At the start of this Weight and Balance series, I discussed the potential consequences of excess weight. There are three major issues: performance, structural integrity and stall speed.
The first two were covered adequately, and the third, stalls, I left untouched. This I did because stalls are intimately affected by changes in both weight and balance. Until the topic of CG location had been thoroughly explored, stalls had to be postponed. It is now appropriate to tackle this last area.
WEIGHT
In a nutshell, increased weight leads directly to increased stall speed.
The area is best understood by reverting the term "stall speed" to its more accurate nomenclature: "minimum flight speed" (MFS). The number refers to the indicated airspeed at which the wing will reach critical angle of attack as a pilot attempts to maintain straight and level flight while continuing to slow down. It is important to note that MFS is not the only speed at which a wing will stall, merely the slowest.
When manufacturers first establish this speed, they do so by adopting a worst case scenario: maximum weight at maximum forward CG. Both factors have the effect of raising MFS.
MAX WEIGHT
As weight increases, more lift is required. Assuming a constant airspeed, the added lift can only be obtained through increased angle of attack (AOA).
In addition to adding lift, this increase in AOA also decreases the physical margin from stall. This is significant: the wing has gotten closer to critical AOA without an accompanying loss of speed. In a logical progression, if the airplane were now slowed until AOA increased to critical, the stall would occur at a higher indicated airspeed than at lighter weights.
This increase in minimum flight speed does not worry every pilot: many do everything they can to avoid flight at even moderately high AOA, so it is not unusual to encounter willingness to exceed maximum weight limits without concern for inadvertent stalls.
To make the liability of over-gross flight a little more clear, let’s add a level turn at 45° of bank.
TURNS
Turns require a continuous application of force to overcome inertia. In airplanes (and birds and gliders), a very clever solution presents itself: the wings are banked, and a portion of the lift so produced is used as the source of this effort. This portion of the total lift is commonly called the "horizontal component."
Thus, in a level turn, the wings do double duty: they continue to overcome gravity and take on the added chore of overcoming inertia.
In order for this to work, the total lift must be increased over that required for wings-level flight. This is accomplished through a deliberate increase in AOA, the result of the increased back elevator you use each and every time you make a level turn.
Thus, just as with added weight, turns require the wing to more closely approach critical angle of attack without the accompanying loss of speed normally associated with stalls.
In a 45° banked level turn, the required lift is increased by a factor of 1.4, indicating the wings are supporting 40% more weight than required for "normal" flight. This in turn requires 40% more AOA (a linear progression), so the wing is now flying much closer to critical AOA than in straight and level flight. In an already overloaded airplane, a relatively small increase in AOA at this point will result in a stall at an significantly elevated airspeed.
The bottom line: turns and over-loading add up to unexpectedly high AOA. If you then add a sudden increase in AOA from turbulence or wind shear, it is likely you will be surprised by an inadvertent stall.
Next month, stalls, the CG and a conclusion of the examination of pitch stability.
