@Maoraigh absolutely, and I think that is @dublinpilot’s main point. Typical moderate turbulence wind shear 15-20 knots can quickly take you from Va into the yellow caution speed range, even while holding a constant attitude and allowing altitude change to reduce airframe stress.
Is turbulence only in vertical plane? Do swirls in the horizontal plane occur, directly affecting ASI reading and causing yaw? I think they do
@Airborne_Again good point, to stay unloaded and ballistic, pitch would need to be at zero alpha! I was thinking around the lateral axis in roll.
RobertL18C wrote:
Va is a function of stall speed, and at 0 G stall speed is zero, and Va, compressibility/flutter aside, could be Vne. The airframe is totally unloaded. However, ballistic Zero G only occurs in aerobatic flights, our puddle-jumpers do not routinely experience zero G.
Va is a function of the 1 G stall speed.
You’re reasoning doesn’t work because once you move the controls, the load will increase. I promise that if you are flying a ballistic (zero-G) flight path at Vne and rapidly apply full up elevator, you will rip the wings off.
@lbra Va is a function of stall speed, and at 0 G stall speed is zero, and Va, compressibility/flutter aside, could be Vne. The airframe is totally unloaded. However, ballistic Zero G only occurs in aerobatic flights, our puddle-jumpers do not routinely experience zero G.
Outside of competition aerobatics, and even an Extra with a symmetrical airfoil will have a slight positive wing incidence, Va in negative G will be lower than Va positive G because the negative G envelope is for a lower stress, eg 1.52 G negative to 3.8 G positive. However the inverted stall speed is likely to be higher as the airfoil camber is now inverted, and ergo negative G Va, despite a lower G limit, will be slower than positive G Va, but not by the same proportion of the square roots of 1.52 to 3.8.
Why do we care about negative G and taking @dublinpilot’s point about yellow arc, our puddle-jumpers are certified to relatively modest gust envelopes of 30 fps. If we get moderate turbulence that puts the airplane into a strong upward wind shear, you can tell by the cockpit headliner smiting your head, we are probably outside the negative G gust envelope, and possibly outside the negative G envelope itself. In the context then some knowledge of what Va under negative G might prove useful, to paraphrase Bertrand Russell. It is actually well below the yellow arc, and below positive G Va.
I was flight testing a turbine deHavilland Otter floatplane a couple of weeks ago, upon which I had had installed my swiveling pitot/static head. It was on a boom made by the airplane operator, so as (as required for testing) to position the pitot head one wing chord ahead of the leading edge of the wing. The pitot head is capable of free swivel 40 degrees in any direction, and was connected to the Garmin G5, so the airspeed is a tape and numeric. ’Looks like this:
The whole arrangement had worked perfectly for many hours of flight testing. The last test point I was to fly was a dive to 110% Vne (160 MIAS on the floatplane version of the turbine Otter). I achieved the speed, and had the right seat pilot take a photo of the ASI as proof. As he did this, I held speed very carefully, it was nicely very steady. I could feel the slightest of buffet, not the least alarming. I told the right seat pilot to have a look around for anything buffeting while I held speed. Suddenly he said “slow down!” in a metered, but none the less urgent tone. “Look at the pitot boom!”. My swiveling pitot head was oscillating up and down nearly a foot either side of neutral position, so yes, I eased the nose up to slow down! Yet, in hindsight, the Asi reading had been perfectly solid during the entire event (so I wasn’t even suspecting the pitot head/boom).
Two lessons learned: My client’s boom is good for only 150 MPH, and, the movement of the pitot tube perpendicular to the direction of flight does not seem to create an error in the readings.
RobertL18C wrote:
Pretty universal that the Va for negative G is significantly lower than Va for positive G. In the vicinity of moderate turbulence might it not be more sensible to use negative G Va? You hear of pilots hitting their heads in moderate turbulence, that is a negative G event.
I think that is an interesting question, I recall the idea of Va is that you stall before you reach the prescribed G limits on air-frame
Most air-frame are only certified for continuous operation with G >= 1 and there are no stalls when -1< G < 1, especially at G = 0 where Va is ill-defined
The only low/negative G that works is G = -1 (flying inverted), I think one can get a good definition of Va that mirrors the ones for G > 1
The other conceptual problem with G < 1 is that speed won’t be constant (except when level inverted at -1G)
I can fly +2G turn or +1G level at constant speed forever but I can’t fly 0G or -2G at constant speed forever
So talking about constant V-speed numbers when G < 1 does not make much sense (except at -1G), if one does complex numbers they will figure that Va = i*VS where i is the imaginary number that comes from plugging G = -1 as load factor under square-root formula 
Yes indeed, I got that backwards. VA is below the start of the yellow arc.
But you are all focused on the wrong part of my comment! My point was just a reminder that if you enter turbulence, you should watch to make sure that you aren’t in the yellow arc. You should only be in the yellow arc if you are in smooth air and turbulence isn’t smooth air.
Many aircraft cruise quite close to the yellow arc, and if you push the nose down to deal with a sudden updraft it’s very easy to find yourself inside the yellow arc in rough air without realising it. My comment was simply a general reminder of that risk.
dublinpilot wrote:
[Va] usually (perhaps always?) higher than the start of the yellow arc.
The other way around! Va is usually (I would say always) much lower than the start of the yellow arc. Lower than normal cruise speed, even.
Just to add to speeds, load and turbulence, one need to know Vb speed =< Va (in POH) and Vno (marked on ASI)
Hence, one sticks to 5kts less than Va and they will do just fine, Vb is close to Va anyway (if weight is high one has to slow more maybe 10kts)
