Thorp Air Command - T18.net

Hello guys,

Doing our final weight and balance. Where do you put the level on the T-18 when you get it up on the scales? Where is the datum? Where should the CG be?

Thanks for the help. Hopefully DAR and airworthiness cert. soon.

Brent Schultz

Congrats Brent! That's a big step. The WL42 should be level. You can lay a level on the doubler running by the cockpit on the outside of the fuse. The datum can be where ever you pick. Most people use the tip of the spinner.
Cubes

Use the datum shown in the plans, usually out ahead of the plane. The station numbers shown in the plans will correspond to the moment locations. Several weight and balance excel spreadsheets have been published in the forum and newsletters.

_________________
Bob Highley
N711SH
SN 835
KLAL

As Bob said go to the NL's . I found several examples of spread sheets to use as a reference and guide to quantify my numbers . I also spent $20.00 for the WINN Balance CD from Spruce . It allowed me to plug in various conditions to instantly see if the AC was in balance . I went thru EVERY condition I could think of and the AC was always in balance . The other nice thing is you can print out copies of each W & B condition . The DAR thought my presentation was very professional ! I did have the standard single sheet W & B form for him to look at . BOOM !

RB

I realize this is an old thread but I thought I would offer my opinion for anyone with similar questions and finds this thread using the search function. There is no such thing as a wrong datum for weight and balance. Pick what ever you want. I will say, as an active A&P/IA, I prefer the datum to be located on a fixed part of the airframe. My reasoning is pretty simple. If I need to measure something all I have to do is level the airplane, drop a plumb bob to the floor and take my measurement from that point and I have the arm. You can call me lazy but if I can eliminate steps while calculating W&B then I can reduce the chance for error. If I am working with a datum in front of the airplane, then I have to take a measurement from a fixed point on the airframe to find the datum or I measure from the fixed point to whatever I am working on and add/subtract the numbers to get the distance from the datum (this is where matching the station numbers comes in handy and station numbers don't move). I would pick the rudder hinge line over the tip of the spinner any day of the week. I do not consider the tip of the spinner as a fixed point, especially on a homebuilt aircraft (my Thorp has had 3 different engines... O-290, Ford V-6 and O-320). A change to nearly any major component forward of the firewall could potentially change the location of the spinner tip and if the corresponding weight and balance change isn't handled correctly it could cause problems down the road. At a minimum, to keep using the spinner tip as the datum after it's location has changed, a new weight and balance sheet would have to be generated because all the arms will either gain or lose length in relation to the spinner instead of just doing a simple revision to record the changes (there is that laziness again).

Sample CG envelopes:
Datum at station 0.0 = 63.5 to 71
Datum at wing leading edge = 8.5 to 16
Datum at spinner tip = 71.5 to 79 or 71 to 78.5 or 73.5 to 81 (could be anything and easily change)
Datum at rudder hinge = -137 to -129.5

DISCLAIMER: The above numbers are for discussion only. Any resemblance to numbers found on a drawing or actual aircraft documentation is purely coincidental. I don't have my aircraft W&B sheet to look at nor do I have a drawing in my possession which shows weight and balance information for the T-18.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Guys,

I used the leading edge of the wing as my datum. I am on my second spinner, which is different from the first one. Also the CG is usually expressed as a % of the wing chord. Using the leading edge makes the calculation simple.

Regards,

Jim

Ditto... I used my leading edge as well, because it is very easy to locate and not subject to change. Once I leveled the airplane, I located BL0.0 on the floor. Then I dropped a plumb bob on each side and drew a line across BL0.0 on the floor. Then I located the tailwheel axle center, and the main wheel axle centers, and took my measurements down BL0. The rest was arithmetic using the drawing as a reference, and confirming with a tape measure.

The tip of the spinner is way right of BL-0.0... while that doesn't really matter, it just bugged me to use that as a datum.

If I remember right, the tail was about 4 feet in the air.