Originally Posted by
Apokleros
I have researched both Va and Vo to the best of my ability looking up part 23 regulations and FAA special bulletin statements about Va. Am I correct in stating that below Va a pilot may deflect one flight control ONCE to its full deflection in stable air and expect the flight control not to come detached from the airframe? Contrary to what most pilots believe, Va has nothing to do with stalling an airplane to prevent overstress of the aircraft structure and flight control surfaces.
Va has nothing to do with deflecting control surfaces and has everything to do with stall speed. Wikipedia has a good simple explanation of the various speeds. You also really need to look at a Vg diagram to understand Va. Here is a website with a good one:
Vg Diagram
Technically, FAR 23 says that Va cannot be less than the stall speed times the square root of the design positive limit load factor. This implies that it can be faster than that speed, but I'm not aware of a case where that is true. If you look at the Vg diagram, you can clearly see that you can exceed the design negative limit load factor at Va. Follow a vertical line down from Va and notice it would hit the yellow and then the red area before it would hit the extended curved line, which is what the wing is capable of. In other words, you can push on the yoke and hit the structural damage regime while still inside the flight envelope. Not true with positive g's, it will stall first. Can you stomp on the rudder at Va and snap the tail off? Maybe. The important thing to remember is that you can damage the aircraft at Va with control inputs. A lot of us were taught that you can "do full deflection in any direction and not damage the airplane". That is false.
Originally Posted by
Apokleros
Vo offers further protection by enabling an airplane to stall before a flight control is stressed beyond its limits. When the manufacturer chooses Va to be equal to Vo, Vo is not specified in the AFM/POH, and only then will Va offer further protection by stalling before overstress of controls is encountered. I am hypothesizing that this is why the Cessna 172 Skyhawk does not have a Vo speed...because Va is most likely equal to Vo.
I am still not certain if choosing a speed below Va but slightly above Vo for steep turns in a Cessna 162 Skycatcher is appropriate. The AFM/POH states that 102 knots (Va) for that airplane is the maximum recommended entry speed for that maneuver, although my co-worker thinks we should do them below Vo. Do you folks think that the FSDO might have anything useful to say about this?
Vo was introduced for the certification of light sport aircraft in 1993 according to wikipedia. The aircraft designer can pick a speed below Va and call that Vo for various reasons. Flight control deflection? Maybe. Tail strength? Maybe. The pilot doesn't know what the criteria that the designer used were. All you need to know is that for whatever reason, the designer chose to further limit the airspeed for reasons specific to that design. That being said, I would say use Vo for steep turns. You are only going to be pulling 2 g's anyway in a 60 degree turn. I wouldn't do them below Vo to give you an adequate buffer above the stall speed.
I would also always have your students compute the various V speeds for their current weight as the published speeds are for gross weight. It is a simple calculation: multiply any V speed by the square root of your current weight divided by gross weight. I'm sure you know that one, but for others here is an example. Published Va is 130 knots at a gross weight of 2100 lbs. Our current weight is 1800 pounds. 1800/2100 is .86 The square root of that is .93 130 knots times .93 is 120.9 or 121 knots. You can then use that .93 as a factor to determine any other V speed such as approach speed, stall speed, Vno, etc. Vne stays the same, however.