V-Tail CG shift

[MondoMor]

Hello,

I'm learning on a GWS Pico-Stick. After a crash several weeks ago broke the stick near the motor, I took what remained and grafted it onto a cheap foam glider body for an experiment. I kept a very similar distance between the wings and tail, and managed to locate the CG right in the middle of the battery pack.

It flew great. The CG location meant that I could swap in different packs of similar size without re-checking the CG.

Recently, I crashed the thing again while flying in the wind. The horizontal stabilizer, weak from previous breaks, snapped lengthwise right where it joined the stick. I'm tired of trying to glue it, so I had the bright (?) idea of making this thing a V-Tail. I trimmed both elevator halves so that they were the same size and mounted them 90 degrees to each other.

I checked the CG (essentially right where it was before), eyeballed the controls to "neutral", set up my controller and tossed the thing. It was my first V-Tail flight, and my first V-Tail crash.

The Pico Stick appears to be tail-heavy - it flies in a very unstable manner forwards, before becoming uncontrollable in pitch.

After scratching my head, I came to the conclusion that it is tail-heavy. Even though I only lost about 1/2" (maybe 5%) of the horizontal stabilizer width, the aspect has changed and now has something like only 70% of its former horizontal stabilizer area.

So if I'm correct, the center of pressure has moved forwards significantly, and is too close to the CG (it didn't swap ends and fly tail-first, so it's not ahead of the CG). If I'm right, I'll need to move the CG forward and/or the wings rearward to regain neutral flying characteristics.

I have a very limited (and mostly intuitive) grasp of this kind of stuff, so I'd like the opinions of wiser folks.

Thanks in advance.

[Ollie]

The area of a V- tail conversion has to be equal to the sum of the horizontal and vertical tail areas for equivalent stability. The way you made the V-tail reduced the stability of the aircraft by moving the neutral point forward. You can regain stability by moving the CG forward so that it is the same distance ahead of the neutral point as before the modification. After regaining stability. the modified tail will provide somewhat less control authority. The pitch and yaw damping will also be reduced. That means that it will take longer for oscillations to settle to the trimmed flight condition after a disturbance.

[Sparky Paul]

How did you set up the -rudder- action?
To do this properly, hold the plane so one surface is vertical, like a rudder, then set the surface directions..
100 times out of 100 it is ALWAYS done wrong the first time!
http://rcgroups.com/gallery/showphoto.php?photo=627&password=&sort=1&cat=500
Although what Ollie says is strictly correct, for your installation such precision will have no discernible effect over what you have now.

[MondoMor]

Thanks, guys. I'm glad my reasoning is pointing me in the right directions. It gives me some hope.


Originally posted by Sparky Paul
How did you set up the -rudder- action?
To do this properly, hold the plane so one surface is vertical, like a rudder, then set the surface directions..
100 times out of 100 it is ALWAYS done wrong the first time!
http://rcgroups.com/gallery/showphoto.php?photo=627&password=&sort=1&cat=500
Although what Ollie says is strictly correct, for your installation such precision will have no discernible effect over what you have now.

HA! I actually sat down and looked at it a good long while before trying to fly it, and got it right according to that picture. I'm glad I finally did something right! Because of the way I mounted my servos, this was the only thing I own that I had to reverse the rudder channel (on the TX) for, which was annoying. But now that it's V-Tail, I no longer need to reverse the servo channel! Of course, I do have to remember to switch over to v-tail...

Too bad I didn't reason out all the consequences before I actually flew. :L

Alas, it's been too windy to fly for a long time here, so I can't test the results of my modifications (cutting out more foam to slide the battery more forward).