jaumet forum posts in last 60 days

mr love said..
Reflex is as Jaumet describes, a turn up on the trailing edge of the upper, low pressure side of the wing. Commonly seen on flying wings to improve pitch stability.... The concave / turn down on the underside ( high pressure) trailing edge is often described as " rear loading" and increases lift and an excellent way to lower the stall speed of a foil. But it does increase the negative pitch moment as Jaunet explains.
Reflex does decrease lift....so to have a less negative pitching moment you may be having to increase your foil area which adds drag. Bear in mind that on our foil configurations where we a standing well above the lifting wing on top of a lever an increase in drag at the bottom of that lever will make the board want to pitch down, it literally is a balancing act.
Jaumet, I would appreciate your opinion on rear loaded profiles for foiling....the pros and cons...

Hey Mr Love,


Loading the aft portion of the profile is known as aft-camber or hook.
It is a common trick in profile design that has many consequences, and as you say, with everything that is design or operation or foils,
its a balancing act.

Increasing the hook of the section increases its Maximum Cl as you are deflecting more flow directly downwards. Its not the only way to increase Max Cl, but its a good one as it does not decrease the cavitation speed like if you were messing with the LE radius. Additionally, it often increases the lift slope of the section, making it ideal for pumping.
The drawback is that the Cm of the profile gets very negative as the Centre of Pressure (CoP) moves very far aft (hence this whole thread).
Additionally, these sections may be physically brittle as the absolute thickness of the profile near the trailing edge may become quite thin.
When designing sections with a lot of hook one must be cautios not to be too eager with potential gains, as overdoing it may lead to flow separation, both in the upper and the lower surfaces.
One can design a very pretty profile with a nice hook that is super optimized for some Cl. However one must be aware that aerodynamics are very non-linear and that this hook strategy does not work in all flow regimes, and so it must be tested in the tools to see if the expected operating conditions favour a some amount of rear-camber or not.

I hope this clarifies your question,
Jaume

Hey guys - I am new around here.
I am a fellow Americas Cup and Hydrofoil designer and I thought I'd say hi with a topic we all have discussed time again.
(Unfortunately I still can't paste images here so you can find the original article in the Takeoff-Technologies Community Hub)

Decoding Reflexed Airfoils
Airfoil reflex is one of the most misunderstood topics in the hydrofoil community. The goal of this article is to clarify what reflex is and how it influences the performance and handling of a hydrofoil.
One of the simplest ways to increase a profile’s maximum lift coefficient (Cl) and tailor its performance to specific operating conditions is by adding camber. Tailoring a good camber distribution increases lift generation while reducing drag at the targeted design points. However, this inevitable also increases the section's nose-down pitching moment (its pitching moment coefficient (Cm) becomes more negative).

To fly in equilibrium, this nose-down moment must be balanced by an opposing nose-up moment which is typically generated by the rear stabilizer or, to some extent, by rider weight transfer.

What Is Pitch Stability, and Where Does Cm Fit In?
For a hydrofoil to fly comfortably, two independent conditions must be satisfied:

- Trim: The foil must be able to trim to its required operating lift coefficient, meaning the total pitching moment is zero (Cm=0) at the corresponding angle of attack.
- Static Stability: The system must be statically stable. If a wave or gust disturbs the craft, it must naturally return toward its trimmed attitude.

Longitudinal stability is governed by the position of the craft's Neutral Point (NP) relative to its Center of Gravity (CG). For positive static stability, the Neutral Point must lie behind the Center of Gravity. The distance between them, known as the static margin, determines how strongly the craft resists to pitch disturbances.

Mathematically, the static margin sets the slope of the pitching moment curve with respect to the angle of attack: dCm /da
- Positive Static Margin: Produces a negative pitching moment slope (dCm/da < 0). If a gust increases the angle of attack, the craft naturally generates a restoring nose-down moment, returning it to its trimmed condition. A larger static margin creates a stronger restoring tendency and a more "locked-in" feel.
- Small Static Margin: Produces a gentler restoring moment, resulting in more responsive, agile handling.

The Role of Airfoil Geometry and Cm
The Neutral Point is determined primarily by the geometry of the complete system: main wing sweep, stabilizer area, fuselage length, and their respective lift characteristics.
By contrast, the airfoil section's pitching moment coefficient (Cm) has little influence on the neutral point itself. Instead, it determines how much balancing force the stabilizer must generate to achieve trim. This is because the section pitching moment remains nearly constant over the normal operating range, changing Cm shifts the entire pitching moment curve up or down without significantly changing its slope.

How Reflexed Airfoils Can Help
A highly cambered airfoil produces a large nose-down pitching moment, requiring the rear stabilizer to generate a continuous balancing force. That balancing force incurs additional induced drag reducing overall efficiency.
By curving the trailing edge upward, a reflexed airfoil reduces the section's nose-down pitching moment, moving Cm closer to zero.

Because the baseline pitching moment changes while the pitching moment slope (dCm/da) remains essentially unchanged, the hydrofoil retains the same static stability but requires less trim effort:

- Lower Trim Drag: Reduced downforce requirement on the stabilizer lowers total system drag.
- Greater Control Authority: The stabilizer retains more control range because it isn't working as hard just to keep the craft in static equilibrium.

Summary
Reflex does not make a hydrofoil inherently more or less pitch stable. Instead, it improves the efficiency with which a stable hydrofoil can be trimmed.

Lets open the discussion!