Aerodynamic and Thrust Considerations
It has been known for some time that landing approach path control by elevator or pitch adjustments does not work for low-aspect-ratio (stubby) straight or sweptback wings. This is due to the variation of drag with airspeed when the lift is equal to the gross weight in level flight conditions. We normally expect level-flight drag to increase rapidly with increasing airspeed, and so it does at cruising airspeeds. At cruising airspeeds height control by pitch attitude changes using the elevator is stable and effective. The throttle can be left fixed.
However, the level-flight drag for any airplane increases with decreasing airspeed near the stall, as a result of induced drag increases and flow separation at high angles of attack. As airspeed is reduced from cruising values level-flight drag reaches a minimum and then actually increases again as the airspeed is reduced still further. The airspeed at which level – flight drag, and thrust required to hold level flight, reach minimums was given the name “minimum drag speed” by Stefan Neumark in Britain (1953).
The increase in level-flight drag near the stall is accentuated for airplanes with low-aspect – ratio wings, leading to increases in minimum drag speed. The minimum drag speed for an airplane with a low-aspect-ratio wing can be well above the low approach airspeed desired for carrier landings. Thus, if an airplane with a low-aspect-ratio wing is on a stabilized descent at a low landing approach speed typically used for aircraft carriers and the pilot retrims the airplane to a higher angle of attack, reducing airspeed, the airplane will rise at first relative to the original path and then settle even faster. The flight path will become steeper, a counterintuitive result.
For landing approaches below the minimum drag speed, where increasing thrust is required for decreasing airspeed in level flight, sometimes called “the back side of the thrust required curve,” pitch attitude control by the elevator is unsatisfactory, even with the throttle used to control height. Thrust control by the pilot or an automatic system (the Navy’s APCS) to hold constant airspeed or angle of attack has been used to artificially create the normal variation of thrust required for level flight.
“Backside” carrier-based approach problems were first recognized about 1950 (Shields and Phelan, 1953). Pilots needed to use higher approach speeds forthe XF-88A andXF3H-1 airplanes than the standard rules of thumb based on stalling speed. Shields and Phelan proposed a fixed-throttle pitch-up test maneuver that is similar to a popup maneuver later adopted as one criterion for minimum carrier-approach speed. The first large-scale organized set of data on minimum approach airspeed behavior for jet airplanes was taken at the NACA Ames Aeronautical Laboratory (White, Schlaff, and Drinkwater, 1957). Carrier – type landing approaches were made with seven straight – and swept-wing jet airplanes, the FJ3, F7U-3, F9F-6, F4D, F-100A, F-94C, and the F-84F. The objectives of the 1957 Ames tests was to find the minimum “comfortable” approach airspeeds for carrier-type landings for these representative jet airplanes.
The reason most frequently given by the NACA Ames pilots for minimum approach airspeeds was inability to control precisely altitude or flight path at lower speeds. However, there was a surprising lack of correlation between the minimum comfortable approach airspeed and the Neumark minimum drag speed. For example, Ames pilots set the minimum comfortable approach airspeed for the Douglas F4D-1 Skyray at 121 knots, while the minimum drag speed is 152 knots. Similar results appeared with the North American F-100A Super Sabre, where a minimum approach airspeed of 145 knots was selected, as compared with the minimum drag speed of 150 knots (Figure 12.2). Clearly, some other factors than inability of the elevator or stabilizer to control height without reversal were critical.
Another set of carrier-approach tests (Bezanson, 1961) found that flight path control of the Vought F8U and Douglas F4D-1 airplanes at low landing approach speeds required use of the throttle and was not satisfactory by angle of attack or pitch control modulation alone. Bezanson found that with thrust modulation as the primary path controller the dynamic characteristics of the thrust control system became important, including such factors as throttle friction and breakout force, throttle sensitivity (pounds of thrust per inch of throttle movement), and thrust time lag following abrupt throttle movements.
In contrast to pure jet engines, turboprops are operated at high RPMs all the time. Thrust modulation is done by propeller pitch changes, with very small time lags. The poor engine dynamic behavior of pure jet engines, particularly engine thrust time lag at low power levels (Figure 12.3), kept U. S. Navy interest alive in turboprop combat airplanes long after the U. S. Air Force had switched to pure jets. For example, the Douglas/Navy turboprop A2D-1 Skyshark made its first flight in 1950, the same year as the start of production on the Boeing/Air Force B-47A six-jet bomber.