Crosswind Limits,
QUESTION 1.
To understand the effect of increasing altitude we need to remember that the principal effect of gusts is to increase and decrease angle of attack. This in turn varies CL and load factor.
The important thing to note is that it is the TAS rather than the CAS that determines response to gusts. In a vertical gust the change in angle of attack is proportional to the ratio of TAS : vertical gust velocity. (The tangent of the angle of attack change is equal to the vertical gust velocity divided by the TAS.
As altitude increases the TAS at any given CAS increases, so the effect of a vertical gust decreases. At 40000 ft ISA for example, the TAS at any given CAS is about twice its sea level value, so the effect of a vertical gust is reduced accordingly.
This is similar to the way aerodynamic (usually simply termed roll) damping reduces with increasing altitude. So as altitude increases the gust load for any given combination of CAS and gust velocity decreases. Statement (i) is therefore incorrect.
The effect of increasing weight is a little more problematic, so we need to consider it from two points of view.
The first factor is that increasing weight reduces the rate at which an aircraft will accelerate upwards or downwards in a vertical gust. If an aircraft hits an upward gust and the extra lift causes it to accelerate upwards, this will partly offset the increase in angle of attack. A light aircraft will accelerate more quickly and so will experience a lower increase in angle of attack.
The second thing we need to consider is the degree to which any given gust will alter Cl and hence load factor, if we ignore the vertical acceleration of the aircraft.
Let's consider 2 identical aircraft, one at 10000 Kg and the other at 20000 Kg. If they are both flying at the same altitude and IAS, then the angle of attack and Cl of the light aircraft will be half that of the heavy one. Let's suppose the angles of attack are 3 degrees and 6 degrees respectively. In straight and level flight they will bot be at 1g.
Now assume both aircraft hit the same gust and it increases their angles of attack by 3 degrees. The light aircraft will experience a doubling of its angle of attack, Cl and load factor. But the heavy aircraft will experience only a 50% increase. So the heavy aircraft will experience 2g while the light one experiences only 1.5g. So increasing weight decreased the gust load, which means that statement (ii) is true.
If we look more closely at the first factor of vertical motion, we can see that the greater aceleration of the lighter aircraft is a direct result of the greater change in load factor.
The fact that light aircraft jump about more in turbulence is due to the greater changes in load factor they experience.
So option c is correct.
QUESTION 2.
The important thing here is that a high aspect ratio wing has a steeper CL : alpha curve than a low aspect ratio wing. So for any given gust induced increase or decrease in alpha, the change in CL will be greater for a high aspect ratio than for a low aspect ratio. So gust load increases with increasing aspect ratio.
You can demonstrate this simply by drawing a steep and a shallow CL :alpha curve on a piece of paper. Now apply the same change in alpha to both curves and see how the CL varies.
The effect of altitude is discussed above.
So option b is correct.
[ 19 January 2002: Message edited by: Keith Williams. ]</p>