Topic: Wing Incidence

Hi All,
I am sketching an own design model based on a 1930's racer sort of idea. This time I am using a NACA 2415 wing section, as opposed to the Clark Y that I usually employ and I thought I would canvas your collective opinions as to the incidence angle I should set the wing at. I am guessing at a couple of degrees positive or thereabouts but if anyone can throw something a bit more scientific in I'd be very grateful!
Thanks,
Jim

The scientific approach would require you to locate the graphs for the 2415's characteristics at various Reynolds numbers (Re).

You also need a Nomograph that determines the CL required for the estimated wing loading and speed.

Knowing the CL and Re number, you then obtain from the 2415's graphs the wing section's angle of attack required.

You then have to obtain the correction factors for the AR (since the wing is not of infinite length) and also for the planform (which corrects for the wing not being elliptic).

All of these numbers and factors are then "cooked" in a formula that gives you the entire specific wing's AoA necessary for level flight at the estimated wing loading and speed.

That angle, is your incidence angle because it allows you to position the wing as required for flight, while still having the fuselage at zero angle, which offers the least drag and looks the best 

OR,

You can try a simpler approach and use 2 degrees incidence for a non aerobatic design, or 1-1.5 degrees for a model that is expected to spend lots of time flying inverted 

I'll do that then. Ta.

I would find a sim design and cheat

To my eyes, apart from 2415 being a bit thicker, the two sections are very similar. Given the inevitable effect of small size on aerofoil characteristics, I would be very surpried if what suited one didn't suit the other - so what no go with what you would use with Clark Y?
Mike

To my eyes, apart from 2415 being a bit thicker, the two sections are very similar. Given the inevitable effect of small size on aerofoil characteristics, I would be very surpried if what suited one didn't suit the other - so what no go with what you would use with Clark Y?
Mike

NACA 2415 is a symmetrical airfoil, Clark Y isn't.
It is for that reason (regardless of thickness) I would not set them at the same angle.
Of course I assume the angle for the Clark Y is usually measured by modellers relative to the bottom surface, for practical convenience though that is actually not the correct angle of attack of that airfoil again, due to it not being symmetrical.

Information from http://worldofkrauss.com/foils/show_compare?sform_contains=2415&items_per_page=50&commit=Compare&id%5B%5D=483&id%5B%5D=1450

                                      CLARK Y                              NACA 2415
Thickness (%)                  11.707                                        15.005
Camber (%)                        3.433                                          2.000
Trailing Edge Angle (%)  15.259                                        19.096
Lower Surface Flatness   71.818                                        43.595
Leading Edge Radius (%) 1.236                                           3.310
Maximum Lift (CL)              1.295                                           1.281
Maximum Lift Angle-of-Attack (deg) 8.500                         11.500
Maximum Lift-to-drag (L/D) 51.615                                      40.672
Lift at Maximum Lift-to-drag 1.180                                          0.991
Angle-of-Attack for Maximum Lift-to-drag (L/D) 7.000          6.500

Its actually semi-symmetrical. the drawing attached shows the datum. It is this line you use to set the incidence (0- the trailing edge.)

Sorry, of course the 2415 is semi symmetrical, was thinking of another airfoil...anyway the point I was getting at was that both airfoils have different zero lift angles of attack (-2 deg for the NACA 2415, and -5 deg for the Clark Y) and that has to be taken into consideration.

NACA 2415 is a symmetrical airfoil, Clark Y isn't.
It is for that reason (regardless of thickness) I would not set them at the same angle.
Of course I assume the angle for the Clark Y is usually measured by modellers relative to the bottom surface, for practical convenience though that is actually not the correct angle of attack of that airfoil again, due to it not being symmetrical.

No it's not. If you superimpose on Clark Y - apart from thickness and max camber locations being slightly different, they are kissing cousins.
Mike

Whoops - the point has already been made.
Mike

Looking at two together, I am surprised that their zero lift AoA differs as much as that. Looks as though it might be down to LE radius, which in model terms can often be at the mercy of sandpaper
Mike

A lot of people assume with Clark Y that the flat bit is the datum. It is not... again, its the centre of the L/E to the trailing edge.

I used Naca 2415 in the early 70's on the "Skydancer" that has come to life again under the banner of PB Models.

Picture of  myself with the original "Skydancer" in 1973 in my profile on this site.

Good choice, does not really need any incidence  as the camber line being semi symetrical give enough incidence for normal RC flying, but might need a bit more for SC models.
The other one that is equally as good which I used before 2415 is 2412

The graph on the link I posted earlier would suggest the CL is roughly equivalent for the two aerofoils at about Clarke Y +1.25 degrees for the Naca section.

Hello Tsr,
Nice work on specifications.  What stands out to me is that we do all that and we get 1/2 of one percent.  Seems to me that flat to curved or symmetrical shows that just getting the model built to 1 degree relative to the fuselage is work enough.  Now just think about all that can go wrong...landing gear,  both wings set identical fuselage error on one side vs. the other...etc...  Much to do about something that at the end will , or may , not even show up.
take care and do not sweat the small stuff.  But in my own opposition, the devil is in the details, so maybe it is worth to spend time grasping the concept and weighting in on the possible effect.  I mean too, that we fly these models with a lot of skill.  Some models I have flown by practice should never have flown.
Albert

I think the point is that when we are copying an original aircraft we can pretty well dispense with worrying about incidence angles as we can copy the original and its wing section and so on and build it all true. When you start to play with your own original design as well as building it square and true you then add in an unfamiliar section and a few other bits and pieces that we are not sure about, then we need to have a bit of faith in what we are doing and that it will work as we want it to, especially when it is sitting for the first time on the runway with the engine ticking over. I agree that we can fly round minor problems and sort them out afterwards, but for that maiden flight if I know that I have calculated that it will all work as I built it and I am confident that the CG is right, then it sure helps me with the butterflies.
I know having flown it that I can tweak the incidence, CG, thrustlines etc to suit afterwards that's part of the test flying, but for that initial throttle opening I need all the confidence in the creation that I can get otherwise I spend all day taxying faster and faster until eventually I get airborne by mistake, flop round a circuit or two and get it back on the ground and then think yippee it flew. Oh actually that was pretty much my normal maiden.