Rudder profile

Guys:

In your opinion, what is the best rudder profile?

Marino

The more I practice, the luckier I get.

Long and skinny?

Travis

Don’t forget to have fun!!!

Rudders are like women.

Some people like long ones, some like short ones, others like them skinny, and still others prefer them fat.

There is no “best” rudder profile. You can beat this topic to death and come out with no better answer than you started with.
Anyone that tells you a certain shape is the “best shape” really has no idea. There are too many variables to give one answer.

For speed, High aspect ratio. The longer the better. For lift, Low aspect ratio.
Corners equal drag no matter what the rudder shape is.

Peter R.
www.climatemodels.com

Visit www.climatemodels.com

Peter:

I like your answer… however, wouldn’t a high aspect ratio rudder be better both for high speed (due to lower drag) and for lift (I was under the impression that high-aspect foils provide more lift)…

Please correct me if I’m wrong.

Marino

The more I practice, the luckier I get.

A thin foil needs to travel faster to create the same lift as a thicker foil for any given speed.

Here is a great program to play with foil shapes

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

Peter R.
www.climatemodels.com

Visit www.climatemodels.com

The best rudder shape for your boat would be the one that provides the most appropriate turning moment for the amount of force applied and drag induced. A balanced spade rudder is the most common design in this area because the forces acting upon the rudder balance themselves (nearly) thereby requiring the lightest forces possible to actuate and smoothly control the rudder. This means smaller servos in the case of an R/C model, consequently less battery drain.

A rudders steers much in the sense of a propellor, that being from deflection, not from lift.
cambered surfaces can create different amounts of drag, but essentialy only detract from performance (remember you can not generate lift without also inducing drag) One of my favorite planes in Uncle Sam’s Flying club was the T-38, a nifty little trainer we flew that was the first plane to fly by angle of incidence alone.

It was the angle of the wing in relation to the fuselage and subsequent line of thrust that kept you in the air, not the airfoil shape. Your rudder works the same as a symetrical wing on a aerobatic plane, it flies by deflection or it would not fly at all. The reverse would also be true that a plane with an asymetric chord could not fly upside down.

You want your rudder to slip through the water with the slightest possible amount of drag, and then when the rudder is called upon to exert itself on the boat, you need to continue to maintain the lowest amount of drag as it is applied. Of course, the very fact that you are applying the rudder means you are creating drag and braking the boat’s forward motion (hopefully forward, it works in reverse as well) Generating the most generous turning moment (without over-powering the boat)while maintaining the lowest amount of drag is the ultimate goal.

Balancing your boat so that minimal rudder input is required is even more important.

www.LudwigRCYachts.com

I have check out the rudders used by the full size classes and they used all three types parallel, elliptical and in between. There didn’t seem to be a boat that was ahead by design. I do use 45% of the keel area to be my rudder area. US-1 meter 35" keel 16" rudder max. Rudder length about 7". I keep getting smaller the lighter the winds. Some rudders down to 9 sq. in. Your boat size or type was never mentioned.

I do not want to go off topic, but some of the statements by Larry caught my eye.
Perhaps I am just not understanding how he has written his reply, but I am confused by several of the statements made. If it is my misinterpretation of the statements, forgive me.

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>A rudders steers much in the sense of a propellor, that being from deflection, not from lift.<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

I agree that a rudder steers the boat mainly by deflection, (the lift it creates in a turn is negligible)
but a propeller does not create thrust from deflection. It actually creates it by lift. The propeller blades are foils. Most of thrust created is on the front side of the foil. A propeller does a lot more pulling, that pushing.

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>It was the angle of the wing in relation to the fuselage and subsequent line of thrust that kept you in the air, not the airfoil shape. Your rudder works the same as a symetrical wing on a aerobatic plane, it flies by deflection or it would not fly at all. The reverse would also be true that a plane with an asymetric chord could not fly upside down.<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

The angle of a wing in relation to the fuselage has little to do with whether it creates lift or not.
More important is the angle of the wing in the relative airflow. (I think this is what Larry was meaning) While it is true that a symmetrical foil, or even a flat board can sustain a body in flight, it is the angle compared to the relative airflow that creates lift.
Lift is the loss of pressure over a surface by an accelerated medium flowing over it at a higher velocity than the same medium flowing over the opposite side.
In other words, a flat board with a positive angle of attack will create lift.
A wing still creates the same amount of lift even when it is inverted. It just creates it in a direction that is not beneficial to keeping the plane in the air.

FYI: I fly full sized planes, I have built my own full sized plane, and I design model planes and boats for a living. Some of the fastest model boats ever run were my designs. Propeller and foil design is something I spend hundreds of hours a year trying to perfect.

Peter R.
www.climatemodels.com

Visit www.climatemodels.com

Peter:

Correct me if I’m wrong. But lift is desireable from the rudder when beating, isn’t it? I’m not sure if weather helm is desireable on a RC sailboat (due to it’s scale), but on big boats, it does pose a difference.

So, I think (I must make emphasis on my complete lack of knowledge here) that the “ideal” rudder must be low drag and high aspect (balancing all factors, of course) in order to work well both when running from and going to weather. I must redo then my original question: which rudder profile in your opinion better acomplishes that?

Marino

The more I practice, the luckier I get.

Any lift you can create when beating is a good thing. The more lift you have, the further to windward you will be at the end of a leg.
Weather helm is a good thing since you can not ?feel? your boat. Weather helm will help lift the boat in to the wind as the wind speed picks up, or the direction changes.
No helm is better as there is no drag, but this has its problems as well. It makes the boat touchy and requires more rudder input to keep sailing in a straight line. Each time you move the rudder, you are slowing the boat down.
A little bit of weather helm that is held at a constant setting is much better for boat speed than moving the rudder around all the time.
The ?ideal? rudder would have the least amount of form drag possible, be just large enough to turn the boat in all conditions, and would only require minimum deflection to hold the boat on a windward course.

If you can get all those things in perfect balance, you will be a millionaire when you sell your super new rudder design to the big syndicates.

If you asked my opinion, I would go for a 12-14% foil with a 7-1 or higher ratio. 3 to 5 degrees of sweep. No corners, and pivot at about 18-20 percent of the average mean chord.

Peter R.
www.climatemodels.com

Visit www.climatemodels.com

:slight_smile:

Thats precisely the type of answer I was expecting! MANY thanks!

I’ll try to convert those general parameters into a foil I can make. We’ll let you know (If I design THE perfect rudder, will share with you some of the $$ :wink: )

PS Any updates on the quote for the shadows I sent you yesterday?

Marino

The more I practice, the luckier I get.

HI Peter,

I love your explanation on how propellors work, I have been seeing those for years in the design rooms at different companies where I was a Flight Test pilot along side “artists conceptions” of how planes were meant to look and how different features of them worked.

Don’t feel bad as the sucking propellor is a favorite everywhere… right along with the Lear Jet was originally designed to be an Israeli fighter. The pilots were always trying to find new ones to bring to the engineers because they all got such a laugh out of it.

FYI I fly real planes too, about 14,000 hours in over 60 different models including fighters, transports, warbirds, and 10 years of Part 121.

www.LudwigRCYachts.com

Part 121???

Marino

The more I practice, the luckier I get.

The FAA breaks down the FAR’s into Parts, and Part 121 covers scheduled Air Carrier aka airlines, others being 135 Air Taxi which covers
19 seat and under and unscheduled as well as freight in smaller a/c, 141 for structured flight schools 61 and 91 deal with Gen. Aviation etc etc.

www.LudwigRCYachts.com

Thanks! Never heard of that (even though I’m an aviation fan!) Learn something everyday…

:slight_smile:

Marino

The more I practice, the luckier I get.