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.

 

The Rotor

For these simulations, utilize the 4-bladed HVAB rotor. The HVAB rotor is a 4-bladed articulated system specifically designed for analysis validation of advanced edgewise rotors in hover and forward flight. The description of the blade is documented in here. The blade planform is based on the High Lift Rotor designed for testing in the Transonic Dynamics Tunnel in the early 2000s. It has a 66.5-inch radius rotor with a 30-degree swept tip beginning at 95% radius, measured along the feathering axis. The blade features a 14-degree linear twist distribution and RC-series airfoils. The blade root chord is 5.45 inches and the rotor has a solidity of 0.1033. Geometry and grid for the HVAB rotor can be found on the file-share page.

 

Unless you choose to predict aeroelastic effects directly, the following assumption for cone and lag angles can be made:

Cone:    

 

Lag:    

 

The order of blade rotation is collective, cone, and then lag. The rotor speed is 1,250 RPM and sea level standard atmospheric conditions may be assumed.

 

The Airframe

The NASA Robin-Mod7 is a generic single-rotor helicopter airframe that has been useful for several helicopter studies over the years. It has a published analytic definition, which makes it convenient for our studies. A representation of the geometry is available on the file-share page. The definition is normalized by half the fuselage length – in other words, fuselage cross sections are defined for sections between x/L = 0 and x/L = 2.0. In these exercises, L = 61.9655 inches. The relative position of the fuselage to the rotor follows the test setup by Overmeyer and Martin and is shown below for convenience. As with Overmeyer and Martin, the rotor shaft is tilted 3.5° so that the fuselage nose is pointed up when the rotor is level. Pylons and hubs are ignored

The following definitions are convenient for the Hover Problem:

 

Gross Weight:    

Vertical Drag Ratio:    

 

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 1,000 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 1,000 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 (1,000 lbf) and find T = f(t), P = f(t) and DL = f(t). The simulation is 2.4 seconds long requiring 50 rotor revolutions.