Originally Posted by
EXDAC
Isn't is true that "track" is ground referenced? How is a change of track (ground referenced) of any significance at all to the movement of the aircraft in a uniform airmass (airmass referenced).
UNDERSTANDING WIND SHEAR
Definitions
Wind shear can be defined as a sudden change in wind velocity and/or direction over a short distance. It can occur in all directions, but for convenience, it is considered along vertical and horizontal axis, thus introducing the concepts of vertical and horizontal wind shear:
- Vertical wind shear consists of wind variations along the vertical axis of typically 20 to 30 knots per 1000 ft. The change in velocity or direction can drastically alter the aircraft lift, indicated airspeed, and thrust requirements when climbing or descending through the wind shear layers.
- Horizontal wind shear consists of variations in the wind component along the horizontal axis – e.g. decreasing headwind or increasing tailwind, or a shift from a headwind to a tailwind – of up to 100 knots per nautical mile. (fig.1) shows how a penetration would appear as an aircraft crosses a cold front.
This weather phenomenon can occur at many different levels of the atmosphere; however it is most dangerous at the lower levels, as a sudden loss of airspeed and altitude can occur.
It is usually associated with the following weather conditions: jet streams, mountain waves or temperature inversion layers, frontal surfaces, thunderstorms and convective clouds or microbursts, occurring close to the ground.
quote from Airbus.
the only matter of concern to the aircraft is the instantaneous rate of change of a component to the steady inertial state of the aircraft. If you suddenly change your direction is the same situation to the wind suddenly changing its direction.
Is it relevant for most training cases? not really, the rate of change is modest, so the inertial accelerations are not noticeable.
Michael V. Cook BSc, MSc, CEng, FRAeS, CMath, FIMA, in
Flight Dynamics Principles (Third Edition), 2013
14.3.2 Wind shear
Wind shear is defined as a time rate of change in wind speed and direction lasting for 10 seconds or more, the assumption being that shears of less than 10 seconds are unlikely to constitute a flying qualities hazard. See, for example Hoh et al. (1982), which includes a useful discussion of wind shear in the context of
flying qualities requirements.
As for steady wind, wind shear presents a problem during take-off and landing and, when encountered, the aircraft will rise or sink according to the relative direction and magnitude of the wind
velocity gradient. Since the effect of wind shear on the aircraft is to cause an upset in normal acceleration, the requirements quantify the wind shear limits in terms of
acceleration response as given in Table 14.3, where subscript
γmax is the maximum power climb angle and subscript
γmin is the flight idle glide angle.
For this thread on the F-50 short landing, this discussion is irrelevant, however, for a flight around jet streams, it is pertinent.
By definition, shear is determined to be a change in a component wind, and for low-level flight that is normally due to wind gust, turbulent boundary layer effects, gust lines or convective phenomena. That is as taught. At altitude and with extreme jet streams, a rate of change of the heading is a change of wind component, the aircraft has inertia and that has to change and takes a finite time to do so. The speed will alter, and recorded flight data shows that occurring. It is at that point that the assumption that the aircraft's inertial frame of reference is only to the airmass has an issue. For the climb and descent through a jet stream, the same issue arises. If the aircraft was bound inertially only to the airmass, a change in the wind would not change the IAS, it would only change the ground speed. That is not observed in the flight data.
For basic teaching we can ignore the fact that Newton wrote stuff in 1665, and assume that the plane is tied only to the airmass, and ignore the fact that it's motion is referenced to "the aether", not the universe/planet. However, if you look at
chapter 1, THE KINEMATICS AND DYNAMICS OF AIRCRAFT MOTION in Stevens et al, 2015, you will note that the frame of reference isn't to cyclone Bob or Irma, it is to the earth, as is an INS, GPS (kind of). p. 3 of the section which is a riveting read, states:
Inertial Frame: a frame of reference in which Newton’s laws apply. Our best inertial approximation is probably a “helio-astronomic” frame in which the center of mass (cm) of the sun is a fixed point, and fixed directions are established by the normal to the plane of the ecliptic and the projection on that plane of certain stars that appear to be fixed in position.
INS, IRU's & ADIRU etc use accelerometer measurements added or subtracted to the platform alignment and original position georeferenced to the earth, and uses that to determine over time the velocity, and over time gives the distance shift which gives actual position. It doesn't need a wind input, it in fact gives that as a calculated output, from the component accelerations to the aircraft from wind components. The wings see IAS for stuff like V squares etc... the aircraft sees the sum of all
fears accelerations.
As difficult as this may be to consider, the assumption that the aircraft miraculously is unbound to the earth's frame of reference would also suggest that as soon as the wheels came off the ground at the equator the aircraft would be doing 900 knots to the west, as apparently its inertial frame of reference is not to the earth. Most times that doesn't happen. For flying around the pattern you can disregard everything and just look out the window and enjoy the view, as the approximation that we are taught from day 1 suffices. That does not suffice at limit cases, where inertial changes are non-trivial.
The aircraft is immersed in an airmass that is tied to the planet and then to whatever reference you wish to take into consideration. The inertial moment of the aircraft is measurable to the earth as an inertial frame of reference. The airmass is tied to the external frame of reference itself.
Stevens B.L., Lewis, F.L., Johnson, E.N., (2015) Aircraft Control and Simulation: Dynamics, Controls Design, and Autonomous Systems (3rd edition), Wiley and Sons.