This set of Advanced Aircraft Design Questions and Answers focuses on “Structures – Material Selection and Properties”.
1. What is creep?
a) Under low sustained stress, it is tendency of some materials to get deform slowly and permanently
b) Under zero stress, it is permanent deformation
c) High lift device
d) Anti ice device
Explanation: Creep is nothing but the tendency of some materials to get deform slowly and permanently under low but sustained stress. For most of materials used in aerospace, creep is a problem at elevated temperatures. Although some titanium, plastics will exhibit creep at room temperatures.
2. Which of the following is correct?
a) Corrosion is major problem in terms of material lifespan
b) Corrosion cannot be considered as major factor at all
c) Lift on corrosive material will always be same as weight
d) Weight of corrosive material will always be same as thrust loading
Explanation: Since the early days of aviation, corrosion of materials has been a major problem. Aircraft materials are exposed to salt water spray, atmospheric moisture, fuels, oils, exhaust, plumes etc. Stress corrosion can lead to fracture at a stress level one tenth of normal ultimate stress.
3. Let us consider the ambient air temperature is 25°C and aircraft has Mach number of 1.5. Find the stagnation temperature.
Explanation: Given, ambient air temperature T1 = 25°C = 25+273.15 = 298.15K, Mach number = 1.5
Stagnation temperature for air is given by,
Stagnation temperature = T1*(1+0.2*M2)
= (25+273.15)*(1+0.2*1.52) = 432.32K.
4. What is the advantage of wood?
a) Easy to fabricate
b) Moisture sensitivity
c) Complexity during fabrication
d) Non availability
Explanation: Woods are easy to fabricate and repair. This is one of the advantages of the wood. Wood has some drawbacks as well such as moisture sensitivity. Today wood is largely used in homebuilt aircrafts.
5. Which of the following is correct?
a) Wood sufferers from moisture sensitivity
b) Wood is not available, it is rare to find
c) Lift of wooden box will be same as drag
d) Wood surface is not sensitive to moisture at all
Explanation: Wood sufferers from moisture sensitivity. Wood is not rare to find. Wood offers good strength to weight ratio and is easy to fabricate. Lift will not be same as drag.
6. For high strength applications ____ is widely used.
a) 7075 aluminum alloy
b) 7089 magnesium zinc
c) 1098 gallium
d) Ge-Zn alloy
Explanation: For high strength application 7075 aluminum alloy is widely used. 7075 is alloyed with zinc, copper and magnesium. Corrosion resistance is lessened by alloying. Aluminum sheet is frequently clad with a thin layer of pure aluminum.
7. Magnesium is used for ________
a) control hinges
b) highest strength application
Explanation: Magnesium is used in control hinges, wheels, engine mounts etc. Magnesium has good strength to weight ratio. It can tolerate high temperatures and is easily formed. Magnesium can be easily formed by casting, forging etc.
Explanation: Above diagram is showing a typical whisker reinforced composite material. Short strand of the reinforcing materials are randomly located throughout the matrix. Chopped Fiberglass is an example of such arrangement.
Explanation: A typical Fiber reinforced polymer is shown in the diagram. It is also called filament reinforced composite. Most of the aircraft structure are made of the frp composite. They provide much better strength to weight ratio.
10. Whiskers are ________
d) isentropic compression
Explanation: Whiskers are Isotropic. Whiskers have same material properties in all directions and hence, termed as isotropic. Isothermal process has constant temperature. Isobaric process has constant pressure that means pressure will not change during such processes.
11. Composite material has only one material.
Explanation: Composite material is formed by using two or more different materials. Composites are used to get desired properties of material. However, they can be costlier than non-composite materials.
Sanfoundry Global Education & Learning Series – Aircraft Design.
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