# What produces Thrust along the line of flight in a glider?

After reviewing discussions and vector diagrams of gliders in flight, the vertical lift component and the vertical drag components seem to produce a steady state, 0 acceleration balance with gravity. The glider is on a straight line flight towards the ground. Would the forward motion of the glider be accounted for by combining the horizontal lift vector and gravity? Would any other forces be involved?

• Gliders are always converting altitude to energy unless there is an updraft. – Ron Beyer Oct 23 '18 at 20:10
• Not quite. True is that a glider in flight always converts potential energy into heat. On top of that, a glider may convert potential energy into kinetic energy, or vice versa. – bogl Oct 24 '18 at 11:01
• And gliders still convert altitude into energy in an updraft. The air mass is rising faster than the glider rate of descent, yielding net "up". – Robert DiGiovanni Oct 24 '18 at 11:06
• In an updraft, the surrounding air is acting a force on the plane, thus doing mechanical work. Kinetic energy of the air is converted into potential energy of the plane. (plus kinetic energy of the plane during vertical acceleration) – bogl Oct 24 '18 at 13:35
• Got it!, that is how force U is created! Glider makes its own "up draft" by falling. Thermal makes it better. Gravity vector is "put to work" creating forward motion and lift from the wing. The glider has a rate of descent. If the air mass is rising faster, glider goes up. But you are the first one to say "the surrounding air acts as a force on the plane". Thanks, just saying "gravity" was unclear. – Robert DiGiovanni Oct 24 '18 at 15:50 V is the airspeed. U and w are the horizontal and vertical components of V. The rest of the vectors are forces; W = weight, K = aerodynamic force, L & D are the orthogonal components of K. Those are all the forces involved in a glide.

• I was hoping someone would say V is a combination of velocity down and velocity forward (created by horizontal component of lift vector). Interesting that, even though acceleration forces are in balance. Can we have a "relative velocity" direction based on falling and moving forward at a constant speed? – Robert DiGiovanni Oct 23 '18 at 20:37
• Better yet, what generates "U"? – Robert DiGiovanni Oct 23 '18 at 20:49
• The projection of the weight vector is the 'thrust' that opposes the projection of the aerodynamic force (drag). As those forces are in equilibrium, the motion is uniform, with a constant speed and zero acceleration. – xxavier Oct 23 '18 at 21:13
• Airspeed is not a force. V would be the trust component. The question is what is providing the thrust component that is opposing drag. – TomMcW Oct 23 '18 at 22:26
• Force U is 90 degrees to gravity there for can not be created by it. The horizontal component of lift is not drawn (why?). Gravity cannot be moving the plane forward any more than gasoline can provide thrust. Is not the wing the "engine" of lift and thrust, as in birds? – Robert DiGiovanni Oct 23 '18 at 23:12

"What produces Thrust along the line of flight in a glider?" Nothing. There is no Thrust involved in gliding flight. Lift, Drag, and Weight form a closed vector triangle, meaning net force is zero, meaning no acceleration or deceleration is taking place. The Weight vector can be resolved into components perpendicular and parallel to the Drag vector, but noting that Weight has a component that is parallel to Drag is not the same as saying that a Thrust force exists.

See these vector diagrams:

The balance of forces in gliding flight can be represented by a closed vector triangle of Lift, Drag, and Weight, with right angle between the Lift and Drag vectors. If we wish, we can also show the airspeed vector, and we can break it down into its horizontal and vertical components. The vertical component of the airspeed vector is simply the sink rate. All three of the above vector diagrams represent the exact same physical situation. We've represented the aerodynamic forces in black, and the gravitational force-- the weight vector-- in green. The left-hand and right-hand diagrams are easier to understand than the middle diagram, because the force vectors are arranged into a closed polygon (a triangle in this case), which shows us instantly that the net force is zero. On the right-hand diagram, we've also included the airspeed vector, and its horizontal and vertical components. Note that K is the glide angle, and note that on the right-hand diagram, the triangle formed by the force vectors and the triangle formed by the airspeed vectors are similar triangles. The right-hand diagram makes it immediately obvious why the ratio of Lift to Drag, is also equal to the glide ratio through still air-- i.e.the ratio of horizontal speed to vertical speed (sink rate). Note that for high L/D ratios, Lift is very nearly as large as Weight, and horizontal speed is very nearly as large as airspeed, so for most practical purposes we can use airspeed / sink rate, glide ratio, and L/D interchangeably.

Note that if we wish, we can break the Weight vector down into two components, one of which is exactly equal in size and opposite in direction to the Lift vector, and one of which is exactly equal in size and opposite in direction to the Drag vector. The latter can be viewed as being somewhat like Thrust, but it's not strictly correct to label it as such.

The lift vector is proportional to the lift coefficient times the airspeed squared. The drag vector is proportional to the drag coefficient times the airspeed squared. The ratio between the magnitude of the Lift and Drag vectors is identical to the ratio between the lift and drag coefficients. For a given fixed aircraft shape, a change in the lift coefficient and/ or drag coefficient represents a change in angle-of-attack. Every possible angle-of-attack of the wing is associated with a specific lift coefficient, a specific drag coefficient, and a specific Lift : Drag ratio. This is true regardless of whether the aircraft is heavy or light-- a change in weight is expressed as a change in the airspeed associated with any given angle-of-attack, but no change in the lift coefficient, drag coefficient, or Lift: Drag ratio associated with any given angle-of-attack.

It is crucial to understand that the above vector diagrams will be valid for flight at some given particular angle-of-attack of the wing REGARDLESS of whether the airmass is rising, sinking, or neither. A glider doesn't feel any difference in the direction and magnitude of the forces acting on it when rising in a powerful mountain wave, than when gliding in still air.

Note also that we're only dealing with the wings-level case. In circling flight, the forces still look the same in a side view, but airspeed and lift and drag are all higher than in wings-level flight at the same angle-of-attack, and the glide ratio is decreased, and from a head-on view we'd see that the part of the lift vector has been diverted to the side.

Back to the simple case of wings-level flight--

What is "powering" the glider?

Work is proportional to force times distance, and power is proportional to force times velocity. In the airmass reference frame, the glider's direction of travel is identical to the direction the airspeed vector is pointing. The only force doing work on the aircraft in this direction is the component of the weight vector that points in the same direction as the airspeed vector. We can break the Weight vector down into one component that is equal and opposite to the Lift vector, and one component that is equal and opposite to the Drag vector. The latter component is what is "powering" the glider, at least as viewed from the standpoint of the airmass reference frame. The latter component of the Weight vector will be equal to Weight * cosine (glide angle). So the Power exerted by this component of the Weight vector is equal to (Weight * cosine (glide angle)) * airspeed). Now, it just so happens that the sink rate is equal to (airspeed * cosine (glide angle)). So we can say that the power exerted by gravity = (Weight * cosine (glide angle)) * airspeed) = (Weight * (cosine (glide angle)) * airspeed) = Weight * sink rate.

As viewed from the ground reference frame, the glider's direction of travel is different when the glider is in rising airmass than when the glider is in still air, so the above formulae for work and energy will only be valid in still air. From the ground reference frame, the glider's direction of travel is sometimes horizontal or climbing, in which cases the Lift vector can be viewed as having a component that is doing work on the glider. But as noted earlier, the direction and magnitude of the Lift, Drag, and Weight vectors remains the same regardless of what reference frame we are using for our velocity vectors.

See related answers to related questions:

"'Gravitational' power vs. engine power" -- https://aviation.stackexchange.com/a/56040/34686

"Descending on a given glide slope (e.g. ILS) at a given airspeed— is the size of the lift vector different in headwind versus tailwind?" -- https://aviation.stackexchange.com/a/56259/34686

"Does lift equal weight in a climb?" -- https://aviation.stackexchange.com/a/56476/34686

• Please take note that in order to maintain velocity, there must be thrust to overcome drag. Gravity straight down can not produce sideways motion. It is only fuel. The wing is the engine. Gliding is powered flight. I would like to learn the L D W triangle, but keep in mind an object that is moving can have 0 acceleration, but still must burn "fuel" to maintain constant velocity. Gravity is the fuel. U and little w are the velocity components created by the engine (the wing) at constant descent. Got what I came for, Thanks All. – Robert DiGiovanni Oct 24 '18 at 6:40
• The issue was that weight can only go straight down and cannot be resolved into vectors. Aerodynamic force pushes the plane sideways. I learned this by taping a penny to an envelope off center and dropping it. It will glide. – Robert DiGiovanni Oct 24 '18 at 6:45
• @RobertDiGiovanni weight can only go straight down and cannot be resolved into vectors. But you asked what propels it along it's flight path. Gravity can be divided into vectors wrt an angled flight path. BTW, if you got what you came for you should accept one of the answers. – TomMcW Oct 24 '18 at 17:26
• "weight can only go straight down and cannot be resolved into vectors. "- sure it can. The force vector called Weight (= mass * grav. constant) can be resolved into an infinite number of pairs of two other vectors, each with a 90-degree angle between them. For the purposes of this diagram someone decided to resolve Weight into two vectors acting parallel and perp. to the flight path. Personally I think it makes a better diagram if you just draw a closed vector triangle involving Weight, Lift, and Drag and be done with it; forget about resolving the forces into pairs of orthogonal components. – quiet flyer Oct 24 '18 at 22:24
• Tom first, I do not accept any answer I do not fully understand. I got what I came for but remain interested. Now Quiet. I understand dividing weight vector W.R.T. plane provides a nice "oh look, that is what is pulling the plane down!", so be patient, this is for fun. I know how gliders work (el and CG and wing). I will now render and answer with just Lift and Weight. But do not worry, drag will be there too! – Robert DiGiovanni Oct 25 '18 at 1:47

In a steady state with no acceleration nothing needs to produce a forward force. Inertia indicates that all forces are equal. This is true whether climbing, descending or maintaining constant altitude.

That given, there will always be a drag force trying to slow the glider along its flight path. To remain at a steady airspeed it is only this drag force that must be overcome. Since a glider produces no thrust, drag must be compensated by angling the flight path downward causing a portion of gravity to counter the drag force. If you divide gravity into x and y coordinates referenced to the flight path it is the portion that is parallel to the flight path that is countering the drag.

This is all presuming no updraft.

• Even in an updraft the forces are exactly the same, and the velocity vector drawn RELATIVE TO THE AIRMASS REFERENCE FRAME is exactly the same. If you happen to be standing in a rising elevator as you look at the diagram, you'll know what I mean. – quiet flyer Oct 24 '18 at 22:19
• @quietflyer Yeah, but an updraft complicates things. The flight path is no longer the same angle wrt gravity and it provides an additional aerodynamic force. I was just keeping it simple – TomMcW Oct 24 '18 at 22:30
• Actually the flight path with respect to the airmass is at the same angle to the horizon in updraft, downdraft, and still air, and the aerodynamic forces are identical in all three situations. – quiet flyer Oct 24 '18 at 23:54

It might be easier for you to understand this from an energy balance standpoint, as follows: The vertical descent rate in feet per second of a glider times its weight yields the energy release rate (foot-pounds per second) being dissipated by drag forces as it descends. Dividing the energy release rate by 550 yields the horsepower required to maintain the glider in its glide. This energy is being dissipated by a drag force applied to the airframe; that force times the airspeed yields the drag horsepower. In a steady-state glide, the descent horsepower equals the drag horsepower.

• Ok, for (academic) fun, we have gotten from fuel (gravity) to energy release (engine), what is the propeller ? What gets the glider moving forward? Very basic, but important, as this is the "incipient glide" Once the aerodynamic (from falling) forces push the nose down and airflow starts over the wing (propeller). Gravity does not do this. Airflow does. Diagram could be better. – Robert DiGiovanni Oct 24 '18 at 10:58
• But I do like the concept of steady state drag horsepower, as it could be used to determine thrust requirement for powered flight. I had imagined Weight x Inverse of glide ratio. Applied it to can a 747 fly on one engine and it was close! – Robert DiGiovanni Oct 24 '18 at 11:01
• Robert don't you remember that this was recently explored in detail here? aviation.stackexchange.com/questions/56023/… and aviation.stackexchange.com/a/56040/34686 . But weight * sink rate -- the approximation discussed in the answer-- is not the same as weight * inverse of glide ratio. – quiet flyer Oct 24 '18 at 22:29
• I was pointing out that weight x inverse glide ratio gave a fairly close answer for minimum power maintain altitude, and indeed, involves sink rate. But thanks, I will review these articles. – Robert DiGiovanni Oct 25 '18 at 1:42

What creates force U? We know the glider is descending vertically at a constant velocity. We know the wing is pitched down at an angle. How does this translate into a horizontal velocity component? As the "wind" from the vertical descent strikes bottom of the wing, some of the energy will move it sideways!

Notice a perfectly circular parachute will drop straight down. With a sinking glider, the horizontal stabilizer will push the nose down, creating an angled surface to the wind. This is how gravity (the fuel) uses the wing (the engine) to create forward thrust (Force U).

In the steady state, the flight path would be the result of forward and vertical velocities (U and w).