Last month I looked at how gross weight affects minimum flight speed, or, as it is more commonly called, “stall speed.” Throughout this piece, those two terms will be used interchangeably.
The argument is straightforward: to maintain level flight at a constant airspeed, any increase in wing loading requires an accompanying increase in angle of attack (AOA).
This change leads to an important observation: the increase in AOA reduces the margin from stall without an accompanying loss in airspeed. The conclusion: increased wing loading ensures the plane will stall at a higher minimum speed than it will at lighter weights.
This reveals that minimum flight speed is a moving target, and in consequence the FAA has had to establish a consistent approach to the markings on airspeed indicators. The adopted standard always assumes a worst case scenario: the bottoms of the green and white airspeed arcs are therefore established for maximum allowable gross weight.
CENTER OF GRAVITY
In addition to gross weight, there is a second important variable in determining minimum flight speed: the location of the center of gravity (CG).
As the CG is moved forward, an airplane becomes increasingly “nose heavy.” This is fairly obvious, although generally incorrectly explained.
Pilots typically accept an intuitive image of an airplane supported by lift and pivoting around the point where that lift originates, as if it were hung there by a string. They then visualize weight (the CG) forward of this point acting to make the plane “nose heavy” by rotating it around the lift point. Is that the way you were taught to visualize this situation?
Unfortunately, it’s inaccurate. The truth is a bit more complex and somewhat counter-intuitive. Start by accepting that planes pivot around the CG, not around the lift point. When the CG is forward of the center of lift, the upward force of the lift tries to twist the plane around the CG, providing the source of the pitch down moment (“nose heaviness”).
When the CG is aft of the center of lift, the reverse occurs, and the lift of the wings exerts a pitch up moment.
In either case, a balancing force is required, so it is a good thing to have a tail and an elevator. Together, they cancel nose-up or nose-down moments and render the entire situation invisible to the pilot.
TAIL DOWN FORCE
When the CG is forward of the center of lift, the pitching moment created by the wings requires down force at the tail for balance. This force inevitably adds to the total weight of the airplane, with the result that the wings must now carry a load in excess of aircraft gross. This is accomplished by increasing AOA beyond that required to simply support the basic aircraft.
There are two direct consequences: added drag and an increase in minimum flight speed. The drag is a natural consequence of added lift, and the change in stall speed proceeds from the fact that added wing loading always increases minimum flight speed.
Staying with the principle of “worst case scenario” outlined for the airspeed indicator markings, it follows that the bottoms of the green and white airspeed arcs are established for full forward CG as well as for maximum gross weight. This explains why stall practice almost always involves indicated airspeeds well below the “official” stall speeds: few pilots practice at full gross and full forward CG.
TAIL FORCE— UP
The opposite side of the balance coin reflects what happens when the CG is aft of the center of lift. Now the wings exert a pitch up moment which must be countered by a matching up-force at the tail. In this circumstance the tail is helping to support the aircraft, so the wings are now holding up less than the basic gross weight.
This reduction in wing loading leads naturally to several conclusions: less AOA is required, drag is reduced, and minimum flight speed is lowered.
Although lower stall speed and reduced drag are both bonuses, aft CG loading can easily be carried too far, with dangerous consequences in stalls. That subject will be covered in depth in the next and last installment.
