Graham's Hobby Website

RC Flight Sim

Flight simulation software is available specifically for RC slope soarers. This page briefly explains the steps needed to create a model instance for an individual glider, focussing on the aircraft geometry, aerodynamics and flight dynamics model.

RC Slope Sim

RC model slope soarers are radio-controlled gliders which are flown in slope lift at hill, mountain or coastal site. Slope soaring sites are typically located on ridges, slope crests and at the head of valleys. Given a suitable wind strength and direction, slope lift results, where the prevailing wind accelerates en masse up and over the terrain. The models obey the same laws of physics as full size aircraft, but operate in a modified fluid dynamics regime, owing to scale effects.

Learning to fly RC slope soarers is weather dependent and can be quite challenging in itself, so it was interesting to come across several PC computer simulations that can assist with the learning curve. The main advantage is that "crashing" only requires a re-boot in software rather than the "re-build", often needed in the real world!

USB RC "Transmitter"

Apart from a PC and simulation software, the main requirement (at very modest cost) is an RC dummy transmitter or "tx" that connects to a computer USB port. This offers control via two gimballed sticks, as per a real RC transmitter.

Dynam dummy USB transmitter for RC flight simulation

The two best free RC soaring simulations are I think CRRCsim and PicaSim. I will concentrate on CRRCsim as it offers the best prospect for bespoke model development and sofware add-ons.

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CRRCsim RC Flight Simulator

CRRCsim has evolved over some years from original software called LaRCsim, a Flight Dynamics Model (FDM) developed by NASA at Langley, Virginia in the 1990's. The Charles River Radio Control Flight Simulator (CRRCsim) includes powered and unpowered models and for the latter has the option of slope lift and/or thermal activity.

The software is open source, comprising a terrain model and corresponding landscape textures, then geometry, texture and flight model data for the aircraft, together with numerical engines to calculate the lift regime and aircraft flight behaviour. Personally CRRCsim has been found to run best on a fast Apple Macbook Pro, with a widescreen HDTV as monitor.

CRRCsim aircraft model files have a detailed structure but are in plain text. It took some time to understand the meaning of all the particular variables, with help from the descriptions in the CRRCsim documentation and threads on the CRRCsim Yahoo forum. This was put to the test later when it came to producing real flight parameters from additional software.

The CRRCsim glider model comprises two main elements: a) a 3D CAD geometry model, providing the glider visuals, and b) a file containing all the data needed for the flight simulation, ie the Flight Dynamic Model (FDM).

3D CAD Geometry

The RC model geometry has been constructed using AC3D CAD software, together with suitable texture graphics to represent the surface appearance. The model representation includes an accurate colour scheme, airfoil cross-section and independent control surfaces, so they can be "moved" in flight. AC3D costs about $90 to buy (http://www.inivis.com/buy.html).

AC3D geometry model of the Flying Wings Ninja, before applying texture

The model surfaces can either be self-coloured within the CAD model or a separate bitmap texture graphic created. This is mapped to the geometry model, therefore referenced within the geometry file.

Ninja bitmap texture graphics

AC3D geometry model of Ninja with bitmap texture applied

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Physics Modelling

The aircraft simulation parameters are a combination of mass and inertia properties, together with aerodynamic parameters for the airfoil(s) and flight dynamics properties for the aircraft as a whole.

The airfoil data can be generated using the program Xfoil, from which lift and drag parameters are obtained. Xfoil is an interactive program for the design and analysis of subsonic isolated airfoils (http://web.mit.edu/drela/Public/web/xfoil/).

Spreadsheet curve fitting to Xfoil lift-drag graph for airfoil parameters

The Ninja airfoil represents a proprietary design and it also proved problematic to completely represent the airfoil shape from tracing the wing root. This leads to anomalies in the Xfoil pressure plots. Instead, Martin Hepperle's MH 60 airfoil for flying wings was utilised, adjusting the thickness and camber in XFLR5 to closely match the Ninja airfoil profile.

XFLR5 is an analysis tool for airfoils, wings and planes operating at low Reynolds Numbers (http://www.xflr5.com/xflr5.htm). The mass moment of inertia properties can be computed in several ways, using the program XFLR5 or by hand calculation.

The primary flight dynamics data requires a subsidiary geometry model to be created in the Athena Vortex Lattice software, AVL. This is a program for the aerodynamic and flight-dynamic analysis of rigid aircraft, developed by Dr Mark Drela at MIT (http://web.mit.edu/drela/Public/web/avl/).

AVL geometry model of Ninja, as basis for FDM determination

Once the geometry model is created in AVL, aerodynamic conditions are applied and the program then determines forces, moments and their derivatives. These outputs then represent the inputs to the flight dynamic model in the CRRCsim simulation.

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FDM Parameters

The AVL outputs include FDM parameters that determine the effects and sensitivity of control movements. In total the CRRCSsim aircraft model requires about forty named parameters to enable the flight dynamics to be modelled.

CRRCsim input aerodynamic data etc for the Ninja flying wing

Note - the two left-hand columns show the different parameter designations in AVL vs CRRCsim. The primary data distinctions are between: pitching moment, lift and drag, plus lateral-, roll- and yaw- stability.

The Ninja Flies!

Well, eventually. It took about six rounds of de-bugging to achieve a satisfactory model. Random tweaking of the FDM was avoided. A January 2013 paper by Luca Gasparini entitled "Evaluation of glider model realism in CRRCsim" helped with checking that the FDM parameters fell were reasonable.

Ninja model flying in the CRRCsim Cape Cod coastal scenery

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Next CRRCSim Model

A follow-on to the Ninja slope soarer build in 2015 was the Quark by Island Models, a 2 metre wingspan aerobatic slope soarer in balsa construction for 4-channel radio control (http://www.islandmodels.ie). This was a step up in complexity from the Ninja, in both the real and virtual worlds. It is waiting to be flown in 2016.

The geometry model required many measurements to be taken off the plans in x-y-z coordinates. Intermediate cross-sections for the airfoils and fuselage were established and then the solid volume formed by extrusion in AC3D.

AC3D geometry model of Quark aerobatic slope soarer

The AVL geometry model for the Quark required the fuselage to be approximated based on round sections having the same cross-sectional area as the original.

AVL geometry model for the Quark

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Add-on Alpine Scenery

A great add-on for CRRCsim takes the form of terrain files for a number of flying sites in the French Alps. These are freely available from Joel Lienard (http://joel.lienard.free.fr/crrcsim/index-en.html).

The CRRCsim world comprises a cubic "skybox" within which the terrain views are projected. Joel has captured panoramic views of Alpine slope sites using a theodolite-mounted digital camera. For each site the photos have been stitched together to create accurate montages, with terrain height determination by several methods, and the panorama projected to yield scenery files consistent with the CRRCsim skybox. The result is excellent graphics and a real sense of space in which to "fly" RC gliders in a mountain environment.

Quark model flying in Joel Lienard's alpine scenary for Col Des Faisses, France

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