I wish that diagram that shows load factor and stall speed as a function of bank angle had never been created. It is valid only under specific circumstances, and has caused a lot of grief not just in online forums but also in oral exams/checkrides.
A wing stalls if, and only if, the angle of attack exceeds the critical angle of attack. Plain and simple.
We have also all heard (I hope) that the airplane can stall at any airspeed, in any attitude. But sometimes I question if beyond rote memorization of this sentence pilots really understand what it means.
Pulling the stick back gets an airplane closer to a stall. Pushing the stick forward buys us margin from a stall. Forget stall speed - if you feel a buffet, push the stick forward. Plain and simple.
If you want to determine stall speed, then load factor is the primary variable to look at. (Aircraft gross weight, configuration, and air density are also factors, but they don't change as much or as quickly as we can change load factor.)
Load factor is lift over weight. Thus, whenever we are asking the wings to produce more lift than the weight of the aircraft, our load factor is greater than 1. That can happen in a turn, that can happen entering a loop, that can happen while transitioning from level flight to a limb, that can happen in the landing flare or during a go-around. When it happens, stall speed is increased from straight-and-level stall speed.
Consequently, we have a load factor of less than 1 whenever we ask for less lift than the weight of the aircraft. That can happen when pushing the nose down to start a descend, or to increase the rate of descent we already had going. That can happen at the top of a loop when gravity adds to the lift produced by the wings, helping us pull the aircraft towards the center of the loop. An aircraft parked on the ramp has a load factor of 0 - no lift is needed.
We can combine things that increase the load factor with things that decrease the load factor, so that the changes cancel each other out. A coordinated turn while simultaneously increasing the rate of descent can be done at a load factor of 1 (and thus no change in stall speed from straight-and-level flight). Note that descending alone isn't sufficient - it's an increase in the rate of descent (or a reduction of the rate of climb) that creates the less-than-1 load factor. So we can only do this for a short time before the nose of the airplane is pointed straight down.
In a steady descent (constant rate of descent, wings level), the load factor is actually a teeny tiny bit smaller than 1. The reason is that there is now a vertical component of drag which helps offset the weight of the aircraft. Similarly, in a steady climb the load factor is also a little bit smaller than 1, because thrust now has a vertical component. But unless we fly something like an F-15 which can climb almost vertically, the difference is so small that it's completely negligible for stall speed. I have some diagrams at the end of a video
introduced in this thread which show the math behind it.
- Martin