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 relative to the airplane’s weight, a lower aspect ratio (long chord/short span), and thin airfoil shape—all of which amount to the need for speed to generate enough lift. The sweptwing is additionally penalized at low speeds because its effective lift is proportional to airflow speed that is perpendicular to the leading edge. This airflow speed 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.
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 at those low speeds. As a jet airplane is slowed toward its minimum drag speed (VMD or L/DMAX), total drag increases at a much greater rate than the changes in lift, resulting in a sinking flightpath. If the pilot attempts to increase lift by increasing the AOA, 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-required curve. The sink rate can be arrested in one of two ways:
- Pitch attitude can be substantially reduced to reduce the AOA 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. 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.