Introduction Propellers Internal Combustion Engines  Gas Turbine Engines (TPs, TSs) Chemical Rockets Non-Chemical Space Propulsion Systems AER 710 Aerospace Propulsion

Lockheed C-130 Hercules

Sikorsky CH-54A Tarhe (“Skycrane”) powered by hp P & W T73-P-1 turboshaft engines

Introduction to Turboprop & Turboshaft Engines Both turboprop [TP] and turboshaft [TS] engines use a gas turbine core engine to drive an output power shaft for a propeller or helicopter rotor The main difference between the two variants is that a TP engine might also produce a fraction of its overall thrust via a hot core exhaust jet, while a conventional TS engine will have a lower exhaust velocity but correspondingly somewhat higher shaft power as the tradeoff TPs commonly employ a free power turbine to drive the speed reduction gear that in turn drives the propeller at a lower rotation speed. This is typically also true for TS engines, in driving the main rotor at a significantly lower rotation speed

Schematic cutaway diagram of a free-turbine two-spool (LP/HP core)/three-shaft turboprop engine. Note the reverse flow arrangement relative to the propeller at left, as a means of avoiding foreign object damage to the compressor, combustor or turbine, and a means for more convenient on-wing hot-section (combustor, turbine) maintenance PT-6

Schematic cutaway diagram of a fixed single-shaft turboprop engine. The example engine drive shaft may run at around rpm, while the propeller shaft rotates at around 2000 rpm

Overall thrust given by a TP engine driving a propeller : One can define an equivalent power P eq that nominally comprises both the shaft and jet power contributions: In the static case (V  = 0), no singularity present in solution:

To avoid the singularity issue at finite airspeeds, assume:, hp assuming 2.5 lb of thrust per horsepower is a reasonable estimate. Note that the core jet power can be as much as 20% of the equivalent power in the static sea-level case, but will drop to less than 5% at cruise conditions, for those TPs that utilize a significant portion of the exhaust jet energy for thrust.

Cycle Analysis of Free-Turbine Turboprop As noted for previous gas turbine variants, some of the baseline equations for turbojet cycle analysis can be utilized for the turboprop (and turboshaft) case Here, let’s look at the one-spool/two-shaft TP engine for introducing the cycle analysis that can be applied for this category of engine. The approach for a turboshaft engine will be similar, with differences noted as we proceed. the power output of the LP free turbine (to the speed reduction gearbox) can be estimated as follows:

The upstream HP turbine must drive the main compressor, such that: so that

Downstream: Recall power demanded by prop: and resulting propeller thrust: Once T 05 known, can find p 05 :

For the case of a turboshaft engine desiring the ideal maximum shaft power output, where for this ideal case, For specific power:

In practice, one will likely need Ma e to be at least 0.3 in order for adequate positive throughput of flow (avoiding transient backflow). In the general (non-ideal) case for either a TP or TS, specific power becomes: For power-based fuel economics, one uses brake specific fuel consumption: Let’s continue the cycle analysis:

In the typical case that a TP (or TS) would not be using an afterburner, one can transfer flow property values immediately from station 5 to station 6 for the nozzle entry. The TP’s core exhaust jet is usually unchoked, such that for a simple convergent nozzle with station 7 as the exit plane,

TP or TS thermal efficiency: TP or TS overall efficiency:

Shaft power chart for example turboprop engine (maximum cruise power throttle setting). Engine performance comparable to Pratt & Whitney PW120, compression ratio  c of 12

Shaft static power chart for example turboprop engine (full takeoff power throttle setting) for takeoff at different outside air temperatures and airfield altitudes. Engine performance comparable to Pratt &Whitney PW120 at takeoff throttle setting

Photos of wing-mounted Kuznetsov NK-12 turboprop engines installed on Tupolev Tu-95MS Bear. Note the use of contra-rotating (4 + 4 blades) propellers, and rearward-directed exhaust, to maximize thrust from each engine.

Rolls-Royce T56 turboprop engine being operated on an outdoor test rig. The reduction gear unit that ultimately rotates the propeller is positioned well ahead of the main body of the turboprop engine

Pratt & Whitney Canada PT6T twinned turboshaft engine in position on the upper fuselage of a Bell CH-146 Griffon helicopter