So time for the final elimination round!!!!!
Who lives? Who dies? Who soars above the rest and who crumples under the forces . . .
So let's have a look at our contenders!
Aquilla:
Fage & Collins 4:
Gottingen 611:
S4094:
All fine airfoils, pretty flat bottomed, with roughly different points on the cord for the max peak, some thicker, some thinner.
One final piece of mystery for all of this . . . Reynolds number. This is a dimensionless measure of how thick the air appears to a wing of it's size relative how quickly it wants to push through. Since this is an exploration through graphs (No! I'm not doing the math!), we'll use a
Re Calculator for this.
Plug in 0.15875 for the cord (6.25" in m), use the higher default temp for the Kenematic Viscosity . . .
v(mph) | Re | ~Re |
10 | 47,275 | 50k |
20 | 94,025 | 100k |
40 | 188,050 | 200k |
100 | 469,915 | 500k |
In this case, for warm(ish) air, at 10mph, Re is roughly 50k . . . so why did I hit those speeds? Each set of polar plots are made for a specific Re and Ncrit (turbulance in the air), and I have polar plots pre-generated at 50k, 100k, 200k, and 500k (and now we know roughly what airspeed that means for this wing on these plots). For Ncrit, we're going to assume the air is smooth -- pick the lowest -- because it makes the charts easier to read (it's good to go back and examine higher Ncrit to see if your selected wing may have trouble in turbulence)
So, 100mph (500k) is crazy fast (it might be able to do it) and 10mph (50k) is painfully near stall, but 20mph (100k) should be comfortably within the envelope. Since I'd be pleased with 20mph for a slow cruise, we can pick Re = 100k data sets and generate the polars . . .
For all of the polar plots below, Aquilla is teal, FG4 is blue, goe611 is red, and S4094 is tan.
First up:
Think of C
L as "how much lift", and Alpha as Angle of Attack. So all else kept equal, held at at ~20mph, if you change the pitch, this is how lift will increase.
For comparison, three things to look for:
- The farther right peak the peak is, the higher the stall angle (generally, the lower the stall speed)
- The flatter the top, the more gently it will approach stall (the more warning you get)
- the higher the peak, the more liifty the wing is.
In this case, Teal and Red have fairly flat stalls, but of the two, Teal is much further right. Tan is further right and higher, but it's peak pretty sharp. So Tan > Red > Teal in liftyness, Tan > Teal > red in stall, but Teal = Red > Tan in gentleness of stall. Blue is erratic and under-performing in this plot, so it's the big looser for this comparison.
There's more we can glean from this plot, but for comparison, we're ready to move on to . . .
Placed here for completeness -- C
d is "how much drag" vs. Alpha, which is still AoA. Most everything you see here can be found easier on other plots . . . but for this plot, a lower and wider "U" is preferable. Also notice the U can be shifted off center to the right from 0-Alpha this means you'll get more drag pointed down than up.
Not much more to see here, so we'll move on to . . .
Now we're getting somewhere! C
L/C
d is the lift:drag ratio.
two things to notice:
- the higher the peak is, the more efficiently the wing can work.
- the wider the curve, the less sensitive the wing's efficiency will be to AoA -- a wider "happy place".
looking at these plots, Red and Teal are once again neck-and-neck for the top, blue makes a good effort for third where Tan is abysmally low. Red is a touch wider, but just a touch.
So for L/D, Red = Teal > Blue > Tan.
We'll come back to this plot later with the winner foil, but for now, it's time to move on to:
This may be the oddest of plots but for flight envelope, it's one of the more telling.
Three things to look for:
- the farther left the vertical part of the "C" is, the less draggy the wing
- the wider the "C" is top-to-bottom, the wider the flight envelope the wing will have.
- the lower the bottom part of the C reaches, the better the inverted performance will be.
In this plot, Tan shines. It's just a bit more draggy than the rest, but it performs "a little draggy" over a much wider range, both normal and inverted, where as none of the other wings will do well inverted -- they'll need a LOT of back-pressure.
Blue once again under-performs (honest, I didn't try to setup this foil for failure
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
), but orange and teal have similar behavior -- orange having a bit more lift, but teal picking up a *slight* advantage in inverted flight and lower drag.
So in low dragginess Teal > Red > Blue >Tan, but in envelope, Tan > Teal = Red > Blue
And finally . . .
This is the weird one. C
M -- the pitching moment -- is a measure of how much pitch stabilization this wing will need from the tail. What makes this weird, the closer C
M near 0 means smaller the tail, and more elevator authority, but less self-correction in pitch and narrower the tolerable CG range. The more negative C
M is the stronger it will self-correct any wobbles in pitch, and the wider your happy CG range can be, but the less authority your elevator will have.
things to look for:
- Never positive. When pitch is disturbed, you'd rather it resist the pitch than add to it. Where it's positive, you can have your CG right, and the wing will act unstably at that AoA like it was tail heavy.
- Sharp, strong transitions (Blue, I'm looking at you) show AoA's that may cause the wing to quickly switch from docile to hyper sensitive -- like a flying wing crossing over it's center-line.
- the lower the curve, the more stable, but the bigger tail it will need.
So for this plot, Tan is a big looser for the instability out on the negative AoA -- pitch down too much and the wing add to the pitch over -- not friendly. Blue is eratic . . . not thrilled with that. Red, as you'd expect from a liftier wing is a lot more stable, but will need a bigger tail to balance itself out. Teal stays negative but not too negative, has some erratic to it, but not as erratic . . .
So Teal > Red > Blue = Tan
So after all that . . . IMO, Teal (Aquilla) and Red (GOE611) are neck in neck. I'm liking the Aquilla foil a touch better because of it's smaller pitching moment and slightly better inverted performance, but I think it's worth pulling each into the model and seeing how they look on the layout.
That's next . . . we'll see how each look on the plane, get a feel for where the servo will hide, and start pondering the foam-board construction.
Sounds like a great thing to pick up . . . mañana . . .