What is the indicated airspeed necessary to exit ground effect on takeoff?

Question

I always thought that a plane needed to be at stall speed or greater to be able to exit ground effect on takeoff. However, after seeing the YouTube video in the link below I am not so sure.

In this video the winds change from a headwind to a tailwind during takeoff and the plane is not able to leave ground effect. The pilot ultimately aborts the takeoff. The pilot was able to achieve about 60 knots indicated airspeed while in ground effect but the plane was not able to accelerate beyond that.

Looking at the specs on the Cessna 172, which was the airplane in the video, 60 knots is the Vx speed of the Cessna 172. So it seems like the pilot could not exit ground effect while flying at Vx?

My question is if there is a particular V speed necessary for a plane to leave ground effect or is it entirely based on acceleration and drag?

Answer 1

The idea that the plane was unable to leave ground effect is not quite right, in fact it's the opposite.

Ground effect results in the plane having slightly more lift (and less drag) at very low altitudes close to the ground, and it typically comes into play when your altitude is less than the wingspan of the aircraft, this is why it is sometimes described as a cushion feeling in the last moments as a plane descends down to ground level.

So for example, let's assume you are stable and approaching the runway at a constant rate of descent, lift and drag will be very much constant. Suddenly however, as you pass down to an altitude less than your wingspan, the aircraft gets a slight increase in lift, and a corresponding decrease in drag (specifically, induced drag).

As a result, your rate of descent will decrease due to the increase in lift, and your airspeed may also increase a little bit, which can give the sensation of the plane floating just as it is about to land. I personally found this a good source for understanding ground effect and induced drag:

https://www.boldmethod.com/learn-to-fly/aerodynamics/what-happens-to-your-plane-in-ground-effect-float/

Going back to the video then, the airplane is actually generating slightly more lift during the take-off roll thanks to ground effect, which essentially is helping to push the plane upwards (hence why I said it's quite the opposite above). In fact, the pilot states that he managed to gain a small amount of altitude before feeling that he had suddenly lost lift and deciding to abort, and that is likely due to primarily the weather conditions, but possibly also supplemented by the fact that he did in fact leave ground effect, and no longer had the benefit of the increased lift (and reduced drag), and hence did not have sufficient lift to sustain the initial climb, especially given the conditions.

However, I think you'd have to say that the main cause of the aborted take-off was the extreme wind conditions which did not allow them to generate sufficient airspeed required for the climb. As the pilot states, the wind changed dramatically during the take-off roll, and you can see the airspeed indicator moving around a lot too.

So in summary, changes in induced drag (which is essentially what causes ground effect) are a function of altitude vs wingspan, and is not directly related to speed.

Answer 2

The key to getting out of Ground Effect is having sufficient amount of thrust to overcome the increase in Induced Drag as you start to move away from the ground. If you don't have the sufficient amount of thrust to overcome the increase in induced drag you will never leave ground effect and that is indeed what happened in the video.

A few observations about the video:

1. The airport in the video is Big Bear Airport in California which has an elevation of 6,752 feet and you can probably bet the density altitude is much higher than the elevation. The pilot in the video blamed the failed takeoff on a tailwind but it was most likely due to high density altitude. A tailwind causes a considerable delay in reaching the necessary indicated airspeed for takeoff but from the video we see that the airplane stops accelerating completely at 60 knots which indicates a lack of necessary thrust.
2. It is a little hard to see from the video but it appears the pilot may have taken off with the mixture in the full rich position which is incorrect in high-density altitude conditions. The correct procedure is to lean for best power before takeoff and that should really be done at any altitude including sea level. Many pilots who learn to fly in the lowlands don't even know how to lean for best power and that may have been what happened here. The pilot may not have been making as much power as he could have.
3. From the video it can been seen that some precipitation was beginning to fall. I learned to fly at high density altitude airports (New Mexico USA) and I know from experience that high humidity greatly reduces the airplane's performance. The amount of power that an engine can create is directly related to how much oxygen the engine can ingest at any given time. The problem with high humidity is that the water molecules displace the oxygen molecules for a parcel of air so the oxygen percentage for that parcel is reduced. Less oxygen means less power. Turbocharged engines essentially force excess air into the engine and as such are much more immune to the effects of high density altitude.

As to the questions about being at Vx and still not being able to leave ground effect, that seems about right. At the absolute ceiling of an aircraft Vx is equal to Vy and the plane will fly level at that speed. This is essentially what happened here in the video so basically he was operating at the absolute ceiling of the aircraft at what density altitude he was at. His absolute ceiling was probably affected by the fact that he wasn't properly leaned in this scenario.

Answer 3

Given that the video did not mention anything being wrong with the aircraft, my money is on the shift in winds being the primary cause for the apparent reduction in airplane performance. I would also blame the pilot for rotating before he had the indicated airspeed for a climb. The time to reject a takeoff is before the wheels leave the ground.

The pilots decision to immediately land was definitely influenced by a healthy concern for the available power to overcome whatever additional unexpected weather conditions might be encountered before climbing over the obstacles beyond the end of the runway.

Looking at the video, if the aircraft was performing normally and if there were no further meteorological changes, it's very probable he would've been able to clear the obstacles. The pilot's problem in the video was uncertainty about both.

So all in all, if there is sufficient runway to land, it's definitely more prudent to do so in that kind of a situation.

As to the question of Vx and flying out of ground effect, the answer is pretty simple. As long as the aircraft stays within the envelope of conditions that allow for a climb at Vx, it'll continue to do so. But the outside world is not static and neither is engine performance.

Answer 4

I propose a thought experiment to answer this question, as follows.

We fly a Cessna 172 with full fuel, baggage, and 2 passengers up to a 5000 feet cruise on a hot day, simulating ground conditions in Winslow, AZ. Now we simulate a high density-altitude takeoff roll by cutting the throttle to idle, adding backpressure to reach the verge of stall, then adding full throttle and full rich mixture and returning the elevator to neutral.

To simulate ground effect we add some flap.

With full power on, the plane then begins to accelerate and as it picks up speed it ceases to descend and instead begins to climb. We dial in some nose-down trim to simulate a takeoff roll on the ground at constant altitude and in response the plane stops climbing and gently descends as it accelerates.

We watch the VSI and airspeed and when the VSI goes positive (thus simulating the takeoff point) we rotate per the book and watch the airspeed. One or two seconds later we simulate flying out of ground effect by pulling in the flap.

To review: we are now at or near 5000' on a hot day in a loaded 172, no flaps, full power, rotated for takeoff at the book airspeed (nose up for standard climbout), and just barely out of ground effect.

In this configuration, will the plane continue to climb, stay in level flight, or stall? Imagine the plane stays in level flight. This is not what the pilot wants, so she adds some backpressure or trims nose-up to enforce the desired climb rate.

NOW: in this configuration, will the plane climb, stay in level flight, or will it stall?

In the last 10 years there have been two fatal accidents at the Winslow municipal airport where transients stopped for fuel and then crashed from low altitude just beyond the end of the runway. One was (I think) a Vari-Eze and the other a Mooney M20 series. The hypothesis in both cases was that the pilots flew out of ground effect and promptly stalled and pancaked in.

Is this plausible?

Answer 5

The indicated airspeed necessary to exit ground effect on takeoff is a not floppy and consistently increasing indication through some certain minimal value. Airspeed indicators tell you more than just any momentary airspeed. The way airspeed develops is at least as important as airspeed itself.

You can almost feel in the video, how the first notion something is off comes to the pilot, as he notices how late the airspeed comes alive. You don't just want to exit ground effect. You want to do all the other stuff that comes after that as well. Vx is something you go through. It's not a destination. Being able to go through it convincingly is as much a minimum requirement as reaching it. There is no warranty on Vx.

It's not like I'm surprised to see I can fly every time I do, but every time the wheels come off the ground, I realize I'm sitting on thin air. All that is keeping me from falling like a brick is sheer magic. No matter how much I learn about how to deal with air and the weather and how to make that magic happen, I should never get the notion I own it, so I don't try, or try not to. Even flying an A380. which I can imagine to be more like a day in the office than like flying a Cessna, doesn't make it own the skies. Vx is like a practical dream, useful yet flimsy. The same thing goes for drag and acceleration. It's a way to understand things, but it doesn't determine what is there.