We are taught early in our flight training that the reason
an airplane stalls has to do with angle of attack (AoA) and
not airspeed. Yet we tend to think, talk, and fly in terms
of stall speeds. The reason is rather simple: light
airplanes have been conventionally equipped with airspeed
indicators, not AoA indicators. Moreover, aircraft
manufacturers provide us with stall speeds in their flight
manuals.
Even though stalling is all about AoA, stall speeds provide
us with convenient reference points for our flight
operations. Unfortunately, the focus on “stall speed”
sometimes leads to confusion and misunderstanding. The
reality is that speed by itself is a poor surrogate for AoA.
But the real confusion that traditionally comes into play is
the affect of weight. Pilots tend to overlook it when
considering "stall speed" and misunderstand the roll it
plays in stall dynamics.
So what is an airplane's stall speed? The answer is, "It
depends."
If we frame the question to include a reference to weight,
meaningful answers follow: What is the stall speed at “X”
pounds? What is the stall speed at “Y” Gs? What is the stall
speed at “Z” degrees angle of bank (bank angle suggests load
factor)? A specific stall speed can be provided in response
to each of these questions.
Weight must be included in any discussion about stall speed;
the two are connected mathematically. The relentless pull of
gravity is balanced by lift from the wing during normal
flight operations such as steady, level flight. This
balancing act is what links an airplane’s stall speed to its
weight.
Lift is one of the forces generated as a result of air
flowing around a wing. Its magnitude depends upon
four
variables:
air density, airspeed, wing area, and coefficient of lift.
As the pilot, you directly control lift by changing the
wing’s AoA. For a constant amount of lift in unstalled
flight, an increase in AoA results in an increase in
coefficient of lift, but a decrease in airspeed. A decrease
in AoA results in a decrease in coefficient of lift, but an
increase in airspeed.
An airplane’s weight at any point during flight is the sum
of two parts: gross weight plus induced weight. Load factor
is a measure of the combined effects of gross and induced
weights. Gross weight consists of all the components of the
airplane plus all the stuff loaded into it, e.g., fuel, oil,
people, and baggage. Think of gross weight as the number
seen on a scale before you take off. It is the same value
used for weight and balance calculations.
By comparison,
induced weight can be imposed by elevator inputs and gust
loads/turbulence. A coordinated, level turn at 60 degrees of
bank in smooth air, for instance, results in an airplane
that feels and acts twice as heavy compared to wings-level
flight.
As
shown in the graphic, lift equals weight (L=W)
when we are in steady, level flight. And when
flying at the critical AoA, we also have the
maximum coefficient of lift. These conditions
create the connection between weight and stall
speed. All things
equal, speed is the only variable that can
change to preserve L=W as weight changes—in this
case, the stall speed at critical AoA. Higher
weight, higher stall speed; Lower weight, lower
stall speed so that lift can continue to equal
the weight.
Working with Stall Speeds
By definition, stall speed (Vs)
is either the stalling speed, or the minimum
steady flight speed at which the airplane is
controllable. Consider the former definition for
our purposes. When at Vs, we are operating at
the wing’s critical angle of attack with maximum
coefficient of lift.
Stall speeds are usually published for an
airplane in particular configurations at maximum
gross weight. For example, the Pilot’s Operating
Handbook for the 1978 Cessna 152 has stall speed
tables for max gross weight with the power off.
Stall speeds are shown as a function of flap
deflection and bank angle. “Bank angle” is an
oblique reference to the load factor necessary
to maintain a level turn. As bank angle
increases, so does the required load factor, and
so does the stall speed. Shallowing the bank
reduces the stall speed. Operating the airplane
at a lower gross weight similarly reduces stall
speeds as well.
Indicated airspeed (IAS) is shown on the
airspeed indicator. In contrast,
calibrated airspeed (CAS) is IAS
corrected for instrument and
installation errors. When performing
calculations involving stall speeds, it
is important to use CAS. This often
requires consulting the airspeed
calibration chart provided in the
airplane’s flight manual to convert
between IAS and CAS.
The Curious Case of Maneuvering Speed
Ok, so what does this have to do with maneuvering speed? The
answer is simple once you know the code. Design maneuvering
speed, Va, is the maximum speed at which you can move a
single flight control to its full deflection without risk of
damage to the airplane. Instead of generating structural
loads that could bend or break the airplane, it stalls
instead. In other words, Va is the stall speed corresponding
to a specified design limit load factor (G-load).
That’s right. Va is just another stall speed.
An airplane’s design limit with its flaps up and no rolling
in the Normal category, for example, must be at least +3.8G.
The corresponding stall speed at this load factor is roughly
double the +1.0G stall speed, or 2.0Vs (CAS).
Think of Va as the “upper limit” stall speed for a given
weight and configuration. In the range of speeds from Vs
through Va (i.e., 1.0Vs–2.0Vs CAS), the airplane will stall
before it can bend or break. The stall acts as an
aerodynamic relief valve to prevent structural damage. Above
Va, however, the airplane could suffer structural damage or
failure before the stall can intervene.
As you play with the interactive graphic, notice how lift,
Vs, and Va follow the weight trend. Increasing weight
necessitates an increase in lift. All other things equal at
the critical angle of attack, the additional lift can only
be achieved with a higher Vs. Conversely, decreasing weight
requires a decrease in lift. All other things equal at
critical angle of attack, the reduced lift is achieved at a
lower Vs. Since Va is a stall speed, it changes as well.
Rich Stowell is the 2014 National FAASTeam
Representative of the Year and the 2006 National Flight
Instructor of the Year.
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