding a gearbox to the shaft allows the fan to spin more often at it's ideal RPMs (the same is done on turboprops). This allows the engine to be more efficient across many flight regimes instead of just one. The downside is that the added gearbox adds size, weight, and cost to an engine, which may end up costing more than the extra fuel needed to fly in inefficient regimes. What's more efficient, props or turbofans? Well, that's kinda like asking "What tastes better, apples or steak?" They are very different machines with very different applications. The piston prop is good because it's cheap and lightweight, and very, very efficient so long as you are not flying high or fast. A turbofan is better at higher speeds than a piston prop and efficiency increases as speed increases due to the nature of the engine (using compression to generate power). A turboprop however is the best of both worlds. It can fly faster than a piston prop and more efficient than a turbofan in the middle flight regimes (0M to ~.5M). It can also fly higher than a piston prop but not quite as high as a turbofan due to the inefficiency of a prop in thin air. Hopefully that answered your question, and a few more of them. Feel free to drop me a line if you want more explaination, I did a project at school last year and after learning all we could about engines we settled on a turboprop and designed it from the ground up. Tim Gallagher Georgia Institute of Technology |
Subject:
Re: Turbofan aircraft engines
From: edcox1630-ga on 12 Oct 2005 20:06 PDT |
I'm afraid I'll have to disagree with tpg2114-ga. With regard to the "design thrust efficiency" of the turbofan engine. I work at dfw international airport. The company I work for is responsible for assisting American Airlines pilots and mechanics in keeping their schedules to and from classes at the American Airlines flight academy and "maintenance hangers 1-4" at dfw. We are also responsible for the same with regard to pilots and their crews which have to get their 6-month and annual "recurrency" certifications at "Flight Safety International" and "C.A.E. Bombardier-Simuflight". The aircraft mechanics that work at "maintenance hangars 1-4" for american airlines have stated that on take off and climb to cruise altitude, the fan blades of the GE-90 and Rolls-Royce Trent engines, used on the Boeing 777 are actually responsible for no less than 80% of the total thrust of the engine. The actual purpose of the fan on any commercial aircraft is to turn the energy of the turbines working gases, which would if left pure, "produce tremendous amounts of noise and fuel consumption at required modern commercial aircraft thrust levels" into "quiet and efficient" thrust. In other words, to produce quiet efficient propulsion. If the fan blades were as inefficient as has been suggested, engine manufacturers would not waste millions developing them. Some of the reasons large diameter fans are used in conjunction with turbine or jet-engines are "Power at take-off", "efficiency at take off and cruise", and "Low noise". It is true that you could power a 747 or 777 with a pure jet engine with no fan at all. But the noise that such a powerplant would generate would be comparable to a shuttle launch. i.e. the Pratt&Whitney F119, the engine of the YF22, according to "Jane's all the World's Aircraft" produces 35,000lbs of thrust but then again so does the Rolls-Royce 535c, the launch engine of the Boeing 757. The difference in appearance of the two is like night and day distinguished most noticably by the 535c's large fan. The fan is turned by the last 4-5 rows of turbines of the engine. With this particular engine the fan accounts for roughly 60 - 70% of "power at take off" or "maximum possible thrust". The engine has a bypass ratio of 4.3 to 1. If instead of having the 535c's on the Boeing 757 you replaced them with the p&w f119's, an engine that produces 90%+ of its thrust from "working gases alone", along with significant amounts of afterburning - the engine/aircraft combination would, limit the airports that you could fly out of, 1. due to the noise generated in full afterburner, "airports have noise restrictions" - and afterburning is what this engine needs to produce the required thrust, 2. greatly increase fuel burn on take-off and climb to cruise altitude. I doubt that any airline would purchase an aircraft that guzzled as much jet-a as this one would. Tpg2114-ga was accurate about the associated problems with variable pitch fan blading. Not that such a set up is impossible. Just that it would be complex and expensive to design and manufacture. |
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