{ "cells": [ { "cell_type": "markdown", "id": "a5a73515", "metadata": { "slideshow": { "slide_type": "slide" } }, "source": [ "# Forward Flight" ] }, { "cell_type": "markdown", "id": "449c23a3", "metadata": { "slideshow": { "slide_type": "-" } }, "source": [ "```{epigraph}\n", "\"The problem of the rotor in steady forward flight is the problem of defining the magnitude and direction of airflow encountered by each blade element\"\n", "\n", "-- Alfred Gessow (Aerodynamics of the Helicopter) \n", "```" ] }, { "cell_type": "markdown", "id": "97225998", "metadata": { "slideshow": { "slide_type": "skip" } }, "source": [ "Discussing about the details of forward flight state aerodynamics is also the perfect time to talk about the 2D aerodynamic characteristics of the airfoil sections that make up the blades of the rotor. You would not be prepostrously wrong in assuming that if the blade sections are made up of an airfoil with better airfoil characteristics then the overall rotor efficiency would be improved. 'Efficiency' of the rotor here refers to just the profile + induced power consumption of the rotor but in literature this term can also be used to refer to the acoustic noise or vibratory characteristics of the rotor. These are not within the scope of this course so we'll just end up focussing on rotor power consumption. But what does 'better airfoil characteristics' precisely mean? Is it just larger $C_l/C_d$? Larger stall angle of attack? The way the question has been posed might suggest that indeed there is something *extra* that needs to be accounted for or considered in order to improve rotor performance in forward flight. What that '*extra*' might be would become clear as the various aerodynamic phenomena, encountered by the rotor, in forward helicopter flight are discussed. " ] }, { "cell_type": "markdown", "id": "676c91de", "metadata": { "slideshow": { "slide_type": "skip" } }, "source": [ "On a conventional helicopter, the rotor is the sole entity responsible for generating thrust to counter the weight as well as the fuselage drag. While this characteristic itself leads to interesting aerodynamics and rotor performance attributes, there are a ton of pecularities arising just from edgewise rotor flight." ] }, { "cell_type": "markdown", "id": "ef0ed79b", "metadata": { "slideshow": { "slide_type": "slide" } }, "source": [ "
UH-60A Helicopter [source]
\n", "Wing tip vortices behind a Boeing C-17 Globemaster [source]\n", "
\n", "Wing tip vortices behind a McDonnell Douglas F/A-18 Hornet [source]\n", "
\n", "Rotor tip vortex trajectories at difference advance ratios [source]
\n", "Free-wake simulation of a 4 bladed rotor in forward flight
\n", "\n", "Rotor descent flight states [source]
\n", "Rotor induced velocity during axial flight states [source]
\n", "Net rotor inflow during axial flight states [source]
\n", "Inflow results based on experimental measurement of different rotors [source]
\n", "Tangential velocity distribution in hover versus forward flight (at $\\psi$ =90°)
\n", "Reverse flow region during forward flight
\n", "Blade section drag due to in-plane velocity
\n", "Rotor in forward flight
\n", "Total helicopter power consumption versus forward speed
\n", "