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Old 01-10-2008, 08:18 PM   #21 (permalink)
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Who- what you described sounded like a soft field takeoff rather than a short field one. But you're right about the way you have to push pretty hard to keep the nose level.

Anyways Metro, the reason ground effect increases performance so much is because it significantly reduces induced drag and thereby reduces the angle of attack needed to create enough lift (or vice-versa depending on your view).

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Old 01-20-2008, 01:04 AM   #22 (permalink)
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Some mention of ground effect in this Heathrow 777 Article
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Old 01-20-2008, 06:58 PM   #23 (permalink)
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During WWII, ground effect saved the lives of many aircrews returning to England over the North Sea in shot-up bombers with one or more dead engines and lots of added drag from the bullet and flak damage. They would steadily lose altitude until they got within 50 feet of the sea's surface, where the altitude loss would stop.
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Old 01-26-2008, 10:03 AM   #24 (permalink)
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Having spent some time on jets a week or so ago, here's a question (which I suspect may be heresy to the pilots here)...

But is it strictly necessary to take off & land into the wind? Is there a threshold wind speed below which it's not necessary?

I did notice that heading on an easterly journey with the prevailing breeze from the west, the takeoff and landing consumed extra time & fuel due to having to head the "wrong" way at take-off for some time and then turn around toward the destination.

Similarly on arrival, we had to overshoot the destination by some distance in order to turn & approach the landing into the wind.
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Old 01-26-2008, 10:40 AM   #25 (permalink)
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Originally Posted by MetroMPG View Post
But is it strictly necessary to take off & land into the wind?
Yes. Even just a 10-knot tailwind will significantly increase your takeoff and landing distances, potentially to the extent that you will not have enough room to abort or go-around if necessary. There are some very specific regulations that define how much room you must have (based on the manufacturer's official aircraft performance specifications for a particular loading and configuration). Safety is a major priority, because things do break and you always need a way out. Having the engine crap out on your car as you barrell down an onramp is inconvenient but is not likely to kill you. Having an engine crap out just before rotation speed on a runway that is not long enough (given the conditions) to abort the takeoff very well may kill you and your passengers.

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Is there a threshold wind speed below which it's not necessary?
Yes and no. I can take off from my local airport (KGGG) in a light plane that might normally only require a 1500' distance to take off with a 20-kt tailwind because I have over 2 miles of runway in front of me. I'll have plenty of space to take off and it's not overstressing the plane at all. But the risks are higher. I'll be going a lot faster on the ground before having enough speed to take off (even though my indicated airspeed at rotation will be the same), and the consequences of a mistake are proportionately greater. The same applies for landing: the groundspeed will be significantly higher (requiring a lot more braking) and the distance required will be much longer (i.e. compare stopping distances in your car at 60mph and 80mph: the distance required at 80mph will be greater than the proportional increase in speed might lead you to believe).

Is it possible to land and take off with a tailwind? Sure, if you have enough room. Is it safe? Well ... Aircraft operators and pilots need to establish a habit and attitude of safety. Even with all of the automation and computer asisstance with which modern aircraft are equipped, a moment of inattention from the pilot can spell disaster.

With respect to aviation, safety needs to come before economy, always. I have a feeling that, in the long run, it's more economical that way.
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Old 01-26-2008, 10:47 AM   #26 (permalink)
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I'm not sure what the mileage improvement from skimming across a lake would be but 30% wouldn't surprise me at all and the optimal speed for skimming would likely be faster than the maximum endurance (most economical) flying speed as well.
Maximum endurance speed is a bit slower than maximum range speed. Both a quite a bit slower than you'd want to be flying in a small aircraft. Check out some charts and explanation here: http://www.eaa1000.av.org/technicl/p...ormance_Charts
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Old 01-26-2008, 11:04 AM   #27 (permalink)
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Thanks for the explanation, drcoopster. I suspected as much, and picked up a few other points (eg. hadn't considered braking distances).
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Old 01-26-2008, 11:07 AM   #28 (permalink)
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drcoopster, I don't want to be disagreeable but for very large airports near large metropolitan areas, I think they tend to have preferred directions based on normal prevailing winds and desired noise reduction strategies and will often let themselves get into very mild downwind landing situations so that they don't have to reverse patterns, or annoy residential neighbors. In addition, they can also permit crosswind situations that would be treacherous for small aircraft if all they have landing there are large commercial jets.

On small airports, or airports with mixed general aviation, the wind totally rules.
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Old 01-26-2008, 11:08 AM   #29 (permalink)
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Here's some more info on the tail wind. It really a matter of certification process for transport aircraft. They are only tested to a maximum component, most of the time this is 10 kts. Traffic has a lot to do with it also. You can get a landing against the flow if traffic is light or late at night and get straight in landing with a light tail wind provided as Drcoopster states there is enough runway. Depending on which type of operations you are conducting (Part 121, Part 135, Part 91) there is a whole slue of regulations, guideline and procedure both company and FAA/CAA imposed that must be followed. Most of which deal with safety.

Here's an interesting look at just how many airplanes are in the sky at a given time. So lots of time you just have to go with the flow.
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Old 02-04-2008, 10:58 PM   #30 (permalink)
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I've never piloted anything real but I do know a few things, as was said a steel plane with turbines off drops like a rock, pretty much, so that's no option.

With commercial airlines it is all regulated, there is this process and a procedure, I would think most pilots simply do as they're told, and for more than one reason. Safety is one but more than anything those big passenger jets are nowhere near as nimble as say a fighter aircraft or even a Cessna... That is, a mistake in the air costs a TON of fuel and time to recover, never mind economy it's the fact it takes so much fuel that only a few not so minor mistakes can put a plane's fuel levels in the critical zone, not to mention it throws it off schedule. So the big jumbo jet is roughly comparable to driving a city bus in traffic, there's just not much left in the way of maneuvers to try and save a few gallons.

Last but not least it's not their petrol, the big guys buy it and pay for it all they do is fly it, there's nowhere near the incentive to save when it doesn't come out of your pocket, no doubt there's the odd one out there who might do it out of principle and I think everyone should, but...

Look at hypermiling a car, so much of it is ludicrous, it compromises safety and even overall economy just to increase mpg... To the point a hypermiler will take the longer route because there are less stops involved and it helps on the gauge when in reality it uses more actual ounces of fuel, stuff like that.

But with all that in mind and in a plane, a smooth approach, the most direct route involving the least throttle changes makes a difference. If on landing the pilot doesn't have to adjust the throttle but once or twice and make the least amount of corrections to hit the ground at the least air speed and mostly in one smooth descent line with little in the way of altitude / aileron and flap adjustments that helps and considerably, when the wheels pop out makes a big difference as well thou a few seconds won't matter much... But I think here a pilot with experience can reduce fuel consumption, whether they do it is another story, some most likely do, others maybe, a few certainly not.


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