A problem determining the correct CG on a homemade glider conversion...

[DavidHyams]

I have been working on a Guillow "Sky Rocket" glider to electric power conversion project. My semi exact calculations put the CG about 2.5" back from the leading edge, the main wing is swept back, so the entire wing width is about 6.5".

My initial test flights were very dissapointing and I suspected that my calculated CG point may be in error. Then by accident, I left the receiver atenna wire, approx 30", hang loose behind the plane during some powered test glides. The nose stayed down for a change and required very little up elevator to achieve a smooth glide path.

I left the atenna wire hang loose again as I tried a more aggressive test. After a sloppy launch it started to fly so I went for it. As long as I kept the speed up I was able to mantained a controled flight. Once in awhile I lost stable flight when turning into the wind, I attributed this to momentary loss of tail drag from the wire being blown at the same rate as the aircraft.

So I assume that my CG point is too far foward and that my aircarft is really nose heavy and the atenna wire drag compensates for this. I originally thought that the plane was tail heavy but I can not figure out how a hanging wire could compensate for this.

Has anyone run into this situation before where a hanging atenna wire allows flight..? And which way am I heavy..?

Thanks
Dave

[Sparky Paul]

What is the wing planform?
Root chord, tip chord, sweep at the tip, span?

[DavidHyams]

Here are my semi exact measurements.

Root Chord = 4.75"
Tip Chord = 2.75"
Sweep Distance = 4.0
Half Span = 11.25"
%MAC Balance Point = 25.1%
CG = 2.75"
Sweep Distance at MAC = 1.86"
MAC = 3.55"
MAC Distance from Root = 5.22"

After checking the numbers again, I discovered a calculation error that puts my CG at 2.75" back from leading edge instead of 2.55" as previously calculated. Since the plane now balances at 2.5" back from the leading edge, the plane appears to be nose heavy at the new CG point.

I would really appreciate some help with my calculations and the puzzle of why I can overcome a bad CG by having the antenna wire hanging.

[Sparky Paul]

Your computations are correct..
The additional moment of the dangling wire helps move the c.g. aft.

[DavidHyams]

Thanks for the confirmation on my calculations. I am now very anxious to get to the field this afternoon and try out my plane now that I have added some tail weight and taped the antenna wire back to the fuselage.

I will have to remember the antenna wire trick in the future.

Thanks for your help, time to go flying...