Aerodynamics Question for Biplane Setup

[Marion]

I have always understood that on a biplane, you wanted the most forward wing to stall before the aftmost wing. The inter-wing struts on the Watage Sopwith Camel (per the instructions) make the aft wing at a higher angle of attack (incidence I mean) than the front wing, meaning the aft wing will stall first. This seems backwards to me.

I understand about downwash, but can this set up be correct??? Or am I missing something here ??

What say anyone ??.....

[Marion]

Anybody have any ideas here ???

[Sparky Paul]

The "front wing stalls first" is a holdover from free-flight and some full-scale practice.
For an r/c plane, it's unimportant.

[Marion]

Thanks for your reply.. I am getting varying opinions on this point. altho yours is the only one on this forum

[agpilot24]

I'd love to hear the other responses too...this is interesting to me. One thought I had was, if the top wing on a slow draggy biplane has a higher angle of incidence it might actually cause a pitchup tendancy when at high AoA, especially if it is mounted high above the fuse like some of the older bipes.....dunno, maybe it was just the way i built that one..

[vintage1]

As far as I can tell on the Sopwith Camel the full size was rigged pretty much both wings the same.


Mind you, it wasn't noted for stability :)

[Marion]

I have been getting several very strong recomendations that the aft wing MUST be at a lower incidence. Also getting a few suggestions that the downwash from the forward wing changes things enough that the aft wing might need to be at a slightly higher incidence.

I suspect the vertical spacing between the wings, as well as the fore-aft spacing, plays a sizeable factor when it comes to the downwash effect.

The Wattage Sopwith Camel assembly instructions result in a higher incidence on the aft wing. There are some reports that the model is not a very stable flyer. I know I got mine into unstable flight on the first try. The previous owner also described filght characteristics that could be unstable flight (He crashes a lot, so it is hard to be sure what really happened)

On my second flight the c.g. had been moved ever so slightly more forward, and things seemed OK until I flew my single conversion receiver too close to another transmitter :-(

My plan of action..... I set the aft wing at a lower incidence (about 1.5 to 2.0 degrees) than the forward wing , and moved the c.g. a little more forward. A dual conversion receiver is installed, to eliminate possible confusion from interference.

It will be a week or so before I can make a test flight - I had hernia surgery yesterday !!

Wow, I really got long winded....

[flieslikeabeagle]

MarionDK, any update on the outcome of your mods? I'm building my own Sopwith Camel and have just discovered the same thing you did ( I measured the interwing struts and found the rear ones longer, setting the aft wing at a higher incidence angle).

I've already made the mod of putting some dihedral into the upper wing, as per other Wattage Sopwith Camel threads on rcgroups.

-Flieslikeabeagle

[50+AirYears]

I think in the old Chuck Cunningham and Andy Lennon articles on Bipe design, they ussually set up one wing with a 1 or 2 degree angle of attack in relation to the other. The choice of wing seemed random. One set of plans I had for a Beech Staggerwing had the lower (Forward wing) set at +2 degreeas and the upper (Back) set at 0. Whatever works, I guess.

[vintage1]

Ther is no doubt that the airflow over the wings is interdependent on a biplane. i.e the lower wing is operating in a higher pressure regume due to downwash from the top and vice versa. Staggering the wings makes it more complicated yet.

Since no one has tried to seriously analyse biplanes since the mid '30's, its hardly surprising you get so many differet points of view.

Persobnally, since they are scale objects, I rig em scale and live with the idosyncrasies.

That's whay I have the transmitter :D

I have been starig at an Alabros B111 scale drawing - that has the ailerons rigged with washout on the full size. Wonder why :D

[Marion]

For whatever it is worth.... I had good luck with the forward (upper) wing set about 2 to 3 degrees higher incidence than the lower wing. Flight was good, and stalls were mild. I also moved the c.g. a little more forward than the plans called for.

[flieslikeabeagle]

5+AirYears, the Staggerwing setup you describe does in fact follow the rule that the forward wing should have the higher angle of attack - since it's a staggerwing, that means the lower wing has the higher angle of attack!

Vintage1, I wonder if you may have raised another advantage of the staggerwing configuration. Since the wings create more downwash than "upwash", the lower forward wing will presumably have less effect on the upper, aft wing. In the traditional arrangement with the upper wing forward, the downwash from that wing is more likely to strongly interfere with the trailing edge of the lower, rear wing.

I really am hoping for more stability than the original full-size Sopwith. With RC planes you have no "seat of the pants" feel, so it takes longer for the pilot to notice an attitude change and compensate for it. Add the fact that models have much smaller moments of inertia, and therefore are able to pitch, yaw, and roll much faster then their full-size counterparts, and I think I could get in trouble easily with a truly scale-flying Sopwith!

MarionDK, I'm still in Wattage Sopwith Camel construction he** (it seems very few things about the kit go together to my liking, and I keep re-engineering one thing after another!), but after scouring the 'Net I decided to do the same thing you did, except I set the upper wing at +1 degree higher incidence than the lower, rather than +2 deg. I also set the stagger to 1 inch - I couldn't tell what Wattage intended, but 1" seemed about right.

I threw away the mess of wood struts Wattage supplied, and cut two interwing spacers out of Sturdiboard (foam core, paper skins) instead. This let me have some assurance that the two wings were in the right positions with respect to each other, something I had little confidence of achieving with the stock construction method.

I'm going with a best-guess CG 1" back from the lower wing leading edge. This is figured from finding the half-way point between the 25% chord location for each wing, which are at 0.5" and 1.5" behind the lower wing LE.

I'm hoping to finish my build today and fly the Camel tomorrow - but this thing keeps taking longer than I expected to finish!

-Flieslikeabeagle