Flight Training - March/April 2020 - 53

POWER UP
AS THE CG MOVES
FORWARD, THE WING
MUST NOW PRODUCE
MORE LIFT, AND
HENCE THE STALLING
AIRSPEED INCREASES.
The tail-down force opposes wing
lift and increases effective weight. As
the CG moves forward, the wing must
now produce more lift, and hence
the stalling airspeed increases (as
the square root of the effective wing
loading). On the other hand, as the CG
moves rearward, less tail downforce
is required and the stalling speed
decreases. A dynamic wing-loading
reduction of 10 percent will cause an
approximate 5 percent reduction of
the stalling speed. This can go as far
as totally unloading a wing in which
case the stalling speed becomes
zero (nothing to lift). Conventional
airplanes are designed so that CG and
center of pressure are quite close on
the longitudinal axis in the normal
operating range. Many conventional
airplanes often pitch down in a stall
because the horizontal tail stalls
before the main wing.
Less tail-down force with an aft
CG also means less tail trim drag,
resulting in higher cruising airspeed
and better fuel efficiency. Modern
long-range jets transfer fuel automatically in flight to dedicated tail tanks,
resulting in noticeable cruise-speed
increases. However, moving a CG too
far aft reduces pitch stability and it
becomes easier to structurally overstress an airplane while maneuvering.
Moving the CG too far forward generates excessive pitch stability, less
maneuverability, and results in serious landing flare control problems:
running out of up elevator, which is
also degraded by reduced main-wing
downwash in ground effect, inducing
an additional pitch-down moment.

THRUST EFFECTS ON STALL SPEED
BY NIHAD E. DAIDZIC

HOW DOES THRUST/POWER affect stalling

speed? Forces and associated pitching
moments in straight-and-level slow flight
are shown below. Although the pitch
angle and the angle of attack are relatively
large, the flight path is horizontal.

factor. The propeller's induced flow
increases with thrust, which also energizes the boundary layers on the wing
root and the horizontal tail. That is not
bad in itself, but often causes sudden and
rapid nose drop as the normally thin-air-

Lift

Lift < weight
Thrust > drag
Thrust

Drag

Relative wind
Weight

Tail-down force
(Tail lift)

BALANCE OF FORCES in slow flight and the effect that thrust has on the stalling speed. The point

through which the resultant drag forces act will depend on the actual angle of attack, airspeed, configuration, control deflections, interference between boundary layers, and other factors.

If we project all forces on vertical and
horizontal axes, we notice that the horizontal thrust component must offset total
drag (including tail trim drag) in unaccelerated straight and level flight. Hence,
the total thrust intensity must be larger
than drag. On the other hand, the vertical
thrust component opposes weight and
less lift is needed, decreasing the stalling
airspeeds. The vertical thrust component
is normally larger than the tail's downforce at high pitch angles. Thrust effects
can go as far as totally removing the need
for lifting surfaces. Many short-takeoffand-landing propeller-driven airplanes
have powerful engines and with efficient
wing boundary layer control the stalling
speeds are significantly decreased at high
pitch angles, allowing for short takeoff
and landing distances.
However, the propeller thrust in
single-engine airplanes adds another
complexity. Added thrust (induced flow
or propwash) is often a destabilizing

foil horizontal tail surfaces experience
sharp leading-edge stall at high AOAs.
Hence, the power-on stall entries are
normally crisper (depending also on CG
location) than power-off stalls.
In many conventional-tail airplane
designs with tractor propellers, the
go-around power in trim-up landing
approach will cause a sudden pitch-up
because of enhanced propeller induced
flow. If not promptly arrested, it may
cause the airplane to stall and enter an
incipient spin (if any sideslip exists).
While added thrust normally reduces
stalling speeds, it may simultaneously
cause serious control problems in conventional single-engine airplanes. FT
NIHAD DAIDZIC is president of AAR Aerospace

Consulting. He is also a professor of aviation
at Minnesota State University, Mankato. He is
an ATP AMEL and an active gold seal flight
instructor with experience in airplanes, helicopters, and gliders.
AOPA.ORG/FT 53

http://www.AOPA.ORG/FT

Flight Training - March/April 2020

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