Eddie Sez:

The reference stall speed is what ends up producing many of the numbers that matter to you, such as V2 and VREF. You should never see it in flight if you have a stick pusher, provided you allow the stick pusher to do its job.

The key takeaway from this discussion, however, has nothing to do with normal flight operations. You should note that VSR is measured during test conditions on a fully trimmed aircraft, 1-G flight, using a measured deceleration. If you are turning, decelerating more than 1 knot per second, or have any other conditions outside what the pilots used to produce these numbers, you aircraft is going to stall at a higher speed.

Before getting into the aero engineering, how about a Stall IQ test?

  1. As the angle of attack is increased, so to does the coefficient of lift until a point where it decreases as rapidly as it increased. TRUE or FALSE?
  2. The wing tends to stall from the root to the tip, giving you adequate control of the ailerons during the initial phase of the stall. TRUE or FALSE?
  3. The aircraft will tend to pitch down once the wing has been fully stalled. TRUE or FALSE?
  4. A proper stall recovery can be made by increasing thrust or decreasing pitch while minimizing altitude loss. TRUE or FALSE?

If you answered TRUE to each question, congratulations, you are qualified to be a Cessna 152 pilot. If you answered FALSE to each question, you are qualified to fly a swept wing, T-tailed jet. See Stalls for more about this.

Most of this comes from 14 CFR 25 as you would expect from a number designed primarily for aircraft certification, but there are other sources as listed in the references at the bottom of the page.


Regulatory

[14 CFR 1.2] VSR means reference stalling speed.

[14 CFR 25.103] Stall speed.

(a) The reference stall speed, VSR, is a calibrated airspeed defined by the applicant. VSR may not be less than a 1-g stall speed. VSR is expressed as:

VSR VCLMAX nZW

where:

VCL MAX = Calibrated airspeed obtained when the load factor-corrected lift coefficient

( nZWW qS )

is first a maximum during the maneuver prescribed in paragraph (c) of this section. In addition, when the maneuver is limited by a device that abruptly pushes the nose down at a selected angle of attack (e.g., a stick pusher), VCL MAX may not be less than the speed existing at the instant the device operates;

nZW = Load factor normal to the flight path at VCL MAX

W = Airplane gross weight;

S = Aerodynamic reference wing area; and

q = Dynamic pressure.

(b) VCL MAX is determined with:

(1) Engines idling, or, if that resultant thrust causes an appreciable decrease in stall speed, not more than zero thrust at the stall speed;

(2) Propeller pitch controls (if applicable) in the takeoff position;

(3) The airplane in other respects (such as flaps, landing gear, and ice accretions) in the condition existing in the test or performance standard in which VSR is being used;

(4) The weight used when VSR is being used as a factor to determine compliance with a required performance standard;

(5) The center of gravity position that results in the highest value of reference stall speed; and

(6) The airplane trimmed for straight flight at a speed selected by the applicant, but not less than 1.13VSR and not greater than 1.3VSR.

(c) Starting from the stabilized trim condition, apply the longitudinal control to decelerate the airplane so that the speed reduction does not exceed one knot per second.

(d) In addition to the requirements of paragraph (a) of this section, when a device that abruptly pushes the nose down at a selected angle of attack (e.g., a stick pusher) is installed, the reference stall speed, VSR , may not be less than 2 knots or 2 percent, whichever is greater, above the speed at which the device operates.


What it Really Means

VSR is a speed:

While VS is defined by 14 CFR 1.2 as "the stalling speed or the minimum steady flight speed at which the airplane is controllable," VSR is further defined as "not less than a 1-g stall speed.


Gulfstream Example

Figure: G450 Stall Speed, Idle Power, Speed Brakes Retracted, Flaps 0°, from [G-450 AFM, pg. 5.1-33].

The G-450, for example, provides several charts for VS at varying configurations at idle power as a function of gross weight and pressure altitude. While not specifically noted, these are presumed to be a 1-G flight which makes them the same as VSR.


References

14 CFR 1, Title 14: Aeronautics and Space, Definitions and Abbreviations, Federal Aviation Administration, Department of Transportation

14 CFR 25, Title 14: Aeronautics and Space, Federal Aviation Administration, Department of Transportation

Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013.