As was pointed out above, some airplanes tend to drop one wing first because of their construction or because they acquired a "bend" sometime in rough service. Others aren't inclined to drop either wing, so they are said to stall "straight ahead".
As Ryan was saying, aerodynamic stall happens by reaching the critical angle of attack (AOA); this can be done in any attitude and any speed. The key is AOA; the angle between the wing and the relative wind. Realize it's not a "wind" per se blowing on the airplane, it's simply the air rushing past as a result of the plane moving forward. In other words, if you stick your hand out of a car window at 60 mph on a calm day, your hand feels 60 mph of "wind", right? Note that the "wind" is coming from straight ahead.
If the concept of relative wind doesn't quite make sense, imagine an airplane in level flight - the relative wind is essentially "in your face" because that is the vector of the airplane, just like the "wind" your hand felt out of the car window. Now picture that airplane in a pure vertical dive. The relative wind striking the front of the wing is still coming from "straight ahead" ~relative~ to the airplane
but to an outside observer, that "wind" would be coming straight up from the earth, right? If our pilot tries to pull too abruptly to return to level flight and exceeds the critical angle of attack, he will stall despite being pointed straight down and indicating a high airspeed. (BTW, this condition is called an "accelerated stall".)
If you're wondering how an airplane ever gets back to level flight from a vertical dive without exceeding this angle, think of it in slow motion, frame-by-frame. As the nose tracks up, the airplane's vector changes. As the vector shifts, so does the relative wind - it's dynamic. If you've ever been out on the bow of a sailboat (Never let go, Jack!) and felt the wind on your face and then felt the relative wind shift to your cheek while the boat was in a turn, that's the concept.
(Here is an advanced part of the concept: if the boat was going straight and a gust caused you to briefly feel wind on your cheek, that's an example of how gusts/turbulence can momentarily change relative wind even though the vector of the boat is the same - one of the reasons pilots carry a few extra knots "for Mom and the kids" in those conditions; they are reducing AOA to increase their margin from the critical angle.)
You've probably also seen relative wind in video where a high performance airplane, like an F-18, is pulling out of the bottom of a loop with an extremely nose-high attitude, but the airplane's vector is still downward ... the airplane is not going where it is pointed. The angle between its vector and its body is a good illustration of high AOA.
Spins require two ingredients: stall and yaw. Take a docile airplane like a Citabria, pull the power to idle and allow the airplane to slow while trying to maintain altitude (not attitude) by easing the stick continuously back. The nose rises and when critical AOA is reached, the wings stall. Lift doesn't disappear, but it is reduced below its ability to support the weight of the aircraft. Because both wings are seeing nearly the same AOA, the nose (aka the heavy end) drops and the wings stay fairly level. With the nose lowered, the airplane's vector becomes more "downhill", AOA is reduced back below the critical angle and the airplane resumes flying.
Take the same situation and add yaw - this can be intentional yaw caused by rudder input or not-so-intentional yaw caused by the build of the airplane, gyroscopic force of the prop, or sloppy piloting. Bottom line is some amount of fishtailing, if you will ... this makes the relative wind "felt" by each wing different - each will have it's own AOA. As a result, one may stall first, and in the stall, if yaw is still present, one will be "more stalled" than the other. As I mentioned, lift doesn't disappear during a stall, it's reduced. If one wing is at a lower AOA, it is "less stalled" and will bring more residual lift to the table than its mate. In layman's terms, that's what causes the rotation, or spin; one wing is providing more lift due to its individual AOA.
The standard spin recovery can be summed up like this: IDLE, NEUTRAL, AFT, RUDDER, STICK, RECOVER.
Check the throttle in idle. Neutralize all controls (rudder and stick). Bring the stick full aft. Apply full rudder opposite spin direction to get the rotation to stop. Use forward stick to reduce AOA and recover from the stall. Quickly recover from the dive you now find yourself in. It took stall and yaw to get into the spin - removing both gets you back to flying.
The Glasair III uses stall strips also. They are aprox 10" strips with a triangular cross-section, one on each wing located at the inner third of the span (ideally preserving good flow out at the ailerons). The presence of the shape affects air flow at that part of the wing and helps induce a stall earlier than it would occur otherwise. During flight test, the strips are taped in place and the plane is stalled. If one wing tends to stall first and drop, the strips are moved laterally until the wings stall simultaneously. Once the right locations are found, the strips are bonded in place.
So, to say that an airplane always drops one wing first or stalls straight ahead depends on a number of things. The largest variable in stall behavior is what the pilot is doing, (creating or removing yaw) at the moment of stall and whether those inputs can have a greater effect than the other factors at play (torque, wing shape, etc). It all boils down to the AOA seen by each wing. And yes, the wing itself may have twist (or wash) that causes different parts of the wing to see different AOAs along its span in a stall, but we'll leave that for another day.
Ken
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