KF-IV CofG

[ofergd]

Hi all.
On a previous post I wrote about installing a Rotax 912 iS on a KF-IV. Since no one had this experience I started digging and designed an engine mount to fit the Rotax ring mount to the FW.
To get good results I have to move the engine forward about 5.5". This will cause the CofG to move forward about 1.3". What is your experience with the CofG of a KF-IV wuth a Rotax 912, is there a problem with forward limit of the CofG? Where does your CofG come out?

[HighWing]

I don't know how much this will help. I added some ribs and gap seals to my empennage which added about six pounds to the tail on my recently completed Model IV. I did the same on one completed in 1998 and then it resulted in an aft CG to the point that I placarded the baggage sack at 25 pounds maximum vs. the 40 lbs. in the book. This time, I took great pains to minimize the weight of the add ons.

With the new airplane, I wanted the whole 40 lbs available in the baggage sack, so I decided to fabricate my own engine mount, moving the engine forward a bit. My spreadsheet told me that I could move it 3 inches forward. I moved it 2.5 inches as a cautious compromise. After weighing the airplane. I found it within the CG envelope but with just me and full tanks, I found it just aft of the forward limit.

I guess I think too much, but I recalled what the airlines did during the Arab Oil Embargo in the early 70s. They loaded their airliners to close to the max aft CG as that is the most efficient loading to reduce drag and give a more efficient fuel burn.

In short, I decided to add five pounds of lead right over the tailwheel mounting bolt to move the CG a bit more aft. with this configuration, my empty weight W/B is 11.96 and my most aft CG loading with myself a 180 lb passenger 36 gallons of fuel and 40 pounds in the baggage sack (1200 lbs. gross wt) gives a CG of 15.69. I have a trim tab and after flying, if I don't trim for landing, the tab is straight aft - neutral with the elevator. I think this shows my W/B pretty close to neutral in flight.

[ofergd]

I might be too early in the building process so I don't know what my plane will weigh and where wil the CG come out. That's why I can't make a spreadsheet like you did. I'm trying to to estimate what will the effect of moving the engine forward will have on the way I will be able to use my plane. What will be the limit with one light weight pilot with minimum fuel? Will I need to add ballast at the tail? What effect will it have on the useful load of the plane?
Does any one have an approximate empty weight and CG of a model-IV with a Rotax 912 and wing tanks. This will be a good point to start the calculation.

[HighWing]

I hope this works, I will attach a PDF file of my W/B spreadsheet. Apparently the forum won't allow a straight excel file so you will have to either create your own Spreadsheet or download the spreadsheet which is available here http://kitfoxaircraft.com/Weight%20&%20Balance.htm I wish the spreadsheet was more specific as to the engine arm and weight so you could more easily do the manipulations.

My modifications:

I have approximately 6 lbs in the tail with two servo motors, gap seals between rudder and vertical stab and elevator and horizontal stab with ribs in all four.

I have aluminum leading edge similar to the LLE that extends from the extruded PVC leading edge 12 inches aft.

I put in a large baggage sack. This is why I have two arms for the baggage sack to more accurately reflect loading.

I have a spring cabane style landing gear, that moves the axle two inches downward relative to the leading edge - no fore or aft movement.

My engine has been moved 2-1/2 inches forward to accommodate the added tail weight.

I found I wanted but didn't really need five pounds of lead in the tail for reasons described in a previous post.