Super Sonic Flow and the Modern Airliner

F/A-18C Super Sonic at 100 feet, a great video. It happens fast, you can see the shock wave as a ball of vapor as the Hornet flashes by. Let’s take a closer look.

sock wave

In this picture you can see the classic shock wave caused by compression of the air. Due to humidity and pressure change the vapor cloud presents a fantastic visual. The first super sonic fighters had a hard time punching through the number due to drag from the wing in the tran-sonic region, .92-.96 Mach for the F-4 Phantom II. It is called Critical Mach and varies aircraft to aircraft. Basically, due to flow dynamics, parts of the aircraft go super-sonic first. It creates a lot of added drag which prevents the entire aircraft from punching through the number into the super-sonic region. Because the wing is producing lift, it creates the biggest wave drag. So do things like the canopy, notice the separate shock wave behind it in the picture above.

Just a bit of techie stuff, the Area Rule in a nut shell:

The area rule says that two airplanes with the same longitudinal cross-sectional area distribution have the same wave drag, independent of how the area is distributed laterally (i.e. in the fuselage or in the wing). Furthermore, to avoid the formation of strong shock waves, this total area distribution must be smooth. As a result, aircraft have to be carefully arranged so that at the location of the wing, the fuselage is narrowed or “waisted”, so that the total area doesn’t change much. Similar but less pronounced fuselage waisting is used at the location of a bubble canopy and perhaps the tail surfaces.

So what does that mean? It means you can arrange the fuselage and wing to minimize the area disruption, drag, and punch the aircraft through the number with less power. Or delay the Critical Mach number, to cruise faster with less power. Look at the fuselage where the wing is joined. By indenting the fuselage, giving it a coke bottle shape, the area is greatly reduced.

3 F 4 1 F 14 in column

Delaying critical Mach also delays creation of a sonic boom. Sonic Boom? That is the thunder clap you hear when the shock wave reaches the ground. Look closely at the picture below.

shock wave 2

Note the ocean surface, the disturbance on the water is caused by the shock wave. The closer you are the louder it is. Again, also aircraft dependent. The loudest Sonic Boom ever recorded I can proudly say was an F-4 at 100 feet. Why? The older aircraft bludgeoned the air into submission. Modern fighters with computer designed fuselages don’t have to. Note the smaller wing and no visible “coke-bottling”.

F-A-18 NASA hard turn

So what does all this have to do with modern airliners? Simple, the airlines want the fastest Mach Cruise for the least amount of fuel burn. They want a high Critical Mach number to get to their destinations quicker. It is not just fuel that is saved; every minute flown is a cost in maintenance, crew costs, etc. Going from a cruise Mach of .78 to .84, for the same fuel burn is a huge savings.

380

Recognize the shape of the fuselage where the wing joins on this Airbus 380?

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leland

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6 comments on “Super Sonic Flow and the Modern Airliner
  1. Karlene says:

    This is fascinating, and the video was awesome. I’m looking forward to the day I can fly Super Sonic Jet. Think it’s possible? I’m thinking that engineers do the impossible, so yes.

    • Leland Shanle says:

      Karlene, I do think it is possible, but it has to fit operationally. The Sonic Cruiser Boeing designed, for example, did not. It wasn’t fast enough and cost/time equation was not worth cost. So it failed. I think a Super Sonic airliner will have to cruise at 1.5 Mach.

      There is a Biz Jet that is getting close.

  2. Great post Karlene. This is so interesting. A few nights ago, a Netflix astronomy show portrayed what the Shuttle would look like, coming and going, at five miles per second at low altitude.

    Love all you do for pilots and aviation.

  3. Awesome article Chip. Your test-flight days are a great supplement to your commercial flying expertise.

    Here’s a song about super sonic commercial flight that I co-wrote, and you might like:

    Farewell to Concorde by Mark L. Berry and Cleveland Brown

    http://marklberry.com/memoir/farewell-to-concorde/

    lyrics

    I always wanted to go flying on Concorde
    But it’s something I can no longer do
    Because now every Concorde that once flew
    has been grounded
    And now within these hangar walls
    they are confounded
    Never to fly again
    Never to fly again
    Never to fly, never to fly again
    So bye-bye
    Farewell to Concorde

    I always wanted to go flying on Concorde
    Because it seemed like just the thing to do
    Riding on a Supersonic Transport in the sky
    Sipping on champagne and sitting next to you, my baby

    (chorus)
    I used to stop and stare with envy in my eyes
    Such a beautiful bird, just to watch it fly
    Once this great machine was the envy of the skies
    Now I’ll never get to ride it before I die

    I always wondered what it felt like
    To fly faster than mach two point zero four
    Sitting on the edge of space
    ten miles up in the sky
    Looking down at the world
    while riding next to you, my baby

    (chorus)
    I used to stop and stare with envy in my eyes
    Such a beautiful bird, just to watch it fly
    Once this great machine was the envy of the skies
    Now I’ll never get to ride it before I die

    Well it’s famous pointy nose
    Now it strikes a permanent pose
    In the Smithsonian
    Air and Space Museum
    Where you can see ’em
    On the hangar floor
    Never to fly no more
    Never to fly again
    Never to fly again
    Never to fly, Never to fly again
    So bye-bye
    Farewell to Concorde

  4. Eric Auxier says:

    Leland,
    Fascinating article! I too hope that we’ll see above M1 in our commercial airliners in our lifetime. I know it’s possible, but as you say, not practical. So, I’ll “settle” for suborbital hypersonic flight LOL!

    Interesting read, Leland, thanks for sharing! Glad to see our BIF team is back on for 15! And btw, great song, Mark L. Berry!

    Eric “Cap’n Aux” Auxier
    capnaux.com

  5. Ron Rapp says:

    I agree that supersonic aircraft are coming, and that they’ll likely be business jets rather than airliners. The smaller passenger compliment and cargo requirement means the tiny fuselage isn’t a problem, and with bizjets, the airplane is simply a business tool rather than a conveyance which must turn a profit in order for the business to survive.

    It’s amazing that the area rule was discovered in the early 1940s, yet in the year 2015 we don’t have any non-military supersonic airplanes.