Get a value that interpolates between two floats based on where character is facing

I'm making a controller script for a paper airplane and I'm currently stuck on trying to make the plane gain speed when facing down but lose speed when facing up.

The ideal way would be having a value that's 0 when facing right or left but drops to -1 when facing up and 1 when facing down (It doesn't really matter w$$anonymous$$ch is 1 or -1 just that they're distinguishable). Then I'd just multiply that value (w$$anonymous$$ch is somewhere in between -1 and 1) by gain and add that to my speed variable like t$$anonymous$$s speed += value * gain.

Here's a picture to better understand.

I already got the information where the plane is currently facing (both in vector form and degrees) but how can I do t$$anonymous$$s?