Aircraft Design Project
AIRCRAFT DESIGN PROJECT
Chapter 1
CHAPTER 1
INTRODUCTION
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1.1Defining a new design
1.2Design process
1.3Conceptual design
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“A beautiful aircraft is the expression of the genius of a great engineer who is also a great artist.”
Neville Shute,
British Aeronautical Engineer and Novelist,
From, No Highway, 1947.
When you look at aircraft, it is easy to observe that they have a number of common features: wings, a tail with vertical and horizontal wing sections, engines to propel them through the air, and a fuselage to carry passengers or cargo. If, however, you take a more critical look beyond the gross features, you also can see subtle, and sometimes not so subtle, differences. What are the reasons for these differences? What was on the mind(S) of the designers that caused them to configure the aircraft in this way?
Airplane design is both an art and a science. In that respect it is difficult to learn by reading a book; rather, it must be experienced and practiced. However, we can offer the following definition and then attempt to explain it. Airplane design is the intellectual engineering process of creating on paper (or on a computer screen) a flying machine to (1) meet certain specifications and requirements established by potential users (or as perceived by the manufacturer) and/or (2) pioneer innovative, new ideas and technology. An example of the former is the design of most commercial transports, starting at least with the Douglas DC-1 in 1932, which was designed to meet or exceed various specifications by an airplane company. (The airline was TWA, named Transcontinental and Western Air at that time.) An example of the latter is the design of the rocket-powered Bell X-1, the first airplane to exceed the speed of sound in level or climbing flight (October 14, 1947). The design process is indeed an intellectual activity, but a rather special one that is tempered by good intuition developed via experience, by attention paid to successful airplane designs that have been used in the past, and by (generally proprietary) design procedures and databases (handbooks, etc.,) that are a part of every airplane manufacturer.
1.1Defining a new design
The design of an aircraft draws on a number of basic areas of aerospace engineering. As shown in the illustration, these include aerodynamics, propulsion, light-weight structures and control.
Each of these areas involves parameters that govern the size, shape, weight and performance of an aircraft. Although we generally try to seek optimum in all these aspects, with an aircraft, this is practically impossible to achieve. The reason is that in many cases, optimizing one characteristic degrades another.
AerodynamicsPropulsion
Light-Weight Control
Structures
In most cases, the design objectives are not as focused. More often, the nature of an aircraft design is compromise. That is, the goal is to balance the different aspects of the total performance while trying to optimize a few (or one) based on well-defined mission requirements.
There are many performance aspects that can be specified by the mission requirements. These include:
- The aircraft purpose or mission profile;
- The type(s) and amount of payload;
- The cruise and maximum speeds;
- The normal cruise altitude;
- The range or radius with normal payload;
- The endurance;
- The take-off distance at the maximum weight;
- The landing distance with 50 percent of the maximum fuel weight;
- The purchase cost;
- And other requirements considered important;
1.1.1Aircraft Purpose
The starting point of any new aircraft is to clearly identify its purpose. With this, it is often possible to place a design into a general category. Such categories include combat aircraft, passenger or cargo transports, and general aviation aircraft. These may also be further refined into subcategories based on particular design objectives such as range (short or long), take-off or landing distances, maximum speed, etc. The process of categorizing is useful in identifying any existing aircraft that might be used in making comparisons to a proposed design.
With modern military aircraft, the purpose for a new aircraft generally comes from a military program office. For example, the mission specifications for the X-29 pictured in figure 1.1 came from a 1977 request for proposals from the U.S. Air Force Flight Dynamics Laboratory in which they were seeking a research aircraft that would explore the forward swept wing concept and validate studies that indicated such a design could provide better control and lift qualities in extreme maneuvers.
With modern commercial aircraft, a proposal for a new design usually comes as the response to internal studies that aim to project future market needs. For example, the specifications for the most recent Boeing commercial aircraft (B-777) were based on the interest of commercial airlines to have a twin-engine aircraft with a payload and range in between those of the existing B-767 and B-747 aircraft.
Since it is not usually possible to optimize all of the performance aspects in an aircraft, defining the purpose leads the way in setting which of these aspects will be the “design drivers.” For example, with the B-777, two of the prominent design drivers were range and payload.
1.1.2Payload
The payload is what is carried on board and delivered as part of the aircraft’s mission. Standard payloads are passengers, cargo or ordnance. The first two are considered non-expendable payload because they are expected to be transported for the complete duration of the flight plan. Military ordnance is expendable payload since at some point in the flight plan it permanently leaves the aircraft. This includes bombs, rockets, missiles and ammunition for on-board guns.
For personal or small general aviation aircraft, the payload includes the pilot as well as passengers and baggage. For business, commuter and commercial aircraft, the payload does not include the flight or cabin crew, only the passengers, baggage and cargo.
1.1.3Cruise and Maximum Speeds
The mission of an aircraft usually dictates its speed range. Propeller-driven aircraft are usually designed to cruise at speeds between 150 to 300 knots. Jet powered aircraft have higher cruise speeds that are normally specified in terms of Mach number. The typical cruise Mach number for business and commercial jet aircraft is from 0.8 to 0.85. This range of cruise speeds is close to optimum for maximizing the combination of payload weight, range and speed. The few supersonic commercial aircraft designs (1) have supersonic cruise speed as their principle design driver and (2) sacrifice range and payload. The cruise Mach number of the Concorde is 2.02. It will carry 100 passengers with a range of 3740 miles, which is considerably less than the aircraft of normal class, which have high subsonic cruise speeds.
Modern military jet combat and attack aircraft usually have a flight plan that involves efficient cruise at high subsonic Mach numbers. This is usually in the range from Mach 0.85 to 0.90. The maximum speed is usually specified in the context of an intercept portion of the flight plan. This has a Mach number that is typically in the range of 2.0.
1.1.4Normal Cruise Altitude
The cruise altitude is generally dictated by the cruise speed, propulsion system and cabin pressurization. An aircraft with an un-pressurized cabin would cruise no higher than 10,000 feet. With propeller-driven aircraft, turbo-charged piston engines can maintain a constant horsepower up to an altitude of approximately 20,000 feet. Higher altitudes are possible with turboprop aircraft, such as the Piper Cheyenne, which have a maximum ceiling from 35,000 to 41,000 feet. The decrease in air density with higher altitude lowers the drag, so that for these aircraft, the cruise range increases with altitude.
At higher subsonic Mach numbers, the turbo-jet engine gives the higher efficiency. For subsonic turbo-jet aircraft, there is an optimum altitude where the fuel consumption is a minimum. This occurs at approximately 36,000 feet. Therefore, it is the best altitude for the most efficient, long range cruise of turbo-jet-powered aircraft.
1.1.5Range
The range is the furthest distance the aircraft can fly without refueling. In a flight plan, range refers to the distance traveled during the cruise phase.The choice of the range is one of the most important decisions because it has a large (exponential) effect on the aircraft take-off weight. An aircraft that is intended to fly across the United States (New York to Seattle) should have a minimum range of 2500 nautical miles. A range of 3500 nautical miles would be necessary for transatlantic flights from East coast U.S. cities to coastal cities in Western Europe. Shorter range transports that are designed to fly between major cities in a regional area (e.g., Los Angeles to San Francisco) should have a minimum range of 500 nautical miles. Twice that range would allow an aircraft to fly non-stop between most of the major cities along either coast of the United States.
1.1.6Endurance
Endurance is the amount of time an aircraft can fly without refueling. With a reconnaissance aircraft, endurance is one of the main design drivers. For a commercial aircraft, a flight plan will include an endurance phase to allow for time that night is spent in a holding pattern prior to landing. For operation within the continental United States commercial aircraft are required to be able to hold for 45 minutes at normal cruise fuel consumption. For international operation, the required hold time is 30 minutes.
1.1.7Take-off Distance
The total take-off distance of the length of a runway needed to accelerate, lift off, and climb to prescribe obstacle height. The obstacle height is 50 feet for military and small civil aircraft, and 35 feet for commercial aircraft. The take-off distance that is required to accomplish this depends on different factors in the design such as the thrust to weight ratio, the maximum lift to weight ratio and the surface of the air field that affects the rolling friction of the landing-gear wheels.
Table 1.1 Typical ranges for different types of aircraft.
Aircraft TypeRange (nautical miles)
Personal/Utility500-1000
Regional Turboprop800-1200
Business Jets1500-1800
Smaller Jet Transports2500-3500
Larger Jet Transports6500-7200
Different designs can fall into standard categories for take-off and landing. A conventional take-off and landing (CTOL) aircraft has distance that is greater than 1000 feet. A short take-off and landing (STOL) aircraft, such as the YC-15 in figure 1.2 can take off and land in under 1000 feet. Both of these would have a ground roll portion during take-off and landing. A vertical take-off and landing (VTOL) aircraft does not require a ground roll.
Personal and general aviation propeller-driven aircraft, which are intended to operate out of smaller airports, need take-off distances of 1200 to 2000 feet. Larger twin engine propeller commuter aircraft, which operate out of medium to larger size airports, have take-off distances from 3000 to 5000 feet. Business and smaller commercial jets have take-off distances of 5000 to 7500 feet. Larger commercial jet transport aircraft require take0off distances from 8000 to 11000 feet. The take-off distance is a function of the altitude of the airport, although the distance at sea level is usually specified. Table 1.2 lists the altitude and runway lengths of some of the major airports in America.
1.1.8Landing Distance
The landing distance consists of the length of the runway needed to descend from a specified height of 50 feet, touchdown and break to a stop. Factors that affect the landing distance are the maximum lift to weight and the surface of the air field, which affects the landing-gear wheels’ braking friction coefficient. The lift to weight ratio directly affects the slowest (stall) speed at which the aircraft can fly. The landing touchdown speed is taken to be a small percentage higher than the stall speed.
Table 1.2 Altitude and runway length of major airports in America
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CityAirportElevation (f)Runway (f)
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AtlantaHartsfield102611889
BostonLogan2010081
ChicagoO’Hare66711600
DenverDenver543112000
Los AngelesLos Angeles12612091
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For commercial aircraft, in a worst case scenario, the landing distance is determined with half of the fuel weight at take-off remaining and with an additional two-thirds distance to account for pilot variability. Even with these measures, the landing distances are almost always less than the take-off distances. Therefore, with regards to airports with available runway distances, the limiting conditions will generally be set by take-off.
1.1.9Purchase Cost
The purchase cost of an aircraft involves the cost incurred in the research, development, test and evaluation (RDT&E) phase of the new aircraft design, and the acquisition (A) or production cost of customer-ordered aircraft. The cost of research and development is amortized over an initial fixed number of production aircraft. Therefore, as the number of production aircraft used to distribute this cost increases, the purchase cost per aircraft decreases. The decision on the total number of aircraft to be produced is therefore an important factor in establishing the purchase price. In some cases, this price and customer competition may be the final arbiters that determine if a design is to be built.
The cost estimates are based on “cost estimating relationships” or CERs. These are simple model equations that correlate a few important characteristics of a larger group of aircraft with their cost. The primary characteristics on which these are based are the weight of the structure of the aircraft, which is a fixed percentage of the take-off weight, the maximum speed at best attitude, and the production rate. From these, we expect that larger, heavier aircraft will cost more than smaller, lighter aircraft. Similarly, aircraft with higher cruise speeds are expected to cost more than slower aircraft.
1.1.10Federal Aviation Regulations
Any aircraft design must consider standards and regulations that are set by government associations. Civil aircraft designed, built and operated in the United States must satisfy the provisions of the Federal Aviation Regulations (FARs). The FARs is continually being updated to incorporate additional requirements that come about due to increased time and experience with existing aircraft. Electronic listings of the FARs can be obtained through a World Wide Web link to the Flight Standards Service of the U.S. Federal Aviation Association (FAA). The exact link can be found through a search under the keyword FAA.
Sections of the FARs that are of particular interest to designers are Air Worthiness Standards, General Operating and Flight Rules and Operations. Air Worthiness Standards Part 23 and 25 in particular define different categories of aircraft (for example, transport or commuter) based on such characteristics as number of passengers and maximum take off weight.
These categories are important in making comparisons to other aircraft with regard to flight performance, or other design drivers.
1.2Design Process
The process of designing an aircraft and taking it to the point of a flight test article consists of a sequence of steps, as illustrated in figure 1.3. It starts by identifying a need or capability for a new aircraft that is brought about by (1) a perceived market potential and (2) technological advances made through research and development. The former will include a market-share forecast, which attempts to examine factors that might impact future sales of a new design. These factors include the need for a new design of a specific size and performance, the number of competing designs, and the commonality of features with existing aircraft. As a rule, a new design with competitive performance and cost will have an equal share of new sales with existing competitors.
The needs and capabilities of a new aircraft that are determined in a market survey go to define the mission requirements for a conceptual aircraft. These are compiled in the form of a design proposal that includes (1) the motivation for initiating a new design and (2) the “technology readiness” of new technology for incorporation into a new design. It is essential that the mission requirements be defined before the design can be started. Based on these, the most important performance aspects or “design drivers” can be identified and optimized above all others.
Following the design proposal, the next step is to produce a conceptual design. The conceptual design develops the first general size and configuration for a new aircraft. It involves the estimates of the weights and the choice of aerodynamic characteristics that will be best suited to the mission requirements stated in the design proposal.
The design will make estimates of the total drag and size the power plant. It will determine the best airframe to accommodate the (1) payload and (2) wing and engine placement. This conceptual design will locate principle weight groups in order to satisfy static stability requirements. It will size control surfaces to achieve a desired degree of maneuverability. Finally the conceptual design will estimate the RDT&E and acquisition costs to develop one or more test articles.
Research, Development and Market Analysis