Simviation Forum - why are jets faster then props?

Feb 4^th, 2003 at 11:02pm

ViNtAgE FlYa Ex Member

why are jets faster then props? lol stupid question it may seem Cheesy

but why??

Back to top

IP Logged

Reply #1 - Feb 4^th, 2003 at 11:24pm

Blade Offline
Colonel
Annapolis, MD

Gender:
Posts: 2477

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/inventors/bljetenginetypes.htm

&&&&Dell 4550&&P4 2.53Ghz &&512MB DDR SDRAM&&GeForceFX 5900 129MB&&60GB HD @ 7200RPM &&PROUD TO BE AN AMERICAN

Back to top

IP Logged

Reply #2 - Feb 4^th, 2003 at 11:33pm

Brett_Henderson Ex Member
EVERY OUTER MARKER SHOULD
BE AN NDB

Gender:

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 "climb" through the air, pulling the plane along. A prop can only cut through the air to a point, where a jet is constantly pushing, no matter how fast its moving.

Once you've eliminated the limits of the prop and have more airspeed to work with, you can get away with smaller wings, further increasing the aerodynamic efficiency of the plane, and you just keep getting faster Grin

Back to top

IP Logged

Reply #3 - Feb 5^th, 2003 at 12:11am

Rifleman Offline
Colonel
" Full size A/C are just
overgrown models ! "
Tropical island in the Pacific

Posts: 6622

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 !

Back to top

IP Logged

Reply #4 - Feb 5^th, 2003 at 9:18am

Brett_Henderson Ex Member
EVERY OUTER MARKER SHOULD
BE AN NDB

Gender:

Quote:

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" Wink

Back to top

IP Logged

Reply #5 - Feb 5^th, 2003 at 10:43am

Felix/FFDS Offline
Admin
FINALLY an official Granddad!
Orlando, FL

Gender:
Posts: 1000000627

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

Grin

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" Wink

Felix/FFDS

Back to top

IP Logged

Reply #6 - Feb 5^th, 2003 at 2:52pm

Brett_Henderson Ex Member
EVERY OUTER MARKER SHOULD
BE AN NDB

Gender:

Oh yeah, I forgot, how silly of me Roll Eyes

Back to top

IP Logged

Reply #7 - Feb 5^th, 2003 at 7:37pm

Loafing Smurf Offline
Colonel
Push it!
Ottawa, Ontario (Canada)

Gender:
Posts: 1905

Quote:

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. Grin

*note* Igniters and nozzle are not in the diagram.

Thats just a liquid fueled rocket in the chemical category.

My online photo album&&http://public.fotki.com/ranger21/&&&&Discover your Smurf name today!&&http://www-personal.umich.edu/~mule/smurfgen.html&&&&;

Back to top

IP Logged

Reply #8 - Feb 5^th, 2003 at 8:39pm

Deputy Offline
Colonel
Hillsboro, Oregon

Gender:
Posts: 2090

Quote:

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. When it hits the tail, has less friction with the air molecules, thus, less drag.

-Brad

Bad boys, bad boys, whatcha gonna do? Whatcha gonna do when I come for you?&&&&Iustita Omnibus&&Justice for All&&&&Women are: attractive, single, mentally stable. Pick two.&& &&Yes, we drive on the right-hand-side of the road. Yes, I parked on the left-hand-side of the road. Yes, I blocked traffic for a picture. &&&&&&

Back to top

IP Logged

Reply #9 - Feb 5^th, 2003 at 9:35pm

katana_1000 Offline
Colonel
a_blesk
patomac,MD

Gender:
Posts: 1803

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" Wink

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;)

&&and yet i cant say it in the chat room:P&&&&http://airliners.net/random.inc&&&&;

Back to top

IP Logged

Reply #10 - Feb 5^th, 2003 at 9:53pm

Loafing Smurf Offline
Colonel
Push it!
Ottawa, Ontario (Canada)

Gender:
Posts: 1905

Quote:

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.

My online photo album&&http://public.fotki.com/ranger21/&&&&Discover your Smurf name today!&&http://www-personal.umich.edu/~mule/smurfgen.html&&&&;

Back to top

IP Logged

Reply #11 - Feb 5^th, 2003 at 10:05pm

Deputy Offline
Colonel
Hillsboro, Oregon

Gender:
Posts: 2090

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.

Bad boys, bad boys, whatcha gonna do? Whatcha gonna do when I come for you?&&&&Iustita Omnibus&&Justice for All&&&&Women are: attractive, single, mentally stable. Pick two.&& &&Yes, we drive on the right-hand-side of the road. Yes, I parked on the left-hand-side of the road. Yes, I blocked traffic for a picture. &&&&&&

Back to top

IP Logged

Reply #12 - Feb 5^th, 2003 at 11:41pm

Rifleman Offline
Colonel
" Full size A/C are just
overgrown models ! "
Tropical island in the Pacific

Posts: 6622

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. Grin

*note* Igniters and nozzle are not in the diagram.

[img]

Thats just a liquid fueled rocket in the chemical category.

One upgrade to the whole idea you present here....in the event of the little boy crouchng under the nozzle trying to strike his match to light the fuel and oxidizer, and winding up finding the sulphur all paste from moisture, they now use a foolproof method of ignition,....the fuel and oxidizer are " hypergolics ".....materials which ingite on contact with each other........been using these types since leaving the moon became a requirement to a successful mission.

oh yeh, one more thing..... airplane engines are called "

air

breathing engines"

Back to top

IP Logged

Reply #13 - Feb 6^th, 2003 at 12:17am

Brett_Henderson Ex Member
EVERY OUTER MARKER SHOULD
BE AN NDB

Gender:

Quote:

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 (thrust pointing in opposite direction),,geez Wink

Can I have the quarter ?

Back to top

IP Logged

Reply #14 - Feb 6^th, 2003 at 2:45am

Cloud Maker Guest

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...................

Back to top

IP Logged

	Search the archive: