Assigned 3/28/91 ----- Due 4/9/91 (13 days!)

    Stormin’ Norm and I were hunkered down in the bunker last night watching reruns of the Baghdad air show when he told me about a little problem that they have with their Apache helicopter gunships. These aircraft are equipped with the Gatling gun rapid fire cannon. This weapon fires spent uranium slugs at a rate of 10 rounds per second from the 4 foot long barrel. The slugs are 25 mm dia. by 2 cm long. The muzzle velocity is about Mach 1. The recoil forces from this gun are substantial, and can interfere with the control stability of the aircraft. They want a device which will mount on the cannon or on the aircraft and which will provide time varying inertial forces to counteract and cancel the inertial reaction forces from the recoil. The only other constraint is that this device be as light as possible to avoid reducing the payload capability of the aircraft. The steel alloy material available for the pivot pins has an allowable shear stress of 15000 psi. The links may be made of any reasonable materials, but the materials and their dimensions must be specified, and the links must be capable of surviving the dynamic loads imposed.

    I told him that the WPI engineering brigade would be able to solve this problem in less than two weeks, So, your mission (should you decide to accept it) is to design a mechanism based on the fourbar Grashof crank-rocker linkage which will provide dynamic shaking forces according to the above constraints, and to arrange the shapes and sizes and materials of the links such that the shaking forces applied to the frame of the vehicle are appropriate in direction and magnitude. The driving torque fluctuations are also of concern as they will rob energy from the cannon drive mechanism and thus should be minimized. The pivot pins must be sized against failure, and the dimensions and materials of the links specified.

    Luckily for you, the Army has loaned us a Top Secret computer program developed for Star Wars research called "DYNAFOUR" (available in the Aptlab and MicroCad Lab). You will need to upgrade your copy to Version 5.1 rev 14 1/1/91. This program will calculate all the forces and torques on a Fourbar linkage for multiple positions. You must supply it the link dimensions, masses and moments of inertia for your design's elements, and also the crank input angular velocity, and any external forces or torques. Note that DYNAFOUR imposes a constant angular velocity on that crank, which assumes that the motor used has sufficient torque available at all speeds. You also have to specify what horsepower will be required of this motor, and the required diameter of the pivot pins based on the dynamic pin forces in your linkage and the material strength. TKSolver will be an invaluable aid for calculating masses and moments of inertia of your links.

    As with any design problem, there is an infinity of solutions possible. You are expected to come up with one solution which will work and, most importantly, to understand how it works or, if it doesn’t work, at least be able to explain in good engineering detail why it doesn’t! To do so you will have to try out many alternate designs and iterate to your 'best' solution. You should expect to go through at least ten iterations before arriving at an acceptable solution. You will need to do some paper and pencil (or TKSolver) calculations for masses and moments of inertia of your proposed links before going to the computer to use DYNAFOUR. The entire project including its kinematics can be designed using only DYNAFOUR but the kinematic accelerations can be more quickly done with program FOURBAR. DYNAFOUR can then be used for the force and torque calculations. Note that a Fourbar data file can be read into DYNAFOUR as well.

    You are also required to document your solution in a professional engineering report which adheres to the "Project Report Specifications" document previously distributed. This report will document the process by which you iterated to your final design as well as the design itself. Do not just describe the final result. Rather show me how you arrived at it, including the failures encountered along the way. This will demonstrate to me that you understand the engineering concepts and the relevant course material. A working cardboard model of at least one plane (one side) of your design is required. Your report should include titled (labeled) plots and tables from program 'DYNAFOUR'. Remember, one purpose of the project report is to convince me that you understand your solution well enough to have been the real designer thereof!

Some suggestions to get you started:

1. Do research before trying to solve the problem! Don't 'shoot from the hip'. Avoid BFI (Brute Force and Ignorance). Engineer it.

2. Investigate the dynamic factors involved

3. Mr. Hrones and Mr. Nelson might help.

4. Cardboard models make designing much easier.

5. THINK about the theory discussed in class and in your text. It IS applicable to this problem!

IMPORTANT! IMPORTANT!