Can anyone explain the differences between a turboprop and piston propellor?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by KC135DELTA, Feb 23, 2010.

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  1. Feb 23, 2010 #1

    KC135DELTA

    KC135DELTA

    KC135DELTA

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    Hello again,

    After searching high and low I can't find an answer to this. A 400hp turboprop engine and a 400hp piston engine seem to have vastly different props. Why is this? Does a turboprop have a higher output shaft speed? Torque differences? Problems with a fully feathering prop on a piston engine?

    400hp piston:[​IMG]

    400hp turboprop:[​IMG]

    Are they really that different? Comments?
     
  2. Feb 23, 2010 #2

    pwood66889

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    Basic differences between gas turbine power plant and a piston (like most all cars) one. The former is a very high speed/low torque proposition. The later has more torque but at lower revs. There is a lot of differences and that explains most of what you see.
    The prop part is just a perceived optimization for a particular air frame.
    Percy in SE Bama
     
  3. Feb 23, 2010 #3

    lr27

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    Are those props really vastly different? Total blade area and diameter may be similar, for all I can tell from the picture. I suspect that the second one is a constant speed (i.e. variable pitch) prop that's feathered.

    It may be that in the second case, there's more of a diameter constraint. Perhaps the turboprop is geared down to run the prop at a lower rpm than the piston engine. In that case, it may provide more low speed thrust with the larger diameter.

    Your picture is of the Extra 500, I think. What's interesting is its little brother, the 400, has a 350hp engine and a prop that looks very much like that on the Lancair in your picture. But I don't think the blade shape has anything to do with the type of engine.
     
  4. Feb 23, 2010 #4

    autoreply

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    Except for the lower RPM (lower noise, bigger prop, higher efficiency), another difference is that most turboprops are flat rated to a lower power.

    The Extra 400 engine for example is thermally capable of about 2.5 times it's 400 hp so it still makes that 400 hp at cruise altitude (FL300 or so) Since the air is way thinner it's beneficial to have a bigger prop. A turbocharged 350 HP piston is already at 75% power at FL200 and won't even make it to FL300.
     
  5. Feb 23, 2010 #5

    KC135DELTA

    KC135DELTA

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    Actually the extra 500 is notoriously bad at altitude because it is not flat rated. Thus why I used it in my example.
     
  6. Feb 23, 2010 #6

    MadRocketScientist

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    The 400hp turbine also needs a heavier aircraft to carry all the extra fuel needed. This would also influence the prop design.

    Shannon.
     
  7. Feb 23, 2010 #7

    BBerson

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    The turbine probably has a slower shaft (propeller) speed. Since all turbines must have a reduction gearbox, they probably gear it down to an optimal rpm somewhat lower than the piston engine.
    The turbine prop needs to feather because the idle speed is very high and landing would be difficult with a huge idle thrust.

    Just my guesses.
     
  8. Feb 23, 2010 #8

    autoreply

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    I stand corrected, I assumed it had the LP version of the PW6, but it has the Allison 250 which does output 700 hp max. According to sources is has some flatrating, but indeed not the amount you'd expect or need :)

    In most turboprops this is indeed the trade-off.

    The weight benefit for a turbine however is (for 350 hp) typically a 300 lbs or so. Cruising at the same power in a 3 hour flight a piston never makes up for that by it's lower fuel burn, so in fact most turboprops needs a lower takeoff mass, only their long-range performance is harmed seriously.
     
  9. Feb 24, 2010 #9

    Jan Carlsson

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    My wild guess is that the turbo prop make 17-1900 rpm at the prop, and the piston engine 25-2700 rpm

    that make a different in prop design, the extra is licenced to FL250
     
  10. Feb 24, 2010 #10

    Dan Thomas

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    The prop goes to low pitch for landing. It can go to totally flat, or to reverse if the pilot desires. Feathering is used only for two reasons: To eliminate drag on a multi when one engine fails, and to help avoid having the breeze turn the prop when the airplane is parked. Turbines have no compression resistance when they're not running, and if the wind turns the prop it does the bearings and so forth no good. Low RPM means no oil to them, and if it's a free turbine the oil pump will be on the gas generator section, which the prop doesn't turn at all. You'll often see the prop tied so it can't turn in the wind. Like this:

    [​IMG]

    Dan
     
  11. Feb 24, 2010 #11

    Starman

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    The main difference is due to turboprop engines being smooth running and the piston engines generating strong torsional shock loads.
     
  12. Feb 24, 2010 #12

    Waiter

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    I would think that the props could be identical.

    Given that the prop speed is the same, 400 hp is 400 hp, the prop doesn't know where its coming from.

    Issues to take into consideration, ground clearence (prop diameter) this will drive how many blades are on the prop.

    Keep in mind, the more the number of blades, the less the prop efficience (Yes, a one bladed prop is the most efficient, and was actually built and tested during WWII)

    ALSO, Prop Diameter (blade length) vs prop speed. You really want to keep the prop tips from going super sonic, this usually means shorter blades or lower prop speed.

    (Supersonic blades generate a LOT of noise and loose efficiency)

    Waiter
     
  13. Feb 24, 2010 #13

    wsimpso1

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    I have seen this a couple times, so I have to comment - Yes the turbine engine runs much faster, but there is a gearbox that makes the output to the prop comparable in both torque and rpm to the piston engine.

    Differences in props? Mostly having to do with the mission and airframe and exactly what power/rpm ranges they run at... There is nothing inherent about one vs the other that requires a completely different blade shape...

    Billski
     
    Last edited: Feb 25, 2010
  14. Feb 24, 2010 #14

    bmcj

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    I propose that many of the points made here (yours and theirs) are valid and play a part in the selection of a prop. Actually, I saw minor differences in the props in KC135's two photos, but I have to ask him what specific difference is he referring to? Number of blades? Full feather capability? Blade length or geometry? Maybe even the color? That clarification will help narrow the answers.

    Bruce :)
     
    Last edited by a moderator: Feb 25, 2010
  15. Feb 25, 2010 #15

    orion

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    As alluded to above, the design of a prop is primarily an exercise in matching the prop's propulsive characteristics to the requirements of the airframe and of course the available power. From the aerodynamic standpoint, the design process is identical regardless of the type of powerplant you're considering. The aerodynamic design process addresses the physical characteristics of the prop including the blade's section, its twist distribution, its chord length distribution, its set in the hub (variable with speed) and of course, its diameter. The proper and optimal mix of these variables enables the designer to best match the prop to the given engine and airframe - this process is identical regardless the type of engine.

    To show why certain props are bigger and others smaller would require a history lesson more so than anything else, since several factors are closely related to the development of the engine and its associated technologies. In short though, the props and the engines (recips) evolved nearly at the same time and the resultant mix of engine rpm and performance, and prop diameter are interrelated. With minor variation most recip engines will turn around 2,500 rpm at cruise, although the application range is somewhere between about 2,000 rpm and about 2,700 rpm.

    Higher rpms are also used (still direct drive) but these are either special engines (automotive) or special application (racing). For these faster turning applications custom props are developed that can still deliver the necessary performance, but at these greater speeds.

    When turbines came along the designers picked a better speed (slower turning props are more efficient) that did a better job of matching the requirements of the engine and the airframe. But no-one has as of yet been able to tell me why turbine manufacturers picked between 1,700 rpm and about 2,000 rpm for their products. The only consistency is that these turbines, which tend to have higher outputs, will be used on larger airplanes, which can swing larger propeller diameters. Spinning them slower reduces noise and also reduces the possibility of the tips going sonic (which is really loud).

    There are also some structural differences between the two applications however often these are minor - it is actually not uncommon to see blades and hubs used interchangeably between similarly rated engines, provided of course that proper engineering is part of the selection process. True, there are some models that should not be used on a recip if they were designed for a turbine however, historical failures were caused by vibratory issues more so than the type of power impulse the recip engine delivers. The turbine prop also has to consider the vibratory environment however that realm tends to be at a somewhat different range than the types of vibration a recip application needs to consider. At times, these ranges and or the structural harmonics are incompatible between the two applications. But again, at times the hubs and blades can be used on either.
     
  16. Feb 25, 2010 #16

    Jan Carlsson

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    Thank you Orion for cleaning up here, I could not have said it better my self, I come up with a short answer on this topic earlier, but was to busy with shovelling snow and a French translation of my propeller design software, just as with building airplane, if you change one thing you have to change a number of other, same with my software, it consist of 2,8 Mbyte of mathematic.

    The difference between the props above is that they are both made of MT, (that was a joke) I just checked the Extra 500, it have a RR 250 rated 450 HP and turn the prop just over 2000 rpm, I guess its mission profile is to fly between FL 120-FL250 and the prop is designed for that.

    There is also an Extra 400 with a 350 HP turbo/intercooler, liquid cooled. It turns 2600 rpm. 4 blade 76,7 inch diameter.

    They burn about the same fuel per hour 22-23 gph at 210 kts
    Fl 200 -250

    So the answer is as before, with more hp or same, with lower RPM it need more blade area, if diameter is limited by ground clearance or tip speed, the number of blades have to increase, if propeller is optimised for high altitude the blades need more area as well.


    Jan
    www.jcpropellerdesign.com
     
  17. Feb 25, 2010 #17

    BBerson

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    Thanks Dan,
    I should have said: The turbine engine needs a prop that can go to a lower pitch for landing, not feather as I said. I don't know what I was thinking.
     

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