This is the original "Plans" method for attaching the chromemoly engine mount to the spar mounted aluminum extrusions. Notice that the engine loads are being applied to the two quarter inch AN fasteners in shear. Notice also that the bolt head and washer are not bearing against a flat surface but against the curved 5/8" diameter tubing.  This method places exceptionally high stresses in one of the two weakest parts of any bolt: the juncture between the head and the shank (the other is in the thread area) . All of the stress is concentrated along a thin contact line tangent to the tubing. Note: only a portion of the engine mount is depicted here and in subsequent images.
 
 
 
 
 
 

Aside from the fact that the bolts are stressed to their limits and subjected to unnecessary high concentrations at the heads, the horizontal bolt has historically interfered with clean fitted cowlings. It's proximity to the edge of the firewall has created problems for more than one builder. Some have gone as far as to build blisters in an otherwise clean cowling, just to clear the fastener.

This method of attaching the engine to the aircraft has no provisions for adjusting thrust angle. If any errors in alignment should occur during construction, the only way to correct them is to construct a new engine mount or attempt repairs to the original one by removing drilled tubes and rewelding new ones. This is a risky business at best, even when the final assembly is heat treated to remove internal stresses.
 
 
 
 
 
 

In this suggested modification, this 4130 Chromemoly extrusion replaces the original aluminum extrusion. It's bolt pattern for securing it to the longerons and main spar is identical to the original aluminum extrusions. The four main engine mount tubes will have to be moved 0.0625" (1/16") in the vertical plane. Two of those same tubes must also be moved laterally 15/16" in order to maintain engine mount alignment. No modifications to the aircraft spar or longeron attachments are needed. The change in the engine mount itself is a welding operation and should be heat treated after the changes are made. In this design, the extrusion is made up of one inch square chrome molly with .120" thick walls and an .120" thick end piece welded in place through which to place the main fastener. The device is then heat treated to relieve stresses using accepted aero-space industry methods. NEVER use an engine mount or related weldement that has not been normalized in an oven. The cut out shown is optional and was incorporated to facilitate the use of socket wrenches and the installation of nuts and washers inside the extrusion. in practice this cut-out has proven to be unnecessary and may be omitted at the builders discretion. The large diameter access hole, sufficient to accommodate a socket wrench will then be a must at the location of every attachment called for in the design (four for each extrusion).
 
 
 

This is what the finished result looks like. That one fastener you see is a standard 3/8 inch AN bolt of appropriate length. The tubular engine mount it passes through is the original one. A small section containing the old cross bolt holes was cut off as needed with a hack saw.  All four corners of the mount are secured in this fashion. This method allows for some limited adjustment of the thrust angle of the engine. If the hole in the 1/8" end plate is made slightly oversize, washers can be used to asymmetrically shim the engine mount to compensate for building errors.  Four extrusions will be required: two left and two right. The main spar or fuselage is not shown in the interest of web speed access.
 
 
 
 
 
 
 

The fastener is in tension as it should be, not in shear. The high concentrations of stress on the bolt heads of the original design have been completely eliminated. The protruding horizontal fastener is also gone, thus making cowling attachment an easier undertaking. Larry Danner, himself a Long-Ez builder and an aerospace engineer,  has suggested that that the extrusion be made from stock slightly larger in cross section, perhaps 1-1/4" or 1-1/2" square, instead of 1". This change will eliminate the need for spot facing the weld area. Spot facing of the welded plate is needed on the 1" design to permit flush fitting of the 5/8" tube to the face. Larry's suggestion eliminates the possibility of sharp edges that the machining would leave behind thus reducing the possibility of stress risers.  We concur and thank him for the suggestion. In fact, the prototype mount used on Long Ez N-67EB and first built in 1988 is a 1" by 1-1/2" extrusion! Whichever size you choose, we suggest that you select an extrusion with a minimum wall thickness of 0.100" to insure adequate weld penetration.

Note: quite some time ago, RAF encouraged owner-builders to replace the aluminum extrusions with steel ones to safeguard against weakeneing of the engine mount and/or the main spar in the event of a prolonged engine fire. Although never tested for resistance to flame, these replacements will obviously withstand higher temperatures than the aluminum ones.  They also will not conduct heat into the spar as readily as the aluminum units of the original RAF design. Still, a thin piece of Fiberfrax, sandwiched between the spar and the extrusion can help reduce conductive heat transfer.
 
 

This animation takes about 60 seconds to load at 56kbaud.

 
 
 

Owners of Long-Ez's or Vari-Eze's whose mounts use the cross drilled fasteners and who do NOT wish to change the extrusions are encouraged  to fabricate special contoured washers like the sample shown here. Since the fasteners are still perpendicular to the loads, these will not eliminate the shearing forces but they will create a large surface against which to clamp and eliminate those high concentrations under the heads.
 
 
 
 
 

Ideally, you would use two per bolt (sixteen altogether), one on each side of the tube. However, placing this washer between the aluminum extrusion and the tube would alter the mounting geometry of the engine mount and create more trouble than it's worth. In actual practice, one per fastener will suffice, that being where the bolt head contacts the horizontal tube member.

These washers are easily made from a few inches of 5/8" round solid steel bar stock. Drill a 1/4" hole down the center and then cut off sections using a 5/8 inch hole saw, applied perpendicular to the center axis of the stock. Make certain that the finished washer has a minimum wall thickness at the crown of no less than 0.100". Dress each one as needed with a round file and/or Dremel tool paying particular attention to the finish of the inside curve. Builders who wish to go one step better can heat each piece to a bright orange with a propane torch before oil quenching. Use all new fasteners and apply with a torque wrench to insure that they are not over-tightend!
 
 
 
 

Free download of construction drawings

Several "A" sized drawings for the original 1" extrusion design are available for free by selecting this link: Engine Mount Mods   . You will need AutoCAD or AutoCAD Light to read them. You can also view them by downloading the E-Drawing utility from the SolidWorks website at http://www.solidworks.com.

None of these graphical representations are photographs. All were modeled and rendered in SolidWorks, an Intel-Windows based, parametric solid modeler.  Materials for this modification were purchased from the Dillsburg Aeroplane Works in Pennsylvania (717- 432- 4589, they have no web site) but anyone with a broad inventory of quality 4130 should be able to supply the needed material.

CCI will not be held accountable for consequences arising from the use of this design. Those who implement it do so with the understanding that they are totally responsible.
 
 

Effective July 2001, A simple addition to the mount of an "L" shaped butress will improve the mount design. It elliminates the need for the welded bulkhead and heat treatment of the steel extrusions thus keeping fabication within the capabilities of the average homebuilder. It also puts the loads in compression on the mount rather than in tension. Best yet, the change works with the previous version mount. Those who have already implimented the previous design can simply add the butress without any need for major alterations to the existing system

JPG copies of the drawings files will make it possible for most anyone to view the drawings without need of CAD software. We are also investigating the use of Adobe Acrobat as an alternative. The revised data will be available soon.
 
 

We received valuable feedback from Howard on the construction of these mounts. His efforts have made it clearer.  We are pleased to acknowledge his contributions and the enhancements that he made possible.