zfennell wrote:ronnie. that idea is pretty clever. using additional surface area to redirect the flow is pretty common. for example. the slot between the jib and main in a sail boat slats on the leading edge of airplane wings flaps on the trailing edge of wings.
but the best example is the addition of extra blades in a propellor or turbine stage to redirect as much flow as possible into the desired direction.
in turbo-machinery, the concept is called "solidity ratio" adding blades will increase the thrust per unit area until you reach a tipping point where the extra drag and interference between blades exceeds any potential lift benefits.
i think what you are suggesting can be done, but this particular approach will add an awful lot of surface area and associated friction before you see any benefit.
you folks have done a remarkable job so far and i hesitate to criticize any attempt to try something new. But if you're selling this idea to Hawaiis, i thought he deserves another opinion before committing time and effort.
Regarding the foilsim app: Thanks I've been playing with it and having fun. i did notice the few quirks you mentioned. dont take the l/d ratio too literally at 0 deg camber for your ellipse. The drag numbers appear to come from a lookup table of experimental data that is not quite symmetrical at zero deg. test your results for pos AOA and neg camber with mirrored values of neg AOA and pos camber. they dont match. the flat plate, as you have surmised, has no theoretical thickness, so the presence of thickness slider is misleading even though it appears to be inactive.
the curved thin plate is the same way , but does allow for camber inputs. thin plates , as you have mentioned , can have very good performance.(especially with camber) compared to 'fat' foils , even rounded leading edges make pretty sharp traing edges.
if you wanted to simulate thickness in a plate, you may have better luck reducing the thickness in an ellipse to 3% . but its still not the same.
thanks again. -bill
Good point about the extra surface area.
I've been trying not to 'sell' the idea - just discuss it as something which might be tried if the normal ellipse wasn't working - before ditching the ellipse.
That's an interesting trick with Foilsim to mirror image the figures and see if the lift/drag figures are inverted. Maybe Foilsim works better with aerofoils than ellipses? When I have some spare time I will do some comparisons.
foilsim is probably the easiest way to get a handle on many of the variables you are considering. i'm sure the calculation is just as accurate for the ellipse as it is for generic foils and flat plates.
the NASA guy says that the calculation is for invisid (frictionless) flow. then they add lots of corrections for skin friction, parasitic (pressure) drag, aspect ratio and reynolds number. all of that is hard to beat w/o the benefit of 3-d boundary layer solvers. and compared to the 'noise' you've already seen from their experimental data. it will be hard to beat anywhere.
the only downside is the inability to plug in real NACA foil shapes. but you should be able to look up lift and drag coefficients for any of those in the other links i mentioned (somewhere).
my suggestion is to start off with your desired input parameters:
- PIck desired velocity and required lift, as starting point
-pick a foil shape for strut and wings. -compute required area, aspect ratio for both. -decide if you can fabricate results while meeting structural requirements. -can your kite generate the required thrust at your chosen velocity? -compare effects for off-design conditions.(i.e. change in wind speed, angle of attack, etc.) modify configuration accordingly and recompute estimates.
Great Job! Looks like a lot o fun prototyping and testing! being a twin tip foil what is the performance loss yoou feel from directional foils. can you compare them by categorie? upwind, stability, speed... etc: Pedro
ronnie wrote:It looks pretty smooth now. You have established that the concept of a twintip foil works.
What is the balance like in terms of front/rear foot pressure?
It is quite balanced, I was expecting a lots of pitch adjustments, but it rides just like a twintip.
That's good news - it would suggest that symmetry is not going to be a problem in that regard and is a big plus for the twintip foil prospects.
The crashes look quite gentle - is that always the case, or is it down to slow speed/light wind? I'm asking because if the twintip is quite balanced, it opens up the possibility of a strapless twintip foil?