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View Full Version : How to measure distance from spinner for W&B?



CarlOrton
11-19-2011, 08:53 AM
Not quite there yet, but my Sonex requires me to measure the distance from the tip of the spinner to the main gear axle (taildragger). What is the trick to making an accurate measurement? I can drop a plumb bob from the spinner, but if I draw a line out to the width of the airplane, I have no idea if it's perpendicular to the centerline of the a/c. If I measure the width of the gear, that could be off because the gear leg distance might be more/less depending on "spread" of the gear legs at any one time.

Sorry if I can't articulate this clearer.

Thanks for any input!

rosiejerryrosie
11-19-2011, 12:41 PM
I'm not sure that I understand your difficulty, but the way I'd approach it is to (1) park the airplane on a smooth, level surface (preferably a concrete floor) (2) Draw a line under the airplane between the center of the two main wheels (3) drop a plumb bob from the spinner and mark where it points on the floor (4) measure between that point and the line you drew between the two mains. This should give you an accuarate measurement of the distance from the spinner to the mains axel. (It makes no difference if the main gear is spread or not what you need is the distance from the spinner to the main axel [if there was an axel that ran all the way across from the left wheel the the right wheel] and the distance between the wheels is immaterial.

Gooney_B1rd
11-19-2011, 12:58 PM
This kind of measurement that is required should be given a procedure by the kit manufacturer. If they ask for a parameter, they should give instructions so that you measure correctly. At least this is the way we did it in the Navy years ago.

CraigCantwell
11-19-2011, 05:30 PM
Carl: Drop a plumb bob from both the spinner tip and the center of the tailwheel mount. Snap a chalk line between the two points. Drop a plumb bob from the center of each axle and snap chalk lines again. You can use right triangle trig between the spinner location and the tailwheel location to the axle lines to verify square. Once you are square, strike a cross mark for the spinner tip on the center line and you can then drop a plumb bob anywhere down the fuselage and have a fuselage station quickly.

If you don't have a helper, use some duct tape on the floor with some 2" masking tape over it and drop your plumb bob into the tape just enough to make a mark. color in with a magic marker and do the same with the axles. You can then strike a line between the axles and measure from the spinner mark to that line. As to being square, simple right triangle trig again.

BTW... Doing CG locations on the production birds out in Dock 9 and the fuel buildings, all we use is tape, a plumb bob and a good steel tape measure. The weights guys have a nice little spreadsheet that does all the calculations for them.

Neil
11-19-2011, 05:50 PM
When you get ready to do your weight and balance you will need to know where to start measuring from. Begin by leveling your airplane. If it is a tail dragger you will need to raise the tail until whatever leveling method set forth by the designer can be employed and level achieved. You must also level left and right if you are checking the rigging at the same time. I start with the rudder post and drop a plum bob to mark that point on the floor. Then measure the firewall for center and drop a plum bob there. Now you can pop a long line out beyond the spinner. Drop the line from the spinner if you wish to establish that point. For weight and balance calculations I usually pick a number that is easy to work with (like 50")and set that point as a point forward of firewall or leading edge of the wing. A line should be also popped that is from side to side to represent the main gear position and you will need to know the location of the tail wheel along this same line. The reason for starting your measurements at or forward of the spinner is so all your math will be in positive numbers. Using a point forward of the firewall or wing leading edge provides more meaningful numbers to other builders since different engine, prop, motor mount combinations will mean that weight and balance datum information from the spinner dimension is really only good for that one airplane.

CarlOrton
11-20-2011, 05:53 AM
Thanks, guys, for the relevant help. I've just had a lot going on lately, and couldn't see the forest for the trees. All good responses; I've got it straight in my head now.