What is the correct fairing position for a tadpole recumbent trike? I have a HP Gekko 26 with a Streamer fairing. Should the fairing be as far forward or as close to my pedal range as possible? Should the trailing end of the fairing be high (inline with my head) or low (inline with my chest)?
Fairings are external body panels that help shape the airflow over you and your ride, whether it be an upright, or a recumbent, bike or trike.
So if you're aiming for more speed via better aerodynamics, then the fairing should be as small as possible AND smooth the airflow over you as much as possible. This means as narrow as possible while not interfering with your pedalling. It should also be high up so there is less vertical surface presented to the airstream (ie your face)
If you're aiming for comfort while commuting, then visibility is important, so a lower trailing edge allows you to see more. Yes it will be not quite as good for slipping through the air, but a crash or an avoidable incident will slow you down more.
Length isn't really that relevant to aerodynamics unless you're getting side winds, where the surface area is larger because the wind is from the side. There's a rule of thumb about the length of any tailbox which should be 6x longer than its maximum width to avoid stalling the windflow as it comes back together.
A final thought about trailing edge height - if you have a sudden stop that throws you forward, will you be better off with the trailing edge inline with your head or your chest? Obviously not stopping suddenly would be best, but we don't always have that option.