Why would I want to lower the stall speed?

Question

Why would a cargo pilot for example want to dump fuel to lower the stall speed? I understand that lowering the weight decreases the stall speed, meaning it is "easier" for the plane to reach this new lower speed and stall, correct? Why would I want that? Don't I want to have a higher stall speed so I could be far from stalling? What am I missing here.

Answer 1

You have misunderstood.

Lowering the stall speed means that, whereas your stall speed at a given configuration was e.g. 150 knots, the stall speed now is 138 knots. This does not mean 'it is "easier" for the plane to reach this new lower speed'. It only means you can slow down more without stalling.

Moreover, keep in mind that dumping fuel lowers all associated approach speeds. Less weight means lower V_ref , hence safer approach and landing (shorter landing distance).

Answer 2

Don't I want to have a higher stall speed so I could be far from stalling?

Unfortunately this is incorrect. Aerodynamic stalls happen when an aircraft’s airspeed is at or below the aircraft’s stall speed for a specific configuration, attitude, phase and condition of flight. A higher stall speed will mean that you will be closer to stalling at any given airspeed. A lower stall speed is desired so that you can maintain a safety margin between your airspeed and the stall speed.

A lower stall speed also means the aircraft can fly and land at a lower airspeed and, in turn, ground speed. This would decrease its momentum, its required landing distance, and (in case of a soft field landing, off airport landing, or crash) its impact force with the ground.

“I understand that lowering the weight decreases the stall speed, meaning it is "easier" for the plane to reach this new lower speed and stall, correct?”

Unfortunately, the second half of this statement is incorrect. The different weight will produce different momentum, and a different amount of force required to change velocity. That does not mean that it will be easier or harder to reach a stall speed with a greater difference from the original flight speed than you would have had with the original stall speed. It just means that you may have to have a greater amount of acceleration to reach the lower speed than you would the higher speed in the same amount of time.

This is all irrelevant, though, since the entire point of lowering the stall speed is to make it harder to stall the aircraft. Especially at your given airspeed. Which means that they aircraft can fly at a lower airspeed without stalling. You do not want to stall the aircraft. This is important during low speed maneuvers such as approach, landing, and turns to final.

To put it in laymen’s terms, the stall speed is the airspeed at which the aircraft would stop flying. Another way to say it is that it is the airspeed at which the wings stop producing lift (or enough lift to keep flying). In reality, the stall speed is the airspeed at which flying straight and level would produce a high enough Angle of Attack that the airflow would separate from the wing surface. This is called the Critical Angle of Attack. Although it can be different for each aircraft, it is generally about roughly 17°. It also does not change for a specific aircraft in its configuration.

Lift is a function of: wing shape; airspeed; and the angle between the line formed by the relative wind and the chord line of the wing (Angle of Attack). Altering any of these would alter the amount of lift produced.

• You alter wing shape with the wing control surfaces. The more camber you have, the more lift you will have.
• You alter airspeed with power and pitch. The more airspeed you have, the more lift you will have
• You alter AoA with wing load. The higher the AoA, the more lift you will have until it reaches the Critical AoA. The more wing load you have due to the forces applied on the aircraft’s mass (such as gravity and g-forces from turns and acceleration/deceleration), the more lift you will need to have to counter the wing load. And, the higher the AoA will need to be in order to produce enough lift to counter the wing load.

Reducing the weight which the wings have to carry (wing load) would reduce the amount of lift that must be produced. Reducing the wing load reduces the AoA that must be maintained to produce lift. The further away from the Critical AoA, the better and safer flight will be. The further away from the Critical AoA, the further you are away from a stall.

In the particular scenario brought by the Original Poster, this all may also be irrelevant. As pointed out by @Peter Kampf, dumping fuel to reduce weight may just be for the purpose of getting the aircraft below its Max Landing Weight. Or, in the case of an emergency landing, ridding the aircraft of as much flammable material as possible in case of a crash.