Turbo jet operating rpm(s)...

[AlexisDetroit]

When a turbo jet engine on, say, a Lockheed C-130 is running while flight does the engine always run at max rpm with the pitch of the prop controlling the speed of the aircraft? Somewhere I was given the impression that turbo jet engines do not change rpm to change air speed.

[SNS3Guppy]

Alex,

The Allison T-56 on the C-130 is fairly unique among turboprop engines, in it's operation. Another common turboprop/turboshaft which operates in a similiar manner is the Garret TPE-331.

A turbofan or turbojet engine ("jet engine") does indeed vary RPM in order to effect a change in thrust. Thrust in a jet is non-linear. This means that at the bottom end of the RPM range, the turbojet engine may be producing a small amount of thrust with large changes in RPM, but near the upper operating limits, very small changes in RPM produce big changes in thrust.

Yes indeed, turbojet engines change speed in order to alter their thrust. The lower end is the idle speed, which usually changes from ground to flight...flight idle speeds are a little higher, and is typically about 50 to 60 percent of the maximum engine RPM. Cruise RPM is typically about 90% of the maximum RPM.

RPM may, or may not be the measurement used to determine what the engine is doing, however. Unlike a turboprop engine where the primary power instrument is the torque gauge, a turbojet engine may use RPM, or may use EPR...a measurement of the pressure difference between what's coming out the back of the engine and what's going in the front. This only measures the engine core thrust, however, and doesn't address what a turbofan is contributing with the fan section...that part is read on the N1 gauge, which is a measurement of one part of the RPM of the engine...not necessarily the RPM of the engine as a whole.

[Tree]

Yes the Alison turboprop on the Herk runs at 100% rpm at all times except when manually shifted to ground-idle mode (kids, do not try this inflight). Adding more fuel via more throttle = coarser prop pitch and more thrust (torque) but always at 100% rpm (prop low oil or pitch lock faults excepted of course!)
Fantastic throttle response gets you out of doo doo immediately with thrust and airflow increase over the top of the wing(limitation is no less than 1 sec. from idle to full throttle and be assured it will do it in 1 sec.)
One sweet engine and the sound is music to the ears. You fly it on constant TIT (turbine inlet temp) so starting out heavy your speed is lower and as you lighten up the speed increases. Interesting flight planning and a very interesting operation. A true pilot's engine and aircraft. Not for Atari Ferrari metrosexual pilots (you need 2 hands on the wheel, 2 feet working the rudder and oh yes you will have to trim elevators/rudder and ailerons and will need a third pilot/FE to keep the engines out of max limits or NTS.)

[Old Fella]

The Allison T56 family of turbo-prop engines are a constant RPM type. The engine drives a propeller via an extension torque shaft through a reduction gearbox. The engine at 100% RPM runs at 13820 RPM reduced to 1020 RPM at the propshaft using a 13.54:1 reduction gearbox. When the throttle is advanced from Flight Idle more fuel is scheduled and to maintain RPM nominally in the 100% range the propeller blade angle increases with the resultant increase in thrust. Although the propeller is fully reversible and constant speed there is no pitch changing lever. The throttle operates through a device known as a co-ordinator which combines outputs to both the engine fuel control unit and the propeller governor. Another lever known as the Condition lever can be used to feather the propeller, unfeather it and shutdown the engine. Earlier C130's did not have a low speed ground idle facility and it was not uncommon to close down the outboard engines to assist in controlling taxi speed after landing. As mentioned by Tree, up until the latest C130 (the J model) the operating crew included a Flight Engineer. The maximum torque allowed on all from the C130A to the C130H was 19600 inch lbs, this being an engine mounting limitation rather than an engine limit. The NTS mentioned by Tree is Negative Torque Signal which comes when ever the propeller tries to drive the engine, for instance on descent. The NTS system will cause the blade angle to increase so as to cause the engine to have to drive the propeller to maintain RPM. I have experienced a situation where the NTS system was mal-adjusted and prevented an air start being performed by driving the prop back toward feather whilst endeavouring to unfeather it.
The Lockheed Hercules is most certainly a true legend in it's own lifetime, even having been flown onto and from an aircraft carrier (the USS Forrestal) 29 times.

[jh5speed]

Turboprops
Some turboprops have independent speed control (via a separate lever : the condition lever is sometimes used as an rpm demand input e.g. PT6?); some have have speed demand as a function of power lever position (speed demand comes via an 'interconnect' characteristic e.g. Dart, Tyne); some are constant speed (T56) as discussed above; some have different speed demanded depending on flight mode (e.g. TP400).

Turbojets/fans
Not really the subject of the question , agreed: rpm is variable - but NL (N1) depending on engine type, can be fairly linear with thrust e.g. mixed turbofan.

EPR
I think cold EPR and combined (integrated) hot+cold EPR have been used on some types. Some issues with icing and system complexity though ...