Engineers, unite!!

[Pilotasso]

I have had a discussion with my father who is a construction engineer and my brother who is a AF pilot student (to take convertion for F-16 soon) about engine propultion principles.

 

They say its the sustaining force of each turbine and compressor blade along with the excess of pressure of gases trapped between the engines exaust and the ambient air that propels airplanes, like a pressure balooning efect (LOL) I used to think the same as a kid. But this IMHO comes after wrong placed empirical thinking.

I have long since been taught that the less pressure behind the engine the better because you can accelerate the gases faster and get the propultion with the oposite reaction. Also using bernouli's equation the faster the gases go the lesser total pressure is. That is why the nozzle exists, to adjust and balance exaust pressure with that of ambient air.

 

They get even more confused when I ask why H-O and scranjet engines work better in vacuum for reaction forward and then they return to the story of the engine blades working like propelers (that on turbojet engines).

 

Would anyone help me to enlighten these misguided minds? :)

[Crusty]

Not entirely sure I understand the question, but remember F=ma

high exhaust velocity means little if its not moving enough mass of gas

not sure how you think a scramjet could work in a vacuum?

apologies if I have missunderstood

[Kenan]

 

They say its the sustaining force of each turbine and compressor blade along with the excess of pressure of gases trapped between the engines exaust and the ambient air that propels airplanes, like a pressure balooning efect

 

:huh:

[Crusty]

:huh:

 

(assuming no other losses through the engine)..the pressure differential seen at the compressor face should equal the thrust?

is that what they are saying

[Kenan]

Jim bu lu qua, pop-pop, clockity clockity clock, bin bin pyook yang ying

 

:huh:

[Crusty]

:huh:

 

good translation:megalol:

[Pilotasso]

Not entirely sure I understand the question, but remember F=ma

high exhaust velocity means little if its not moving enough mass of gas

not sure how you think a scramjet could work in a vacuum?

apologies if I have missunderstood

 

Let me refrase, My father says its not F=ma, but rather the exaust smashing into ambient air.

[Swede]

Thrust = (Mass/sec of air + mass/sec of fuel) x Airspeed of the nozzle - mass/sec of air x air velocity + (pressure at exit - ambient pressure) x Nozzle Area.

Thrust Equation

[britgliderpilot]

Let me refrase, My father says its not F=ma, but rather the exaust smashing into ambient air.

 

He's wrong.

 

 

The thrust of a jet engine (or indeed any aero engine) comes from accelerating the mass flow through the engine. My University lecturers say so, Rolls-Royce's excellent book The Jet Engine says so.

 

 

Pressures and where the thrust is loaded internally within the engine, I'd have to read up on . . . . but it wouldn't take too long to find the answers/equations and post them.

[britgliderpilot]

Thrust = (Mass/sec of air + mass/sec of fuel) x Airspeed of the nozzle - mass/sec of air x air velocity + (pressure at exit - ambient pressure) x Nozzle Area.

Thrust Equation

 

In other words, acceleration of the massflow PLUS difference in pressure*area at nozzle exit.

[Pilotasso]

In other words, acceleration of the massflow PLUS difference in pressure*area at nozzle exit.

 

Isnt the nozzle gas pressure made as close to ambient air as possible?

What I have always read in books is that high ambient pressure behind the engine=bad because that brakes the exaust gas, thus high altitude good for accelereation in the engine but on the other hand bad for combustion efficiency.

[Guest Cali]

:huh:

 

LMAO, I think you have to be the funniest person on these forums!!!

[warthogmadman987]

Let me refrase, My father says its not F=ma, but rather the exaust smashing into ambient air.

 

 

Ya, i have heard people say that before, i actually as a little kid always wondered why they had to have gas and a flame to run an airplane engine. I always thought you could just have and air pump push the air over the wing. I quickly learned that was virtually impossible. :)

[Kuky]

as far as I know, in any nature law, one simple equation must always comply:

 

Energy in = Energy out

 

This to me means the thrust achieved is basically chemical energy of the fuel converted through combustion in the engine chambers into high pressure at the nozle... the combustion has to be very quick for the pressure to be very high as if it is too slow the time over which the combustion is achieved is too great and all yuo'd get is a "weak" burning process.

[Gripes]

as far as I know, in any nature law, one simple equation must always comply:

 

Energy in = Energy out

 

This to me means the thrust achieved is basically chemical energy of the fuel converted through combustion in the engine chambers into high pressure at the nozle... the combustion has to be very quick for the pressure to be very high as if it is too slow the time over which the combustion is achieved is too great and all yuo'd get is a "weak" burning process.

 

Now, finally something my thick scull can absorb:)

[Crusty]

Isnt the nozzle gas pressure made as close to ambient air as possible?

What I have always read in books is that high ambient pressure behind the engine=bad because that brakes the exaust gas, thus high altitude good for accelereation in the engine but on the other hand bad for combustion efficiency.

 

Its true that they try and achieve a as low a exhaust pressure as is practical..if pressure at exhaust is to high then the engine basicaly just chokes up

the same effect as if you stuck a banana up your cars exhaust

Basically, the pressure at the exhaust has to be lower than the pressure at the turbine face or obviously gas would not flow

[Kuky]

what ever preassure is obtained in the combustion chambers, there will always be some loss of this converted energy before it exits at the nozzle. This is because no energy conversion is 100% efficient for any engine type. The preassure is therefore lower at the nozzle then initial conbustion chamber, but this must be higher then the ambient air at the nozzle. Preassure of the nozzle has to be higher then the external air.

 

Laws of physics are nothing but laws of nature... we know they exist (if they are discovered) and we can explain how they work... but not why they exist.:pilotfly:

[strikeout]

What causes propulsion engines to generate thrust

 

Any kind of reaction engine generates thrust because of the forces acting on the engine components. These pressure forces within the engine act on the compressor, turbine and casing. Thrust is not directly caused by the flow of gas, but by the direct 'static pressure' of the gas on engine surfaces.

 

The lower the net force acting on the surfaces in the engine, the less thrust is developed.

 

When the reaction fluid is moving supersonically, pressure waves in the flow cannot push directly backwards, so the fluid must be expanded laterally against a nozzle surface of some kind to gain more thrust.

 

Thanks.

[Crusty]

Well explained, Strike

[Guest HamstersForRent]

All this stuff has been said above, And I'll jump on the bandwagon as well

 

Thrust is the unbalanced force caused by the difference in momentum of low velocity inlet air with high velocity outlet air. Pressure is identical at inlet and outlet (ambiant pressure)

 

F=m(v_exh-v_in)

 

where F= Thrust

m=mass flow rate of air

v_exh= exit air velocity

v_in=inlet air velocity

 

this force acts on the components of the engine as stated above, and in turn on the aircraft.

 

Also using bernouli's equation the faster the gases go the lesser total pressure is. That is why the nozzle exists, to adjust and balance exaust pressure with that of ambient air.

 

The nozzle is also important as it allows the exhaust gasses to be accelerated beyond mach 1 (relative to the engine). without the nozzle, exhaust speed would be choked at Mach 1 (relative to the engine) due to compressability of air at this speed.

 

Isnt the nozzle gas pressure made as close to ambient air as possible?

What I have always read in books is that high ambient pressure behind the engine=bad because that brakes the exaust gas

 

If the pressure is significantly higher at the nozzle "exit" than ambiant, shock waves will be generated. Entropy generated across the shock wave will results in a decrease in exhaust gas velocity, and when plugged back into the formula above will result in less thrust.

 

On the other hand though, your father and brother aren't totaly off the mark with their 'exhaust smashing' idea.

The exhaust air will be slowed and in the process will exert force on the ambiant air, but it isn't this reaction force which pushes the plane.

[Pilotasso]

Cool replies, thanx all. :)

[Kuky]

Thrust is nothing but an Action-Reaction of applied force law... it's got nothing to do to with outside air pressure but the energy released from fuel combustion in the engine, and all the energy in form of preassure being directed in one direction creating equal amount of energy in the oposite direction... thrust