This set of Aircraft Design Multiple Choice Questions & Answers (MCQs) focuses on “Fixed-Engine Sizing”.
1. Based on existing engine or based on new design, aircraft sizing can be done.
Explanation: Aircraft sizing is process of estimating the take-off gross weight of the aircraft. Aircraft can be sized either by using any existing engine or by developing new design to meet the required parameters.
2. Existing engine used for sizing is called as __________
a) fixed engine
b) unfixed engine
c) differential engine
d) rubber engine
Explanation: When existing engine is used for sizing of the aircraft then the engine is called fixed engine. Fixed engines are any existing engine which can almost fit to our requirements.
3. The thrust developed by fixed engine will be ___________
a) fixed during sizing
b) unfixed during sizing
c) independent of engine
d) zero at sizing
Explanation: Thrust will be fixed during sizing if fixed engine is used. Fixed engine is any existing engine that is available for use. However, when we use fixed engine we are bounded with some constraints such as fixed thrust. Hence, we cannot vary thrust as a rubber engine.
4. Size of fixed engine can vary during sizing.
Explanation: Any existing engine has pre-defined size and diameter for their design. Fixed engine will produced finite thrust and has finite thrust loading. We cannot alter the size of the particular fixed engine.
5. Why we use fixed engine for aircraft sizing?
a) Cost effective
b) Can be stretched
c) Can be rubberized
d) Can provide variable thrust at sizing
Explanation: Fixed engine is more easily available than a rubber engine. Any existing engine can save millions of dollars which was going to be spend in developing the new engine. However, we cannot stretch it during sizing but they are more cost-effective than the rubber engine.
6. Fixed engine sizing method is adopted for aircraft. If thrust per engine is 20000 unit and thrust loading T.L. is 0.8 then, find the take-off gross weight. Given aircraft has 2 engines.
a) 50000 unit
b) 58000 unit
d) 32589 unit
Explanation: Given, number of engines N = 2, thrust per engine T = 20000 unit and thrust loading T.L. = 0.8
Now, take-off gross weight is given by,
W = N*T / (T.L.) = 2*20000/0.8 = 50000 unit.
7. As a designer let’s consider that we want to build an aircraft weighing 28000 units at take-off. How many engine will require if our thrust loading T.L. is 0.71 and thrust per engine is 10000 unit?
Explanation: Given, thrust per engine T = 10000 unit and thrust loading T.L. = 0.71
Now, take-off gross weight is 28000 units.
Now, number engines required is given by,
N = W*T.L. / T = 28000*0.71/10000 = 1.98.
We cannot use 1.98 engines so we will take the nearest integer. Hence, the answer will be 2.
8. An aircraft needs to accelerate from 0.8M to 2.0M. What should be the mission segment weight fraction if fixed engine sizing is used?
Explanation: For an aircraft that accelerates from 0.1M to 0.8M, the required fuel fraction is 0.9805.
Similarly, for 0.1M to 2.0M we required a fuel fraction of 0.937 typically.
Now to find fuel fraction for aircraft from 0.8M to 2.0M is given by,
Fuel fraction = (required fuel fraction for 0.1M to 2.0M)/(required fuel fraction for 0.1M to 0.8M)
= 0.937 / 0.9805 = 0.956.
9. Which of the following is disadvantage of fixed engine sizing?
a) Often requires compromise in mission range or performance
b) Less costly compared to rubber engine
c) Availability is more than rubber engine
d) Has no disadvantage
Explanation: In general, for any reason if our weight needs to be alter during sizing then we need to provide optimization for such changes. In most of cases we need to alter the range or any other mission parameters.
10. Which of the following is correct?
a) W0 = N*T per engine / Thrust loading
b) W0 = Number of engine
c) W0 = T per engine / Thrust loading
d) W0 = 1 / Thrust loading
Explanation: For fixed engine sizing if we allow range to vary then, we can simply estimate take-off gross weight as W0 = N*T per engine / Thrust loading. Here, N is the total number of engine of an aircraft.
Sanfoundry Global Education & Learning Series – Aircraft Design.
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