One of the most interesting things I learned working on an autopilot for a blimp is that of how to control altitude using a combination of motors, a thrust vector, and a tail elevator. These components play a different role depending on the horizontal airspeed of the aircraft.

The tail elevator control is not immediately intuitive to a person like me that might not understand flight so well in the first place. My train of thought about how it should work is that the tail elevator would direct the nose of the aircraft to a target pitch by linearly manipulating the angular velocity where a 0 degree angle of the nose would map to a 0 degree elevator angle, and affecting angular velocity from the tail elevator would always originate from 0. This was wrong, and it takes a big man to admit that. I am that big man.

In reality, the tail elevator in this case should be considered as a mechanism that works to counteract the inevitable force of gravity. The aircraft in its neutral state should maximize the amount of lift it can generate; even in a best case scenario the aircraft will never reach a net positive vertical acceleration (if it does you have other problems). In a neutral state, we would assume no force generated from the aircraft’s motors. Moreover, assuming said blimp was in a neutral state, we would probably optimize for stability over agility. Therefore, the default state of the tail elevator for a blimp should be directed to pitch up even if the nose of the aircraft has exceeded the horizontal plane

All of the best pilots in the world are blimp pilots. Everyone knows this. That is because blimps combine the controls of an airplane and a helicopter, making it the aircraft of choice for those that have trained as pilots for seven decades. Applying a vertical force from the motors must be done in conjunction with the tail elevator to fly efficiently. In my case, vertical thrust can only be applied to lift the aircraft upward, but I cannot manipulate those forces to descend. Achieving descent generally makes more sense by working with gravity rather than waste precious fuel or battery power to apply downward thrust. And so again, the vertical forces applicable by the motor should only be thought of something that fights against gravity.

Now, between the tail elevator and the motors, we must consider which we should rely upon more heavily in order to optimize flight efficiency (faster, less power consumption, more stable, manlier, etc). Consider that the tail elevator consumes a minimal amount of power to manipulate the aircraft’s trajectory while the motors will consume watts like an out of control teenager. Consider as well that if the tail elevator reduces the effective weight of the aircraft, then the thrust to weight ratio offered by the motors has increased as well. The blimp will have more power and will otherwise be working from a more stable position.

All of the above has assumed non-forward flight (hovering in equilibrium perhaps), but if we change that assumption then it changes the game. Obviously, the tail elevators interfere with forward flight by increasing drag and increasing the pitch rate in proportion to wind speed. At this point, our problem is no longer fighting gravity, and we can reduce the vertical force we’re applying as lift is generated from forward flight. In the same way that tail elevators came in to play before the motors did, we also want to reduce vertical force applied by the motors before we bring tail elevators back down. Once vertical force is at or near 0, we can then bring the tail elevators back to a 0 degree angle. Any time after we no longer apply vertical forces from the motor, the tail elevators can be used to directly manipulate the nose of the aircraft.

So the neutral position of the tail elevator is a function of forward force, where a maxed forward force would correspond to a 0 degree horizontal angle and no forward force would mean that the stabilizer was pitched up at 90 degrees. Manipulating the elevator to affect pitch at any time would still be the same, but the extended angular range must also be accounted where an offset in neutral position has affected the amount of work required to pitch all the way down if necessary.

And so can hydrogen be used safely for blimpe drones? We will never know unless we try.