Aircraft Carriers

Discussion in 'Stability' started by Keough, May 13, 2011.

  1. Keough
    Joined: May 2011
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    Keough Marine Systems Designer

    How do aircraft carriers maintain stability during aircraft takeoff and landing? I'm assuming these jets are about 10-30 tonnes in weight (no idea of true values), and travel from around the CL at the aft to liftoff the port side ahead of amidships. This amount of weight must cause some stability issues in the vessel itself, and of course are accounted for, but in what ways?

    This is my first post of the forum. I am a new graduate in Marine Engineering Systems Design, and have a bit of background in naval architecture/stability analysis. I was recently watching discovery channel show Mighty Ships on an aircraft carrier and got to thinking about this issue. Do they use conventional systems and also just ballast down to raise the KB and lower the KG, or is there more going on?
     
  2. DCockey
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    DCockey Senior Member

    Displacement of a Nimitz class aircraft carrier is around 100,000 tons. Max take off weight of a F18 is around 25 tons. Data found on internet.

    25 tons is 0.025% of 100,000 tons. To put that in perspective 1/2 lb is 0.025% of 1 ton. I wouldn't expect moving 1/2 lb around on 1 ton boat to have a noticeable effect on trim.

    One skill which is valuable as a working engineer is being able to come up with a quick estimate of whether something may be signficant or or can be ignored using whatever information is readily available.
     
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  3. Submarine Tom

    Submarine Tom Previous Member

    Good post DC. In addition, the aircraft weight is almost entirly on the wings by the time it reaches the bow, reducing the load on the vessel to near zero minimizing pitching effect.

    -Tom
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Spot on :cool:
     
  5. jehardiman
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    jehardiman Senior Member

    Concur
     
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  6. Dean Smith

    Dean Smith Previous Member

    stabilty is not strictly proportional to disp?
    An anology may be the richter scale?
    or the righting arm of a 100 ft sail boat is not 5 times greater than a 20? it is many times more
    This is a question, not a critism
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Incorrect.

    The GZ curve, and how we calculated the righting levers come from knowing where the centre of gravity is, KG, and the location of the metacentre, M, or KM.

    To find KM, is straight fwd when we know where the KB is, that is easy to work out as it is simply the centre of buoyancy. So we need to know the relationship between KB and KM, which is thus:

    BM = I/V

    V = volume, ie the displacement
     
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  8. DCockey
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    DCockey Senior Member

    A Nimitz carrier is a about 1000 ft long, so a 1/50 scale model would be around 20 feet long and have a displacement of around 0.8 ton. (Displacement scales as dimensions cubed). A 1/50 scale model of an F18 would have a max takeoff weight of around 0.4 lbs. The angular change in pitch or heel from moving the full size and scale model F18 from stern to bow or from side to side on the full size and model carriers respectively would be the same (assuming the weight of the model carrier was distributed so the CG height stayed the same in scale.

    That's a simple way to do a quick analysis of this.
     
  9. cthippo
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    cthippo Senior Member

    The other factor is that the boat moves some anyway, the pilots just get used to it.
     
  10. Dean Smith

    Dean Smith Previous Member

    very well my boy, go to the top of the class:)
     
  11. Lurvio
    Joined: Jul 2009
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    Lurvio Mad scientist

    Just a little tidbit of information on Fighter Jets. I got a training for the FA-18 Hornet weapons systems in the army (pioneers, had to know how to diable the bombs it the bird fell down). The Finnish airforce has the Hornets in use, we don't have carriers, but the plane is build for it. According to the instructor, the landing gear is buildt so that the plane can be dropped 10 meters in free fall before the shock absorbers bottom out. And they better be, they call the landing a 'controlled crash' or something in the same effect.

    Here's a pilots description of a carrier landing, worth reading.

    How to land a jet plane on an aircraft carrier

    Lurvio
     
  12. Squidly-Diddly
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    Squidly-Diddly Senior Member

    even in calmest seas, the swell acting on the hull would be

    several times(maybe 100X?) the weight of a plane landing.

    Yes, the ship does roll and pitch, but not that much, but I hear it is a factor in landing.
     
  13. DCockey
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    DCockey Senior Member


  14. Submarine Tom

    Submarine Tom Previous Member

    You know what you call three birds on a log floating on the ocean?

    That would be a Canadian aircraft carrier! (We don't have any aircraft carriers... but we used to).

    -Tom
     
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