radio-controlled aircraft produce 90 N (20 lbf) of thrust. The GE90-115B engine fitted on the Boeing 777-300ER, recognized by the Guinness Book of World Records as the "World's Most Powerful Commercial Jet Engine," has a thrust of 569 kN (127,900 lbf) until it was surpassed by the GE9X, fitted on the upcoming Boeing 777X, at 609 kN (134,300 lbf). Concepts Thrust to power The power needed to generate thrust and the force of the thrust can be related in a non-linear way. In general, . The proportionality constant varies, and can be solved for a uniform flow, where is the incoming air velocity, is the velocity at the actuator disc, and is the final exit velocity: Solving for the velocity at the disc, , we then have: When incoming air is accelerated from a standstill – for example when hovering – then , and we can find: From here we can see the relationship, finding: The inverse of the proportionality constant, the "efficiency" of an otherwise-perfect thruster, is proportional to the area of the cross section of the propelled volume of fluid () and the density of the fluid (). This helps to explain why moving through water is easier and why aircraft have much larger propellers than watercraft. 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: 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: This formula looks very surprising, but it is correct: the propulsive power (or power available ) of