Back to basics: thrust = drag

[LongRoadBob]

I’ve never quite come to terms with the idea that an airplane flies, at a constant rate when thrust = drag.

weight = lift somehow intuitively seems right, no problem with that, but the thrust = drag intuitively feels like the plane ought to have no forward movement. I know it is wrong, but still...

Even steadily climbing thrust = drag, but lift > weight.

I suspect I’m not the only one. If an airplane is accelerating then at that point thrust > drag.
So, the only model I can make this work with is hard to take, that the original impetus of excess thrust sets the airplane in forward motion (producing lift also because of airspeed and the wings) and similar to a rock thrown on an icy lake, with very little friction just keeps on going from the original force of throwing it. But with the rock on ice, there “drag” is slightly more than “thrust” so even though the rock travels far, eventually it stops. Where in an airplane if the forces are equal that original excess thrust means it continues forward.

Except that seems wrong. Can fly 200 NM just on one push and since nothing is totally perfect as it is in theory, little pockets of differing density, etc. would mean it wasn’t perfectly balanced and one would need a new push now and then.

Anyone have an enlightening way to illustrate what is really happening, or how to understand the idea better?

 

[Salty]

If you were in the vacuum of space and pushed off from your vehicle, there would be no drag, and no thrust, but you would continue to move away from the vehicle at the same speed.

when thrust equals drag, there is just enough thrust to counteract the drag and keep your speed constant. With less thrust than drag you will slow down because there is not enough thrust to overcome the drag.

 

[Dana]

Newton's law: An object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.

Your rock on the ice has an initial force applied to it to get it moving, thereafter the only force on it is friction, slowing it down until it stops. But if it had a good tailwind, it might keep moving if the force from the wind is equal to or greater than the friction against the ice.

Thrust in an airplane isn't the "initial" impetus, but it's applied continuously. The airplane accelerates, and drag increases as it accelerates, until the drag has increased to the point that it's equal to thrust, at which point the net force is zero and acceleration stops.

 

[Blatham489]

Even steadily climbing thrust = drag, but lift > weight.

Not correct. If lift is greater than weight, the plane will accelerate vertically.

The simple formula is F=ma, F is the NET of all forces acting on an object. If there is no net force, there is no acceleration (whether positive or negative) which is a change in velocity. Even at Mach 3 cruise, thrust=drag.

 

[Capt. Geoffrey Thorpe]

You need thrust to counteract the drag to maintain a constant speed. If thrust were less than drag, you would slow down. If it were more, then you would speed up.

 

[Van Johnston]

Park your car on a level road, put it in neutral, get out and push until you have it going at a constant velocity and keep it going at that constant velociy. Then you will feel in your bones thrust = drag. In this example 'drag' is the rolling friction plus any aerodynamic drag. You'll probably be going so slow the aerodynamic drag will be negligible.

 

[LongRoadBob]

Newton's law: An object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.

Your rock on the ice has an initial force applied to it to get it moving, thereafter the only force on it is friction, slowing it down until it stops. But if it had a good tailwind, it might keep moving if the force from the wind is equal to or greater than the friction against the ice.

Thrust in an airplane isn't the "initial" impetus, but it's applied continuously. The airplane accelerates, and drag increases as it accelerates, until the drag has increased to the point that it's equal to thrust, at which point the net force is zero and acceleration stops.

okay. Now we’re getting somewhere that last paragraph hit home for me.

I felt silly writing the complete thought I had which was the rock on ice, if I added a little motorized engine and propeller to the rock, fixed rpm calculated to thrust = friction/drag for a given speed, I would still need to push it to get it going at least up to that speed. But as you say in that last paragraph, like air, the friction would not be perfectly the same on all areas, so if it hit a rough patch would need more rpm to equal friction and drag.

That last paragraph helped a lot, thanks!

 

[Larry in TN]

the thrust = drag intuitively feels like the plane ought to have no forward movement.

Since we're talking about an airplane that's already in-flight, i.e. moving...

What would prevent it from continuing to move if thrust = drag?

thrust=drag, or lift=weight, means that there is no acceleration. It doesn't mean that there is no movement.

 

[LongRoadBob]

Since we're talking about an airplane that's already in-flight, i.e. moving...

What would prevent it from continuing to move if thrust = drag?

thrust=drag, or lift=weight, means that there is no acceleration. It doesn't mean that there is no movement.

I got that, to a point, but as I noted, air density differences, etc, mean drag is not constant and I couldn’t see how one “push” way back at the time we went to cruise could last for a long distance as we fly. Newton’s laws are ideal, in the real world we would expect imperfection and small extra or less drag throughout, so Dana’s answer helped me to confirm that it is an ongoing process, that thrust = drag is for periods of time, but need more impetus now and then to keep it going and equal. That it is a model but this way makes sense that it also is changing the whole time.

 

[RyanB]

I’ve never quite come to terms with the idea that an airplane flies, at a constant rate when thrust = drag.

There’s always two forces acting on us all the time. It’s simply Newton’s third law of motion at work. Whenever two objects (or forces in this case) interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

It is true that air density changes with altitude, thus decreasing drag on the airplane, but remember, just as air density decreases with altitude gain, so does horsepower and thrust produced by the engine.

 

[Stewartb]

Chew on this. Go-fast guys work to minimize drag. STOL guys work to maximize it. Different missions, different priorities.

 

[skier]

Chew on this. Go-fast guys work to minimize drag. STOL guys work to maximize it.

The goal of STOL aircraft is not to maximize drag, but rather to achieve stall at the slowest speed possible.

L=1/2*rho*V^2*S_w*C_l

at stall L=W

where:
L= Lift
W = weight
rho = air density
V = velocity
C_l = lift coefficient
S_w = wing area

We can rearrange that equation to find that
V_stall = sqrt {2*W/(rho*S_w*C_l)}

So to achieve L=W at the slowest speed possible STOL aircraft are designed to maximize C_l and S_w while minimizing weight.

Increasing C_l requires things like flaps, slats, etc, which all increase drag. Increasing S_w increased the wetted area and increases drag.

So drag isn't something that STOL aircraft are designed to maximize, but rather relatively higher drag is a byproduct of the other goals of a STOL design.

 

[Stewartb]

Do you have a very high performance stol airplane? Have you experimented with streamlining versus landing performance? Great big flaps dropping to 70*? Maximizing drag was an exaggeration, but not as much as some guys think. I like dirty gear, big tires, a long flat prop, etc. It makes the plane work.

 

[Dan Thomas]

Do you have a very high performance stol airplane? Have you experimented with streamlining versus landing performance? Great big flaps dropping to 70*? Maximizing drag was an exaggeration, but not as much as some guys think.

Maximizing drag without any lift return on it would hurt takeoff performance.

 

[Stewartb]

Big horsepower fixes that. My new Cub motor will make 235hp. Drag allows very steep landings. Power provides very steep takeoffs. Fun stuff.

 

[Tantalum]

There’s always two forces acting on us all the time

The battle between good and evil?

 

[Tantalum]

your body exerts a downward force on the chair and the chair exerts an upward force on your body

The part that confuses people is that if you push the lever on your chair and go down 6 inches then during that transit time the forces are still equal.. it's not like your body is pushing down HARDER on the chair then it pushes up.. the actual seat you are sitting on still pushes up equally on your bottom .. and the little cylinder contracts with equal pressures on both sides of it..

 

[Dana]

The pressure isn't equal on both sides. When you push the lever it lets gas out or whatever so the force holding you up is momentarily less than the force of gravity pushing you down, so you accelerate downwards... until the pressure on the bottom increases enough to momentarily be more than your weight so you decelerate to a stop. You might even bounce a time or two at the new position as things stabilize... sounds familiar, huh?

 

[RingLaserGyroSandwich]

Big horsepower fixes that. My new Cub motor will make 235hp. Drag allows very steep landings. Power provides very steep takeoffs. Fun stuff.

By itself, the extra drag isn’t helping you regardless of how powerful the engine is. Well, I can think of one exception. The extra drag will change the pitch attitudes you need in various phases of flight which could help with visibility, and possibly landing technique, in some circumstances.

 

[luvflyin]

I’ve never quite come to terms with the idea that an airplane flies, at a constant rate when thrust = drag.

weight = lift somehow intuitively seems right, no problem with that, but the thrust = drag intuitively feels like the plane ought to have no forward movement. I know it is wrong, but still...

Even steadily climbing thrust = drag, but lift > weight.

I suspect I’m not the only one. If an airplane is accelerating then at that point thrust > drag.
So, the only model I can make this work with is hard to take, that the original impetus of excess thrust sets the airplane in forward motion (producing lift also because of airspeed and the wings) and similar to a rock thrown on an icy lake, with very little friction just keeps on going from the original force of throwing it. But with the rock on ice, there “drag” is slightly more than “thrust” so even though the rock travels far, eventually it stops. Where in an airplane if the forces are equal that original excess thrust means it continues forward.

Except that seems wrong. Can fly 200 NM just on one push and since nothing is totally perfect as it is in theory, little pockets of differing density, etc. would mean it wasn’t perfectly balanced and one would need a new push now and then.

Anyone have an enlightening way to illustrate what is really happening, or how to understand the idea better?

Inertia. Thrust gotta equal drag so inertia can do it's thang.

 

[chemgeek]

The pressure isn't equal on both sides. When you push the lever it lets gas out or whatever so the force holding you up is momentarily less than the force of gravity pushing you down, so you accelerate downwards... until the pressure on the bottom increases enough to momentarily be more than your weight so you decelerate to a stop. You might even bounce a time or two at the new position as things stabilize... sounds familiar, huh?

Exactly. There goes that physics stuff again. For anything to be set into motion, there needs to be a net force applied.

 

[LongRoadBob]

Newton's law: An object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.

Your rock on the ice has an initial force applied to it to get it moving, thereafter the only force on it is friction, slowing it down until it stops. But if it had a good tailwind, it might keep moving if the force from the wind is equal to or greater than the friction against the ice.

Thrust in an airplane isn't the "initial" impetus, but it's applied continuously. The airplane accelerates, and drag increases as it accelerates, until the drag has increased to the point that it's equal to thrust, at which point the net force is zero and acceleration stops.

You need thrust to counteract the drag to maintain a constant speed. If thrust were less than drag, you would slow down. If it were more, then you would speed up.

There’s always two forces acting on us all the time. It’s simply Newton’s third law of motion at work. Whenever two objects (or forces in this case) interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

It is true that air density changes with altitude, thus decreasing drag on the airplane, but remember, just as air density decreases with altitude gain, so does horsepower and thrust produced by the engine.

just quoting some, though not all of he helpful replies. These in particular helped me.
The part that bothered me most was the wrong thinking that the initial “push” as being sufficient.
That didn’t seem right.

For a student specially the balancing act of the forces in flight can be difficult to grasp. That they all interact, and there are different ways to achieve the same results. I remember clearly being baffled early on when my instructor had me at the same airspeed ascending, descending, and flying straight and level. It blew my mind.

So thrust = drag seems to make more sense to me now. Thanks all! There were other enlightening posts, just quoted a few.

 

[Rushie]

If there were no drag then the thrust would cause you to continue accelerating. When you’re flying along at a constant velocity the drag is countering the thrust exactly.

You know that in outer space, away from the gravitational pull of a star or planet, you can turn off your engine and you will continue moving at a constant speed indefinitely. That’s because there is no air, so no drag, hence no thrust is needed, but yet you’re still moving.

As your plane encounters air, to continue at the same speed it must start applying thrust at the same rate. (For the thought experiment, ignore gravity and the weight-lift part and the issues of atmospheric re-entry).

 

[Stewartb]

Y’all sure make it difficult. When in steady level cruise trust equals drag. Increase thrust and you climb. Decrease thrust you descend. That’s because lift-gravity have a relationship with thrust-drag. Flight is three dimensional.

 

[Rushie]

Y’all sure make it difficult. When in steady level cruise trust equals drag. Increase thrust and you climb. Decrease thrust you descend. That’s because lift-gravity have a relationship with thrust-drag. Flight is three dimensional.

Well yes, the confusion comes from the force being split into vector components along the x and y axes: thrust-drag the x and lift-weight the y. Increase thrust you climb, or go faster, or even descend depending on where you point the nose. We won’t talk about z right now.