In my admittedly limited experience that never works out very well (not if you want an exact copy). Even when the guy lifting the coords has decades of experience with precision measuring it still introduces errors that you may not be able to detect but the air can. So you have to smooth the raw coords which may produce a good profile but it's no more similar to the wing you lifted the coords from than the raw numbers from your splash. The lift and moment of your copy will be pretty close to the original wing but the drag is harder to duplicate because imperceptible surface irregularities can have big effects on the boundary layer and sometimes there will be a bump in the velocity gradient that belongs there but whoever (or whatever) smoothed the raw coordinates didn't know that and cut it off.Or take profiles from a SparrowHawk.
That would be fine for NACA 4 and 5 digit profiles and those from any other source before 1935 but NACA 6 and most other profiles designed after 1950 get a bit finicky about the leading edge bump that that method could produce and the waviness that bondo will develop if you keep it for a few months. The method of transferring the profile from your splash to the material you're making the part from can also introduce waves (a penciel line as amazingly thick and of inconsistent thickness when you're trying to work to machinist tolerances). And don't forget the skin offset, that's another chance to screw up the final shape.I copy profiles by the following process:
Place a 2” wide strip of 3M Scotch packaging tape over the section to be replicated.
Cut a piece of 1/2” plywood to within 1/4” of the profile.
Mark LE and TE vertical lines on the plywood.
Use Bondo to fill the gap between the tape and the plywood.
Remove the plywood with Bondo.
Sand the Bondo flush with the sides of the plywood.
Use the plywood with Bondo as a template to transfer the profile as needed.
Indeed, but even relatively small variations in airfoil shape can change it's characteritics quite a bit, depending on what the variation is.The biggest problem is that one persons "Close Enough" is someone elses "Way Wrong!"
INDEED! but what if a first time builder built the one you copy off, which was also copied from some manualy typed in co-ordinates from a dubious source previously.But you errors in copying is probably going to be no worse than the average first time builders errors in construction.
There always comes a point of diminishing returns for perfection when people have to step back and say "Thats close enough"
The biggest problem is that one persons "Close Enough" is someone elses "Way Wrong!"
Why go to all the trouble of lifting a good airfoil from a molded wing if you're not going to even try to build it to the tolerances demanded by such airfoils? You're not going to get the performance that that molded wing can deliver so you might as well pick something out of a catalog that is less difficult to build and will probably out-perform your sloppy copy.There always comes a point of diminishing returns for perfection when people have to step back and say "Thats close enough"
Let say, an airfoil (a set) for a microlift 12 metre sailplane of 145 and 345 empty and gross weights, with a rectangulare shape and a 30 inches chord. If that make sense, what would be a suggestion?
I would copy the Greg Cole airfoil on the Windward Performance SparrowHawk.
So what airfoil(s) would you recommend?That would be fine for NACA 4 and 5 digit profiles and those from any other source before 1935 but NACA 6 and most other profiles designed after 1950 get a bit finicky about the leading edge bump that that method could produce and the waviness that bondo will develop if you keep it for a few months.
For example KIWI303 the KIWI itself ended up copying the same airfoil over and over again, WITH ERRORS, then in the end it could no longer fly!
The Supersonic Nosecone is still intact though
YEAH THE BEANS I FORGOT THEM! Sixty Bean Power to the tin....optimised for rocket propulsion, but forgot beans weren't available until after european colonisation!
Thank you for posting that, Hot. I looked for it earlier today but did not find it.Time to post this PDF by Orion again?
Selecting an airfoil or set of airfoils is up to the designer. First he has to outline the mission. Then round up candidate airfoils and their polars. To me the most important airfoil characteristics are CLmax, CL at minimum drag, and lift to drag ratio but I don't do sailplanes so you may have different priorities (for instance you may want the lift to equal the weight of your glider at your circling speed and best power factor).So what airfoil(s) would you recommend?
The ideal airfoil is one that works optimally with the wing plan and use. One negative of the original NACA airfoil studies and publications is that many designers have subsequently been sub-optimizing wing design to airfoil selection. Much attention is paid to selecting "the best airfoil" based on 2-D performance data, and then that airfoil is used to design an aero-mechanically sub-optimal 3-D wing.what is the ideal aerofoil ... laminar flow ... equal pressure at the entry as the exit ... forward camber and rear thickness ... and what realy is praticle to achieve and how critical ... or just a general foil thats good enough across the board ...