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16.20 Handed Out: Lecture 36
DESIGN PROBLEM
(and….HOME ASSIGNMENT #9)
You and your team of colleagues are employed as structural engineers at Small-Planes-R-Us, Inc. The board of directors has committed the company to producing a new small two-seater airplane. Your engineering manager is responsible for the fuselage design and specifically assigned your team to work on the aft fuselage between the wing and the tail. She has asked you to submit a preliminary design for this section of the fuselage in about three weeks. The aerodynamics group has decided to go with a circular cross-section and it is divided into two regions by a "floor" to attach some instrumentation as specified by the avionics group. The preliminary sizing work has defined the diameter of the cross-section near the wing region (aft fuselage station 0) to be 180 cm, and at the tail connection point (aft fuselage station 250) the diameter is 90 cm. The length of this section is 250 cm.
FIGURE 1 Side view of aft fuselage.
z
x
Floor
250 cm
Wing Location
(Station 0) Tail Location
(Station 250)
180 cm (^) 90 cm
Fall, 2002
You have been asked, specifically, to make recommendations for the areas and materials to be used for the stringers and the thicknesses of the floor and skin of the fuselage. This is to help your engineering manager prepare for a "Design-Build Team" meeting where representatives from the various aspects of the engineering divisions as well as manufacturing and marketing representatives will confer to better define the design.
The preliminary sizing and configuration work has defined a cross-section for the aft fuselage. This configuration consists of skins connecting four stringers: one at the top of the fuselage, one at the bottom, and one on each side of the floor at the floor level. In a discussion with your group, your manager has provided you with the following information as well as the accompanying figures:
FIGURE 2 Cross-section dimensions.
180 cm
Floor
Station 0
Station 250
Floor
90 cm
z
y
Fall, 2002
- The aft fuselage may be considered as a cantilevered beam clamped at station
- There are rigid frames to maintain the cross-section at regular intervals. You are not being asked to worry about the frame location and design at this time.
- Buckling is not to be considered at this stage but will likely be an important consideration in the next stage of the design.
Preliminary material information
The following material properties have been furnished by the materials department:
Material E (^) L [GPa] νLT σL allowable^ [ MPa] τLallowable^ [ MPa] ρ [ g/cm 3 ]
Gr/Ep laminate 110 0.4 500 ---- 1.
Aluminum 70 0.3 350 180 2.
- Further questioning of the materials department yields a working shear modulus of the graphite/epoxy laminate of 6.0 GPa.
- The material allowables are valid in both tension and compression. These represent values that have been reduced from mean material ultimates due to statistical and other considerations.
- The cost per manufactured pound of graphite/epoxy structure is $150. The cost per manufactured pound of aluminum structure is $70.
Preliminary load information
The loads on the aft fuselage are primarily due to the lift loads on the horizontal stabilizer and the rudder/vertical stabilizer. (The fuselage is not pressurized.) The loads can be considered to act at a point. There are three types of loads at the tail location (station 250) which may be considered to act separately for the purposes of preliminary design:
- The first load condition is at take-off and consists of a symmetric 2500 N load on each horizontal stabilizer acting in the negative direction at a distance of 75 cm from the center of station 250.
Fall, 2002
- The second load condition is an anti-symmetric maneuvering load of 1000 N per horizontal stabilizer. These loads also act at a distance of 75 cm from the center of station 250.
- The third load condition is again a maneuvering load. This consists of a load of 1000 N on the rudder. This acts 100 cm above the plane of the floor at station
Preliminary design criteria
A preliminary assessment of usage has been conducted based on historical data as well as a survey of past and potential customers. The following design criteria for the aft fuselage have been initially set:
- Under normal conditions, the bending deflection in the z-direction at station 250 must not exceed 1% of the length of the fuselage between stations 0 and 250.
- The twisting at the tip must not exceed 1° under normal operations.
- An ultimate factor of safety of 1.5 is to be employed when considering the operating loads/stresses in comparison to the material allowables.
The Job
Your group's job is to provide the complete specifications of your preliminary design, with dimensions of all the areas involved, material used, and the weight and cost of the resulting structure. You should highlight any key tradeoffs such as that between weight and cost that would result by this choice of design and provide suggestions pertaining thereto.
How and What...
This assignment is divided into two parts. The first part is Home Assignment #9 and is a separate document handed out with this document. This assignment poses a general stiffener-skin configuration similar to those to be considered in Design Problem #4. Your team is asked to determine the cross- section properties of the configuration and the stresses and deflections that arise
