Perhaps it would make the semantics easier to deal with if the work done were thought of as USEFUL work done. Certainly the work done by the helicopter engine is converted into useful work by it's rotor if it accelerates enough air to allow it to hover and prevent an acceleration toward the earth's core at a rate of 32' per second per second. Likewise for any work done by a machine that can be directed or converted to useful purpose such as in the electrical power generation (energy conversion) plant.
In the case of thrust horsepower (a term of somewhat limited usefulness itself) calculation for a jet airplane, thrust required to sustain a given flight speed is simply multiplied by the flight speed (converted to the appropriate units of measurement) to arrive at a thrust horsepower as per the basic equation: Power = force X distance/time. Let's say two identical airplanes producing equal engine thrust of 20,000 lbs are compared using the same formula. One airplane is configured for landing while the other is clean. Both airplanes will stabilize at the speed where total drag is equal to thrust applied. One airplane might be doing 150 kts while the other does 300 kts. With the same thrust applied in both cases, the thrust horsepower figure is about double for the "clean" higher speed airplane. You can see that thrust horsepower is a rather poor form of of comparison for the purposes the thread originator has in mind. Perhaps it would be far more descriptive to solve for equivalent shaft horsepower in presenting relative comparisons between jet engines and the more familiar auto engines to the non-aviation public. Unfortunately, I cannot presently locate my powerplant theory books to find the applicable formula at the moment. IIRC, the idea is to find the additional shaft HP needed to duplicate the value of the jet pipe thrust with additional shaft ouput. This amount is then added to the known shaft horsepower output of the engine as measured on the shaft and referred to as ESHP. Torque can be measured directly on some TP and TS engines but require factory test data for TF engines because torque is not read directly since it would be of little or no operational value. As Smokey points out, for jets, it's all about the thrust!
On the subject of a conversion factor for HP of an engine to lbs of thrust produced by a propeller or rotor driven by that engine, that too seems to be somewhat more complex a problem than it might appear at first glance. The relationship of power input to propeller or rotor thrust output is dependant on many factors that extend beyond the depth of my studies, but a few things seem apparent when considering the following:
By making some rough estimations, typical GA trainer engine/prop combos appear to produce no more than about 3 lbs static thrust/HP. At Reno last year, the Texan II military turboprop trainer had a placard claiming 3,500 lbs ST on about 1,000 HP. Carter Copter claim 1,140 lbs. ST on 240 HP for it's propeller. The UH-60A would hover at a TOW of 20,000 lbs at about 90% torque from it's two 1,543 SHP engines. With some of that torque being used to drive the tail rotor, It seems safe to say that It's rotor had to produce at least 8 lbs ST/SHP. Larger and slower turning props and rotors appear to convert a greater proportion of input power into thrust output. Perhaps there are reduced losses from slippage, spanwise flow and compressability effects when the delta V across the disc is reduced and the area of the disc increased. I would very much like to read some comments from someone well-schooled in the black art of propeller or rotor design!
Best regards,
Westhawk