Keel : Rudder Length

[yokebutt]

Thanks Tom, that makes sense.

Furthermore, I do have a confession to make. I realized a while back that the reason for all this acrimony is that we all have our own ideas of how to define weather helm. And of course, without having a precise definition in common of what it is, we are all just talking past each other.

But then again, I didnt see any harm in letting Dougs stomach acids eat away at his esophagus for a while.

Yoke.

Well, I'll be damned-will wonders never cease...

[Skippy]

Yoke: ... we all have our own ideas of how to define weather helm. And of course, without having a precise definition in common of what it is, we are all just talking past each other.

Absolutely, that's an excellent thing for anyone to remember. I think a lot of disputes just about everywhere boil down to that, people just talking about different things. Every participant in any conversation is responsible for trying to understand what other people mean by what they say. On a technical forum like this one, novices sometimes use terms with imprecise, colloquial meanings, or ideas that may be based on common misperceptions. I like having both experts and beginners here, and I would expect to see this issue to crop up again in the future.

[yokebutt]

Dear Doug,

I truly have no issue with most of your ideas, in fact, I think they are frequently quite brilliant. However, I do get the impression that you are not particularly inclined towards objectively evaluating matters at hand, but would rather prefer to push your own point of wiew without further consderation. I'd strongly recommend that you read a couple books by Dr. Feynman, perhaps you'll find a wider way of wiewing of the world.

Yoke.

[RHough]

Originally Posted by tspeer

You are absolutely right with respect to trim yawing moment.

The question then becomes, "What happens when the yacht is disturbed from trim?" This is where the helm-fixed vs helm-free issue comes in, and how much workload is required to maintain course.

If the rudder is sized so that the change in yawing moment per change in leeway from both the keel and the rudder is equal and opposite to the yawing moment caused by the increased pressure on the sail rig that caused the leeway angle to increase in the first place, then the yacht will track straight without any operator change to the helm angle. This will give the impression of a well-balanced boat.

If the rudder is larger than this balanced size, when a gust hits and the leeway angle increases, the yacht will want to turn down and the helm has to be moved to leeward to compensate. If the rudder is smaller than the balanced size, then the helm has to be moved to windward to produce the lift necessary to compensate for the change in yawing moment from the sail rig. In both cases, the yacht will be brought back into equilibrium by the pilot. But the workload to do so will be increased, and the yacht's handling qualities will be perceived as degraded compared to the balanced yacht.

In aircraft design I'd think of this as positive or negative dynamic stability. I'm working out keel and rudder areas for a design and have decided to consider the foils in relation to each other independent of the hull.

The problem I run into (based on self-learned glider theory) is that the tailplane (rudder) has to generate a pitch down moment to cancel a pitch up disturbance. The easy way to get this balance in an airframe is to have the CG at or forward of the 1/4 cord point of the wing. The wing has a higher AOA than the tailplane and in steady state the tailplane's lift is opposite the wing.

A sailboat going to weather needs to have the lift of both foils acting in the same direction. In a glider that means moving the CG aft of the 1/4 chord point so the tailplane load is positive. Can positive dynamic stability be achieved with symmetrical foils when both are positively loaded?

[tspeer]

Originally Posted by RHough

...
The problem I run into (based on self-learned glider theory) is that the tailplane (rudder) has to generate a pitch down moment to cancel a pitch up disturbance. The easy way to get this balance in an airframe is to have the CG at or forward of the 1/4 cord point of the wing. The wing has a higher AOA than the tailplane and in steady state the tailplane's lift is opposite the wing.

This is only partially correct. For static stability (the tendency to return to trim), the center of gravity has to be ahead of the neutral point, not the quarter chord of the wing. Only a flying wing or canard configuration would have the c.g. ahead of the quarter chord - aircraft with aft tails have the c.g. well aft of the quarter chord. The reason is you have to consider the pitching moment from the fuselage and tail/canard as well as the wing.

It's possible to have a stable aircraft with a lifting tail, too. Many freeflight models are set up this way - it's not unusual to see c.g. locations near 100% chord for gas-powered models. It requires a large tail, though, on the order of 25% of the wing area. But the tail will be more lightly loaded than the wing. They go this route to avoid excessive speed stability, because they need to climb at high speed and glide at low speed.

A sailboat going to weather needs to have the lift of both foils acting in the same direction. In a glider that means moving the CG aft of the 1/4 chord point so the tailplane load is positive. Can positive dynamic stability be achieved with symmetrical foils when both are positively loaded?

Yes, it's done every day! The vast majority of sailboats are designed this way.

[RHough]

Originally Posted by tspeer

This is only partially correct. For static stability (the tendency to return to trim), the center of gravity has to be ahead of the neutral point, not the quarter chord of the wing.

Leave it to you to point out the difference between the neutral point of the aircraft and 1/4 chord of the wing.

The FF model has to operate in two very distinct modes. High speed near zero lift and low speed minimum sink. During during the launch phase the thrust vector provides stability, in glide phase min sink requires an aft cg to load the tailplane. If the airframe fails to make the transition from high speed climb to low speed glide they tend to tuck and "re-kit" themselves.

What I overlooked in my train of thought is that a glider's speed changes with AOA where the foils of a sailboat operate at the same speed when AOA is changed.

I'll have to spend some more time thinking about how the load on the foils changes with changes in yaw moment while the speed remains constant.

Sailboats and math R hard ...

[tspeer]

Yaw stability is generally not that much of an issue with sailboats. Most of the problems we've been talking about have to do with yaw trim and its effect on performance, not yaw stability.

[RHough]

Originally Posted by tspeer

Yaw stability is generally not that much of an issue with sailboats. Most of the problems we've been talking about have to do with yaw trim and its effect on performance, not yaw stability.

OK.

Doesn't yaw stability effect performance? In that reducing the number and amplitude of yaw corrections needed would make the boat both easier to sail and faster?

Greater yaw stability should mean that the optimum yaw trim is attained a higher percentage of the time.

[tspeer]

Originally Posted by RHough

OK.

Doesn't yaw stability effect performance? In that reducing the number and amplitude of yaw corrections needed would make the boat both easier to sail and faster?

Greater yaw stability should mean that the optimum yaw trim is attained a higher percentage of the time.

Yes, I suppose so. I guess for the boats with which I'm acquainted, when you get enough rudder area for low-speed maneuvering, etc., there's enough area for stability.