Building a super low aspect ratio high chord unifoil.

[fluidity]

I was inspired by the thread by Horst Sergio who was promoting unifoiling: viewtopic.php?f=201&t=2408431.

But then I went my own way and came up with some ideas that didn't quite mesh with the "generally accepted" idea that high aspect foils are amazing.

Gliders use high aspect foils. Fighter planes use low aspect foils. Aeroplane fuselages often are designed as low aspect foils themselves. Barges are wide. Speed boats are narrow. On investigation I found that low aspect wings are renowned for low speed high lift capabilities and this ties in nicely with our experiences with kites, a low aspect ratio kite develops more low speed power than a high aspect kite of the same area. This is because the longer chord produces a higher affected volume if you use a 1:1 scaled wing profile shape. That affected volume has a mass relationship to the affected air or water. Theoretically we can drop the relative thickness to chord ratio lower than in a high aspect wing and still get the same lift or get greater lift at lower speeds with the same area. To me, these are sounding like good outcomes for wing powered hydrofoiling

A lesser aspect ratio results in a more nimble foil that rolls more easily, making turns theoretically easier. A long enough monofoil allows direct design-in of a mast socket and increases stability so that the interruption to water flow of a stabiliser is no longer needed to provide comfort from pitching stability.

With the above design considerations I was keen to design a foil meeting these requirements and try it out. So a few nights ago I designed the foil, last night and this morning I modified it for 4 part 3D printing to fit inside my printer's build voulme. This morning I started it printing but forgot a brim around the edges, this afternoon after work and before my wing session on my massive second DIY wing,I found the build had reached about 14mm high and corners were lifting. So I resliced it, now with brim and a heat shield skirt. I've increased the base plate heat from 45 degrees to 70 and it's building anew.

Infill is cubic to create closed pockets. Estimated build time from prusaslicer is about 4 and a quarter days. Estimated unclad weight is approx 1.5kg for a foil of 3.36 litres volume. Hopefully my new slicing settings will work and I'll get 4 well constructed pieces... otherwise I'll glue up some plywood and CNC route out some foam, ply or Paulownia
These wing pieces are quarters like directions of the compass looking down, NW, NE, SE, SW. Hard to stop warping so it will be interesting to see how they look by tomorrow morning.

Today, out on the waves advancing my wingsurf foil learning every time my foil wore a chunk of seaweed i was thinking about the very different shape of this foil I'm building now. And no stabiliser to go fancy dress party mode either Just the mast...

[fluidity]

Making it real!
Showing 27mm high, wing profile is 40mm at it's thickest. The profile is maintained right through to the wingtips but scaled to match the reducing chord towards the wingtips.

You can see the heat shield in use to protect the print from thermal shock and to maintain warmth from the build platform at it's more elevated than usual temperature.

The cubic infil is also visible, it's the only infil I know of that has the capability to form useful sealed pockets, all the other sealed infill types extend to the full height of the model but with cubic infil and a small layer height, the pockets are small enough that a water leak won't spread very far. There's a brim enabled too, just like a brim of a hat, the brim prints down on the heated build plate and gives extra adhesion to the corners that are first to cool. I'm using Marvell 3D black PLA+ which has better sandability and impact resilience than regular PLA.

In these pictures you can also see the foundation building around where the holes for the bamboo skewerlette sections will later go to join the front and rear sections of the wing together. I use a slow setting superglue after redrilling the printed holes to ensure they aren't blocked.

Even though there is a lot of open space within the pockets in the model, the pockets are 3D, not 2D and the walls of the pockets which I print at 0.55mm thickness + the infil perimeters next to the outside two perimeters all contribute to weight. PLA is about 1.36 x more dense than water, I'm expecting this print to use one and a half KG or one and a half reels of PLA. At some time I'll pause it while it's working on infil or the skirt, remove the remnant black filament and swap it out with another reel. Still undecided on whether I'll put one of my other colours on... I have a fair number of reels of different colours. I'll be using a lot of reinforcement near the centre of the wing but very little further out, so I'm still undecided, If I switch to white later then I can use an under layer of a print polyester. I have some steam punk and other patterns in my stock.

[Trent hink]

This is super interesting. Please keep us updated on your progress and results!

[fluidity]

Failed to reload the black filament before bed several nights ago and had the dissappointment of waking up, hearing the printer and then suddenly realising, I'd meant to swap it out the night before but forgot.
So I got about 50% printed. Then lowered the printer to chech the actual height I'd printed to, trimmed that amount off and re-sliced the upper part. However attempting to print for several hours with static PLA+ in the heating chamber has partially obstructed the nozzle and reduced the flow, I'll need to remove and clean the nozzel before I can get any more good prints. Meanwhile I decided to concentrate on a thinner plywood CNC cut one which took me 3 nights on the CNC to complete and left me time to glass the under surface as well.

This one fits in a box shortened from 50mm thick to 38mm thick. However the chord is less due to the gull wing profile. I think it's a good test of a high chord wing(600mm span, 450mm chord) as the thickness is only about 23mm at it's maximum. Thickness on a scaled in-use hydrofoil profile would likely work out a lot thicker but I'm interested with this one in the more ignored component of lift which thickness doesn't contribute so much towards. I think people have so fixated on the Wright brother's rediscovery of air profiles from bird's wings that coanda effect and angles of attacks have received less understanding by most of us.

I'm still trying to understand the theory of foils and lift but what I am clear on is that rounded(Coanda effect friendly) leading edges are essential to higher angles of attacks without stalling and that a longer top surface for lift is only a small part of the story, unless we have zero degrees angle of attack then there are much more significant mass-shift effects in play as a wing slices though water that takes time to move away. Water would far rather just sit there than form turbulence without external energy input, that being our weight, transferred to the foil.

This will be my Xmas hydrofoil present to myself

[fluidity]

Did 3 coatings each side,
Centre and mast socket reinforcement, vac bag, sand,
Full wing twill glass, vac bag.
Recoat while still slightly sticky.
Fast cure in bag in sun,
Wet sand both sides, 180 grit.
Extract 40mm 3-hole aliexpress/gong type mast foil stub.
Trim mast sleeve with dremel type tool sitting on spacer plywood foot.
Sand top of mast sleeve tidy.
Hose off and take to inlet for a quick hand held test
https://youtu.be/G_ReKxUM_dE

Stability? It's still quite pitchy compared to a wing/stabiliser combo. At speed it's going to be massively more pitch stable than a high aspect unifoil wing though.
Roll? easy roll control due to narrow wingspan. Not sure about lift. I gave it very little thickness so lift will be more about attack angle than profile. Waiting for wind to test now though, I made the switch to my long mast with USA box but i will need high wind to test it on a sinker kiteboard I made several years back. Will be challenging at my newbie level but I'm optimistic

[jaros]

Great project Fluidity! I am looking forward to hear how this rides. Hope it rides as nice as it looks!

[fluidity]

I decided my foil was ready enough to test(still not happy with the finish but the paint takes about 5 days before it's ready for polishing... whoops!)
Because I want to move ultimately to a USA box foil mount I have a second(3-holes) mast with the flat base so I mounted it to an old kiteboard and had a go...
It's a sinker, probably only about 10 liters and a couple KG weight, one floppy handle strap, huge concave. Easy to mount the mast base and I put surf wax on the deck.
However I learned some things:
1. One loose strap does not allow me to twist the board under my body.
2. The slightly positive bouyancy of my foil is a real nusciance.
3. I need substantially more bouyancy in the board, reason being, that it's the bouyancy of the board pushing it flat against my body that gives me control of mast and foil deep under water when I'm trying to balance it- and this board didn't have enough! Aso the convex deck worked against me controling the board roll under water

So my next try for a low volume board will likely still be less than my weight but it will be at least 50 litres and NOT an old kite board

[fluidity]

First opportunity to use new foil today. At 1800 instead of 2600 sq cm, and in only around 16 knots with some bigger gusts, my 105 kg and the Naish 5.4 which is not rigid enough for good pumping, it was a mission to get on foil. With the 2600 I have been in the position of light wind, barely white capping, a few pumps of board and wing in a gust and off. By the time I got the monofoil on the board the wind had picked up.. a bit but not much. At first I had only a few very short glides. It is nimble, playful, responsive to my feet and easy to control both roll and pitch, at 450mm long there really is no issue at all with pitch control, it's quite forgiving. Faster than my bigger foil which I expected but I think I have a lower attack angle before stall. Probably a combination of my weight on a smaller surface area and that I thinned to 72% of my design thickness to be able to CNC it from only 2 x thicknesses of 18mm plywood. Next time it's blowing over 20 knots it will be my go to foil, I really like it's responsiveness, I think it will teach me foil control skills much faster than the big foil because it reacts faster. I could feel the absense of the stabiliser in the yaw axis, it has no inclination to stay going in one direction like a stab-foil, it's all up to the rider to take control of with no gentle correction from a fuselage.
Meanwhile I'm reprinting my true-to-design version which will be 40mm at it's thickest after cladding with composites. 8.9% thickness to chord ratio. This should improve my low speed lift and also increase my attack angle before stall.

I found it doesn't like to ride as high as my high aspect ratio foil which I commonly ride up to about 100 mm under the surface before sucking air, the smaller foil needs to run a bit deeper as it can suck air from lower down. I didn't get much of a feel for pumpability, I needed a lot more speed for takeoff and as it had started raining the waves were only small, best technique was wait for a *really* good gust and pump like crazy into the trough of a small wave trying to pass. Again, I think this is a thickness issue. Because I thinned it down to CNC and because it's a smaller foil, I don't get the strong feeling of lift i get from the big foil. This basically means I'm trying to operate it with insufficient wind and it's stalling instead of lifting at low speeds.

Regarding the 450 long chord and how it feels, I have plenty of time to adjust my angles, hand wing loading etc I didn't really get a good feel for my balance points on the board but it felt more immune to my forward/back positioning of my front foot which surprised me. I'm used to being able to slow down to almost walking speed on the big foil, with this one I really needed to maintain speed to stay up. At this stage I'm calling it a success.

[joriws]

Did you see this?

https://projet--dared-com.cdn.ampprojec ... amily/amp/

[fluidity]

Did you see this?

https://projet--dared-com.cdn.ampprojec ... amily/amp/

Thanks, no, I hadn't seen that.
He has a very abrupt transition ftom the high aspect part to the low aspect tail which leads me to suspect he may not have a well fused foil profile for the middle part. I could be wrong but if I am then there will be a lot of release turbulence on the sides of the tail back of where it blends to the short chord parts of his wing. Pity he doesn't post side angles.

His notes on the perfomance characteristics match mine though. After I redo my wing to my design thickness it should have a bit more lift.