Aircraft carriers - ramps - makes less than zero sense to me

Discussion in 'Physics & Math' started by Jonathan Ainsley Bain, Apr 11, 2022.

  1. Jonathan Ainsley Bain Registered Member

    Messages:
    6
    There are basically two types of fixed-wing aircraft carriers.
    Those that use a 'ramp' and those that use a catapult to
    help with the launch of the aircraft.

    Now the catapult (steam or electric) is not the issue here.
    They seem fine ; but are only used by the French and Americans.

    But for the life of me, that ramp which the rest of them use makes
    less than zero sense. Because in order to gain that extra height
    the aircraft must lose horizontal velocity.

    But a loss in horizontal velocity INCREASES the chance of a stall.
    And horizontal velocity is what the catapult adds to assist the aircraft.

    Jumping in the air is not flying. The wing requires horizontal velocity
    to generate vortexes which cause lift. Flying up, is not lift.
    So actually it would be better to not have anything than that ramp.

    After all, the American navy does not seem to use ramps anywhere
    from what i can see. And it certainly is a much cheaper solution.
    That catapult is a huge maintenance problem.

    Thoughts, anyone?
    (Please no search-copy-pastes, if you really understand this, in your own
    words or in a graphic image you have made yourself.)
     
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  3. DaveC426913 Valued Senior Member

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    Yes, I have a thought. Here it is: Do you think its reasonable to demand others write their own answers, while you can't be bothered to simply Google your own questions?


    "In aviation, a ski-jump is an upward-curved ramp that allows aircraft to take off from a runway that is shorter than the aircraft's required takeoff roll. By forcing the aircraft upwards, lift-off can be achieved at a lower airspeed than that required for sustained flight, while allowing the aircraft to accelerate to such speed in the air rather than on the runway. "
    https://en.wikipedia.org/wiki/Ski-jump_(aviation)
     
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  5. exchemist Valued Senior Member

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    The principle is that by imparting upward velocity, the aircraft can become airborne at an airspeed below that needed to generate enough lift to support its full weight.

    Although this upward velocity decays away, because immediately after launch the wings are not generating enough lift to support the full weight of the plane, by the time it has gone completely the aircraft will have further accelerated in flight, to the point where the lift has become enough to support it fully. So the ramp "buys time" for the plane to accelerate further, whereas, without it, it would drop on leaving the deck, potentially into the sea.

    It is not the case that the ramp causes the aircraft to lose horizontal velocity, since it is accelerating under full take-off thrust from its engine(s). The deflection of some horizontal acceleration into vertical acceleration will reduce the rate of horizontal acceleration, for a second or so, as the aircraft traverses the ramp, that's all.

    The ramp works. If it didn't, it would not have been fitted to the Royal Navy's two new carriers. Here is one of them:-

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  7. Jonathan Ainsley Bain Registered Member

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    Funny, when I put this question to actual F15 pilots,
    the majority agreed with me.

    Are you saying that loss in horizontal velocity does not increase the chance of stall?
     
  8. Jonathan Ainsley Bain Registered Member

    Messages:
    6
    Any 'time bought' will be lost in having to regain velocity
    and in so doing its horizontal velocity having decreased
    will increase the chance of a stall.

    You copy-pasted that image very well, though.
     
  9. exchemist Valued Senior Member

    Messages:
    11,260
    You didn't read my reply, evidently. There is no loss in horizontal velocity.

    The photo illustrates my point that the ramp has been chosen for new builds. So there is no question that it works, whatever your F15 pilots - whether real or imaginary - may say.

    P.S. There never has been a naval version of the F-15, so its pilots would probably not be trained on the operation of a ski-jump take-off.
     
    Last edited: Apr 11, 2022
  10. billvon Valued Senior Member

    Messages:
    20,651
    Well, no. They are accelerating the entire time, and generally the angle of the ramp is such that they can continue to accelerate even on the ramp. (In other words, the available thrust exceeds the normal force exerted by the ramp.)
    Well, yes - flying up is flying, by definition. The goal is to KEEP flying.

    Here's how it works:

    Let's take an aircraft with a stall speed of 120 knots in its normal takeoff configuration. That means it must maintain 120 knots to maintain level flight. It can absolutely fly faster than that, in which case it has enough additional lift to climb. It can absolutely fly slower than that, but then must begin to descend, since its weight exceeds the available lift.

    The aircraft starts its takeoff roll. It accelerates to 95 knots. If there was no ramp, when it got to the edge of the deck, it would not lift off - it would have to start descending, since it is not at 120 knots yet. Can it reach 120 knots before it gets to the water? Maybe - and there are videos of bolter aircraft doing just that. But you better hope there are no big waves!

    However, this carrier DOES have a ramp. It hits the ramp and is deflected upwards about 100 feet. Now it is in the air, headed upwards, while flying at 90 knots. (Slight reduction in acceleration due to the ramp.) It cannot maintain its climb, so it noses down, and after a few seconds, is in level flight. If it was still at 90 knots, it would continue its descent. However, those few seconds have allowed it to accelerate to 120 knots, and so it can maintain level flight. Now it accelerates to 130 knots and begins to climb.
     
  11. DaveC426913 Valued Senior Member

    Messages:
    16,925
    It is an interesting and rather alarming fact that pilots do in fact get flight theory wrong.

    There was a physics challenge that went around years ago about "can a plane on a conveyor belt take off?" (Google for the riddle specifics; read at your leisure.)

    A lot of pilots got it wrong.
     
    Last edited: Apr 11, 2022
  12. billvon Valued Senior Member

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    20,651
    Nope. Which is why ramps work.
     
  13. billvon Valued Senior Member

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    20,651
    I am sure they did. Most pilots don't understand physics very well. If you want proof of this, google "downwind turn."
     
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  14. Seattle Valued Senior Member

    Messages:
    7,738
    The OP argument is a bit like arguing that aircraft carriers with ramps don't work and therefore don't exist.

    A ski jump is like a ramp in reverse (trading gravity for forward thrust). There is no doubt that if a skier going off a large jump had an engine the skiers would keep "flying". Not a perfect metaphor but close enough.

    You don't know "most F16 pilots" and I doubt if most would argue against the existence of those aircraft carriers. You may have talked to a drunk F16 pilot engaged in inter-service braggadocio but that's about it.

    The purpose is obviously to eliminate the complexity of catapults, to reduce the length of the runway (and therefore the ship length) and it probably reduces the choice of aircraft that can be accommodated on that carrier.

    The trade-off in reducing the rate of acceleration for time and altitude to accelerate to a level flight speed (and beyond) is obviously achievable.

    What a silly question, the way it's worded.
     
  15. exchemist Valued Senior Member

    Messages:
    11,260
    Indeed. One curiosity of this poster is his injunctions not to quote from sources, or provide links to other sources of the information he seeks. If we all took that attitude, we would not be able to learn much, it seems to me. In fact, as it happens, he could easily have answered his own question here, by looking it up on the internet for himself. Very odd.
     
  16. Yazata Valued Senior Member

    Messages:
    5,612
    It's all a matter of airspeed. In conventional aircraft takeoffs, the aircraft accelerates down the runway until it reaches takeoff velocity and then rotates upwards and continues accelerating as it climbs out, still accelerating while supported by the lift of its wings.

    The trouble with an aircraft carrier is that it's too short for an aircraft to reach takeoff velocity when powered only by its own engines.

    So one solution is to use catapaults that accelerate the aircraft more rapidly than it could accelerate itself on such a short takeoff run.

    The other solution is the ramps. The idea here is to launch an aircraft that's still going too slow to fly upwards on an arching ballistic trajectory. Remember that it's still accelerating due to its engines running full-bore. So it arcs up and maybe down some, all while it's going faster and faster. It reaches takeoff velocity and climbs out. Think of it as a way to virtually extend the length of the runway up into the sky a little.

    Stalling isn't the issue since if it leaves the carrier deck going too slow to support itself aerodynamically, it's already the equivalent of stalled. The idea with the ramp is to give it additional time to accelerate so as to correct that. It obviously works, since the British, Russians, Chinese and Indians are all using carrier ramps successfully.

    Remember that this only works with high performance fighter planes. They have a very high thrust to weight ratio, they accelerate rapidly and some of them are even capable of accelerating straight up. If a conventional airliner or general aviation aircraft tried this ramp trick, it would belly flop into the ocean because it doesn't have the ability to accelerate quickly enough once it leaves the ramp. If a fighter plane loses thrust on takeoff, it will splash too.

    Choosing ramps instead of catapaults was controversial in Britain. Many thought that these new carriers of theirs should do things the American way with catapaults, which would increase the variety of aircraft the carrier could launch beyond high-thrust fighter planes. That's why France's similar sized carrier can launch Hawkeye airborne radar planes and the British carriers can't. The US carriers also launch "COD" (carrier onboard delivery) aircraft, which are small transport planes to carry small cargo and personnel to and from carriers at sea. Though the US might be going to VTOL Ospreys in that role, which is something the British could also do with their ramp-carriers.
     
    Last edited: Apr 12, 2022
  17. Yazata Valued Senior Member

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    5,612
    F-15's don't fly from carriers. I can imagine a pilot who launches from long runways with a full takeoff roll being a little nervous about launching at less than takeoff velocity after such a short roll.

    If the aircraft leaves the carrier flight deck below takeoff velocity, it's already effectively stalled. It's the short takeoff roll and insufficient acceleration that causes that, not the ramp. The ramp just tosses the plane up into the air giving it more opportunity to accelerate.
     
    Last edited: Apr 12, 2022
  18. DaveC426913 Valued Senior Member

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  19. Yazata Valued Senior Member

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  20. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

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    Going
    • WITH (same) travel direction OR
    • AGAINST (opposite) direction
    Which then relies on
    • length of belt
    • acceleration capacity of aircraft

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  21. DaveC426913 Valued Senior Member

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    No.

    The plane takes off.
     
  22. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

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    ¯\_(ツ)_/¯ OK

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  23. billvon Valued Senior Member

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    None of that matters. The only thing that matters is the aircraft itself.
     
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