Airfoil and dihedral on a J3 Cub

[GOK]

Hey guys,

I opened a similar topic in the scale electric forum but didn't get much feedback so thought this might be a more appropriate place to post this.

I am about to make my transition into the larger planes (will electrify them). I am undecided between a 1/6 and a 1/5 scale J3 Cub. I need to purchase locally due to high tax and shipping charges etc. What I could locally find was a Graupner (Blackhorse) 1/6 scale and a World Models 1/6 and 1/5 scale Cubs. I am a bit confused though because these models have different airfoils and dihedrals.

The 1/6 Graupner j3 has a semi symmetrical airfoil and some amount of dihedral (don't know exactly how much but minimal). The WM 1/6 has semi symmetrical airfoil no dihedral and finally the WM 1/5 one has a flat bottom wing with no dihedral.

So here are my question

- Does the full scale j3 Cub have dihedral?
- What difference would flat bottom/semi symmetrical and dihedral/no dihedral make in a 1/6 - 1/5 scale J3's flight characteristics?

I've been reading about some nasty tip stall habits for 1/6 WM Cub for example (I think Clipped Wing though). Can this be due to semi symmetrical and no dihedral?

I'm just after a sedate flyer with simple loops, rolls and stall turns here and there, but am quite confused with the different versions of this model.

Thanks in advance for all the feedback.

Cheers,

GOK

[bwalt822]

"tip stalls" cant really happen for a straight wing, the inboard will always stall first, it wouldnt really matter what type of airfoil is used.

A cub will probably be nice and gentle no matter how you slice it and because its a high wing it has sort of an effective dihedral. If you are really concerned about stall behavior just put a little bit of washout on the wings.

[kcaldwel]

The dihedral angle on the full scale J3 is very small, and set in the rigging instructions. It has basically a flat wing:

http://www.toni-clark.com/english/modelle/piper/piper3v.htm

Rigging instructions:

http://amisaviateurs.free.fr/COMMON/DOCS/PC_ServiceBulletinsPiper/BS1.pdf

The full size uses the USA 35-B airfoil. I'm not sure what your descriptions of semi-symmetrical mean to you, but here is a plot of the full size airfoil:

Dihedral would make the airplane more roll stable, and also allow it to turn without ailerons.

Kevin

Edit: The difference between a "flat bottom" and a "semi-symmetrical" airfoil may be complicated, but generally, the SS will have less camber. This will mean the airplane will fly a bit faster, and it will likely have less pitch change with power changes - this can be somewhat trimmed out with the thrust line, etc. The real thing climbs at high power, and descends at low power, something most people don't want in a model. Power is used for altitude control, pitch for speed control.

The full scale J3 has about 0.64 degrees of dihedral each side (2 3/8" over a 17.625' semi-span).

[Texas Buzzard]

The full-scale has a USA-35B airfoil. For all practical purposes a model J-3 with a wing in the 4 to 6 ft wing span range will fly very well with most any airfoil that is close to a Clark Y (flatbottomed).
Look closely at your airfoil and see if it is actually semetrical OR is the leading edge just a "Phillips" Entry.( lower surface rounded up but flat on bottom).
If I were building the J-3 at about a 4 to 6 foot w.s. I would put in about 2 degrees dihedral in each wing. That would make the slow speed flying a lot more relaxed. 2 dgrees per wing will appear to be a "flat" wing in the air. Be safe. Dihedral is your friend.

[DT56]

You might want to look at this article on the Cub wingtip design and stall behavior. (Scroll down to the construction specs section)

The NorthStar improved Cub has a different tip design that provides better stall behavior, without the drag producing washout required in the standard Cub wing.

http://www.customflightltd.com/introltr.html

If your ARF Cub is built without washout, you can add some by rigging a slight amount of reflex (up) into the ailerons. (Ailerons only, not the flaps, if a Super Cub)

[GOK]

Thank you very much for the feedback and the pointers everyone.

Cheers,

GOK