Basic Acceleration

The Basic Acceleration performance 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. STK's Aviator capability adheres to these values when possible, but in procedures where the turn is non-level the values may be adjusted to maintain the correct relationship between these interrelated parameters. Select one parameter to manually define; the other parameters are calculated relative to the parameter that you specified.

Table - Level Turns Parameters

Climb and Descent Transitions

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

Table - Climb and Descent Transitions Parameters

Parameter	Description
Max Pull Up G	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.
Max Push Over G	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.

Accel Maneuver Mode Field

You can toggle if the aircraft turn or push/pull calculations use atmosphere density scaling. Alternatively, you can define specific parameters used in the turn calculation for more realistic results.

Table -Accel Maneuver Mode Options

Always Ignore Flight Path Angle Check Box

When you are working at the design limits of an aircraft model, you may commonly encounter problems with:

• pushing over at high path angles
• pulling up at low flight path angles

For example, an aircraft flying at high altitude and high speed may not have enough control authority to push over as a procedure requires. Or, the aircraft may need to violate another constraint such as the procedure ceiling.

In these situations, you can select the Always Ignore Flight Path Angle check box to ignore load factor limits. This option enables you to suspend these limits without needing to change to the aircraft model that you are using.

Attitude Transitions

Aircraft attitude is determined using a 123 Euler angle sequence of Bank, Angle of Attack The angle between the body X axis and the projection of the velocity vector onto the body XZ plane. The velocity vector is the velocity of the object as observed in the object's central body fixed coordinate system., 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.

Table - Attitude Transitions Parameters

Parameter	Description
Roll Rate	The standard roll rate - the rate at which the aircraft bank angle changes - of the aircraft in a turn. When STK's Aviator capability 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	The pitch rate when transitioning between attitude modes, between procedures, and between uncoordinated maneuvers when necessary.
Sideslip/Yaw Rate	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 you 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 four aerodynamics strategies to choose from:

• Basic Fixed Wing

• High Fast

• External File

• Simple (Disabled)

Each of these strategies are defined below.

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 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 + (1e-4 * Total Drag Index)

For example, suppose that you define the Cd-0 of an aircraft as 0.02. In the 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 STK's Aviator capability 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.

Table - Basic Fixed Wing Aerodynamics Strategy Parameters

Parameter	Description
Reference Area	The area of the lifting surface of the aircraft.
Compressible (High Speed) Flow	Select this check box to define the aerodynamic parameters with respect to high speed, air compression conditions (e.g., supersonic flight).
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.
Linked	Select this check box to link the Lift and Drag factor values so that they are equalized.
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 four calculators included in the Basic Fixed Wing aerodynamics strategy - the Lift Coefficient Calculator, the Cl-Alpha 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.

Table - Basic Fixed Wing Aerodynamics Strategy Calculators

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 True Airspeed: the speed that the aircraft is moving relative to the airmass that it is flying in.), 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.
Cl-A...	This calculator determines the Cl-Alpha lift coefficient based on values that you provide for the wing geometry of the aircraft. The computed Cl-Alpha value is displayed at the bottom; the Cl-Alpha field in the Basic Fixed Wing window will be updated with the displayed value when you click OK. This calculator operates independently of the Lift... calculator; you can only implement the computed value of one of these calculators.
Drag...	This calculator determines the induced drag coefficient (K) and zero life drag coefficient (Cd-0) based on values that you provide for the relevant properties. The calculator uses this information to determine the coefficients. If you select Set on OK, then the respective fields in the Basic Fixed Wing window will be updated with the displayed value when you click OK; otherwise, the values will remain stored in the calculator but will not be implemented.

The Plot Aero... option enables you to evaluate how parameter changes affect the vehicle's performance.

High Fast

The High Fast aerodynamics strategy uses thrust to generate a lift vector, which provides the ability to track fuel burn during lift. Additionally, it generates the forces perpendicular to the velocity vector to provide maneuvering.

Parameters

Parameter	Description
Drag Area	An aircraft's drag is a function of dynamic pressure. Drag area is calculated by multiplying drag coefficient by the reference area, which is then multiplied by dynamic pressure to understand the aircraft's drag force.

The Plot Aero... option enables you to evaluate how parameter changes affect the vehicle's performance.

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.

Table - External Aerodynamics Strategy File Parameters

Parameter	Description
Reference Area	The area of the lift surface of the aircraft. This parameter is defined for both the Forward Flight and Takeoff and Landing modes.
Remove invalid rows	When this option is checked, STK filters out invalid rows prior to loading the file and only loads the valid rows. If you uncheck the option and reload the file, STK loads all rows unfiltered. Invalid rows are classified as any rows where ClAlpha, K, or Cd0 are less than zero.
Linked	Select this check box to link the Lift and Drag factor values so that they are equalized.
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.

The Plot Aero... option enables you to evaluate how parameter changes affect the vehicle's performance.

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.

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. AGI recommends that you fine tune these separate parameters so that they result in a faithful representation of actual performance.

There are four propulsion strategies to choose from:

• Basic Fixed Wing

• High Fast

• External File

• Simple (Disabled)

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.

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.

Table - Basic Fixed Wing Propulsion Parameters

Parameter	Description
Minimum	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	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	The rate at which 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	Select this check box 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	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	A scalar value applied to the thrust for parametric analysis.
Fuel Factor	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 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.

The Plot Prop... option enables you to evaluate how parameter changes affect the vehicle's propulsion performance.

High Fast

The High Fast propulsion strategy provides propulsive force, which overcomes drag and provides the force to accelerate.

Parameters

Parameter	Description
Lift Specific Impulse	The measure of how much thrust force the aircraft gets per unit of fuel flow.
Max Thrust Lift	The maximum lift force that your system can generate.
Propulsive Specific Impulse	The potential energy of a propulsion system, measured in seconds. Specific impulse measures how long an engine exerts a continuous pound of force until fully burning through a pound of propellant.
Max Propulsive Thrust	The max thrust when the system is at max throttle.

The Plot Prop... option enables you to evaluate how parameter changes affect the vehicle's propulsion performance.

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.

Table - External Propulsion File Parameters

Parameter	Description
Speed Changes	The rate at which 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	Select this check box 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	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	A scalar value applied to the thrust for parametric analysis.
Fuel Factor	A scalar value applied to the fuel flow for parametric analysis.

The Plot Prop... option enables you to evaluate how parameter changes affect the vehicle's propulsion performance.

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.

Table - Simple Propulsion Parameters

Parameter	Description
Max Thrust Acceleration	The rate at which the aircraft speeds up at maximum throttle.
Min Thrust Deceleration	The rate at which the aircraft slows down when the thrust setting is at a minimum.
Scale by atmospheric density	Select this check box 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	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	A scalar value applied to the thrust for parametric analysis.
Fuel Factor	A scalar value applied to the fuel flow for parametric analysis.

Dynamics/Moments

The Dynamics/Moments tab offers a SimpleAero strategy, which is intended to capture aerodynamic control surfaces. It enables you to specify the maximum torques that can be generated at a specified flight condition, and the minimum level of attitude control.