We all have to think before we speak!!

Your friend made a committment to build the plane for free(less material expenses) You upheld your part of the bargain by paying the materials. He could have at any time backed out if it turned out to be overly difficult or time consuming. He choose to complete the project.

You didnt owe him anything outside of the agreement. Out of consideration however, you offered him something for his effort.
It was the honorable thing to do.

If he is unhappy, there is very little you can do about it.