Aircraft Design Questions and Answers for Experienced people on “Thrust-Weight Ratio-2”.
1. Thrust loading is defined as ____________
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
Answer: a
Clarification: 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
Answer: a
Clarification: 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.
3. At idle cruise condition, thrust loading is equal to ______
a) drag to lift ratio at cruise
b) drag to power ratio
c) lift to drag ratio
d) aerodynamic efficiency
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: Given, Thrust loading at cruise T/W
Thrust loading at cruise T/W = 1 / (L/D)
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
Answer: a
Clarification: Given, maximum lift to drag = 12.
Thrust loading at cruise T/W = 1 / (L/D at cruise)
For, jet aircraft cruise lift to drag = 86.6% of maximum lift to drag = 86.6% of 12 = 0.866*12 = 10.392
Now, thrust loading at cruise,
Thrust loading at cruise T/W = 1 / (L/D at cruise)
= 1/10.392 = 0.096.
7. Power available for prop will vary with density.
a) True
b) False
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: b
Clarification: 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
Answer: a
Clarification: Given, Power loading P.L. = 12.
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
Answer: a
Clarification: Given, thrust loading at cruise T.L. = 0.9, prop aircraft.
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