kiwi307 said...

[The other 2 points of interest for a design on paper are, 1) the centre of gravity with pilot in place, and 2) the centre of lateral resistance. Neither of these have I been able to find anything other than a "suck it and see" method (except huge amounts of complicated maths). Bill Finch showed me his method about 29 years ago and I still use it.
Centre of gravity, balance the whole yacht (with pilot) on a brick and move the yacht back and forwards till you have the CofG noted, it's where it balances.
Centre of Lateral resistance is found by tying a rope on and try to drag the whole yacht sideways (peagravel or wet grass is easiest). With a bit of luck and a fair bit of empirical testing (otherwise known as messing about) you will find this point.
If all 3 points line up, ie Cof E directly above the C of G which co-incides with the CLR, you will have a really sweet yacht with minimal steering input to keep straight. Strangely it also seems to be the fastest ones too!

Gizmo said...

Infact you should be looking at is "Aerodynamic Centre"
www.grc.nasa.gov/WWW/K-12/airplane/ac.html

also keeping in mind the "Centre of Pressure"
www.grc.nasa.gov/WWW/K-12/airplane/cp.html

"It has been found both experimentally and theoretically that, if the aerodynamic force is applied at a location 1/4 chord back from the leading edge on most low speed airfoils, the magnitude of the aerodynamic moment remains nearly constant with angle of attack."

Gizmo said...



That's actually a method used 25+ years ago... things have moved on since then just like plastic seats and near vertical steering on land yachts, what you are measuring in that method is the "Centre of Gravity" of that object. It might have been a guide then but hardly reliable particularly in the current information technology age.
www.grc.nasa.gov/WWW/K-12/airplane/cg.html

Infact you should be looking at is "Aerodynamic Centre"
www.grc.nasa.gov/WWW/K-12/airplane/ac.html

also keeping in mind the "Centre of Pressure"
www.grc.nasa.gov/WWW/K-12/airplane/cp.html

"It has been found both experimentally and theoretically that, if the aerodynamic force is applied at a location 1/4 chord back from the leading edge on most low speed airfoils, the magnitude of the aerodynamic moment remains nearly constant with angle of attack."

The "Centre of Effort" as it has become known DOES vary up and down this has been known for years and is NOT a static point, water sailors have used this concept in strong wind sailing to reduce the healing motion of the yacht.

Gizmo said...

Landyacht, The picture shows by feathering the top of the sail the CE moves down and with a raked mast and strong down haul it will then move forward just a bit.

The lift on an airfoil is a distributed force that can be said to act at a point called the center of pressure. However, as angle of attack changes on a cambered airfoil, there is movement of the center of pressure forward and aft. This makes analysis difficult when attempting to use the concept of the center of pressure. One of the remarkable properties of a cambered airfoil is that, even though the center of pressure moves forward and aft, if the lift is imagined to act at a point called the aerodynamic center the moment of the lift force changes in proportion to the square of the airspeed.

en.wikipedia.org/wiki/Pitching_moment

As we have said before land sailing is a bit more than just bunging a sail on a 3 wheeled frame, most things in the sport have been a case of trial and error, and it has changed a lot over the years