172driver wrote:
Don’t you mean Va, maneuvering speed?
No. Va is the max speed for a full deflection of the controls. It’s usually (perhaps always?) higher than the start of the yellow arc. The yellow arc signifies a speed that you should only exceed in smooth air.
While it’s covered in training, it doesn’t really come up very often for students because they don’t tend to do training in anything but smooth air. Hence my taking the opportunity to remind the wider audience that might be reading this of the importance of the often forgotten yellow arc!
It’s usually (perhaps always?) higher than the start of the yellow arc
The majority of GA is in the standard envelope category of +3.8 g to 1.52 g. The envelope typically shrinks with flaps to +2.0 to -0.0 g. Va is basically stall speed times the square root of the g envelope limit. The venerable Warrior has a stall speed of 50 KIAS flaps up and MAUM. Multiplying the square root of 3.8 and 50 gives 97 KIAS. However Va is in a range of 88 KIAS to 111 KIAS depending on mass. Vno, top of the green arc, however is 126 KIAS. The Bonanza has Vno of 167 KIAS but Va of 134 KIAS, and this is a type in the Utility category.
@Pilot DAR might know why Va is stated as 111 KIAS, which implies the envelope is closer to 5 g! 111/50 squared is around 5.
Am not aware of a GA type where Va is higher than Vno, and would suggest that is never the case.
The Harvard interestingly quotes Va for different axis: roll/longitudinal is quite high, am thinking 190 mph, pitch/lateral more modest around 140 mph and yaw/normal an even lower 95 mph.
As performance starts to be affected by compressibility and transonic effect there are further effects on Va.
Va is maneuvering speed and as @RobertL18C states is calculated w the stall speed as base. It is also the rough air penetration speed to which you slow down in turbulence. It varies with weight and is not indicated on the ASI. At least here there is quite a lot of emphasis on it during training.
I am not aware of any V speed (marked or not) between Vno and Vne, IOW inside the yellow arc.
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)
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.
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.
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 
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.
@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.
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.
