Rudder design. How?

[Little Rascal]

Was it Sven Yrvind?

Thats the chap, thanks!

Just another thought (on topic )

There are different versions of NACA 0008. The 0008 designation just refers to an symmetrical 8% thick foil.

The thing you need to decide is where to put the maximum thickness. As said before, if it's further forward you'll have a higher stall angle because of the fatter radius on the leading edge.

Putting the maximum thickness further aft, like 40-50% of the chord, is an attempt to reduce drag by preserving laminar flow. (After the widest point the flow has increased skin friction: drag)

The penalty here is that the smaller radius, sharper leading edge tends to cause early seperation at high angles of attack (ie stalls quicker.)

Laminar flow foils have to be made extremely accurately and smoothly to be any good, so I would say you are unlikely to reap the low drag benefits with a homebuilt foil. So I personally would go with a fatter leading edge version.

Youre only taking tiny differences between them though - possibly not even noticeable. As you know, a flat plate still works fine...

Also - does your blade have any taper? ie. are the leading and trailing edges parallel?

Secondly, does it have any sweep? ie is the leading edge parallel to the pivot axis?

It makes a bit of difference to the pivot point position.

 

[caiman]

Little Rascal-my understanding is that the maximum thickness is always at 30% of the chord,irrespective of the foil section.As you say,the 0008 stands for 00=symetrical,08=maximum thickness is 8% of the chord,but located at the position equaling 30% of the chord.The amount of lead area of the rudder is determined by building the rudder so that it's axis is at the position equalling 15% of the chord.The leading edge is zero%.ie,the measurements are taken from here.

Please correct me if I am wrong,I'm still learning all this.
Cheers

 

[Joker]

What is the advantage [if any] in 'sweep' in a rudder?

 

[nathanlee]

Also - does your blade have any taper? ie. are the leading and trailing edges parallel?

Secondly, does it have any sweep? ie is the leading edge parallel to the pivot axis?

It makes a bit of difference to the pivot point position.

My new design is parallel to the pivot, but the original wasn't. The whole thing swept back aft, which has raised some concerns over whether the centre of effort will be correct for the hull on my new version. The rudder shaft is vertical.

The new design is literally a rectangle of 53x44cm, mounted to the stock in a way which creates the required 15% balance. The rudder is, overall, a bit deeper than the original, but does extend aft as much.

Actually, that's probably not quite true. Since the two rudders have essentially the same surface area, and the old one had 25% forward of the pivot, the new one does have more area aft, although the maximum distance aft is less since it's a rectangle and not an aft sweep trapezoid. Does that make sense?

 

[Little Rascal]

Caiman.

That's not wrong: if the designation is just 0008 then it's safe to assume max thickness is 8% at 30% of the chord.

In a 4-digit series each number refers to:
First: camber - 0 (there isn't any - it's symmetrical)
Second: position of max camber as % of chord - 0 (as above)
Third and fourth: Max thickness as a % of chord - 08 ie 8%

The NACA 4-digit series is a pre-war airfoil family. The 5-digit and 6-digit series are a more sophisticated set developed during the war to preserve laminar flow and reduce profile drag.

However some 4-digit series symmetrical profiles have been modified to also give better laminar flow and reduced drag. These have an extra 2 numbers added eg NACA 0008-34.

The fifth number refers to the sharpness of the leading edge. 6 being unmodified and lower numbers being sharper.

The sixth number refers to the position of maximum flow velocity - effectively the max thickness as a % of chord.

So NACA 0008-34 is the standard 8% symmetrical foil but with a sharper leading edge and max thickness at 40% of the chord.

You can compare the two here
NACA 0008: www.worldofkrauss.com/foils/1502
NACCA 0008-34: www.worldofkrauss.com/foils/1726

A more extreme example here of two NACA 0010s. Both have unmodified LE radius but one has max thickness at 40%
NACA 0010-64: www.worldofkrauss.com/foils/1722
and the other at 60%
NACA 0010-66: www.worldofkrauss.com/foils/1720

In practical application the differences are quite subtle and depend on accurate reproduction of the airfoil coordinates in construction.

Leading edge radius refers to the 'sharpness' of the forward edge and isn't anything to with the pivot axis position or amount of area forward of it.

 

[Little Rascal]

Yup.

I was asking because if you had a tapered blade you would need to work out the mean aerodynamic chord - a rectangle makes it much simpler.

 

[Little Rascal]

What is the advantage [if any] in 'sweep' in a rudder?

Rake in the pivot axis changes the 'feel' - aft rake is heavier.

Sweep of the blade shape (ie trapezioid) has an effect on the lift distribution and stall characteristics. It's also one way of moving area about for optimum CLR (centre of lateral resistance) for helm balance.

Taper can increase efficiency by giving better lift distribution and reduces the induced drag off the bottom of a wide tipped foil. A low aspect ratio constant chord foil like a Corribbee rudder will have a lot of drag off the tip. In skegs and keels there's also a structural advantage with the bigger root chord.

 

[caiman]

Little Rascal-Thank you for an excellent reply.
Cheers