Aerodynamics - thundercat racing inflatable

Discussion in 'Hydrodynamics and Aerodynamics' started by Roflhat, Oct 13, 2015.

  1. Roflhat
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    Roflhat Thundercat - 70.10mph

    Cutting away deck area near the bow isn't an option, as the copilot on the boat uses their weight to act as ballast, moving from the stern to be bow as required. I would say I have a copilot about 50% of the time, but I'd like it to be stable with or without a copilot.

    i was thinking of using aluminium tubing for the struts to the foil and then getting some foam which I can form into the shape of a foil and then fibreglass over it. I could also fibreglass over the struts supports.
     
  2. jimburden
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    jimburden Junior Member

    This is not an aircraft.

    A fast boat is operating between a semi hard almost road like surface that is exceedingly uneven, worse than any all terrain vehicle ever had to encounter. The Thundercat is the deepest Vee sea sled like cat I have ever seen and should have the equivalent of a softer suspension in these Vees. But it still will hit some random wall waves hard. A person at 65 MPH can ski on the heels of shoes so when a wave strikes it either bounces the bow into the air or the water has to be taken over the top or penetrated. For the heavy narrow ultra deep Vee boat I was proposing as a replacement for the Thundercat design it would penetrate through the wave instead of being so light per unit of foot print area it has to go over the top. In smoother water all fast boats are planing over the surface. Another choice is like a New zealand cat demonstrated on utube to set the whole boat up on air, hydrauic or spring suspened sponsons or pontoons and let them follow the waves without as much affecting the bounce of the upper suspension supported hull. This is no different than all road and off road, railroad and aircraft landing gear do. This is no different than the legs of a water skier bending to take up shocks.
     
  3. jimburden
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    jimburden Junior Member

    Airplanes do need greater mass ahead of Lift but this is not an airplane. This is like a race car or fast alterrain vehicle on a very rough surface.
     
  4. Jim Caldwell
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    Jim Caldwell Senior Member

    Downforce in the tunnel like an Indy car.
     
  5. jimburden
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    jimburden Junior Member

    Down force in the stern defeats the lifting force.

    This is a good idea with some kind of active control actuation you might suck the underside down like a race car cornering. If the boat gets too high above the wave tops or the bow angle rises too much and a flexing flap bend upward across the underside of the transom. This would have to be enough reduced pressure to oppose the emerging aerodynamic lift at the bow. Possibly thirty years ago I thought that all fast flatter hull racing boats should have an automatic active wing far forward on the bow controlling bow height. Similarly a small spring loaded hydra foil might reach down into swell troughs and prevent the hull from dropping and catching the next slope. Human control might produce over control interacting with all the variables of on coming waves and boat ossolations. Even automated controls need variable dampening to prevent over control or at least shock absorbers. The problem is flow separation often causes such rearward surfaces operating in turbulent boundary air to fail to do anything useful after flow attachment is lost, they stall out. Even the proposed rear spoiler wing operating in the turbulent air of a lifting hull can fail to provide any support. The reason these things work on race cars is because the tracks and path of the cars are fairly level which is rarely an option over water. So many possibilities and problems is why I think of a totally new type of boat that handles all of these problems without the appendages. If this Thundercat had longer side hulls protruding out twenty feet ahead with no center hull and weights on their bows it might have the about the same drag but way more stability
     
  6. tspeer
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    tspeer Senior Member

    Adding a fairing behind the engine cowl will probably reduce the drag somewhat. Any drag reduction helps. But I wouldn't expect a big difference in performance, especially since the engine is in the wake of the pilot.
     
  7. groper
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    groper Senior Member

    It would require alot of engineering/fabrication, but you can largely mitigate somewhat the pitching moment in what is essentially a low aspect ratio wing (the tunnel) by creating a reflex profile instead of a straight section. With a reflex profile, total lift will be reduced but stability increased. You may find it slower with copilot because you may benefit from the extra lift and riding the edge of the stability envelope. With just 1 person aboard, the extra stability would likely result in better total average speed especially in rough conditions where stability is more important.

    This could perhaps be done by adding a surface directly under the existing tunnel if it cant be replaced entirely. The added suface may only need to cover 50% of the tunnel, for example, adding a slightly convex surface to the rear 50% of the tunnel etc.

    What do you think Tom?
     
  8. markdrela
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    markdrela Senior Member

    Adding reflex by itself does not add aero pitch stability. Moving the CG forward is what adds pitch stability. But in practice you need to do both, since pitch trim is also necessary.

    To keep the physical effects clear, this is a better way to think about it:
    * To increase passive aero pitch stability of a given surface planform (i.e. without adding a tail) it is necessary to move the CG forward.
    * To maintain aero pitch trim as the CG is moved forward, it is necessary to simultaneously add reflex.

    Having said that, on a boat it's probably better to NOT have perfect aero pitch trim, but instead have a slight amount of nose-down trim, so that the boat tends to return to the water after it bounces into the air. In that case too much reflex is bad, and zero added reflex may also be OK.
     
  9. philSweet
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    philSweet Senior Member

    How does the prop thrust vector fair in all of this? It doesn't just track the shaft axis. As I understand it, as the boat pitches up, a "normal" flatish prop would tend to continue to push horizontal, so in boat coordinates, the thrust vector would depart downwards and create an unstable pitching moment. I would imaging clever prop work wrt rake can minimize this, but it is a pretty big force to contend with compared to the 15lb/sqft aero potential.

    Any formula for thrust angle as a function of inflow angle? I've seen one for turbine generators, but not for boat props.
     
  10. Alumination
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    Alumination Junior Member

    Keeping the Center of Gravity ahead of the Center of Aerodynamic Lift will go a long way to adding stability. It will also help you keep your angle of attack below 5 degrees which will help keep Aerodynamic drag reasonable.

    Is your new motor able to run to redline?
    Have you tried different propellers?
     
  11. powerabout
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    powerabout Senior Member

    Negative rake props?
     
  12. tspeer
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    tspeer Senior Member

    I wonder if, instead of a reflexed trailing edge, giving the stabilizer positive camber or a positive incidence angle would help. The idea would be to use aerodynamic force aft to reduce the hydrodynamic lift and shift the planing area forward by reducing the pitch angle. One could start with setting the stabilizer at zero lift (regardless of what its camber was), and then experiment with changing the incidence angle to see the effect on stability and performance.
     
  13. powerabout
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    powerabout Senior Member

    I cant see that there is a problem, thats the class and thats that.
     
  14. groper
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    groper Senior Member

    My father has designed an ekranoplan which was developed exactly like that.... the reflexed tunnel wing with aft stabilizer. He has published a paper on it but I'll have to find it. He has always laughed at the racing boats which keep flipping over and can't understand why they keep running the same flawed designs...

    In ground effect , the center of pressure is further aft than the same wing at altitude. As the wing climbs, the center of pressure moves further forward and so the pitch moment increases.

    The cog problem is easily solved with a tiller extension to the outboard motor so the pilot can sit further forward
    ..
     

  15. powerabout
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    powerabout Senior Member

    If it went faster with cog forward then they would be doing just that
    The weight of the hull at that speed has determined the angle of attack to get as much boat out of the water as possible before air drag becomes significant
    Its right on the edge
     
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