Difference between thrust and power...

Tristan

Tristan

Well-Known Member
Hi,

I seem to have trouble getting my head around these two different concepts, or clearly distinguishing between the two. I know that thrust is force, and power is the rate at which you apply that force over time. But I still get confused between the two.

One example my textbook gives is a tractor and a ferarri climbing a hill. THe ferrari has more power because it covers more distance over time, that I get, but how would the tractor produce more thrust than the ferrari if it's going slower?

Anyone got good examples to explain the difference between the two concepts?

Cheers
 
I'm just taking a stab here but not all power is transferred to the ground like the Ferrari.
 
:yeahthat: :yeahthat: :yeahthat: :yeahthat: :yeahthat: :yeahthat:
I was taught that power is the energy the engine produces, while thrust is what eventually propels the vehicle at the tail end of things[no pun intended], because all that energy from the engine does not equate to the amount of thrust.
 
Tristan said:
Anyone got good examples to explain the difference between the two concepts?
Click to expand...

You've probably asked one of the most difficult concepts to explain *intuitively* in all of aviation. One of the problems, I think, is that we already have layman definitions of thrust and power which makes them synonymous, but according to the precise, scientific defintions, they're very different.

Mathematically,

Power = Thrust * velocity

so whenever you know the thrust of an airplane, you can figure out how much power it's developing by multiplying that number by how fast the airplane is going. Thrust is the more fundamental concept, because without thrust, you have no power.

One way I think about it is that power incorporates not only the concept of thrust, but also the velocity that the thrust produces. This is useful in certain contexts, because some aircraft performance parameters depend on that velocity.

For instance, your angle of climb depends on how much excess thrust you have. The velocity of the aircraft along that flight path is irrelevant. If my climb angle is 45 degrees, it remains 45 degrees no matter how fast I'm flying.

However, rate of climb depends on power. If my climb angle is 45 degrees, the faster I fly along that flight path, the greater my rate of climb is.
 
thrust is what moves the plane, it comes from power. Power is what the engine eventually creates by the turn of the blades or the propellers, some of it is waste and the rest is thrust that pushes the plane forward.
 
mrivc211 said:
Power levers are in props

Thrust levers are in jets


simplicity just works best sometimes
Click to expand...

Actually, Boeing, MD and Airbus seem to refer to them as "throttles" in their literature....

Anyway, read what TGrayson wrote, it's about as simple as it can be and actually understand it.
 
Let's picture a rectangular block sitting on a flat, frictionless surface.

1) Assume that the block is rigidly attached to the surface in some manner so that it cannot move. Perhaps it is nailed to it. If you shove against that block will all your might, you will impose a very large force on the block. Since the block cannot move, however, it remains at rest and its kinetic energy remains zero.

2) Now assume that the nails are removed and the block is free to slide. If you apply the same force as before, the block will move in the direction you shove it. As your force moves the block through a distance the block gains speed and therefore kinetic energy. The faster the block moves the faster the force can deliver more kinetic energy. Power is the rate at which the kinetic energy increases.

In a light aircraft the reciprocating engine generates power in the crankshaft, which is carried to the propeller. From here part of the power goes into developing thrust at the aircraft's speed and part of it is transferred to the airflow as it passed through the propeller.
 
This is a difficult subject to explain since it is natural to assume thrust comes from the engines and the engine output is rated in horse power so it must be the same thing. There is truth to that but the difference between thrust and horsepower is subtle but important for understanding airplane performance.

Thrust is a force so you could say an airplane has 600 pounds of thrust or whatever it is. But the thrust available will decrease as the speed increases. Since you can only get thrust by pushing on the air and you can't push on the air as hard if it is already flowing in the direction you are trying to push it so thrust decreases as airspeed increases. Drag is sort of like negative thrust. So when you plot thrust versus airspeed and drag versus airspeed on a chart they converge at a point which is the maximum speed of the airplane.

Power is work done over time, and work is a force applied over a distance. So you move an object and push on it with 20 pounds of force over the distance of 1 foot and you have done 20 foot pounds of work (not to be confused with pound feet of torque which is completely different). A horse power is 33,000 pound feet of work per minute. So an airplane with 180 horsepower can do 5,940,000 pound feet of work per minute. So it could lift 5,940,000 pounds to a height of 1 foot off the ground in one minute or 1 pound 5,940,000 feet off the ground in one minute. So you could figure your airplanes climb rate ignoring propeller efficiency and drag based on horsepower and weight. But you can take drag into account if you know the airplanes speed and the pounds of drag it is experiencing and propeller efficiency will be some coefficient.
 
Killtron2000 said:
Thrust is a force so you could say an airplane has 600 pounds of thrust or whatever it is. But the thrust available will decrease as the speed increases. Since you can only get thrust by pushing on the air and you can't push on the air as hard if it is already flowing in the direction you are trying to push it so thrust decreases as airspeed increases. Drag is sort of like negative thrust. So when you plot thrust versus airspeed and drag versus airspeed on a chart they converge at a point which is the maximum speed of the airplane. .
Click to expand...

Me thinks you might want to think about this a little more! For starters, thrust has nothing to do with "pushing on air", otherwise a rocket wouldn't work in a vacuum....
 
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