Maritime Surveillance Aircraft

This was a group project for my aircraft design class. We made a preliminary design of a Maritime Surveillance Aircraft (MSA), designed to loiter for long periods of time over open water. The main tradeoff in this design was the engine choice: turboprop vs. turbofan. Turbofan engines were selected based on the current market for MSAs as well as the desired performance of the plane. The interior of the aircraft can also be reconfigured as a regional airliner. The performance of our airplane was evaluated on a typical regional airliner mission, and it was found to outperform the CRJ-200 aircraft in cruise speed and service ceiling. 

Design Proposal

Maritime surveillance needs around the world are rising, and many countries are looking to replace their aging fleets. To meet the needs of these customers, most designers seek to repurpose existing airframes for maritime surveillance use. Many business jets and turboprops in the medium to large commuter weight class are now being converted to Maritime Surveillance Aircraft (MSA) in order to capture an additional market with the same airframe. Because the market for regional turboprops and bizjets is much larger than the market for MSAs, it makes sense to optimize an airframe for the larger market and add modifications to meet the requirements of the smaller one. The Anthony Series of aircraft utilize the same airframe, with the LAS-Anthony as the MSA variant, and the TER-Anthony as the regional commuter variant. The only differences between the LAS and the TER are the fuel/payload capacity and internal configuration. All other performance parameters are the same. 

 

Requirements

Because we are designing a dual purpose aircraft, there are two sets of requirements being targeted in parallel. There are the MSA requirements, and there are the commuter requirements. Some of the MSA requirements are 5000lb payload (2000 expendable), 300-600nm mission radius, 6hr time on station, 60%-75% mission efficiency, Mach .6-.82, loiter altitude 0-3000, cruise altitude 30k-45k, TO/Ldg distance of 4000-6000, and stall speed of 85-140ktas. The commuter requirements were determined from competitors in a comparable weight class.

 

Design Methodology

The design of this aircraft concept was guided by an intensive Excel spreadsheet and a MATLAB program. The aircraft is being designed to maximize MSA mission performance requirements, while simultaneously evaluating the aircraft performance on a commuter mission for benchmarking with competitors. 

  1. Excel's built in iterative solver was used to calculate a gross takeoff weight (MTOW) estimate based on initial guesses of aircraft geometry, propulsion, and mission requirements.
  2. A design point (wing loading and thrust-weight ratio) was selected based on the mission requirements. With the MTOW estimate, required thrust and wing area could be determined.
  3. The main wing planform geometry was then defined, and an airfoil selected based on its drag bucket geometry and the mission C_l migration.
  4. The fuselage size and shape were defined based on mission payload volume requirements. Using an original MATLAB GUI, component layout within the fuselage was visualized and to aid in optimizing the layout to reduce fuselage length and associated drag.
  5. A complete drag build on the plane was performed for each mission segment. 
  6. The horizontal and vertical stabilizing surfaces were given a preliminary size estimate based on moment arm requirements and competitor aircraft dimensions. The horizontal and vertical stabilizers were then positioned for adequate effectiveness during stall and spin recovery.
  7. Engines were sized to meet cruise thrust performance and static takeoff thrust performance. A propeller was selected based on calculations using the Hamilton Standard propeller charts (turboprop only).
  8. Takeoff and landing lengths for AEO and OEI were calculated along with balanced field lengths.
  9. Enhanced lift devices were selected and sized to optimize takeoff and landing performance.
  10. Design load factor was determined based on performance maxima and regulations.
  11. Bending loads for the wing and fuselage were calculated using the design load factor and component arrangement.
  12. Structural members (longerons, spars, skin) were designed and materials selected for each.
  13. An empty weight fraction was calculated using correlations.
  14. Static stability analyses were performed to place internal components, the main wing (center of lift), and the tail surfaces.
  15. CAD and detailed design of specifying component locations (landing gear, engines, etc).
  16. A cost analysis was performed to determine the production cost of the aircraft.

 

Design Tradeoffs

The main question to answer for this project was whether to use a turboprop, or a turbofan engine. The issue is that each engine is only efficient in a certain flight regime. Turbofans are only efficient during cruise and have poor SFC during loiter, resulting in increased fuel load and weight. Turboprops are more efficient in loiter than turbofans, but they are much slower, resulting in a lower mission efficiency. So throughout the project we were essentially designing two airplanes: a turboprop and a turbofan. Another tradeoff question to answer was regarding the payload. No MSAs in this weight class have sonobuoy capability. There is a choice to design for sonobuoys as a competitive feature, resulting in a slightly larger weight than the competitors, or eliminate the sonobuoys and attempt to minimize the weight. The other main tradeoff is between the cruise speed, engine size, and mission efficiency. Increasing the cruise speed will also increase the mission efficiency, but this will require a larger engine, resulting in more fuel and weight. I have a feeling that we will be more equipped to address these tradeoffs once we start the financial justification process. But is important to keep these tradeoffs in mind at all times, and to continually explore the design space.

Based on market research and MSA competitors, our team decided to go with a turbofan design. The market indicates that there is a growing demand for regional commuter jets, partially stimulated by the recent drop in oil prices. Additionally, our aircraft was designed to cruise on the upper limit of the range, in which turbofans are the obvious choice. 

One aspect of the design that puzzled us was the longitudinal stability and control calculation. The procedure was to calculate the forces required for level flight at a variety of static margins, and plot a number of scatterplots for different parameters that varied with static margin. Then parabolas were fit to those scatterplots and stability at any center of lift location could be determined at any center of lift location using those curves. The center of lift location was optimized to produce a stable aircraft with the least trim drag. However, in order to keep our aircraft stable and lower the trim drag, the tail had to be moved very close to the wing and made quite large (as you can see in the photos). This increased drag caused our weight to increase as well, and overall had a very negative impact on the design. In addition, our static margin was extremely high (50%-75%). I am still trying to figure out exactly why this was happening. I also hope to write a MATLAB script to calculate longitudinal stability. Excel is a very poor tool for this type of calculation.

Specifications

LAS-Anthony (MSA) [Competitor Benchmark: Bombardier Challenger 605]

MTOW: 61,000lb [48,200lb]

Empty Weight: 33,000lb [26,985lb]

Payload: 4920lb, 1920lb expendable [1350lb, 0 exp.]

T/W: .44 [...]

W/S: 90 [...]

Length: 88.7ft [68.42ft]

Wingspan: 80ft [64.33ft]

Engine Selection: 2x General Electric CF34-10A, (thrust scaled by 78.5%), 13,000lbg/eng TO [...]

Design Cruise Mach: .82 [.80]

Service Ceiling: 42,000 [41,000]

Crew: 11 [...]

Mission Efficiency: 62% [N/A]

Design Mission Radius: 500nm [N/A]

Loiter Altitude: 2,000 [N/A]

Materials: Fiberglass skin and wing spar, 2024-A1 Aluminum longerons [...]

 

TER-Anthony (Commuter) [Competitor Benchmark: Bombardier CRJ-200]

MTOW: 61,000lb [53,000lb]

Empty Weight: 33,000lb [30,500lb]

Payload: 13,500lb [13,500lb]

T/W: .21 [...]

W/S: 87 [...]

Length: 88.7ft [87.8ft]

Wingspan: 80ft [69.6ft]

Engine Selection: 2x General Electric CF34-10A, (thrust scaled by 78.5%), 13,000lbg/eng TO [2x General Electric CF34-3B1, 8,729lbf/eng TO]

Design Cruise Mach: .82 [.74]

Service Ceiling: 42,000 [41,000]

Passengers (Crew): 50 (4) [50 (4)]

Design Range: 1650nm [1644nm]

Materials: Fiberglass skin and wing spar, 2024-A1 Aluminum longerons [...]