LOW SPEED FLIGHT
The jet airplane wing, designed primarily for high
speed flight, has relatively poor low speed
characteristics. As opposed to the normal piston
powered airplane, the jet wing has less area, a lower
aspect ratio (long chord/short span), and thin airfoil
shape—all of which amount to less lift. The sweptwing
is additionally penalized at low speeds because the
effective lift, which is perpendicular to the leading
edge, is always less than the airspeed of the airplane
itself. In other words, the airflow on the sweptwing has
the effect of persuading the wing into believing that it
is flying slower than it actually is, but the wing consequently
suffers a loss of lift for a given airspeed at a
given angle of attack.
The first real consequence of poor lift at low speeds is
a high stall speed. The second consequence of poor lift
at low speeds is the manner in which lift and drag vary
with speed in the lower ranges. As a jet airplane is
slowed toward its minimum drag speed (VMD or
L/DMAX), total drag increases at a much greater rate
than lift, resulting in a sinking flightpath. If the pilot
attempts to increase lift by increasing pitch attitude,
airspeed will be further reduced resulting in a further
increase in drag and sink rate as the airplane slides up
the back side of the power curve. The sink rate can be
arrested in one of two ways:
• Pitch attitude can be substantially reduced to
reduce the angle of attack and allow the airplane
to accelerate to a speed above VMD, where steady
flight conditions can be reestablished. This
procedure, however, will invariably result in a
substantial loss of altitude. |
• Thrust can be increased to accelerate the airplane
to a speed above VMD to reestablish steady flight
conditions. It should be remembered that the
amount of thrust required will be quite large. The
amount of thrust must be sufficient to accelerate
the airplane and regain altitude lost. Also, if the
airplane has slid a long way up the back side of
the power required (drag) curve, drag will be
very high and a very large amount of thrust will
be required.
In a typical piston engine airplane, VMD in the clean
configuration is normally at a speed of about 1.3 VS.
[Figure 15-12] Flight below VMD on a piston engine
airplane is well identified and predictable. In contrast,
in a jet airplane flight in the area of VMD (typically 1.5
– 1.6 VS) does not normally produce any noticeable
changes in flying qualities other than a lack of speed
stability—a condition where a decrease in speed leads
to an increase in drag which leads to a further decrease
in speed and hence a speed divergence. A pilot who is
not cognizant of a developing speed divergence may
find a serious sink rate developing at a constant power
setting, and a pitch attitude that appears to be normal.
The fact that drag increases more rapidly than lift,
causing a sinking flightpath, is one of the most
important aspects of jet airplane flying qualities.
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