Can you explain the decalage in a bi-plane?

All the bipes I've seen have the upper wing with slightly more incidence than the lower wing, so it stalls first.

The decalage (note the spelling) for most biplanes is set up just like Steve says, for exactly that reason. Because most biplanes have positive stagger (i.e.: top wing slightly further forward than the bottom wing), stalling the top wing first will shift the center of lift aft, which tends to hold the nose down and delay any stalling of the bottom wing. Of course if you're building a Beech Staggerwing or a Sorrell Hyperbipe (negative stagger), you would want to set it up with more incidence on the bottom wing to create this same effect.

One notable exception to this arrangement was the famous Bucker Bu-133C "Jungmeister" ("Young-my-stir", German for "Young Master"). Its incidence was 0.0 on the top wing, +1.0 on the bottom wing, with positive stagger. This had two effects. Because of some complex interactions I won't go into right now, there was no change required in elevator trim between upright and inverted flight. In addition, in a positive-G stall, the bottom wing stalled first, shifting the lift forwards, causing the nose to go up even more, triggering a stall of the top wing as well. The reverse happened if down elevator was added for recovery. As a result, the Jungmiester stalled like a lightswitch, either "on" or "off". This gave it extremely crisp entries and exits to stall-based maneuvers such as spins and snap rolls (" flick rolls" for you Brits). Most airplanes will recover predictably from a snap roll 9 out of 10 times, but watch out for that 10th time! Because of this, it's very important to have lots of altitude when doing these types of maneuvers, just in case. The Jungmeister would recover predictably 10 out of 10 times, making it ideal for this sort of dangerous nonsense. You can see a Jungmeister (modified with a modern flat-opposed engine instead of the old radial) doing exactly this in the movie "The Great Waldo Pepper", in the sequence showing "the famous ace Ernst Kessler" doing a 10-turn "death spin" at an airshow, recovering with the wheels almost touching the ground. It's been said that the Jungmeister was the only aircraft that could safely (if there is such a thing for this kind of stunt) do this, since it was the only aircraft that could be counted on to recover at exactly the right moment, EVERY time.

This fantastic ability for precise stall-based maneuvers proved to be the Jungmeister's undoing in competition. The man who compiled the dictionary for aerobatic maneuvers, Jose L. Aresti, whose book established the difficulty factors and scoring for modern aerobatic competition, flew a Jungmeister. Consequently, he gave low difficulty factors to the snap maneuvers Jungmeisters are good at, and high difficulty factors to the "yo-yo" vertical maneuvers that spell trouble for the Jungmeister's draggy (but extraordinarily strong and light) wire-braced wings. As a result, the modern rules favor the cleaner (but not as good at snap maneuvers) monoplanes, with their low drag airframes and internally braced wings.

Don Stackhouse @ DJ Aerotech
djarotec@bright.net
http://www.bright.net/~djwerks/