Sheet 02
A file you keep, not an account. Nothing is stored on this site.
Re
From the Wing Sizing Sheet, or
.
mm
m/s
m ASL
°C
% RH
Standard atmosphere at this elevation. Enter the actual temperature if you fly somewhere hot.
A section of your own
Selig or Lednicer, the two formats every airfoil site hands out. Nothing is uploaded; the file is read here.This reads geometry only. Coordinates say what a section is shaped like; they do not say what it does. Lift, pitching moment and stall behaviour come from a wind tunnel or a panel code at your Reynolds number, and nothing here can stand in for either — which is why the nearest matches below are offered as shapes that resemble yours, not as data about yours.
Where each section works
Every bar is the Reynolds range a section is actually happy in. The red line is yours. If a bar does not cross it, that section is the wrong tool no matter how good its polar looks.
Ranked candidates
The table behind these cards
| Rank | Section | Family | Thickness %c | Camber %c | Re from | Re to | Cl max | Cm0 | Cm source | Stall | Build | In band here | Drawn for |
|---|
The rows are the ones showing above, in the order they are ranked, so what you take away is what you were looking at. Reynolds bounds are the useful band each section holds over, not a hard limit. Cl max and Cm0 are indicative figures at model Reynolds numbers; Cm source says whether the pitching moment was measured or computed, which matters most on a tailless model where it is the whole argument.
Three things the polar will not tell you
Pitching moment sets your tail
A cambered section pushes the nose down all the time, and the tail has to carry that load
forever. The number is Cm0. At -0.05 a normal tail copes. Past
-0.20 you are committing to a big tail, a long boom, or both.
Reflexed sections sit near zero or slightly positive. That is the whole reason they exist — a tailless aircraft has nothing to trim against.
Thickness is a structural decision
A 9% section on a 200 mm chord leaves 18 mm of spar depth. At 14% you get 28 mm, and spar stiffness climbs with the cube of that depth. Thin sections are not free.
Thin trailing edges are also the first thing to warp, ding and go wavy — and a wavy aft section at low Reynolds numbers costs more than the shape ever gained.
Surface finish outranks the section
Below about 150,000 the difference between a good section and a great one is smaller than the difference between a smooth wing and a lumpy one. A well-finished Clark Y will beat a badly-built SD7037 every time.
Get the shape held accurately and the covering tight before agonising over the last few points of lift-to-drag.
Where people go wrong
The errors this method actually invites, and what it does about each. Every one of them is already a warning somewhere — in a note that appears when your own figures trip it, in the scope statement, or in the equations behind the page. Collected here so you can read them before you need them.
Choosing on maximum lift alone. Reynolds fit is 42% of the ranking score and lift is a fraction of the rest, because a section outside its useful band does not deliver the polar it was measured with. The highest Cl max in the list is no use at a Reynolds number it was never tested at.
Trusting a computed pitching moment on a tailless model. Twelve of the twenty-seven pitching moments here are computed rather than measured, and every card says which. On a flying wing Cm0 is the entire argument for whether the thing will trim — filter to the measured ones and see what is left.
Reading a maximum lift figure off a high-Reynolds table. NACA 2412 is often quoted near 1.6. Measured at an effective Reynolds number of 110,000 it is 1.03. Every figure in this dataset is at model Reynolds numbers, which is why they look low against the handbook.
Treating the top row as the answer. Build demand is 12% of the score, so a section you cannot build accurately can outrank one you can. A badly built good section is worse than a well built ordinary one, and that trade is yours rather than the ranking's.