Where should I put it?
Last week someone asked the question about top mount vs. under-slung battery mounting for racing quads. This started plenty of debate as you would expect, a lot of which in my opinion was complete garbage, so this got me thinking.
It turns out that the problem isn’t as simple as people might think but I have a theory as to the reasons why an under-slung battery makes for a better racer! First of all let’s start with the basics, battery mounting high or low changes the center of gravity (CG) of the quad as it’s the majority of the weight. So, the real question is, do we want a high or low CG?
Up top or down low?
The other basic principle we have to consider is where the center of lift is (i.e. where relatively speaking the quad is being ‘suspended’ in the air by the motors). The center of lift will be inline with the props, however depending on the combination of thrust can sit anywhere between the 4 corners. The stability and characteristics of flight is ultimately controlled by the distance and positioning of the CG from the center of lift.
So my theory here is that there is an important reason why a low CG is better for race characteristics.
Yaw-Roll inertia – When yawing the quad in forward flight, with a low CG the inertia of the system (which acts at its CG) will try to continue on its path, this will tend to roll the quad into the direction you turn. For example you yaw left, the craft will try to bank left naturally. With a high CG this will be the opposite, you yaw left, the craft will try to bank right. These FC these days will correct this and keep it level if tuned perfectly, however consider that a high CG will mean that when you yaw left and roll left to make a coordinated turn, the roll command will be fighting the inertia which is trying to roll the other way which will be less efficient.
My other reason for low battery is that in forward flight (99% of racing!) the quad will be in a nose down angle of attack moving forward. If the battery is on top it is more likely to take the full brunt of a head on crash with an obstacle. With the battery low mounted the quad frame is more likely to be the first point of impact. There is an argument as to which is better (hit the battery save the quad, or hit the quad and save the battery) but in my experience the batteries come out worse in a collision than the quad.
Update – after some thought, G (in the comments) had a valid point. Even aerodynamic stability as well as the inertia stability is handled by the flight control in such a way that relatively speaking the CG position shouldn’t have an effect on the natural stability of the craft. This is why I have corrected this article.
Lets explain why: Point 1 – Consider my original diagram of CG vs thrust center, this theory applies only in a static balance case, if the center of lift remains in the middle the CG position can affect the ability of the quad to balance on this point. This explanation still stands true, but what happens in reality is that the FC corrects for this balance by adjusting thrust and changing the center of lift so the quad keeps it angle, and this would happen with a high or low CG. In the case of an ‘under-tuned’ quad this affect might be noticeable, but with current setups this is unlikely to be a big issue.
Point 2 – The other aspect which G pointed out is the ‘rocket pendulum fallacy’ a common misconception about stability of rockets with high or low CG. The quad has inertia centered at its centre of mass (related to CG) the misconception is that when it tilts that there is an instability created with high CG system as the thrust direction is pushing the craft over more as it tilts more. Well this isn’t the case as according to Newtons Laws (equal and opposite reaction) the inertia of the quad will just react only against the thrust direction that is applied to the mass, as the quad tilts the thrust also tilts with it so it is no more or less unstable with a high or low CG.
My Conclusion: Swing it low
Low battery is better for racing. It offers improve yaw-roll inertia and is more efficient and natural for coordinated turns and also for minimising the chance of head on impacts with the battery.