Catamarans High Speed Blow Over - Causes & Solutions

Discussion in 'Hydrodynamics and Aerodynamics' started by kidturbo, Sep 11, 2013.

  1. Petros
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    Petros Senior Member

    Daiquiri,

    I do not think you read my post very carefully. I did recognize, and pointed out, that there is competent designers of these boats, (and certainly no lack of budget) they could not achieve those speeds if they were not. It is clear the boats are controllable and stable when in their normal operating envelope, but they clearly go unstable and depart from control very fast when they get airborne out of the ground effects.

    I understand there is a rapid change in the stability conditions (that is clear), but this kind of thing is handled in for example supersonic aircraft where similar rapid changes in the forces occurs when transitioning from sub sonic to super sonic flight conditions. It was a killer at first, until it was understood better, and accommodated. Now it is routine from both super cruising transport aircraft and military aircraft.

    It can be done, I guess the question is how common is it for them to get outside their stable operating envelope? and if some accommodation was made to the design for that condition, how much would it affect their speed when operated normally. That I suspect might be the crux of the problem, and the basis for my inquiry.

    you have not added anything useful in that regard.

    Also note that I did not say it has to be maneuverable or controllable when "in-flight", just that it be stable. Making a shape stable outside of ground effects is relatively easy, no reason it could not be done on these boats. It does not have to be an efficient flyer, just stable. You do not seem to understand what I am referring to. If it in the air, and if you want to land upright and pointing in the correct direction (which these boats clearly do not now), than it has to maintain aerodynamic stability until it can transition back into ground effects in a controlled manner.

    It is a fact that they are very unstable when in the air.

    They suffer rapid and violent "departure" when they get airborne. That is a fact, so how would you propose to fix that? Putting a drag device on it to rapidly slow it down does not seem a very good solution. The OP was inquiring on ideas to correct or mitigate the loss of control when they get airborne.

    Active controls might do it, but a simpler solution might as well. What do you suggest?
     
  2. Petros
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    Petros Senior Member

    this is not correct, this is true of some early designs, and some poorly designed aircraft. But for well over a half a century, control in a stalled condition is what consumes a lot of time and effort of the design staff. Some aircraft in fact, like an aerobatic aircraft, are specially designed to maintain full control when fully stalled, in a flat spin, or even traveling backwards! (as in a "hammerhead" stall). So you are obsolete in understanding control of an aircraft in a stall. when the aerodynamic and inertial forces are larger than the control forces, you loose control of the aircraft.

    Beside, I am not referring to control of the boat when in the air, I am only referring to stability. Understand the difference, a free flight glider, a paper airplane, a kite on a single string, all have stability, but have no means of controlling their flight. That is what "free flight" means. these speed boats are not just in free flight, but also are wildly unstable and are in fully departed mode. If you want to save lives, win races, and keep the boats together, that has to stop. It must maintain stability, whether actively with flight controls and stability computers, or with passive shapes that maintain stable flight conditions.

    These boats are in fully flight mode, the only forces acting on them are aerodynamic when airborne. If you want to keep them pointing down the course and to return to their normal operating envelope, you will have to create an aerodynamic solution.
     
  3. daiquiri
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    daiquiri Engineering and Design

    Thank you for these kind words, as usual.

    I understand very well what you wrote. This, in particular:
    Now, listen. The CoG of the hull is in the range of 25-30% of the LWL, from the transom. For a 15 m cat it is around 3.5-4 meters from the transom. Once airborne, out of GE and at high angle of attack, the aerodynamic force can have a point of application up to around 20% from the leading edge of the tunnel. Which means around 7-7.5 meters ahead of the CoG.

    Please let me know what are these so simple (yet ignored by everyone but you) alterations to the hull which will allow the CoG to get back in front of the CP in these conditions?

    I have some ideas but they are probably obsolete, so I suggest to explore your fresh and revolutionary ones.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    How about.......install a top-shelf GPS unit right at the bow, when altitude exceeds a set threshold, engine power is automatically cut till it falls below that threshold, the drag of that part of the boat still in the water, and/or including the drives, will act to pull the nose down again. Or maybe no GPS is accurate enough. Just an idle musing.
     
  5. kidturbo
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    kidturbo Junior Member

    Thanks for the kind welcome. Great to find the knowledgeable input I was expecting.

    No disrespect to any of the very brilliant designers or builders out there, but I have to agree with Petros when it comes to these big cats being a very badly designed aircraft... I recall seeing photos that Mystic boat Daiquiri has listed in a wind tunnel, also walked around their finished product a time or two. Amazing machines to admire in person, and the vertical stabilizer addition to one of the race boats sure wasn't meant for esthetics.

    So the industry has defiantly been catching on and I consider Mystic a leader at this time. But the evidence shows they will still easily depart, stall, and fall on their transom like all the rest. If their CG was spot on as suggested, then why don't they recover automatically and land gracefully like a 110 Cessna? I understand some compromise for speed. They fly beautifully across the water. 50' of waterline with a contact area equal to a pair of jet ski's. However if they get out of their sweet spot, your just along for a ride. No matter who's award winning design your onboard.

    That's why I posed these questions. Beauty is one thing, safety another. Going forward I expect we'll see great things, but what to do about today's designs. These boats will be around for years, and get re-rigged multiple times. CG gets moved all around, drives changed, each impacting how they perform. If they're on the edge from factory, then a little trial and error change could get pretty risky for the next owner.

    Flight control surfaces are probably gonna show up soon, and a reasonable solution for retro fitting some existing craft. Just how much surface area, and where best to place them that addresses the departure issue.

    Cutting the power typically doesn't cause a correction fast enough, unless you could brake the props and increase drag within milliseconds of bow lift. But your on the right track. :cool:

    daiquiri, I see you've found some of the data you requested, here is my favorite designers list for your review.

    http://www.nor-techboats.com
    http://mysticpowerboats.com
    http://dcbperformanceboats.com
    http://marinetechnologyinc.com

    Ok my brains parched, back to the pro's..
     
  6. powerabout
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    powerabout Senior Member

    its already in the air so there is no drag from the props
     
  7. powerabout
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    powerabout Senior Member

    the boats are driven via a prop in the water and some ground effect on the hull
    the CoG is where it is to make that work
    if the launch is correct then the landing will be correct
    but when its in the air and changing trim where do you suddenly get your flying controls from?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Yes, I expect that would apply a lot of the time, but if it were possible to deploy "countermeasures" when the elevation of the bow reached a pre-determined height above sea level as indicated by GPS, ( big swell height could upset that ), it might include shooting some kind of weighted mini-drogue out of the stern, which retracts automatically and quickly, when the engines power is restored as the bow drops.
     
  9. powerabout
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    powerabout Senior Member

    It couldnt be done with height it would need to be done with height difference from bow to stern and even then you would have to be measuring the speed of rotation to predict it was a blowover which also means it would need to include a measure of how far from the water the drives are as if they hit the water it could well prevent the blowover
    You would need a Lockheed flight computer I would think and even then you would need to work out how to stop the rotation, I would think something based on the water which means to slow it enough to generate an opposite rotation may well result in a stuff?
    Who's going to test it??
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    "Do not try this at home" :D It did occur to me that it might cause "stuffing" (reminds me to put the free-range chicken in the oven shortly), but I'm not sure about that. The idea could be tested on scale models I suppose, but it seems to me the water is there and why not use it to arrest the rotation.
     
  11. El_Guero

    El_Guero Previous Member

    ground effect IS FLIGHT ...

    if an airplane is inside of its envelope, there is NO NEED for 'recovery.' It IS flying.
     
  12. El_Guero

    El_Guero Previous Member

    Use a gyro .... to control attitude.

    I think what you are missing is their need for speed exceeds their desire to live.
     
  13. powerabout
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    powerabout Senior Member

    well yes
    but whats the gyro ( 3 axis laser ring) going to control?
     
  14. El_Guero

    El_Guero Previous Member

    attitude.

    what you will notice on the badly designed lifting devices (boats), is the tendency for the bow to lift, climb, and stall backwards.

    The gyro would keep the nose down. attitude.

    The problem of using true lifting surfaces to control is multiple.

    ONE - FAA.

    Two, a boat pilot is NOT a flight pilot.

    Three, even a lot of flight engineers NEVER design for proper stall. And I have never read of any engineers designing for a 'soft landing' from a stall.

    Canards, could be used, but when stalled would give a nose in dive.

    Horizontal stabilizers give a different set of problems, but when stalled, go in tail first.

    Both normal flight controls at your desired altitude give NO TIME for recovery.

    Hense, the best attitude control system would be a good sized gyro. It would also give control of YAW.

    wayne
     

  15. powerabout
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    powerabout Senior Member

    clearly the attitude but tell me what us the gyro connected to??
     
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