The Basic Forces of Flight

The 4 basic forces acting on our quad are:

  1. Gravity, which always pushes down towards the ground
  2. Drag (in the form of air resistance), which always pushes opposite the direction of movement
  3. Thrust which is provided by the motors/props

So where is the 4th force? Since quads do not have wings to provide lift, Thrust is actually broken into two parts:

  1. Thrust that provides movement (for lack of a better term, we’ll call it displacement thrust)
  2. Lifting thrust which offsets gravity

Total thrust will always have to be broken down into these 2 components. From here, we can see what effect more thrust can do for speed.

Since gravity is always constant, we will always have a set amount of thrust going toward offsetting gravity. If you have 3000g total thrust on a quad that weighs 600g, then 20% of the thrust (600g/3000g x 100%) will go towards lift and the rest towards movement. If we have 6000g of thrust, then only 10% will go towards lift.

Breaking Down the Thrust

If we set the throttle so we only put out 600g of thrust, then we have just enough thrust to hover as long as our pitch is at 0 degrees (the thrust is perpendicular to the pull of earth’s gravity). If we keep the throttle the same, but start pitching forward, our thrust is now at an angle relative to the pull of gravity. Without getting into the trig, we are now using a portion of our 600g of thrust to propel the quad forward. Therefore, we now don’t have the full 600g of thrust to offset gravity. This is why we need to increase throttle the more we pitch forward. Let’s say we had to increase it to 800g.

Now that we have adjusted our throttle to keep our altitude consistent, we can see that 200g out of the 800g of thrust is being used to push us forward. It’s easy to see how the 600g is going toward lifting our 600g quad, but what does this 200g of forward thrust do? Just like lifting thrust offsets gravity, displacement thrust offsets the drag force. The faster the quad travels, the more drag force acts upon it. In our case, the quad will keep accelerating until the force of drag equals our displacement thrust.

Later in the series, I will show how we can use the drag equation to figure out our velocity and the effective ways to reduce drag. However, first we need to make sure our quad is utilizing all motors equally which is the next topic in this series.

Part 4: Center of Thrust and CG

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