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

  1. Jeremy Harris
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    Jeremy Harris Senior Member

    Not quite. Pusher propulsion is more stable than tractor propulsion and air is incompressible at sub-sonic speeds, so behaves very like water in most respects. I know it sounds counter intuitive, but the effect of fitting the propeller at the rear adds stability in pitch and yaw. Anyone who has flown a pusher aircraft will tell you the same, that pitch and yaw stability decreases significantly when thrust is is reduced. The first time I flew a pusher aircraft I was quite surprised by how significant this additional stability from thrust is, it makes landing a pusher more challenging, as when you reduce power on approach you suddenly have to work a lot harder to maintain stability.

    One reason is that the column of faster moving fluid (air or water) behind the aircraft or vessel tends to increase its effective length, and length improves yaw and pitch stability.
     
  2. Tiny Turnip
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    Tiny Turnip Senior Member

    ?? typo, Jeremy? if not, an expansion would be appreciated!
     
  3. portacruise
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    portacruise Senior Member

    May be true as regards the referenced topic, as we all know about compressed air tanks, etc., which are extraneous to the prop discussion.

    I wonder about the relative importance (to efficiency) of having undisturbed water in front compared to behind a prop?

    Porta

     
  4. Jeremy Harris
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    Jeremy Harris Senior Member


    Not a typo, no. In free flow (i.e. when not contained in a sealed vessel like a scuba tank) air is incompressible at speeds up to around transonic, just below supersonic. Compressibility only becomes an issue with things moving through air at these very high speeds.

    This means that there are no compressibility effects on air flow past things like relatively slow propeller driven aircraft, other than those at the tip of the propeller blades (and then only if the prop is big enough and turning fast enough for the tips to reach those speeds).

    The same basic equations apply to movement of a body in water or air (or any other fluid) in the speed region where flow is incompressible. It's one reason that you can use low speed water tunnels to simulate relatively high speed air flow, provided you correctly adjust the results for the difference in density and viscosity.
     
  5. Jeremy Harris
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    Jeremy Harris Senior Member

    Generally, flow disturbance into a propeller creates a fair bit of efficiency loss, which is one reason why pretty much all aircraft propellers have a faired spinner at the centre.

    The viscous drag over the body aft of the prop is proportional to viscosity as well as velocity, though, so with water being almost 50 times more viscous than air, viscous drag from increased flow velocity can have a greater effect in water than it does in air.

    If the body in front of the prop is well faired and designed to feed clean flow into the propeller, then the propeller will work as well as a pusher as it does as a tractor, in terms of pure propeller efficiency.

    The efficiency difference between the two then comes down to the difference in viscous drag over the body, which will be lower if the local flow velocity is lower, as in the pusher case.

    Many of the propulsion pods around seem to be a bit crude and not well designed in terms of minimising losses. The Azipod is a good example, as it isn't that well faired, but generally it seems that few, if any, electric pod propulsion manufacturers pay much heed to efficiency. Take this one as an example: http://www.mastervolt.com/press-releases/the-new-podmaster-from-mastervolt/

    ZF took hydrodynamics seriously when they designed their pod drive (albeit not electric): http://www.zf.com/corporate/en/prod...er_product_ranges/boats/poddrives/drives.html with a well faired body ahead of the prop, with very little to disturb the flow into the working area of the prop.
     
  6. Tiny Turnip
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    Tiny Turnip Senior Member

    Thanks Jeremy, and Porta - a bit more context made all the difference.

    Adrian
     
  7. portacruise
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    portacruise Senior Member

    Thanks, Jeremy.

    Hum, no significant REAR fairings on ZF unit or some of the pusher aircraft I looked at. Seem like there should be something elongated and tapered behind, along the lines of a previous prop fairing discussion on this same list....

    Porta


     
  8. Submarine Tom

    Submarine Tom Previous Member

    Wayne,

    Are you looking for the one in the picture?

    It's not Andreas's design.

    You can find more by Googling: Stevenson Project Hydrofoil.

    Cheers,
     
  9. Gib Etheridge
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    Gib Etheridge Junior Member

    Andreas,

    Thank you for your advice, and I'm flattered that you would want me to test your prop, but it appears that you will be done with your boat and will have tested it for yourself long before I am done with mine. I don't get as much time to build boats as I would like and I still need to finish the rowing dory I'm currently building.

    Jeremy,

    Would you consider trying to give me an estimate of what speed I can expect using the info. provided in post 1787? After reading the entire thread I think I know what info you need, and I think it's all there. I'll bet that you can do it right off the top of your head.

    Thanks in advance, just in case.

    Gib
     
  10. Jeremy Harris
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    Jeremy Harris Senior Member

    True, and they might get a small gain by using some form of fairing to reduce turbulence, but it might be quite small in practice and outweighed by other losses.

    The benefit can be quite pronounced on a fast pusher aircraft, where the energy recovery from the turbulence aft of the hub is greater than the added viscous drag from the fairing, but it may well be that in water the situation is reversed, with the added drag penalty outweighing the gain. Certainly on low speed pusher aircraft adding a fairing seems to make no appreciable difference in practice, and just becomes a bit of a nuisance when doing routine checks on the prop hub and fastenings.
     
  11. Jeremy Harris
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    Jeremy Harris Senior Member

    Sorry Gib, but it's a lot more complex than that. To get a resistance curve that would be any use to you would mean modelling the hull shape to get an accurate estimate of wetted area, then using something like FreeShip or Michlet to derive the curve. I've never been super-fast at using Michlet (Rick Willoughby was the one here who seemed to be able to do a Michlet run in half an hour or so!) and would need to spend a fair bit of time drawing the hull up and loading the lines into one or other programme.

    The general rules of thumb allow estimates of likely maximum displacement speed (the best known probably being Fr X Sq Rt waterline length), but this doesn't help when trying to get an estimate of speed when you have a given amount of propulsive force, for that you need a better idea of the hull resistance at lower speeds.

    Maybe one of the NAs here could help, as my background is aerodynamics and physics rather than ship and boat design. I'm strictly an amateur when it comes to small boats!

    If you have an idea of the wetted area of the hull, perhaps by doing some measurements around it from the waterline, then I could do a very rough resistance curve using just viscous drag from the wetted surface, corrected roughly for the length and beam of the hull. It wouldn't be too accurate, but might give you a ball park feel for the thrust needed for a given speed.
     
  12. Scheny
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    Scheny Junior Member

    Hello Submarine Tom:

    Have been away for the weekend. I would recommend "digital" servos with metal gearing and ball bearings. This combination is the toughest around.

    Maybe you have a look at http://www.kobbyking.com, its a big (and cheap) online discounter in the US. They also show the maximum torque in Kg (1kg/cm => 0.1Nm) for different voltages and the control speed (s/x°).

    The "XL" servo category:
    http://http://www.hobbyking.com/hobbyking/store/__291__189__Servos_and_parts-X_Large_Servo_50g_.html

    I also recommend to have a look at the review comments. Some servos are marked as metal geared, but this sometimes only means only the last stages. These guys are using them on rough stuff like jumping 6 feet high with a huge buggy and will post, when they still hold.

    Maybe sort them by weight. A 300g servo will hold 40kg, while the 50g are not so interesting in your case.

    You can also buy a 10$ servo tester. Later on I recommend not to use more than 50Hz actualization rate, as some servos will burn otherwise and limit the pulse length between 1ms (full left) and 2ms (full right). The servos use the length of the pulse as the position information. Some can be used from 500ms to 2500ms, but some others will be harmed.

    Just tell me when you need more info.

    Andreas
     
  13. Scheny
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    Scheny Junior Member

    The picture I posted is the "Interflight Hydroflier". It has major similarities to Tom's project, as they use a displacement bulb with variable lift (flooding) to achieve maximum stability for all speeds.

    The other similarity is the fact, that it was the first "low speed" hydrofoil. It only uses 5hp for slow speeds, but today there are even better designs around.

    There are the "Quadrofoil" which takes 2 people at 4kW:
    [​IMG]

    And the "Waterblade" with also 2 persons at 4kW:
    [​IMG]
     
  14. Scheny
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    Location: Vienna/Austria

    Scheny Junior Member

    Reply for "why planes have tractor configuration":

    It is true, that it is much simpler to design a tractor than a pusher. There are many different reasons, why they choose a tractor:

    1.) A prop at the rear is subject to harm by loose gravel, etc. The tires are throwing them up and the blades are damaged quite easily.
    2.) A pusher has a limited prop size in order to make the plane rotate enough (pitch up for start and landing). As mentioned in point 1, you also have to add additional clearance (see LH-10 Elipse, BD-5 and Aceair Aeriks 200).
    3.) For a pusher you have to design a laminar flow over the hull to preserve prop efficiency. Otherwise also the noise goes up very much.
    4.) A canard pusher cannot use flaps (most designs don't have them) which makes start and landing roll longer.
    5.) Engine cooling is rather tricky.
    6.) In a crash, the engine is pushing from behind, rather than absorbing energy.
    7.) A pusher has most of the area before the center of gravity, making directional stability worse.
    8.) A canard pusher has better efficiency because all foils add lift (normally the tail reduces lift), but the vortices of the canard make turbulent flow on half of the main wing, diminishing the effect mainly.
    9.) A pusher with the tail on a boom requires instant control input in case of an engine out, or it stalls instantly (the thrust axis is so high, that the plane instantly pitches up).

    These are only a few points, but I will explain it more deeply if anyone has questions.

    In sum, it is EASIER to design a tractor, but I also favor a good designed pusher. The problem is, that there aren't many around.

    Andreas
     

  15. portacruise
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    portacruise Senior Member

    Scheny:

    Thanks, very interesting.

    Also that the situation appears reversed (despite pushing issues) as far as mounting for jet aircraft engines... Only a few older fighter jets with limited passenger room that have forward intakes?

    Porta

     
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