C172 "Autoland" Procedure

labbadabba said:
Respectfully, no. There is no cushion of air. Lift remains unchanged. What DOES change is the lift:drag ratio caused by the near elimination of induced drag when subject to ground interference. This results in the 'feeling' that the plane suddenly has more lift.
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http://en.wikipedia.org/wiki/Ground_effect_(aerodynamics)

You should probably correct this Wikipedia entry then. Other than the fact it claims that induced drag is reduced, it's all wrong.
 
Sac Arrow said:
http://en.wikipedia.org/wiki/Ground_effect_(aerodynamics)

You should probably correct this Wikipedia entry then. Other than the fact it claims that induced drag is reduced, it's all wrong.
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I absolutely should correct it because it's false. The cushion effect is at best a nominal factor in winged aircraft.

When a wing is flown very near the ground, there is a substantial reduction in the induced drag. Downwash is significantly reduced; the air flowing from the trailing edge of the wing is forced to parallel the ground. The wing tip vortices that also contribute to Induced drag are substantially reduced; the ground interferes with the formation of a large vortex.


Many pilots think that ground effect is caused by air being compressed between the wing and the ground. This is not so. Ground effect is caused by the reduction of induced drag when an airplane is flown at slow speed very near the surface.
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http://www.pilotfriend.com/training/flight_training/aft_perf.htm
 
labbadabba said:
Respectfully, no. There is no cushion of air. Lift remains unchanged. What DOES change is the lift:drag ratio caused by the near elimination of induced drag when subject to ground interference. This results in the 'feeling' that the plane suddenly has more lift.
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Please explain a soft field takeoff.
 
MAKG1 said:
Please explain a soft field takeoff.
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Wouldn't it be the same thing? More advantageous L/D ratio... Same amount of lift but less drag. Airplane flies earlier on same amount of lift.
 
Kritchlow said:
Wouldn't it be the same thing? More advantageous L/D ratio... Same amount of lift but less drag. Airplane flies earlier on same amount of lift.
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No it doesn't. Flying level in ground effect means lift = gravity.

Can you do the same thing at 3000 feet, at the same speed?
 
MAKG1 said:
No it doesn't. Flying level in ground effect means lift = gravity.

Can you do the same thing at 3000 feet, at the same speed?
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Flying level ALWAYS means lift equals weight, regardless of altitude.
 
MAKG1 said:
Yup. Very hard to see how just reducing drag is going to make that happen below the 10 deg flap stall speed.
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Why would that be hard to see? A rising airplane is doing so because of excess lift. Does that mean that MORE lift is being created in this case? No. It means that there is less drag to counteract the lift. For a plane's attitude to change that means that one or more of the four forces is out of balance.

Remember the parabola graph L/D +L/D max?

For example let's say our lift to drag ratio is a hypothetical 2:2 in normal cruise flight. If we add the fact that the aircraft is 5 feet off the ground it reduces the drag coefficient. Now we're perhaps looking at a L/D ratio of 2:1. Because the value is weighted more heavily on the lift side rather than the drag side of the ratio the aircraft will rise. But the amount of lift remains unchanged.
 
Reducing drag will indeed improve a glide ratio... In this case "ground effect float".

I hate having the last post on a page. See last post on last page to answer you last question... :D
 
labbadabba said:
Why would that be hard to see? A rising airplane is doing so because of excess lift. Does that mean that MORE lift is being created in this case? No. It means that there is less drag to counteract the lift. For a plane's attitude to change that means that one or more of the four forces is out of balance.

Remember the parabola graph L/D +L/D max?

For example let's say our lift to drag ratio is a hypothetical 2:2 in normal cruise flight. If we add the fact that the aircraft is 5 feet off the ground it reduces the drag coefficient. Now we're perhaps looking at a L/D ratio of 2:1. Because the value is weighted more heavily on the lift side rather than the drag side of the ratio the aircraft will rise. But the amount of lift remains unchanged.
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That means you need less thrust to maintain speed (or accelerate in this case).

Most of us do soft field takeoffs by flying level just over the runway surface to accelerate. There is no excess lift. It's equal to gravity in that case. So, if ground effect doesn't change the lift, you should be able to fly level at the same configuration and speed at 3000 AGL, just perhaps not accelerating so much due to the higher drag.

If you try it, you'll get a different result.
 
You're able to accelerate because you can channel the excess energy generated by a favorable L/D radio into downforce to maintain level flight. That excess energy is then put on the thrust axis.

Just because lift and drag are on different axes doesn't mean affecting one won't effect the other.
 
labbadabba said:
You're able to accelerate because you can channel the excess energy generated by a favorable L/D radio into downforce to maintain level flight. That excess energy is then put on the thrust axis.

Just because lift and drag are on different axes doesn't mean affecting one won't effect the other.
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Tell us, when you do a soft field takeoff and you've leveled off for acceleration, is your pitch angle positive, zero or negative?
 
As you accelerate you'll need slight forward pressure to maintain level flight. Lift will increase as a result of increased airspeed (thrust).

If the plane pitches up induced drag and additional form drag will become factors and the benefit of ground effect will be minimized.
 
labbadabba said:
As you accelerate you'll need slight forward pressure to maintain level flight. Lift will increase as a result of increased airspeed (thrust).

If the plane pitches up induced drag and additional form drag will become factors and the benefit of ground effect will be minimized.
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If you are in level flight, lift is NOT increased. It is the same.

Forward pressure is not a pitch angle. And you could have trimmed that away.

The answer is that the pitch is quite close to zero initially and becomes negative quickly, all in level flight. At negative pitch, your thrust works against you, and there must be some force to oppose it or you're going down.

Unless you can somehow make drag negative, you don't have any way to keep your aircraft off the runway below the stall speed without getting lift appropriate to a higher speed.
 
As I said, as you accelerate the more air passes the airfoil and the more lift is generated. To keep the airplane from climbing you must apply forward pressure. Since the excess energy is not applied to climbing you'll accelerate even more.

The point is, drag IS almost reduced to zero. Typically in ground effect particularly in landing, you're generating very little form drag due to airspeed and no induced drag.

So when you say if you have negative drag you don't have any way of keeping the plane off the runway, this is precisely what happens when ballooning in ground effect.
 
I read this thread and am back to believing we really don't know what makes airplanes fly
 
I'm sure someone will soon chime in with "shorter path" theories to explain lift :)

I've never seen a PPL-level studybook explain correctly how lift is created. They all share the nonsense about either bernoulli, shorter path, or some other incorrect way to explain what the wing does.
 
mtuomi said:
I'm sure someone will soon chime in with "shorter path" theories to explain lift :)

I've never seen a PPL-level studybook explain correctly how lift is created. They all share the nonsense about either bernoulli, shorter path, or some other incorrect way to explain what the wing does.
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I've never seen any pilot oriented textbook that gets it right...
 
Jimmy cooper said:
Try google. Answer is quickly forthcoming.
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Well, of course. My statement was more to indicate that sources that are anywhere near correct are usually tailored to the engineer audience, not the pilots.
 
I don't know about auto land, but I learned to fly in my straight tail 172 and the crusty old army veteran CFI taught me to land at night by simply putting in a little power on short final, setting the plane up in a landing attitude and letting it find the runway on its own.

He was a firm believer (and so am I) that the runway is 6 feet higher at night that is during the day.
 
Stingray Don said:
Best answer I've seen.... :dunno:
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My understanding is, flight is a precise balance of the four forces that are in play when we fly. They are illustrated here-

5de1823ea85c48c822f9e01259d22ba4.jpg
 
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