Setting the CG on the FT Mighty Mini Arrow

[Gryf]

Hi, all.

I'm just finishing up an Arrow, which I hope to bring to Flite Fest this year. It was mentioned in another thread that the Arrow can be tricky to balance, since using the fingers-on-the-CG dots method isn't too reliable, as the dots are on the bottom and the Arrow tends to topple off of your fingers.

So I used the following method this morning, and hopefully some of you can try it as well. I read that somebody else had balanced the Arrow using modeling clay packed in the leading-edge wing roots, so I came up with this trick -

1. I had already added a drop of hot glue to each balance point for the fingertip attempt. This time I stuck a small nail into the bottom of the wing at each point:


2. Using a piece of pushrod wire, I fabricated the lifter seen here:


3. Now I added clay to either side of the upper nose...


4. ...and lifted the Arrow with the wire, adding clay until it balanced:


5. Once it was balanced, I cut a couple of holes in the bottom for packing in the clay. Note that you can use the access holes provided in the SpeedBuild Kit. I hadn't cut those out of my scratchbuild since I'm not planning on FPV:


6. I used a Popsicle stick to pack the clay into each side:


7. Once the clay was all stuffed in, here's what we have. I'd originally planned to replace the cutouts, but finally decided to leave the clay exposed. If I need to add or remove clay later, it will be easily accessible:


8. Again with the wire, and she appears to be balanced nicely. But again, if I need to adjust the ballast later it will be easy. Once I was done, I pulled out the nails.


So there you go. Of course, if my maiden takes its usual course, I'll bash all this artwork out of shape on the first "landing"... so cross your fingers!

Cheers,

Gryf

 

[makattack]

Hey Gryf, nice job improvising a CG device that works for you! I'm a little surprised by how much clay you need on your FT Arrow. I just built one (admittedly a SBK bought via the FT STEM program) and found that with a 850mah 3S lipo (the only one I have thin enough to fit comfortably in the pod) it balanced out almost perfectly with the battery up towards the nose of the pod. There was still a little space for a board camera, but I will maiden it without FPV gear. I'm using a 2204 motor on my build.

 

[Gryf]

Makattack -

Yes, I was also a little surprised at the amount of weight required. In fact, the thought occurred to me while I was writing the article above, "Now I DID have the battery installed, right?" But yes, I had an 800 mAh battery pushed as far forward as I could get it.

Kind of makes you wonder if Josh scoped out the weight with FPV gear in mind, and without the camera and TX, you have to compensate with ballast.

Anyway, nailing the CG according to the indicated balance points is one thing... having it fly correctly is another. I've balanced other models according to plans, only to have them fail humorously when launched. I built a Baby Blender SBK a couple of years ago, balanced it meticulously, and watched it act really tail-heavy and fall out of the air with every launch. My FT Mustang never did fly very well; same story. So who knows. "The proof of the pudding, etc." Stay tuned for the maiden. Naturally, as soon as I got the Arrow ready for testing, the wind picked up and storms are moving in.

Gryf

 

[tomatwalden]

I've just finished building my mini arrow (scratch build) and have found it massively tail heavy too. Admittedly I don't have the electronics in yet, but just putting an 800mah LIPO in the front (without motor or ESC yet at the back) still results in it being tail heavy. With the motor fitted at the back, I can only imagine how bad it will be without ballast like this.

Out of interest, the plans say the dry weight is 218g. I weighed mine in at 190g so the foamboard I've used is clearly lighter. However, I don't see how the relative weight of the foamboard would change the CoG at all. It will be interesting to see what it's like with the electronics installed, but I'm anticipating having to add weight too.

I'm also pondering fitting different winglets - ones that perhaps don't hang out the back quite so much - or perhaps having in-board winglets instead (a bit like the ones of the middle of the eFlite Opterra).

 

[makattack]

Interesting. With my FTArrow built from a SBK in the white (not waterproof) adams foamboard, my CG marks are about 170mm/5.5in back from the nose (no extensions), or 55mm/2.25in from the motor cutout trailing edge. It's a little bit in front of the tabs that secure the center side walls to the bottom of the wing.

I find my arrow, which is pretty light even with a 850mAh 3S battery installed and FPV equipment, is a bit too light to fly in strong winds. I once made a mistake of flying it in 20mph winds and it was pretty much unflyable. Anytime I turned too sharply away from the wind, it would blow it over into a hard to control roll.

 

[bazsound]

Its not as sensitve as you thinnk.i fly with 850mah baterys and 15gofweight up front. Lost the weights and forgot flew fine.


Ivr flown it wih the cg 1 inch from the firewall .. although i do like twitchy.

The finger method works fine as it doesnt hsve to be super acxueatw

 

[bazsound]

Makattack -

Yes, I was also a little surprised at the amount of weight required. In fact, the thought occurred to me while I was writing the article above, "Now I DID have the battery installed, right?" But yes, I had an 800 mAh battery pushed as far forward as I could get it.

Kind of makes you wonder if Josh scoped out the weight with FPV gear in mind, and without the camera and TX, you have to compensate with ballast.

Anyway, nailing the CG according to the indicated balance points is one thing... having it fly correctly is another. I've balanced other models according to plans, only to have them fail humorously when launched. I built a Baby Blender SBK a couple of years ago, balanced it meticulously, and watched it act really tail-heavy and fall out of the air with every launch. My FT Mustang never did fly very well; same story. So who knows. "The proof of the pudding, etc." Stay tuned for the maiden. Naturally, as soon as I got the Arrow ready for testing, the wind picked up and storms are moving in.

Gryf

Yes i think the Cg with FPV gear comes into the design, but that still seems like an awefully large amount of weigtht to be adding....

It might just be the picture but it looks nose heavy without any weight.

 

[Liam B]

The CG is pretty darn forgiving on that plane. I've ran 1000mah batteries all the way up to 2200's in it (without canopy and FPV, of course). I just wish that it was slightly bigger. Then I could fit a larger battery and FPV both into it

 

[bazsound]

The CG is pretty darn forgiving on that plane. I've ran 1000mah batteries all the way up to 2200's in it (without canopy and FPV, of course). I just wish that it was slightly bigger. Then I could fit a larger battery and FPV both into it

Yikes 2200mah batteries was that not a pig to fly!

Shouldnt be too difficult to scale up, downlaod the pds plans full size and print out tiled at say 120%.

or build the other FT wing which is much larger

 

[Liam B]

Yikes 2200mah batteries was that not a pig to fly!

Shouldnt be too difficult to scale up, downlaod the pds plans full size and print out tiled at say 120%.

or build the other FT wing which is much larger

It was! Wing rock everywhere!

I might do that.