I think your main issue is that the rotating parts of the chopper are a part of the main body.
to make everything easier, separate the heckrotor and the rotor from the main chopper so you have 3 separate objects.
The bones take care of keeping everything together.
From there you can do a 2 keyframe animation for the rotation.
When you're in the graph editor, you can linerize the graph to get a constant rotation.
I'll do a quick video on it so you can try it for yourself.
This video link should be live in a few hours. Just keep trying if it isn't - I'm uploading it now.
https://youtu.be/BXwSKP8LSAMIn the video you'll see a few main steps.
1) deleting the existing armature (sorry)
2) duplicating the helicopter object two times
--- Deleting the body and top rotor (rotor_rear)
--- Deleting the body and rear rotor (rotor_top)
--- Deleting the rotors (body)
--- You'll note i'm selecting & deselecting the existing vertex groups
3) rotating the helicopter to lie as close as i can to the XYZ axis - trust me when I say it makes animating easier
4) creating a new armature
--- create main body bone
--- create top rotor bone, parent to the main body bone
--- create rear rotor bone, parent to the main body bone
5) parent helicopter parts to the correct bones
6) open the dope sheet and animate
--- you'll see me struggle with this in the video since i'm not an expert with it haha
--- on the graph sheet, once you create 2 keyframes, it draws a line between them. this line defaults to a smooth falloff.. Normally great for regular animations, bad for constant speed rotations
--- I modify the tangent angles of the start and end keyframes to make a linear path between keyframes (There's probably a better way to do this, but I don't know it, and this way has worked for me in the past)
7) I played with the animation framerate to smooth out the animation since we're only rotating 90 degrees over a few keyframes.
You'll have to define the same FPS in your QC file when you compile.Other things to note:
You shouldn't do a complete 90 degree rotation because the start and end keyframe are in the same spot - which will display the rotor in the same spot twice. If you look close enough you can see this in the video.
This is hokey, but we can get a smoother animation with some math: say you want 5 keyframes per 90 degrees.. that's 18 degrees per keyframe. so you want to rotate the rotor 90-18 for a total of 72 degrees.. So the next keyframe is actually 0, but since you have 4 rotors, it mimics the full 90 degree rotation.
Hopefully you can follow along with the video for yourself and learn. Try it over the weekend and if you can't get it, I'll upload what I have and you can just animate the rear rotor.