So I have the unifoil 3 pack. A symetrical foil(back foot), flat(balanced), and a inside(front foot)
These are just different camber numbers, more camber creating more downforce. I understand that much.
My question is, lets say you shim these stabs for equivalent takeoff pressure (shim the symetrical for a little more downforce, flat neutral, and the iniside a little less) what happens over the speed range? Does one ramp up front foot pressure at higher speed more than another?
Seccondary question - negating the front foot back foot question(that can be solved independently with shim) is one of these objectively better in a specific scenario. Maybe the symetrical is better for high speed and low turbulence and the inside is better overcoming ventilation, turbulence, and stall?
Each % of camber is roughly equivalent to 1 degree of AOA, so if you have 3% camber, you would need to shim the symmetric section 3 degrees to get the same downforce. All sections have an AOA where they are optimized to operate, the game is to keep the foil operating inside the optimal zone (the drag bucket) as much as possible if you are trying to minimize drag.
In your scenario, you would be shimming to achieve the same CL, so you would likely see more drag with the symmetrical section (since you are probably outside the drag bucket much of the time) unless the stab is larger than optimal for your setup.
Below are the lift/drag curves of the same foil section, one with camber (green) and one without camber (blue). The cambered one is optimized to operate at a CL beween -0.2 to -0.8. The symmetrical section is optimal around +0.3 to -0.3. The modelling I’ve done says that the stabilizer operates in the -0.4 to -0.6 range most of the time with modern small HA stabs.
I don’t see any good argument for a symmetrical section. If anything, a higher cambered stab will have less drag at high speeds since stab CL increases as you go faster.
Gotcha, so from a drag perspective the camber is better than symmetrical. Question now is how much. I’ll get the calipers out and figure out - for a section of those dimensions- how much camber results in a flat sided foil so I can try to get an idea of what the camber number is for the middle tail.
One thing you said has me thinking about front foot pressure increasing at speed…. If the cambered wing increases the CL as speed increases wouldn’t that result in front foot pressure ramping up more as speed increases. So maybe the trimmed symmetrical foil is slower but better at keeping foot pressure consistent at speed.
If it’s truly flat sided (like a thruster side fin) camber is half the thickness, so a 10% section would have 5% camber. I would be surprised if it’s a true flat sided foil, most likely there’s a bit of shape on the inside as well. Measuring very small foils like that is pretty tricky
the CL vs AOA curve is basically linear until you get close to stall. The main difference will be that the cambered section has a better L/D ratio in it’s optimal range, and will also make a bit more ultimate lift before stall.
This is a graph of CL vs Alpha (angle of attack) of the same two sections. You can see that the slope is basically the same with about a 3 degree offset, so you need 6 degrees alpha on the symmetrical section to get the same lift as 3 degrees alpha on the cambered section. Just adding drag will reduce front foot pressure on it’s own, so I suppose if your drag goes up faster than downforce it could help balance things out. Flex is something to consider, I felt up a friends uni stabs a while back and my impression is that they are pretty thin and flexible. A heavier rider can develop 100lbs of downforce at speed, I would imagine those would flex a lot but not sure what that does to performance, could be good or bad depending.
