Editing Thrust (section)

===Thrust to propulsive power===
A very common question is how to compare the thrust rating of a jet engine with the power rating of a piston engine. Such comparison is difficult, as these quantities are not equivalent. A piston engine does not move the aircraft by itself (the propeller does that), so piston engines are usually rated by how much power they deliver to the propeller. Except for changes in temperature and air pressure, this quantity depends basically on the throttle setting.

A jet engine has no propeller, so the propulsive power of a jet engine is determined from its thrust as follows. Power is the force (F) it takes to move something over some distance (d) divided by the time (t) it takes to move that distance:<ref>{{cite web
| url = http://www.aerospaceweb.org/question/propulsion/q0195.shtml
| title = Convert Thrust to Horsepower
| first = Joe
| last = Yoon
| access-date = 1 May 2009
| archive-url = https://web.archive.org/web/20100613154846/http://www.aerospaceweb.org/question/propulsion/q0195.shtml
| archive-date = 13 June 2010
| url-status = live
}}</ref>

:<math>\mathbf{P}=\mathbf{F}\frac{d}{t}</math>

In case of a rocket or a jet aircraft, the force is exactly the thrust (T) produced by the engine. If the rocket or aircraft is moving at about a constant speed, then distance divided by time is just speed, so power is thrust times speed:<ref>{{cite book |title=Introduction to Aircraft Flight Mechanics |first1=Thomas |last1=Yechout |first2=Steven|last2=Morris |isbn=1-56347-577-4}}</ref>

:<math>\mathbf{P}=\mathbf{T}{v}</math>

This formula looks very surprising, but it is correct: the ''propulsive power'' (or ''power available'' <ref>{{cite book |title=Understanding Flight |first1=David |last1=Anderson |first2=Scott |last2=Eberhardt |publisher=McGraw-Hill |isbn=0-07-138666-1 |date=2001}}</ref>) of a jet engine increases with its speed. If the speed is zero, then the propulsive power is zero. If a jet aircraft is at full throttle but attached to a static test stand, then the jet engine produces no propulsive power, however thrust is still produced. The combination [[piston engine]]–propeller also has a propulsive power with exactly the same formula, and it will also be zero at zero speed – but that is for the engine–propeller set. The engine alone will continue to produce its rated power at a constant rate, whether the aircraft is moving or not.

Now, imagine the strong chain is broken, and the jet and the piston aircraft start to move. At low speeds:

<blockquote>
The piston engine will have constant 100% power, and the propeller's thrust will vary with speed<br />
The jet engine will have constant 100% thrust, and the engine's power will vary with speed
</blockquote>