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An airplane has an engine that pushes its flight. What force pushes a glider to fly? Is it gravity? I think the flight of a glider is driven by gravity, but some people disagree with me. They say gravity acts vertical, it has no component in the horizontal direction. Am I wrong?

Please note that this question is not specifically asking about what (thrust-like) force acts opposite to the drag vector. The direction of a glider's flight path as viewed from the earth may not necessarily be opposite to the direction of the drag vector-- for example, the glider may be travelling horizontally over the ground at constant altitude, or may be rising straight up in ridge lift or wave lift-- and this question may be construed as asking what force is pushing the glider along the direction of the flight path as viewed from the surface of the earth.

quiet flyer
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enbin
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    Comments are not for extended discussion; this conversation has been moved to chat. – Federico Mar 22 '20 at 17:20
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    ASE users with voting privileges-- please reconsider your vote. This question should not have been closed. Since a glider can move along a trajectory (as seen from the ground) that is not opposite to the drag vector, this question legitimately invited consideration of forces other than forces that would act along the direction that a thrust vector would act, and thus is not a duplicate of https://aviation.stackexchange.com/questions/56352/what-produces-thrust-along-the-line-of-flight-in-a-glider/ . Some interesting content was posted that would have been off-topic to the other question. – quiet flyer Mar 22 '20 at 18:45
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    (Ctd) This includes Peter Kampfs entire answer https://aviation.stackexchange.com/questions/75470/an-airplane-has-an-engine-that-pushes-its-flight-what-force-pushes-a-glider-to/75478#75478 , as well as much of the content in this answer https://aviation.stackexchange.com/questions/75470/an-airplane-has-an-engine-that-pushes-its-flight-what-force-pushes-a-glider-to/75480#75480 . Please respect the hard work of ASE contributors and vote to re-open this question that is not actually a duplicate. – quiet flyer Mar 22 '20 at 18:45
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    How many times is this question going to be asked? – Romeo_4808N Mar 25 '20 at 06:59
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    I still do not see how this would not be a duplicate – Federico Mar 29 '20 at 12:09
  • @CarloFelicione Because there has been controversy. Many people think that gravity is not the force of glider movement. – enbin Mar 29 '20 at 22:12
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    What I'd suggest is going in an searching the archives here for this. Quite often these kinds of questions have been asked in the past. – Romeo_4808N Mar 30 '20 at 01:09
  • The appropriateness of the closure of this question as a duplicate is now a topic on the ASE meta site -- https://aviation.meta.stackexchange.com/q/4067/34686 – quiet flyer Apr 05 '20 at 17:08
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    @CarloFelicione _ I just searched through the archives under "glide" and "glider" and I see no question that duplicates the question as it stood at the time of your above comment, i.e. after the March 22 edits. I find no question that invites consideration of the problem from the point of view of forces acting along the direction of the flight path as viewed from the ground. See https://aviation.meta.stackexchange.com/questions/4067/is-the-question-an-airplane-has-an-engine-that-pushes-its-flight-what-force-pu/4071#4071 for more consideration along these lines. – quiet flyer Apr 06 '20 at 14:31
  • Since this question is obviously not going to be re-opened anyway, I'm tempted to roll back the March 22 addition (which is the entire second paragraph) as it is not obvious whether or not it follows the authors original (or current) intent for the question. However, noting that he was still engaged on the topic a week later (see his March 29 comment), I guess he would have rolled it back himself had he so desired, so I'll leave it alone. – quiet flyer Apr 07 '20 at 14:39
  • @enbinzheng I think you're missing that all aerodynamic forces are defined relative to the arimass. If the air is lifting relative to the ground (e.g. in thermals), the glider will still be descending in the airmass but can be ascending relative to the ground – Manu H Apr 21 '20 at 16:43
  • This question was admittedly rather vague in its original formulation. It would have been better to leave a comment asking the questioner to clarify whether he was speaking about the flight path as viewed from the airmass reference frame or the earth reference frame or both, before leaving any answers. Also to clarify whether or not the question was intended to be confined to steady-situations (constant airspeed and constant trajectory.) – quiet flyer May 01 '20 at 15:51

7 Answers7

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Airplanes do not fly because of their engines. Airplanes and gliders both fly because their wings turn thrust into lift and drag.

That simplistic answer, of course, just begs the question "how can you generate thrust without an engine?"

A glider is constantly trading altitude (potential energy) for airspeed (kinetic energy). Energy over time equals force, which in this case we call thrust. Airplanes do the same thing when descending. Both can also trade energy in the opposite direction, but only briefly until the wings stall.

The big difference is that an airplane can also turn fuel (chemical energy) into thrust, which allows sustained level or climbing flight. Gliders don't have that option.

StephenS
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  • Glider turns gravity into thrust, similar to aircraft engines. – enbin Mar 21 '20 at 06:28
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    The glider turns lift into thrust by having its lift vector point forward. That in turn is done by flying on a sloped glide path. It is actually that lift force which propels the glider. – Peter Kämpf Mar 21 '20 at 08:02
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    @PeterKāmpf How can lift tilt forward with a positive AOA? – StephenS Mar 21 '20 at 13:24
  • well, neglegting "small scale" vortices (smaller than the plane), then the airflow vector is the negative of the aicraft velocity vector, they are in the same direction.. – Apfelsaft Mar 21 '20 at 17:09
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    @StephenS: Wings do not convert forces, wings deflect airflow. In case of stationary flight (unchanged velocity vector), the forces are in an equilibrium (sum of all force vectors equals zero). Gliders have no thrust, so that's zero. When gliding, the sum of the weight vector (m*g_vector) and the aerodynamic forces (lift_vector+drag) cancel each other in stationary descent. And, btw - energy is the integral of a force along a path, no time involved here. – Apfelsaft Mar 21 '20 at 17:13
  • @CarlBerger What if the vector sum of these forces is not zero? Was the glider pushed in this case? Who pushed it? – enbin Mar 23 '20 at 00:36
  • @enbinzheng If the sum of all force vectors isn't vanishing, then there is a net force acting on the airplane mass. That means acceleration in this direction, and the airplane is not any more in a uniform motion. Any flight maneuver would lead to that. – Apfelsaft Mar 26 '20 at 20:06
  • @StephenS Peter Kämpf is wrong. Lift is perpendicular to the flight path and it is impossible to provide the force to advance the glider forward. – enbin Mar 26 '20 at 22:26
  • @CarlBerger Peter Kämpf is wrong. Lift is perpendicular to the flight path and it is impossible to provide the force to advance the glider forward. – enbin Mar 26 '20 at 22:26
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The force that pushes a glider is a component of its weight. More precisely, it's the projection of the vector weight W on the glide path. It's exactly of the same value of drag D, if the glider is flying at a constant airspeed, with no accelerations. In the picture, all vectors are forces except for U, V and w, that are horizontal-, total-, and vertical airspeed, respectively...

enter image description here

$W$ has two components, a component $W_n$ perpendicular to $V$, and a component $W_t$ parallel to $V$. $W_t$ is the force that pushes the glider, corresponding to the thrust that drives a powered plane.

xxavier
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Let's do a Gedankenexperiment.

Fly a glider on a horizontal flight path. It will slow down and stall.

Now fly the same glider in a vertical dive. It will accelerate downwards, pulled down by gravity.

Now think of gliding flight as a superposition of both states. Mostly horizontal flight with a bit of vertical dive on top. Shouldn't it be obvious that gravity keeps the glider moving?

But gravity only provides the vertical component of acceleration. The glider in a dive is only accelerated downward, not forward. Only when the pilot pulls on the stick and adds some lift on the wing will the glider experience a forward acceleration.

There you have it. Since the glider is moving along an inclined flight path, its lift vector is tilted forward which provides a bit of thrust. Of course, the lift vector is only needed because the glider flies in Earth's gravitational field, so more gravity or more mass would require an increase in lift, which again means more forward thrust. But gravity is only indirectly involved by setting the lift requirement. What really pulls the glider forward is its forward lift component.

But the glider is not only accelerated forward, but a bit downwards, too. Drag, after all, is also tilted and provides a bit of upward force. That bit needs to be balanced which is done by gravity. In a tilted reference system, it is indeed gravity which is tilted and pulls the glider forward along the tilted longitudinal axis. But if we remain in the standard earth-fixed system, only lift and drag are tilted, and lift pulls the glider forward. Gravity only contributes a downward fraction, the size of which depends on the glide ratio.

Now consider the glider in an upward moving packet of air in which it flies along a flight path which is pointing upwards. But still the pitch attitude of the glider is a bit nose down in order to tilt its lift vector forward. In this condition we now have an upward sloping flight path and still it is only lift which pulls the glider forward.

Peter Kämpf
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  • Suppose the earth has no atmosphere, and you throw the glider horizontally. How does the glider move? Does it move vertically downwards, or move forward and downward along a parabola? If you say it moves along a parabola. So why does it move along a parabola when there is no lift? Not only that, but it also has acceleration. why? – enbin Mar 21 '20 at 11:38
  • Was thinking the same thing, how did they land on the moon? If you throw a glider or a golf ball (shape does not matter) in a vacuum horizontally, it moves down on a parabola because it accelerates vertically from gravity, while maintaining constant horizontal speed (no drag). This is why the ungainly LEM "flew" down to the moon from orbit with rocket power and thrusters to safely land. The modern day Falcon 9 uses drag and rockets to return to earth. Note, in orbiting, the horizontal velocity is enough for the object to "fall", but never reach the ground. – Robert DiGiovanni Mar 21 '20 at 13:00
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    @enbinzheng - If the glider is in a vacuum, then it is a glider in name but not action. A thrown rock does not glide. – Dean F. Mar 21 '20 at 14:38
  • @DeanF. It moves along a parabola because of gravity and initial velocity. – enbin Mar 21 '20 at 15:38
  • @quietflyer I like your explanation with the glider in rising air. Even in rising air it will keep the nose down in order to tilt its lift vector while its flight path is straight or pointing up. No forward component of the weight vector involved, and still the glider flies forward. – Peter Kämpf Mar 21 '20 at 21:48
  • @enbinzheng - “ It moves along a parabola because of gravity and initial velocity.” Let’s define velocity. Velocity can only be defined in vectors (direction and magnitude). If the object being moved/thrown has an initial velocity whose direction is exactly opposite of gravity, it will not move parabolically. It will go straight up, then straight down unless it is acted upon by another force. If the direction of the initial velocity is exactly the same as gravity, it will not move parabolically. It will move in a straight line unless acted upon by another force. – Dean F. Mar 21 '20 at 22:08
  • @enbinzheng - Long distance shooters use this same principle. The vertical acceleration of gravity is a constant. This is regardless of the initial force created by the expanding gasses in the chamber, the mass of the projectile, the muzzle velocity of the projectile, or the shape of the projectile. The arc created by the projectile is not perfectly parabolic due to the fact that although the force of gravity is creating a constant acceleration, neither the force of drag nor the projectile’s velocity is constant. They are calculable. But, they are not constant. – Dean F. Mar 21 '20 at 22:16
  • @enbinzheng - “Suppose the earth has no atmosphere, and you throw the glider horizontally. How does the glider move? Does it move vertically downwards, or move forward and downward along a parabola?” So to answer your question.. If you throw a glider in a vacuum, it will move in a parabola. If you drop a glider in a vacuum without any horizontal movement, it will drop like the proverbial stone. The same gravity is acting on the glider in both examples. Its vertical acceleration and vertical speed components will be the same in both examples. The introduction of atmosphere changes the scenario – Dean F. Mar 21 '20 at 22:54
  • @DeanF. On the path of the parabola, the speed of the object is also increasing. So there is a gravity component in this direction. – enbin Mar 21 '20 at 23:24
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    @enbinzheng - In a vacuum, the vertical component of speed would increase due to the accelerating force of gravity. The horizontal component of speed would not increase at all. In a normal atmosphere this would be different due to the forces inherent with movement through the atmosphere. This is basic physics which is highly documented, demonstrable, and reproducible. This is one of the first labs done in any freshman university physics class. – Dean F. Mar 21 '20 at 23:28
  • @DeanF. So when you throw an object horizontally in a vacuum, does it increase speed in the direction of the tangent of the parabola? Is this the most basic physics problem? – enbin Mar 21 '20 at 23:39
  • @enbinzheng - When you throw an object horizontally in a vacuum, the object accelerates vertically towards the center of mass of the attracting object. The acceleration is constant. Therefore, the speed increases at a constant rate. In a vacuum, the horizontal speed is constant. It neither accelerates nor decelerates. In a vacuum, the object would travel indefinitely until the vertical component pulled the object to the surface of the attracting object. – Dean F. Mar 22 '20 at 00:31
  • @DeanF. Is there any acceleration in the tangential direction of the parabola? – enbin Mar 22 '20 at 00:44
  • @enbinzheng There's only acceleration directly in the tangential direction if you throw it vertically. When thrown horizontally, the tangent of the path is going a different direction than the acceleration. Acceleration points down, but the movement has some horizontal component always because it had initial velocity that doesn't change horizontally. – JMac Mar 29 '20 at 05:24
  • @JMac The actual path an object takes is a parabola, so the object has acceleration in the tangential direction. So the acceleration of the object along the tangential direction is increased. – enbin Mar 29 '20 at 05:55
  • @JMac A parabola is a curved path, and objects moving along the curved path must have acceleration. Otherwise, the path of the object's movement will not be a parabola. – enbin Mar 29 '20 at 11:34
  • @enbinzheng The object hasn't accelerated in the tangential direction. The tangential direction has a horizontal component unless it's thrown vertically. It's accelerated vertically only. The horizontal velocity doesn't change. The acceleration also isn't increased, it's constant in the vertical direction. If the acceleration was acting along a tangent, it wouldn't even be a parabola. The parabola is because the acceleration is only vertical while the path has a horizontal component. – JMac Mar 29 '20 at 14:00
  • @JMac Well, you're right. So what do you want to explain based on this? – enbin Mar 29 '20 at 14:12
  • @enbinzheng That there isn't acceleration in the tangential direction. – JMac Mar 29 '20 at 14:18
  • @JMac I ask you a question: Is there a speed change of the object along the tangent direction? – enbin Mar 30 '20 at 04:53
  • @enbinzheng There's a change in speed of the object in the vertical direction. There's no change in the horizontal direction. I think it would be weird to call that "a change in speed along the tangent direction". – JMac Mar 30 '20 at 12:17
  • @JMac Don't think it strange, is there speed in the tangent direction? – enbin Mar 30 '20 at 12:58
  • @enbinzheng I wouldn't say there is a change in speed along the tangent direction, because to me that implies that the acceleration is in the same direction as the tangent; which it isn't. There is a change in speed in the vertical direction; which has an effect on the overall speed of the object. – JMac Mar 30 '20 at 13:07
  • @JMac https://physics.stackexchange.com/a/540250/176092 – enbin Mar 30 '20 at 13:50
  • @enbinzheng I've seen that. I still think it's important to be careful with your wording. "A change in speed along the tangent direction" heavily implies to me that the acceleration is in the same direction as the tangent. A clearer way to word it would be "there is a component of acceleration in the tangential direction"; but to me that is quite different than "there is acceleration in the tangential direction". – JMac Mar 30 '20 at 13:54
  • @JMac There is acceleration in both the tangential and normal directions. – enbin Mar 30 '20 at 14:01
  • @enbinzheng Again, I think it's much clearer to say there is a component of acceleration in those directions. The net acceleration is purely vertical. – JMac Mar 30 '20 at 14:03
  • @JMac Tangential and normal accelerations can replace vertical accelerations. – enbin Mar 30 '20 at 14:14
  • Let us continue this discussion in chat. – JMac Mar 30 '20 at 14:16
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Enbin Zheng is correct, the gravity force is vertical, always vertical, and cannot account for horizontal motion at all. What does?

Let's take the bicycle first. On flat ground it has a gravitational force of 1 G, but does not move vertically. There for, it has an equal upwards 1 G force of terra firma (the ground) holding it at "ground level". Taxiing aircraft also experience this phenomena, as well as tractor trailers, which is why the lowly semi is around 4x more fuel efficient than the best freighter aircraft (trains outdo us by another 4x). No need for lift here.

Now put the bicycle on an inclined plane, it will "glide" down the hill with no peddling power. It is the repulsive force of the ground, not gravity, that accounts for the forward motion! Just draw the vectors.

On to gliders. No terra firma once airborne (and no peddling). The repulsive force to gravity is vertical drag. Every unpowered aircraft has the downward force of gravity pulling it down through the atmosphere. Every glide starts with falling. The drag vector, like the gravity repulsive vector of the bicycle, opposes gravity.

But downward velocity is required for vertical drag. We pay for our drag with the h (height) in the potential energy formula mass x gravity x height.

So now tilt the glider forward by offsetting its center of gravity and center of vertical drag. (Accomplished in aircraft design with horizontal stabilizer area). We now have horizontal motion. A glide slope. It is only then the more efficient wing can take over lifting duties and "glide" the plane. Notice the Lift vector from the wing now has a "forward" horizontal component.

Notice also that a plane has much more profile area when viewed from directly underneath than from head on. This means much more drag is produced from falling a given distance than moving forward. A little like squeezing a wet mellon seed between two fingers. Behold its flight!

A good wing and a well designed glider use this "forward push" with amazing efficiency, as do the greatest of the "motor gliders", modern airliners.

PS:I have learned to account for forward motion with weight vector when using plane reference. Pitched down, the weight vector can be decomposed to a "thrust" line component. This matches the logic of converting potential energy to kinetic energy by diving (works for the bicycle too).

Now for Wn. It has been shown that Wn is the force against the surface of a sliding block on an inclined plane and indeed has significance in determining drag (friction). As the plane increases, Wn decreases and Wt increases. As Wn decreases, friction decreases. Air drag is negligible. For an airplane at flying speed, air drag is not negligible, and the aircraft is held on its "plane" (glide slope) by Lvertical and Dvertical opposing W. Lhorizontal opposes Dhorizontal in a steady state glide.

Robert DiGiovanni
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In order for a glider to fly, it must generate lift to oppose its weight. To generate lift, a glider must move through the air. The motion of a glider through the air also generates drag. In a powered aircraft, the thrust from the engine opposes drag, but a glider has no engine to generate thrust. With the drag unopposed, a glider quickly slows down until it can no longer generate enough lift to oppose the weight, and it then falls to earth.

For paper airplanes and balsa gliders, the aircraft is given an initial velocity by throwing the aircraft. Some larger balsa gliders employ a catapult made from rubber bands and a tow line to provide velocity and some initial altitude. Hang-glider pilots often run and jump off the side of a hill or cliff to get going. Some hang-gliders and most sailplanes are towed aloft by a powered aircraft and then cut loose to begin the glide.

The powered aircraft that pulls the glider aloft gives the glider a certain amount of potential energy. The glider can trade the potential energy difference from a higher altitude to a lower altitude to produce kinetic energy, which means velocity. Gliders are always descending relative to the air in which they are flying.

Gliders are designed to be very efficient, to descend very slowly. If the pilot can locate a pocket of air that is rising faster than the glider is descending, the glider can actually gain altitude, increasing its potential energy. Pockets of rising air are called updrafts. Updrafts are found when a wind blowing at a hill or mountain has to rise to climb over it. Updrafts can also be found over dark land masses that absorb heat from the sun. The heat from the ground warms the surrounding air, which causes the air to rise. Rising pockets of hot air are called thermals. Large gliding birds, such as owls and hawks, are often seen circling inside a thermal to gain altitude without flapping their wings. Gliders do exactly the same thing.

NASA: Gliders

Super
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  • As long as there is gravity, there will be gravitational potential energy. So the thrust of the glider flight comes from gravity. Right? – enbin Mar 21 '20 at 07:08
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    @enbinzheng yes. See xxavier's answer, it has a good picture of this. – Jpe61 Mar 21 '20 at 07:16
  • @Jpe61 His answer seems to be wrong. What is U? Where does it come from? – enbin Mar 21 '20 at 07:54
  • @enbinzheng - U is the horizontal component of Lift. See Robert DGV’s bicycle example. Gravity is involved. But gravity can only provide force downward. Forward motion must be provided by a horizontal force. In a non-powered, parabolic flight, the forward motion has already been provided by other means such as the initial launch by hand, rocket, or engines. Without that initial thrust, the flight would be straight up then straight down unless acted upon by another force besides gravity. With an atmosphere, that force is the air itself. Google ballistic experiments and air friction. – Dean F. Mar 21 '20 at 14:46
  • @enbinzheng - Another example of this is flight inside uprising air such as an updraft/thermal. A pilot can adjust their both their horizontal and their vertical speed by changing the vectors of the total lift force. If you change pitch or bank, you will redirect lift. Thereby changing the intensity of the force in the horizontal. Gravity itself has not changed. At a given altitude, it is more or less a constant. The change in velocity in this case is independent of gravity. As a matter of fact, lift can be produced when there is no gravity. All you need is moving air. – Dean F. Mar 21 '20 at 15:03
  • @DeanF. The lift is perpendicular to the glider's path, so it is impossible for the lift to pull the glider along the glider's path. So you are wrong. – enbin Mar 21 '20 at 16:12
  • @enbinzheng - Lift is perpendicular to the airfoil. The force of lift is divided into vertical and horizontal components in most cases. That is why lift supports less of the aircraft’s weight when the aircraft banks or pitches. Let’s not forget the vectors of lift during a loop or barrel roll. As I mentioned earlier, a good start to your research and argument may be found in the references I listed in an earlier post. – Dean F. Mar 21 '20 at 23:36
  • @DeanF. https://www.grc.nasa.gov/www/k-12/airplane/drag1.html The third paragraph reads: "Drag acts in a direction that is opposite to the motion of the aircraft. Lift acts perpendicular to the motion. "https://www.grc.nasa.gov/www/k-12/airplane/lift1.html Note the first paragraph: "Lift acts through the center of pressure of the object and is directed perpendicular to the flow direction. " – enbin Mar 22 '20 at 00:09
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"What force pushes a glider to fly?"

It depends on what reference frame we are using.

Newton's first law of motion is Force = Mass times Acceleration. In linear straight-line flight, acceleration is zero, so net force is zero. No net force exists in linear (straight-line) gliding flight.

We should then consider the exact meaning of the words "What force pushes a glider to fly?"

These words are basically asking what are all the forces that exist that exert a component parallel to the flight path, acting in the generally forward rather than the generally backward direction. (Note that the question did not use the word "thrust", which has a different, specific, well-defined meaning in aviation. "Thrust" is zero in gliding flight.)

The answer to the question depends on precisely what we mean by "flight path".

If we are looking at the flight path through the airmass, there is no such aerodynamic force, but gravity does exert a force component that acts against the direction of the drag vector, i.e. parallel to the flight path and acting in the generally forward direction, even though gravity is purely vertical. So there's one answer.

If we are looking at the flight path relative to the ground, the answer gets more complicated, and depends on whether the glider is descending, climbing, or maintaining constant altitude. Consider the case where a glider is maintaining exactly constant altitude in slope lift. Now what force is providing a component acting parallel to the trajectory, in the generally forward direction? Not gravity. And while the net aerodynamic force acts straight up vertically, the component of the net aerodynamic force that we call the lift vector does contain a component that acts parallel to the trajectory in the generally forward direction, and thus opposes the component of the drag vector that acts parallel to the trajectory in the generally backwards direction. But take care not to confuse this statement with a claim that lift is actually helping to oppose drag-- that's not the case. Lift and drag are orthogonal (i.e. are perpendicular to each other).

The argument immediately above may strike some readers as an arcane way of playing "games" with components of vectors. But in truth the same could be said of the claim that gravity is helping to "push" a glider through the air in some way. The key point is that in straight-line gliding flight, lift, drag, and weight form a closed vector triangle, with zero net force. Which components of this triangle may be considered to be contributing a component that "pushes" forward along the trajectory, depends upon which reference frame we are viewing the trajectory from.

The analysis gets even weirder if we are looking at the trajectory relative to the ground, and the glider is climbing. There are even valid reference frames where the glider is moving backwards. Now what is the direction of the force component that we would consider to be purely a "pushing" force?

For example, consider a glider that is slowly ascending straight up relative to the ground in powerful mountain wave lift. This happens often. Since the lift and drag vector are always defined relative to the airmass rather than relative to the ground, for a given steady-state airspeed they retain the same orientation in space for a given angle-of-attack of the wing, just as the glider retains the same pitch attitude in space, no matter which way the airmass is moving relative to the ground. As the glider rises slowly straight up, it's obvious that the lift vector and the drag vector now both contain components that act in the direction of the trajectory relative to the ground, while the weight vector does not. Similarly, it's not hard to imagine a case where the glider is drifting backwards and climbing along such a trajectory that only the drag vector contains any component that acts along the directory of the trajectory relative to the ground.

The idea that a component of the lift vector is helping to pull the glider forward along the aircraft's trajectory as viewed from the ground is only true when the aircraft's achieved glide ratio is better than the L/D ratio, or when the aircraft is climbing (unless the aircraft is drifting backwards along a climb path that is flatter than the direction of the drag vector). If there is no tailwind, this means that the air must be rising.

It's usually most useful to focus on the glider's flight path through the airmass rather than the glider's trajectory relative to the ground, or relative to other reference frames, but all of these points of view are technically valid.

When we are using the glider's flight path through the airmass as our reference frame, the answer to the question ""What force pushes a glider to fly?" is "the component of the weight or gravity vector which acts parallel to the flight path." In this reference frame, neither lift nor drag are exerting any "pushing" force, i.e. any force acting parallel to the flight path and pointing generally forward rather than backwards.

Note that a related question may be asked-- "what powers a glider in flight"? Work is force times distance, and power is work per time. Again, the answer will depend on whether we are looking at the work done along the direction of the trajectory relative to the airmass, or the direction of the trajectory relative the ground. In the former case the answer is simply "the component of the weight vector that acts parallel to the flight path", while in the latter case the answer depends on the direction that the airmass is moving relative to the ground.

Related ASE questions and answers (including some useful diagrams)

What produces Thrust along the line of flight in a glider? (Question)

What produces Thrust along the line of flight in a glider? (Answer)

'Gravitational' power vs. engine power (Question)

'Gravitational' power vs. engine power (Answer)

quiet flyer
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Momentum and inertia.

The others (gravity, lift, drag) are not sources of energy, but they do modify the forces of energy. The actual forces involved are momentum and inertia.

KWallace
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