Flying boats

Discussion in 'Boat Design' started by Benoit Lescot, May 25, 2010.

  1. Benoit Lescot
    Joined: May 2010
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    Location: France

    Benoit Lescot New Member

    Hi everyone, first post on this forums !
    I'm designing a Flying boat, as in 6 engines, 13m wing span, 2T takeoff weight, and I'd like to dig a bit more into the design of my hull, conventional hulls have a single or double step, about 5-10% of the beam and it's quite simple, however I beleive research has led to more modern designs such as the faired steps used on the saro princess for example, I'd like to know if you guys know of a CFD program able to analyze my hull from 0 up to takeoff speed with results for total hump resistance, hump speed and possibly trim during that time, basically a CFD reproducing what NACA did in their Langley bassin.

    I've looked briefly at some boat design softwares but it seem they are doing their analysis based on statistical data from documented boat hulls.. don't think I can extrapolate that to my flying boat hull..

    any comments welcome !
  2. Easy Rider
    Joined: Oct 2009
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    Easy Rider Senior Member

    I think probably the most important thing is to achieve the perfect angle of attack relationship between the wing and the hull. With speed and elevator I think you'll get more AOA control and range w a single step.

    Easy Rider
  3. tunnels

    tunnels Previous Member

    My first siggestion is for you to look on you tube under flying boats and Float planes is another place to look . ' They have hull shapes that are quite differant to a ordinary boat plus they need a touch of aerodynamic shape to them as well . so they have a dual purpose Hydo dynamic lift as well as aero dynamic lift to !You will aso see the forms of stepped hull shapes from early time to quite recent jet powered flying boats . Another consideration is weight and what do you intend to build it out of ? My choice would be Carbon fibre with a nomex type core really strong and very light weight !. I hope you can find a good programs for your computer !!! :confused: :p
  4. Jeremy Harris
    Joined: Jun 2009
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    Jeremy Harris Senior Member

    As someone with a float plane rating and experience of aircraft design, my view would be that the aerodynamic issues dominate when at, or near, step taxy speed. Most float planes change attitude markedly as they come up on the step and need a fair bit of elevator authority to get them there. If you can find a way to minimise the pitch change from displacement to on the step then that would be a great benefit, as it would reduce the chance of a PIO leading to porpoising.

    My limited experience of putting floats on aircraft tells me that the position of the step relative to the longitudinal C of G is the major factor. Too far forward and you don't have enough elevator authority to get on the step, too far aft and it gets as twitchy as hell in pitch.

    I've always thought that a two step design might have an advantage in terms of pitch stability and broadening the on-water allowable C of G range. I'm not sure I'd trust any CFD analysis though, as it's an area where I suspect little in the way of proof of simulation accuracy has been carried out.

    At least one company in recent years has been working on a similar sized seaplane. They opted to use some big radio controlled scale models (around 1/5 scale I think) to refine the sea keeping elements of the hull. I believe this approach worked well, and gave results that were pretty close to the way the real hull/fuselage performed.

  5. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    6 engines? why so many? Could you provide a rough sketch?

    what is the intended purpose of the flying boat?
  6. Stumble
    Joined: Oct 2008
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    Stumble Senior Member


    As I understand it the move to more than 2 engines was dictated by the maximum size of engines available, not desirability. Now I could see 3, maybe even 4 engines on a large plane, but 6 sounds like added complexity not added safety.
  7. Guest62110524

    Guest62110524 Previous Member

    of course you a re correct, 6 engines would exceed the weight he talks of But I posted that cos I like seaplanes:) even spruce goose had only , um what was it 8 engines?
  8. MatthewDS
    Joined: Mar 2010
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    MatthewDS Senior Member

    It was my understanding that the large number of engines seen on the giant flying boats was due to the low power of the engines available at that time.

    Today we have much more powerful engines for a given size/weight, so I wouldn't think more than two would be required.

    This wikipedia page describes something similar to what the OP describes:

    (Ooops, sorry Stumble, almost the same post, didn't see yours)
    Last edited: May 25, 2010
  9. Jeremy Harris
    Joined: Jun 2009
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    Location: Salisbury, UK

    Jeremy Harris Senior Member

    Another reason for multiple engines, other than the availability of big ones, has to do with propeller erosion. You need to keep the props well clear of spray, or else they erode very quickly. This limits the prop diameter, which in turn limits the power they can efficiently absorb.

    Using smaller diameter, faster rotating, multiple blade props to get around this problem tends to be less efficient, particularly at low speeds, such as accelerating up on to the step, when you really need as much thrust as you can get. One way around this was to use several engines, mounted as high as possible. Each could then be fitted with a decent sized prop for the engine power and collectively they could shove a far greater mass of air backwards. The net result was good take off performance, something that was often more important when operating in restricted harbour areas than cruise efficiency.

  10. HJS
    Joined: Nov 2008
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    HJS Member

  11. Benoit Lescot
    Joined: May 2010
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    Location: France

    Benoit Lescot New Member

    thanks all for your input, just a little clarification, this 6 engines seaplane is a replica (4 seater) of a beautifull 40s era french seaplane, hence the 6 is invariable, also I have been researching documents on flying boat design since about 6 months now so I have done a lot of reading and have about 100 times more to do, conventional hull design (the one with a basic 90 deg "vertical" step) is very well documented, I have some book like the Thurston one explaning all that, but then I found some "new technologies" as of faired steps and with added ventilation which I'd like to integrate in my design (even if this deviates from the original) as I beleive cleaness and lowest drag possible are of essence.
    Also I've read a lot of NACA reports, probably more to read but further away from the subject.

    What I am trying to find out through this post is a software solution to predicting hump resistance, speed and spray patterns.

    Of course if no software solution exists or is reliable enough I will go for a scaled down RC model but I hope in 2010 we have something that can help..

    Has anybody worked with Fluent for example ? would this software enable me to analyse the water interference from a hull accelerating from say 0 to 90kts ?
  12. Submarine Tom

    Submarine Tom Previous Member

    I hope you get some responce to your query, I'm afraid I don't have it.

    No matter the outcome, I would strongly encourage you build a RC version.

    If you're proposing a ground effects boat/plane, as I believe you are, then wave height is critical.

    One contact while airborne could be your last...

  13. tunnels

    tunnels Previous Member

    WOW:D the first thing i found is so interesting and will take days to read and partly understand what there . :) !:D
  14. WickedGood

    WickedGood Guest

    Just copy this one

    1 person likes this.

  15. tunnels

    tunnels Previous Member

    youtube !

    I had a look on Youtube under the suject PBY and theres some very interesting footage of the manufacture and assembly about the old Catalina Flying boats being built , the assembly , and test flights landing and taking off etc i spent a good afternoon watching videos .
    Interesting during the filming of the assembly are shots of the bare hull bottom and all the frames set up and then stringers attached so theres lot to see in the hull shape etc etc And with people crawing all over it you would even be able to roughly work out sizes etc etc
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