Rc sailrocket

Discussion in 'Hydrodynamics and Aerodynamics' started by Glueandcoffee, Feb 9, 2021.

  1. Glueandcoffee
    Joined: Feb 2021
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    Glueandcoffee Junior Member

    Planning to make an rc version of sailrocket with the objective of setting a world record for rc sailing.. i believe the current record unofficially is held by an rc ice boat at about 52km/h.
    I'm acutely aware that
    1. Water is a difficult thing to move through
    2. At 50kmph
    3. When you are made of balsa wood and thin fiberglass and plastic and other not ideal materials
    4. By someone building their first rc boat
    5. On a sub par budget.

    I think I have an ok grasp of the physics but I'll put it to the forum for validation. I'm Also aware that I don't know what I don't know so I will just leave a few photos and you can ask the questions or give advice or tips for me. 20201227_231549.jpg 20201227_231701.jpg 20201227_231741.jpg 20201227_232205.jpg 20201227_232513.jpg 20201227_232550.jpg 20210111_161013.jpg 20210201_154524.gif Snapchat-1478658112.jpg VideoCapture_20210103-183743.jpg VideoCapture_20210103-183710.jpg Snapchat-68656781.jpg 20201227_231549.jpg 20201227_231701.jpg 20201227_231741.jpg 20201227_232205.jpg 20201227_232513.jpg 20201227_232550.jpg 20210111_161013.jpg 20210201_154524.gif Snapchat-1478658112.jpg VideoCapture_20210103-183743.jpg VideoCapture_20210103-183710.jpg Snapchat-68656781.jpg
     
  2. messabout
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    messabout Senior Member

    Being aware that you don't know what you don't know is a sign of intelligence and a grip on reality. You have made a good start here.

    Your drawings show a vehicle that reminds me of a book I have around here someplace. The book was printed in the 1950s and its' title is : The 40 Knot Sailboat. The author was a US naval officer with an imagination and a quest for discovery. He made several models similar to the ones you have pictured. His observation about the limited speed of a sailboat was that the hull is the object that sorely limits attainable speed.

    The record at 52 km/hr translates to about 32 MPH here in the colonies. That is a very fast model sail boat. The first thing to chew on is scale. To achieve the most speed you will surely need a large model. Going fast with a surface supported model is achievable of course. Some of the model RC power boats can top 150 km/hr. But they have the advantage of enormous power to weight ratio. Sailboats not so much.

    The difficulty that I can imagine is that you will need an unusually flat water surface at the same time that we have a very brisk wind. That is not impossible but you may have to have the patience of a Buddhist monk to find that condition.

    Foiling is clearly the way to begin. I will be interested in your progress. One of our members is a foil fiend who has built many models. Some of them impressively fast. He is Doug Lord. If you can connect with him, that will be a valuable asset for you.
     
  3. Will Gilmore
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    Will Gilmore Senior Member

    I am interested in following your project. I have some thoughts, but no background to give them weight.

    You mentioned being aware that water is a difficult medium to move through and messabout agrees that foiling is the way to go. However, at these high speed, air also offers a considerable amount of friction to overcome.

    One of the problems with sailing on water that sailing on ice has an inherent advantage over, is the need for lateral resistance. You necessarily have an appendage dragging in the water to provide lateral resistance. An ice boat gets its lateral resistance from its very efficient skate. If you can achieve good lateral resistance with a minimal depth and surface area that is more like an ice boat skate and still have adequate lateral resistance and combine that with an aerodynamic hull above the water, I see no reason you can't approach ice boat speeds.

    -Will (Dragonfly)
     
    Last edited: Feb 10, 2021
  4. Glueandcoffee
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    Glueandcoffee Junior Member

    Although the weather hasn't been with us in months the usual wind is from the south west here. Coming down a short valley across the sand bar which is only about a meter higher than high tide which allows the wind to stay pretty consistent and doesn't chop the water.
    I have a single foil foil on the rear which is almost definitely too big . 3"root 2" tip 14 " span on the section bellow the radius of the L .roughly 8to12% thickness of the chord. The foil was sanded from a piece of tough pvc so the aerfoil sections are far from perfect. On airfoil tools.com I chose an airfoil which closely approximates what I was aiming for in terms of thickness and camber and Reynolds number the foil would be at at 50kmph in water and got a Cl of about 0.6 at 4.5° angle of attack.
    Using the lift equation
    lift= Cl x 1/2 density x velocity ^2 x surface area. In si units.
    We get 0.6x500x196x0.0225=
    1323 newtons. Divide that by 9.81 gives aprox 135 kg pulling the foil into the water against the sail.
    Now I'm not saying this is best or worst case scenario but it is a scenario which is possible in theory. But I don't think the boat is built for those loads. Solution: chop the foil to a much smaller size to match the sail power at that speed. I know Paul Larsen ended up chopping the foil down to be far smaller than it was originally designed for.
    Anyone spot any major flaws in my understanding of the force it might experience ?
     

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    Last edited: Feb 13, 2021
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  5. Doug Halsey
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    Doug Halsey Senior Member

  6. Glueandcoffee
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    Glueandcoffee Junior Member

    Thanks . I've posted in rc sailing groups on Facebook and ian holt has replied. I think he has has his own ideas about a the practicality of an rc sailboat that only uses one tack.

    Yes I've learned as much as I can from mini 40s and searching the internet I can only find 2 examples of an rc sailrocket. Both were designed and built by the sailrocket team. I've been in contact with Paul larsen and malcolm Barnsley and they were very helpful but even after their input I still haven't managed to build the boat to its potential. But in saying all that the boat hasn't touched water so nobody knows,... it might do 70kmph. Unlikely. But I don't know.

    I'm just waiting for the servos and reciever to arrive in the post.
     
  7. Doug Halsey
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    Doug Halsey Senior Member

    The40KnotSailboat.jpg
    This is the book.

    My Dad was inspired to build something similar when I was a teenager. It was amazingly fast in spurts, but tricky to get trimmed right, partly because it was freesailing (not RC), and partly because the solid wingsails gave no visual clues for guidance.

    I can't really guess the top speeds, but I'm pretty sure they couldn't have been close to the 30kph levels that the Mini40's are currently seeing.
     
  8. Glueandcoffee
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    Glueandcoffee Junior Member

    What do you mean solid wing sails don't have visual clues for guidance. ?
    Like backing the leading edge of the sail when undersheeted or tell tails flapping when stalled or oversheeted?
     
  9. messabout
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    messabout Senior Member

    The author, Bernard Smith, sailed his models in the Washington D.C. reflection pond, among other places.
     
  10. Glueandcoffee
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    Glueandcoffee Junior Member

    I'll be sailing my one on fountainstown back strand. Ireland.
     
  11. Glueandcoffee
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    Glueandcoffee Junior Member

    So right on que the remote control and servos arrived and I had to throw something together to get a feel for it. And this was the result.
     

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  12. AlexanderSahlin
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    AlexanderSahlin Junior Member

    I think it is a very good idea to build a model Sailrocket. Such a craft will have the potential for a very high speed, also in model size. This is because a a configuration like Sailrocket has cancelled the heeling moment, so you can handle a sail-force several times the weight of the boat. Model-boats need to have very low weight / sail-area, otherwise take-off will be hard. You can certainly do much more that 52 km/h with a model like the one you are building.
    You write that you designed your foil for a cl at 0.6. This is a very high cl for a surface-piercing hydrofoil at high speed. My experience from such foils at such high cl is that they ventilate on the suction side, and lose most of the lift. I suggest about half or even less of that cl. If the surface-piercing part of the foil has lift, you may also have to install anti-ventilation fences. If you know Malcolm Barnsley, you can certainly get useful info from him about the foil-design from Sailrocket. I am quite sure they used a base-ventilated section for their foil. The advantage of such a section is that you can avoid cavitation while you still have enough thickness for the bending moment on the foil. Such base-ventilated sections work excellent in model scale. The high speed and small submergence makes the pressure-drag from the air-bubble small relative other forces on the foil.
    You will make life easier by starting with somewhat oversize foils. When everything works and the boat can take off to high speed, you can try to reduce the foil-size to the optimum.
    Looking forward to see a report from your tests here!
     
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  13. Glueandcoffee
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    Glueandcoffee Junior Member

    I reccon that cl is quite high. It was only a guestimation based on another guess of the foil having 4.5° aoa. But after doing 0 experiments on any parts of the boat ,estimates are my best shot at working out forces , angle of attack, apparent wind angle, leeway angle, coefficients of lift , coefficients of drag etc. How would you go about reducing coefficient of lift on the foil.reduce foil aoa or reduce sail power or increase foil area. Or is there another way that I'm unaware of.
    There is a plan to put fences if it ventilates but until then I'm keeping it simple stupid.
     
  14. AlexanderSahlin
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    AlexanderSahlin Junior Member

    This is not so complicated, the foil will adjust it's aoa, so you get the cl required for equilibrium. When you start from rest, you have much higher apparent windspeed on the sail than the boatspeed, so there you may have a quite high cl on the foil. And when the boat has accelerated to say, twice the windspeed and you are reaching at say, 105° from the wind, you have the same apparent windspeed as the boatspeed. And this is quite close the conditions that e.g. Sailrocket is designed for. As the forces from the sail and the foil in the water are almost perfectly aligned here, the sum of them is zero. As water is 800 times denser than air: Sfoil x CLfoil = Ssail x CLsail/800 for this case.
    Assuming CLsail =1, I get that you would need some 11 sqm. of sail to get CLfoil=0.6
    However, on your pictures the sailarea looks like more in the 1 to 2 sqm. range. And then you will get a much lower CLfoil for the foil you have built. I think it is a good starting point. When you have made your model take-off to high-speed sailing, you can start experimenting with reducing the foil-area.
    But the man who has the practical experience of this is Malcolm Barnsley, who did lots of model testing on a RC-model before building the full-scale Sailrocket.
     

  15. Glueandcoffee
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    Glueandcoffee Junior Member

    Yeah. I suppose that's true. I knew it would sail in equilibrium but wasn't sure where that equilibrium would be. Could you explain the equation you wrote. I think I understand it but I'm not sure. Why assuming cl=1. How did you get to 11m^2. Yeah sail area is around. 0.9 m^2.
     
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