In this blog post, I am going to look at some basic aircraft design principles and how they have shaped Passerine Aircraft’s approach to building a drone.
Flight is Easy
At its simplest, flight is just about balancing four forces: lift, weight, thrust and drag.
- Lift — The force in the upwards direction.
- Weight — The force trying to bring you back to the ground.
- Thrust — The force forwards from the engines.
- Drag — The force trying to slow the aircraft down.
If all four of these forces are equal, the aircraft will stay at constant speed and altitude.
Lift is generated by lifting surfaces, like an aeroplane’s wings. Any object moving through the air can generate lift depending on its shape. A car, for instance, can experience a degree of lift and that’s why an aerofoil (the wing shape on a boot) is placed on faster cars. Aerofoils generate lift but they are mounted upside-down, so that they push a car into the ground instead of lifting it into the air.
The amount of lift created is related to the size of the lifting surface, and the speed it is moving through the air. Basically, a big wing moving slowly will produce the same lift as a smaller wing moving faster. For an object to fly it needs to have more lift than its weight to leave the ground.
Moving on to drag. We are all familiar with drag in some form. When running, you can feel the wind push against you. This is drag and it is created by you moving through the air. It can also be created when generating lift. So, lift does work against an aircraft in some way. All the drag on an aircraft needs to be equalled by the thrust from the engines — thrust being the force the engines generate to move a plane forward.
Good aeronautical design dictates that the drag on an aircraft should also be significantly less than the weight of the plane. In fact, for a fixed-wing aircraft, the drag should be less then a tenth (1/10) of the weight. If an aeroplane is designed purely to minimise drag, this ratio can be even larger. The lift to drag ratio is an important number in aircraft design.
For example, one important use for it is to determine the glide slope of the aircraft. The glide slope is the distance forward the airplane covers for every metre/foot it drops in altitude. e.g. If an aircraft has a lift to drag of ten to one (10:1), it will travel 10 meters forward for every 1 meter it descends (with no thrust). Gliders have extremely high lift to drag ratios and are therefore capable of staying in the air for very long periods of time. Even though they have no thrust.
At Passerine Aircraft, we designed our drone to minimise drag, because drag decreases the efficiency of a craft. A plane with a lot of drag needs a lot more energy to travel the same distance as a similar size plane that has less drag. Minimising drag with our drones means they can fly further and faster.
So, once you’re in the air, flying is quite easy, however…
Take-off is Hard
Flying is easy because you just need to maintain enough speed to keep the plane generating lift. At the start of a take-off, you just have a giant paperweight. So, you must have thrust, and preferably lots of it. You can have the most aerodynamic, lightest aeroplane in the world, but without thrust, you are going to just sit on the runway. As you are well aware, thrust is only a part of the story.
Cars have lots of thrust, but they don’t leave the ground, so that’s where ingredient two comes in. You also need to generate enough lift to overcome the weight of the aircraft. As I mentioned earlier, lift depends on the speed of the air moving over the wings on the aircraft.
Now, there are two basic ways of getting this speed. The first method is to accelerate along the ground until you pick up enough pace. This is what most fixed-wing aircraft do and it’s why we have runways.
The advantage of this approach is that you don’t need a lot of thrust to do this. So, it’s possible for this type of aircraft to be designed for very efficient flight. In the animal kingdom, an albatross basically does this. They pretty much run along the water until they manage to get airborne. They are some of the most amazing flying creatures, but they really struggle to get into the air.
The other option is to move your lifting surface, instead of moving your aircraft. A helicopter’s blades are its lifting surface, and when you move them fast enough, you generate enough lift to take-off. In nature, a hummingbird demonstrates this quite well.
Moving a lifting surface is how all rotorcraft work. Another way of thinking about this is that lift is being generated by pointing your thrust up. This does make take-off quite easy. In fact, if you have enough thrust you can make anything fly. The problem is that you now have way more thrust than you need for normal flight, which means the aircraft is no longer flying efficiently.
Planes that reach a compromise between these two methods are known as Vertical / Short Take-off and Landing (V/STOL) aircraft. The Harrier Jump Jet is an example of one of these. Making a successful VTOL aircraft is a difficult problem and there have been many attempts to do it. Really interesting designs have come out of the challenge of VTOL, and I will have a look at some of them in a future blog post.
Skipping the take-off phase
At Passerine Aircraft we are taking a different approach to solve the “short take-off with efficient flight” problem. Our solution is to just “skip” take-off… well jump, during the take-off phase. Many birds already do this. The initial speed needed is provided by their legs, which are then retracted during flight, so that they don’t create drag and hinder the bird’s flight performance.
We are just mimicking birds, like the heron above, and getting an aeroplane to jump off the ground.
Making a craft that can take-off and land anywhere has been done. And making one that is very efficient at flying long distances has also been done. Combining those qualities is new. Our aim is to have an aircraft that doesn’t need a runway, but is also efficient at flying. It’s the best of both worlds and has so many exciting applications.