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Tuesday, January 20, 2015

Battery placement experiment for best balance

Hi everyone -

After watching Dave Power's "3 Axis Balance: Battery-Booster-Seat" video the other day, I realized I still needed to work on getting my plane even more neutrally balanced than I had already been working on.  Here is Dave's video.


I'm not so worried about getting my planes to hover, but eventually I will experiment with aligning my battery with the wing plate as Stephan has already discussed awhile back in one of his posts.

If you have been following the theme of many of my recent posts, I am striving to set my planes up to achieve neutral balance and thus far have done so by concentrating only on the pitch axis, I have still been stuck in my old philosophy with respect to the roll axis.

What I mean by this is that probably 90% of the park jets I have ever flown, I have put the battery all the way to the right side of the electronics bay to help counter torque roll in order to have zero trim in the roll if possible.  Here is a picture of what I mean.  This is because I was still stuck in the mentality that I have to balance my plane so that I have zero trim and not balance it first and then trim as required.  It makes a big difference in how my planes fly as I shed this old way of thinking.


As I have been dialing in my planes using neutral CG in the pitch along with other precision setup techniques learned from Dave Scott's Airplane and Radio Setup manual (have my own copy now, so life is even better... :)), I have noticed that the plane is much more responsive in all three axes.  When I say responsive, I don't mean twitchy, but as my planes become more balanced and responsive, they are also affected more by other factors that can adversely affect this optimal balance such as the battery being placed too far right or left of center line in the roll axis.


So here is what has been happening thus far when flying with my battery all the way to the right.

The plane has much greater tendency to want to tip stall to the right when slow and too much control input is applied.  Also, I noticed in loops that it often would want to spill out to the right as speed bled off, so my loops were not quite as smooth and uniform as I was liking.  Rolls were also not balanced in both directions.  I know torque roll can impact roll behavior, but with the battery so far to the right, my right hand rolls were tighter and smoother than my left hand rolls because the plane was having to lift that unbalanced weight as I tried to roll left.  Because I was still stuck in my old "zero trim is good" mentality, I just worked harder to fly my planes and compensated for these shortcomings.

So just a couple numbers first to put things in perspective.  I like to fly with a 2200 3S battery in my planes as shown in the picture above about 99% of the time, I just like the weight and wing loading it brings to my planes and helps with wind stability and penetration.  Since most of my planes are in the 20-22 oz range and the battery weighs 6.9 oz, it makes up about 33% or 1/3rd of my plane's total weight which is pretty significant.  So by having it so far to one side of roll center line, it would be like going for a hike and packing all your heavy stuff to one side of your back pack, you would feel very imbalanced and so does the plane.

So the last couple times at the field, I have been placing my battery as close to dead center as possible in relation to the center line like in the picture below.


Before I started doing this, I zeroed all my trims to ensure that all control surfaces were still dead center.  I did find one aileron and one rudder not quite centered, so after centering them, I got on with the testing.  

Right away, my plane felt more balanced, easier to fly and the handling smoother, regardless of direction.  The tip stalling to the right was pretty much eliminated as was pulling to the right in loops and my roll balance was almost completely equal both left and right.

I did have to put a couple clicks of right trim in the roll to help counter torque roll and I am good with that.  I am changing my philosophy that trim is my friend and not my enemy like I always thought it was.  Of course this has to be applied with reason I think, if I needed to carry half the trim I have available to me around, then there might be something not right with my plane.  This of course is always a bit of a crap shoot with the scratch built planes I fly, not every one of them is going to be perfectly straight and true and each plane is subtly different.

So I guess next up might be to experiment with the "Battery Booster Seat" and see how that affects the balance of my planes.  Stay tuned... :)

Cheers,

Scott




8 comments:

  1. Scott,
    On one of my prototypes I stuck a Dubro rod through the nose and one into the tailplane. With the battery all the way in the back in our current set up and ESC as high up as possible, I found the plane pretty well balanced in the central axis.
    For prior versions of the Mig with the battery forward a booster would be needed as the fuselage base plate diverges from the wing plate--i.e. the battery sits lower the further forward it is. I'm not sure how you would rig up a "top bottom balance." A symmetrical design with centralized components should be balanced for roll.

    I could see setting up a battery with an ESC on top of it to determine where they are balanced as unit. That would be simple to do. If you don't beat me to it, I'll rig it up when I get home.

    Stephan

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    1. Interesting stuff, Stephan, I probably won't get to anything like that right away as my priority is our other project I am working on right now. It will be something I will play around with eventually, but not a huge priority, kind of the last piece of the puzzle, but thus far just making the change in where the battery is in the roll axis is already making a big difference.

      I guess with each plane after determining where the battery goes for best neutral balance, a person could build a customized "battery booster seat" for each plane by taking measurements and angling the little "box" properly to keep it in line with the wing plate depending on how the bottom of the fuselage is angled in relation to the wing. It would probably make the electronics bay a little crowded for other components like ESC, Rx and servo wire bundle especially since we both like to fly with 2200 3S batteries, but with a little patience and ingenuity it could be done.

      Cheers,

      Scott

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    2. This is a good discussion. May I suggest that we have two topics: balance and center of mass. We set fore to aft "balance" by setting up the C of G. We should then ALWAYS set lateral balance (roll axis) by holding the ready to fly airplane by the tip of the nose and a center point in the tail. Of course every heavy component is located centrally to optimize lateral balance (I'm teasing you Scott. That battery should be in the middle of the fuse). A way to fix lateral balance is with wire brad or tiny finishing nails pressed into the wing tips and then glued. We now have an airplane with a good C of G and one that won't dive off to one side when the rudders and ailerons are neutral.

      Now to the battery booster. Assume the battery is in the middle of the fuselage (from side to side of the fuse) and we want to raise up the battery off the fuse floor. Why? I would suggest that we are working to minimize the distance of every heavy component from the center of mass so that less force is required to either roll or loop the plane. Make sense? If we think about the battery as a weight located at a distance away from the center of mass, it will require less torque (torque is force times distance) if the battery is is close to the center of mass.

      As I was thinking about this I realized: what is the desired center of mass and center of lift and the thrust line relationship? A simple design would be to make them all coincident. So the location of the wing will be dictated by the thrust line. I THINK our RCP planes have a shoulder wing so the wing is slightly above the line of thrust. So raising the battery a little off the floor would be good. But maybe below the wing so we're a different required torque when pulling the plane up vs. down (positive and negative angles of attack). I need to ponder this one a bit.

      Dave

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  2. So David Scott likes a little positive angle of incidence so the plane flies level. Our RCP planes appear to have zero degrees of wing incidence. They appear to be mid wing and the thrust line is coincident with the wing. While this is not ideal I believe Scott Lott has compensated for this by placing the wing, thrust line on the same plane and then placed the wing in the middle of the fuse.

    Hmmm, I think we've hit on a common design characteristic that Scott Lott has in all the recent RCP planes. Clever guy!

    Dave

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    1. Gents -

      Interesting discussion about balance and center of mass. I remember getting this explanation from a NASA site I found the help me understand it better (ie, I think this site was geared for school kids, so perfect for me... ;) ) http://www.grc.nasa.gov/WWW/k-12/airplane/rotations.html In short, here is what it says about centre of gravity

      "In flight, any aircraft will rotate about its center of gravity, a point which is the average location of the mass of the aircraft."

      In a lot of our discussion about balancing our planes as neutrally as possible, Stephan and I have also been talking about what we coined as polar moment of inertia (PMI) as a short form to discuss the distance from the front of our battery to the tip of the motor. In the way we build our planes, our ESCs, servos, Rx and wire bundles are also within this span. I am sure there might be a more technical way to describe this but it helped us discuss the relationship between centers of mass, gravity and thrust (again the center of thrust part is probably not correct, but we used it to discuss where the motor was located in relation to everything else) as we modified and tested our planes.

      The reason we found this important as we experimented with motor location on the plane and then the subsequent location of batteries, ESC, servos, etc was as we got everything closer to the CG, the plane became much more maneuverable and better balanced. It does however come with a bit of a cost in stability as very slow in high speed, aggressive maneuvers, the plane could break loose more easily than a nose heavy plane with a longer PMI. We have found that the electronic components alone account for 60+% of our aircraft weight, so where that is located has a marked difference on how the plane flies through our testing.

      Great discussion, learning more and more every day... :)

      Cheers,

      Scott

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    2. Scott, I think you have to be careful equating a smaller PMI with "better balance". You can definitely balance the plane just as well regardless of the PMI. The smaller PMI simply means less torque is required to rotate the plane. Generally speaking, people would say things with greater rotational inertia are more "balanced" - or more correctly, more stable. For example, one puts their "arms out" in order to help balance on a narrow footing - increasing their rotational inertia. It's definitely true that a plane with a smaller PMI will rotate faster, which will provide greater maneuverability, and I appreciate that this may feel more "balanced" in the air, for the pilot.

      As far as the CoG, CoL, CoT discussion goes, just based on your school kid NASA site quote, you can see that if they aren't all lined up, a change in lift or thrust will affect the dynamic balance of the plane, by causing a change in the rotational torque being applied by the thrust and/or lift. In the case of the low battery, the thrust line is above the CoG, so an increase in thrust will cause a downward pitching moment. However, being that the distance from the CoG to the CoT isn't that far in these mid-motor jets, the pitching moment isn't that large. Decreasing the PMI by moving the motor forward and battery backward brings the CoT and CoG even closer together, further reducing the moment. However, when trying to hover the thrust is fluctuating a lot, and the dampening factor from the control surfaces is minimal due to the lack of airflow, so even a tiny moment can have a significant effect - thus bringing the CoG up in line with the thrust line using the battery booster seat.

      I wish I had more time to fly so I could back up my theorizing with some actually flying. I know you all have more flights on a single plane than I have in total, so for now all I can do is argue semantics with you ;)

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    3. Chris, thanks for the thoughtful comments! We all should strive to fly as many flights as Scott!

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    4. Hi Chris -

      Thanks for the feedback, very valid and thoughtful points. You are correct, smaller PMI and good balance don't have that much to do with each other until you get to the point where you want to achieve maximum performance with a park jet in my experience. It does however come with a cost like most things in aerodynamics, improve in one area to the detriment of another. My experience is that as I have achieved neutral balance and tightened up the PMI, I do lose some of the stability that I was used to when I flew my planes nose heavy with longer PMIs.

      Not to say that dependent on the plane it will suddenly be twitchy or uncontrollable, but I do notice a loss in stability with that setup. The good news is that the reward is well worth the risk as my plane requires less effort (control input and surface movement) to maneuver, tracks much truer and bleeds off less energy as it transitions from maneuver to maneuver and is over all much more agile.

      KF size also has much to do with how these flat wing park jets fly as well if a pilot chooses to use them we have discovered. Stephan has exhaustively researched and hasn't been able to find a golden rule for swept wing planes like Migs and Sus, but through experimentation we have discovered that in the neighborhood of 40% of wing chord along the fuselage and tapered to 35% at the wingtip gives best performance. Since the wingtip is swept back so far behind CG, having less KF at the wingtip makes for a better performing plane. Not enough KF and you lose it's value, too much and the CL can get moved too far back, plane gets unstable at slow speeds and is harder to pull out of high speed dives, at least in my experience.

      Additionally, we have found that having the trailing edge of the KF meet the fuselage equal to where the CG runs through the pitch axis seems to give best performance.

      It takes a little more time to find that neutral balance point in a plane and I have found that at some point once I am close to it, the plane will tell me where it's CG is. I have found as I get to that neutral point the plane is very happy and responsive, but as I stray away from that even by a little bit, the decrease in performance is noticeable and my plane is not as happy.

      I'm still working hard to understand all the science behind what is going on with my plane, but I learn a lot being "hands on" at the field and testing and discovering all the subtle nuances that make my plane fly better and more "honestly" (ie less expo, less throws, proper balanced and trim).

      Lots to learn, but having a blast doing it!

      Cheers,

      Scott

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