Overview

Project Merlin began with the design concept to build a wide flight envelope aircraft. In order to do this, several major technologies were included in the design to give the aircraft the desired range of speeds and flight characteristics. The two main technologies that were included in this design are the use of variable wing sweep and thrust vectoring.

The design began on the white board and in the computer. A combination of hand calculations, Matlab programs, and 3-dimensional SolidWorks models with computational fluid dynamics (CFD) analysis found that the current design would be able to satisfy the design objectives. The weight of the plane is estimated to be about 60 pounds at take-off and a small jet turbine engine will provide the thrust. These were the two starting numbers that the rest of the design was centered on.

Aerodynamic

The aircraft configuration, body shape, and airfoil selection were the first aspects of the aircraft that were designed. The overall design came from a combination of looking at existing aircraft with similar missions and by reading several aircraft design books. Aircraft data for many existing aircraft was taken from the internet and compiled into a spreadsheet to look for trends among the different airplane characteristics (thrust to weight, aspect ratio, airfoil, etc.).

One of the first decisions made was to go with the canard design instead of using a horizontal tail. The canards are important to the design for several reasons. First, the canards offer a safety net for the aircraft. When an aircraft with canards stalls, it loses lift on the canards before the wings. When this happens the nose of the aircraft drops down and the aircraft will increase its speed and regain control. This makes the aircraft easier and safer to fly. Having canards also adds to the lift of the aircraft. A rear tail has to be angled slightly down so that it produces downward force acting against the lift created by the wings. This is necessary to balance out the moment about the center of gravity that the wings create. Canards need to produce lift to balance the moment that the wings create. The thrust vectoring will allow us to fly at much higher angles of attack and lower speeds but traditional control surfaces (ailerons and elevator) lose their effectiveness at higher angles of attack. As a result, the canards will also be used to maintain control the aircraft in these flight conditions.

The body of the aircraft was designed to be a lifting body. The fuselage produces about the same amount of lift as one of the wings, giving Aesalon high lift and low drag when the wings are swept.

During the initial design stages CFD calculations were run using COSMOSFlowWorks, an add-on to the SolidWorks 3D modeling package. After each set of CFD runs the shape of the fuselage, size and position of the wings and canards, and complexity of the model were changed. The shape of the aircraft, as well as a pressure plot from the fourth such iteration, is shown in Figure 1.

By the eighth design iteration (Figure 2), the fuselage was at its final outer shape and the wings and canards had been properly sized and placed. The engine inlets and thrust vectoring mount can be seen, as well as the cuts in the wings and tail for the ailerons and rudder. Figure 2.3 shows the forces on the aircraft components and Figure 2.4 shows the moments on the entire plane for different center of gravity positions, both with respect to angle of attack. Using this data, it was determined that the center of gravity of the airplane needs to be less than 1.3 meters back from the tip of the nose in order for it to be statically stable.

he forces in Figure 3 were used to size the servos needed for all of the control surfaces. The pressure distribution obtained from the CFD runs was used to characterize a flush air data system that reads pressures from ports in the nose cone and, through the use of a neural network, converts the raw pressures into angle of attack, airspeed, and pressure altitude measurements. Additional verification tests of the CFD to assist in the characterization of the air data system were performed in the Engineering Physics wind tunnel. The wind tunnel model was manufactured using rapid prototyping facilities available courtesy of the Mechanical Engineering department.

Figure 1 – 4th design iteration results

Figure 2 – 8th (final) aerodynamic design iteration

 

Figure 3 – 8th aerodynamic design iteration force curves

Figure 4 – 8 th aerodynamic design iteration moment curves

 

Figure 5 – Wind Tunnel Testing

Materials

The skin of the Aesalon prototype is made out of aerospace grade fiberglass. The fiberglass was chosen because of its high strength and light weight. The process for manufacturing the skin was also a strong point for the fiberglass. Carbon fiber would be an upgrade that might be considered for the future.

Fiberglass has a very high tensile strength relative to its weight but it does not resist bending very well. Project Merlin has designed Aesalon to use fiberglass as a structural skin. This means that the skin will be the surface that interfaces with the surrounding air to produce the lift for the plane but there will not be any bulkheads, as in an aluminum aircraft. The skin will also act as the supporting structure for all of the loads on the aircraft, internal and external. It has been found, using CFD analysis, that the fiberglass will be able to withstand the forces that will be applied to it during flight. The skin is approximately 1/16 of an inch thick. Using only a few main supports, the aircraft structure has more than sufficient strength, tensile and bending. Figure 6 shows the layout of the bottom of the fuselage, the fiberglass skin and the fiberglass support structure.

Figure 6 – Bottom Fuselage Layout

The manufacturing process for the fiberglass is also a benefit for this design. The fiberglass begins as a cloth and after applying a chemical resin to the cloth the material hardens into the form that is desired. A mold is needed to hold the cloth and resin in place while they dry. A mold can be created for any shape desired and the fiberglass will conform to fit the mold. The fuselage of Aesalon was designed in a virtual world were it is easy to make curved surfaces. This shape is very significant to the lifting characteristics of the aircraft, as seen in Section 2.2 on Aerodynamics. It would have been incredibly hard to manufacture an aluminum frame and skin to fit the shape of the Aesalon fuselage. Therefore, the ability to easily form curved shapes based on an easy to build mold was a must for this design.

After building a prototype of the aircraft, it has been found that our original weight goals were much more difficult to achieve than first anticipated. The main source of the aircraft weight is the skin. In order to cut this weight, by up to half, carbon fiber will be used as a substitute for the fiberglass. The carbon fiber is stronger and weighs about a 1/3 less than fiberglass. Using a vacuum bagging technique and reducing the thickness of the skin the carbon fiber would cut the weight of the skin up to 50%.

Variable Sweep Wings

The wings on Aesalon will be able to sweep back. This feature was chosen to increase the top speed that the aircraft will be able to reach. During slow flight the wings will have a sweep angle of 10° back from perpendicular to the fuselage. As the airplane speeds up the wings will automatically sweep back to a maximum of 50°. This variable wing sweep will decrease drag by approximately 10%, according to the CFD, and will increase the aircraft’s top speed by 15 mph. The top speed will be a little over 200 mph.

As mentioned in previous sections, the wing sweep will occur naturally. This is due to the new and innovative design using springs to control the sweep of the wings. The configuration is made up of a cable attached to the root leading edge of each wing. The cable is then wrapped around a pulley and attached to the springs. As the forces on the wings increase (when the speed increases) the moment about the pivot point will grow to the point that the springs will begin to compress and the wings will sweep back. This method of sweeping the wings saves on weight, power, computing, and complexity over using electric motors, a program to determine the optimal sweep angle, and a complex mechanical system to sweep the wings.

Propulsion and Thrust Vectoring

In order to achieve a wide flight envelope it was necessary to use a jet engine for propulsion. The jet engine that Aesalon was developed around is a 36 pound thrust jet engine. The engine is a product of the SimJet Company in Denmark. This micro turbine engine will give Aesalon, take-off weight of about 60 pounds, a thrust to weight ratio of close to 50%. This is much lower than that of a fighter airplane but higher than a cargo or commercial aviation airplane. A thrust to weight ratio of 50% will give the aircraft relatively good handling properties while also allowing for a significant payload capacity.

A picture of the engine used in the current design is in Figure 7. This engine has a 4.8 inch diameter and is about 12 inches long. It also weighs about 3.7 pounds. This engine was chosen from a large field of turbines for remote controlled aircraft. The SimJet model was the proper thrust for the aircraft we were designing and also had a very good price for the thrust that it produced. This engine would be used in the actual production version of the Aesalon aircraft.

Figure 7 - SimJet Nexus 3600 Micro Turbine

 

To lower the minimum speed that Aesalon can achieve a thrust vectoring system has been added to the vehicle design. Thrust vectoring gives the operator or controlling computer the ability to control the direction of the exhaust gases flowing out of the engine nozzle. Using this technology, it is possible to maintain controlled flight at much lower cruise speeds. The thrust vectoring on Aesalon is expected to lower the minimum speed of the aircraft by approximately 15 mph. This is a significant expansion of the flight envelope. Using two paddles mounted to the engine nozzle, the flight computer will be able to redirect the exhaust flow as needed to help with pitch control and especially for high angle of attack flight.