Originally Posted by
NZScion
The diameter of the fan doesn’t have much to do with it. What is important is the volume of air passing through the core, which of course varies from one engine type to another. The flight manual for each type should contain the information you require, including an envelope (speed vs altitude) where a windmilling relight is possible.
To elaborate a bit, it's the bypass ratio more than the fan diameter. The earlier generation of high bypass engines had pretty good windmill starting characteristics, but starting with the higher bypass on engines like the PW4000/112" and GE90, the combination of less air going through the core and the core being more highly loaded (higher pressure ratios to generate the work to spin those big fans), windmill N2s dropped relatively to airspeed, meaning you had to go faster to get adequate N2s for a windmill start. You need an N2 of about 10% before there is adequate pressure from the engine driven fuel pump to operate the fuel control and open the high pressure shutoff valve (varies slightly between engines, but you need ~300 psi to open the shutoff valve in the fuel control).
You could reliably windmill start at CF6-80C2 at 220 knots (we demonstrated down to 200 knots, but needed to allow some margin for the inevitable deterioration that occurs in service) - the GE90 needed 270 knots.
On the GEnx, we needed to depressurize the engine driven hydraulic pump during in-flight starting to get enough N2 to reliably start the engine at 270 knots.
3 spool Rolls engines are also a problem and require relatively high speeds for windmill starts since the incoming air has lost so much energy going through the first two spools that it doesn't spin N3 very fast.