The critical trade-space: Stability vs. Drag
Everyone with even a basic level of experience in foiling would agree with a simple statement that larger tails are more stable and “slower” while smaller tails are more twitchy and maneuverable with better glide.
This essay will attempt to explain the physics behind why this is the case. And additionally provide the next level of detail to fill in some blanks left by the overly simplified statement above.
What is stability?
In air, an arrow flies straight with excellent stability. This is because it has a heavy arrow-head in the front, and tail feathers all the way in the back. What really matters are the locations of the “Aerodynamic Center” versus the “Center of mass”. Stable systems have the center of mass in front of the aerodynamic center. The further ahead the center of mass is, the more stable the system. An arrow pushes this distance to the maximum.
Stability can also be thought of as the opposite of maneuverable. Cessna trainer airplanes are designed to be very stable and forgiving for beginner pilots. Fighter jets are often fundamentally unstable and require complex control systems to even allow a human to control them. Stability is traded directly against Maneuverability.
What is the role of the front wing and the rear tail?
The front wing entire purpose is to hold us up in the water - it provides positive or upwards lift. The tail wing’s entire purpose is to provide stability. It does so by producing reverse lift or down-force. It could be confusing to understand how a down-force could create stability, but it does tie back to the “arrow” explanation above.
This diagram of an airplane could help explain how the down-force of the tail wing reacts against the rider’s center of mass which is in front of the front wing lifting center.
Just as in the arrow example, the most stable system will have the rider furthest forward in front of the front wing. A minimally stable system will have the center of mass of the rider just ahead of the front wing lifting center. If the center of mass is behind the lifting center, the system will be unstable and require active input to keep it flying.
The airplane or hydrofoil is a lot like a teeter-totter rotating about the center of lift. The tail down-force on its long lever arm has to balance out the entire mass which is on a fairly short lever arm. If we increase the down-force, our mass can sit further out on the teeter-totter.
Hopefully, it makes sense that a larger downforce will support a center of mass which is further forward.
It’s not only about size
If we want to increase the stability (increase distance from lifting center to center of mass), we have a few ways to achieve that with the tail. We already discussed increasing the size (area) of the tail wing which would make it produce more down-force. We could increase the angle of attack of the tail wing (shimming for more angle), which increases its down-force. A different foil section could potentially achieve the same result. Or, we could instead increase the distance to the tail wing (longer fuselage), which gives it a larger lever arm even without increasing the down-force.
Why does a more stable system introduce more drag?
Both the front wing and tail wing incur two types of drag: parasitic drag due to form and surface area, and lift induced drag.
The parasitic drag generally goes along with the total wetted surface area. So a larger tail wing, or a longer fuselage has more surface area creating drag.
But less understood is the damage done by lift induced drag when increasing stability. Lift induced drag is a ratio of lift depending on how efficient the wing design is. So for a given lift produced by a wing, it will be associated with a given drag.
But the drag coming from increased stability is not only coming from the larger tail with its increased down-force, but it is also coming from the extra drag on the front wing due to its need to counteract the additional down-force with associated extra lift. So increasing the tail’s down-force not only increases the tail drag but also the front wing drag.
What about the effect of speed?
If you want your front foil to provide a given force to counteract your weight, it will require a certain angle of attack depending on the speed. At slower speeds, more angle of attack is needed (nose up attitude) and at faster speeds a lower angle (nose flatter or down). See image below.
Your body naturally adjusts the angle of attack constantly to hold mast height above the water by shifting your weight slightly fore/aft. These angles can be very slight, as little as a few degrees across the entire speed range.
Unfortunately, on our simple fixed foil systems, the tail comes along for the ride as the angle of attack of the front foil is adjusted to match its speed. As speed increases, and the angle of the front wing comes down - the angle of the tail wing goes up! Which means it makes more and more down-force as speed increases. The best tail wing will be well matched to the angle of attack range of the front wing, and set up to be stable across the entire speed range. But in practice is is nearly impossible to completely match the characteristics across the speed and angle of attack range.
Why is the low speed capability worse with a small tail?
The most common knowledge says that bigger tail wings enable slower stall speeds for a given front wing. But with enough tail angle (shimming) a small tail wing can do the same thing. Its all about getting enough down-force to be stable at those slow speeds.
Most modern foils are able to produce a lot of slow speed lift with fairly high angles of attack - several degrees nose-up attitude. At that high angle, the tail has a very low angle of attack producing very little down-force, or sometimes even positive lift. This creates an minimally stable or unstable system that can be difficult or nearly impossible to ride. So for a lot of foils, the slow end of the spectrum is limited by the tail, not the front wing.
Shimming the tail angle up or going with a bigger tail can really help keep the front foil working at these very slow speeds, except the penalty here is a lot of drag and excessive stability (locked in feel) at higher speeds.
What is the perfect setup for me?
Every rider needs to figure out for themselves what they want to optimize for. There are no single magic answers, but a thorough understanding of how to adjust and change the variables can help a rider home in on the best setup.
Some riders specifically want less stability which could also be thought of as maneuverability. Other riders specifically want a lot of stability for learning or for ease of riding.
Some riders would rather have less drag, while other riders finding extra drag a benefit to staying in the pocket of waves with plenty of power to overcome the drag.
Some riders are going to prioritize minimal drag at the highest speeds for racing, and others are going to prioritize low speed lift or stability to getting out of the water. Most are going to want a balance between these two.
Tail style, tail size, tail shimming, and fuselage length are extremely powerful variables that allow a given rider to maximize the things most important to them.
