Weathercocking in the air ?
Quote from AndyGiov [Feb26/14:50]:
In the aeronautical engineering slang... weathercocking is possible once airborne as well, not only while on the ground!
Indeed, "weathercock stability" is the nickname given to tendency of the aircraft to "naturally" align its nose into the wind...
[Unquote]
Sorry for the late reply, and it's going to be lengthy, and sometimes didactic (for the new guys). It seems your knowledge of dynamics theory is greater than mine. The rustiness of the comparatively little aerodynamics I was taught has already been demonstrated on this forum. But I do have a career of practical flying experience to draw on.
WEATHERCOCKING IN FLIGHT ?
As I understand it, you are saying that – if, for example, an aircraft with aerodynamic directional stability is airborne and on a westerly heading, flying within an air mass that is moving from north to south – a series of horizontal gusts from its right will each induce a sideslip, leading to a slight movement of heading towards the wind. A series of these events would have the effect of gradually bringing the heading of the aircraft round towards the north, i.e., into the wind. As the wind rises and falls, the gusts striking the tail could be regarded as sometimes positive, sometimes negative. So your proposition is counter-intuitive, but in the dynamics of fluid it has the ring of truth.
Presumably, the above is a theoretical long-term effect. My flying experience is admittedly limited to conventional aeroplanes from 30 - 380 kts IAS, 30 - 500 kts TAS, and in wind speeds up to about 180 kts; but I've never noticed this effect in practice. This may be because the pilot will always correct sideslip with rudder, but the corrections required from horizontal gusts are minimal on large aeroplanes. This applies particularly on a swept-wing airliner with yaw dampers, where pilots are discouraged from making rudder inputs in symmetric flight, especially at high speed.
For modern tail-less aeroplanes, which may have inadequate aerodynamic directional stability, my presumption is that artificial stability is achieved by asymmetric deployment of computer-controlled, wing-mounted air brakes. Such an aircraft, like the B-2, might be less subject to the phenomenon you describe?
DRIFT IN FLIGHT
What is evident to the aircrew in flight, of course, is the phenomenon of drift. An aeroplane on the approach to land, with no sideslip, at 120 kts TAS in a beam crosswind of 40 kts, tracks over the ground at nearly 20 degrees different from its heading. That difference of track (TRK), compared with heading (HDG), is known as the drift angle. "Left" drift is the result of a crosswind from somewhere on the R/H side of the aircraft; bow, beam, or quarter.
An aircraft flying at 480 kts TAS, pointing directly across a 160-knot jetstream, experiences the same angle of drift as the example above. [For the uninitiated, both examples are routine on jet aeroplanes.]
WEATHERCOCKING ON THE GROUND
To define the expression "weathercocking", we need to remind ourselves what a weathercock is, and how it works.
A weathercock is essentially a weather vane (wind-direction indicator), normally mounted on the top of slim buildings, such as church steeples. Its shape represents the profile of a male chicken, wings folded. [The male chicken is known as a "cock" in England; a "rooster" in America.] The male displays large tail feathers, particularly when courting females. On the ground, standing, the large tail means that it has to face ("head") into any stiff wind; to avoid being turned and blown over.
Like the rooster on the ground, a weathercock "heads" into wind. This is because, in place of two legs, it has one central leg in the form of a rod, which pivots on a bearing that is fixed on the building.
In the case of a conventional aircraft on the ground, the rod of the weathercock is replaced by the main gear legs. [Gliders often have just one.] If it is stationary on a windy day, the brakes are released, and any wheels on other legs allowed to castor, it will be free to rotate on its main leg(s). If it is provided with vertical tail surfaces to provide directional stability in flight, and if the wind is strong enough, the aircraft will turn to face into the wind. This is known as "weathercocking".
The tendency for this conventional aeroplane to turn into the perceived wind persists as long as its weight is fully or partially supported by the main gear legs, whatever its speed along the ground. At higher IAS, the rudder is more effective in controlling this tendency, whereas a steerable nosewheel (or fixed-straight tailwheel) becomes less effective as the ground speed rises.
PS: Reviewing the above, my ears burn – imagining the chorus of comment that I am labouring the bleedin’ obvious. But even matt_hooks – for all his knowledge of aerodynamics – was confusing weathercocking and drift early in this thread (posts #6 – #11). So it may not be so obvious if you are an aspiring aviator, whose current experience is only virtual.