Transonic Flight

Question 1

Define transonic flight ?

Answer 1

Part of the free stream airflow around an aircraft is supersonic and part is subsonic

Question 2

What speed range does transonic flight typically occur?

Answer 2

Mach 0.75- Mach 1.2

Question 3

What makes the transonic range so complex ?

Answer 3

Design features having to cater for subsonic flight without compressibility AND supersonic flow with compressibility effects

Question 4

What are the commercial benefits / operational benefits of this speed range?

Answer 4

Efficient fuel burn, Long haul , large payload , affordable.

Question 5

What is the only factor to affect speed of sound ?

Answer 5

Temperature ( formula LSS =38.94 x square root of (273-actual temp)

Question 6

What is compressibility of air?

Answer 6

As an object moves through an air mass it creates disturbances which propagate through the air at the speed of sound.
At low speed the disturbances propagate ahead of the object and create a ‘pressure warning’ which in turn creates air flow direction changes well ahead of the leading edge.
At or above the speed of sound the pressure field cannot influence the airflow ahead as the pressure disturbances can not be propagated fast enough, thus a compression wave forms.

Question 7

When does compressibility become an issue for aircraft?

Answer 7

At some speed below the speed of sound (as low as M0.7) where due to the aerodynamic shape of wings to produce lift, the local speed increases and reaches M1.0

Question 8

What is aspect ratio?

Answer 8

Ratio between wing span vs chord.

Question 9

What type of aspect ratio is most desirable on transonic aircraft and why?

Answer 9

Medium to high aspect ratio

• enables flexibility in the wing and subsequently a smoother ride during turbulence
• wings with high aspect ratio have lower induced drag as it is a function of CL and aspect ratio

Question 10

Define MAC?

Answer 10

The chord drawn through the geographical centre of the wing plan area located along the longitudinal axis.
It is not the average chord, it’s the chord through the centroid of the area.

Question 11

Why is MAC often the primary reference for longitudinal stability and used for weight and balance calculations?

Answer 11

Due to wing sweep.

The smaller the sweep angle, the less effect MAC will have on aircraft stability

Question 12

How does wing sweep effect longitudinal stability?

Answer 12

Less stable due to the large pitching moments created by changes in weight / lift on the wing. Countered by horizontal stabiliser forces. (No sweep means minimal shift in lift /weight forces ).

Question 13

Describe washout and it’s application in transonic flight ?

Answer 13

Gradual change in angle of incidence from wing root to tip. Greatest incidence at the root, (where the wing is strongest) and least at the tip

• The wing will stall at the root first keeping control authority over the tips.
• More spin resistant

Question 14

Describe dihedral and it’s application in transonic flight ?

Answer 14

Is wing angle to the horizontal plane.

• Is used to increase lateral stability by producing more lift on the dropping wing and less on the upward moving wing to restore level flight.
• Reduces effect of dutch roll

Question 15

Why do transonic aircraft have swept wings?

Answer 15

• Delay onset of all compressibility affects
• Mcrit will increase
• Peak drag rise is delayed
• Any change in drag, lift or moment coefficient will be reduced

Essentially, it enables the aircraft to fly faster before suffering the penalty of high speed flight

Question 16

What are some of the disadvantages of swept wings?

Answer 16

• Combining sweep with taper creates a strong tendency to stall at the tip first resulting in reduced lateral control, a nose pitch up due to forward shift in CoP
• Strong tendency for span wise flow at high CL (takeoff and landing)
• Reduces effect of ailerons and flaps compared to straight wing
• Increased design complexity especially at root where twisting effect is carried through the spar

Question 17

Why is MAC often the primary reference for longitudinal stability and used for weight and balance calculations?

Answer 17

Due to wing sweep.

Question 18

What is the taper ratio?

Answer 18

The ratio between the root and tip chords

Question 19

Why is wing taper implemented and what are the advantages?

Answer 19

In order to create natural distribution of lift along the span.
It reduces the vortices therefore reduces induced drag

Question 20

What is Area Rule?

Answer 20

A reduction of cross sectional area of the fuselage to accomodate the cross-sectional area of the wings/engine pylons etc

Question 21

What are the advantages of area ruling?

Answer 21

Drag is a function of the aircrafts total cross-sectional area. By reducing the total area you reduce drag and increase speed for a given level of thrust.
At higher speeds approaching Mcrit, by making the cross-sectional area more streamlined and uniform it, the shock wave and associated wave drag is reduced

Question 22

What are the advantages of winglets?

Answer 22

They improve the aircraft due efficiency by reducing the induced drag spiral produced at the wing tips.

Question 23

Dihedral design is used on transonic wings to ..?

Answer 23

Provide roll stability (lateral stability) and reduce Dutch roll severity.

Question 24

Wing sweep does what to the airflow?

Answer 24

Divides it into 2 components , chord wise and span wise flow. Because the chord wise flow is only a portion of the entire relative flow (depending on degree of sweep ) it delays the air speeding up over the wing chord for any given airspeed and delays mcrit being reached.

Question 25

What amount of sweep is generally needed to produce any significant benefits?

Answer 25

30-35degs.
(30 degrees =8% increase in Mcrit .
45 degrees = 20% increase in Mcrit)

Question 26

Why is Area rule used on tail mounted engine design ?

Answer 26

Reduced drag. increases laminar flow between the engine and fuselage , reducing drag.

Question 27

What is a modern development of the winglet?

Answer 27

Raked wingtips. On increased span wings with raked tips , the same efficiencies can be gained.

Question 28

A super critical wing does what primarily?

Answer 28

Delays formation and reduces the strength of the shockwave on the upper wing surface. (Fly faster with less thrust )

Question 29

What are conventional tail design pros and cons ?

Answer 29

• lightweight design
• can blanket vertical stab, requires larger vertical stab
• can experience downwash from wings
• cannot fit rear mounted engines

Question 30

What are the T-tail pros and cons ?

Answer 30

• vertical stab needs to be able to support horizontal stab
• can be smaller though because it’s out of downwash / blanketing areas
• enables rear mounted engines
• susceptibility to Deep stall, must be mitigated

Question 31

A dorsal fin does what? (Note , is the lower part of the leading edge of the vertical stabiliser)

Answer 31

Improves efficiency of the vertical stabiliser at high angles of attack (yaw) by creating a vortex and thus stalling at higher angles of attack

Question 32

Ventral fins / strakes do what?

Answer 32

Improve lateral stability at high speed as they are not blanketed by the wing. (Decrease unwanted positive lateral stability and improve roll control ). And improve directional stability.