The Gun-Smash and Atlas Series: A Senior Design Project (Part 1)

In order to graduate from RPI, each engineering student must participate in a senior Capstone design project. As I described in a previous blog entry, I was a member of a 9-person team designing a two-craft helicopter system to compete in this year's AHS RFP. I'll break down our design of the Gun-Smash helicopter in Part 1 here, and describe our Atlas lifting system in Part 2.

Design Philosophy

The mission requirements for this project were that two rotorcraft vehicles needed to operate as a system such that they could carry 75% more payload than either could accomplish alone. They needed to deliver a payload 100 nautical miles from the starting location, with a 10-miute hover period mid-mission. Both vehicles then needed to return to the initial location. The system needed to accommodate ISO containers, military vehicles, and other large machinery.

A brief aircraft configuration study (more detailed in another blog) was performed in order to weigh our various options. We compared the tandem rotor design to the traditional SMRTR (single main rotor tail rotor) method, as we felt these would be the best potentials for our goal. Using MATLAB code to compute estimated numbers, the tandem configuration was deemed a better option due to the lesser power loss.

Rotor and Hub

As a foundation for the design, we chose to use Boeing's CH-47 Chinook. It consists of two rotors in tandem, rotating in opposite directions to cancel out the moments caused by the blades. In addition, we integrated the use of flex beam technology, which is mechanically simpler than the current design; fewer parts mean less potential of failure. We suggested they be manufactured from fiberglass rather than carbon fiber based on the needs of the craft.

In order to carry a larger payload, more lift is necessary. One of the ways we accomplished this was by using new airfoils with updated blade twist geometry. After significant research, we concluded that the use of the VR-12 and VR-14 airfoils would increase the performance standards of the current VR-7 and VR-8 design. We allowed for 11 degrees of twist from root to tip, with 18-degree twisting outboard to 7-degrees at the tip.

Another aspect we focused on were some solidity factors. We decided not to increase the rotor diameter. By leaving this dimension unchanged, we did not modify the necessary space to store one of the craft. We did increase the chord of the blade form 2.5 feet to 3.5 feet to lower the lead-lag moment on the rotor. Initially, we planned on integrating 4 blades into each hub and rotor assembly. After more careful thought and consideration, however, we found that the added weight was unproductive and less efficient than the 3-blade design.

Engine and Transmission

Many modifications were made to the Chinook design to reach the engine and transmission assembly of the final Gun-Smash vehicle. We upgraded to two Allison AE1107C engines, which are currently used in the V-22 Osprey. It's high output, with a capacity of 4 gallons per engine. The oil system consists of a tank, pump, cooler, particle detector, redundant filtration, and a separator. Aluminum was selected for the casing since it is good with heat, lightweight, and inexpensive. The gear stock, however, was improved to Inconel 625 since it is very tough and forms a layer of passivation at the surface. A dry sump system was chosen for lubrication.

Fuselage and Landing Gear

We proposed a change to the fuselage of the helicopter and intended to model it after the S-64 Skycrane. We realized quite quickly, however, that this was not as feasible as we had anticipated. By removing a large portion of the body, we were also detracting from its utility for other purposes. The only major change from the Chinook was the fore and aft hooks, which were reinforced for the added payload.

Join me next time for details on the Atlas and for other details about the rotorcraft system!