Hobby king had two small cameras perfect for this ! the wingcam, and the wingcam 2. Unfortunately the wingcam 2 doesn't show in stock anymore. The wingcam was around 50? and I paid 85 for the 2 version. but the resolution was SO good! I used it for years but something finally went wrong.. it had been in about 5 crashes and had gone 160 mph on a jet ! Most of the footage on my youtube taken the last few years is with the same camera.

Here is some footage taken with the Wingcam 2. Oh, by the way if you want good clear footage electric or turbines are the way to go. I built this WW1 1/4 scale electric mainly to be a big camera plane when we fly a Dawn Patrol...

https://www.youtube.com/watch?v=FZeN8Ei3orw