What is ground effect?
Ground effect is a term used to describe the aerodynamic phenomenon of increased lift and a decrease in the induced drag that is generated by your aircraft’s wings when you are flying close to a flat surface like the ground or water.
What causes ground effect?
Let’s explore a little aerodynamic theory for a moment, so that we can understand the science behind ground effect.
As every good pilot remembers learning, the air pressure on the top surface of the wing is less than that on the bottom. This difference in pressure creates a circular flow of air from the high-pressure area below the wings to the lower pressure area above the wings. Although this movement of air happens along the entire trailing edge of the wing, it is most apparent near the wing tips. We call these areas of circular airflow “vortices.”
When you are flying at cruising altitude, the vortices are large. They cause more air deflection or downwash. When the air is deflected by the wingtip vortices, that angles the relative wind direction downward. Remember that lift is always perpendicular to the relative wind direction, so the lift vector shifts back correspondingly. Since a vertical lift vector is more efficient than a tilted vector, the vector shift gives you decreased lift and increased drag.
As you get close to the ground, the aerodynamics change. The wingtip vortices size decreases and their shape becomes more elongated rather than circular, resulting in less downwash. The decreased downwash doesn’t angle the relative wind direction as much, and the lift vector remains closer to vertical. This more efficient near-vertical lift vector provides increased lift and decreased drag.
How does ground effect impact me?
On landings, you will generally start to experience ground effect when you are within ½ your total wing length from the ground. The increased lift and decreased drag will cause your aircraft to seem to hover in the air near the ground, maintaining its speed and altitude longer than it otherwise would. You will often hear this phenomenon described as being “like floating on a cushion of air.”
For takeoffs, you start out in ground effect, where the increased lift and decreased drag help you to build airspeed. Your aircraft may become airborne at lower than rated takeoff speed. As you begin your ascent and climb out of ground effect, more power must be used to maintain your rate of ascent.
Because the impact of ground effect increases the closer your wings are to the ground, low-wing aircraft like a Piper Warrior experience more ground effect than a high-wing Cessna 172.
How can I use ground effect to my advantage?
The two most common instances when ground effect can come in really handy if you know how to use it to your advantage are short field and soft field takeoffs. It has also come in useful to help pilots in emergency situations.
Short Field Takeoffs
For a short field takeoff, the focus is on achieving your best angle of climb (Vx) speed as quickly as possible. Every little bit helps, and if you know how to use ground effect to your advantage, you can get the boost you are looking for. As soon as you have enough speed to raise your wheels off the runway, do so to help decrease drag. Maintain a low angle of attack (AoA) to decrease drag and increase acceleration. Follow your POH guidelines for flaps. This will usually be whatever gives you closest to a 15-degree AoA as that is most efficient for the majority of planes. Too much flap will increase drag. Lower the nose once you have a positive rate of climb and let ground effect help the horizontal engine thrust to build speed quickly.
Soft Field Takeoffs
The issue with a soft field is that until your wheels clear the ground, you are subject to high levels of wheel drag thanks to snow, mud, sand, or whatever other non-optimal surface you are taking off from. With a soft field takeoff, your focus is on decreasing wheel drag as soon as possible. This means that you must get the wheel in the air fast. To do that, the nose angle must be as close to maximum as possible. The challenge is holding that angle as you bounce along the uneven soft runway.
Once you get the wheel in the air, you need to keep it there so you can start picking up speed. Once you are airborne, slowly and carefully ease the nose down to allow acceleration while keeping the wheels above the runway. Initially, you will be flying very slowly, but thanks to ground effect, you will be able to keep the plane in the air, paralleling the runway until you pick up enough speed to safely climb out of ground effect.
Timing is everything with a short field takeoff. If you ease the nose down too soon, your wheels will be dragging in the soft ground again. Wait too long, and you will start climbing out of ground effect too soon before your speed is high enough to sustain non-ground effect flight.
Extending low altitude cruise range
There is historical precedent for using ground effect to your advantage when needing to extend your flight range in the case of an emergency. In World War II, B-59 aircraft flew long flights over the Pacific Ocean from the Mariana Islands all the way to Japan. Fuel margins were already tight, and if an engine failed, the increased drag could decrease the aircraft’s range enough that the pilot would be unable to make the return flight.
Eventually crews learned to fly low over the ocean, staying in ground effect for long distances and extending their range enough to make it home.
Common ground effect mistakes to avoid
We always prefer to learn from someone else’s mistakes rather than our own, so let’s review the most common ground effect mistakes and how to avoid making them.
Rotating prematurely – a hazard with increased potential during soft field takeoffs – can cause you to leave ground effect before you have accelerated to a safe climb airspeed. If you haven’t reached your safe climb speed (typically Vx) when you leave ground effect, your plane may quickly sink back towards the runway. Avoid this mistake by getting your airspeed up prior to rotation and to leaving ground effect.
Too fast an approach speed
When coming in for an approach to landing, carrying too much speed into your flare can cause you to float. This will extend your landing distance – a potentially serious problem depending on the length of your runway. Avoid this mistake by being mindful of your speed and confirming it prior to flare.
Trying to force the plane to land
When you feel the cushioning sensation of ground effect below you, you may be tempted to offset it and force the plane to land, especially if you have carried too much speed into your approach. Unfortunately, this can lead to the plane repeatedly touching down, then bouncing back up off the runway. This dangerous “porpoise landing” has led to the demise of many aircraft. Avoid this mistake by maintaining an appropriate descent rate and speed. Don’t try to force the aircraft to land. If you do start to porpoise a landing, quickly execute a go-around.
Ground effect is an aerodynamic phenomenon that if properly understood can be used to our advantage. The key is to understand what it is, what causes it and how it affects our aircraft. Armed with this knowledge, we can safely and effectively take off and land more smoothly.
Learn more about Ground Effect and other factors of flight in the FAA Pilot's Handbook of Aeronautical Knowledge FAA-H-8083-25B.
> B-59 aircraft
Typo. s/b B-29?