October 06, 2017, ©. Leeham Co: In the last several Corners, we wrote about research around laminar flow for aircraft. It’s research to lower the aircraft’s drag. Why is this important? How large are the different drag types and what can be done about them?
To find out, we will spend some Corners looking into the drag of an aircraft and what is done to optimize the drag for different aircraft types.
When an aircraft flies through the air, it generates drag. Drag is the force that tries to slow the aircraft down. Therefore, we need engine thrust to keep the speed constant.
In normal cruise flight, the thrust of the engines is exactly the size of the drag, but pulling in the opposite direction.
At the same time, the aircraft lift is exactly the size as the gravity force on the aircraft mass, once again in the other direction.
If we can reduce drag, we can reduce engine thrust. Reduced engine thrust means less fuel consumption per covered distance. We can fly the same distance using less fuel or fly longer for the same fuel.
For each new generation of aircraft, the expectation is reduced fuel consumption and reduced CO2 emissions (directly proportional to the fuel burn).
Reduced fuel consumption can be achieved through more efficient engines, reduced airframe drag or both. This is why reduced drag is an important part of airframe development.
We will go through the important drag components, how important these are during different flight phases and what can be done to reduce them.
We will constrain the discussion to subsonic airliners. These have limited supersonic flow areas during cruise, and we will discuss these under transonic drag. But we won’t go any deeper into supersonic drag.
We will do the discussion as simple as possible, without losing the gist on how the drag is created. But we will not go into the detailed physics, and we will skip some drags that are less important.
At the base, we can divide the aircraft drag in those that comes from the aircraft’s size and those that comes from the aircraft’s weight.
In coming Corners, we will go through these drag classes and talk about how big a problem they are and what is done in modern airliners to keep them low.
From the column: “In normal cruise flight, the thrust of the engines is exactly the size of the drag, but pulling in the opposite direction.”
Doesn’t thrust need to be higher than the drag to maintain forward movement? It seems to me that if the engine thrust and the drag were equal during flight, the aircraft would just not move forward (opposite and equal energy). Shouldn’t the engine thrust be greater than the drag to overcome the drag and generate forward speed?
Force = change in momentum over time
Momentum = mass * velocity
Net horizontal force = thrust – drag = 0
This means there is no change in momentum during cruise time, so velocity (speed) remains the same. (Ignoring the gradual loss of mass in the plane from burning off the carried fuel to generate the thrust.)
Laminar flow has been the Holy Grail for all of the length of my career, perhaps even all of the length of your life.
My very limited knowledge is that one barrier was contamination of the airfoil surface with dirt and bugs. Might be less today with good washing of them to get the tiny amount of drag reduction cleanliness might provide.