So when you are flying with half a tank, by yourself, and you enter some bad turbulence pulling back to 105 is good enough to keep the wings on? I guess at least you won't take anyone with you if that were to ever happen.
Notice what I said in my first post:
Sorta depends on what you're going for here. At 105, no matter what the weight, the control surfaces will remain attached to the airplane when you deflect them fully or abruptly. That's what Va is intended to ensure.
Va, as defined by the FAA, is a speed solely designed for protecting the control surfaces. It is not intended to protect you from turbulence or excessive load factor. Va, legally, can be set well above the speed that will protect you from exceeding the load factor limit.
Notice how Va is defined in the front of the C172 AFM:
"Do not make full or abrupt control movements above this speed"
That's why it reads that way.
The reason that confusion exists on the subject is that aerodynamics books refer to a speed called "maneuvering speed" that is defined as the sqrt(max load factor) * stall speed, which will indeed protect you from overloading the aircraft. Va is not intended to be this speed. However, Cessna has always made them the same and Piper often gets close.
There is a new V-speed that is intended to give you what you thought you had in Va. It's called "Vo". Newly certified airplanes should have this V-speed.
As for turbulence, slowing down to Va or Vo is probably excessive. Va will not necessarily prevent you from exceeding the load factor limit, but Vo will. However, the aircraft can withstand a larger load factor when the load factor does not result from deflection of the flight controls. The reason is the load is more evenly distributed about the aircraft, from what I understand. If you see a complete Vg diagram, it will often show part of the envelope exceeding the load factor limits. These are the gust lines that must be calculated for aircraft certification.
The thing about Vo in turbulence is that you don't want to stall the airplane any more than you want to bend the wings. If you know you have a little more leeway on the airspeed thing, you don't have to do either.
If you're skeptical, bring up the Part 23 Certification regs and do a search on Va.
One thing that always bugged me is the placard in the 172, it just lists Maneuvering speed as 105 but does not list Va for the lighter side of the operations. I guess that is why it comes with a three inch thick POH in the backseat.
I'm not sure what exists on the type certificate, but arguably there is only one Va for the aircraft, at max gross. Perhaps the placard is a reflection of that.
Actually since all V-speeds are calculated at max. gross weight, you could recalculate any V-speed for your aircraft's weight using that formula.
