Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Aerospace Propeller airplanes vs jet airplanes

  1. Dec 14, 2009 #1
    why propeller airplanes have superior take-off performance than jet airplanes?
     
  2. jcsd
  3. Dec 14, 2009 #2
    Who told you they do? Takeoff performance is a function of thrust to weight ratio. At low advance ratios, propellers perform worse than they would at cruise. Some props are specifically designed as "climb" propellers, but then have lower cruise speeds.

    I'm not sure how jet engines thrust vary with forward airspeed. Fred can answer that for you better than I.
     
  4. Dec 14, 2009 #3
    okay! this guy at the other forum makes this point which is quite confusing to me.
    here's is what he says


    " All aircraft's Climb Rate depends upon excess Power available. For the jet, Power, and excess Power increase as the aircraft fly at higher speeds, with excess Power reducing at very high speed as Drag builds up, and the Power Required increases. That's why jet aircraft Climb at very high speed, typically at, and sometimes above, normal cruising speed. For the same reason in REVERSE, best Climb speed for a piston aircraft is somewhat below cruising speed.

    All of this also explains why 'prop' aircraft generally have superior Takeoff performance (High low speed Thrust) with modest cruise performance, whilst their jet cousins experience have inferior Takeoff performance, but superior cruise performance "

    is he correct?
     
  5. Dec 14, 2009 #4
    I'm not sure what the thrust of a jet engine looks like as a function of speed, but he may well be correct.
     
  6. Dec 14, 2009 #5

    okay. i'll wait.

    thanks cyrus
     
  7. Dec 14, 2009 #6

    FredGarvin

    User Avatar
    Science Advisor

    It does increase as ram inlet pressure increases. However, this somewhat implies that they have very minimal power/thrust at low speeds which is wrong. This is tantamount to saying a red car is faster than a blue car. It makes no sense without stating a lot of other factors.
     
  8. Dec 14, 2009 #7
    all right.
    thanks fred.

    & guys! please help me on this one too.

    what are various important take-off characteristics of an aircraft? i mean like on what factors of an aircraft does it depend?

    and suppose if : Two identical aircraft of equal weight, one powered by turbojet, the other by turboprop, are lined up on parallel runways. Both will fly their maximum rate of climb performance. how do we compare their AOA and TAS?

    my answer to this is ' AOA for the prop is higher & TAS for the jet is higher'.

    m i correct?
     
  9. Dec 14, 2009 #8

    turbo

    User Avatar
    Gold Member

    We have small airports around here with short runways. Having had clients fly in with both small jets and turbo-props (and conventional prop-planes) I can assure you that the folks with the private jets needed a lot of room to get off the ground, while the folks with the turboprops jumped off the runways quite promptly. I always assumed that being able to change the prop pitch allowed the turbo-props to "claw" their way through the air and gain altitude quickly. Could be wrong about that, but turbo-props seemed to be pretty muscular at take-off.
     
  10. Dec 14, 2009 #9

    FredGarvin

    User Avatar
    Science Advisor

    You are NEVER going to find compatible aircraft to compare like that. Of course a business jet needs longer takeoff runs. It is not designed for short take offs like a prop aircraft. It is, for the most part, designed to cruise efficiently as possible at much higher speeds and altitudes than a prop driven aircraft.

    These comparisons are not apples to apples or even apples to oranges.
     
  11. Dec 14, 2009 #10
    The main reason turboprops generally have shorter takeoff and landing distances is because turboprops are usually designed for for "puddle-jumping," with greater efficiency at lower altitudes than pure jets. The latter are designed primarily for high-altitude, long-distance cruise.

    To achieve maximum efficiency at high-altitude, long-distance cruise, one requires a smaller wing (less drag), and engines which are right-sized for cruise. The compromise results in a higher take-off velocity, hence a longer runway, and generally poorer climb performance.

    If small jets always had a two mile runaway available, they'd have even a smaller wing! However, they need to be able to land at regional airports, and so are designed with 5,000 ft runways in mind.

    The associated nature of a turboprop's better performance at low altitude is that they're generally designed for shorter runways, closer to 3,500 feet. As such, they have larger wngs for a lighter wing loading, lower stall speeds (and takeoff speeds), and more power to weight ratio. However, they're also not as efficient, either, for long range cruise.

    Then again, they're usually not designed for long-range cruise...

    Those which are have very similar performance capabilities as do pure jets, as well as very similar weaknesses.

    Thus, bottom line, it has more to do with what environment in which the airplane is designed to fly than the type of powerplant. Historically, turboprops got the nod at lower altitudes because in the old days, jets were turbojets, and no bypass about them. Their low-alititude performance was horrible.

    These days, even smalll high-bypass turbofans offer very good low-altitude performance.

    In fact, the six scimitar-shaped blades found on the C-130J, combined with their RR AE2100D3 engines, make it somewhat of a hybrid between straight turboprops and high-bypass turbofans. One might even call them a "very high bypass turbofan" or an "unshrouded bypass turbofan." The engines alone increased their efficiency over the C-130E, with its -7 Allison engines and four-bladed props by more than 50%!
     
  12. Dec 15, 2009 #11
    okay its all about the design.
    i got the point.

    i have few more questions but before asking them i need few pre requisites. so i'll get back to them once i have some thorough knowledge.

    you guys are really helpful.
    thank you so much.

    see you guys soon.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook