View Full Version : Power vs thrust

Gin Slinger
19th Jun 2002, 10:52
Can anyone provide an lyman's analogy between power required (to accelerate or hold a TAS value) and thrust.

I think I have it clear in my mind but I'd just like to hear others ideas on how to express this relationship in plain language.

Many thanks, GS.

19th Jun 2002, 18:51
Thrust is a force ie it shoves things along. Force equals Mass x Acceleration. The more force you apply the greater you can accelerate a mass. Alternatively, for the same rate of acceleration you could move a larger Mass. Energy is used when a force is applied to make something happen eg accelerate

If you achieve something by applying a force eg move an object then you have done 'Work'. Work equals the Force x Distance.

Power is the term used to describe how quickly Work is done ie Work divided by Time. Think of it as the rate the energy is used to do something.

Thrust will have been needed to accelerate to the speed but without drag the thrust will have to be removed to maintain the desired speed. Alternatively, if there is a counteracting force (Drag) the acceleration will reduce to zero ie maintain a constant speed, once the counteracting force is equal in magnitude to the 'desirable' force (Thrust).

Accelerating something is a way of transfering Energy into it. The faster the object goes, the more energy it now contains. The amount of Energy tranferred increases by the square of the speed ie double the object's speed and you quadruple the energy stored within the object. That's why even reducing a crash speed by a few knots makes a significant difference to the damage done. The energy has to go somewhere during the sudden stop. It gets used up in deforming the metal, deforming the passengers & other contents, noise, heat etc etc

The faster you can transfer this energy into the object - the more power you have - then the sooner the object will reach its final speed. Increased Power lets you do this by allowing you to increase the size of the Force used to propel the object.

With less power available you could only apply a smaller Force. You would eventually get to the target speed but it would take a longer time & greater distance.

Since Drag is an increasing factor eventually the two forces would balance and IAS (actually EAS but for simplicity's sake...) would be constant.

To go faster you would have to provide more power to supply the force needed to accelerate once more. Remember though that the final 10kt increase in IAS requires a LOT more power than the first 10kts (the energy proportional to the square of the speed again).

The thrust needed maintain speed is proportional to Drag. Drag, in turn, is related to IAS.

Power required to do provide this Thrust is proportional to TAS. At S.L. ISA, IAS is the same as TAS but as altitude is increased TAS increases for a constant IAS.

The result is that more power is needed to maintain a given IAS at altitude than when maintaining the same IAS at S.L.

Hope my ramblings haven't confused you further!


Genghis the Engineer
19th Jun 2002, 20:25
Thrust = force.

Drag = force.

Thrust = drag, keep speed same. More thrust, get faster, less thrust get slower.

Power = thrust x speed.

Drag gets more as you go faster, less as you slow down.

So, you increase power, which increases thrust, and get faster. But as you get faster, thrust gets less for the same power, and drag increases. Eventually they balance out and you're at a constant speed again.

Similarly, reduce power, and it slows down until drag is the same as thrust again.

This of-course assumes that you keep trimming for level flight all the time, but might help.


19th Jun 2002, 21:59

I appreciate the fact that you are trying to keep it simple, but some of your comments are misleading.

When accelerating from VS to VMP, both drag and power required decrease.

When accelerating from VMP to VMD, power required increases but drag decreases.

When accelerating from VMD, both drag and power required increase.

If Gin is working towards his ATPL POF or PERF exams, the above facts could make the difference between getting 74% or being happy.

Genghis the Engineer
19th Jun 2002, 23:20
Fair cop, everything I've said is only true if you stay on the right hand side of the drag curve.


Gin Slinger
21st Jun 2002, 01:34
Keith - don't worry, he didn't confuse me - your caveats are entirely logical to me.

21st Jun 2002, 16:03
Genghis :

>But as you get faster, thrust gets less for the same power

Why does the thrust get less ?


Gin Slinger
21st Jun 2002, 17:27
Because of more profile drag??? [power = thrust x drag]

21st Jun 2002, 18:10
Power and thrust are dimensionally different. One pound force of thrust @ 550 feet per second is equal to 1 horsepower. Power equals force times velocity.

21st Jun 2002, 18:41

The way I understand it, the amount of thrust has nothing to do with the amount of drag, (or vice versa).

If that's true, then *speed* is the resultant of thrust - drag.

So whether the plane is sitting still or at cruise speed, isn't a full-throttle engine producing the same amount of thrust ?

21st Jun 2002, 23:19

I think Genghis's comment refers to the arithmetic, rather than aerodynamics of any particular aircraft or propulsion system.

Propulsive power = Thrust x TAS.

If for example you are producing 50000 ft lb /min of propulsive power when flying at 100 ft/min, you have a thrust of 500 lb, because 100 ft/min x 500 lb = 50000 ft lb / min.

If this thrust accelerates you to 1000 ft/min with no change in propulsive power, you will then have a thrust of only 50 lb, because 50 lb x 1000 ft / min = the same 50000 ft lb / min.

In reality the thrust produced by a propeller aircraft decreases rapidly as TAS increases. So as TAS increases, propulsive power increases to a peak at a fairly low TAS, then decreases as TAS continues to increase.

In a jet (for the purposes of the JAR POF exam) thrust is approximately constant as TAS increases. So propulsive power increases approximately linearly, as TAS increases.

For the AGK exam however you need to remember that jet thrust really decreases as you accelerate from zero, levels off at about 250 Kts as ram effect kicks in, then returns to its original value by about 500 Kts. These figures are of course all generalisations used for examination purposes.

23rd Jun 2002, 00:31
Thrust doesn't require any energy. You could hang a 747 of the end of a suitable metal bar. That bar would be providing thrust equivalent to the weight of the 747. What's more impressive is it can do it all day without overheating.

When that thrust starts to move the aircraft then everything changes. The metal bar can't do this apart from a tiny movement caused by the shrinkage as heat moves out of the bar.

You can begin to see that the thrust of an engine is only loosely related to its power.