Basic Performance Models

Basic Acceleration | Basic Climb / Descent | Basic Cruise | Basic Landing | Basic Takeoff

There are six basic performance models in Aircraft Mission Modeler - Basic Acceleration, Basic Climb, Basic Descent, Basic Cruise, Basic Landing, and Basic Takeoff; all of them are provided with Aircraft Mission Modeler and are integral to every aircraft model as the "Built-In" performance models in the Aircraft Properties window. Each of these models and their parameters are defined below.

Basic Acceleration

The Basic Acceleration model is comprised of three tabs - Basic, Aerodynamics, and Propulsion. The Basic tab defines the basic turning, climb and descent transition, and attitude characteristics of the aircraft, while the Aerodynamics and Propulsion tabs allow you to select and define strategies to model attitude and propulsion characteristics, respectively.

Basic

The Basic tab is comprised of three sections - Level Turns, Climb and Descent Transitions, and Attitude Transitions - as described below.

Level Turns

The values specified for these parameters are the level turn values for the aircraft. Aircraft Mission Modeler will adhere to these values when possible, but in procedures where the turn is non-level the values may be adjusted in order to maintain the correct relationship between these interrelated parameters. Select one parameter to manually define; the other parameters will be calculated relative to the parameter that you have specified.

Field	Description
Turn G	The standard G force of the aircraft in a turn.
Bank Angle	The standard bank angle of the aircraft in a turn.
Turn Acceleration	The standard acceleration of the aircraft in a turn.
Turn Radius	A fixed turn radius that is independent of the aircraft's speed.
Turn Rate	The standard turn rate.
Scale by atmospheric density	Select to consider dynamic pressure when calculating turn radius; the aircraft will perform with lower G force at high altitudes because of reduced lift.

Climb and Descent Transitions

The values specified for these parameters define the G force of transitions between climbing or descending and level flight.

Field	Description
Pull Up G	Defines the force normal to the velocity vector used to transition into a climb or to transition out of a dive into the next flight segment. The minimum value is 1.05 G. Low values increase the likelihood of terrain impact when a procedure is defined with a high rate of descent close to the ground.
Push Over G	Defines the force normal to the velocity vector used to transition into a descent or to transition from a climb to the next flight segment. The maximum value is 0.95 G. High values increase the likelihood of exceeding the ceiling when a procedure is defined with a high climb rate at an altitude close to the ceiling.
Scale by atmospheric density	Select to consider dynamic pressure when calculating G force; the aircraft will perform with lower G force at high altitudes because of reduced lift.

Attitude Transitions

Aircraft attitude is determined using a 123 Euler angle sequence of Bank, Angle of Attack, and Sideslip, originating from a velocity aligned, nadir constrained set of axes. Attitude rates may be violated in the case of very short - or zero distance - procedures.

Field	Description
Roll Rate	Defines the standard roll rate - the rate at which the aircraft bank angle changes - of the aircraft in a turn. When the Aircraft Mission Modeler violates the specified Turn Roll Rate, the probable cause is an unrealistic climb or descent model, or the use of climb, descent, level turn and speed change parameters that aren't well matched to the roll rate parameters of the aircraft.
AOA/Pitch Rate	Defines the pitch rate when transitioning between attitude modes, between procedures, and between uncoordinated maneuvers when necessary.
Sideslip/Yaw Rate	Defines the yaw rate when transitioning between attitude modes, either triggered by changes in the acceleration performance model or between takeoff/landing, normal flight, weight-on-wheels, or hover mode.

Aerodynamics and Propulsion Analysis

The Aerodynamics and Propulsion tabs of the Acceleration performance model function together as an analysis system that performs a trim calculation as the aircraft flies to compute lift, drag, thrust, throttle, and fuel consumption parameters at any given flight condition for the specified trajectory. The input to this system is the mission flight path while the outputs are the aerodynamics and propulsion data. The fuel flow is integrated into the weight of the aircraft, but these values do not directly influence the flight path.

The system will generate warnings when AOA limits are exceeded or when thrust deficits exist indicating the aircraft design is not capable of flying the path specified, but the path will still be flown as designed; this feature allows users to explore an aircraft design (perhaps based on high accuracy wind tunnel and engine test data) and its suitability to perform a desired mission.

Aerodynamics

The Aerodynamics tab is used to define the methods used to compute lift, drag, angle of attack, sideslip and intermediate/derived values. There are three aerodynamics strategies to choose from - Simple (Disabled), Basic Fixed Wing, and External File. Each of these strategies is defined below.

Simple (Disabled)

The Simple aerodynamics strategy disables the dynamic calculation of angle of attack and sideslip and instead defines the aircraft's attitude using one of two basic methods. Select Helicopter or Fixed Wing from the Aircraft operating mode drop-down menu.

If you select Fixed Wing, the aircraft x-axis will remain parallel to the velocity vector at all times, resulting in a constant angle of attack of zero; the sideslip angle is always zero.

If you select Helicopter, the angle of attack will be opposite the flight path angle, resulting in a negative angle of attack when climbing and a positive angle of attack when descending.

If the Simple aerodynamics strategy is selected it will force the propulsion strategy to be set to Simple as well.

Basic Fixed Wing

The Basic Fixed Wing aerodynamics strategy calculates angle of attack dynamically; the sideslip is always zero. To utilize this strategy you will need to define Forward Flight and Takeoff and Landing Mode settings, which - combined with the weight of the aircraft defined in the Aircraft Configuration window - will be used to calculate the angle of attack over the course of the mission. Ignoring effects such as a laminar drag bucket, the drag polar for a finite wing can be modeled as a parabolic arc with the following equation:

Cd = Cd-0-total + K Cl^2

where:

Cd-0-total = Cd-0 + Configuration Drag Index Scale * Total Drag Index

For example, suppose that you define the Cd-0 of an aircraft as 0.02 and leave the Configuration Drag Index Scale at the default of 0.0001. In the aircraft configuration window you have defined two external fuel tanks with a drag index of 20 each and a base drag index of 50. The total parasitic drag would then be calculated by AMM to be:

Cd-0-total = .02 + .0001*(20+20+50) = .029

The induced drag coefficient is then added to the parasitic drag coefficient to produce the overall drag coefficient.

The parameters of the Basic Fixed Wing strategy are defined in the following tables.

Parameter	Description
Reference Area	The area of the lifting surface of the aircraft.
Cl-0	The coefficient of lift at zero angle of attack.
Cl-Alpha	The slope of the coefficient of lift curve.
Min AOA	The minimum angle of attack possible.
Max AOA	The maximum angle of attack possible.
Cd-0	The coefficient of drag of the lifting surface at zero angle of attack.
K	The coefficient of induced drag.
Configuration Drag Index Scale	The scale value used to convert the Drag Index calculated in the Aircraft Configuration window into a real aerodynamic value. The Drag Index and its Scale can be used to facilitate adapting flight manual data to real aerodynamic coefficients.
Lift Factor	A scalar value applied to the aircraft's lift surface area for parametric analysis.
Drag Factor	A scalar value applied to the aircraft's drag surface for parametric analysis.

There are three calculators included in the Basic Fixed Wing aerodynamics strategy - the Lift Coefficient Calculator, the Drag K calculator, and the Altitude/Airspeed converter, which is contained within the Lift Coefficient Calculator. All three calculators allow you to define values that will be calculated or converted, and which will then be automatically propagated to the appropriate fields in the performance model.

Calculator	Description
Lift...	This calculator determines the Cl-0 and Cl-Alpha lift coefficients based on values that you provide for the Reference Area - the area of the lift surface of the aircraft - and two sets of data for Weight, Altitude, True Air Speed (TAS), and Angle of Attack (AOA) - which define two coefficient of lift values. The calculator uses this information to determine Cl-Alpha and then Cl-0, which are displayed at the bottom; the Cl-0 and Cl-Alpha fields in the Basic Fixed Wing window will be updated with the displayed values when you click OK. The calculator saves the last set of values that were applied and will use them regardless of the values currently in the coefficient fields; if you have manually entered coefficient values, the calculator cannot extrapolate from that value to populate the whole equation, so the calculator will simply remain set to the last values defined for it.
Altitude/Airspeed...	This calculator converts airspeed, with respect to altitude, from several possible formats to the one that you have selected for the True Air Speed field. Both the Altitude and True Air Speed fields will be updated accordingly when you click OK. If you change the altitude, the last velocity field that you selected will be held constant.
K...	This calculator determines the drag K based on values that you provide for the Aspect Ratio and Oswald Efficiency Factor. The calculator uses this information to determine K, which is displayed at the bottom; the K field in the Basic Fixed Wing window will be updated with the displayed value when you click OK.

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External File

The External File aerodynamics strategy calculates angle of attack dynamically using aerodynamic data supplied by an .aero file.

Click the ellipsis buttons to browse to the files that you want to use to define the Forward Flight and Takeoff/Landing aerodynamics strategies. Additional parameters are described in the following table:

Parameter	Description
Reference Area	Defines the area of the lift surface of the aircraft. This parameter is defined for both the Forward Flight and Takeoff and Landing modes.
Configuration Drag Index Scale	Defines the scale value used to convert the Drag Index calculated in the Aircraft Configuration window into a real propulsion value.
Lift Factor	Defines a scalar value applied to the aircraft's lift surface area for parametric analysis.
Drag Factor	Defines a scalar value applied to the aircraft's drag surface for parametric analysis.

Propulsion

The propulsion tab is used to define the rate at which the aircraft will speed up or slow down and provides a method for computing the fuel flow; this involves computing the thrust requirements and the throttle setting for any given flight condition which in turn requires a full aerodynamics calculation.

The propulsion models provided with STK separate the acceleration and deceleration speed changes from the thrust available as computed by the models. This is done for ease of use and to allow for quick construction of flight paths without constraints imposed by the propulsion system. We recommend that users fine tune these separate parameters so that they result in a faithful representation of actual performance.

There are three propulsion strategies to choose from - Simple (Disabled), Basic Fixed Wing, and External File; if the acceleration model is using a Simple aerodynamics strategy, then the Simple propulsion strategy will be the only one available. Each of these strategies is defined below.

Simple (Disabled)

The Simple propulsion strategy specifies how fast the aircraft should accelerate or decelerate, but does not compute thrust or fuel flow. The following table describes the additional parameters that can be defined for a simple propulsion strategy.

Parameter	Description
Max Thrust Acceleration	Defines how quickly the aircraft speeds up at maximum throttle.
Min Thrust Deceleration	Defines how quickly the aircraft slows down when the thrust setting is at a minimum.
Scale by atmospheric density	Enable to scale acceleration/deceleration performance by the ratio of density at altitude to sea level density raised to the Density Ratio Exponent.
Density Ratio Exponent	Defines the relative impact of atmospheric density on the aircraft's acceleration/deceleration performance. A supercharged/turbocharged engine with a variable pitch propeller may have an exponent close to zero, while a non-turbo/supercharged engine may have an exponent closer to 1; 0.7 is a common setting for turbine powered aircraft.
Thrust Factor	Defines a scalar value applied to the thrust for parametric analysis.
Fuel Factor	Defines a scalar value applied to the fuel flow for parametric analysis.

Basic Fixed Wing

The Basic Fixed Wing propulsion strategy calculates propulsion dynamically. Select a propulsion method from the Mode drop-down menu - Jet or Propeller. For jet engines you will specify net thrust; for propellers you will specify net power. The following table describes the parameters of this propulsion strategy.

Parameter	Description
Minimum	Defines the minimum thrust or power and associated fuel flow that the engine is capable of producing. Click Calculate... to open the Minimum Thrust Calculator.
Maximum	Defines the maximum thrust or power and associated fuel flow that the engine is capable of producing. Click Calculate... to open the Maximum Thrust Calculator.
Speed Changes	Defines how quickly the aircraft speeds up at maximum throttle and how quickly the aircraft slows down when the thrust setting is at a minimum.
Scale performance to atmospheric density	Enable to scale thrust and acceleration performance by the ratio of density at altitude to sea level density raised to the Density Ratio Exponent.
Density Ratio Exponent	Defines the relative impact of atmospheric density on the aircraft's thrust and acceleration performance. A supercharged/turbocharged engine with a variable pitch propeller may have an exponent close to zero, while a non-turbo/supercharged engine may have an exponent closer to 1; 0.7 is a common setting for turbine powered aircraft.
Propeller	If the engine mode is set to Propeller, define the number of propellers, their diameter, and their RPM.
Thrust Factor	Defines a scalar value applied to the thrust for parametric analysis.
Fuel Factor	Defines a scalar value applied to the fuel flow for parametric analysis.

The Basic Fixed Wing propulsion strategy contains a calculator called the Minimum/Maximum Thrust Calculator, which can be launched by clicking Calculate.... This calculator determines the minimum or maximum thrust or power based on values you provide for the aircraft's engine capabilities. You can click Configuration... to open the Aircraft Configuration window and edit the aircraft's station and fuel tank configuration. The calculator uses the information that you provide to determine the minimum or maximum thrust and power required, and the thrust angle. The thrust or power value is propagated to the Basic Fixed Wing window upon clicking OK; click Cancel to close the calculator without changing the currently defined thrust or power value.

External File

The External File propulsion strategy calculates propulsion dynamically using propulsion data supplied by a .prop file.

Click the ellipsis buttons to browse to the file that you want to use to define the propulsion strategy. Additional parameters are described in the following table.

Parameter	Description
Speed Changes	Defines how quickly the aircraft speeds up at maximum throttle and how quickly the aircraft slows down when the thrust setting is at a minimum.
Scale by atmospheric density	Enable to scale acceleration/deceleration performance by the ratio of density at altitude to sea level density raised to the Density Ratio Exponent.
Density Ratio Exponent	Defines the relative impact of atmospheric density on the aircraft's acceleration/deceleration performance. A supercharged/turbocharged engine with a variable pitch propeller may have an exponent close to zero, while a non-turbo/supercharged engine may have an exponent closer to 1; 0.7 is a common setting for turbine powered aircraft.
Thrust Factor	Defines a scalar value applied to the thrust for parametric analysis.
Fuel Factor	Defines a scalar value applied to the fuel flow for parametric analysis.

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Basic Climb / Descent

A Basic Climb or Basic Descent performance model is comprised of a simple set of parameters that define the flight characteristics of the aircraft while climbing or descending.

Parameter	Description
Ceiling Altitude	Defines the maximum altitude above Mean Sea Level that may be specified as a procedure altitude for the aircraft. The ceiling describes the maximum altitude at which an aircraft can sustain level, non-accelerated flight. In general, an aircraft will not exceed the ceiling, but the ceiling could be violated by the Aircraft Mission Modeler in some instances, where required to complete a maneuver.
Airspeed	Define the aircraft's speed while climbing or descending; select a reference from the drop-down menu - true airspeed (TAS), calibrated airspeed (CAS), equivalent airspeed (EAS), or Mach number.
Altitude Rate	Defines the constant rate at which the aircraft will climb and descend once established in a steady climb or descent.
Use Aero/Propulsion Fuel Flow	Select to use the fuel flow calculated by the aerodynamics and propulsion strategies defined in the active acceleration performance model; if the acceleration performance model is not set to calculate fuel flow, then fuel flow will not be calculated for this performance model, either.
Fuel Flow	Defines the amount of fuel used per unit of time selected.
Initial Level Off for Acceleration	If enabled, the aircraft will level off and accelerate or decelerate until its airspeed is within the defined Relative airspeed tolerance, and only then begin its climb or descent. This models common pilot practice of achieving the aircraft's optimal climb or descent airspeed at level flight before beginning a climb or descent.

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Basic Cruise

A Basic Cruise performance model is comprised of a simple set of parameters that define the flight characteristics of the aircraft during level flight.

Field	Description
Ceiling Altitude	Defines the maximum altitude above Mean Sea Level that may be specified as a procedure altitude for the aircraft. The ceiling describes the maximum altitude at which an aircraft can sustain level, non-accelerated flight. In general, an aircraft will not exceed the ceiling, but the ceiling could be violated by the Aircraft Mission Modeler in some instances, where required to complete a maneuver.
Default Altitude	Defines the aircraft's default cruising altitude in feet. You cannot specify an altitude below sea level (0). If a procedure is specified that requires the aircraft to fly below sea level, the performance models in effect at sea level will be applied to that procedure.
Airspeed Type	Defines the airspeed reference of the performance model - true airspeed (TAS), calibrated airspeed (CAS), equivalent airspeed (EAS), or Mach number.
Use Aero/Propulsion Fuel Flow	Select to use the fuel flow calculated by the aerodynamics and propulsion strategies defined in the active acceleration performance model; if the acceleration performance model is not set to calculate fuel flow, then fuel flow will not be calculated for this performance model, either.
Minimum	Defines the minimum cruising airspeed and fuel flow.
Max Endurance	Defines the cruising airspeed and fuel flow that will provided the maximum flying time possible for the aircraft.
Max Range	Defines the cruising airspeed and fuel flow that will provided the maximum flight range possible for the aircraft.
Maximum	Defines the maximum cruising airspeed and fuel flow.

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Basic Landing

A Basic Landing performance model is comprised of a simple set of parameters that define the flight characteristics of the aircraft during a landing.

Field	Description
Landing Speed	Defines the speed to which the aircraft decelerates during approach and while on the landing glideslope; select a reference from the drop-down menu - true airspeed (TAS), calibrated airspeed (CAS), equivalent airspeed (EAS), or Mach number. If the approach fix is too close and/or the speed at approach is too fast, the aircraft may not be able to achieve landing speed by the point of touchdown - a warning message will be displayed if this occurs.
Sea Level Ground Roll	Defines the distance the aircraft travels along the ground while decelerating to a stop at sea level.
Use Aero/Propulsion Fuel Flow	Select to use the fuel flow calculated by the aerodynamics and propulsion strategies defined in the active acceleration performance model; if the acceleration performance model is not set to calculate fuel flow, then fuel flow will not be calculated for this performance model, either.
Fuel Flow	Defines the amount of fuel used per unit of time selected.

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Basic Takeoff

A Basic Takeoff performance model is comprised of a simple set of parameters that define the flight characteristics of the aircraft during a takeoff.

Field	Description
Takeoff Speed	Defines the speed to which the aircraft accelerates on its ground roll for takeoff; select a reference from the drop-down menu - true airspeed (TAS), calibrated airspeed (CAS), equivalent airspeed (EAS), or Mach number.
Sea Level Ground Roll	Defines the distance the aircraft travels along the ground while accelerating to takeoff at sea level.
Departure Speed	Defines the aircraft's speed upon leaving the ground; select a reference from the drop-down menu - true airspeed (TAS), calibrated airspeed (CAS), equivalent airspeed (EAS), or Mach number.
Takeoff Climb Angle	Defines the aircraft's angle of ascent while taking off.
Use Aero/Propulsion Fuel Flow	Select to use the fuel flow calculated by the aerodynamics and propulsion strategies defined in the active acceleration performance model; if the acceleration performance model is not set to calculate fuel flow, then fuel flow will not be calculated for this performance model, either.
Accel Fuel Flow	Defines the aircraft's fuel flow rate while accelerating during takeoff.
Departure Fuel Flow	Defines the aircraft's fuel flow rate at departure speed.

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