KF 4-1200 Weight and balance

[Jerrytex]

I am about to weigh my plane as I am getting close to getting my AW certificate. I have been reading everything I can find on weights and balancing and I think I have a grasp on the concept. I was playing with the Excel spreadsheet on the Kitfox site, just kind of plugging numbers in. The site says that for a KF 4-1200 the CG is 10.2 forward and 16 aft. So if my thinking is correct...when I weigh my plane, hopefully the CG will fall in between 10.2 and 16 right? And then when I load it, it should still fall between 10.2 and 16 correct? As I change the numbers, I noticed that the closer I kept the empty CG to 10.2, the more room I had for a heavier passenger and baggage. Seems this is a good thing? Is it advisable to rearrange things to get the empty CG as close to 10.2 as possible? Also the Datum says lead edge. Mine has the plastic streamlined lead edge under the fabric. So do I measure from the lead edge of the plastic or measure from the spar?

Thanks Jerry

[DBVZ]

I think you mostly have the right idea. Look at where the fuel and seats and baggage loads are located. You want to be within the CG range at min and max loading. Your DAR will check this. From what I have seen, not done it myself, you will need to calculate an Empty Weight CG, Most Aft Weight and CG (loaded with max load in baggage and seats etc), Most Forward Weight and CG, Flight Test Weight and CG (with your real loading for flight testing). I believe all need to be within the CG range and less than the max gross weight. The idea is to show you can load the fuel and seats and baggage in ways that work. For example, if you have such a heavy engine that the CG is forward of the limit with just a light pilot in the left seat, you may need to add ballast to the rear so the Most Forward CG comes into the proper range. You may also find that the aircraft will fly better if you keep it in a tighter range than the design limits. Some just like to fly better with the CG in the center to slightly aft in the CG range - less trim needed, so less drag from the trim tab etc. For each flight, it is necessary to know that at both TO and LDG (with full fuel and at min fuel) you will be within the limits at all times.

Others with experience doing this, or perhaps a DAR, can correct anything I get wrong; but that is how I understand this is supposed to be done. You probably ought to call your DAR to confirm what he will expect to see.

FYI: I am aware of a guy who recorded his W/B with his DAR and the FAA at 1320, for an AVID Mk IV (Heavy Hauler wing) with a kit design weight limit of 1150. 170 pounds added to the max load. No issues with the DAR or the FAA, and still LSA legal. Taking an 1150 to 1320 may be a bit excessive, but I think it is fairly common to take the 1050 and 1150 to 1200 or 1250, especially when they are planning to add floats. Taking a Kitfox 1200 to 1320 may also be common now? I don't know. The thought was that it gives you some additional utility, by reducing your margin on the + and - g-forces. Someone on the forum may want to comment on that too.

[jtpitkin06]

Jerry,
You don’t need to measure anything from the wing spar to complete the weight and balance. The empty aircraft is weighed on the wheels and you use the datum points shown on the weight and balance form to do all your calculations.
John Pitkin
Greenville, TX

[Dave Holl]

Yes that is correct but do you not need to know the position of the wheels from the datum for your calculations?
Dave

[DBVZ]

Yes that is correct but do you not need to know the position of the wheels from the datum for your calculations?
Dave

Did you get instructions with the kit for doing this? Do it by the instructions and not by some comments on this forum from someone like me who may not have done it or may not have done it right. I have not done a Kitfox, but for the Piper the empty W/B was done with the tail aligned to flight trim, weight at the 3 points with the measured arm for each (both mains the same we hope) and calculate the total weight and moment and CG. The loaded calculations did not require the tail in flight trim, since they are just adds to the empty CG. Unless you are doing something unusual and you want to verify exactly what the arm is for your fuel, seat loads, and baggage. In that case, you could do each one independently with the plane in the same trim each time, and that would let you calculate exactly how much the weight increased and shifted the CG, and from that you could calculate the exact arm for each. Depending on the gear configuration, tail up or tail down could shift the empty CG quite a bit because you shift an arc of airframe material from behind the mains with the tail down, to in front of the mains with the tail up. You can see it when you lift the tail. Tail weight would be a little higher with the tail down than when in flight trim, with that weight shifted to the mains when in flight trim.

[desertfox1]

Hi Jerry,

Just go to Kitfoxaircraft.com and look under technical FAQ.
The model 4 W&B spreadsheet is right there with all the info
you need. And yes, you drop the plumb lines for the datum
from the leading edge including the plastic former. Use a
carpenters square on the floor to mark the axle centerlines.

Have fun.

Phil

[HighWing]

I am about there as well with my IV and have been reviewing the plan. We will be weighing the V next weekend. I think the datum question has some validity. I don't know the answer, but suspect the answer would be the leading edge as that is what is suggested by the spreadsheet. And that is what I will use. The Kitfox Aircraft spreadsheet also has a disclaimer:

Builder is to verify all dimensional information for their aircraft.


I was planning on verifying the dimensions.

Lowell

[DBVZ]

The Spreadsheet, and the FAA Manual. Nice of Kitfox to make it convenient.

http://www.kitfoxaircraft.com/Weight%20&%20Balance.htm

Almost a 13,000kb PDF, but the manual has the theory and instructions all there. Including some instructions about leveling to flight trim for different aircraft. Leveling screws, or use the top of the door frame, or whatever was in the TCDS for leveling.

[jtpitkin06]

Refer to the "Final Assembly" section of your build manual. It should have sample W and B forms, datum for the wheels and sample W and B.

It also specifies leveling the aircraft and the level reference.

In addition to the instructions, verify the position of the landing gear and tailwheel or nose gear for your specific aircraft.

John Pitkin

[wannafly]

Just a side note. After you have completed the weight and balance and verified everything here is what I do. Here in Canada you must carry a current weight and balance. I went to the Kitfox website and printed off various sheets that will match differant weights.
example: 1-me and full fuel,
2- me, lite passenger, fuel to take me to 1200 pounds
3- me, heavey passenger, less fuel to 1200 pounds
4- a couple of blank ones to fill in as needed

Then when I go flying I take the sheet that reflects that day. Keeps the officials happy. Hope this helps

mic