Hover Focus Problem

The intention of this problem is to deep-dive into the hover simulation of a rotorcraft in the proximity of a ground plane. Select one, several, or all aspects of the problem to test your analysis methods. In the absence of a detailed, publically available database, practitioners are encouraged to share results for comparison with helicopter enthusiasts across the globe.

 

For these simulations, utilize the 4-bladed HVAB rotor with the following assumption for cone and lag angles:

Cone:    

 

Lag:    

 

and the following definitions:

 

Gross Weight:    

Vertical Drag Ratio:    

 

Assume sea level standard atmosphere and a rotor speed of 1250 RPM.

Step 1: HOGE Assessment – Rotor Performance, Download, and Augmented Thrust

  • For the isolated HVAB rotor, run a thrust sweep (suggest θ75 = 6°, 8°, 9°, 10°, 11°) to determine thrust, power, distributed thrust , distributed torque , and pressure coefficient at radial stations r = 0.875, 0.900. 0.973, and 0.990.

  • Repeat the calculation in Step 1A for the installed rotor case for θ75 = 10°

  • Compute the augmented thrust, the added thrust produced by the rotor as a result of the presence of the fuselage. Consider the power produced by the rotor of the installed case.

  • Compute the download acting on the aircraft due to the rotorwash. Report the download as a percentage of the thrust; report the augmented thrust as a percentage of the download.

  • Extra credit: Repeat the installed calculations for θ75 = 6°, 8°, 9°, 10°, 11° and find DL = f(T) and ΔT = f(T)\

 

Step 2: HIGE Assessment

  • For a flat ground plane, compute installed hover performance
    for a gross weight of 1,200 lbf. Consider rotor height above
    ground from h/D = 0.5 to 3.0. Find thrust, power, distributed
    thrust , distributed torque , and pressure coefficient at radial
    stations r = 0.875, 0.900. 0.973, and 0.990.

  • Find DL = f(h/D).

 

Step 3: Groundwash Assessment – Part 1

 

  • For a flat ground plane, compute the ground wash for installed hover performance for a gross weight of 1200 lbf and a rotor height above ground of h/D = 0.5. Note: this is one of the cases in Step 2A.

  • Plot the ground velocity profile at azimuth locations of 0, ±45°, 90, ±135°, and 180°, and radial locations of 0.5D, 1.0D, and 1.5D.

  • Extra Credit: How do the results of Step 3A differ for an isolated rotor?

 

 

 

 

Step 4: Groundwash Assessment – Part 2

Repeat Step 3 with the hillside ground plane.

Step 5: Impact of Headwind

Repeat Step 3 with 3- and 6-knot headwinds. For this exercise, do not consider changes to rotor flapping or add cyclic for rotor trim. Instead, report changes to rotor forces and moments.

Step 6: Generalized Impact of Winds

Repeat Step 5 with 3- and 6-knot winds from ±45°, 90, ±135°, and 180°. 

Step 7: Descent Simulation

Consider a descent from h/D = 2.0 to h/D = 0.5 following the velocity profile described by:

 

V = 6.9271 - 6.9271 x COS(π/1.2 x t)

 

Keep gross weight constant (1,200 lbf) and find T = f(t), P = f(t) and DL = f(t). The simulation is 2.4 seconds long requiring 50 rotor revolutions.