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NANYANG TECHNOLOGICAL UNIVERSITY

SEMESTER 2 EXAMINATION 2009-2010

AE4105 – ROTARY WING AIRCRAFT

April/May 2010 Time Allowed: 2½ hours INSTRUCTIONS 1. This paper contains FOUR (4) questions and comprises FOUR (4) pages. 2. Answer ALL questions. Support your answers with formulae and figures whenever appropriate. 3. This is a CLOSED-BOOK Examination. 4. Marks for each question are as indicated. 1. The Boeing V22 Osprey is a tiltrotor helicopter as shown in Figure 1 below. It has 2 rotors at both ends of its wing. On the ground, the V22 rotates its engine/rotor combinations upwards such that, it could lift off from the ground in similar manner to the dual rotor helicopter (Figure 1(a)). In the air, the V22 rotates its engine/rotor combinations 90° forward, such that, its rotors become large propellers to move forward (Figure 1(b)) in similar manner to the conventional plane.

Figure 1: Boeing V22 Osprey tilrotor

The V22 has maximum take-off weight of 27,400 kg. It has 2 turboshaft engines, each delivered maximum power of 4,590 kW, at which each rotor spins at 397 rpm. Each of the V22 rotors is 11.6 m in diameter. The solidity of each rotor is approximately 0.05. The average sectional lift curve slope along all its six rotor blades is 2π. (Photo & information, courtesy The Boeing Company)

Note: Question No. 1 continues on page 2.

1(a) V22 in helicopter mode 1(b) V22 in plane mode

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(a) Consider the case when this helicopter hovers above the ground at the height of 10m. Estimate the induced velocity and the figure of merit at maximum power. You may use actuator disk theory to do calculation, and use the induced power correction factor of 1.15.

(10 marks)

(b) Consider the case when this helicopter climbs vertically at its maximum climb velocity of 11.8 m/s (out of ground effect) and at maximum take-off weight. Estimate the power required. You may use actuator disk theory to do calculation.

(10 marks)

(c) Assume each of the three rotor blade is straight, of constant chord, uniform cross- sectional shape but twisted linearly. Estimate the pitch angle at 75% radius required to hover as in condition (a).

(5 marks)

These equations may be useful 3 2

2t

PiCC κ

= , 2

NcR NCR R

σπ π

= = , 2

i Th

v CR

λ = =Ω

(in hover motion)

2

12 2

i C C

h h h

v V Vv v v

⎛ ⎞= + +⎜ ⎟

⎝ ⎠ (in vertical motion)

212

tip

root

r

T lr rC C r drσ

== ∫ where r y R≡

7512 3 2T lC C α

θ λσ ⎡ ⎤= −⎢ ⎥⎣ ⎦ for linearly twisted blades and uniform inflow

and 2IGE

OGE T const

P zP R

=

⎡ ⎤=⎢ ⎥

⎣ ⎦ (Betz’s ground effect equation)

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2(a) Figure 2 below shows the stabilator under various flight conditions. Explain (i) the use of stabilator and (ii) how it works or help in these 4 flight conditions as shown in the Figure

(10 marks)

Figure 2

(b) Explain, by means of sketch(es) or graph(s), the effect of forward speed on the power of typical helicopter, and hence, the vertical climb speed. Show the effect of increase gross weight, increase altitude and one engine-inoperative (OEI) situation. Why does the pilot of the heavy helicopter (e.g. Chinook) need to fly forward near ground before climb up?

(15 marks)

3. An OH-6 Cayuse helicopter has a take-off mass at 1,400 kg. The main rotor diameter is 8.33 m, and the rotational speed is around 400 rpm. The rotor solidity (σ) is approximately 0.127. Maximum engine available power at sea level is 236 kW. Assuming an induced power factor (κ) of 1.15, average blade drag coefficient of 0.01, and equivalent flat plate drag area for the fuselage (fs) of 1.0 m2,

Note: Question No. 3 continues on page 4.

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(a) Calculate ideal induced power (in kW) at sea level. (10 marks)

(b) Calculate figure of merit at sea level. (10 marks)

(c) Determine inflow ratio when the helicopter flies at sea level at a speed of 40 m/sec.

(10 marks)

(d) Estimate the maximum vertical climb rate (in fpm) when the helicopter flies at 1,000m at a mass of 1,200 kg.

(10 marks)

Altitude, m

Temperature, K

Pressure, N/m2

Density, kg/m3

Speed of Sound, m/s

0 288.16 101325 1.2250 340.3 500 284.91 95461 1.1673 338.4 1000 281.66 89876 1.1117 336.4 1500 278.41 84560 1.0581 334.5 2000 275.16 79501 1.0066 332.5 2500 271.92 74692 0.9570 330.6 3000 268.67 70121 0.9093 328.6

4. When a helicopter follows closely the contours of the earth at and below tree-top

height, it is called nap-of-the-earth flight. A military requirement for the helicopter is to have the capability of performing pull-up at 1.75g for three seconds to clear obstructions and push-over at 0 g for two seconds to return to nap-of-the-earth heights. During the pull-up and push-over maneuvers, the pilot may need to pitch, roll, and yaw the aircraft. Briefly discuss the ability of a helicopter having a simple, two-bladed, teetering rotor to satisfy this requirement. What design changes, if any needed, would you suggest meeting this requirement.

(10 marks)

End of Paper

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