Aerodynamic Forces Caused by the Ground Effect

I would first like to discuss the characteristics of measured variation in aerodynamic forces caused by the ground effect and suggest their principal causes.

The design of the crafts aerofoil contributes to the performance due to the fact, different aerofoils cross sections have altered aerodynamic features for example, the lift to drag characteristic with a difference in angle of attack. Thus crafts that fly at slower speeds have been designed to have thicker cross sections whilst high speed crafts have been designed with a thin cross sections (aspect ratio also affects performance).  

The movement of an aerofoil when travelling through air creates a moment about the aerodynamic centre of the aerofoil. We would call this the pitching moment which is the result of pressure transfer on the aerofoil. To make flight stable and efficient, the pitching moment needs to be balanced. To overcome this variation of aerodynamic force, we would add another lifting surface on the craft either a tail plane on the back or canards at the front.  

The ground effect disturbs the pitching moment which is produced by the aerofoil. The changed air flow acting upon the aerofoil moves the aerodynamic centre positioning of the wing thus this pitching moment occurs.

Convenient aerofoils produce nose down pitching when in flight. The ground effect results in an increase in this moment thus a larger stabilising force is needed to keep this pitching moment balanced. As mentioned before, the craft would require a larger tail plane or canard to keep the craft balanced (this would decrease the crafts efficiency due to increase of weight).      

From previous data that I have collected in the past and through observation from multiple experiments, I would find that flight in extreme ground effect can move the aerodynamic centre to the half chord position. This causes design difficulties as the need to control the craft over a greater pitching moment increases drag, design complexity and weight.

The wing in ground effect stability with respect to the crafts height is achieved when there is a decrease in height thus an increase in lift (vice versa). These force parameters allow the increased lift to help return the craft to its original height. Fundamentally if the craft is troubled in flight, the lift force should help to restore the craft to its original height.      

I had also examined and observed vortices generating when the aerofoil was further away from the ground (ground effect). My data also showed that flying near to the ground is more efficient which I could also see through slow motion footage that I had taken. This means that flying closer to the ground is much more efficient due to the elimination of disturbed airflow vortices (this is due to the downward pressure being stopped from above the aerofoil). A vortex is created when the airflow swirls around the wingtip of the aerofoil, this normally generates a pushing force to the wing which makes it more difficult for the craft to remain in a steady flight. Hence why the solution is to create/install aerodynamic winglets on wing tips which reduce the disturbed airflow vortices.

Now it is vital to understand that if the air speed data I had calculated had an error of 10% associated with them, what effect this would have on the lift generated (what is the relationship between lift and velocity?). To answer this, myself and my colleague had collected data from the manometer at several angle of attacks, heights and velocities.

Our data showed us a unique relationship between the lift/drag and velocity. I had concluded that when the velocity was increased, both the lift and drag had also increased (lift always being greater than drag hence why lift increased with each pitch increase).  

When taking data for the ground board effect (height of 75mm), you would see that the fundamental theory works that lift increases dramatically whilst induced drag is decreased (ground effect comes into play). This cushions also helps reduce vortices as mentioned before (proven through data and practical footage).

Now when looking at both the recorded and calculated data for velocity readings from the manometer instrument, I would find that there is a slight error with what we had calculated to what we had read from the manometer. This is due to human error whilst recording data (eye sight or instrumentation increment may not be small enough to get accurate displayed data). My data showed that the calculated largest error is 4%, which is within the 10% targeted parameter.  

So what actions would a pilot take to mitigate the influence of ground effects on low level flight (especially during landing?).

Ground effect also cause problems to the craft and pilot when taking off or when coming to land. When the pilot is travelling down the runway at high speeds, the ground effect will may result in the craft becoming airborne before the recommended take-off speed has been achieved. Now the craft might take off from the ground but it will encounter a hard time getting out of the ground effect hence why an increase in angle of attack is needed (maintain the same lift coefficient) as shown in my data that a higher angle of attack and more thrust at a certain degree with help generate more lift (pilot must consider; climb rate, take off speed, flaps and weight before take-off).

On the other hand, we know that the ground effect increases lift and decrease induced drag as mentioned earlier thus when a pilot is coming to land, the increase of lift will make it much harder for the pilot to land due to flaring. If the craft is near to the ground with a constant angle of attack, the craft will have an increase in lift coefficient thus require less thrust hence floating/gliding may occur. This is why during landing, the pilot must reduce the crafts thrust and make sure that the correct trim/flap settings have been applied.