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

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

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

obviously , the nozzle part.....

the reason being......
it is shaped to accelerate the flow of hot gases....
might be they are using convergent - divergent type nozzle...

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

I don't know any thing about jet engines, just thinking out loud here
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.

Very beautiful layman explanation of banging :wink: here by chandan
http://www.rcindia.org/fuel-and-engines/wren-70-turbine-strip-down/msg74759/#msg74759

Why is it that the Jet engine is so efficient at high altitudes where the air density is less?

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

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

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

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.

Cheers
 

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.

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.

Ps
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?

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 

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...

   

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... 

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.

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 ,

Be advised , most , if not all explanation on the net sucks, incomplete and misleading,

Alam,
It is not turbine ,

answer plz........

dunno!!

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.

Cheers

Place reserved for Brayton Cycle image (sent it to anwar bhai to do the honors)

[Admin Note] Picture uploaded.

Now look at the brayton cycle and tell me which part is propelling the aircraft forward ? The reason I being hard to get is to make you'll think and come up with the correct answer

alright

the one part that propels the aeroplane forward is the compressor, why ? simple explanation , ahead of the compressor is low pressure and behind it is very high pressure (Some times, depending on the compression ratio, upto 13 atmospheres), that is why most attachments to the RC aircraft from the engine side is at the end of the compressor. real aeroplane transfers thrust to the airframe through a pin in called King Pin

See Rep 25 above you will know what i am saying

Most critical aspect in a jet engine is the metallurgy, nobody shares this technology, even in rc engine, itis the turbine metal that matters, TIT, turbine inlet temp, is again critical, thermal stresses can lead to creeps and crystallization of turbine alloy,

and this thread

http://www.rcindia.org/gas-glow-nitro-planes/intro-to-turbinejet-engine-flight/

ENGINE HANDLING

Introduction

1.       The wide variety of gas turbines in service, each having certain engine characteristics, means that the information in this post must be of a general nature only.  Manuals give precise details of engine handling for a particular engine installation and cover both normal and emergency operation.

STARTING AND GROUND RUNNING

Precautions

2.       Whenever possible the aircraft should be headed into wind for all ground running.  During prolonged ground running the aircraft should never stand tail to wind as hot gases may enter the air intakes and lead to overheating. 

3.       Furthermore, standing tail into wind causes a back pressure, which slows the gas stream and can also lead to overheating.  Starting tail into wind should normally be avoided. 

4.       The surface of the ground ahead of the intake should be free of loose objects and equipment, and all personnel should be well clear of the intakes.  Wherever possible, engine intake guards should be fitted to minimize the ingestion risk. Fire extinguishers should always be close at hand. 

Starting

5.       Modern Engine starting sequences are fully automatic and only require to switch on engine services and initiate the start cycle. 

6.       If the engine fails to start or the engine temperature exceeds the allowed start-up limit, the fuel cock should be closed immediately.  The Manual gives guidance on the number of starts that may be attempted, and the time interval between them, before it is necessary to investigate the fault.

After Starting

7.   Idle running checks vary , but usually include checks for fire, normal gas temperatures, rpm, and normal oil pressure. 

8.       Turboprop Engines.    The basic starting procedures are the same as those for turbojet or turbofan engines.  The rotor must always be in the ground fine pitch setting for the start, otherwise the load on the starter motor will be excessive.

TAXING

Engine Considerations

9.      Throttle handling should be smooth and considered.  Rapid and frequent opening of the throttle is to be avoided.  The initial response of an aircraft to throttle movement may be slow and considerable power may be necessary to start the aircraft moving.  Once underway, idling rpm is usually sufficient to maintain momentum.  In some cases idling rpm is more than sufficient and the aircraft speed may slowly increase. 

10.      Directional control whilst taxiing is by the use of rudder and/or nose wheel steering.  Differential throttle may also be effective in maneuvering multi-engined aircraft but allowance should be made for poor engine acceleration.

11.      Throttle movements should be made slowly and a careful watch maintained on the engine temperature during prolonged periods of taxiing.

TAKE-OFF

Engine Considerations

12.      When conditions dictate a short take-off run the throttle should be smoothly opened to take-off power against the brakes, then the brakes released. 

13.      Some engines may tend to stall or surge under crosswind conditions, because of the uneven airflow into the intake.  If this happens the throttle should be closed and if necessary the aircraft should be turned into wind until the engine responds satisfactorily to throttle movement.

14.      Certain conditions to rpm, gas temperature and / or torque that should be achieved at take-off power to indicate if the engine is producing full thrust. 

15.    These indications are the ‘placard’ figures and are worked out on engine installation, and are used to determine the thrust degradation of an engine during its installed life. 

General

16.      In spite of the ECU, the rpm for a given throttle setting may tend to increase with altitude/temperature.  The throttle may therefore have to be closed progressively as the height increases to maintain constant rpm.

GENERAL HANDLING

17.      The principles of gas turbine handling are determined by the fact that this type of engine is designed to produce maximum thrust and efficiency at one rpm - usually 100%.  Malfunction of the engine is often associated with acceleration, or with operating conditions that differ widely from the optimum.   Device such as the ECU is incorporated primarily to assist the change in the conditions.  A malfunction of these devices should not prevent successful control of the engine provided that greater attention is paid to throttle handling and the preservation of a good flow into the compressor.

18.      In some cases flame-out can occur if the throttle is opened too rapidly. 

Surge

19.      High altitude surge may occur above 7,000 ft elevation, when flying at a low speed and high rpm under very low temperature conditions.  Symptoms are , substantial bang, fluctuating rpm, higher than normal gas temperature, and a considerable loss of thrust.  Closing the throttle and increasing the Speed by diving effects a return to stable conditions. 

Mechanical Failure in Flight

20.      If the engine fails because of an obvious mechanical defect the immediate action should be to shut down the engine and carry out a dead stick landing.

Pump Failure

21.      If the booster pump fails through either a pump malfunction or an electrical / ECU failure, in some engines a bypass system allows fuel to flow from tanks by gravity, or by suction from the engine driven pumps.  However, since the purpose of a booster pump and the header tank  is to prevent vapour locking and cavitation of fuel, and to maintain a satisfactory supply of low pressure fuel to the pumps, certain handling precautions should be taken.

22.      At high throttle settings and high ambient temperatures rpm may fluctuate and a flame-out could occur. At high level an immediate flame-out is possible, and this possibility is increased with AVTAG if it was at high temperature on take-off (note: the use of AVTAG and related fuels has declined considerably).  Detailed procedures for individual aircraft may be found in the manual, but general precautions are:

(a)   Reduce rpm, this will reduce the chance of damage to the pump because of fuel starvation, and will reduce the chance of cavitation.

(b)   Avoid negative g, because the fuel is gravity fed.

(c)   Descend as this reduces chance of vapour locks.

(d)   If a flame-out has occurred plan dead stick landing .

ENGINE ICING

23.      Centrifugal compressor engines(most RC Engines are this type) are relatively insensitive to moderate icing conditions.  The combination of centrifugal force, temperature rise, and rugged construction found in these compressors is effective in dealing with all but severe engine icing.

24.      Axial flow compressors are seriously affected by the same.  Heavy icing can cause an excessive gas temperature leading to turbine and engine failure, and the breaking off of ice can cause engine surge and mechanical damage. 
Effect on RPM on the Rate of Icing.

25.      For a given icing intensity can be reduced by decreasing the rpm.

Flame Extinction.

26.      Flame extinction may be caused by overfuelling, underfuelling, interruption of the fuel flow, or insufficient idling speed. 

APPROACH AND LANDING

27.      A powered approach is necessary on jet aircraft to ensure a quicker thrust response if it becomes necessary to adjust the glide path by use of the throttle.  30-40% will give enough power for use in the approach configuration.  The rpm should be kept at or above this figure until it is certain that the runway can be reached.  When going round again from a powered approach the throttle should be opened smoothly to the required power to prevent engine surge.

28.      If the decision to go round again has been made after touch-down, or just before, when the rpm have fallen below  the minimum approach figure, the throttle must be opened very carefully until the rpm reach the minimum approach figure, otherwise the engine may surge.  When opening up under these conditions the engine takes longer to accelerate to full power.  Engines that are controlled electronically are independent of the rate of throttle movement, as the engine will only react depending on the signal from the control unit, which in turn will only accelerate the engine at a rate dictated by conditions.

STOPPING THE ENGINE

29.      After landing, the engine can usually be shut down immediately upon the aircraft reaching its parking position.  A check should be made to ensure that the gas temperature and rpm have stabilized before following the shut down procedure detailed in the Manual for type.

Subsequent posts will explain JET ENGINE PERFORMANCE


1.      Although jet engines are rated in kilo-newtons and -propeller engines in kilo-watts, both types are assessed on the power produced for a given weight, fuel consumption and frontal area, popularly know as Power to Weight Ratio

2.      Since the thrust developed by the gas turbine is dependent on the mass of air entering the engine, it follows that the performance of the engine is influenced by such variables as
(a) forward speed
(b) altitude
(c) climatic conditions
(d) The efficiency of the intake, compressor, turbine and exhaust nozzle

Comparison Between Thrust kN (Force) and Shaft Power kW  (Power)

1.      Because the jet engine is rated in thrust and the prop engine is rated in equivalent shaft power, no direct comparison can be made without the use of power conversion factors.  Factors converting the shaft power developed into thrust, or the thrust developed in the jet to shaft power may be used, thus, converting power to force or force to power.

2.      In the SI system of units, 1 W = 1 Nm/s, so the conversion of thrust to power requires the aircraft velocity, in mtr/sec, to be taken into account.  For an aircraft travelling at 150 m/s, and the engine producing 40 kN, the thrust to power conversion is as follows:

   40000      X   150   =   6000000   =   6000 Kw

Now for a prop engine powering an aircraft at the same velocity, 150 m/s, with a propeller efficiency of 60%, and producing 6,000 kW the engine rating will be:

   6000      X   100 /  60   =   10000  Kw

Therefore, in an aircraft travelling at 150 m/s 1 kN of thrust =  250 kW of power.

Question

Is RPM vs Thrust Linear in a jet engine ? if yes why, if not why.


Shoot, dont google

Nandan and other jet fliers could contribute 

Answer to the Question Above 

Effect of Engine RPM on Gas Turbine Performance

At minimum engine rpm, below which the engine will not be self-sustaining, all the available power is absorbed by the turbine in order to drive the compressor.  It is not until high engine rpm is reached that the output becomes significant (See image).  The conversion of fuel energy into gas energy is poor at low rpm, but improves rapidly to become most efficient between 90-100% rpm.


Question two:- What happens to Specific Fuel Consumption Vs RPM

Excellent information sir,

One question is why is the jet turbines less efficient as compared to the turbo prop with he same turbine in both.

cheers,
Asutosh

We are Going into the realm of entrophy enthalpy, bypass ratio, cold bypass etc etc which i want to avoid, want to keep it specific to RC Jet engine Basics (Not to design one)

Thanks Girish & Ashutosh, good you guys found it useful