Aircraft Performance - Part 5 - Example Calculations

Completing either a performance calculation or mission analysis requires three essential ingredients:

• Basic statistics on the airplane's size and configuration
• Engine thrust and fuel performance throughout the flight envelope (including max, military and part power conditions)
• The airplane's drag polar, including the effects of stores drag, Mach number, and g-loads

A central element of this simulation will be the predicted engine performance.  For many of the jet engines now flying, performance information is readily published at static, sea level conditions.  Such statistics as maximum and military (or dry) thrust, specific fuel consumption, overall pressure ratio, fan pressure ratio, bypass ratio, and air flow are all published for many of the engines developed during the 1980s and in the decades before.  These statistics provide a basis for extrapolating engine performance across to other design conditions.  Again, the objective here is not to precisely simulate everything that goes on inside of a modern jet engine, or to design its components.  We are not seeking that level of detail.  Moreover, any functional limitations - such as maximum pressure limits - will be unknown.  But such an extrapolation will allow an engineer to draw some conclusions regarding how the engine, and aircraft, will perform across a broad variety of flight conditions.

There are two software packages currently available to the general public that would allow us to perform precisely this sort of extrapolation: AEDsys, produced by the American Institute of Aeronautics and Astronautics (AIAA) as a teaching tool; and GasTurb, produced by a retired engineer from MTU Germany.  The GasTurb software offers a more comprehensive tool set, but is also more difficult to use.  For the limited objectives of the example calculations portrayed here, AEDsys will be used.

The F100-PW-220 engine can be used as an excellent first benchmark.  This is the engine that today powers many of the older F-16 models, and most of the world's F-15 aircraft.  Many of the basic operating parameters for this engine are readily available from the open literature for sea level static (SLS) conditions.  This includes the overall pressure ratio, fan pressure ratio, bypass ratio, air flow, specific fuel consumption, even the burner exit temperature.  The only major parameters for which estimates would need to be made would be the component efficiencies, the afterburner temperature, and turbine cooling air and leakages.  Employing these basic parameters, its possible to generate an engine simulation for the off-design performance of this engine.

To assess the quality of this model, a comparison can be drawn against publicly available data for the F100-220 available from NASA publications.  Comparing the predicted and published thrust lapse (thrust at off-design conditions divided by the sea level static thrust), we can see that the analytical simulation provides a fair estimate for the performance of the engine across a wide range of Mach numbers and altitude.

Building on this successful experience, we can assemble a similar simulation for the F110-GE-100 engine, which equips the Block 40 F-16C.  Like the F100, statistics are publicly available for overall pressure ratio, fan pressure ratio, bypass ratio, air flow, and specific fuel consumption.  Unlike the F100, there is no publicly referenced value for the burner exit temperature - but we know that the F110 was a contemporary of the F100, employing similar technologies, and that the burner temperatures must be of similar magnitude.  There is therefore sufficient detail to develop an analytic model for this engine.

Similarly, basic statistics and configuration data - including airframe and fuel weights - are readily available for the the Block 40 F-16C.

Ordinarily, an outside analyst would need to make projections from the published airframe data to estimate the zero-lift drag coefficient and Oswald's efficiency.  Not a difficult task, but one which would add uncertainty.  However, in this particular example a significant amount of the data necessary for projecting the drag polar of the F-16 has also been published under various NASA reports - so airframe-calibrated data is already available.

Combining this collection of airframe statistics, projected engine performance, and drag polar data, we can  begin to perform an example mission profile assessment - in this instance for a hi-lo-hi mission.  Comparing between the published and predicted combat radius of the aircraft, we can see that the two values are in close agreement - with a predicted value of 770 nm compared to 760 nm from the published literature.  This comparison further affirms that with sufficient aircraft and engine data, and some fundamental design relations, we can draw some fairly accurate projections for the mission capabilities of a particular aircraft.

This assessment however, only touches on the mission profile capabilities of the airplane.  To complete the complementary performance assessments for this aircraft, we would need to be able to assess the effects of flight conditions on the drag polar of the aircraft.  This includes Mach number effects (for which published data does exist), and g-load effects.  The latter data is more difficult to come by, since it is not an area of particular concern for commercial aircraft.  There is, however, at least one published resource - again coming from NASA - that documents the effects of g-loading on the drag polars of the F-15, F-16 and F/A-18, allowing us to calibrate prediction methods for these effects.

In summation, there is a broad body of published data for both aircraft and engine operation, available for many of the aicraft flying today.  Even many Soviet aircraft, once shrouded in secrecy, have come into public view with the fall of the Berlin Wall - drive by the Russian manufacturers as they seek to secure export sales.  Significantly, this same data is not readily available for some of the most recent aircraft flying today.  Many of the basic statistics for both the airframe and engine of the F-22 and F-35 remain unpublished, to prevent a potential adversary from performing precisely the type of analytic studies that are illustrated here.