Airfoil determination

[LABstar]

I would be very pleased if someone could help me to determine a airfoil. The red line on the image shows the original root rib airfoil from Dan Palmers C-119 *Boxcar*. I assumed that this would be a NACA profile but apparently I err :confused: The black dotted line shows a NACA 4421

Has anyone a idea what for a airfoil Dan Palmer has used for his Boxcar ?

Also interesting that he used only a wing wash-out from 1° The angle of attack from the root rib is 5° and at the wing tip 4°

http://www.labsun.de/img/RootRibComparison.jpg

( Autocad2002 : http://www.labsun.de/img/RootRibComparison.dxf )

( CorelDraw11 : http://www.labsun.de/img/RootRibComparison.cdr )

If it isn't possible to determine the correct airfoil is there something to be said against the 4421 ?

Thanks in advance for any tips.

LarS 8^)

[Ollie]

"Has anyone a idea what for a airfoil Dan Palmer has used for his Boxcar ?"

For models, use thinner airfoils. I suggest NACA 4412 or NACA 4415 rather than NACA 4421. Too much thickness of airfoil for models. Too much drag and too much thrust to overcome high drag.

[LABstar]

Thanks for your quick response Olli :) That is nearly the same recommendation as from my german buddies ;) so it would be probably a good idea to use 4415(8) for the root and a 4412(5) at the wingtip. That is a fat, slow flying transporter. A fast flying characteristic is undesired ;)

[Langenase]

... Dan Palmers C-119 *Boxcar*. I assumed that this would be a NACA profile but apparently I err :confused: The black dotted line shows a NACA 4421
.....
Has anyone a idea what for a airfoil Dan Palmer has used for his Boxcar ?



Do you mean Fairchild C-119?, maybe this can help:
http://www.ae.uiuc.edu/m-selig/ads/aircraft.html

Fairchild 110 C-119, R4Q ......(Root) NACA 2418.....(Tip) NACA 4409

I think that the NACA 2418 closely ressembles your redline airfoil. Hope it helps.

[BMatthews]

Thanks for your quick response Olli :) That is nearly the same recommendation as from my german buddies ;) so it would be probably a good idea to use 4415(8) for the root and a 4412(5) at the wingtip. That is a fat, slow flying transporter. A fast flying characteristic is undesired ;)

Slow is good but you'll get that from a low wing loading rather than the airfoil.

15% is as thick as we can go with a model and not suffer from severe separation bubbles. 12 to 13% is actually better. If you're looking for a slower airfoil that will let the model look more like the staggering along look of the prototype then switch airfoils to something like the Eppler 201, 210 or Selig 4233. Any of those will slow down and carry a lot of weight nicely.

[Ollie]

Low reynolds number is the model problem (low size X low speed). The lower the reynolds number, the airfoil is better with thinner.

[LABstar]

These are really precious answers. Thanks a lot :)

And its naturally true that a low wingloading has more effect to the airspeed then the airfoil. The rough estimated liftoffweight is around 20 kg and that means a wingloading of around 155 gr./dm² which finally needs an airspeed of 20 - 22 m/s for level flight. . . . theoretically.

Are these additional infos helpful to limit the scope ?

Till now I have 6 proposals :

(obviously) Original : NACA 4418 - 4409 (I like that but sadly to thick)

+ NACA 4415 - 4412 (better)
+ NACA 2418
+ Eppler 201
+ Eppler 210
+ Selig 4233

:confused: . . . unfortunately I'm not in the condition to hit a decision. I'm absolute reliant on you :o

regards from Berlin :)

LarS 8^)

[Langenase]

Some "Profili" tests show that for a gesstimated Rn=155400 (400 mm chord, 20 Km/h min. stall speed), best results correspond to the Selig 4233....the original NACA2418 should be the worse. :confused:

[LABstar]

@Langenase : Thanks for your effort ! I see your diagramm but unfortunately it says nothing to me :( When I have a look at the airfoil which Dan Palmer had used I can hardly believe that a SELIG4233 should be the best. The selig profil looks to me like a nice airfoil for a fast big glider with a higher wingloading . . .

http://www.labsun.de/img/RootRibComparison-2.jpg

Sorry that a airfoil blockhead like me has such doubts. But I wonder why Dan Palmer has used such a thick airfoil ? He is a experienced architect, he should know what he do :confused: What are the disadvantages of such a thick airfoil like in the plans ? Is the stall speed to high, has it a worse tendency for tip stalling ? what is wrong with this thick airfoil.

From the practical point of view a thicker airfoil would give me much more static strength. I have 2 heavy tailbooms, with engines, retracts and a tail which are bending my wing . . .

LarS 8^]

[Langenase]

LABstar

Nice beast you're building!
In model gliders an 8% airfoil can be considered thick!. So don't worry about the S4233, it's a very thick and proven airfoil. As BMatthews previously said, at our low Rn, airfoils of more than 15% thickness do have severe problems in keeping the airflow attached at low speeds, and besides, are very draggy, these are the main reasons that make optimized sections such Selig, Eppler, MH, etc. give much better results in models. But if your aim is to maintain scale (or even stand-off scale), then go with an airfoil that allows you a configuration that ressembles the original. A bonus added in doing so should be a stronger structure in a very sollicited area of your Flying Boxcar. I haven't taked the effort of measure the wing area of the C-119 and from the 155g/dm2 calculate your AUW, wingspan, and wing chord at root. Could you please give us those data and (guesstimated) minimum speed?. I'll try to make some calcs with Xfoil :)

[Langenase]

Ooops! Sorry, did'nt see the 515 mm chord........

[Langenase]

Done. Never fly by numbers....
Well, at increased Reynold number (speed 5.5 m/s, 51.5 cm chord), much better results for the chubby 4418 and 2418.
Cl=Coefficient of Lift
Cd=Cefficient of drag
I think that the diagram showing the airfoils at different Alpha (Angle of Attack) is self-explanatory, S4233 is a bit critical at AoA=8º, but we soarers usually don't want to reach such values! Other airfoils stall at 13ºAoA and have a much more predictable and mild stalling behaviour, better suited for your type of plane. Now the decission is yours, hope all this messing has helped a little. I want to see your airplane!!! 20 Kg aagh!
BTW In Deutschland, is necessary for such big model airplanes to obtain special permissions?

[LABstar]

I'm very pleased with your results. Thank you very much Langenase :) In a german forum some buddies discussed this case with different profiles also and finally come the same result :) They also agree that a 4418 is at all a good choice, with no wing wash-out (I hope this is the correct term for decreasing of the angle of attack) and no changing of the airfoil to the wingtip.

Alternative they recommended a 4418 at the root with 5° angle of attack and a 4412 at the wingtip with 3.5°. That should improve the efficiency of the aileron and improve the stall behavior of the model and is close to the configuration of the full size plane :D

I think I choose the 4418 for the whole wing. This gives me a lot of static strength and will allow me to fly this model pretty scale. I haven't the intention to do loops or inverted flights ;) Maybe drop some sweets or parachutes instead of sowing fullmetal from a Gunship ;)

Depending the rules in germany. Some years ago they increased the max. liftoffweight for modelairplanes up to 25 kg. Only the airfield needs a permission. So up to 25 kg you don't need a special permission.

Here are two nice shots from the Original :

http://www.labsun.de/img/jcollars1lg.jpg

http://www.labsun.de/img/ac119_07.jpg

These are some (extrem minimized :D ) previews from my upscaling work with Corel Draw 11 :

http://www.labsun.de/img/C119-FuseSideView.jpg

http://www.labsun.de/img/C-119-Former-Plan.jpg

http://www.labsun.de/img/Tailboom-100606.jpg

and at least this one shot from my last model (still a Hangar Queen :( ). A Jerry Bates B-26 Marauder in 1:8 scale. If you like you can register by RCScalebuilder.com and find in the Forum there (Jerry Bates section) a long thread with many images (pretty more actual then the documentation at my website :o )

B-26 Marauder Thread on RCscalebuilder.com (http://www.rcscalebuilder.com/forum/forum_posts.asp?TID=1038&PN=1)

http://www.labstar.de/Graphics/B-26/Model/RCline/B26-Marauder-Final-23.jpg

best regards from Berlin,

LarS 8^)

[Langenase]

Schöönn!!!!
Congratulations Lars. I'm very impressed with your Marauder, and with your plans too! :) .Talking of the wing, just one last comment, the reason for to give washout is the same than to progressively change the section towards the wingtip: maintain enough control in roll at speeds near the stalling (ailerons), avoiding the stall of the outer panels of the wing when the inner part stalls thus keeping control in ailerons. Two ways of attain the same result. In your case I intend you'll have slotted Fowler flaps at the inner section of your wing, and that minimizes the need for washout or geometric change in section/airfoil, just drop some degrees of flaps and hop! relative AoA of inner wing increases automatically while outer panels remain away from stalling. Another trick for increase transversal stability in flight could be give both ailerons 1 or 2 clicks of up obtaining a result similar to built-in washout but with poorer overall performance of the wing. This, I suppose, doesn't matter you, so go with the 4418 and don't worry no more!. Anyway your magnificent C-119 will for sure be a very good flyer as I can be seen from your working skills... :) Nice detail in the Marauder's co-pilot. Congratulations again :)

Saludos
José Antonio