AAE 451 Spring 2006
SDR Report
(Group II)
Mike Dumas
Adam Naramore
Ben Scott
Gaetano Settineri
Tim Sparks
David Wilson
Table of Contents
I. Executive Summary...... 1
II. Introduction...... 2
III. Trade Studies...... 5
IV. Concept Generation...... 11
V. Engine Analysis...... 25
VI. Preliminary Sizing...... 34
VII. Current Sizing Estimates………………………………… 37
VIII. Conclusion…………………………………………………40
Appendix A………………………………………………………41
Appendix B………………………………………………………43
Appendix C………………………………………………………48
Works Cited……………………………………………………..51
EcoJet Inc. Page 1 of 51
I.Executive Summary
"Here we have a serious problem: America is addicted to oil, which is often imported from unstable parts of the world," President Bush, State of the Union Address, 28 January 2006. As President Bush stated America truly is addicted to petroleum and petroleum based energy sources, and as a result a national effort has begun to help wean American off of its oil addiction. With this in mind, the goal of this project is to design an alternative fuel aircraft for either the business aviation (BA) or the general aviation (GA) market. Research is being done into the application of bio-fuels in current propulsion systems, battery powered or battery supplemented propulsion, and fuel-cell powered propulsion.
EcoJet Inc. is designing a BA aircraft targeted to fractional ownership companies, companies such as NetJet and Flexjets, and any other business aircraft operators. With this market in mind EcoJet will be working toward a fast, reliable, alternatively fueled business aircraft.
The primary concept is a box wing design aimed to carry six passengers with a1700 nautical mile range and a cruise mach number of 0.85 which should pique the interests of business aviation operators. In addition to long range and high speed performance, EcoJet will also seek to create an aircraft that is sleek and comfortable for the passengers, as well as being easy to operate and maintain. Lastly, minimizing both the acquisition and operating costs will be critical because of the target market. With all of these factors in mind this paper will seek to provide a general sense of the current concept as well as justification for many of the important decisions that have been made concerning various aspect of the current concept.
II. INTRODUCTION
System Definition Review, Purpose and Format:
The purpose of the system definition review (SDR) is to provide a more detailed “picture” of the aircraft under design by EcoJet Inc. The main content of the SDR will be the trade studies and concept selection process used to arrive at the current aircraft concept. The trade studies will help to show the reasoning and justification for the current design requirements as well as the current concept being carried forward. This paper will provide a brief review of the business case, a listing of the current design requirements, an overview of the concept selection process, a look into the propulsion system analysis completed thus far, the methods used to reach preliminary sizing estimates, and a review of the preliminary sizing and cost data.
Business Case Review:
The target market of EcoJet Inc. is fractional operators such as NetJets and FlexJets and corporate aviation departments. Current predictions show that the price of oil will steadily increase over the coming years, and as a result EcoJet sees a potential market for alternative fuel based aircraft in the next ten to twenty years. According to the Roll-Royce Forecast shown on the next page in Figure 1 (Rolls-Royce.com) the largest fraction of business jets expected to be delivered between 2003 and 2022 are aircraft in the “light-medium” category.
Figure 1: Rolls-Royce Outlook 2003-2022
Aircraft that fall under the “light-medium” classification typically have a maximum range between 1,600 and 2,000 nautical miles and are primarily used to fly coast line air routes and one stop cross-country routes. Aircraft in this category include the Cessna Citation XLS, the Raytheon Hawker 400XP, and the Bombardier Learjet 40XR. There is a large variety of aircraft already in this market, but by offering an alternative fuel based platform EcoJet can anticipate a substantial share of the market as the price of oil based fuels continues to increase. The proposed market capture target has been set at 2.5% of the current market share this equates to approximately 25 aircraft per year for each of the first five years of production. This proposal is conservative when compared to data of aircraft sales from JetSales.com, but a conservative estimate is justified because currently petroleum based fuels are still competitively priced. Based on trends from JetSales.com after five years of production, assuming adequate initial market capture and exposure, a 5% market capture target has been proposed, which will account for about 50 aircraft sales per year.
Current Design Requirements:
The design requirements for the aircraft have changed since the system requirement review (SRR) was completed. In Table 1, shown below, the current design requirements are summarized; note the distinction between target or goal values and the threshold or limit values. The changes made from the requirements presented in the SRR have come as a result of feedback and analysis completed since the SRR. Based on these design requirements EcoJet Inc. feels that the aircraft that will be put into the market will be very competitive as is, and as the price of petroleum based fuels increases in the coming years this aircraft has the potential to quickly become the best selling aircraft in its class.
Table 1: Current Design Requirements
Parameters: / Targets: / Thresholds:GTOW (lbs) / 20,000 / < 25,000
Cruise Mach / .85 / > .77
TO Field Length / 3500ft / < 4000ft
Passenger Cap: / 6 / ≥ 4
Range / 1700 nm / ≥ 1500 nm
Acquisition Cost / $10 million / ≤ $15 million
Operating Cost / $1,000/hr / ≤ $1,500/hr
III. Trade Studies
After creating an extensive aircraft database, historical trade studies could be performed. Thirty-two aircraft are currently in the database and are listed in Appendix A. Some of the trade studies are listed here, and several more are in Appendix B. There will be a translucent red bar over the area of the curve that we have set our mission design requirements and thresholds or the area of the curve initial results have put us in.
Figure 2below plots for all of the aircraft the maximum takeoff weight versus acquisition cost. The current design goals of designing this aircraft to be 20,000 lbs for $10M is currently exactly on the trend line at the left most edge of the red box. Our current acquisition cost of $8.64M and a weight of 21176 lbs match 2 Learjet models to the left of the red box. With time and calibration, the cost of the design aircraft will increase, but our target and threshold values fit very well with this trend.
Figure 2: Maximum Takeoff Weight vs. Acquisition Cost
The acquisition cost was plotted versus the nominal cabin volume of each aircraft in Figure 3 on the next page. The nominal cabin volume is simply the cabin height, cabin center width, and the cabin length multiplied together. This value is larger than the actual cabin volume but enables each aircraft to be compared equally. The current estimate of the design aircraft’s nominal cabin volume is 840 ft3. If this value is placed on the trend line, the acquisition cost is $15 million. This cost is at the upper threshold of the mission requirements. The cabin volume may have to be made smaller based on the previous trade study with max takeoff weight, the design aircraft is well within target of the acquisition cost.
Figure 3: Acquisition Cost vs. Nominal Cabin Volume
Figure 4 on the next page plots the maximum takeoff weight versus the range with maximum fuel and the range with maximum payload. The range with maximum payload trend line crosses the red box about at the design aircraft’s current maximum takeoff weight. The range with maximum fuel trend line crosses the red box at our design goal. Based on the mission requirements, the design aircraft is on target to meet its goals.
Figure 4: Max Takeoff Weight vs. Range with Max Fuel and Range with Max Payload
A Cost Effectiveness Index (CEI) was calculated using a cost performance method for business aircraft by Amir Moghadam and Ken Farsi. This method is charted for each aircraft in Figure 5 on the next page. The basic premise behind this method is calculating the ratio of a performance index over the acquisition cost of the aircraft. The values were scaled to 1.0 based upon aircraft that are in a similar weight and range class. Any aircraft above 1.0 has a range greater than transcontinental.
The performance index is made up of three parameters: the nominal cabin volume, the cruise effectiveness, and the takeoff effectiveness. The cruise effectiveness is a product of the speeds, payload weights, and ranges at max payload and at max fuel. The takeoff effectiveness is the greater value of the takeoff distance, the accelerate/stop distance, or the takeoff field length. A higher CEI value means that an aircraft has greater performance per dollar then an aircraft with a lower CEI value.
A few conclusions can be taken from this chart. First, range has a significant affect on the CEI value. Second, the design aircraft should aim for a CEI value ranging between 0.1 and 0.2. Finally, there is a large difference in CEI values between aircraft manufacturers regardless of design requirements.
Figure 5: Cost Effectiveness Index (Unitary) versus Aircraft
Based on sizing equations from chapter 3 in Raymer’s book, sizing trade studies were done to see the effect of range, payload and lift to drag ratio for a bio-diesel powered aircraft compared against an aircraft using conventional Jet-A. These trade studies were done to get rough estimates of the gross takeoff weight and understand how gross takeoff weight will vary when certain design parameters are altered. Figure 6 on the next page illustrates how the design range affects the gross take off weight for various payloads for a fixed lift to drag ratio.
Figure 6: Range-payload vs GTOW
The trends show that for an aircraft using bio-diesel the GTOW increases more rapidly with range compared against Jet-A fuel. This study estimates that for a 2500 nautical mile range, an aircraft using Jet-A can carry 400 more pounds of payload and still achieve the same GTOW. Similar trends were found when varying both the range and lift to drag ratio while keeping the payload fixed, and also when varying the payload and lift to drag ratio while keeping the range fixed. These graphs can be found in AppendixB.
A similar set of trade studies were done for acquisition cost estimates. The cost model was found by fitting a regression curve to graphical data taken from the aircraft database that was compiled. The curve is a function of estimate GTOW and the design cruise Mach #. For all the trade studies, the Mach # was set at the design requirement of 0.85. Figure 7on the next page shows the cost estimates for varying ranges and payloads for a fixed lift to drag ratio.
Figure 7: Acquisition cost vs Range-payload
The trends found for the GTOW are also found for the acquisition cost. This is due to the direct dependence acquisition cost function on GTOW. Similar graphs were generated for varying the range and lift to drag ratio and varying the payload and lift to drag ratio. These graphs can be found in Appendix B.
The goal of these trade studies was to obtain basic trends for various sizing parameters. They are in no way intended to be accurate sizing and cost models. A more thorough sizing code was used more detailed sizing estimates and is explained in section VII.
IV. Concept Generation
Pugh’s Method
Pugh’s Method was used to aid in concept generation and selection for the alternative fuel business aircraft design. In the end, two concepts were chosen to bring forward into further analysis in the coming weeks.
Criteria for the concept selection were pulled from the QFD matrix for the purpose of ranking concepts. Table 2below discusses why each criterion was chosen for concept selection analysis.
Table 2: Concept Generation Criteria Discussion
Criteria / ReasonsCabin Height
Cabin Width / Cabin dimensions are very important for the desired customers of this aircraft. According to Troy Dowman, Raytheon Aircraft Co., executives will sacrifice extra cost and purchase hours in a larger aircraft if the cabin better suits their needs (i.e. standing height, working space, etc.). The design requirements call for a large cabin based on a traditional fuselage.
Takeoff Length / The design mission specifies a takeoff field length goal of 3,500 feet. This ability enables an aircraft to fly into small regional, executive, and large municipal airports to bypass major airport congestion while also allowing executives to fly closer to their actual destination.
Turnaround Time / Any aircraft with the dual purpose of being flown for profit and executives must minimize time on the ground. This criterion includes passenger on/offloading, refueling, payload loading, and any checklist time. Also, FAR 91.1059 states that the maximum duty period for each pilot is 14 hours within a 24 hour period. The turnaround time must be minimized to enable pilots to fly the full mission range and return within the required time of 14 hours. Otherwise, an alternate crew must be found to fly the return flight, possibly delaying passengers.
Time to Overhaul / An aircraft that must be overhauled and maintained more frequently or for a longer duration costs the owners, crew, and customers more money than needed. Aircraft that have lower maintenance costs are going to compete well in the market place regardless of some disadvantages.
Aircraft Length / The overall aircraft length determines overall crew and passenger comforts, and payload room. However, increasing this value too much can limit terminal or ramp access and create excess fuselage weight
TO Weight / This value is directly related to cost. An aircraft that weighs more will require more fuel and thus a higher operating cost. The plane may fly passengers very comfortably and suit all their needs, but when the marketplace is business jets, minimizing cost becomes a priority.
Range / The design mission goal is 1,700 nautical miles. The concepts must be able to reach this goal in minimal time and fuel.
# Seats / The cabin must be large enough to seat 6 passengers in equal comfort. As a business jet, the seats need to be large, comfortable, and maneuverable to aid in passengers working together. This value significantly affects cabin size, aircraft length, weight, and costs.
Ceiling / The mission design goal is 45,000 feet. This value enables the cruise portion of the flight to occur above regular commercial airline operations and above most weather phenomenon.
Climb Rate / For the specified takeoff field length, an aircraft must be able to take off the desired runway and climb above a 50 foot obstacle in as short of a distance as possible. The climb rate needs to be large enough to accomplish this task in any airport an executive might desire to fly in.
Cruise Speed / Much emphasis has been placed on the desired cruise Mach of 0.85. This value will enable businessmen to fly to their destinations quickly. FAR 91.1059 states that normal flight time for a flight crew of 2 may not exceed 10 hours with an extension up to 12 hours. Our concept must be capable of flying the full range, including reserves, and return within the 10 hour flight time limit.
Operating Cost
Acquisition Cost / In the business market, operating cost is more important than acquisition cost. Aircraft must have quick, cheap turn around period to the benefit of not flying on an airline. Acquisition cost is a one time item. Since aircraft can be flown for decades, a minimal operating cost is essential to maintaining profit margins.
After all the concept drawings were created, they were split into four main categories: a traditional wing, a blended wing, a box/jointed wing, and a supersonic cruiser. The Cessna Citation XLS was chosen as the baseline to use for the first run through Pugh’s Method. In the second run though, the traditional concept was chosen for the baseline case to compare with.
For each criterion, a rating was given to each concept. A (+) was given if the concept would be better than the baseline case. A (-) was given if the concept would be worse than the baseline case, and a (S) was given if the concept would be the same for that criterion for the baseline case. The cost criteria were ranked on a different system. The four concepts were rated for appearing to have the highest cost (value of 1) and the lowest cost (value of 4).
Concept 1 – Traditional Aircraft
The traditional case has the body of the Cessna Citation XLS but with the performance characteristics of the project mission design requirements. Figure 8below is a sketch of the traditional concept aircraft.
Figure 8: Concept 1 - Traditional Aircraft
Table 2: Concept 1 - Traditional Aircraft - Pugh's Method