exam 2011-01-18

Student number:

Course code: AE4 -240 AE4-211 (please encircle the appropriate course code)

1 Exam AE4-240/211 January 18, 2010

Question 1 Aeroelasticity

Consider the span wise lift distribution at a given lift coefficient of an undistorted, aft-swept wing at Mach 0.85. During cruise distortion by bending occurs (the distortion through torsion itself is neglected).

a) In a front view, show the position of the front and the rear spar due to the bending of the wing under 1-g

loading.

b) Sketch in the figure below the position and orientation of the tip airfoil in jig shape and under 1-g

conditions.

jig shape 1-g conditions

z

yjig shape

x

z

x

z

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2 Exam AE4-240/211 January 18, 2010

c) Sketch in the following figure the spanwise lift distribution for the same lift coefficient of the wing in

undistorted (jig-shape) conditions and for the wing under 1-g conditions. Clearly state which line

corresponds to which situation.

d) Explain what happens to the position (both lateral and longitudinal) of the aerodynamic centre of the wing

when it deforms from its jig shape to its 1-g shape.

e) Indicate the difference in lift-curve slope between a rigid and a flexible wing at high Mach numbers and

explain the cause for this difference.

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3 Exam AE4-240/211 January 18, 2010

Question 2 Aeroelasticity and Mach effects

In the figure below the lift-curve slope of the A300 wing (CLαw) is plotted as a function of Mach number. Two wings

are considered: one being flexible and one being rigid.

a) For the rigid wing, explain the gradual increase of the lift curve-slope with Mach number.

b) For the rigid wing, explain why the lift-curve slope has a maximum around M=0.85 and decreases beyond

that point.

c) Carefully explain why the flexible wing has a lower lift-curve slope than the rigid wing over a wide range of

Mach numbers.

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4 Exam AE4-240/211 January 18, 2010

d) Carefully explain why the lift-curve slope is higher at 11km than at sea level.

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5 Exam AE4-240/211 January 18, 2010

Question 3 Deep stall on BAC-111

To the left the BAC-111 is depicted. This aircraft experienced two crashes

due to deep stall (one with fatal results), the first one 1963. The deep stall

originated from the nacelles

a) Explain the aerodynamic phenomenon of deep stall in relation to

.

the geometry of the BAC-111.

b) Carefully explain why controllability is significantly reduced during a deep stall.

c) Name two aerodynamic measures that can be taken to ensure sufficient longitudinal stability at high angles

of attack and hence avoid deep stall. Explain your answers

d) Name one non-aerodynamic measure that can be taken to avoid the angles of attack at which deep stall

occurs?

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6 Exam AE4-240/211 January 18, 2010

Question 4 Engine intake

The intake of a jet engine needs to function properly during take-off, cruise, and one-engine-inoperative condtions.

a) Why is for the intake of a jet engine between the throat and the fan face a diffuser desired?

b) In the figures below draw the stream tube in front of the intake in take-off and cruise conditions. In

particular indicate the following items in both figures: position of the stagnation point and the critical area

for separation

high-speed low-speed

c) An important measure for the intake effectiveness is the total pressure recovery of the flow between the

highlight and the fan face. Explain why a loss of total pressure reduces the efficiency of the engine in high-

speed conditions.

d) In order to keep the pressure recovery close to 1, a rule of thumb is to keep the average throat Mach

number below M=0.8 in all conditions. Explain what happens to the flow when the average throat Mach

number increases beyond M=0.8 and how this affects the total pressure recovery.

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7 Exam AE4-240/211 January 18, 2010

e) Explain why in the figure below the pressure recovery improves when the contraction ratio grows.

Low-speed inlet performance (belonging to e and f) Fan cowl drag (belonging to g)

f) What are blow-in doors and why do they increase the pressure recovery at low taxi speeds and a

contraction ratio of 1.1?

g) Explain why, in the figure above, the larger contraction ratio result in higher cowl drag at M=0.98.

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8 Exam AE4-240/211 January 18, 2010

Question 5 Forward Swept Wings

Near the end of WWII the Germans applied the swept-wing concept to attain higher speeds. An example is the

Junkers, Ju-287, which was a jet-powered bomber (see figure below). This aircraft had the peculiar feature that the

wings were swept forwad.

a) Explain the general principle of a swept wing using a sketch (below) and a clear description.

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9 Exam AE4-240/211 January 18, 2010

b) Explain the aerodynamic reason why a forward swept wing was preferred over an aft swept wing?

Carefully, elaborate the advantageous consequence this had on the lateral stability and control.

c) Name two important drawbacks of forward swept wings and explain the physical causes of each these

drawbacks.

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10 Exam AE4-240/211 January 18, 2010

Question 6 High-lift devices

In the figure below you see three profiles with equal length at a given angle of attack. The profiles are positioned

such that the trailing edge of the ‘predecessor’ is positioned above the leading edge of the proceeding profile with a

2% slot in between them. When the pressure distribution of each of the profiles would be measured separately from

the others the dashed lines would be the result

a) Sketch the pressure distribution over the three profiles now that they are positioned together.

b) Give two reasons why the slots between individual components are beneficial for achieving a high

maximum lift coefficient. Motivate your answers.

+1

cp

-3

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11 Exam AE4-240/211 January 18, 2010

c) The effect of Reynolds number on the minimum pressure coefficient is shown in the next figure. Carefully

explain the relationship between the minimum pressure coefficient and the Reynolds number.

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12 Exam AE4-240/211 January 18, 2010

Question 7 ATR 42/72

The ATR 72 and its smaller version (the 42) are twin turboprop airplanes that have experienced some difficulty in

icing weather conditions. In June 1986, an ATR-42 on a training flight pitched down unexpectedly. The FAA blamed

the incident on ice on the plane's tail and warned pilots to avoid a certain flap setting. On different occasions the

plane displayed a sudden rolling motion leading in at least two cases to catastrophic crashes.

a) Carefully explain how a certain flap setting can cause the sudden nose-down pitching moment?

b) Carefully explain how icing conditions can cause a sudden rolling motion of the airplane.

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13 Exam AE4-240/211 January 18, 2010

c) Carefully explain how aileron deflection to correct the rolling motion can aggravate the situation. To

support your answer, make a sketch in the space below

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14 Exam AE4-240/211 January 18, 2010

Question 8 DC-10

In the picture below you see two versions of the DC-10. Studies were performed on reducing the peak pressure

coefficients between the fuselage and centre nacelle at the fuselage aft-end. The studied configuration showed

supersonic flow and the associated drag due to shock waves. In the final configuration (top picture), this had been

reduced considerably. The later version (bottom picture) had a larger fairing between the wing and the fuselage as

well as a fillet between the fuselage and the vertical tail surface.

a) What aerodynamic design goal is served

by applying these fairings and fillets?

b) Carefully explain the effect that the addition of the wing-body fairing has on the local flow and

subsequently on the drag of the airplane in cruise conditions.

c) Explain how an additional fillet could help to reduce the peak pressure distribution between the vertical

tail, the fuselage and the aft nacelle.

d) Not all airliners chose the option of having additional fairings and fillets when they bought a DC-10, even

though their drag reduction was proven. Explain why.

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15 Exam AE4-240/211 January 18, 2010

Question 9 Isobars on swept wing

For the design of a military trainer, a wing-fuselage combination is evaluated for a swept wing of constant profile

(also shown). In the cruise condition the pressure distribution over the top side of the wing is indicated in the figure

below. The next design feature is the addition of air intakes, shown with dashed lines.

a) In the figure below, draw the pressure distribution over the top surface at the root and at the 70% semi-

span based on the isobaric pattern above.

Root 70% semi span

air intake

cp

x/c0

1

-0.6

-0.4

-0.2

0.2

0.4

cp

x/c0

1

-0.6

-0.4

-0.2

0.2

0.4

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16 Exam AE4-240/211 January 18, 2010

b) In the figure below sketch the spanwise form drag distribution that you expect for this wing.

c) Indicate the effect of the added engine intakes on the isobaric pattern by drawing the isobars on the left

wing in the first figure of this question. Explain below why you think the isobaric pattern changes according

to your drawing.

d) What is the effect of the addition of the intakes on the total form drag of the wing? Explain your answer.

drag

coef

.from

pressure

distr.,Cd

(~)

y/b

+

-

0 1

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17 Exam AE4-240/211 January 18, 2010

Question 10 Propeller slipstream effects

Consider a high-wing airplane with just one propeller engine attached to one of its wings. In the figure below, the

yawing moment coefficient is plotted as a function of the spanwise propeller position for 4 different situations. In

the sub questions reference is made to this figure.

a) On the flip side of this paper, fill out the table related to directional equilibrium and the propeller

slipstream.

b) Flaps up, tail on. Explain why the spanwise position of the outboard-down propeller has a smaller effect

(shows a shallower slope) on the yawing moment coefficient than the inboard-down propeller.

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18 Exam AE4-240/211 January 18, 2010

outboard down inboard down Indicate direction of rotation

Lift distribution of wing alone

Lift distribution of wing + slipstream velocity

Lift distribution of wing + slipstream velocity + slipstream rotation

Resulting trailing vortices

Resulting cross flow at the vertical tail

Resulting yawing moment about center of gravity

Note: use the size of arrows to indicate the magnitude of forces/moments/vorticitity/velocity

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19 Exam AE4-240/211 January 18, 2010

c) Flaps down, tail on. Explain why the deployment of flaps increases the yawing moment coefficient of the

airplane significantly.

d) In the figure below, at the indicated marks, draw the local drag force, using the size of the arrow as an

indicator for the drag magnitude. Also indicate the thrust force.

Resulting drag force

e) Flaps up, tail off. Explain why the rolling moment coefficient is larger for the outboard-down propeller than

for the inboard-down propeller.

x x x x

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20 Exam AE4-240/211 January 18, 2010

Question 11 Slats on a swept wing

In the figure below you see three geometrically identical wings for a business jet. Wing A does not have any slats.

Wing B has slats between the 40% semispan and the tips. Wing C has slats over the entire span of the wing. Wing A

has a clean (no flaps) CLmax of 1.20.

a) In the figure below, draw the lift curves for wing B and C, clearly showing which line corresponds to which

wing. Indicate the expected stall angle-of-attack as well as the expected stall behavior beyond CLmax for

each wing.

wing A

wing C

wing B

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21 Exam AE4-240/211 January 18, 2010

b) Explain which of the two wings has better stall characteristics in terms of longitudinal behavior of the

airplane. Motivate your answer.

c) Explain which of the two wings has better stall characteristics in terms of roll behavior. Motivate your

answer.

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22 Exam AE4-240/211 January 18, 2010

Question 12 Swept wings

In the figure on the flip side of this page a modern swept wing for a high-subsonic transport aircraft in a low-wing

configuration is presented.

a) Sketch the design pressure distributions at the three locations that are indicated in a. Annotate your

sketches such that you clearly indicate key characteristics in the pressure distributions.

b) Sketch the ideal spanwise loading, the spanwise lift coefficient, and the spanwise moment coefficient in b.

c) Sketch the typical shape of the wing profiles at three different spanwise locations in c. Pay close attention

to the following characteristics: Incidence angle, camber, Leading edge contour, Thickness, Position of the

thickest point, Front/aft loading. Make sure that the pressure distributions that you sketched under (a)

belong to the geometry of the profiles that you sketch here.

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23 Exam AE4-240/211 January 18, 2010

cp(-)

(+)

cp(-)

(+)

cp(-)

(+)

cl c

CLc

cl

cm

(+)

(-)

y/b/2

y/b/2

y/b/2

0

0

0

1

1

1

x/c0 1

z/c

x/c0 1

z/c

x/c0 1

z/c

a

b c

root

kink

tip

η=

η=

η=

η=0.75

η=0.75

kink

root

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24 Exam AE4-240/211 January 18, 2010

Question 13 Thrust reverser on IL-62

Thrust reversers are usually not deployed in flight but only with all wheels on the ground. However, in the figure

below you find the outboard reversers on an Ilyushin Il-62 deployed in-flight. It features 4 engines, mounted two by

two at the rear fuselage. This is a standard procedure on this high-speed aircraft type.

a) Explain why this aircraft type deployed it’s reversers in the air already

b) Explain why only the outboard nacelles are fitted with reversers. Why not the inboard engines?

c) The goal of thrust reversers, in general, is to maximize reverse thrust. Identify two other the design goals

that oppose this fist goal.

exam 2011-01-18

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