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Thread: Swept Back Wings

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    Swept Back Wings

    So here is the dumb question of the day, why do commercial jet airliners have their wings swept back ? I thought that was only required when you wanted to go fast, like faster than the speed of sound.

    Richard

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    Re: Swept Back Wings

    Quote Originally Posted by Richard6 View Post
    So here is the dumb question of the day, why do commercial jet airliners have their wings swept back ? I thought that was only required when you wanted to go fast, like faster than the speed of sound.
    Well, you've answered your own question

    Yes, sweep makes sense for high speeds. If you sweep a wing 45 degrees the chord your airflow sees grows by 41%, but the thickness remains the same, so you've just created a relatively thinner wing, which delays transsonic effects which can cause higher drag or controllability issues. That's in a nutshell why airliners have sweep and the aircraft that are far enough from the speed of sound (like many regional turboprops) have simple straight wings.
    Aude somniare

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    Re: Swept Back Wings

    The velocity of the air flowing around the nose of the airplane and the leading edges of the wings is accelerated to a higher speed than the forward speed of the airplane so when you get to Mach 0.8 or so there will be some supersonic flow on some parts of the airplane. This is called the transonic regime. Shock waves start at points where the local flow is supersonic and the shock waves propagate away at an angle called the Mach angle. It takes a lot of energy to make the shock waves in the first place and when a wave from one part of the plane hits another part it at least doubles the wave drag. The shock waves radiating from the nose form a cone and to keep the wave drag down it's necessary to keep all downwind parts behind that shock wave (AKA inside the Mach cone). Although technically the Mach cone only develops when the sound energy can't propagate forward because the plane is going faster than the sound can the the wave is mostly swept downwind and can be seen as a cone with a sweep angle of about 37 degrees. As you go faster the sweep angle of the shock waves gets steeper. The entire volume of air inside the cone has been accelerated by the shock wave so that the airplane is actually flying subsonicly inside the cone but any part that sticks out in front of the cone has supersonic air hitting it. To fly efficiently at wave inducing speeds means keeping as much of the airframe as possible inside the Mach cone and that's what actually sets the sweep angle of the wings on high speed airplanes. That cosign of the sweep angle stuff is valid for shock waves that form on downwind parts of the wing but the Mach cone which is shed by the nose is the first one to deal with and it sets the leading edge sweep
    Norm
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    Re: Swept Back Wings

    Commercial airline wing design has nothing to do with body nose shock angles however the answer is correct for many fighter and other high speed aircraft configurations. This used to be a driving factor for design throughout the fifties and sixties however is not so much any more.
    "To live is to learn; to learn is to live" (author unknown)

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    Re: Swept Back Wings

    Quote Originally Posted by orion View Post
    Commercial airline wing design has nothing to do with body nose shock angles however the answer is correct for many fighter and other high speed aircraft configurations. This used to be a driving factor for design throughout the fifties and sixties however is not so much any more.
    I'm sure you're right but why bother making the leading edge parallel to the Mach angle at cruising speed anymore then? Check any 3-view you have handy. Is it just tradition? Really, I'm asking, I don't know another good reason that they would all group around a specific angle
    Norm
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    Re: Swept Back Wings

    You gain some dihedral stability, slightly shorter wingspan for area, put the CG in a good place while mounting the wing in a desirable place, looks cool. An emergency in the higher speeds under the Mach, you might just want a little extra room incase it gets close to punching through. Everything is so conservative now but did they not push the DC8 through 1.0m a couple of times on purpose. You must be a Slowtation fan.

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    Super Moderator orion's Avatar
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    Re: Swept Back Wings

    Quote Originally Posted by Norman View Post
    I'm sure you're right but why bother making the leading edge parallel to the Mach angle at cruising speed anymore then? Check any 3-view you have handy. Is it just tradition? Really, I'm asking, I don't know another good reason that they would all group around a specific angle
    There's a substantial difference between shock formation as prescribed for fighter configurational evolution and the flow seen around a commercial fuselage. Since the commercial aircraft is subsonic, the transonic flow does not show significance until some distance aft of the nose, at which point you get a "normal" shock, which is nearly perpendicular to the surface and has a limited extent of effect.

    The angle that is evidenced in wings is a configurational balance between the need for sweep due to transonic effects, structural issues and aerodynamic efficiencies. The wings utilize similar geometries from one airplane to another simply because they are all designing for a nearly identical flight regime.

    My ex-partner was the aerodynamicist on the 727 and 737 programs - interesting stories. But the wing and wing root configurational designs are a function of localized effects, not any form of body generated shock cone issues.
    "To live is to learn; to learn is to live" (author unknown)

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