parasitic drag vs induced drag..... could someone please explain the difference to me

Discussion in 'Aviation' started by pinsog, Oct 25, 2013.

  1. pinsog

    pinsog Member

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    I thought I knew so much after reading about WW2 airplanes for 30 years and then I get on this sight and feel like a retard (my apologies if I offended any other retards out there by using the term retard)

    Could someone please explain the difference between parasitic drag and induced drag? Thank you
     
  2. Matt308

    Matt308 Glock Perfection
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    Parasitic drag for you retards is just a formula away from induced drag. Recommend not using parameter "retard".

    End of lesson.
     
  3. Shortround6

    Shortround6 Well-Known Member

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  4. pinsog

    pinsog Member

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    I don't feel any smarter than before. Is it like people saying HP is more important than torque when in fact they are interconnected rpm x torque/5252=HP?

    The reason I ask is in the 1000 to 1200 hp long range interceptor post, someone said the Spitfire had higher parasitic drag but the Mustang had higher induced drag. I have NOOOOO idea what they mean.
     
  5. pinsog

    pinsog Member

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    Thank you Shortround6,

    I saw your message when I posted mine. That explains it a little better.
     
  6. GregP

    GregP Well-Known Member

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    #6 GregP, Oct 25, 2013
    Last edited: Oct 26, 2013
    Parasitic drag is composed of form drag, skin friction drag, and interference drag. Form drag is the drag caused by the form of the aircfcraft pushing the air out of the way. Skin friction drag is caused by less than perfect skin smoothness, such as non-flush rivets, scratches, dirt on the surface, etc. Interference drag is caused by the interseciton of shapes (mixing of airflow at intersections), such as a wing sticking out 90° (or at whatever angle) from the side of the fuselage. Parasitic drag starts out small and increases as speed increases.

    Induced drag is the dag caused by the production of lift by the lifting surfaces, which can include the fuselage as well as the wings and tail. Induced drag is typically high at low speeds and decreases as the speed increases.

    So the drag at low speed is mostly induced drag while the drag at high speed is mostly parasitic drag.
     
  7. Shortround6

    Shortround6 Well-Known Member

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    See the graph in the link, Both change with speed even on the same airplane.

    Not all airplanes follow the same curves. Without test results for the actual speed/s in question it is hard to say which was higher. we do know the Mustang had less total drag but how it was divided up?
     
  8. JtD

    JtD Member

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    Induced drag is pretty easy to estimate fairly accurately. Again here's a link to wiki: Lift-induced drag - Wikipedia, the free encyclopedia. If you check the formula, the only thing unknown will be the wing efficiency, which doesn't vary by a wide margin in comparable flight states. It's also what the Spitfire has over the P-51, because of the elliptical wing shape, outside of the lower weight. Once you know induced drag, and total drag, you know parasitic drag.
     
  9. drgondog

    drgondog Well-Known Member

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    #9 drgondog, Oct 27, 2013
    Last edited: Oct 27, 2013
    Drag has two distinct 'buckets'.
    Drag due to Pressure Distribution and Drag due to Shear Stress. Typically parsed and categorized as noted below:

    Drag due to Lift (Induced drag associated with the lifting surfaces - primarily the wing but also canards, horizontal stabilizer, wing twist,etc.)
    Form Drag (Drag associated with separation of flow such as boundary layer build up over wing or separation of flow aft of a nacelle, etc)
    Parasite Drag (All the drag components associated with profile/surface of the airplane including nacelles, masts, landing gear, gaps in control surfaces, skin friction, Radiators, etc. Any element of the airframe and wings that are exposed to airflow)
    Compressibility Drag (Drag due to transonic and supersonic drag rise, also includes propeller tip drag contributions)

    Within Induced Drag, there are several factors which contribute to Induced Drag. They are Coefficient of Lift based on airfoil selection - which in turn is a function of (weight, wing area, velocity and density of air).

    There then Aspect Ratio and Plan Form (which accounts for deviation from Minimum Induced Drag elliptical plan form). All airfoil data is expressed as Infinite wing span with zero tip losses. Many plots are adjusted for Aspect Ratio by changing the plot to reflect a finite wing with varying span to Area ratios. So, the wing span/wing area as well as planform ranging from Ellipse to Delta, to Trapezoidal are included in the calculations.

    To account for such variations, an Oswald Factor is inserted to reflect the overall efficiency of the wing tip geometry to the total Induced Drag of the Wing.

    Spit versus Mustang
    The Spit had bulgy 'stuff like radiators, gaps in wheel covers, radiators, cannon barrels , poorly designed windscreen (up through Mk IX), rough camo paint, plus a wing with much higher profile drag as well as parasite drag due to rivets and gaps in sheet metal surfaces combined with greater wing surface area.

    Mustang positives - no bulgy stuff, well designed canopy windscreen, flush wheel door covers, optimized radiator cowl design, second degree upper and lower airframe lofting profile, smaller wing surfaces with all gaps and rivets filled, sanded and painted.

    Spit 'positives' - Thin wing delaying onset Mach Divergence Drag/Transonic Drag although the Mustang with max wing thickness to wing chord ratio at 45% vs Spit 24% meant the velocity gradient was less on the Mustang suggesting that onset Mach Divergence Drag was delayed when compared to conventional wing designs at same free stream velocity.

    The Spit had a slightly better plan form design re: tip losses/Oswald factor

    Summary - Mustang had 30+% less Zero Lift Drag but slightly higher Induced Drag at top speeds - yielding significantly higher speeds at all altitudes for same engine HP.
     
  10. stona

    stona Well-Known Member

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    #10 stona, Oct 27, 2013
    Last edited: Oct 27, 2013
    Your description of the finish of a Spitfire wing is inaccurate, certainly after the introduction of the smooth (Type S) cellulose paints in 1942. The Air Ministry in conjunction with Supermarine and sub contractors went to great lengths to establish exactly how much of the wing had to be filled sanded and smoothed (in a multi step process which took 50 man hours, one quarter skilled, for each aircraft) to have the minimum detrimental effect on performance.

    [​IMG]

    This does not reflect your description:

    "....rough camo paint, plus a wing with much higher profile drag as well as parasite drag due to rivets and gaps in sheet metal surfaces..."


    The Ministry also spent considerable efforts along with manufacturers in developing and improving smooth paints with a suitable camouflage finish.

    The RAF in turn devoted considerable resources to instructing and informing maintenance personnel in how to maintain the finish on so called high speed aeroplanes, of which the Spitfire is obviously one.

    Cheers
    Steve
     
  11. drgondog

    drgondog Well-Known Member

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    Steve - I hereby stand corrected on the surface prep for 1942 era Spits. Thanks for the article.
     
  12. OldSkeptic

    OldSkeptic Active Member

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    #12 OldSkeptic, Oct 27, 2013
    Last edited: Oct 27, 2013
    A major srouce of induced drag is the formation of wingtip vortices. Wikipedia gives a simple and stright forward summary:

    Lift-induced drag - Wikipedia, the free encyclopedia
    So, taking the Spit vs Mustang again as an example, the Spit's induced drag was certainly lower*, but the Mustang's parasitic (all the other surfaces on the plane) was considerably lower, especially radiator drag, which was much higher on a Spit than the Mustang (in fact probably at least half, maybe more, of the total parasitic drag difference could be accounted by the radiator drag alone).
    The one exception was probably that the Spit had a lower tailplane drag, which offset the difference a bit.

    * Because of the elliptical shape and airfoil change near the tips, noting that they both had washout.
     
  13. drgondog

    drgondog Well-Known Member

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    The tip Vortex is a phenomenon of finite span wings.

    'It' creates Drag by altering the flow field about the wing in such a fashion as to alter the surface pressure distribution in the direction of increased drag. Because the tip vortex has a downward component of velocity it has the effect of 'tilting' the lift vector 'backwards' (if you will accept this analogy) so that an incremental force acting parallel to the free stream is created (incremental drag)
     
  14. stona

    stona Well-Known Member

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    No worries, it's the point of a forum, to exchange information! Glad I could help.
    Cheers
    Steve
     
  15. OldSkeptic

    OldSkeptic Active Member

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    Easier to think of air as particles. Basically air rushing in, but because the plane is moving it 'seems' to look like a spiral falling behind the plane. Remembering there are no 'sucking' forces involved, only 'pushing ones.
    The wing is pushing air out of the way, at the tips it rushes in horizontally as well as vertically.
     
  16. stona

    stona Well-Known Member

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    What you want for less "draggy" tip vortex patterns is a nice elliptical knife-edge wing tip ........like on ......:)

    Cheers

    Steve
     
  17. drgondog

    drgondog Well-Known Member

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    The Spit had a 'better plan form' closer to elliptical but also diminished its approach to elliptical efficiency by introducing wing twist.

    The trapezoidal plan form of the Mustang with tip chord to root chord ratio was pretty close from perspective of Oswald factor.

    The 109 with no twist was perhaps more efficient than both....
     
  18. N4521U

    N4521U Well-Known Member

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    I must say.............
    This innocent question has turned into a great thread of information.

    Well done exchange of well thought out information, and just proves why this forum is so well attended.
     
  19. OldSkeptic

    OldSkeptic Active Member

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    Not really, wing twist and profile changes (both of what the Spit had) can actually reduce induced drag, if done right of course.

    Remember the twist makes the tips more perpendicular to the air flow, ie a lower angle of attack. This in itself will reduce drag, at the cost of lower lift in level flight.
    This also helps stalling, since the tips have a lower stall speed than the inner wing, again because of that varying angle of attack.
    The greater the angle of attack the higher the stall speed (with the usual caveat, or weasel words, "all other things being equal").

    The Mustang had wing twist too, but the Spit combined that with a profile change. Those 2 factors gave the Spit its gentle stall, combined with the shape and thinness this maintained low induced drag even with such a low wing loading.

    The price paid was the complexity, so you didn't get something for nothing.

    Now the 109's slats, were not your modern ones, all hydraulically and computer controlled. They were on or off, often quite abruptly too.They did mean a very simple wing with a good landing stall speed at the price of control issues when one or more slats came into operation in higher speed maneuvers (even in landing with cross winds and so on ...brr). There was also a drag issues due to gaps (etc) in the leading edges and of course maintenance, dust, snow, ice and mud clogging up the slats were not a lot of fun.

    You pays your money and you takes your choice.
     
  20. bobbysocks

    bobbysocks Well-Known Member

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    which wing design for the 109...some had a rounded and some were squared off....or didnt it matter in this case?

    do you know how much drag is reduced by removing the scoop on the mustang? or what the translation into gain of mph would be? a lot of the later reno racers removed the scoop and had the rad cooled a different way. might not have been ( and probably wasnt ) feasible for a ww2 fighter... its just something i have always been curious about.
     
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