More Aerodynamic Musings

Angle of Attack (AoA) vs Incidence/datum and aircraft “sit”. 

The Angle of Attack is the angle made between the airflow meeting the wing and the airfoil maximum chord line. 

Wing incidence is the angle between the aircraft longitudinal datum set by the designer and the airfoil maximum chord line. 

The maximum chord line joins the foremost point of the L.E with the rearmost point of the T.E. The aircraft longitudinal datum is defined at the design stage and is a line drawn through the longitudinal axis in side elevation. 

The tailplane setting angle is the angle that the tailplane is set in relation to the wing incidence and determines the aircraft “sit”. See also `Note` below. The job of the “sit” is to present the minimum drag profile to the airflow at the aircrafts` normal operational airspeed. As an extreme example see the Armstrong Whitworth Whitley bomber of the late 1930`s. One has to ask the question,” Was this the result of poor aerodynamic design?” as the fuselage certainly does not seem to be presented to the airflow at the min drag angle. On model aircraft the “sit” is determined by the designer and, generally speaking, there is not much one can do to alter this. However if your model appears to be sitting tail down, especially in a turn, it can be caused by the model being flown too slowly, ie low power and holding the stick back to maintain height,  or having the balance too far aft. Note. See also “Some Aerodynamic Musings” under the Cmo explanation which also determines the Tailplane Setting Angle. 

WING TWIST OR WASHOUT.  Wash-out is defined as the gradual reduction in incidence towards the tip of the wing. As the wing approaches the stall angle (Maximum AoA) it is the inboard section which therefore stalls first, leaving the outer section still lifting. As lift becomes insufficient to maintain level flight the aircraft loses height, the nose drops and the outboard section begins to generate lift before the inboard section so helping to prevent a wing dropping. Notice the use of the phrase “stall angle”. 

The phrase “stall speed” is used all too often and is used to explain that the wing has reached a low enough speed to stall. While this is partly true a wing will/can/is able to stall at any speed. All that is required is, even at Vne (Never Exceed Speed), a large enough up elevator movement (high speed stall). The speed at which an aircraft stalls (Vs) is dependant on its weight but the stall angle will never change as it is a phenomenon of the particular airfoil section used.

Washout in full size civil aviation swept wing aircraft. Engines are now universally wing mounted and airframes are mostly (generally speaking) of aerolastic construction. This means that every airframe component moves and twists with changing aerodynamic loads. It will be noticed that the engines have the intakes way out in front of the wing L.E`s and it can be imagined that with each engine on a 747/400 for example, weighing in the region of 5500kgs this, due to the afore mentioned type of construction, will impart some twist to the wing…..washout and for the same reason we use it!

I operated for some years on the Super VC10 and that had a clean wing with the engines mounted at the tail end. This aircraft was also of aerolastic construction and twist it certainly did. The wing was so bendy and twisty that below a certain weight, 132,135kgs I think the figure was (Peter Maillard may remember) and below a certain height (16000ft, Peter?) the ailerons were upfloated by means of a switch on the flight deck. This had the effect of pushing down on the wings so relieving the air loads. Above these parameters the weight of fuel in the wings did the job.

An exciting phenomenon which was exhibited, but only during training flights, was what was termed “Mach tuck”. This was also caused by the flexibility of the wing and occurred at a mach number of M.93. (Peter, help!). The normal operating mach number during service operations was M.84. The aircraft speed was increased to the Mne (Never exceed mach number) and the aircraft then, and very suddenly, went into a nose down trim…….mach tuck. The drill was to slam the throttles shut and deploy the airbrakes to slow the aircraft. The airbrakes were the only things which were capable of slowing the machine down even thought the engines were decelerating, which took 4-5 seconds. In this time the speed would have increased due to the enormous inertia forces and the Mach crit. would have been exceeded (Critical mach number). Mach crit was the speed at which the local mach number reached Mach 1 which was at the tailplane where it crosses the fin and if this occurred the fin and tailp[lane could have parted company.

But why the mach tuck? In straight & level flight, as airspeed increases, the wing AoA decreases due to retrimming ( for all airfoils) and the centre of pressure moves rearwards. In the SVC10 case the wing was twisty enough for the rearward pressure to twist the wing (L.E down) so that the zero lift angle (AoA at which all lift ceases) was exceeded and the wing went into negative lift.


Mike White