Simviation Forums

why are jets faster then props? lol stupid question it may seem  :D but why??

Ok, this is where it gets a tad bit complicated, so bear with me here. The propeller is spinning in the air right, thats ALOT of force on the blades, trying to pull them out, called centrifigal force. A propellor can only turn so fast. This is where the jet engine comes in. It has rows upon rows of blades, pushing the air back, so not as much strain on the blades compared to a prop. By doing this the jet blades can turn at much higher revs. Also accounting to the fact that you add fuel to the air and you ignite it, you get more added thrust by the heat. This is why a jet can go faster. But there are limits also, you can't go mach 6 with a jet engine, it reaches a limit also, so here comes the ram jet and scram jet. These two engines are similar but the scram jet has more power. Basically if you want to go really fast you use these. The ram jet is based on a deisel engine, there are no spark plugs. If you look straight through a ram jet engine, you dont see anything, it gets small in the middle, and then it gets bigger in the rear, there are no fan blades. What happens is that at high speed air is rammed, hence the name, down the intake, where it is compressed. As it is compressed, fuel is being added to the mixture as well. The intense amount of pressure causes the air to explode, and ignite the fuel, which propells the aircraft. If you remember the movie red october, with the catapiller drive, almost no moving parts, well this engine is the same thing but for the air, no moving parts. By using a ram jet, you can go infinite amounts of speed, BUT that all depends how long before the fuel runs out, or the plane rips apart around you. Below are some examples.

Jet Engines
http://inventors.about.com/library/inve ... etypes.htm

Usually it's because, jet engines of the same weight as piston engines have more HP, thrust, wattage (or how ever else you measure power), but mainly it's because jet engines just keep applying accelleration until whatever load they're carrying reaches it's aerodynamic limit. Props (including turbo-probs) are like little wings

trying to overcome the drag induced by a propellor which has its root travelling sub-sonic and its tip travelling supersonic is the main limiting factor in why there is a limit to the airspeed obtainable for a propellor driven aircraft........drag is the biggest factor in Jet aircraft also.....drag climbs by a huge factor as airspeed increases......until we find a way to reduce this drag and raise the power level of our motive force, we will be still travelling around the same speeds as we do now in the air......in the vacuum of space, its a whole 'nother ball game !

in the vacuum of space, its a whole 'nother ball game !

Yup, in a vacuum even the tiniest jet (rocket actually, as there is no air for the air/fuel mixture) could accelerate a 747 to incredible speeds. With no friction, the thrust vector (acceleration) just keeps accumulating in the form of velocity. However, you need to "turn around" when your 1/2 way to your destination and start "decelerating"  ;)

Brett - you're wring, of course, haven't you seen the Star Wars movies?  That's how its done...




[quote]

Yup, in a vacuum even the tiniest jet (rocket actually, as there is no air for the air/fuel mixture) could accelerate a 747 to incredible speeds. With no friction, the thrust vector (acceleration) just keeps accumulating in the form of velocity. However, you need to "turn around" when your 1/2 way to your destination and start "decelerating"

Oh yeah,  I forgot, how silly of me    

in the vacuum of space, its a whole 'nother ball game !

Yes, another ball game.

Airplane engines are called "breathing engines" because the fuel combust with the surrounding oxygen (in general fire needs oxygen) In space, chemical rockets need an oxidizer to complete the combustion with fuel.

Let me take you through a general process of the rocket engine.

*ranger sings in the tune of that bone song. (you know, "the hip bone is connected to the.....')*

The fuel and oxidizer is pumped into the injectors, the injectors causes vaporization. Then the vaporization causes mix-ing. The mixing is ignited by the igniters. The igniters causes combustion. The combustion produces hot ga-ses. The hot gases go trough the nozzle. And then hotthrustprducinggasses comes out.

trying to overcome the drag induced by a propellor which has its root travelling sub-sonic and its tip travelling supersonic is the main limiting factor in why there is a limit to the airspeed obtainable for a propellor driven aircraft........drag is the biggest factor in Jet aircraft also.....drag climbs by a huge factor as airspeed increases......until we find a way to reduce this drag and raise the power level of our motive force, we will be still travelling around the same speeds as we do now in the air......in the vacuum of space, its a whole 'nother ball game !

I want to add on to this. Another thing is the vacuum trail. When a propeller spins, it creates forward thrust. When the fan spins in a jet engine, it creates foreward thrust. What it also does, is at the exhaust, it creates a whilrpool effect.

[quote]

Yup, in a vacuum even the tiniest jet (rocket actually, as there is no air for the air/fuel mixture) could accelerate a 747 to incredible speeds. With no friction, the thrust vector (acceleration) just keeps accumulating in the form of velocity. However, you need to "turn around" when your 1/2 way to your destination and start "decelerating"

and exactly how do you decelerate in an enviorment with no air if u have one little rocket engine pushing the plane forward?(first person to explain this gets a shiny new quater and a shiny new learjet;)

If a thruster applies force directly opposite of the direction it is moving, it should slow down. Objects with more momentum would require more force to slow down an object in space.

That lear jet sounds mighty tempting, but I'll take the quarter. LOL...I just wanted to say that, even if the anwser is wrong.

I cant think much of a better analogy right now, because of my son. . .


Think of a skateboard. Push it on a flat level surface. Its moving, so how is it going to stop? Either it will hit something, or a constant opposite reaction (gravity) will slow it down. In space, a small can of oxygen could easily stop the shuttle in space. It has a constant equal and opposite reaction.

Newton's Law applies here - For every action, there is an equal and opposite reaction.

[quote]

Yes, another ball game.

Airplane engines are called "breathing engines" because the fuel combust with the surrounding oxygen (in general fire needs oxygen) In space, chemical rockets need an oxidizer to complete the combustion with fuel.

Let me take you through a general process of the rocket engine.

*ranger sings in the tune of that bone song. (you know, "the hip bone is connected to the.....')*

The fuel and oxidizer is pumped into the injectors, the injectors causes vaporization. Then the vaporization causes mix-ing. The mixing is ignited by the igniters. The igniters causes combustion. The combustion produces hot ga-ses. The hot gases go trough the nozzle. And then hotthrustprducinggasses comes out.

and exactly how do you decelerate in an enviorment with no air if u have one little rocket engine pushing the plane forward?(first person to explain this gets a shiny new quater and a shiny new learjet;)

I said, "Turn around" , or turn the engine around

 Interesting replies here. I'm surprised that nobody mentioned stall speed . Jet engines, having more blades per row and more rows are more efficient at moving air than propellers. About the best a prop can do is use a countr-rotating system. Because props are so inefficient they have a tendency to stall around mach1 due to compression of the air in front of the blades. Jet engines stall too, but it takes a more severe event, such as introducing hot gas to them. Hot air being more sparse will play hell with a fast spinning turbine blade, causing a compressor stall. If your lucky, the engine will re-ignite. If not, well...................