A friend of mine recently stated that flight times were reduced when flying with a tail wind, and increased when flying with a head wind. We spent about twenty minutes going over pseudo-physics, but couldn’t convince each other that either of us was right.
From my limited understanding of Physics, I would assume that a plane has to travel faster with a tail wind. But I might be wrong and defer to the SD to provide a solid, physics-related answer.
You don’t need advanced physics to explain this. Go out on the street during a heavy windstorm. Do you go faster when walking with the wind or against the wind?
With a head wind it would create more lift and make the aircraft lighter…creating the POSSIBILTY for it to go faster. But it would also create a lot more resistance, so it will go faster with a tail wind.
Note that the difference in direction is non-linear with wind speed. Consider that a Cessna flying 100 kt into a 100 kt wind is stationary…flight time will be infinite, same airplane with a 100 kt tailwind is making 200kt groundspeed, so flight time would be halved over the winds-calm case. Thus a tailwind can help to a certain degree, but a very strong headwind can be a really serious problem.
Isn’t this analogous to a boat traveling with the current vs. against the current. With a tailwind, the plane is traveling “with the current” and should fly faster which reduces the travel time. I don’t see where the OP disagrees with his friend.
This is just wrong. An airplane has a top speed based on thrust and drag. The drag is determined by the size and shape of the plane - it doesn’t change. The thrust is based on the type of the engine - it doesn’t change. The plane will go pretty much at the same airspeed regardless of the wind speed or direction.
What can change is the groundspeed. A tail wind will cause a higher ground speed and a head wind a lower ground speed.
No, the plane doesn’t need to go faster if there is a tail wind. I’m assuming you mean that it must go faster to provide the same lift as with no wind.
This can get a little tricky. Suppose an airplane with a takeoff airspeed (speed relative to the air mass) of 60 kt. is sitting on the runway facing a headwind of 20 kt. The airspeed indicator already reads 20 kt. so you only need to gain 40 kt in order to take off. On the other hand, if the same plane is on the runway with a 20 kt tail wind it must gain 80 kt in order to take off. However, in both cases the airspeed at takeoff will be 60 kt. This is why airplanes take off into the wind. You cover a lot less ground to gain 40 kt than to gain 80 kt and so you don’t need as much runway for takeoff. It works the same way for landings. You can use a much shorter runway to land into the wind than if you land downwind.
Once you get into the air, as others have said. the only effect wind has is on navigation. It has no effect on flight characteristics.
That is the most appropriate analogy. A common misconception is that planes are flying in space and merely acting on the air. That is not true. Planes fly IN the air just like boats travel in the water and airspeed is the only thing that matters.
This is where we get into airspeed versus ground speed. A Piper Cub can fly head on into a 60 mph headwind and be dead still or maybe even flying backwards in reference to the ground. The plane doesn’t care and knows nothing about the ground. It just needs the appropriate airflow over its lift and control surfaces. Reverse the wind and that tiny plane would be going 120 mph over the ground but the flight is exactly the same from an aerodynamics point of view.
A 747 may cruise at about 600 mph. However, if you go into the cockpit at 35,000 feet, the air speed indicator won’t read a speed anywhere near that fast. That is because the air is thinner at that altitude and the plane couldn’t give a rat’s ass about how fast the ground is passing below. The number or air molecules passing over it over second are all that counts (and it is the most important thing to the pilot too because the plane operates on airspeed and not ground speed).
Not to mention that the airspeed indicator under-reads significantly at altitude.
You may be at 25,000 with a true air speed of 250 kts, the air speed indicator might read 160 kts, and there may be 50 kts tailwind giving you a ground speed of 300 kts.
And just to complet this, it’s the indicated airspeed that is important for flight. The true airspeed is important for navigation.
If a plane stalls at 45 kt indicated at sea level it also stalles at 45 kt indicated at any altitude. As you go up for a constant true airspeed the indicated airspeed drops. Eventually the indicated speed equals the stalling speed and that’s as high as you can go.
If you want to make any turns you have to fly at some lower altitude and have a reserve of indicated airspeed in level flight so that when you lose speed in the turn you won’t stall…
Thanks David Simmons. I haven’t finished my pilots license yet but reading about all things aviation including theory has been my passionate hobby since I was young. I have never heard the part above put quite that way but it makes perfect sense and made some things click beyond that simple example.
There is an exception to this. The air speed indicator over-reads slightly at high altitudes due to the less dense air being more easily compressed. So the indicated stall speed does increase a little.
The indicated stall speed also increases at high altitudes and true air speeds where developing shock-waves cause the airflow to depart from the upper surface of the wing sooner than normal. Shock-waves can develop well before the speed of sound.
Also, the stall speed increases in a turn so loss of speed is not required although it exacerbates the problem.
ETA: Just to clarify. The air speed indicator under-reads significantly compared to true air speed at altitude. It also over-reads slightly from what it would read in the same pressure at sea level.
N.B.: If you fly into a 30-knot headwind then return the same distance, now with a 30-knot tailwind, your overall flight time will be longer than if there were no wind. (Nobody asked, but this is a classic brain-teaser.)
I interpreted the statement to mean that he thinks a plane must travel faster through the air with a tail wind in order to maintain lift.
Not so in the steady state. The airplane, as was pointed out, is immersed in the air and travels with it, like a balloon. Call this the plane’s “balloon speed.” If the wind suddenly changes direction it takes the plane a little while to respond and attain the new “balloon speed” and during that time the flight characteristics are changed. If this happens close to the ground, as in wind-shear incidents, the results can be disasterous.
Shagnasty. My example was greatly simplified, as was later pointed out by 1920’s Style “Death Ray”. However it was close enough for a general feel for why you can only go so high.
The maximum height can also be limited by the engine at a lower altitude. Engines don’t get as much air at high altitudes as a low and so they eventually run out of enough power to maintain flying speed. However, even if there is plenty of power you still can’t go above the point where your IAS is the stalling speed.
It seems you’ve omitted the part of the explanation that covers why there should be a limit on true airspeed.
The typical explanation offered here is that flutter will occur at a more or less constant true airspeed. At some reduced pressure, flutter speed and stall speed coincide, and you are in a “coffin corner” where the only option is to descend.
(There’s some evidence that this view of flutter is oversimplified, but it’s a common and generally safe approach.)
There’s no inherent reason why you need to lose speed in a turn. By contrast, it is hard to execute a turn without increasing the load factor, which in turn increases stall speed.