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« on: August 16, 2011, 06:20:17 AM »
rcpilotacro
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This thread is dedicated to understanding of jet engines. My request is keep it to the topic and understand/ contribute/ discusss/ ask questions on jet engines
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« Reply #1 on: August 16, 2011, 06:25:34 AM »
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For parts and basic explanation of how it works read this thread
http://www.rcindia.org/fuel-and-engines/wren-70-turbine-strip-down/msg74674/#msg74674

Ps
Pics, courtesy, Nandan
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« Reply #2 on: August 16, 2011, 06:28:26 AM »
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Question asked was

Which part of the jet engine propells the aircraft forward and why? Shoot without refering the Internet, read the previous thread for some answers
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« Reply #3 on: August 16, 2011, 09:07:14 AM »
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obviously , the nozzle part.....
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« Reply #4 on: August 16, 2011, 09:14:45 AM »
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the reason being......
it is shaped to accelerate the flow of hot gases....
might be they are using convergent - divergent type nozzle...
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« Reply #5 on: August 16, 2011, 09:22:56 AM »
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Nope not the nozzle or exhaust, let me repeat the question , which part propels the airplane forward.? Let me list the subparts of a jet engine starting from front 1. Compressor 2. Diffuser 3. Combustion chamber 4. Turbine 5. Exhaust nozzle

Your answer has to have a logic as well, don't be shy to shoot
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« Reply #6 on: August 16, 2011, 10:03:25 AM »
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I don't know any thing about jet engines, just thinking out loud here Smiley
Combustion chamber, because that's where thrust is generated part of which is used to run the turbine which will in turn run the compressor to complete the cycle.
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« Reply #7 on: August 16, 2011, 11:51:58 AM »
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Very beautiful layman explanation of banging :wink: here by chandan
http://www.rcindia.org/fuel-and-engines/wren-70-turbine-strip-down/msg74759/#msg74759
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« Reply #8 on: August 16, 2011, 01:44:27 PM »
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Why is it that the Jet engine is so efficient at high altitudes where the air density is less?
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« Reply #9 on: August 16, 2011, 07:07:29 PM »
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Quite contrary - a turbine actually will produce less thrust at higher altitudes because of less denser air = less oxygen
Turbine efficiency is similarly derated at higher temperatures
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« Reply #10 on: August 16, 2011, 07:19:37 PM »
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from what i know, about 85% of the air entering the jet engine bypasses the actual combustion chamber.
If all the cool air went into the combustion chamber, it would get too cold to keep running.
the proportion of air entering the combustion chamber can be varied.it mixes with the hot gases coming out of the combustion chamber expands and adds to the thrust.
A piston engine invairably doess not have enough oxygen to completely burn all the fuel hence there is carbon monoxide in the exhaust.
This problem is compounded at high altitudes where about 5000metres ( 15,000ft) the airdensity is 50% of that at MSL.Hence superchargers etc.
Also props need to push air back to generate thrust. hence less air to push back, less forward thrust ( also less drag).
So the piston and prop engines have a lower cieling.
But don't go away thinking that turbine engines are superior to piston engines in all respects.
The american Abrahms M1 battle tank is being redesigned to have a diesel powerplant. the present version has a turbine engine while the indian arjun has a piston engine.
regards
Avijit
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« Reply #11 on: August 16, 2011, 09:23:31 PM »
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The typical turbine engine used in the commercial planes are Turbo fans where large volumes goes through the outside of the turbine providing cooling and portion goes through the turbine. This makes it a lot more efficient and quieter.

The Typical turbines used in models are turbojet where most of the Thrust goes through the turbine and combustion chamber.

The Engines are more Fuel Efficient in the higher Altitudes as they are cooler and re
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« Reply #12 on: August 16, 2011, 09:27:40 PM »
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Sorry pressed the post button without completing.

The Engines are more Fuel Efficient at higher Altitudes as they are cooler and run at higher rpms. The maximum efficiency of the turbines are rated at 90% rated rpm.

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« Reply #13 on: August 16, 2011, 09:57:51 PM »
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I think the jury is still out on this. You are right in saying that their maximum efficiency is at 90% RPM. But that has been done taking the considerations of high altitude in mind! A similar engine would be more efficient if designed for use at ground level.

Drag is much reduced at high altitudes. The engines are optimised for these conditions. So (possibly slightly) reduced efficiency is traded off for the benefit of reduced drag.  For an airline operator, the consideration is total cost of fuel/km flown. This way the cost is minimised.

I would like to add a rider that I am not - nor claim to be - an expert on these matters. Just my inputs on what I have gleaned over time.
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« Reply #14 on: August 16, 2011, 10:49:33 PM »
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Alright, before this high altitude Efficiency thing gets out of hand , here is the answer
(a) the indicated air speed at height is less whereas the true airspeed is more, now what the hell is this ? Simply put, the amount of air molecules passing over the wing , for the actual speed it is doing with respect to the ground decreases with height, so? So, there is something ram pressure rise, while the prop efficiency continues to drop, jet efficiency increases due this ram pressure rise, however thrust will reduce due decrease in density ( increment in thermal Efficiency due reduction in temp as we go higher is there, but the reduction in density is more) , now due reduction in mass flow the turbine inlet temperature rises ( TIT is the most critical part of a jet engine) to control TIT the fuel flow is reduced by the ECU ( engine control unit) but, at higher altitude for much lower amount of molecules the airplane is experiencing , the true speed it does with respect to ground is more, since aircraft drag depends on the molecules it encounters flying higher helps in reducing drag, ram pressure rise at intake, reduction in drag, reduction in fuel flow, increased thermal efficiency, all put together improves KPL of the jet engine, poor prop doesn't enjoy such benefit.

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I hope I made sense, couldn't have simplified any further, read it more than once if required, shoot your questions, rest assured they will be addressed.

Pls
Question 2, there is a limit to this increment in efficiency with height, one is airplane related , that is called MCrit, we will discuss that in our basic aerodynamics thread, second limiting factor is engine related, which is limited by something called Tropopause, what do you understand by that?
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« Reply #15 on: August 16, 2011, 11:05:34 PM »
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Ok
One clue for question no 1 , please refer Brayton Cycle, now give me the answer, best answer will be rewarded with showers of praises  Wink
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« Reply #16 on: August 17, 2011, 03:13:42 AM »
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answer for question no.2 ...

We know that jet engines lose their thrust as we go higher ,at 25000 ft they loose almost half the power... therefore they require turbochargers at that height to maintain their power...

Tropopause is the height after which the temperature doesnt decreases anymore..that is roughly around 36000ft (when the air-temp. is almost constant)... now as the matter of fact jet engines have an advantage  of lower temperature as altitudes increases .. this benefactor is no more there... this result at quicker rate of Jet-Thrust decay... Tubro charger have a  big significance here..This is the maximum limit to which the turbo-chargers can support the maximum power rating of the engine...

   

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« Reply #17 on: August 17, 2011, 04:15:14 AM »
shadman_alam
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Q1)Which part of the jet engine propells the aircraft forward and why?

answer: its turbine

 1. Compressor
2. Diffuser - doesnt have any exit or inlet to outside
3. Combustion chamber - its inside the engine.. it cant be for obvious reasons
4. Turbine
5. Exhaust nozzle- it only directs the  outgoing air at high speed (by creating a orifice effect)


the funny thing is the work done by the compressor and turbine is almost the same..because they share a common shaft.. .. but the propulsion(thrust) generated by turbine is much more than the compressor... that is because the after combustion the air is hot and it expands and therefore the pressure inside the rear end is much higher... 

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« Reply #18 on: August 17, 2011, 05:39:34 AM »
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Alam, excellent attempt, a few clarifications
(a) there are no turbochargers in a jet engine, they are in piston engine, not even in turbo prop, why , unlike the Rc jet and turbo starter of real life jet which is a centrifugal compressor(sir frank whittle's first design) real life jet engine has multiple stage axial compressors, however what works like a turbocharger is the airintake design. There is something called critical, sub- critical & super- critical operation of an Airintake, critical operation is desirable. air intakes operate close to critical to capture max pressure rise due to ram air( something like a ramjet)

Answer on tropopause is bang-on.
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« Reply #19 on: August 17, 2011, 05:52:30 AM »
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As regards air getting bypassed after compressor and mixing after turbine, yes this is true. Not always though, how?
There are three types of jet engines
(a) turbo jet , where all the air the compressor compresses goes through the combustion chamber and burnt, no air is bypassed.
(b) turbo fan. This has two sub category called low bypass and high bypass, commercial jets are high bypass, high bypass is more economical , they are like a prop and jet, disadvantage is , they , for a given thrust are much bigger and wider. Low bypass are used in modern day fighters, they mix variably.
(c) third is a turbo prop. They are these ATR airplanes , most energy drawn is used to drive the prop, residue is jet thrust (here lies the clue also for the first question) . They two have two major subtypes , direct drive and free coupling. As name suggests direct drive directly drives the prop, free coupling has a separate turbine driving compressor and another one driving the prop.

Hope that explains ,
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« Reply #20 on: August 17, 2011, 05:54:14 AM »
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Be advised , most , if not all explanation on the net sucks, incomplete and misleading,
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« Reply #21 on: August 17, 2011, 05:57:52 AM »
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Alam,
It is not turbine , Wink
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« Reply #22 on: August 17, 2011, 08:22:30 AM »
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answer plz........
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« Reply #23 on: August 17, 2011, 12:07:43 PM »
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dunno!!
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« Reply #24 on: August 17, 2011, 05:22:24 PM »
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If we remove the Exhaust cone of a turbine then there is minimal thrust produced which means that the plane is not going to go forward even if the Turbine is going full speed and everything else is in place.

On fitting the Exhaust Nozzle, the Thrust is created with the Air Velocity. This is why I had mentioned Exhaust is the answer to what component propells the plane forward.

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