STK Premium (Air) or STK Enterprise
You can obtain the necessary licenses for this tutorial by contacting AGI Support at support@agi.com or 1-800-924-7244.
Capabilities Covered
This lesson covers the following STK Capabilities:
- Aviator Pro
Problem Statement
STK's Aviator capability allows you to model the performance characteristics of rotorcraft as a distinct type of aircraft from a fixed wing aircraft. You can create a rotorcraft model in the User Rotorcraft Models catalog of the Aviator catalog interface or catalog manager.
The Rotorcraft properties window is used to define the global settings and performance models of a rotorcraft model in your User Rotorcraft Models catalog. Any changes that you make in this window will be applied to the rotorcraft's definition in the catalog.
The Rotorcraft properties window is comprised of two tabs - Performance Models and Aero/Propulsion. The Performance Models tab defines the basic turning, climb and descent transition, cruising, and attitude transition characteristics of the rotorcraft , while the Aero/Propulsion tab allows you to select and define strategies to model attitude and propulsion characteristics, respectively.
When you are done editing the rotorcraft's properties, click Save to save any changes you have made and click Close to close the window; you will be prompted to save any changes before closing the window.
Performance Models
As with aircraft models, the performance model parameters of a rotorcraft define its behavior in flight.
| Parameter | Description |
|---|---|
| Max Altitude | The maximum altitude at which the rotorcraft is capable of operating. |
| Default Cruise Altitude | The rotorcraft's default cruising altitude. You cannot specify an altitude below sea level (0). If a procedure is specified that requires the rotorcraft to fly below sea level, the performance models in effect at sea level will be applied to that procedure. |
| Descent Rate Factor |
The actual descent rate of the rotorcraft, before applying either the Max Descent Angle or Min Descent Rate properties. The factor is multiplied by the altitude change rate calculated at zero throttle. If the rotorcraft descends too quickly, the Max Descent Angle will constrain the calculated descent rate. However, the Max Descent Angle can prevent the rotorcraft from descending at all if its airspeed is too slow, and in that case the Min Descent Rate will be applied. |
| Max Climb Angle | The maximum pitch angle of the rotorcraft's flight path while climbing. |
| Climb at cruise airspeed | Select to define the climbing airspeed of the rotorcraft using the cruise airspeed of the procedure in which it is climbing; otherwise, the rotorcraft will always climb at its maximum endurance airspeed. |
| Max Descent Angle | The maximum pitch angle of the rotorcraft's flight path while descending. |
| Min Descent Rate | The minimum rate at which the aircraft will descend once established in a steady descent. |
| Max Load Factor G | The maximum load factor that the aircraft can bear while maneuvering in formation. |
| Roll Rate | The standard roll rate - the rate at which the aircraft bank angle changes - of the rotorcraft in a turn. When Aviator 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. |
| Sideslip/Yaw Rate Dot | The rate of change of the yaw rate. |
| Max Transition Pitch Angle | The maximum pitch angle of the flight path when transitioning between forward flight and hovering. |
| T/F Max Flight Path Angle | The maximum pitch angle of the flight path when the rotorcraft is engaged in terrain following flight. |
| T/F Terrain Window | The time interval over which terrain points are sampled when the rotorcraft is engaged in terrain following flight; smaller values result in a rougher ride and are more resource-intensive with respect to computation. |
| Compute Delta Alt | The Compute Delta Altitude property defines the maximum change in altitude in a computed segment before the data is sampled again and a new segment is begun. |
| Max Safe Airspeed | The maximum cruising airspeed of the rotorcraft. |
| Max Safe Translation Speed | Defines, along with the minimum cruise airspeed, the maximum translation speed of the rotorcraft. The maximum translation airspeed is calculated as the minimum of the minimum cruise airspeed and this property. The minimum cruise airspeed is defined as the speed at which the rate of decrease in power required is minimum as speed increases from 0 to forward flight speed. |
| Always Ignore Flight Path Angle for Climb and Descent Transitions |
When you are working at the design limits of an aircraft model, you may commonly encounter problems with:
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. |
Aero/Propulsion
The Aero / Propulsion tab of a rotorcraft model function together as an analysis system that performs a trim calculation, as the missile 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 missile, but these values do not directly influence the flight path.
The system will generate warnings when 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. limits are exceeded or when thrust deficits exist indicating the missile design is not capable of flying the path specified, but the path will still be flown as designed; this feature allows you to explore a missile design (perhaps based on high accuracy wind tunnel and engine test data) and its suitability to perform a desired mission.
| Parameter | Description |
|---|---|
| Rotor Count | The number of rotors |
| Fuselage Flat Plate Area | The flat plate area for the fuselage |
| Rotor Diameter | The diameter of the rotor |
| Tail Rotor Offset | The offset of the tail rotor. |
| Blades per Rotor | The number of blades on each rotor |
| Tail Rotor Diameter | The diameter of the tail rotor. |
| Blade Chord | The chord of the blade. |
| Blade Profile Drag CD0 | The drag coefficient when the rotor disc does not generate any lift. |
| Rotor Tip Mach The ratio of the aircraft's speed and the speed of sound at the aircraft's altitude, with local atmospheric conditions. | The Mach number of the advancing blade tip. |
| Blade Profile Drag K | The induced drag coefficient, which accounts for how lift generation impacts drag. |
| Induced Power Correction Factor |
The slop factor that accounts for losses. This is typically between 1.15 and 1.25 (meaning somewhere between 15% and 25% losses). |
| Type | The type of the Powerplant. Options include Electric, Turboshaft, and Piston. |
| Max S/L Power | The maximum power at sea level. |
| Max S/L Fuel Flow | The maximum fuel flow at sea level. |
| 3D Model File | The 3D model used in the STK graphic windows. |
| Default Configuration | Click this button to load the default configuration associated with the selected 3D Model file. |
Aerodynamics
A rotorcraft's Aerodynamics parameters define the physical characteristics that are used to compute lift, drag, angle of attack, sideslip and intermediate / derived values.
The Induced Power Correction Factor is used to convert between the power required and the thrust required. There is generally a 15% loss (the default value for this property) when converting power to thrust in most rotorcraft.
Powerplant
A rotorcraft's Powerplant parameters define the rate at which it will speed up or slow down and provide a method for computing the fuel flow. There are three propulsion types to choose from - Electric, Turboshaft, and Piston. The Turboshaft and Piston propulsion models allows you to define the maximum power and fuel flow of the rotorcraft's engine, while the Electric model only requires you to define the maximum power (for the obvious reason).
Dynamics/Moments
The Dynamics/Moments tab allows you to specify control parameters for the aircraft. These allow calculation of aircraft performance characteristics, but are not used to calculate trajectory. Required Control Authority allows you to specify what angular rates are expected to safely control the vehicle. Control Surface Buildup allows Aviator to calculate control forces available in each axis and is used to compute flight profile Control Force Required and FullControl.
Aviator calculates control forces as torques throughout the flight. The max force available for the torque is determined from Surface Area, CLMax, and flight conditions based on:
| Item | Description |
|---|---|
| F | Force |
| p | Air density determined by aircraft altitude |
| V | Aircraft velocity |
| S | Surface area determined above |
The max available control torque along an axis is the product of this force and the proscribed Moment Arm Length.