5
$\begingroup$

Does a powered aircraft fly faster when in a head wind or with a trailing wind? The question revolves around the head wind should provide 'better' lift, and trailing winds have very little to 'push' against.

$\endgroup$
  • 4
    $\begingroup$ It might not be obvious from the answers, but "how fast" is not a simple question. Through the air (airspeed), it will be the same with headwind or tailwind. Over the ground, it will be airspeed + tailwind or airspeed - headwind. A bit simplistic but roughly accurate. $\endgroup$ – Simon Mar 28 '15 at 17:48
  • 1
    $\begingroup$ @Simon good point. OP should respond and clarify, though I suspect if he understood the difference between groundspeed and airspeed, he would not actually be asking this question in the first place. My guess is he means groundspeed. $\endgroup$ – The111 Mar 28 '15 at 23:02
15
$\begingroup$

A tailwind provides a faster groundspeed.

The aircraft moves through the air so whether the ground is moving relative to the mass of air has no relevance to the aerodynamics of an aircraft at cruising altitude.

a British Airways passenger jet approached supersonic speed this week as it rode a surging jet stream from New York to London.

from The Telegraph, 10 Jan 2015

They mean groundspeed - so the term supersonic is moot.

The Boeing 777-200 jet reached a ground speed of 745mph as it rode winds of more than 200mph across the Atlantic. At ground level, the speed of sound is 761mph.

Aircraft often use the predominantly westerly† jetstream when flying from the USA to Europe, From Japan to California or across the USA. Flying the other way they pick an altitude to avoid it. enter image description here Wikipedia

The Jetstream is a high speed, high altitude, wind that blows moderately reliably in one direction.

enter image description here Metcheck

† Westerly means "blowing from the west" - towards the east.

$\endgroup$
  • $\begingroup$ I remember seeing that story about the BA flight and wondering why it was considered newsworthy. Both times that I've flown from Asia to the U.S., we've had ground speeds that would have been well over Mach 1 in still air. It makes a difference when you have a 180 mph tail wind. LAX->HKG was 15 hours, while HKG->LAX was only 12 (77W both ways.) $\endgroup$ – reirab Mar 29 '15 at 1:15
  • $\begingroup$ @PieterGeerkens: as we operate in a coordinate system tied to the earth, that doesn't matter at all. It is true that relative to an inertially fixed system the earth rotation is faster than any wind. $\endgroup$ – Ross Millikan Mar 29 '15 at 15:22
12
$\begingroup$

Headwind is good for take-off and landing. It adds extra speed for free at the start of the take-off run, rsp. allows to come in slower for landing, reducing the landing distance.

Tailwind is good for enroute flight. It pushes the aircraft forward and adds extra miles per hour for free. The speed relative to the surrounding air is the same at the same power setting and altitude regardless of wind speed, but when measured relative to the ground the wind speed must be added to the air speed.

But on the way back from a tailwind leg the aircraft has to fly in a headwind, and the added time and fuel expense is higher for the sum of both trips in comparison to trips made when no wind is blowing. Why? Let's assume the aircraft will fly at the same power setting, and will achieve 200 mph in still air. The trip to a destination 1000 miles away will take 1000 / 200 = 5 hours in still wind for a roundtrip time of 10 hours.

Now add to that a 50 mph tailwind, and the trip will take 1000 / 250 = 4 hours. On the return trip we need to subtract the wind: Now it will take 1000 / 150 = 6 hours 40 min to fly back. The total flight time with wind is 10 hours 40 min, and those 40 min of longer flight time consume proportionally more fuel and crew time.

Tailwind is only good when you don't plan to return, or when the wind will have changed when you go back.

$\endgroup$
  • 5
    $\begingroup$ Or when you can take a different route to avoid the winds, as RedGrittyBrick describes with jet streams $\endgroup$ – raptortech97 Mar 28 '15 at 18:38
  • 2
    $\begingroup$ Well, you lose something. On a flight PRG-CDG (westbound) with easterly wind, you get extra speed during the flight for free, but you take off and land the opposite direction, making a half-turn on both ends, which costs some time. If you get a westerly wind, then you don't turn at all during the whole flight, but you get a headwind all the time that slows you down. $\endgroup$ – yo' Mar 28 '15 at 22:56
  • 1
    $\begingroup$ @raptortech97 Or when you merely fly at a different altitude with less wind on the way back. This is a pretty common practice in GA, at least. Wind can vary a lot by altitude. $\endgroup$ – reirab Mar 29 '15 at 1:18
5
$\begingroup$

Constant wind does not affect the lift or drag of an aircraft.

As stated by others, you will travel faster in relation to the ground with tailwind but your airspeed will stay the same no matter what wind you have.

Shortly after you are in the air, the aircraft travels with the wind, rather then with the surface of the earth. The aircraft wouldn't notice any wind. For an airborne aircraft, the air always stands still. Wind is only the movement of air in relation to the ground, and the aircraft doesn't relate to the ground once it is airborne.

If you are having tailwind, the aircraft will travel with the same speed through the air, but with an increased speed over the ground. This is why you take off into the wind: Your airspeed (the speed the aircraft travels through the air, so the sum of windspeed and groundspeed) is higher with a lower ground speed.

Here the maths: Let's say you have 10kt headwind and you are standing at the runway. When standing still your airspeed is already 10kt eventhough your groundspeed is 0kt. As you start moving, the airspeed will always be 10kt more then your groundspeed.

Conclusion: The wind doesn't make an airplane faster in relation to the air, but in relation to the ground the wind can change the speed of an aircraft, but not by producing more lift/drag, but by simply moving the aircraft along in the air.

$\endgroup$
3
$\begingroup$

The question actually has nothing to do with aerodynamics. It's a matter of relative motion, which is traditionally considered physics, but IMO is really nothing more than simple vector arithmetic.

Consider this equivalent question:

Before you board your airplane, you walk through an airport. You approach two parallel "pedestrian conveyor belts," one moving away from you and one moving toward you. Which one should you walk across if you want to get to the other side faster?

Now replace your body with the airplane, and the conveyor belts with "wind" (moving masses of air). Does that make it more clear?

$\endgroup$
  • 1
    $\begingroup$ This is a bad analogy, because while you get easier on the belt that moves in the same direction as you do, the plane gets easier on the belt moving towards it. $\endgroup$ – yo' Mar 28 '15 at 23:02
  • 1
    $\begingroup$ I am not sure what you mean by the words "get easier," could you please use more specific words? Disregarding takeoff and landing (which makes sense since OP specifically mentioned a plane flying), the plane will make better groundspeed, i.e. arrive at its destination sooner, with a tailwind, i.e. on the belt moving in the same direction it is moving. $\endgroup$ – The111 Mar 28 '15 at 23:05
  • 1
    $\begingroup$ Yes, take-off and landing are preferable in a headwind only because of runway distance constraints. However, if you are interested in getting from point A to point B as fast as possible and have very long runways, then tailwind is still the way to go. :-) OP seems most concerned with speed, and with vehicles already in flight. $\endgroup$ – The111 Mar 28 '15 at 23:10
  • 1
    $\begingroup$ Probably because planes are not designed for traveling on the ground at high speeds. And most crashes happen on takeoff and landing, and crashing is preferable at low groundspeeds. :-) So to answer your question in one word: safety. But in terms of pure math, my statement is accurate. $\endgroup$ – The111 Mar 28 '15 at 23:16
  • 1
    $\begingroup$ Yes, there is a maximum safe speed when on the ground (having to do with tires/brakes/etc.) Also, you want to reach V1 quickly so that if you had to abort, you still have plenty of room to stop without setting the brakes on fire. From a pure efficiency standpoint, though, you're right that just taking off in the direction you want to fly is more efficient. $\endgroup$ – reirab Mar 29 '15 at 1:24
2
$\begingroup$

When looking at the physics, the speed of an aircraft is the speed relative to the air. For this, it is not relevant how the air moves relative to the Earth - or any other planet, for that matter.

The aircraft does not even "know" the direction of wind.

So there can not be a difference in the aerodynamics, in a very fundamental way.

If you ask about the speed relative to the ground, you can just add the vectors of both speeds.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.