what i dont inderstand is:
why does hot air produce more trust than normal air (talking abour full scale aircraft).
so why did the 707 engines work less eficently than 747.
and what about after burners????

very easy answer to the 707 vs. the 747 engine...straight turbojet versus a high bypass engine...plus I think advancements in technology had a lot to play in it.
As far as hot vs. cold air , cooler air is more dense therefore it produces more thrust. However your question is a bit vague. hot air from a jet engine is from the combustion inside the engine Im sure you are aware of that. So Ireally dont understand your question.

Pete

They are not the same type of engine.

HOT AIR EXPAND SO MORE VELOCITY, MORE THRUST. 707 IS A TURBOJET SO IT USES MORE FUEL AT SHORTER DISTANCE . THE TURBO FAN PRODUCE ALMOST THE SAME POWER WITH LOWER FUEL USE AND LONGER DISTANCES.

FOR EXAMPLE A DYNMAX DUCTED FAN PRODUCE 12 TO 13 STATIC THRUST AT 20K RPM. A MODEL 12 # TURBINE AT IDLE PRODUCE 2 # OF THRUST AT 33.000 OR 50.000 RPM. IF YOU COULD INSTALL THE DYMINMAX FAN UNIT ONTO A MODEL JET ENGINE YOU COULD GET MORE ALMOST TO SAME POWER AT A LOWER RPM AND USE LESS FUEL.

Not necessarily the case. Depending on the compression ratio of the machine (turbine in this case) you can compress hot air at a smaller rate than cold air and yield the same density.
But then again a model jet engine does not have the capacity of changing compression rates.

Relative humidity is more of a factor than temperature for a jet engine. the more O2 you can get to the expansion stage, the more thrust. Viceversa, more H20 means less O2 meaning less Thrust~equivalent to the leaning-reachening of a reciprocal engine when climbing/descending.

So: cool air + high humidity thrust can be lower than hot dry air thrust. Just a thought on Thermo I.

David

yeh i should have said.
why is hot air more powerful than normal air

Personally, I've never thought that, people full of hot air really can't be trusted at all

Re: cold air vs. hot air

The energy of a given mass of air is a function of the specific heat of the gas X its temperature. The higher the temperature, the more enery ity has (which can be expanded through a nozzle to generate THRUST). An afterburner is a classic example of this physical relationship.

Peter

and then there was water injection (the JT9's comes to mind).
At times the degradation in performance was a better option than dropping the payload to carry the water..............

And what does this energy in the form of heat get you? Nothing but design headaches. The energy is useless unless it is converted into thrust, by way of pressure. Pressure - also another way air holds energy. More exhaust temp does not necessarily mean more thrust. More pressure means more thrust. In fact, the heat leaving the engine in the exhaust is wasted energy, a by-product of heating the air to gain the pressure increase. A more efficient engine does a better job of converting heat into pressure.

John,

You need to read my previous posting with a little more attention to detail.
I said that the increased energy due to the heat addition is converted to thrust by EXPANDING/accelerating it through a nozzle (i.e. the convergent-divergent nozzle that is typically seen at the back end of an afterburner. The large increase in thrust that is obtained by means of an afterburner is a result of the heat addition to the gas, not a "pressure rise". In fact, the combustion process in an afterburner results in a net LOSS of stagnation pressure of the gas. Don't confuse constant volume combustion (that occurs in a CI engine, which does lead to a pressure rise) with constant pressure combustion (that occurs in a gas turbine engine) that leads to an increase of the enthalpy/energy increase of the working gas.

Peter