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Jets vs Props

  1. Oct 31, 2012 #1
    So I've always wondered about how the characteristics of Jets and Props differ. I've always thought of Jets as being low volume high pressure where as props of being high volume low pressure.

    I have a 750cc Jet ski that uses a water pump impeller as propulsion and it accelerates very quickly faster than almost all speedboats that use a conventional propeller. It doesn't have alot of top end though and has a top speed of 45mph. The speedboats take alot longer to accelerate but they will eventually surpass me.

    Conversely on airplanes, all the military supersonic planes use turbojets or turbofans. A big heavy cargo plane like a C-130 uses a turboprop though. From my understanding there are physical limitations that prevent a propeller driven airplane from reaching supersonic speeds, and I think that they are limited to lower altitudes. I've also heard that propeller driven planes get better fuel mileage.

    So maybe the real question here is thrust vs lift?
     
  2. jcsd
  3. Oct 31, 2012 #2

    rcgldr

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    and a larger, heavier C-5 uses turbines.

    Having the tips of a propeller go supersonic is an issue. The air force tried this. Wiki article:

    http://en.wikipedia.org/wiki/Republic_XF-84H

    A turbofan is another option.

    http://en.wikipedia.org/wiki/Turbofan

    Speed seems to be the main reason (the other is cost) for choosing between propeller, turbo prop, turbofan, turbines, ramjets, and scramjets.
     
  4. Nov 1, 2012 #3
    Okay, you are throwing around a lot of terms but I'm not sure that you understand what they are. For starters, the jet on your Jet ski is not the same as the jets used by airplanes. It is a gas motor otto cycle driven compressor pump that shoots water out through a nozzle as a "jet". The kinds of jets in jet airplanes run on what is called the brayton cycle. incoming air is compressed by a compressor, then the compressed air has fuel added to it which is combusted and adds heat energy to the flowing air, the expansion of the air occurs over a turbine which sucks a bit of energy out of the flow in order to power the compressor, and the rest of the added heat power from fuel is used as thrust out the back of an exit nozzle. In turbojets, the blades that you see at the front of the engine do not generate thrust. Propeller planes can only move as fast as the air they displace moves so its harder to get them to go super sonic but it can be done. Other than that, jets are more efficient at high altitudes where air density is low, and propellers are more efficient at low altitudes where air density is high.
     
  5. Nov 1, 2012 #4

    Nugatory

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    Quite true, but also true that if you are looking at any contemporary large commercial aircraft, the blades that you see at the front are indeed moving air and generating the bulk of the thrust - you'll be looking at high-bypass turbofan engine.
     
  6. Nov 1, 2012 #5
    Good point, but I feel like it is revealing of inner processes that the blades on bratyon cycle engines that people see can produce thrust but they don't need to in order for a jet engine to work, and in fact, air entering the combustion chamber is stagnated.
     
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