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Discussion in 'Boat Design' started by Guest625101138, Jul 14, 2008.

  1. Submarine Tom

    Submarine Tom Previous Member

    Good question Dennis, thank you.

    The short answer is no.

    But, to reduce drag (the power sucker, and remember, my objective here is a hydrofoil for the layman that can be pedalled for an hour) the angle of attack will decrease as speed and lift increase.

    The vehicle will want to rise as velocity increases, but controlling the ride height will flatten the foil, decrease lift, decrease drag and keep the bulb and foils at depth avoiding a surface wave (more drag).

    This is unconventional.

    Another benefit of pivoting forward foils is roll control. The centre canard will have the largest influence on roll but in order to reduce the size of the canard (drag) I will experiment with independant port and starboard AOA variations.

    This is all rather complex and something I was really trying to avoid but as I crunched the numbers I realized it was necessary.

    Interestingly, the radio controlled modelling world has been a wealth of information and their interest has lead me to consider full fly by wire with micro-processor controls to balance on a single strut.

    The flyak is brilliant but note the surface disturbance over the foils at speed. It is a compromise and fine balance in lift, drag, power.

    And that is hugely skilled power I might add, he is a very skilled paddler/athlete.

    I am neither...
     

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  2. Coach Dave
    Joined: Jun 2011
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    Location: Malabar, FL

    Coach Dave Junior Member

    fly by wire

    I noticed that you have the prop in a tractor configuration instead of a pusher. I am used to seeing torpedoes and subs with props mounted aft. I suspect that is to prevent wash from the prop from creating turbulence around the nose of the bulb and increasing drag. If you shape the bulb carefully you may be able to take advantage of natural laminar flow over most of its surface and reduce drag.

    Since you are looking at active controls to achieve stability you could also vary the buoyancy of the bulb. At rest or low speed the bulb is flooded with water. That makes it easier to keep the bulb underneath you. As speed increases you inflate a bladder to displace the bulb's ballast. Maybe the bulb could provide 50% of the buoyancy required to lift the hull and the hydrofoil provides the other 50% at your 1 hour cruising speed. Reducing the lift requirements of the hydrofoil allows you to reduce its area, increase its aspect ratio and reduce drag.

    As an alternative to the microprocessor-based active controls have you considered surface skimmers? They are simple mechanical devices that work effectively for human powered hydrofoils.

    I think you have an interesting and challenging project.

    Dave
     
  3. Submarine Tom

    Submarine Tom Previous Member

    Dave,

    Thanks for your encouragement.

    Flooding/purging the bulb has been the idea all along, that's why it's there.

    It's the corner-stone of the design.

    I have settled on 33% buoyancy, about 40 litres.

    Surface skimmers combined with micro-processor electronics.

    It's got a lot of adjustability and dual control options.

    It is a test bed. The final, or next generation will be based on a 27' rowing scull hull.

    This will allow for a greater lift-off velocity (hull speed) so, smaller foils.

    Laminar flow is not all it's cracked up to be.

    The bulb will have a rough surface finish.

    I'm still toying with the idea.

    Subs etc put them there for protection and convenience.

    Clean water for prop efficiency may be key with the limited power available.
     
  4. Jeremy Harris
    Joined: Jun 2009
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    Location: Salisbury, UK

    Jeremy Harris Senior Member

    Prop position makes a significant difference to efficiency and stability. A tractor prop (at the front of a body), as Dave says will be less efficient, because the flow over the body immediately aft of the prop will be faster than boat speed, so drag losses will be greater. A pusher prop (at the aft end of the body) minimises additional drag.

    Another benefit is that a pusher prop tends to add stability. The column of faster flowing fluid behind the prop extending behind the immersed body and acts a little like a large tail fin, increasing both pitch and yaw stability by making the apparent length of the body greater than it really is. If you talk to any microlight (ultralight) trike pilot they'll tell you this effect can be pretty big; cut the power on a trike with a pusher prop and the thing get a whole lot less stable.
     
  5. Scheny
    Joined: Feb 2012
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    Location: Vienna/Austria

    Scheny Junior Member

    @ Submarine Tom:
    When it comes to fly by wire, I am an senior expert in that area. So if you need advice, I can help you. I have nearly finished a fly by wire stabilization platform for small planes where 11 different sensor inputs are combined to get an accurate sense of yaw, roll, pitch and altitude using quaternions, Euler-calculations and Kalman-filtering. The whole system only needs around 80$ hardware. For my project it already shows the data precise to half a degree, but the active envelope protections are not yet all programmed.

    In your special case, I would suggest cheap ultrasonic sensors for maintaining altitude. Maybe evacuating the bulb from water can be done by suction? Air can get in through the strut and water sucked out through holes in the rear.

    Andreas
     
  6. Submarine Tom

    Submarine Tom Previous Member

    Well that's certainly interesting Jeremy, thanks.

    Do you feel the effects would be as prominent knowing the prop is two feet in diameter and only 80 RPM?

    Andreas,

    Great to know.

    I'm struggling to get the info out of the radio controlled model world.

    My build is a test bed for a second generation build to follow.

    Your costs sound ridiculously low. Does that include servos?

    I'm planning to use compressed CO2 to purge the bulb bladders at speed.

    I was considering using a pressure sensor in the pod to control altitude but am leaning more to a mechanical surface wand instead.
     
  7. Scheny
    Joined: Feb 2012
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    Location: Vienna/Austria

    Scheny Junior Member

    Hi Tom!

    As a pressure sensor I can highly recommend the BMP085. It is used in nearly 50% of all cars for measurement. The 80$ are CPU and sensors only (sensor is a clone of freeIMU, www.varesano.net). The freeIMU comes with complete software library and has a pressure sensor integrated (the BMP085 was used in V3.5, actual version is the V4.3).

    The only drawback I see in your design, is the short length between the foils. When I designed my prop, I could see that increasing the area by 50% didn't rise the drag by 50% but only by 2-3%. This is because of the angle of attack was getting significantly lower, making induced drag less.

    With your design, I think that the active control will need high input because of the short lever, resulting in high induced drag. Sailplanes have small rudders on long booms for minimizing drag. I think you will need that too.

    The most common foil design, the "Canard", has the advantage that altitude automatically balances. With increasing speed, the main wing rises and the angle of incident (not attack!) gets lower as the skimmer (surface wand) holds the canard foil at constant altitude resulting in a negative angle of the boat at speed.

    Additionally you could exchange a heavy single strut by two small struts. as the lever for structural bending force is then in the range of ~2m instead of 20cm (one strut in the front, one in the back), the struts can be really tiny.

    I would personally prefer a design like that:
    [​IMG]

    Just tell me what information you need on the RC sector. I will try to help you.

    Greetings, Andreas
     
  8. Submarine Tom

    Submarine Tom Previous Member

    Agreed Andreas,

    I crunched the numbers last night and you are correct.

    I'm back to my previous design.

    I've seen this one you sent the picture of.

    Way too much wetted area for 100 watts.

    You raise many good points, thank you.
     

    Attached Files:

    Last edited: Dec 10, 2012
  9. Submarine Tom

    Submarine Tom Previous Member

    So, Andreas, has it been R/C gear you've been working with?

    Yes, please advice on what servo and sensor I should use to stabilize roll.
     
  10. Submarine Tom

    Submarine Tom Previous Member

    Jeremy,

    Would you feel the same knowing it was an 80RPM, 2' diameter prop?

    Would you comment on the advantages of tractor props on azipods?

    Thanks
     
  11. Jeremy Harris
    Joined: Jun 2009
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    Location: Salisbury, UK

    Jeremy Harris Senior Member

    Yes. The prop generates thrust by accelerating the flow of fluid through it, so the local fluid velocity immediately aft of the prop will always be greater than the local velocity forward of the prop. Viscous drag is proportional to the square of fluid velocity, so if you increase the velocity locally over the pod, as in a tractor design (like the Azipods) then you increase the drag.

    On the Azipod this effect is probably inconsequential, as these are big ship propulsion units where the small increased drag from having them in tractor configuration is offset by other factors. On a low power system the difference may well be appreciable though, especially if ultimate efficiency is being sought.

    The Azipod is an unusual case, because the pod diameter is very large relative to the propeller diameter. This means that if they operated them the other way around a large part of the prop would be working in dirty water from the pod flow. The real problem they had was that, in order to get the motor inside the pod, the pod had to be very fat. Had they been able to use a smaller pod diameter, and move the propeller further back from the fattest diameter section, then they would have had better efficiency from a pusher arrangement.

    As with all things, this was an engineering compromise, where accepting the greater drag from the tractor configuration was the lesser of two evils, as the turbulence from the pod would have had a greater impact on efficiency if they'd gone for a pusher.
     
  12. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

    Thanks for those comments, Jeremy.

    I wonder why most prop air planes are front tractor configuration, with very few rear pusher designs around. Prop warplanes to low power rubber band models all have a large blunt bodies directly behind the prop. Is it because of the difference in the fluid density of air vs. water?

    Thanks,

    Porta

     
  13. El_Guero

    El_Guero Previous Member

    Andreas,

    I tried to find your design on line and could not. Would you tell me a little more about it?

    Thanks

    Wayne
     
  14. El_Guero

    El_Guero Previous Member

    Porta,

    Yes. Fluid compression doesn't happen in water like in air, that results in different design necessities.

    But, the reason more aircraft do not push is not directly related to fluid dynamics.

    Take something long, broom handle, and hold it in your hand. Now, push it. If your thrust is the least bit off center you will immediately see it move off of center.

    When you pull on the same rod, the rod will 'straighten' out, rather than push out.

    In water, it is easier to push back against the water to straighten out your vessel. It is not so easy with air. So, pilots, AND designers have to be better when dealing with pushers, than with pullers.

    Wayne
     

  15. Jeremy Harris
    Joined: Jun 2009
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    Location: Salisbury, UK

    Jeremy Harris Senior Member

    I think it's largely an accident of history. Engines are heavy and need to be close to the main lifting surface. There were major stability problems with early canard aircraft (like the Flyer) and designers quickly discovered that turning the aircraft around, with the engine and main lifting surfaces at the front made pitch control easier. Once the first tractor aircraft were shown to be more stable and easier to control than canard designs of the time they became established as being the way aeroplanes "should" look.

    It wasn't until around 30 years ago that this wisdom was questioned, by free thinkers like Rutan, who showed that canard designs could be made just as stable and more efficient. Unfortunately, commercial aircraft design is like boat and ship design, it's inherently conservative and resistant to major change. The few commercial aircraft that adopted canard designs failed commercially, probably because people were reluctant to accept something that looked a bit radical. This nearly killed Beechcraft, when they introduced the Starship. The design and shape was just too radical, and despite its efficiency and performance they only sold around a dozen of them, I think.
     
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