Weight & Balance

November 2013

I have done everything possible to keep my Patrol as light as possible and today is the day I find out if I accomplished it.  Most Patrols have been coming in 1,120 and 1,200 lbs.  My goal is to have one closer to Bob's prototype Patrol which came in at 1,027 lbs.  Since his had a fixed pitch prop and mine has a constant speed prop, mine will likely be a bit heavier, maybe somewhere around 1,050 lbs. so that is my goal - 1,050 lbs.  Let's see how I did.

My EAA chapter has a simple set of scales that involve a 4:1 ratio device that allows you to weigh it on regular bathroom scales. My first goal was to assure the scales were as accurate as can be.  To assure accuracy, I used a digital scale to calibrate the other three scales.  The digital scale is really accurate.

To calibrate all the scales, I weighed my friend Mickey on the digital scale and then weighed him on the other three scales, calibrating each scale to read the same with him on each one.
weigh1

Now with all the scales reading Mickey's weight of 179 lbs. exactly the same as the digital scale, it was time to place a calibrated scale under each wheel and set the airplane in the flight level condition.

You can't just jack up each main wheel because the jack is in the way of the 4:1 scale device. In order to pick up each wheel, we used the old fulcrum and lever trick that we all learned in elementary school
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Now with a 4:1 scale device under each main wheel, the tailwheel was set on a single scale.  Since the tailwheel is within the weight limits of the scale, no 4:1 device is needed for it.
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We had to put a block of wood under the tire to make the fuselage perfectly level.  The scale is adjusted to read zero with just the block of wood on it.

Next we checked that the fuselage was perfectly level at the bottom longeron just behind the main gear mounts. Here is Burl checking it with a level:
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Here is a picture of the 4:1 device with the scale under it. This looks pretty crude but is fairly accurate:
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 Note the plumb-bob hanging down from the leading edge of the wing.

Here is an overview of the whole set-up for weighing the airplane.:
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Next we took measurements from the Plumb-bob line to all the critical locations:
Now with all of the weights and all of the measurements from the datum (the leading edge of the wings) its time to do some calculations. A lot of head scratching and noodling going on here:
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Here is what we came up with.  The Patrol came in at 1,061 lbs.  A little shy of my goal of 1050 but still a very respectable weight. Here is our report:

Bearhawk Patrol N316BP Weight and Balance                       

Max Gross Wt = 2000 lbs.

Datum = Leading edge of wing                       
Forward CG Limit = 10.5"  -  16%                       
Aft CG Limit = 21.5"  -  32.5%                       

Empty Weight
Weight Arm Moment
Main Gear R 492 -2 -984
Main Gear L 492 -2 -984
Tail 77 203 15631
Front Seat 0 10 0
Rear Seat 0 46 0
Fuel 0 22 0
Baggage 0 75 0
Totals 1061 13663 CG= 12.88

Here is a PDF showing the empty weight figures and some sample loadings both forward Cg and max aft CG with Max Gross weight.  Click here for the PDF

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