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Figure 13.41 shows a series of expansion waves in a supersonic airflow. After passing through the bow shock wave, the compressed supersonic flow is free to expand and follow the surface contour. As there are no sudden changes to the airflow, the expansion waves are NOT shock waves. A supersonic airflow passing through an expansion wave will experience the following changes:-

•The airflow is accelerated; the velocity and Mach number behind the expansion wave are greater.

•The flow direction is changed to follow the surface.

•The static pressure of the airflow behind the expansion wave is decreased.

•The density of the airflow behind the expansion wave is decreased.

•Since the flow change is gradual there is no “shock” and no loss of energy in the airflow. An expansion wave does not dissipate airflow energy.

Sonic Bang

The intensity of shock waves reduces with distance from the aircraft, but the pressure waves can be of sufficient magnitude to create a disturbance on the ground. Thus, “sonic bangs” are a consequence of supersonic flight. The pressure waves move with aircraft ground speed over the earth surface.

Methods of Improving Control at Transonic Speeds

It has been seen that control effectiveness may decrease in the transonic region if conventional control surfaces are used. Some improvement in control effectiveness may be obtained by placing vortex generators ahead of control surfaces.

However, alternative forms of control such as:

•an all moving (slab) tailplane

•roll control spoilers give better control in the transonic speed region.

These types of control are explained in Flying Controls Chapter 11. Control surface buzz is sometimes remedied by fitting narrow strips along the trailing edge of the control surface, or it may be prevented by including dampers in the control system or by increasing the stiffness of the control circuit.

Because of the high control loads involved at high speeds and the variation in loads through the transonic region, the controls will normally be fully power operated with artificial feel.

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13 High Speed Flight

Flight Speed High 13

The table in Figure 13.42 is provided to summarize the characteristics of the three principal wave forms encountered with supersonic flow.

Supersonic Wave Characteristics

TYPE OF WAVE	OBLIQUE Shock wave		NORMAL Shock wave						EXPANSION wave

	DEFINITION	A PLANE OF DISCONTINUITY,	A PLANE OF DISCONTINUITY,
		INCLINED MORE THAN 90º	NORMAL TO FLOW DIRECTION
		FROM FLOW DIRECTION
	FLOW DIRECTION	TURNED INTO A PRECEDING	NO CHANGE	TURNED AWAY FROM
	CHANGE	FLOW		PRECEDING FLOW
	EFFECT ON VELOCITY	DECREASED BUT STILL	DECREASED TO SUBSONIC	INCREASED TO HIGHER
	and MACH NUMBER.	SUPERSONIC		SUPERSONIC
	BEHIND WAVE
	EFFECT ON STATIC	INCREASE	GREAT INCREASE	DECREASE
	PRESSURE and DENSITY
	EFFECT ON ENERGY	DECREASE	GREAT DECREASE	NO CHANGE ( NO SHOCK )
	OF AIRFLOW
	EFFECT ON	INCREASE	INCREASE	DECREASE
	TEMPERATURE
		Figure 13.42 Characteristics of the three principle wave forms

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Sweepback - Fact Sheet

Sweep Angle: The angle between the line of 25% chords and a perpendicular to the root chord.

Purpose of Sweepback: To increase MCRIT.

A Swept Wing Increases the Critical Mach Number (MCRIT).

All other effects from a swept wing are by-products, most of them disadvantages. However, the benefits from a higher MCRIT outweigh the associated disadvantages.

By-products of Sweepback

1.Increased tendency to stall at the tip first - minimized by fitting wing fences, vortilons or saw tooth leading edges.

•Tip stall can lead to pitch-up, a major disadvantage.

•Pitch-up can give the tendency for a swept wing aircraft to Super Stall.

•Aircraft that show a significant tendency to pitch-up at the stall MUST be fitted with a stall prevention device; a stick pusher.

Close to the stall, ailerons and coordinated use of rudder should be used to maintain wings level because the use of rudder alone would give excessive rolling moments. (VSR is adjusted so that adequate roll control exists from the use of ailerons close to the stall).

2.When compared to a straight wing of the same section, a swept wing is less aerodynamically efficient.

•At a given angle of attack CL is less.

•CLMAX is less and occurs at a higher angle of attack.

•The lift curve has a smaller gradient (change in CL per degree change in alpha is less).

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C L	HIGH ASPECT
	RATIO

LOW ASPECT RATIO

(or sweepback)

Figure 13.43

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13 High Speed Flight

• Swept wings must be fitted with complex high lift devices, both leading and trailing

		edge, to give a reasonable take-off and landing distance.
		◦◦	The least efficient type of leading edge device is used on the inboard part of the
			swept wing to help promote root stall.
		• Because of the higher stalling angle of attack, the fin or vertical stabilizer is swept to
		delay fin stall to a greater sideslip angle.
		• A swept wing must be flown at a higher angle of attack than a straight wing to give the
		required lift coefficient; this is most noticeable at low speeds.
		• One of the few advantages of a swept wing is that it is less sensitive to changes in angle
		of attack due to gust or turbulence; a smaller change in Load Factor for a given gust will
		result.
		3.	A swept wing makes a small positive contribution to static directional stability.
		4.	A swept wing makes a significant positive contribution to static lateral stability.
		5.	At speeds in excess of MCRIT a swept wing generates a nose-down pitching moment;
	13		a phenomena known as Mach Tuck, High Speed Tuck or Tuck Under. This must be
			counteracted by a Mach Trim System which adjusts the aircraft’s longitudinal trim.
	High
		6.	The hinge line of trailing edge ‘flap’ type control surfaces are not at right angles to the
	FlightSpeed
			airflow, which reduces the efficiency of the controls.

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