ArcticChiller wrote:
Va can also save your wings if you keep your controls neutral.
Va gives a good protection in turbulence, but in really bad turbulence the speed will fluctuate a lot. A strong gust with a headwind component will momentarily increase airspeed, and if you are flying Va, then you will suddenly be flying above Va and get a large positive g-load. In that scenario you may not be protected.
In strong turbulence I will tend to fly between stall and Va, a little closer to Va. The turbulence might make the wing stall some times, but that will not overstress anything and will not last long. At the same time the margin up to Va will protect against strong gusts with a headwind component.
Apart from that, I think it is generally accepted that by far the most inflight breakups are due to the pilot’s control inputs, rather than gusts only.
RobertL18C wrote:
This Flying magazine article has some useful comment on Va – note its applicability to 50 fps vertical gust speed and stabilator/elevator strength.
Unfortunately, what I read in that article doesn’t correspond to what I read in the actual regulation. I’m not sure which one they used. That Airbus is surely Part 25 but it mentions normal category later on. I read Part 23 which I imagine is more relevant to most of us here. That article is also rather old. I checked the initial issue of CS-23 from 2003 as well but who knows how old version they used, perhaps it used to be different.
First of all, the specification gives you only minimum value for Va (design maneuvering speed) AFAIK (23.335). That minimum is a clean (power off) stall speed at the limit maneuvering load factor. Common fallacy is to present that minimum as the universal value for Va. It can be higher than that, there is actually no maximum (well, aside from Vne/ Vmo) and the specification even says it doesn’t have to be higher than Vc (design cruise speed).
I haven’t seen anywhere in Part 23 or CS-23 (granted, I haven’t read it cover to cover, so to speak) Va used in the context of gusts. Yes, there is a corner at Va because it’s a corner of the maneuvering envelope. However, I have no idea where they got this: __"Va is a calculated airspeed based on the actual gross weight of the airplane and the wing’s response to a 50-foot per second wind gust, or movement of the elevator."__ Maybe you can point me to that section.
That article also mentions that vertical surfaces have to withstand full displacement of the rudder control and then return to neutral. But when you look at 23.441, you see that that statement is only true for commuter category (while the article implies it’s true for normal category). For normal category, you only need that full deflection.
And that’s just what I noticed while reading it (I checked my memory was serving me well and also so I can provide the section numbers for reference).
On the question of a Va that applies to all flight controls for full deflection in one axis (not combined I think we all agree), why do aerobatic aircraft have lower entry speeds for flick type manoeuvres? Usually significantly below rolling or looping manoeuvres. Perhaps it’s the combination of rudder and pitch, or it maybe related to the load limit of the fin?
The Flying article cited earlier implies the applicability of Va is quite nuanced.
Transport aircraft on the other hand have turbulence speed which is above Va. Am not sure they want to explore speeds below Va when cruising near buffet onset boundary. There is probably a Mach speed related to Va and hopefully the jet crew can advise.
IIRC, the Airbus in NY referred to in the article did not break ‘only’ because of pilot input, but because of an additional force, or torque rather, on rudder and elevator caused by the wake turbulence of another airliner that had taken off from, I believe it was La Guardia, shortly before.
Isn’t it rather true that most or all of the speed limits for aircraft relevant to structural integrity are valid only so long as they aren’t met by reality. They are determined in an “either-or” logic. The whole discussion is a little bit academic, in my eyes. The load limits are valid, but the ceteris paribus condition under which they have been tested is not.
These speeds pretend that the situation is an “either, or”. Either a) pilot input, but (please) otherwise calm air, or b) gusts, but (please) no input by the pilot, and c) of course, (please) only one rudder at a given time.
In reality, we fly at cruising speed, maybe for quite some time, burn fuel and get lighter (which causes all the V’s to go down), then expierence sudden turbulent air (i.e. in descent), and move rudder, elevator and aileron all at the same time to keep the plane level. And while we should just keep the yoke steady, we act on it, in order to react against a sudden force here and there, which makes it only worse.
Which means, eventually, that we should fly a lot slower in heavy turbulence then what the POH says is theoretically possible.
Martin wrote:
First of all, the specification gives you only minimum value for Va (design maneuvering speed) AFAIK (23.335). That minimum is a clean (power off) stall speed at the limit maneuvering load factor.
I can’t see how it could ever be higher than this minimum in any typical aircraft.
If you fly faster that this minimum Va, there is an angle of attack that will exceed the limit maneuvering load factor.
The only way to prevent this from happening is somehow limiting elevator authority to prevent that angle of attack from being reached.
So in any aircraft with normally controlled elevators (i.e., not-FBW, or with another means to limit elevator travel at higher speed) Va cannot be lower than the the stall speed at the limit maneuvering load factor, and it cannot be higher.
Or am I missing something?
AdamFrisch wrote:
Va is a regulatory number as well. […] So Va is what FAA says it is, it’s not always a purely aerodynamic number. I’m sure there are other aircraft that has had the same FAA treatment.
It’s used in the specification and IMHO that’s what it primarily is (it’s used to restrict the design). I don’t think it’s that useful to pilots, it’s not a friendly number you can easily apply. Newer aeroplanes have Vo, operating maneuvering speed. If you look inside an aerobatic plane, it’s not without a reason you’ll see a placard listing entry speeds for individual maneuvers.
I’m assuming this happened when the manufacturer was defunct. An aeroplane has to have certain qualities at that speed. If they believe it doesn’t, lowering it is logical recourse (however, it’s a question what they hoped to achieve and what the impact actually was given how misunderstood this speed is). Out of curiosity, what is the clean stall speed and positive limit maneuvering load factor?
Cobalt wrote:
So in any aircraft with normally controlled elevators (i.e., not-FBW, or with another means to limit elevator travel at higher speed) Va cannot be lower than the the stall speed at the limit maneuvering load factor, and it cannot be higher.Or am I missing something?
It can be lower if roll or yaw are limiting rather than pitch.
RobertL18C wrote:
why do aerobatic aircraft have lower entry speeds for flick type manoeuvres?
It could be due to asymmetry of loads.
RobertL18C wrote:
Transport aircraft on the other hand have turbulence speed which is above Va.
Even Part 23 specifies Vb (design speed for maximum gust intensity). When it was introduced is a question. Logically, older designs won’t have it.
Cobalt wrote:
I can’t see how it could ever be higher than this minimum in any typical aircraft.
Since it’s written that way, it has to be possible. I think you can make the structure as strong as necessary to handle the prescribed maneuvers at a Va you chose without having to increase the design limit maneuvering load factor. Does that mean that the whole structure is good for whatever load factor corresponds to that Va?
I think one way to limit it would be to make it so the force required is higher than what the regulation allows (the pilot limit or whatever it’s called).
Airborne_Again wrote:
It can be lower if roll or yaw are limiting rather than pitch.
In theory, yes. The regulation prohibits this. (23.335)
Martin wrote:
think you can make the structure as strong as necessary to handle the prescribed maneuvers at a Va you chose without having to increase the design limit maneuvering load factor.
Ah – so you could build the aircraft in excess of the certification requirement, say, for 6 G (wing + other structures), but only certify it in the normal category (so you don’t need to meet the aerobatic certification criteria, e.g., spin characteristics) .
So now you can have a Va higher than the minimum (Va min is stall at 3.8G).
Not sure why you would do that in practice, but I see your point now.
