Category Airplane Stability and Control, Second Edition

A jet or rocket stream issuing from a nozzle acts like a hydrodynamic sink on the surrounding free-stream cold air flow. H. B. Squire and J. Trouncer (1944) produced beautiful isocline maps of the free-stream deviation angles around a jet (Figure 4.6). The sense of the deviation angles is for the surrounding free-stream flow to feed into the jet. Squire and Trouncer’s calculated deviation angles are parameterized in terms of the ratio of jet to free-stream velocities. The larger the velocity ratio, the larger is the deviation angle.

If airspeed is reduced from a trim value at a fixed throttle setting, the ratio of jet to free-stream velocity increases. This increases the free-stream deviation angles into the jet at any given location. In the common case in which the jet passes under the horizontal tail, this increases the effective downwash angle as the speed is reduced. This in turn provides a nose-up pitching moment at speeds below trim, a destabilizing effect. Forward neutral point shifts of as much as 10 percent of the wing mean chord are found for airplanes whose jet exhausts are forward of the horizontal tail. Conversely, only minor stability effects are measured for jet exhausts behind the horizontal tail.

Squire and Trouncer’s calculated stream deviation angles into a jet are for a jet stream at the same temperature as a free stream. A correction is needed to apply their data to the heated jets that come from actual jet or rocket engines. The equivalent cold jet velocity ratio is related to the actual jet velocity ratio by a function involving the ratio of the jet temperature to free-stream temperature.

The longitudinal, lateral, and directional stability of wings and bodies in combi­nation are the isolated characteristics plus effects that reflect modification of the flow by interference. In the longitudinal case, upwash ahead of the wing and downwash behind the wing change the body local angles of attack that enter into the Munk momentum theory calculations. Munk’s apparent mass theory for bodies was extended by Hans Multhopp (1941) to account for the nonconstant fuselage angle of attack due to the wing’s flow field. Gilruth and White (1941) used strip theory for this modification.

Stability and control designers have known for some time that whether an airplane has a high or a low wing influences static directional and lateral stabilities. There was an organized study of this at NACA starting in 1939 as a part of a broader attack on the factors influencing directional and lateral stability. The wing position part of the study was completed in 1941 by House and Wallace.

Distortion of the wing’s spanwise lift distribution and trailing vortex system due to sideslip has the following systematic effects:

Low wing airplanes: Static lateral stability is reduced by about 5 degrees of equivalent wing dihedral as compared with mid-wing airplanes. This rule of thumb has lasted to the present day. Static directional or weathervane stability is increased.

High wing airplanes: The reverse of the low wing case. Dihedral effect is increased by about 5 degrees, weathervane stability is decreased.

Cross-Flow Concept The cross-flow concept aids in understanding aerodynamic forces for an airplane in sideslip. The total velocity vector VEL of an airplane in sideslip can be resolved into a component U along the X or longitudinal body axis and a component V along the Y or lateral body axis. The U component gives rise to a symmetric flow, while the V component gives rise to a hypothetical flow at right angles, along the Y body axis. The component flows add together to make up the total streamline pattern of the airplane in sideslip.

The V or cross-flow component is represented in Figure 6.2. This figure provides an explanation for the effects of high and low wing positions on stability. The effects are the result of the distortion of a wing’s span load distribution in sideslip. Undistorted wing span load distributions feature sharp gradients of load with spanwise distance at both wing tips.

Local shed vortex strength is proportional to this gradient, resulting in the familiar wing tip vortices. The flow of air from higher to lower pressure determines the sense of vortex rotation. Thus wing tip vortices rotate to create downflow, or downwash, inboard of the wing tip.

The center vortices shown in Figure 6.2 are the result of the local span load distortion due to wing-fuselage interference in sideslip. Center vortex rotations for low and high wing arrangements in sideslip are seen to be consistent with the observed stability changes noted above.

The Grumman/American AA-1B Yankee, its Tr-2 trainer version, and the Tiger four-seat variant are clever, innovative personal airplane designs. Compared with most airplanes of the type, which are built as riveted metal structures, metal-to-metal bonding on these airplanes eliminates drag due to rivets and skin waviness and the points of stress concentration common to riveted structures. More than 2,000 AA-1B’s have been built, under several designations. Yet the AA-1B has compiled a sad record of crashes as a result of unrecoverable spins. The AA-1B has a flat spin mode that leads to a high-impact vertical crash with the fuselage level, a crash that has made paraplegics out of student pilots and their instructors.

The airplane’s three-view diagram suggests that NACA spin recovery criteria were quite disregarded in the original design. The horizontal tail is mounted low on the fuselage, providing very little tail damping ratio, or TDR (Figure 9.7). By NACA correlation, this would promote a high angle of attack, or flat spin. Also, there appears to be virtually no unshielded rudder area, the NACA URVC factor.

The AA-1B’s poor spin and recovery characteristics are recognized in the 1975 version of the owner’s manual. In both the operating procedures and operating limitations sections

the legend “SPINS ARE PROHIBITED” is displayed. This is followed by the recovery technique in the event of inadvertent spins, and the note:

If recovery controls are not briskly applied in the first turn, more than one additional turn will be required for recovery. For quick recovery, apply full anti-spin controls as the spin begins, before one turn is completed.

A later version of the owner’s manual has wording that reflects actual NASA flight-testing experience:

There is evidence that permitting the airplane to go beyond one turn without initiating proper recovery procedures can allow a spin mode to develop from which recovery is not possible.

To illustrate this point, the 1/15 December 1990 issue of Aviation Consumer reports that American Aviation test pilot Bob Hommel was forced to jump when a modified AA-1A failed to recover during a spin test. The airplane was said to have completed over 100 turns before crashing. There is no reason to think that the AA-1B airplane is unique in having recovery problems in spins that go beyond one turn. James S. Bowman, Jr., writes as follows:

I think it is important to mention that all normal category airplanes are tested for one-turn spins only and if taken beyond one turn, recovery may be slow, or there may be no recovery at all.

Hypersonic flight is generally understood to mean flight at Mach numbers above 5.0. The only experience with manned airplanes at hypersonic speeds has come from the North American/NASA X-15 and space shuttle Orbiter programs. Stability and control phenomena at hypersonic speeds are qualitatively no different than at moderate supersonic speeds. There is the same relative loss in the effectiveness of lifting stabilizing surfaces relative to fuselage-destabilizing moments. The high altitudes at which hypersonic flights are carried out lengthen the periods of uncontrolled motions, always a piloting problem.

The influence of the propulsion system on aerodynamic forces and moments is expected to be more extreme in powered hypersonic flight than at lower speeds. The reverse is also true in that slight sideslip angles could cause severe inlet problems, depending on details of the design. Sufficient control surface authority may be required to overcome yawing, pitching, and rolling moments caused by engine inlet unstarts precisely at altitudes where control moments are low because of low air density.

However, the most pressing stability and control problems of hypersonic airplanes are probably encountered at low speeds, as a result of the unique design features that go along with hypersonic flight. Wing slats or drooped leading edges that could improve low-speed longitudinal and directional stability are apparently ruled out because of aerodynamic heat­ing problems at seams in the forward lower wing surface. A hypersonic passenger airplane for long over-ocean flights remains an interesting, but probably distant, goal for aviation planners. The aerodynamic research that has gone into this concept so far has quite prop­erly dealt mostly with performance and aerodynamic heating. Conceptual designs that have been published show configurations that look like stretched-out space shuttle Orbiters.

While proper stability and control design, supplemented by artificial means such as control centering devices and wing levelers, are fundamental to safe airplanes, some

safety deficiencies can apparently be made up with the right kind of cockpit displays or instruments (Loschke, Barber, Enevoldson, and McMurty, 1974). They reported that on a light twin-engine airplane, a flight director display is of significant benefit during ILS approaches in turbulent air.

Heavy pilot workload during such approaches had been found in an earlier survey of general-aviation airplanes, making precise instrument tracking difficult even for experienced instrument pilots. However, the flight director instrument, which combines inputs from attitude and rate gyros and in effect tells the pilot how to move the controls, reduces somewhat this excessive workload. Even greater improvements in tracking during ILS approaches in turbulence are found when the flight director display is combined with an attitude-command autopilot.

The experiences of an airline pilot in operating heavily automated passenger-jet airplanes is relevant to the improvements provided by flight directors and automatic pilots for general – aviation airplanes. The question is how to use these devices to enhance safety for the average pilot under all operating conditions. William M. Ferree of Mount Vernon, New Hampshire, writes as follows (1994):

I’ve been a professional pilot for over 20 years and currently fly the Boeing 757 and 767. A high level of automation gives these excellent airplanes capabilities that would have been remarkable a few years ago. However, the design breaks down when some significant change of plan is introduced, which may happen because of an equipment failure or, more commonly, because of difficulty with the air traffic control system or the weather. The problem is that unless the computer is reprogrammed in these situations, it is useless. And reprogramming must often be done while landing preparation is being completed, which is an extremely busy time.

Ferree goes on to note that the 757 and 767 autopilot/flight director control panel consists of a few knobs for selecting things such as airspeed and altitude and many identical square push buttons. The panel cannot be operated by feel. The pilot must look at it in order to operate it. The argument for readily reprogrammed equipment in commercial-transport airplanes must be even more valid for general-aviation flight directors and automatic pilots.

Today’s heavily automated passenger jet airplanes have multifunction cathode ray or flat – screen displays of all flight and engine instruments, quadruplicated as backups for failure. Yet, a few old-style instruments are carried as additional insurance. All Boeing transports from the 707 on have small standby vacuum-driven gyro horizons, just to the side of the central instrument panel. This will give several minutes of reliable indication after a power failure, due to the inertia of the gyro’s rotor.

Another question related to automation of passenger jets is whether automation is re­ducing the competence needed in pilots. Wyatt Cook, an American Airlines pilot, reports that in flight training at the Dallas facility, one of the two pilots is required to be on raw data, meaning the VOR and ILS radio guidance signals. William H. Cook, Wyatt’s father, writes, “The basics [VOR and ILS] require a lot of work.”