Here is a cool idea. At the average pump angle, which is higher than you’d expet, put a foil shape in the cross section to optimise the flow over the fuse.
I would be interested in seeing the various cross sections across the brands at the same sliced angle. This is super interesting. I vividly rememeber the first time on a Code 850s after the AFS Silk 850v1 mono and the feeling of more turbulence. I am now used to it and don’t notice it, but I wonder if this might play into it.
Fascinating
I do think some part of how awesome the F4 foils are is related to the very small fuselage profile. The CODE is pretty chunky, which I guess is also a good thing if we care a lot about durability.
I’ve often felt that the fuselage probably doesn’t have to be as stiff as they are now, but I haven’t had the means to try making a few different smaller profiles to see if a little flex is an issue.
I agree that more care could be given to the profile of the fuselage, but it seems to me that overall profile size is the first challenge
Skinny fuse is next years skinny mast
I setup a quick app where you can compare the cross section of various foils
Eg below which is a rough stab at the Code cross section.
Crispfoils explained the angle mechanism but what I’ve ended up on is no doubt super naive approach, but you can replicate his foil shape as per the second screenshot
A very crude estimation of drag, mostly relevant at much lower speeds is my current theory
The cross-section argument is more relevant at slow-speed. It’s probably worth a lot near the stall boundary where the wing is forcing 11° of trim, and largely irrelevant by 15 mph. Above about 16 mph the residual difference between a rounded rect and an ellipse is just perimeter. (eg it’s wetted area that is the key force). (estimate numbers)
h × w 9 mph 20 mph Area Rounded rect 45.0 × 35.0 9.03 N 15.50 N 1489 Aerofoil, same size 45.0 × 35.0 6.01 N −33% 13.19 N −15% 1076 Aerofoil, matched 53.3 × 40.9 7.10 N −21% 15.81 N +2% 1417 The same-size row looks better but has half the vertical stiffness and 28% less material. Matched = stiffness match
The aerofoil cross-section is worth about a fifth of your fuse drag when you’re slow, and by 20 mph it’s gone entirely, slightly negative, because the matched aerofoil ends up 41 mm wide and carries more wetted area, which is all that matters once the angle has collapsed.
I don’t think you should be making the aerofoil matched. If you go taller and less wide, its probably fine for stiffness of the fuselage in the direction we care about (up/down loads not side to side).
If anything to get apples to apples, match the sectional inertia about the horizontal axis for each of the cross sections. That’s what translates to bending stiffness and strength
There are definitely gains to be made in the fuselage cross section but it is also always a trade off.
Axis was probably the earliest to push the width super low on their production Alu fuselages.
If you go too thin/sharp on the top and bottom edge. You could create extra vortex/pressure drag or even flow separation during turning.
The fuselage also dictates the flow around the center of the rear wing which could change the cross section.
Would be an interesting CFD study to measure flow angle in different riding styles and adjust fuselage shape and rear wing position to optimize for measured speed, pitch and yaw angles.
Then you have deflection/flex- depending on the discipline you may want a fuselage that is stiffer or softer in certain directions.
The optimal for most riding is probably somewhere between a circle, double ended airfoil, and a standard airfoil.
Interesting point on the interaction between the fuse and the stab, and the flow separation.
Initially i dismissed the idea as likely only relevant to pump foiling, but then realised that prone foiling I’m mostly operating at as slow speed as possible, and with relatively high angles, and a small stubby stab.
+1 for some CFD, would be a great engineering project for a student
Is the length not even more of an effect? I was wondering that anyway… is a smaller fuse length more efficient? I feel like using the XXS (Code Foils), I seem to gain efficiency for higher speeds, but lose a bit of the ability to step onto the tail in a turn, compared to the S. I also feel like obviously the pump cadence is shorter… which would make it (the shorter fuse) actually a bit less efficient in pumping as you have less glide? Or is my intuition wrong there?
