# Aircraft Design Questions and Answers – Thrust Weight Ratio-2

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This set of Aircraft Design Questions and Answers for Experienced people focuses on “Thrust-Weight Ratio-2”.

a) ratio of thrust produced by an engine to the weight of the aircraft
b) ratio of lift to thrust
c) ratio of drag to thrust
d) ratio of lift to weight

Explanation: Thrust loading is nothing but the thrust to weight ratio. It is defined as the ratio of thrust produced by the engine to the weight of the aircraft. Thrust loading will vary from aircraft to aircraft. It can be between 0.2-0.6 typically.

2. What is thrust matching?
a) Comparison of the engine’s available thrust at cruise to the estimated drag of aircraft
b) Comparison of the engine’s available thrust at cruise to the estimated weight of aircraft
c) Comparison of the engine’s available lift at cruise to the estimated drag of aircraft
d) Comparison of the engine’s available power at cruise to the estimated drag of aircraft

Explanation: Thrust matching is nothing but the comparison of an engine’s available thrust at cruise to the estimated drag of aircraft. It is used for better initial estimation of the thrust to weight ratio.

a) drag to lift ratio at cruise
b) drag to power ratio
c) lift to drag ratio
d) aerodynamic efficiency

Explanation: At idle cruise condition, lift = weight and thrust = drag. Hence, thrust loading T/W = D/L = Drag to lift ratio at the cruise. Aerodynamic efficiency is the ratio of lift to drag. Thrust loading at cruise is inverse of aerodynamic efficiency.

4. For an aircraft cruise aerodynamic efficiency is 10. Find the thrust loading at cruise.
a) 0.1
b) 0.01
c) 0.4
d) 0.5

Explanation: Given, Aerodynamic efficiency L/D = 10
Thrust loading at cruise T/W = 1 / (L/D) = 1/10 = 0.1.

5. A typical jet airliner has cruise thrust to weight ratio of 0.5. Determine at which aerodynamic efficiency this airliner is flying.
a) 2
b) 3
c) 4
d) 5

Hence, Aerodynamic efficiency (L/D) at cruise is,
L/D = 1 / (T/W) = 1/0.5 = 2.

6. A jet aircraft has maximum lift to drag as 12. Find the value of the thrust loading at the cruise.
a) 0.096
b) 0.0069
c) 0.96
d) 0.0096

Explanation: Given, maximum lift to drag = 12.
For, jet aircraft cruise lift to drag = 86.6% of maximum lift to drag = 86.6% of 12 = 0.866*12 = 10.392
= 1/10.392 = 0.096.

7. Power available for prop will vary with density.
a) True
b) False

Explanation: Power available is affected by the density at a given altitude. Based on density ratio power available decreases with increasing altitude given that the engine is not supercharged.

8. Consider thrust is given as 120 N of force then, find the value of thrust in terms of pound of force.
a) 26.97 pound of force
b) 125 pound
c) 12.56 lb
d) 120 KN

Explanation: Thrust in pound of force = Thrust in newton*0.225 = 120*0.225 = 26.97 lb of force.

9. If weight of an aircraft is increased then, thrust loading will _______
a) increase
b) decrease
c) will not change
d) exact half always

Explanation: Thrust loading of the aircraft is defined as the ratio of the thrust of the aircraft to the weight of that aircraft. Hence, thrust loading is inversely proportional to the aircraft weight. Hence, if weight is increased then the corresponding value of thrust loading will decrease.

10. A twin turbo prop is flying with power loading of 12. What will be the maximum velocity?
a) 48.225 mph
b) 48.225 m/s
c) 48.225 km/h
d) 225.48 m/h

P.L. is given by,
W/hp = 1 / (a*Vmaxc), for twin turbo prop a=0.012, c=0.5.
Hence, maximum velocity Vmax is given by,
12 = 1 / (0.012*Vmax0.5)
0.012*Vmax0.5 = 1/12.
Vmax0.5 = 1 / (0.012*12) = 6.944
Takin log at both sides,
0.5*ln (Vmax) = ln (6.944) = 1.937
Now, taking anti-log,
Vmax = e(1.937/0.5) = e3.875 = 48.225mph.

11. An A/C is cruising with thrust to weight of 0.9. If A/C is powered by prop engine then, find maximum lift to drag?
a) 1.11
b) 2.11
c) 3.12
d) 4.221

For prop aircraft maximum L/D = Cruise L/D
Hence, T.L. = D/L.
Hence, cruise L/D = 1/(T/W) = 1/0.9 = 1.11.

12. An A/C is flying with thrust loading at take-off is 0.9. If thrust at take-off and cruse is equal then, determine the thrust loading during cruise.
a) 0.941
b) 0.951
c) 0.961
d) 0.98

Explanation: Given thrust at take-off = thrust at cruise = T
(T/W)cruise = (T/W)take-off * (Wtake-off / Wcruise).
For cruise Wcruise = 0.956 * Wtake-off
Hence,
(T/W)cruise = (T/W)take-off * (Wtake-off / 0.956*Wtake-off) = 0.9/0.956 = 0.941.

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