Reflex, camber, concave and aft loading foil section questions

A lot of talk lately about reflex curve, which I think is the word for the hollow in the trailing end of the bottom surface of the foil. As seen in newer Armstrong foils, but more subtle in many others. Questions: what does it do? My theory is it increases lift more with a given change in AOA, so less drag as you slow down(?). Also, K2 tails have a fair bit of it, which is interesting, not sure what that would do since the AOA changes in the opposite direction. Anyone with AFS notice the same in their tails?

I’ve heard that referred too as “camber”.

I guess they are related, but different. Reflex is focused on the aft third or so I think. I recall Erik saying something about (as I remember)”high camber high aspect reflex foils”, in a recent episode, suggesting they are separate.

All asymmetric wing sections have camber, camber is the bend in the mean line of the section and can be plotted as the half thickness along the chord. A mast section has a straight mean line, a wing section has a cambered mean line.
Reflex is when the mean line of the section has a bit of an “s” curve shape to it. This is generally used to reduce pitching moment and shows up as a concave section on the rear-upper surface like this one, usually for flying wings where the section needs pitch stability:


Concave on the rear bottom is a result of aft-loading of the foil by moving camber aft. When the mean line is cambered toward the aft section of the chord, you see this as concave on the bottom:

In general, these sections can have a broader range, with good bottom end and top end. The downside is increased pitching moment since the center of pressure moves aft at lower angles of attack.

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Interesting. Would a positive shim on the tail kind of emulate Reflex? Essentially you extend that curve to the tail. Or do they have different implications? What would be the implications of reflex on the front wing vs tail?

To the OP - the AFS Silk tail that I have are pretty flat. They don’t have any of the pronounced “reflex” you refer to at the trailing edge.

@J_L can it be possible that it has both concave on the rear bottom and rear top? I haven’t seen the new HA in person, but from this video, a “concave” is described on both the bottom, and top.

Some lightsport planes have negative flap settings during cruise flight for less lift (drag) at higher speeds so they can gain a few knots. Having concave on the bottom of the wing helps with low speed lift so combining the two really expands the range of speed. On top of that, I’ve noticed the last 1/4 of chord length on the Katana has the slightest bit of concave to it. I imagine it’s got to help reduce turbulence between the converging water flow and reduces drag as a result.

Reflex on the main wing would reduce the amount of downforce required from the stabilizer to some extent. Chord length has a major impact on the moment though, so it’s not as big a deal on an HA wing with a short chord length.

For stabilizers, remember the high pressure side is on top, so a concave top surface means an aft loaded section, not a reflex section. On paper, a concave top surface stab is likely to have a little bit better L/D ratio, but it’s going to be harder to fabricate and finish than a flat/convex surface

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That’s interesting re afs tails. Word on the street is takuma is coming out with new tails closer to the 178 etc. I wonder if they saw foils with that section after finalizing the front wing and just made the tails like that! They work fine but I’m guessing not as good as could be. It’s really a shame there isn’t the same brain trust dailing in takuma as there used to be. Only a few of us have tapped our fuses for other tails.

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Yes, it’s possible - I’ve seen the new HA on the beach, but haven’t taken a close look at the top surface. The bottom surface is very lightly concave

My understanding is that Americas cup foils run with negative flap at high speed as well

Yes, that’s right about the rear portion, it’s about tapering the pressure gradient as it exits the foil

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So would this be reflex curve?

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Hard to tell, but check my k2 40. This is the high pressure side and the curve in question is the gap near the trailing edge.

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No, that’s an aft loaded section, looks like a KT prototype

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Looking closer at the internet it looks like reflex is when the trailing edge goes upward, not downward like my tail(pretending it’s not a tail). So reflex is for reducing pitching moment I guess. What are aft loading sections for do you know?

I’m currently driving cross country and have been noticing the foil sections on the huge blades of the wind farm turbine generators. They have a pronounced reflex curve, a lot like the newer hydrofoil designs.

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I wish the title of this thread could be edited.

What we are talking about is NOT reflex. I don’t think any foils people are riding have reflex, (with the possible exception of the Triton monofoil though I haven’t seen it).

The features being referenced in this threas are known as AFT CAMBER or CONCAVE on the bottom aft surface

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The Armstrong foils have bottom concave and trailing edge reflex which friends find allow them to wing much smaller foils and stabs.

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This is a rough approximation, but basically, if you start with a nice simple symmetrical foil, a NACA 0012 for example, and say I want to improve lift to drag at my optimum operating point. So you add camber. But now your stall characteristics start to get a bit aggressive. So you pull the point of maximum camber aft. Then you realize your top end has taken a hit because your zero lift drag has gone up considerably due to the camber. So you pull your center of thickness aft also. So now your stall angle and thus max lift coefficient go up. By this point it is looking like that aft loaded foil section. But you dont need allll of that bottom end so you thin out the section a bit. And now your top end is better. But your stall characteristics just got really aggresive. So you go around and around tweaking these and many other parameters until your test riders say you nailed it.

In actual fact, with xfoil making inverse foil section design very accesible, most foil designers decide what kind of pressure profile they want on the top and bottom of the foil and out pops the foil section to deliver that target, for a given set of conditions. It often ends up looking like the aft loaded section all the same because the goals and trade offs described above remain the same. With an additional one being to avoid major low pressure peaks on the upper side of the foil as this will make it have bad ventilation characteristics.

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Thanks for sharing, can’t wait to get a look at one in person. True reflex entails a very aggressive pressure recovery that can cause quite some stall issues, especially for low Reynolds number applications unless the boundary layer is tripped to turbulent at just the right point. Seems like a terrible match to our application, very curious to see how these issues were mitigated.