Author Topic: Weight & Balance  (Read 3579 times)

0 Members and 1 Guest are viewing this topic.

Offline flyingwaldo

  • Moderator
  • Newbie
  • *****
  • Posts: 274
    • View Profile
Weight & Balance
« on: November 29, 2013, 12:27:06 PM »
On a Long EZ where to measure for CG of pilot, passenger and fuel?  My tanks are 10 gallons smaller than stock.....
Thanks!

Offline BruceHughes

  • Newbie
  • *
  • Posts: 71
    • View Profile
Re: Weight & Balance
« Reply #1 on: November 29, 2013, 12:47:19 PM »
Do you have an owner's manual?   Look around page 27 -30 (I am working from memory).
What are the changes to cause a lower fuel capacity?   Why were the changes made; that
will make it a bit difficult to get ultimate accuracy but the manual should be a lot of help.

Bruce Hughes   :)

Offline go ez

  • Newbie
  • *
  • Posts: 92
    • View Profile
Re: Weight & Balance
« Reply #2 on: November 30, 2013, 04:32:48 AM »
Really you need to do a full from scratch W&B on the a/c after it is finished  - first empty, then with say half and then full fuel to derive  the fuel fuselage station for your aircraft to use for ops flight W&B calcs. Dont automatically use Rutan handbook data for fuel, pilot and P2  position and even canard leading edge etc etc etc unless your a/c conforms to Rutan's prototype a/c planform dimensions

You could work it out by knowing the geometric shape & depth of the tanks but its a stretch to do accurately and sufficiently such that you are happy to use that data for the life of your aircraft (and your life too!).


Regards
Steve

Offline flyingwaldo

  • Moderator
  • Newbie
  • *****
  • Posts: 274
    • View Profile
Re: Weight & Balance
« Reply #3 on: November 30, 2013, 10:29:27 AM »
Steve,
Thanks for verifying what I suspected!   The builder used the poh dimension for fuel knowing he had changed the cg of the fuel.  Not sure how I'll find the right moment arm.   Will look more closely to the plans this AM but so far have measured a couple of the arms they used and wrote in the poh and they don't land on the same datum point so I need to start from scratch to be sure hence my question about how to find cg of pilot, passenger and fuel using a new datum.  General thought seems to be belly button position for bodies, fuel will be tougher....
DV

Offline Marc Zeitlin

  • Newbie
  • *
  • Posts: 34
    • View Profile
    • COZY Builders Web Site
Re: Weight & Balance
« Reply #4 on: November 30, 2013, 11:25:53 AM »
General thought seems to be belly button position for bodies, fuel will be tougher....
Get a good set of scales. NOT bathroom scales. Calibrated aircraft or auto racing scales. Level the aircraft longerons in a closed hangar (NO BREEZE).Measure the position of the nose wheel and main gear axles in relation to the aircraft datum points and aft face of the firewall - correlate and make sure you know where the axles are. Weigh the airplane EMPTY. Weigh the airplane with YOU in it. Weigh the airplane with whomever else will fly the plane in it. Weigh the plane with someone sitting in the back seat. Weigh the airplane with 1/2 fuel and full fuel.

Once you have all these weights (and with three scales, you'll know the weight distribution on each axle), you can then calculate all the moments in each configuration and back out the moment arms for the front seater, back seater, and fuel. You will find that the moment arm for humans will move around depending upon the shape of each human - long legs, long torso, etc. will make a difference - can be up to an inch or two.

Offline flyingwaldo

  • Moderator
  • Newbie
  • *****
  • Posts: 274
    • View Profile
Re: Weight & Balance
« Reply #5 on: November 30, 2013, 06:45:19 PM »
Thanks Marc.  I only had temp use of the scales for weighing the plane and couldn't do what you describe that would have given me the data to find what I need.  Now I can visualize how that works.  Write the static equation and solve for the unknown.  I did all the measurements after the scales went to the next plane and found a difference in main gear location and both were different than what's been used on all previous W&Bs....  Forgot to check the position of the instrument panel with the canard and wing to verify it's correct.  Will do that next.
Thanks again!
« Last Edit: November 30, 2013, 09:47:07 PM by flyingwaldo »