New low drag ruder and turnfin

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Christian Lucas

Well-Known Member
Joined
Aug 23, 2012
Messages
339
Hi,

i want to start a discusion about a lower drag ruder and turnfin. On a simple waterflow test stand i messured 1/3 to1/4 of drag using a disc that rotates free in a ballbearing . The disc is from a rollingcutter knife so it is sharp . The differenc i messured first fix it so it dosn't rotate and than let it free running by the waterflow. Maybe this will lower drag on a straight away boat . For a oval racing boat we have to look about the strearing posibility .It will stear but as it has not the better lengh to wide ratio it will have less stearing power . I have not test it jet on a boat but next time when the wether will zurn to springtime i prove it . What are your thoughts ?

By the way , you can also drive the disc by e motor like a paddlewheel to get no drag and you can use such disc under a streamlined sponson as the surface that is in contact with the water . For this you have to give it a low angle of attace and a flat V position to let only one side be in contact .

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interesting.. I have thought about reduced drag rudder and TF many times.. never though of a round rotating part,.. very interesting..
 
Hi,

the easy option to get such Disc is when you go to the next pizza Italien restaurant . They have such pizzacutter in diferent disc dia .

Have fun and enjoy next pizza.

Have done a small extra test with a motor driven disc . Such ruderdisc will ad small power to your drive system but ruderdrag is going to zero ,no it ad trust .

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Hello Christian,

I am afraid your test set-up will not accurately replicate the conditions in a SAW boat. Due to the small diameter of the disc, the wetted surfaces will have different loads from the inside and outside edges. So changing angle, chord length and depth will not yield results consistent with what happens with the rudder blade is symmetrically loaded. An alternative may be a water tank with pump and a square nozzle large enough to encompass the blade shape. You would have to devise a load cell or drag measurement to predict load. my 2 cents.

Tyler
 
Hi Tyler,

yes ,that's right and for this i will test next week at the tecnical university in Munich such ruder design . They have a high speed water test tunnel with free watersurface here where students messure high speed aerodynamic flow . My friend Dr.Christian Rößler can't waiting to see results and as he is a aerodynamic Dr. of aerplanedesign he definatly say that it will worke.He is a modelairplane pilot as well and won worldchampionchip in pylonracing F5D class. Maybe the aspect ratio will not be so good for a ovalracing boat as they have to drive real turns but for a straight away boat you lower drag segnificant high. I have also a no drag system with a counterrotating propelled boat that used a tail rotor of a helicopter . The tailrotor trust the boat in any direction i want and has as it worke in air no waterdrag .
 
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A rudder or fin has a force componant in the X direction. A stationary disc will also have a force in the X direction.

A rotating disc will gain a force componant in the Y direction. Because of this, no net reduction of force will be realized if water force is used to turn the disc.

There could be a force reduction in the X direction if an electric motor is used to turn the disc.

In fact, the X direction force could reverse and be added to the force component of the prop.

The powered rotating disc could be a sole propulsion force.

That would bring up the argument of added power to the propulsion system.
 
Christian,

I believe I misunderstood what you were trying to accomplish. I thought you were trying to build a test set-up to be able to measure drag on a rudder blade attached to the disc and using the current measurement from the motor to judge the drag as the rudder blade circulated in water. A poor man's rudder dyno. After rereading it I understand you want the disc as a rudder directly.

One of the challenges with SAW riggers has been reducing the lift from the rudder so only the prop or other aero features lift. With a rounded leading edge, I can only assume this will add even more lift.

In my SAW riggers with single prop, the rudder has to be extremely rigid to counter the "prop walk" effects. At high speeds, I have stripped steel gears and bent 3mm pushrods which gives a good indication of the side forces in action.

If you have access to the high speed water tunnel a great comparison would be the traditional wedge rudder with sharp trailing edge to an aero foil shaped rudder. This always stumps the aerodynamicists why the aero foil has more drag in water compared to the wedge rudder. If I had access to such a facility I would be testing the optimal wedge angle, chord length and depth for lowest drag with a fixed amount of side load capacity to keep the boat straight.

Good luck on the experiment,

Tyler
 
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Hi,

the easy option to get such Disc is when you go to the next pizza Italien restaurant . They have such pizzacutter in diferent disc dia .

Have fun and enjoy next pizza.

Have done a small extra test with a motor driven disc . Such ruderdisc will ad small power to your drive system but ruderdrag is going to zero ,no it ad trust .
if you spin it put in water and the water moves that is not zero drag. From what i understand about physics nothing is free. Reduced maybe but not zero. Even spinning it must displace the water it is going thru. current boat setups work against the rudder drag so a minimal of a setup change may be needed or redesign. Watch what happens to a hydro when the rudder breaks at speed. Apart from the spinning factor, the diameter will need to be big enough to turn the boat which will increase the length of the disk in the water. My thinking is the only true test would require a dynamic hull / control test.

I had tried a .12 rigger with a steerable strut on a rigger but it needed more servo to keep the drive straight (increasing weight) than i was willing to go to. even with full opposite rudder and a 1" skeg installed or not the boat would spin out 3 ft from launch. Love to see that tried on a single gas rigger (LOL) Since the prop is not submerged the prop walks if there is no opposing force or corrected angle. Design a boat and prop that wont walk on the water and you will solve all the problems cause you wont need a rudder to keep it straight. I am only touching on the complexity of what you are trying to do. Even a performance rc plane require the thrust angle adjusted to fly right.

I am sure the results from your test will be interesting but you must go deep to get the results. I look forward to hearing more on this cause it makes me think

razor blade prop in a minimal hydro drag cort nozzle anyone?

Just my 2 cents
 
Hi,

many thanks of your intrest and though about a spining disc. And yes Mark the free spining disc has not zero drag but it is reduced. As i can't wait to get some results i have build a simple test system . I mount a ballbering holder at the end of a 3mm carbonrod and secured it with cyno ,so it is fixed. At the other end of the rod i mount a sping that let move the rod in any direction. This i fixed in a clamp . At 2/3 of the rod i sleve a plastic on ,tht's the point where i place a scale to messure the drag force. Now i let flow a waterstream from my bathroom faucet with a constant flow and dia of waterstream. With a easy to remove tape i can fix the disc from spinning or let it spin.

To get a comarison i have also made the same with a wedge ruder made from a kitchen knife. See pictures to compare size and aspec ratio . The ruder is from my 3S hydroplane class similar to the US P class .

The results are : Fixed disc has 2,1 gramm drag and only 0,9 gramm drag when spin free . That is 42% drag of the non spining disc.Disc dia is 7 cm made of hardend steel 0,8 mm thick. The wedge ruder has a drag of 2,4 Gramm .The wedge ruder is only 1,2mm thick at the trailing edge. Size is 6 cm high and 1cm wide at the lower end and 2 cm wide at the rudershaft.

Watersream from the faucet has a dia of 2 cm . I don't know the flowspeed , i will look how i can do it in my poormans testequpment.

For me the results make me to worke up curiosity and i will test such disc on a modelboat.

I will also think about lift as Tyler said . But i think it is possible that the disc will have no higher lift than a ruder as the flow on the leading side will lift but at the trailing side the water flow will be going up as the surface of the disc let the water stick on the disc surface and will counter the force of the leading side. There is a lot to do . I nedd allso the force of the disc it can give when it will force a boat true a turn . And what about a powered disc as Andy is right it ad trust . It's ok if the power comes from the powerbatterie in fast elecric driven boat but with ic engine?

Tomorow moore pic of the testequipment .

A link to toxic rocket artikel of shape of drag making things in dragboats, http://www.toxicrocket.com/docs/Part_3.pdf .

So Mark , a counterrotating propellersystem and you will need only a smaller ruder or no ruder if you take a tail propeller from a helicoopter.

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