weight & Balance

[spacewalker]

To all the math wizers!....

When computing W&B on a tail wheel aircraft that is tail heavy, it's common to move more weight forward of the datum. In my case, moving the engine forward by increasing the arm of the mount works. When I do this, the weight on the tail wheel decreases which is natural and understandable. My question is how does one compute the amount of weight that transfers from the tail wheel to the mains as the arm for the engine increases?

Thanks
Chuck

[Dana]

Determine where the new CG is relative to the wheels. The portion of the weight on each wheel is inversely proportional to the distance from the CG to the wheel. For example, if the CG is 1/4 of the distance back from the main wheels, the main wheels will support 3/4 of the weight and the tail wheel will support 1/4.

Of course the actual numbers will be different between level flight and on the ground, with more weight on the tail in 3 point attitude.

[gmatejcek]

The weight shift formula is w/W = d/D where w is the weight shifted, W is the weight of the plane, d is the distance the CG moves, and D is the distance the weight is moved. An easy way to remember it is "Little w (the small weight) over big W (the big weight) equals little d (the shorter distance the CG moves) over big D (the greater distance the smaller weight moves). So, in your case to solve for how far to move your engine, you might consider:

Distance to move your engine = empty aircraft weight times how far you need to move the empty cg divided by the weight of the engine, prop, and accessories.

[lowandslow]

Spacewalker - an interesting question. Take a look at the attached excel file and see if it does the trick for you. Well, shucks, after putting the file together and then signing up for this forum with plans to send you the file, I discover that it won't send excel files. (No good deed should go unpunished.) So, send me an email to rvsoar at gee mail dot com and I'll do it that way. (Unless the software from which this forum is built offers direct email or PMs; either way would be fine - I just need an email address.)

[spacewalker]

Just a follow-up to my original post. Not being a math wizard, I did the next best thing. I kept moving the engine fwd with 1 inch sq tubing & threaded rod. Then, reweighing the plane. The final result was the engine has to be moved fwd 12 inches in order to keep the CG within the 5 inch range when fully loaded or when down to 3 gals of fuel. Thanks to those that responded with suggestions. Now back to metal cutting, grinding and filing. Ugh

Chuck

[lowandslow]

And, you didn't ask for the spread sheet (to save you lots of time, cutting, grinding and fitting), why?

[Frank Giger]

And, you didn't ask for the spread sheet (to save you lots of time, cutting, grinding and fitting), why?

I'll bet it's because a really cool little setup using 1" square tubing and a threaded rod just begs to be used and adjusted bit by bit.

Plus it's empirical data - no worries if the math in an excel spreadsheet is wrong due to a botched formula.

[rwanttaja]

Just a follow-up to my original post. Not being a math wizard, I did the next best thing. I kept moving the engine fwd with 1 inch sq tubing & threaded rod. Then, reweighing the plane. The final result was the engine has to be moved fwd 12 inches in order to keep the CG within the 5 inch range when fully loaded or when down to 3 gals of fuel. Thanks to those that responded with suggestions. Now back to metal cutting, grinding and filing. Ugh

I'm kind of shocked that an existing, successful design required the engine be moved a full foot forward. Day-yum. If it were a Fly Baby (which the Spacewalker quite resembles), I'd think something was seriously wrong.

Is the plane using something other than a small Continental? Or do you have an unusual loading case?

Ron Wanttaja

[FlyingRon]

Moving weight forward of the of the current CG is what you mean. The Dataum is an arbitrary point and could be anywhere on the aircraft from the tailwheel to the spinner.

The computation of weight forward of the datum is the same as any other point in the plane.

[martymayes]

Spacewalker: Just to cover the obvious, you did have the fuselage level when weighing the plane?

12" is substantial if things are mostly aligned with the plans. That much extra length in front of the c.g. will be somewhat destabilizing in both pitch and yaw and you might find that undesirable.