Finding glider reachable area / catchment area with custom slope?

Question

I'd like to produce maps for gliders in mountains.

I'm looking to display the reachable area by a glider taking off from a given point (obviously with the corresponding DEM).

For a first approach I'll suppose that we are in static air, no thermal ascent, no wind.

So like water gliders only go down.

Gliders have a specific slope (the glide ratio).

I started thinking that my problem is similar to a flood inundation analysis. But I cannot find how to input a specific slope in any of the hydrology modules I explored.

@Spacedman propose a first approach by creating a raster representing a shallow cone centered on the take off point and having the glide ratio as slope. It is a good approach without any obstacles.
Imagine you have an obstacle on the straight line for example there is a cylinder.
The area behind this cylinder may appear as "reachable" but the distance behind the cylinder from the take off point is (straight D)+𝚷(cylinder_radius). So obviously there is 𝚷(cylinder_radius)/glide_ratio difference in height.

Is there another approach that I could use?

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Implementation of approach without any obstacles:
G is glide ratio (ie: ∆D/∆z)
(x₀,y₀,z₀) is taking off point
(x,y,z) are coordinates of a point on the glider reachable area
(x,y,zₑ) are coordinates of the projected point on the earth DEM
Hₑ is height over the ground

Obviously if Hₑ≥0 it is reachable otherwise not.

Answer 1

The only solution I found is to generate a geotiff/tfw world file with Matlab/Octave. This is my sample code for a glider with ratio 7 taking off from the "Aiguille du midi" near Chamonix Coordinates are in official french RGF93 (EPSG:2154)

%G is glider ratio (7)
%x0,y0,z0 is the take off point (Aiguille du Midi / Chamonix / France / EPSG:2154) 
%r is the x resolution in meters (30m like SRTM)

G=7;
x0=1002092;
y0=6538705;
z0=3604;
r=30;
x=x0-(z0*G):r:x0+(z0*G);
y=y0-(z0*G):r:y0+(z0*G);
Xmin=x(1);
Ymax=y(end);
[X,Y]=meshgrid(x,y);
z=z0-sqrt((X-x0).^2+(Y-y0).^2)/G;
Z=uint16(z);
imwrite(Z,'cone.tif');
fid = fopen('cone.tfw','w');
fprintf(fid,"%f\n%f\n%f\n%f\n%f\n%f\n",r,0,0,r*(-1),Xmin,Ymax);
fclose(fid);
%For viewing the mesh uncomment next line
%mesh(x,y,z);

This is tested with Matlab R2018b and GNU Octave 5.1.0, the generated cone.tif works in QGis 3.6.2

I'd like to find a more automatic solution. Hope someone can help.