Peter, what I've done for a mast and last 3 wings is to print out the profile from the site mentioned before, stick it on some thin PVC sheet and cut out a 'stencil'. Then I start sanding the corecell from the one tip, sliding the stencil along as I go. Where the stencil gets "stuck" i mark with a pencil and sand down that area. This takes some time as you can imagine, but I end up with a fairly accurate core. I want to make a copy-carver jig for my Dremel router to speed up the profiling of the core, but that's another discussion...foam-n-fibre wrote: ↑Thu May 25, 2017 2:54 amHere's a question about building these - when you refer to how much sanding you have to do on corecell, how are you getting your finished shape accurate? Are you using a hotwired bed in the right shape of the foil, or a CNC'd bed, or a full mold? Surely no one is sanding corcell and wrapping it in carbon and coming up with an accurate finished shape without something to make sure the shape is accurate in the finished product.
Is there a best method for doing this?? I should add that for the one foil I built, I had a flat bottom, vacuum bagged onto a curved sheet of plexiglass, and for my ability level it seemed to work just fine.
Thanks Kosta,kostantin wrote: ↑Thu May 25, 2017 8:43 am...
It is like I said previously, flat bottom profiles get nasty on higher speed. You will find air detachment and stall, flat bottoms create a lot of drag and have problems on higher aoa's. If I should make a building recommendation. Find someone that is able to cut styrofoam wings. Reduce the profile thickness from the thickness of your carbon.
You will get three pieces. Mold top, mold bottom and the wing. This is a good basis to start. Modern profiles are sensitive when it comes to building errors. By the way, don't vakuum bag styrofoam. It will look like toast afterwards.
I am only new also , built a foil late last year and really enjoying after the learning process. Over the last months I have been playing around on some software XFLR5 which allows you to analysis profiles in 2d and 3d. Even though Aquila9.3 will work it is not the most efficient profile. I have a number of images from the software generations which illustrate the differences. I have used Aquila9.3 , Tom Speers H105 and RG08 which is similar. The first set of images is only for a fixed polar at 2deg AOA.mopman365 wrote: ↑Tue May 23, 2017 2:14 pmHi,
Deciding on a profile for my next (3rd) main wing. Seems everywhere the EPPLER 817 profile is suggested as the best for low-speed hydrofoils. My first one was AQUILA 9.3% that has a flat bottom - so less sanding required for the Corecell core. Second one was the EPPLER 817 and it's a bit of a mission to shape (and also makes the allignment more difficult).
My question is, why everyone's using Eppler 817 when a simpler profile like AQUILA 9.3% seems to have better L/D performance? Am i missing something:
Blue line is Aquila that has better L/D from 2.5 degrees and more:
Good Question. T Speers has optimised his H105 for low speeds <40knots.
Good question about cavitation. The first graphs are only the 2d profile. All you can calculate from inputs Reynolds Number and a single or range of AOA. Graphing illustrates pressure curve over top and bottom surfaces. Profile image illustrates boundary layer (in red) and Cm (large green arrow). If you run software with a range of AOA's you can animate the profile to show the changing of the boundary layer and pressure.mopman365 wrote: ↑Tue May 30, 2017 10:03 amGreat info thanks GrantL! Will have a more detailed look through the screen-shots, but this looks exactly like the information I was after.
"Aquila9.3 has a very high pressure gradiant at the leading edge , more likely to cavitate at tip . The graph demonstrates this with the green arrows length and direction of force."
The green arrows (top graphs)? Is there anyway to see from your software at what angles/speed cavitation is likely?
I can also see from the graphs I posted earlier, that the Eppler is only more efficient at AOA > 2.5 degrees. From 0-2 the Speer/Eppler profiles are better. So your 3D model is showing the cruising angle is around 2 degrees?
Interested in the XFLR5 software. Do they have a trial period before buying?
Users browsing this forum: ger1 and 2 guests