Do winglets decrease ground effect?

[Matt13C]

My very basic understanding of ground effect is that as you get within a wings length of the ground, the wing tip vortex is reduced. So if the point of a winglet is to reduce the vortex, is the effect of ground effect also reduced?

Some background on my understand of flight which is how I came to that question, please correct me where wrong. I am not an engineer, just trying to piece together what I have read on circulation, Bernoulli and wing tip vorticies.

As a wing starts to generate lift, it creates a rotor or circulation of air behind it causing an opposite rotation around the wing, where air in theory rotates backwards under the wing and back over the top. This rotation over the wing is the same "height" above and below the wing as the length of the wing. As the higher pressure air below the wing spills over the side of the wing, it wraps up and over into the down washing air on top, causing drag, decreasing lift. As you get closer to the ground, less air spills out from under the wing, decreasing the drag, increasing lift. How far off, in a basic sense, is that?

 

[shdw]

is the effect of ground effect also reduced?

Ground effect is an exponentially increasing percent reduction in induced drag. Winglets do not change this percent reduction.

Have a look at page 379 in AFNA and reference the section on ground effect. The chart depicting the exponential increase I've mentioned is on page 380.

As a wing starts to generate lift, it creates a rotor or circulation of air behind it causing an opposite rotation around the wing, where air in theory rotates backwards under the wing and back over the top.

The vortices (induced drag) behind the aircraft do not cause circulation. Circulation is a result of the differing pressures above and below the wing. The vortices are a result, not the cause, of circulation.

This rotation over the wing is the same "height" above and below the wing as the length of the wing.

I don't recall ever reading anything that supports this. I'll see if I can dig anything up, will be a few days.

 

[Matt13C]

Ground effect is an exponentially increasing percent reduction in induced drag. Winglets do not change this percent reduction.

Have a look at page 379 in AFNA and reference the section on ground effect. The chart depicting the exponential increase I've mentioned is on page 380.

The vortices (induced drag) behind the aircraft do not cause circulation. Circulation is a result of the differing pressures above and below the wing. The vortices are a result, not the cause, of circulation.



I don't recall ever reading anything that supports this. I'll see if I can dig anything up, will be a few days.

Thanks. I can't remember where I read the height above the wing, maybe I just thought it did because that's the distance from the ground when ground effect begins.

 

[shdw]

I just thought it did because that's the distance from the ground when ground effect begins.

I was thinking the same thing. If you find that source I'm all ears. I'm still learning when it comes to the flow patterns. I'm not an engineer either, just a self study who just enjoys reading technical texts.

 

[Cessnaflyer]

Interesting read. I'll have to come back when I am on real internet.

 

[Matt13C]

Ground effect is an exponentially increasing percent reduction in induced drag. Winglets do not change this percent reduction.

Have a look at page 379 in AFNA and reference the section on ground effect. The chart depicting the exponential increase I've mentioned is on page 380.


The vortices (induced drag) behind the aircraft do not cause circulation. Circulation is a result of the differing pressures above and below the wing. The vortices are a result, not the cause, of circulation.



I don't recall ever reading anything that supports this. I'll see if I can dig anything up, will be a few days.

I am reading the page on ground effect, and I still feel that a winglet would reduce the impact of ground effect. My understanding is, the winglet decreases the strength of the wing tip vortice thus reducing induced drag. According the the article, this is the same thing ground effect does.

"These effects are illustrated by the sketches of figure 6.9. As a result of the reduced tip vortices, the wing in the presence of ground effect will behave as if it were of a greater aspect ratio. In other words, the induced velocities due to the tip (or trailing) vortices will be reduced and the wing will incur smaller values of induced drag"

So if the winglet already is reducing the tip strength, wouldnt that mean that, if everything else is held constant, that a plane without winglets would see a greater reduction in induced drag, or greater ground effect, than a plane with winglets?

 

[shdw]

I am reading the page on ground effect, and I still feel that a winglet would reduce the impact of ground effect.

In a purely numerical sense, yes. In a percentage sense no.

Assume a 50 percent reduction in induced drag, or roughly 1/10 the wingspan height. For simple numbers consider an airplane, airplane 1, with winglets produces 10 induced drag. The same airplane, airplane 2, without winglets produces 100 induced drag.

Airplane 1 has an induced drag reduction of 5. A small number, but still 5 is half of 10; or a 50 percent reduction.

Airplane 2 has an induced drag reduction of 50. A number that is certainly much larger than airplane 1's reduction of 5. However, 50 is still half of airplane 2's starting value of 100; or, again, a 50 percent reduction.

The impact is 50 percent in both cases. The numerical difference does change, but the numerical difference between a 747 and a 172 is different too even though the percent change is identical. It's not proper to classify winglets as reducing induced drag in ground effect as ground effect is a percent based, not numerical based, reduction. However, there is nothing wrong with saying an airplane with a lower value of induced drag will have a lower numerical change in it's drag when entering ground effect.

 

[Baronman]

I can say from a pilot's perspective having flown 2 models of aircraft, each one I've flown a winglet version and a non-winglet version. The winglet version of each seems to float more, that is to say has more ground effect. I couldn't give a quantifiable number but it's enough to notice the trend.

 

[shdw]

The winglet version of each seems to float more, that is to say has more ground effect.

As one would expected. Consider the numbers from the example above. The aircraft with winglets has 5 induced drag and the one without has 50. If two identical aircraft have a mod that reduces induced drag then the aircraft with less induced drag will float more.

 

[Matt13C]

In a purely numerical sense, yes. In a percentage sense no.

Assume a 50 percent reduction in induced drag, or roughly 1/10 the wingspan height. For simple numbers consider an airplane, airplane 1, with winglets produces 10 induced drag. The same airplane, airplane 2, without winglets produces 100 induced drag.

Airplane 1 has an induced drag reduction of 5. A small number, but still 5 is half of 10; or a 50 percent reduction.

Airplane 2 has an induced drag reduction of 50. A number that is certainly much larger than airplane 1's reduction of 5. However, 50 is still half of airplane 2's starting value of 100; or, again, a 50 percent reduction.

The impact is 50 percent in both cases. The numerical difference does change, but the numerical difference between a 747 and a 172 is different too even though the percent change is identical. It's not proper to classify winglets as reducing induced drag in ground effect as ground effect is a percent based, not numerical based, reduction. However, there is nothing wrong with saying an airplane with a lower value of induced drag will have a lower numerical change in it's drag when entering ground effect.

Ok, I get what you are saying. So the winglet lowers the overall induced drag, but the reduction % in ground effect is the same. Which would also explain @Baronman statement above that the winglet version floats more. The plane is starting out with much less overall induced drag, making it more susceptible to float, as less lift is being stolen by drag.

 

[dasleben]

I can say from a pilot's perspective having flown 2 models of aircraft, each one I've flown a winglet version and a non-winglet version. The winglet version of each seems to float more, that is to say has more ground effect. I couldn't give a quantifiable number but it's enough to notice the trend.

Yeah, I've been floating the 767 winglet version a fair amount lately. Just wants to keep flying.

 

[Matt13C]

So winglets decrease induced drag, but is the reduction in size or strength? I ask because in the diagram on page 380 the vortices decrease in size when in ground effect. However, if the reduction in drag in ground effect is the same %, this leads me to believe it's a reduction in strength not size, is that correct? If a wing tip changes the size of the vortice, then would the wing need to be lower to the ground before the reduction in drag takes effect? So a non-winglet aircraft receive a boost from ground effect earlier but more gradually and the winglet aircraft would get a reduction lower to the ground but at a quicker rate to make up for the same % drop in drag?

 

[shdw]

less lift is being stolen by drag.

So lift counters drag now?

So winglets decrease induced drag, but is the reduction in size or strength?

Let me get back to you on this one. It is too late to stick my nose in another book at the moment. Bed!

 

[Matt13C]

So lift counters drag now?

Ha not directly, but if drag is reduced overall from the winglets, wouldn't that mean that at every angle of attack, you'd be generating more lift?

 

[shdw]

Ha not directly, but if drag is reduced overall from the winglets, wouldn't that mean that at every angle of attack, you'd be generating more lift?

You'd be generating lift equal to your weight, but you'd have less drag at each AOA.

Think two dimensional for a moment. A block is pushed across the ground slows down because of drag in the form of friction. The airplane glides over the runway with engines giving some forward force, thrust, and drag acting to slow it down.

Ok now back to three dimensions. There is no more lift than there is weight in any steady state condition. Drag, drag, think drag!

 

[Hellboy888]

I dont think so.
as the ground impact is more compared to flow given by air, Their main purpose is to prevent the high pressure air at bottom of wing.

 

[Matt13C]

I believe it to be otherwise, but I'd love to continue this discussion. Ground effect is caused by a reduction in induced drag. If winglets themselves cause a reduction in induced drag, then there is less total induced drag for ground effect to impact. The % reduction would be the same, but the total reduction would be reduced.

This is totally made up and I am not sure if it is the proper way to describe induced drag. But -
If a wing has winglet-less wing has an induced drag ratio of say 20% wing span, and ground effect reduces this by 10%, then we see a new induced drag of 18%, 2% drop. If winglets reduce overall induced drag to 15%, then ground effect takes its 10%, then the new induced drag is 13.5%, 1.5% drop.

 

[Hellboy888]

Interesting stats.
I am not sure that the reduction in ground effect is the primary objective of winglet .
may be it is helping as per the stats you stated,but not sure only because of winglet .

 

[shdw]

I am not sure if it is the proper way to describe induced drag.

Not really, induced drag is a bit more complicated than a percent span ratio would lead someone to believe. In fact, it's not a percent span ratio at all despite wing span playing a role in its calculation.

It would be more accurate, if you wish to stick with a ratio, to provide the ratio of induced drag to total drag. However, it gives little insight toward the point you're trying to make.

Sticking with fixed totals is likely best. Let's say aircraft A has winglets and aircraft B is identical, but doesn't have winglets. At landing speed aircraft A has 80 induced drag and aircraft B has 100. Inches from the ground each aircraft experiences, for simplicity sake, a 50 percent reduction.

• Aircraft A started with 80 and now has 40. A 40 point reduction in induced drag.
• Aircraft B started with 100 and now has 50. A 50 point reduction in induced drag.

So, interestingly, if the question were, which has a greater fixed value reduction in induced drag? Our answer would be Aircraft B without winglets. If we asked, however, which had a greater percent reduction; the answer is they are the same. You'll also note that even their differences in induced drag decrease: a 20 point difference out of ground effect is cut in half to only 10 when in ground effect.

In either case, what is important to know is that drag slows you down when you're trying to land. Of particular importance is the time you're settled in flare waiting for that last bit of speed to dissipate. During these moments the form of drag that dominates is induced drag. Since the aircraft with winglets has less of it, it would be more likely to float. A pilot flying would likely notice little differences in any other phase, but they would almost certainly notice that, in two identical aircraft, the one with winglets will float more.

 

[Polar742]

Yeah, I've been floating the 767 winglet version a fair amount lately. Just wants to keep flying.

Yes, because it is the winglet causing the float, not Hack McHackerton at the controls...