Why is the stall speed of an aircraft a specific speed?

Question

I've read in many places that the stall speed is xxx knots for a specific plane.

Is the stall speed of any aircraft fixed or does it change depending on other conditions such as weight, density of air, temperature, humidity, etc.?

Could an aircraft stall at a higher flight speed, depending on the angle of attack?

Secondly, shouldn't lift reduce as speed reduces? So why does the aircraft lose complete lift only at a specific speed?

Answer 1

The stall speed of an aircraft is not fixed; it changes depending on several factors:

1. Weight: More weight means more lift required to maintain flight, which increases the stall speed. A lighter aircraft will have a lower stall speed.
2. Air Density: Air density decreases with altitude, which means the stall speed in terms of "indicated airspeed" (IAS) remains the same, but the "true airspeed" (TAS) at which the aircraft stalls increases.
3. Center of Gravity: A more forward center of gravity requires more lift and can increase the stall speed.
4. Load Factor (Bank Angle): When an aircraft is in a turn, the load factor increases, and so does the stall speed. This is why an aircraft can stall at higher speeds during steep turns.
5. Flaps and Wing Configuration: Lowering flaps increases the wing area and changes the wing's camber, allowing the aircraft to stall at a lower speed.
6. Air Temperature and Humidity: These factors affect air density, which in turn affects the stall speed.
7. Icing on the Wing: Ice formation on the wings can disturb the smooth airflow and reduce the wing's lift capability. This increases the stall speed because the aircraft must fly at a higher speed to produce the same amount of lift as it would without ice. Ice can also increase drag and affect controllability, making stalls more sudden and harder to recover from.

Also,

Why does the aircraft lose complete lift only at a specific speed?

The aircraft does not lose all lift at a specific speed; instead, it loses lift when the angle of attack exceeds the critical value. The stall occurs due to exceeding the critical angle of attack, not merely a reduction in speed. The "stall speed" is the speed at which, under a specific set of conditions, the wing reaches that critical angle of attack. This speed is variable based on weight, configuration, and other factors.

For example, here's the chart for the stall speed of a Cessna 172 by bank angle. It's only one factor, but it shows how the stall speed is changing with bank angle and flap configuration.

(Source)

As the bank angle increases, the load factor does too. This makes the stall speed increase. With flaps this increase in stall speed is reduced slightly, but that also increases drag so more power will be required.

• KIAS = Knots Indicated Airspeed
• KCAS = Knots Calibrated Airspeed

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Articles I'd recommend for you to read on this topic -

1. https://skybrary.aero/articles/true-airspeed
2. https://skybrary.aero/articles/indicated-airspeed-ias
3. https://skybrary.aero/articles/stall

Answer 2

Stall is the configuration of airstream around aerodynamic shape that cause detachment of the airstream from the shape surface. The airflow abruptly changes from laminar to turbulent. As a consequence, the force ballance changes abruptly (from dominant lift to dominant drag)

You can have (wing) stall, controls stall and engine stall. But all work exactly the same.

For wing it means loss of lift, for engine it means lost of thrust (propeller, fan blade) or engine stop (compressor blade stall). For controls it means no effect of the inputs.

The state of the wing (propeller, blade, contol) is defined by relative air speed, angle of attack, current wing (propeller, blade, wing-control assembly) geometry, air density and incomming airflow (laminar, turbulent or mixed). Those are all the variables affecting the stall conditions.

The condition easiest to control is airspeed, then AoA and wing geometry. Rest is out control.

So for one airplane - one registration number, one flight - you have infinite ammout of stall speeds.

Luckily it is quite well behaving function of two/three variables, considering all other reasonably constant.

That's why you have a table of stall speeds.

Flaps are designed to alter stall properties. Higher flaps mean lower stall speed and allow higher AoA than no-flaps. In expense of higher drag.

Angle of attack affects the stall speed significantly, so the stall speeds are tabelated for given flaps and listed AoA.

If you have ice on wings (propeller, fan, blade,...) such tables are of no use - stall speeds are (much) higher, unpredictably higher. In turbulent air (storm cloud, behind another plane,...) such tables are of no use either - you want to get out of such dirty air as soon as possible and regain control.

Secondary parameters are:

• Weight - Heavier plane needs higher AoA to maintain ballance (higher lift demand) and is easier to stall. (Nose down means descending)
• Ballance - Aft centre of gravity "pushes" nose up naturally increasing AoA. Must be countered by elevators. (Elevators are closer to their nose-down limit)
• Yaw, bank - those alter the net force ballance out of the "sideview plane" reducing the wings' efficiency and ballance. (in other words, increase the stall speed)
• Damage, icing - altered geometry means reduced efficiency and increased stall speed
• air pressure (altitude) - higher pressure mean higher air density at the same temperature. Higher altitude means lower air density and higher stall speed.