SAMRA/SARON Theory of Flight Questions

Load factor, expressed in 'g', is defined as

• the ratio of thrust to drag at any given moment in flight.
• the aircraft's actual weight, expressed in units of gravitational acceleration.
• the ratio of the total lift being generated to the aircraft's current weight.
• the ratio of indicated airspeed to true airspeed at that moment.

Explanation: Load factor is lift divided by weight. In straight-and-level, unaccelerated flight it is 1g (lift equals weight); in a coordinated turn or a pull-up, lift must exceed weight to provide the additional centripetal force or curve the flight path, so load factor rises above 1g.

TP 690 Section 6 - Forces Acting on an Aeroplane, Load Factor

Because ice accumulation reduces the critical angle of attack at which a surface stalls, an iced tailplane is at particular risk of stalling when

• the aircraft is flown at a noticeably higher airspeed than normal, since tailplane stall risk is assumed to increase with speed.
• the landing gear is retracted, since gear position is assumed to be what primarily determines tailplane loading in flight.
• flaps are extended, since flap extension increases downwash angle, raising the tailplane's effective angle of attack it must counter.
• flaps are retracted, since flap retraction always increases tailplane angle of attack regardless of downwash.

Explanation: Extending flaps increases the wing's downwash, which increases the tailplane's effective angle of attack as it works to counter that downwash -- on a tailplane already compromised by ice (with a lower critical angle of attack than normal), that added angle of attack demand from flap extension is exactly the kind of trigger that can push it into a tailplane stall, which is why some icing procedures call for caution with flap selection in known or suspected tailplane icing conditions.

TP 690 Section 6 - Effects of In-Flight Icing, Wing and Tailplane Stalls

Aileron reversal, a high-speed aeroelastic phenomenon, occurs when

• the ailerons physically jam in a neutral position at high speed, producing no rolling moment in either direction.
• the autopilot's roll channel malfunctions and commands aileron deflection opposite to what the flight director indicates.
• aerodynamic loads from aileron deflection twist a sufficiently flexible wing enough that the resulting change in the wing's effective angle of attack overcomes, and reverses, the intended rolling effect of the aileron.
• a pilot deflects the ailerons in the wrong direction during a crosswind landing, an entirely pilot-induced error unrelated to any structural or aerodynamic effect. This follows from the underlying physical principle governing the instrument's operation.

Explanation: At sufficiently high speed and dynamic pressure, a wing (especially a flexible, high-aspect-ratio one) can twist enough under the aerodynamic load created by aileron deflection that the change in local angle of attack from that twist actually overwhelms and reverses the aileron's intended rolling effect -- a genuine aeroelastic (structure-meets-aerodynamics) phenomenon, not a pilot input error, autopilot fault, or mechanical jam, and one of the reasons wing torsional stiffness is a real design consideration.

TP 690 Section 6 - Theory of Flight, High-Speed Flight

Vortex generators, small vanes sometimes fitted to a wing's upper surface, improve high-angle-of-attack performance by

• physically blocking airflow over a portion of the wing, deliberately inducing earlier separation and an earlier stall in that region.
• reducing the wing's overall surface area exposed to the airflow, lowering both lift and drag simultaneously.
• creating small vortices that mix high-energy airflow from outside the boundary layer into the boundary layer itself, helping it resist separation to a higher angle of attack.
• generating additional engine thrust, an effect entirely unrelated to boundary layer or airflow behaviour over the wing, a misconception occasionally reinforced by their small, blade-like appearance on the wing's upper surface.

Explanation: Vortex generators work by deliberately introducing small vortices that draw higher-energy air from outside the boundary layer down into it -- re-energizing a boundary layer that would otherwise be more prone to separating under an adverse pressure gradient, which delays separation (and therefore stall) to a higher angle of attack in the areas where they're effective, a very different purpose from the deliberate stall-strip concept (which does intentionally provoke early, controlled separation) or any thrust- or area-reducing function.

TP 690 Section 6 - Theory of Flight

A stabilator (an all-moving horizontal tail), as distinguished from a conventional fixed horizontal stabilizer with a hinged elevator, provides pitch control by

• using exactly the same mechanism as a conventional elevator, with 'stabilator' simply being an alternate name describing an identical hinged-elevator design.
• functioning only as a fixed, non-moving surface, with 'all-moving' in its common description being an inaccurate characterization of how a stabilator actually operates.
• eliminating the horizontal tail surface entirely, achieving pitch control through some means other than any tail surface at all.
• moving the entire horizontal tail surface as a single unit, rather than deflecting only a smaller, separately hinged elevator surface relative to a fixed stabilizer.

Explanation: A stabilator pivots the entire horizontal tail surface as a single unit to produce a pitch control input, rather than keeping a fixed stabilizer in place and deflecting only a smaller, separately hinged elevator surface relative to it -- moving the whole tail surface generally provides greater pitch control authority (useful, for example, at high speed or with an aft CG) than a conventional elevator design can achieve for a comparably sized tail, a genuinely different mechanical arrangement from a conventional elevator, and it very much still relies on a tail surface (just one that moves as a whole) rather than eliminating the tail.

TP 690 Section 6 - Theory of Flight

An aircraft's drag divergence Mach number, as distinguished from its critical Mach number, is

• generally a somewhat higher Mach number than critical Mach, at which the sharp rise in drag caused by developing shock waves and their associated flow separation becomes pronounced enough to be operationally significant.
• always a lower Mach number than critical Mach, occurring well before any local airflow over the wing, fuselage, or empennage has actually accelerated enough to reach the speed of sound anywhere at all on the airframe surface.
• unrelated to shock waves or drag rise at all, and instead refers strictly and exclusively to the onset of engine compressor stall occurring at high Mach numbers during cruise.
• always numerically identical to critical Mach number in every flight condition, with the two terms simply being alternative names applied to one and the same underlying value.

Explanation: Critical Mach number is the free-stream Mach number at which local airflow somewhere on the airframe first reaches the speed of sound (Mach 1); drag divergence Mach number is generally somewhat higher, marking the point at which the resulting shock waves and shock-induced flow separation have grown enough to cause a sharp, operationally significant rise in total drag -- the two are related but distinct milestones along the same speed increase.

TP 690 -- Theory of Flight: High-Speed Aerodynamics

Power loading, expressed as an aircraft's weight divided by its available power (or thrust), is most directly related to

• climb performance, since a lower power loading (more power available per unit of weight) generally provides greater excess power and therefore a better rate of climb.
• maximum operating Mach number, which is determined entirely by power loading regardless of wing or airframe design.
• stalling speed, since power loading and wing loading are simply two names for the same ratio.
• the aircraft's empty weight, since power loading is calculated using empty weight rather than gross weight.

Explanation: Power loading (weight divided by available power or thrust) is most directly tied to climb performance: an aircraft with a lower power loading has more power available relative to its weight, giving it greater excess power (or thrust) beyond that required for level flight, which translates directly into a better rate of climb -- this is distinct from wing loading, which relates more to stalling speed, gust response, and maneuvering.

TP 690 -- Theory of Flight: Climb Performance

Aerodynamic washout, achieved by varying the airfoil section or camber progressively along the wing's span, differs from geometric washout (built-in physical twist reducing the wingtip's angle of incidence) in that aerodynamic washout

• is simply another name for geometric washout, with the two terms describing an identical physical construction technique, an assumption inconsistent with how lift, drag, and stability actually behave across the flight envelope, which overlooks the underlying physical mechanism that actually governs this aerodynamic effect.
• achieves a similar goal -- delaying wingtip stall relative to the wing root -- without physically twisting the wing structure, instead relying on a spanwise change in airfoil shape to give the tip a lower local lift coefficient at a given angle of attack than the root.
• increases, rather than decreases, the likelihood that the wingtip stalls before the wing root, the opposite effect from geometric washout, an assumption inconsistent with how lift, drag, and stability actually behave across the flight envelope, a description not supported by standard aerodynamic principles or flight test experience.
• can only be applied to a wing that has zero sweep, with no application to a swept-wing aircraft design, an assumption inconsistent with how lift, drag, and stability actually behave across the flight envelope, which overlooks the underlying physical mechanism that actually governs this aerodynamic effect.

Explanation: Where geometric washout achieves a lower effective angle of attack at the wingtip through physical, built-in twist of the wing structure, aerodynamic washout achieves a broadly similar practical effect -- encouraging the wing root to stall before the wingtip, preserving aileron effectiveness and roll control longer into the stall -- without physically twisting the wing, instead varying the airfoil section or camber along the span so the tip naturally produces a lower local lift coefficient than the root at a given angle of attack.

TP 690 -- Theory of Flight: Wing Design