The intention of this article is to describe a portion of the process required to engineer a frame and engine mounting system for a single engine aircraft.

Figure 1: Many Design Options are available. For example, the above image depicts two options for mounting to the firewall.

The mounting system is the primary interface between the powertrain and the frame. Therefore, it is vital to determine the vibration isolation characteristics. Many mounts are available to isolate these frequencies. However, for this project, my client chose an elastomeric mount. This is primarily a rubber mount that can withstand large amounts of deformation under load with the ability to almost retain its original shape when the load is removed. Rubber is a viscoelastic material which enables it to be used as an isolator and as a damper with excellent results. Rubber engine mounts can be implemented through a passive system, or an active system with a control loop adjusting stiffness based on sensor input to the computer. Many times, these are used in conjunction with one another to provide design redundancy.

Requirements:

  1. Determine force isolation, often encountered in rotating or reciprocating machinery with unbalanced masses.
  2. Determine motion isolation……minimize the transmitted vibration amplitude so the mounted equipment is shielded from vibrations coming from the supporting structure.
  3. Ensure the natural frequency of the air-frame and support equipment is lower than that produced by the power plant.

Goal:

Achieve low vibration transmission to the airplane structure and occupants.

Mechanical System Considerations:

As engineers, we are constantly weighting out design options. For this challenge, mount stiffness must be as low as possible. However, this causes increased static deflection. Also, handling and maneuverability are enhanced with higher stiffness, lower dampened engine mounts.

Figure 2: Mech Model for Elastomeric Engine Mounts

Modeling Setup:

Six DOF to be excited due to inertial forces and torque produced by the main crankshaft.

Figure 3: Engine 6 DOF

Figure 4: Spring dampener on each axis

 

Supporting Theory:

 Utilizing Voigt’s & Maxwell’s Model we can accurately depict the system.

  1. The model consists of a single degree of freedom system where the spring and damper are represented by the stiffness