STOBAR Carrier Ski-jump Simulator

© Artyom Beilis, 2015 - CC-By, JavaScript Code - MIT License

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Abstract | Flight Model | Results | Conclusions | Simulator | Graphs | Flight Data | References

Abstract

Short Takeoff But Arrested Recovery (STOBAR) is one of the methods of designing a modern aircraft carrier. The aircraft takes-off using its own power rather than with the help of a catapult (CATOBAR). In order to takeoff from a limited deck space an inclined ramp or ski-jump is used. When the aircraft leaves the carrier from the ramp, it still does not have enough speed to be fully supported by its own wings, but it has a positive climb rate. The aircraft continues to accelerate losing its climb rate but gaining more airspeed and thus increasing the lift efficiently extending its "runway" length significantly.

There is a common misconception that heavily loaded aircraft cannot operate from STOBAR carriers and their capabilities are restricted to only very limited payload. This simulation allows to investigate various takeoff settings and find out whether specific aircraft is capable to takeoff from an STOBAR carrier and with which load.

It was clearly shown that F-18E/F at maximal allowed gross weight can takeoff from an STOBAR carrier within reasonable Wind Over Deck (WOD) requirements. Thus STOBAR carrier layout does not impose severe limitations on the maximal takeoff weight.

Flight Model

This simulator models the aircraft behavior from the initial acceleration on the deck of the carrier to the point the aircraft can sustain the flight safely. The modelling consists of 3 major phases: (a) acceleration on the deck of the carrier and the ramp (b) reaching the optimal angle of attack (AoA) (c) gradual acceleration until the air speed and the climb are sufficient for a safe flight.

Deck Acceleration

On the deck following forces are taken in account: thrust, drag, and gravity (on the curved part of the ramp). The drag coefficient that is used for 0 AoA acceleration is assumed the same as for drag with partial lift. In reality the drag should be significantly lower, but it is not modeled due to lack of data.

The ramp is considered having a form of an arc of a constant radius - it isn't best shape - but as we don't actually try to simulate gear loading it isn't that important.

Pitching Up

When the aircraft leaves the ramp its AoA isn't the optimal one, thus it takes some time to get to required AoA. Based on flight test data in^[3] on the F-18E leaving the deck, typical AoA behavior is pitching up at maximal pitch rate usually overshooting the optimal AoA reaching the limit and then relaxing the pitch to the optimal AoA [3] pages 84 to 88. So the pitch is modelled as full pitch-up until the AoA and then relaxed pitch-down at half of the pitch rate. The pitch rate limit used for the simulation is 12 deg/s^[3]. See figure below:

Partial Lift Acceleration

The plane exits the ramp with positive climb at ramp angle A_r, once the optimal angle of attack reached the pilot keeps it constant. At the ramp exit the speed is below the minimal required limit and thus only a partial lift is avalible, once aircraft accelerates it gains more lift and looses some climb rate until the minimal required air speed and positive climb rate are reached - the point the aircraft can sustain its flight

The forces that operate on aircraft are

• g - Gravity
• T - Thrust adjusted according to α - AoA to air speed V direction
• L - Lift = L_coef * V² - perpendicular to the air speed - V
  Note: L_coef is calculated under assumption that engines are at maximal thrust giving some vertical lift, i.e.:
  g = L_coef * V²_req + T*sin(α)
• D - Drag = Lift / (Lift-to-Drag ratio) in the opposite direction of the air speed V

Lift to Drag Calculations for F-18E/F

In order to calculate Lift to Drag rate with half flaps we used the single engine failure climbout flight test data^[4]. We assumed standard day atmospheric conditions, filed takeoff configuration with half flaps with maximal available drag index.

According to the chart: 66,0000lb weight at 12° AoA with single engine failure at maximal thrust we have ~700 feet/minute climb at ~165 knots with half flaps and gear down.
Given that:
Climb/Speed = (Thrust-Drag)/Weight we get 3.56ms/84.88ms = 0.041 = (22,000lbf - Drag ) / 66,000lbf
Drag = 19,230lbf
Lift/Drag = 66,000/19,2300 = 3.43

F-18E/F Results

Testing

F-18E/F was tested for possibility to operate from a ski-jump ramp. Two conditions were proposed for minimal Wind Over Deck requirements^[5], see fig. below:

1. Zero Minimal Climb
2. Zero Altitude Loss

The minimal WOD was found that hold the required condition

Data Used

Optimal Speed (knots):

Lift to Drag ratio^[4]

Wind Over Deck Requirements

Zero Climb Condition (knots):

Zero Altitude Loss (knots)

Conclusions

It was found that F-18E/F is capable of operating from STOBAR carrier even at maximal takeoff weight. Also Wind over Deck requirements for high loads aren't low they are reasonable.

Simulator