I knew I needed to do the CG bar project the moment I saw Josh and Josh from Flite Test talk about it. So, I jumped right on it and got it done about two years later. But now that I have it done I really wished I had made it sooner. It could have saved so many crashes.
CG, or Center of Gravity, is one of those dark arts subjects. If you’ve come to us from the quad world, you may wonder why we spend so much time talking about finding CG? Maybe we should just call it the C-G-Spot? It’s dark and mysterious and no man ever knows exactly where it’s at. Anyway, let me show you a photo.
This is my lovely Bixler 2. Here’s the CG bar in action. The plane perfectly balances on the two pencils, so, all’s good, right? Nope, there is no battery in this Bixler 2. If you were run with the plane holding it by the nose then let go the nose would shoot straight up and it would crash. There would be no glide to speak of. This would be known as being tail heavy.
What we’re looking for is the point of balance where the nose isn’t heavy and the tail isn’t heavy. There is usually a small range where the CG can work. We move weight around in the nose to get the CG right. Usually, there is a spot in the owner’s manual that states where the CG is located. You can usually start here, but it’s more like a suggestion than gospel.
What’s this? I own another Bixler 2? Don’t tell my wife! So, here’s a real world example. This Bixler 2 flies like a dream. It’s my LOS Bixler 2. And yes, I have her dialed in. The INAV Bixler 2 has been having problems getting a good autotrim. So, I used the CG bar to figure out where the CG was on my most recent good flight of the LOS Bixler (with the flight battery still in the plane). It turned out to be exactly on the front edge of the spar cover (the side closest to the nose).
I did the same with the most recent flight with the new INAV Bixler 2 and found that the plane was slightly tail heavy. INAV could mask the issue by correcting it which made finding it a bit more problematic. The correct CG made the plane more efficient and thus more flyable. After a little more refinement the INAV Bixler is flying more like my LOS Bixler, a real pleasure to fly.
Also note that the nose has to be exactly perpendicular to the CG bar. If it’s skewed your reading will be skewed, and thus wrong.
How to make a CG Bar
This CG Bar Project is really simple to make, and I’ll show you how.
I went to the local hardware store and bought what’s called a “Stud.” The term here in the States is a “2×4” which refers to its size. 2”x4”x8’. In metric it’s 38 x 89 mm x 2.4 m. It’s the basis for construction. This rather large piece of wood is fairly cheap, it was around $5. So, the million dollar question is if you actually need a CG bar that is this big? It’s large enough to measure 3 meter planes if you made it all 2.4 meters long. But the reality is that I only needed 1.4 meters as nothing I fly is larger than 2 meters. Thus it was cut down to use 1.4 meters.
For simplicity sake, everything from here on out will be in metric units. For simplicity sake, 100mm=10cm= ~4 inches.
With this CG Bar Project, you’re not making a scientific instrument
If you were going to make this a scientific instrument it would most likely have to be made out of machined metal or a very rigid plastic. You would have to have very tight tolerances. And your results wouldn’t change all that drastically.
The issue is that wood warps, especially as it gets longer. So, when I picked out a stud I laid a few on the ground and found the one that had the least amount of warp. As we cover the next part, if you drill a hole and it’s off by a couple millimetres you don’t have to throw the entire project away.
Follow these steps:
Step 1- Cut the wood to size
If you want the wood tool to measure 1400mm you should add 100mm to both sides and make the cut 1600mm. Then measure in 800mm. Mark it, this will be your center.
Step 2 – Make one long line down the center of the stud.
Without getting overly complicated if the stud is 89mm wide, just measure up 45 mm from each end and draw a very long line between these two points. This is the long line that runs down the center of the stud.
Step 3- Cut the wood down to size
Since I cut the wood exactly at 1600mm, I measured 800mm and called that center. I marked where that was. From the center point, measure out 100mm (around 4 inches) and mark the spot on the long line, measure the next 100mm and do the same. The furthest spot was 700mm away from the center (~28 inches). This is the exact location for each hole.
Step 4- Drill the holes
Before we drill any holes we should figure out what you’ll be drilling. There are a couple directions you can go.
If you can find dowel stock at your hardware store and have a drill bit that matches maybe this is the way to go. If you can find 8mm dowel stock, it would be good, It has to be at least 150mm long so the plane doesn’t rest on its belly. 8mm drill bits aren’t hard to find, even in the United States. Yet, this isn’t the best solution. The problem is that you’ll probably lose the dowel at some point. If you don’t have the dowel stock on hand you will need to go buy more.
This was the suggestion from Flite Test, and I have to say it’s an ideal solution for the CG Bar Project. You can always find another pencil. Just make sure both aren’t used so they are the same length. There are some deviations around the world in the size of pencils. In the United States the school pencils seem fairly standardized. I happened to own a very large set of drill bits. The size that worked best was size N. It’s exact size is 7.67 mm or .302 inches. You may want to try an 8mm drill bit as it’s only .33 mm larger than a size N. It may not fit as snug but it will still work. Drill test holes to test how well the pencil fits before doing your final drilling.
Drilling your CG Bar
If you can get your hands on a drill press you can assure the that the holes will be made perpendicular to the wood’s surface. You can also set the depth of the drill. I let mine go in half-way down to the bottom. If you’re doing it by hand, use the level on the drill if it has one and good luck to you! For a CG Bar Project like this, hit up someone who has a drill press for help.
A quick finish
Just mark on each side the distance between the corresponding hole on the other side (200mm, 400mm, 600mm, etc). For example, the first hole left and right of center should be 200mm apart from each other, so mark both of these 200mm Call it a day, your project is done.
I went the extra credit route. I painted it with old paint I had laying around. It worked out great. I first used a few coats of white paint until things looked finished. Then I painted the middle to the first 100mm on either side red. Then I put labels on each hole using Brother P-Touch, it was very clean. The last step was to use some outdated oil based lacquer and it turned the project a deep tint of yellow. Suddenly the project looks 40 year old! Oh well, it’s still rock-solid. It doesn’t have to be beautiful.
If I did the CG Bar Project again
What I would have done differently if I were to make another one is to use an electric sander with 240grit sandpaper and made the surface smooth as possible. Then I would have applied a couple of coats of paint. Re-sanded and applied a final coat of paint. I would have applied a coat or two of lacquer, sanded the project with 240 grit sandpaper. Applied a third coat. Sanded it with 600 grit sandpaper and then applied a 4th coat.
The only issue was that the paint and lacquer went down the holes , so I had to re-drill them again. That was fast and easy.
What it’s like to use.
I was wondering if this project was an elaborate waste of time? But, all I had to do was use it once and figure that, no, I didn’t waste my time. I would find CG the old fashioned way, put my fingers under the plane and see where it balanced. It’s okay for a quick check, but when you’re setting a plane up and trying to dial in CG, you want more accuracy. My fingers are much wider than the top of a pencil eraser so it’s easy to be way off the mark.
What I found was that I was able to really dial in the CG. The largest issue with this is the same issue you would have with the old fingers under wings technique, the plane can easily be skewed. If the nose isn’t absolutely perpendicular to the CG Bar the CG will be off, and thus, no good. This may be one of the largest issues with CG that people may be having. When I find CG, I place pins at the exact spots and then give it a quick visual check over to make sure I’m on the right track.
Once the plane is perpendicular it’s pretty fast and very accurate to find CG. The most important thing is that I can now sit back and breathe. I can really take my time to find the exact right spot and more precisely work the balance. Hey, we sometimes spend 20 minutes getting the current sensor calibrated, why not spend five minutes to check CG?
My real world test
With the Bixlers, when I got the CG right on the newer INAV plane I took it up for a flight and saw an immediate improvement. At one point in the past the CG was so off that I was flying east and a gust of wind came along and within a second I was flying west. Now the plane is tracking better. It’s heavier than the LOS Bixler, but that’s to be expected. It’s getting me closer to the final Autotune.
Back when I first started with INAV I would get a plane up in the air with acro mode, throw on an autotune and wondered why it flew like crap. Now I check over the autotrims carefully, many times over. I make sure the values are between 1450 and 1550 for each servo before moving on to the autotune. I find that the CG is a make or break thing for getting a good autotrim. Once it’s dialed in, everything else seems to fall into place.
CG is one of the fundamental issues we have with people presenting problems in INAV Fixed-Wing Facebook group. The problem is, by the time people present it to us it’s often hard to figure out. The plane pitch oscillates, has yaw wobbles or has some other problem. Often they would never dare fly it in manual. People ask us what value they can change to fix it, when the reality is that they need to reset their PIDs and start over with CG. This CG bar project gives you a great tool to do just that.
This comes back to the core value of what INAV is about. It’s a 90/10 relationship. 90 percent of how well the plane flies is how you have it setup in the first place. The mechanical trim is 90% of the success of INAV. Once the plane flies well with a mechanical trim, the flight controller can really do it’s magic.
My mom wasn’t a very good cook. She did so many things so wonderfully and was amazing as a teacher and a person, but cooking wasn’t her passion. My wife wasn’t much better at cooking when we first got married. Within the past 10 years she’s become an amazing chef. Many of the meals she makes are restaurant quality. The funny thing, the groceries haven’t changed all that much. True she now has better appliances, pots and pans and utensils. But what has changed is her approach to cooking. She spends a lot more time on prep work.
I really can’t think of a better analogy for INAV. If you truly want a better flying experience, spend more time on the prep work. Don’t use the flight controller as a high tech means of fixing fundamental issues with the plane. Work your autotrims, keep updating your mechanical trims until the plane flies well with servo positions between 1450-1550, then do your autotune. You’ll love how your plane flies.