The engine mount

(Click the image for a larger version)

Mounting pads at the upper longeron have additional bolts to help transfer the load though the wooden fire wall. As can be seen in several of the detail photos of the welding jig, the central tube (1/2" x 0.063) passes through the fire wall in the manner of a bushing. In this way, the mounting pad tube acts to increase the bearing strength of the mount on the fire wall. The four 3/16" bolts add additional bearing area.

Behind the upper mounting pad is a piece that ties together the pad, the upper and lower shear plates, and the longeron. It is welded from 0.063 plate, bedded in epoxy/flox, and bolted to the firewall structure.

I welded the mount in two halves. The sides of an engine mount are symmetric, with one additional diagonal tube that joins the two sides together. Using a symmetrical jig to weld the two halves together allows better access to the individual clusters. A second jig (which I forgot to photograph) holds the two halves together in proper alignment to allow the last diagonal to be fit and welded.

The mount is 22.5 inches long. I have included a portion of the Excel spreadsheet that I used to calculate my preliminary weight and balance and show why I picked that length.

The design case is me with an average sized person, a small amount of baggage, and a full load of fuel. A 22.5 inch long engine mount assures that I will be in my allowable CG range. Two heavy people will be able to fly; however, there will be a fuel and baggage restriction to maintain CG and gross weight in spec. I was less concerned with the forward CG case, but a minimum pilot weight of 140 can be accomodated.

(The calculated stick-fixed neutral stability point is 34.5 %)


CG Range - referenced from the firewall
15 %27.65  
30 %34.56  

As measured on 12-17-01
 Moment ArmWeight 
Left Main23.6251964630.5
Right Main23.6251844347.0
(Not yet installed/built)
Header tank8.375216.8
Wing Finishing35.00010350.0
(Battery not included, used for final balance)
Engine related
Mount length22.5  
 Moment ArmWeight 
 Empty CG24.1 

Design Case
 Moment ArmWeight 
Header Tank10.00018180.0
Wing Tanks41.0002409840.0
  30.0 % 

Just me
 Moment ArmWeight 
Header Tank10.00018180.0
Wing Tanks41.0002409840.0
  23.8 % 

Two Heavy People
 Moment ArmWeight 
Header Tank10.00018180.0
Wing Tanks41.0001807380.0
  29.7 % 
(Limited to approximately 30 gallons of gas, no baggage)

Forward CG limit
 Moment ArmWeight 
Header Tank10.00018180.0
Wing Tanks41.00000.0
  14.8 % 
Minimum pilot weight w/out ballast of approximately 140 pounds

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