Features of the Aircraft Design - Essay Example

Aircraft Design 8-26-2006 Topic Review of crash-worthiness of the aircraft design Features of the aircraft design that contribute to the crash-worthiness. In this paper, the crashworthiness of the aircraft design is analysed. Crashworthiness means the ability of the aircraft to protect the passengers during a crash. In order to study the crashworthiness of the aircraft design without using crash outcomes, the deformation patterns of the aircraft structure can be studied. The acceleration forces on the aircraft during a crash can be simulated by computer. Human body models can predict injury probabilities during a crash of the designed aircraft. Injury criteria are used, which correlate forces with injury risk. Although composite materials weigh less than aluminum and have more corrosion resistance, which will lower maintenance, composite materials also have more failure characteristics in high-energy crashes (Langevin, 2003). Composite materials are brittle, and lack plasticity following an impact, so that a change in configuration may be needed in an aircraft design to ensure that crashworthiness criteria are met. An important aspect of aircraft impact survivability is the strength of the passenger seats. The response of seat structures to impact loads shows a need for higher static seat strength. There are load-limiting devices that can be implemented so that the loads transmitted to passengers during a crash will be minimized. The structural assembly can also be modified to decrease transmitted loads. Aircraft subfloor systems can be developed with high-strength materials that hold the passenger seats during impact, and also contain a crushable layer that will absorb energy. The crushable layer is most important in the vertical direction, for improving human tolerance of the impact. This subfloor platform will also distribute loads across the fuselage evenly. Energy absorbing seats are also effective at reducing loads transmitted to passengers. A comparison of two similar real-life crash events shows that energy absorbing seats and restraint systems can mean the difference between walking away and not surviving (Langevin, 2003). Another aspect of an aircraft design’s crashworthiness is its ditchability, or its ability to emergency land in water. There is usually a great loss of life in ditching however; statistics show a 60% survivability during ditching (Kebabjian, 2006) - some sources list a much higher survivability rate. Intentional ditching of commercial aircraft is rare and often occurs after fuel exhaustion; small aircraft ditching is much more common. As far as aircraft design considerations go, large underwing turbofans have been seen as having problematic ditchability ("Fall-back System...", 1999). In our aircraft design, engines mount above or on the wing, and there shouldn’t be the same ditchability problem. Other aspects of the aircraft design that can affect ditching include the aircraft’s ability to float until rescue arrives; even sections of fuselage that can float after aircraft breakup have been known to save lives. In order to avoid electrical fires, which are the next worst aircraft scenario after total structural failure, the idea of a flight essential electrical bus has been advanced ("Fall-back System...", 1999). These systems are sometimes called a virgin electrical bus. This electrical bus will fall back to an electrical selection of "get-you-home" connections in case of emergency. The hardware for the virgin electrical bus includes " interface components, wiring, connectors, control panel, etc." ("Fall-back System...", 1999) and is estimated at 1,800 lbs for a widebody airplane. Aircraft designs that allow rapid evacuation improves crashworthiness during post-crash fires or ditching; features such as accessible exits and wide aisles can improve this aspect of crashworthiness. Aircraft design should also include a fuselage structure that will remain intact after an impact; cabin furnishings should remain intact to prevent injury or hindered evacuation; and the cabin itself should remain livable as long as possible. To achieve this last goal, all aircraft materials should be as non-flammable as possible. Thermal/acoustic insulation in the lower half of the fuselage should have over 5 minutes of burn-through protection. For improved crashworthiness, the aircraft should be designed so that impacts at any angle on its surfaces do not lead to structural failure. In addition, the aircraft must stay intact during rollover. The structural components of the aircraft design that are involved in the aircraft’s crash dynamic response include: “landing gear, pylon/engine, wing box structure, fuselage, fuel distribution system, floor structure, seats/restraint systems, cabin interior, and entry and escape doors” (Brende and Widmayer, 1982, p.37). The wing box structure includes fuel tankage and primary load carrying elements. Each of the structural components listed above participates in crashworthiness considerations; there may be interactions between the structural components as well. Topic 2 - Design of the fuselage structure The fuselage, or main body of the aircraft design must accommodate the desired number of passengers and amount of cargo, as well as attaching to the wings and tail assembly. The fuselage design is therefore a major part of the economics of the aircraft. The desire for increased capacity in order to create increased revenue must be balanced with passenger comfort and need for space. For instance, smaller seat size will reduce passenger comfort while increasing aircraft capacity. In the case of our aircraft, the plan is for four passengers and 1 pilot, making a total of 5 seats. It is a good design tip that seats lie abreast in order to prevent the fuselage from becoming unnecessarily long. In this case, either two passenger seats on either side of an aisle or two rows of one seat on either side of the aisle would be a good design. This decision can affect the dimensions of the cabin and the outer diameter of the fuselage. The fuselage must also have space for a galley and small toilet. This could be implemented by placing a small galley behind the pilot seat, and the small toilet in the rear of the plane, behind the row of passenger seats. Using the dimensions of an average seat, the fuselage width and length can be calculated. Calculations of the fuselage fineness ratio and wing span should also be made at this point. The design and development of aircraft fuselages are most affected by new combinations of materials and structures. Fuselage weight can be cut by as much as 30% when using composite materials instead of aluminum ("Multi Disciplinary.."). The materials of our aircraft are as follows: The main structure of the aircraft will be aluminum alloy; this includes the fuselage skin and wing skins. The fuselage spars and ribs may be made of a composite material instead of aluminum alloy. The empennage and control surface will definitely be composite. Underneath the passenger cabin will be a high-strength platform that will retain the seats in case of an impact. This subfloor system will also contain a large vertical layer of crushable material, in order to increase crashworthiness. The fuselage geometry affects drag in a few different ways. A small fineness ratio for the fuselage will have less wetted area; but when the cabin is fixed there is more wetted area. A high Reynolds number and large tail length add structural weight, although aerodynamics is improved for long thin fuselages. In our fuselage design, the different areas considered are: cross-section dimension for the cabin; and fuselage length and shape. The cross-section shape of the fuselage is usually circular; this reduces bending loads. A typical cross-section of fuselage for smaller aircraft with just one seat on either side of the aisle is: aisle height of 5 feet 5 inches; and maximum width of the cabin of 5 feet 6 inches. Passenger cargo can be estimated by allowing 4 cubic feet per passenger. On our plane, this would amount to 20 cubic feet of passenger cargo. In addition, there may be extra space available for revenue cargo. The outer diameter of the fuselage is found after adding the size of the fuselage frame, stringers and insulation thickness. This outer diameter is often estimated by multiplying the inner cabin diameter by 8%. In our case, taking 5 feet 6 inches as the cabin diameter, the outer diameter of the fuselage would be 0.44 feet, resulting in a total diameter of 5.94 feet. Using a seat width of 20 inches, aisle width of 18 inches, seating one passenger on each side of the aisle, an equal height and width of the cabin, and floor height offset 15% below the cabin center, an online calculator (Alonso and Kroo, 2005) estimates fuselage width and height of 5.7 feet and a fuselage wall thickness of 2.74 feet. The fuselage shape includes a tapered nose, constant section of the passenger cabin, and a tapered tail cone. The total length of the aircraft is 33 feet. The layout is important for maintenance. The fuselage shape must avoid separation and shock waves. Large fineness ratios of the nose and tail cone will minimize flow accelerations; these fineness ratios should be: above 1.5 for the nose; and between 1.8 and 2.0 for the tail cone. Fuselage layout and sizing will also be constrained by a number of federal regulations on seating and doors, ditching and emergency evacuation, etc. For a five seat aircraft with 40 inch seat pitch, 1.8 nose fineness, 2.5 tail cone fineness, 0.8 feet extra aft space and 1 foot extra forward space, using a calculator program (Alonso and Kroo, 2005) : the fuselage length is calculated to be 33 feet; wetted area 475 feet; nose length 10.3 feet; tail cone length 14.2 feet; and cabin length of 8.47 feet, with 2 rows of seats. These results match our original design of 33 feet total length, however the other results may differ The manufacture and assembly of the fuselage structure can involve three sections (forward, mid and aft). These can be manufactured separately, which has the advantage of making manufacturing simpler; however, final assembly faces a challenge to mate the sections. Topic 3 - Cost for through life support and maintenance requirements The cost of through life support and maintenance requirements for our aircraft design can be estimated by matching it with a similar existing aircrafts data. The existing aircraft should match our designed aircrafts mass, range, and passenger capacity as closely as possible for this estimated cost to be accurate. One model that can be used to this effect is the Piaggio P180 Avanti, which resembles our aircraft design. This model is considered a high- quality low-cost twin turboprop airplane, seating seven passengers maximum plus a crew. To estimate the aircraft maintenance cost, it is helpful to first consider aircraft manufacturing cost, which measured in the required dollars per pound ($/lb.) and is estimated for the airframe weight; this is added to the engine cost. Figure 1 shows how the different components of the aircraft contribute to the manufacturing cost. Each of these components also has a potential contribution to maintenance costs, whenever a part needs replacing or repair. Figure 1- Distribution of Airplane Manufacturing Costs (Alonso and Kroo, 2005). Basic Structure (Wing, Fuselage, Tail) 41.5% Propulsion System including Engines 17.1% Furnishings including Lighting 14.5% Avionics (Communication and Navigation) 12.7% Flight Control and Guidance Systems 5.3% AC Power System 2.4% Hydraulic and Auxiliary Power Systems 2.1% Air Conditioning and Pressurization 1.9% Landing Gear, Wheels, Tires, Brakes 1.7% Miscellaneous Systems and Components 0.8% Aircraft design considerations can significantly lower maintenance costs. For instance, the use of composite materials in the aircraft design will lower maintenance requirements and costs due to their higher corrosion resistance, as compared to aluminum. Presumably composite parts will not have to be replaced as often. Maintenance costs saved will include both labor and materials. Other aircraft design considerations include how aircraft cost increases with the weight of the designed aircraft; this may also have an effect on the through life support and maintenance costs. Take-off mass should be reduced wherever possible; a large component of take-off mass is the payload mass. The other large component of take-off mass is fuel mass; the fuel weight of our aircraft is 1100 lb maximum. The maintenance budget of an aircraft is hard to estimate because of unexpected repairs, service bulletins for the airframe and engines, and airworthiness directives. Maintenance will be required on the airframe, avionics and passenger cabin equipment, and engine; there will be on-site technical assistance needed and labor costs for removal and reinstallation as well. Avantair, the provider of Piaggio Avanti, the aircraft which most closely resembles our aircraft design, has made an agreement with Jet Support Services, Inc. for a cost guarantee on airframe maintenance cost. In this way, Avantair has a stable maintenance budget. Obtaining an agreement of this type would be a consideration for our aircraft design. To get an idea what other aircraft maintenance costs are, the Cheyenne I was considered. The maintenance costs of a Cheyenne I are estimated to be $389, 550 for 900 trips of 350 NM (TGAviation, 2005). Maintenance per flying hour for the Cheyenne I is $265. The Cheyenne is a twin turboprop, so the maintenance costs for our aircraft design, which is a twin turbojet, should be on a similar scale. Presumably, our aircraft design would have very similar maintenance costs per flying hour because of a matching passenger capacity; the Cheyenne I can carry five passengers, just as in our aircraft design. For through life support estimates on our aircraft design, similar arrangements are considered here. For instance, a £12 million contract has been made for engine support on a turboprop engine (Frey, 2003). This through life support includes logistic and technical support for the turboprop engine until the aircraft out-of-service date. However, this engine is in use with military training aircraft, and therefore would probably be a much larger contract than our aircraft design would need, since our aircraft design is for a small business aircraft. This atleast puts an upper bound on what can be expected for the cost of through life support for our aircraft design. The cost of providing the through life support for our aircraft design would consist of the labor involved in providing expert logistic and technical support for the aircraft throughout its lifecycle. And this can be assumed to be below the £12 million contract level mentioned above. References Alonso, J. and Kroo, I. (2005) Aircraft Design: Synthesis and Analysis. Retrieved 27 Aug 2006 from http://adg.stanford.edu/aa241/AircraftDesign.html Anderson, D.F. and Eberhardt, S. (2001) Understanding Flight. New York: McGraw-Hill Professional. Brende, O.B. and Widmayer, E. (1982) Commercial Jet Transport Crashworthiness. NASA Langley Research Center. Caiafa, C. and Thomson, R.G.(1982) Designing for Aircraft Structural Crashworthiness. Journal of Aircraft. Vol 19, No 10. Cutler, J. (1999) Understanding Aircraft Structures. Malden, Mass: Blackwell Science. “Fall-back System Seen as Means of Bypassing Electrical Fires” (25 Jan 1999) Air Safety Week. Frey, B. (2003) Turbomeca and UK MOD sign £12 million contract. Press Releases. Turbomeca Safran Group. Future Flight, Greener Design. Design Workbook. University of Southampton. Retrieved 28 Aug 2006 from http://www.futureflight.org/downloads/designlogbook.pdf#search=%22fuse lage%20design%22 Jenkinson, L. R. and Marchman, J.F. (2003) Aircraft Design Projects. Oxford: Elsevier. Kebabjian, R.(2006) Statistics. Retrieved 28 Aug 2006 from http://www.planecrashinfo.com/cause.htm Langevin, G.S. Crashworthiness. NASA. Retrieved 26 Aug 2006 from http://oea.larc.nasa.gov/PAIS/Concept2Reality/crashworthiness.html Multi Disciplinary Fuselage Design. Retrieved 27 Aug 2006 from http://www.lr.tudelft.nl/sia/publications/acoustics%20&%20mechanics/Leo nardo-Times.doc Sheldon, J. (1999) Intake Aerodynamics. Malden, Mass: Blackwell Science. Shore Aviation(2006) 1992 Piaggio P180 Avanti. Retrieved 27 Aug 2006 from http://64.233.161.104/search?q=cache:7Eks9UfGb3kJ:www.aso.com/i.as o3/aircraft_view.jsp%3Ftask%3Dad_frame%26aircraft_id%3D91516%26r eturn_url1%3D/i.aso3/+piaggio+p180+avanti+cost&hl=en&gl=us&ct=clnk& cd=9 Smith, L.G.(2004) Avantair: Soaring in Style. ValueRich Magazine. Retrieved 25 Aug 2006 from http://www.valuerichonline.com/mag/04fall/story.php?id=avantair TGAviation (2005) Super Cheyenne I Conversion. Retrieved 27 Aug 2006 from http://www.tg-aviation.com/pdfs/SC_I_Conversion_ brochurerev.pdf#search=%22%22m aintenance%20cost%22%20Piaggio%22 Read More