I have already touched on this somewhat in previous posts, but wanted to discuss it a bit further and combine all my thoughts on weight distribution in a park jet either by design or by choice when building your plane.
I would suspect weight and balance has been important in air planes since the Wright Brothers first took flight. Unfortunately, over time, some serious aviation accidents have happened either due to incorrect weight and balance calculations before flight or weight shift during. Of course not that anything that dramatic applies to your foam park jet, but hopefully it drives home the idea of how important weight distribution is to the proper balance of your plane and most importantly it's overall performance.
Perhaps I should have covered this a little earlier, but I like to use this definition of center of gravity from this NASA website.
In flight, any aircraft will rotate about its center of gravity, a point which is the average location of the mass of the aircraft.
Although a bit simplified (sometimes simplified is good :)), this diagram shows how theoretically all the axes of pitch, roll and yaw intersect through the center of gravity.
For the longest time, I only thought of a plane's weight distribution along the pitch axis, to get the plane to balance so that it wasn't nose or tail heavy. Then as Stephan continued to do more research, he taught me the importance of vertical balance or how the weight is located in relation to the wing plate top and bottom. Stephan made this video awhile back to figure out how to design the plane to be as neutrally balanced in the vertical axis as possible. This science went into the Mig-FA and continued into the Mig-35B.
So let's get back to how the design of the park jet can affect how you are able to distribute the weight of your plane. In the last post, I discussed motor location, this has a very significant impact on how the weight is distributed along the longitudinal axis of the plane, affecting pitch balance and performance.
Other design factors are where you are able to place your servos, servos of course are not as heavy as a motor or battery, but the further back they have to be placed in relation to the CG, more weight has to be placed ahead of the CG to compensate. For example, the rudder servos for the unique "under rudders" on the RC Powers F-22 V5 have to be placed behind the prop slot in order to be able to run the linkages without interfering with the prop and/or the elevon linkages.
So back to vertical balance as discussed in Stephan's video above. Some park jet designs have all the weight on top of the wing plate, the wing plate actually acting as the bottom of the electronics bay. This tends to make the plane very "top heavy". This can cause the plane to want to roll over aggressively on it's back in turns, loops and other vertical maneuvers. I have also found it causes issues in the yaw axis at the top and bottom of loops as the plane will "dart" sideways one way or the other. Tip stalls also come on much more quickly when slow in turns or when coming in to land, so it is important to keep more speed on the air plane than the wing loading would normally dictate.
Here is a picture of a plane that has all the electronics (other than the elevon and ailerons servos) on top of the wing plate, over half the weight of the plane is above the wing plate in this example.
As mentioned above, lowering the battery on the Mig-35B was important to us after seeing the improvement in stability and forgiveness it provided the Mig-FA. The Mig-35A (the first Mig-35 we designed and sold), was in fact a bit top heavy which was one of the reasons it was for more intermediate to advanced pilots as it was not quite as forgiving as the Mig-FA and Mig-35B. By lowering the weight in the Mig-35B along with some other changes, it became even more stable, more forgiving and the low end of the speed envelope was improved.
To get a sense of how a plane that is neutrally balanced handles, this picture has the battery installed, so just as it would be for flight. I am holding it by the prop nut and the nose so that it can rotate just like when you would balance a prop. As you can see, the wings are perpendicular to the ground. This plane is about as neutrally balanced as you can get, so it's movement in all three axes in quick and effortless with very little control surface deflection required. For a precise, agile and highly aerobatic flight experience with almost no bad habits, neutral balance is ideal in my experience.
I think it also important to have a plane or set up the plane where the battery can be as close to "straddling" the centreline as possible. For a very long time, I always flew with my battery well over the right of the electronics bay to counter torque roll because I was stuck in a mindset that I always had to have "zero trim". This actually caused me to set my planes up so they were unbalanced with far too much weight to one side making rolls and turns unbalanced and the plane more tricky to fly. I now always try to put my ESC on the right side as it is heavier than the receiver and wire bundle of servo wires, then place my battery along the centreline. Hopefully you can see what I mean in the picture below. This gives my planes a much better balance and more even, predictable performance. Sometimes depending on the design, the electronics bay may be too narrow for this type of layout, so it is something to look at when choosing a plane or when building your plane depending on the performance you are looking for.
In the next article, I will discuss stabilizers (horizontal and vertical) and control surfaces.