Balancing a KA7

[Stuart Ward]

Hi Guys,
I would like some help to confirm the COG of a KA6 that I am putting together for a friend
The KA6 is an ARF, 4m span and 390mm root cord, and the wings are swept forward by 150mm at the tips.
The instructions say to balance the plane at 100mm back from the leading edge at the root, which is 25% root cord.
I ran the numbers through an online COG calculator and the result said to set the balance for a start position at 15% forward of MAC at 29mm back from the leading edge at root cord. And 5% of MAC ended up at 55mm back from the leading edge.
That is nowhere near 100mm that is in the instruction manual.
Can anyone shed some light on their experience with this plane and where to balance it?

Stu

[Andrew Ray]

As far as I am aware the K6 shouldn't have any sweep of the leading edge, it should run straight from tip to tip and seems that you need to correct the forward sweep. If you input the calculations based on no sweep does the calculation come out as per the kit instructions?

Andrew.

[Stuart Ward]

As far as I am aware the K6 shouldn't have any sweep of the leading edge, it should run straight from tip to tip and seems that you need to correct the forward sweep. If you input the calculations based on no sweep does the calculation come out as per the kit instructions?

Andrew.

I knew I should have taken some notes rather than trying to write from memory at work
Correction: it's a Ka-7

[Andrew Ray]

Can you correct the thread title?

The kit instructions seem to me to be wrong in that case, I have come across this a few times with ARTF power models where the indicated balance point is too far aft, makes for an interesting if not hairy flight.

I have a 5m Pat Teakle ASW17 where the balance point in the instructions is too far forward and I am slowly bringing it back to the point indicated by an online CogG calculator. The maiden flight with such a forward balance was very stable but I was running out of elevator at anything over 30º angle of bank. The online calculator appears to be reasonably accurate in my case.

Good luck.

[John Vella]

I hope I can help as an ex instructor on K7s. To find the correct c of g on the wing plan form you must determine the Mean Aerodynamic Chord. On this wing with its double taper and forward sweep it is at approximately one third span. So Mark off the chord line at this point and the C of G is around a safe 30% of the chord. Run a line from this point at 90 degrees to glider centre line to Root Rib and Mark your C of G datum point. As a matter of interest the instructor On the full size K7 sits On the C of G and when he sends the student solo there is no trim change. Apart from the appalling view from the rear seat it was a delightful glider to fly.
Regards John.

[Stuart Ward]

John and Cliff,
thanks for your help. we would have had a disaster had we followed the instruction in the ARF manual. The COG is further forward than we expected. we will start at the leading edge and work from there back to a point where it handles nicely.
Stu

[Jilles]

You can use the on line form on the link below
Fill out the figures of your model, for the wing sweep 'S' enter the figure as a negative number e.g. -250
Works rather well



http://rcplanes.000webhostapp.com/cg_calc.htm

[Stuart Ward]

You can use the on line form on the link below
Fill out the figures of your model, for the wing sweep 'S' enter the figure as a negative number e.g. -250
Works rather well



http://rcplanes.000webhostapp.com/cg_calc.htm

Thanks
That's the Calculator that I used to work out that the balance point. It was 29mm back from the leading edge and not 100mm as in the manual

Its a great little tool
Stu