Weight and moment problem that is beyond my education level!

[helicopterguy]

The basic gist: We are preparing to crane an aircraft on top of some concrete pylons in order to conduct some testing. What I need is to be able to calculate the weight at both the forward and aft landing gear. Calculating CG is easy but this one has me stumped. Here are the figures I have to work with:

Weights are in lbs and distance from datum in inches.

Basic acft weight:
Fwd gear 14707 @ STA 245
Aft gear 8008 @ STA 515

Items Added
Equipment 2588 @ STA 33.5
Pilots 480 @ STA 75
Sling 850 @ STA 320
Fuel 2000 @ STA 317

I know the items added above can be simplified as:
Items Added 5918 @ STA 305.2

So with the additional weight added what is the weight at the forward and aft landing gear?

If anyone can help me out with the formula I would be very appreciative!

Thanks,

Mike

 

[GregLocock]

"I know the items added above can be simplified as:"

No they can't. The dimension is wrong. It should be 173.8

All you are doing is taking moments about the origin.

So call the additional front force F, and the rear one R

F+R=5918

F*245+R*515=5918*173.8

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[helicopterguy]

Greg, thanks for your reply but what I was saying was that the items can be combined to a single added weight to simplify things. And the way that is done is by taking each item and multiplying the weight by the arm to get the moment, dividing the sum of the moments by the sum of the weights will give you the arm "CG" of all of the items added.

Mike

 

[GregLocock]

Yes, and you posted the wrong number.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[helicopterguy]

It may be more simple to look at the problem as:
Take an aircraft with a total weight of 28633 lbs with a CG at sta 332.92. If scales were placed at stations 245 and 515 what would they read?

 

[helicopterguy]

yep i didnt see my own error. the 2588 should be at sta 333.5 not 33.5

sorry