When installing conventional certified engines into proven aircraft designs the process is very simple. One simply buys the mounting frame and, if the firewall preparation is accurate, the installer simply bolts the frame to the engine and firewall. Given that both the firewall and the mounting frame is accurate, the process should only take a few hours. When dealing with an auto conversion there is much to be considered and the process is just another in the long list of things to do. Many folk have asked me how I fabricated my engine mount for the EG33 auto conversion. This page is dedicated to this process.

The first concern was where to attach the frame to the engine and where to locate the decoupling rubbers. I chose to take a non-orthodox method. By non-orthodox I mean that I chose to locate the decoupling rubbers (Lord Mounts) on the firewall and not on the engine. In adopting this approach there is no differential motion between any two parts of the firewall backwards installation, even with the most severe vibration. This gave me the advantage of choosing any available hard bolt patterns on both the bottom and top of the engine without having to provide rubber isolation at each point. The schematic diagram to the right of this text shows the side view of the engine mounting frame. Notice how the lower and upper mount connects directly to the engine at the PSRU bell housing thereby transferring the propeller loads directly from the PSRU to the firewall. The engine does not carry any propeller load.

Pitch moments are taken up by the bed mount arrangements afforded by the lower pairs of mounts. The short tube, connecting the two lower mounts close to the engine, is a 1/2" 0.035" wall tube and is there to prevent the long lower tube from flexing. This anti-flex tube is deliberately located at a non-central location to prevent resonant vibration of this longer support tube. In general the engine mount is fabricated from 5/8" dia. x  0.049" wall 4130 steel.

A cross bracing arrangement is also in place to afford torsional rigidity. These anti-torque tubes are visible in the  photographs shown later in this web page.

Fabrication of this mounting frame was accomplished by mounting the engine to the work bench whilst hanging a dummy firewall above the engine. In this configuration it was possible to set up the appropriate angles and tube lengths whilst making it possible to tack weld the tubes in place. The picture, to the left, indicates the engine in this configuration with the lower two mounts in an early stage of fabrication. (Click on the image for a higher resolution image.) The image clearly indicates the lower tube being attached to the bell-housing end of the engine. This location also supports the through bolt for the PSRU attachment thus providing a good structural integrity between the mount and the PSRU/prop loads.

The view to the left is taken from under the engine and clearly shows the two cross braces which ensure that the engine torque is coupled to the lower mounts and the engine forward attachment.

The six firewall mounting points are locates as follows:

• Lower:  waterline   4.5,    Butt line 14.0

• Mid:      waterline 10.9,    Butt line 11.0

• Top:      waterline 31.8,    Butt line 11.0

(Note: I increased the height of the turtle back by two inches to give more internal headroom at the front and rear of the cockpit.)

The lower mounts are in exactly the same location as directed by the plans and have no additional reinforcement. 

The mid mounts are located just below the spar and are visible in the following picture. A glassed plywood gusset is incorporated between the underside of the spar, and the firewall just inboard of the mounting point so that considerable reinforcement could be applied between the spar, gusset and firewall. The engine mounting point is then located centrally into this reinforcement.

The upper mounting points have the most significant reinforcement since the attachment to the firewall is far away from any substantial structure. The box frame is attached to the spar and the firewall on both sides of the structure. Significant lay-ups run front to back of the spar and then up the firewall to finish at the top front. Having completed those lay-ups a plywood board (glassed on both sides with 2x BID @ 45 degrees) is placed as a diagonal coupling the front of the spar to the firewall just below the mount. This provides a very strong triangulated structure. The total thickness of glass fibre at the engine mount is 1/4". 

The hole to the left of the upper engine mount is for a Cannon Mil Connector which will connect my engine electronic sensors to the ECU located on the inside of the firewall.

Notice that the cut outs for the centre section spar do not extend to the edge of the fuselage. This was done to allow the stresses from the new mounting arrangement, to be carried down into the spar without structural compromise. (The three Aluminium tubes are the hydraulic lines for the retractable gear.)

The strength of these upper engine mounting locations is significant and provides for a roll over structure in addition to the engine mounting function. The figure to the left illustrates the engine mounting arrangement for the mid and upper engine mounts. The thicker lines shown on the diagram indicate the lay-up arrangement. The BID lay-ups are at 45 degrees and fold around to the two side cheeks whereas the UNI lay-ups only run vertically between the side cheeks.

Last Updated:    Thursday August 31, 2006