Physics enabled pawn works in 4.15 but not in 4.21

I have modelled a helicopter using physics nodes to drive it. I have got it to work pretty nicely in 4.15.
It has a root component static mesh (its body) with physics enabled. Below this it has more static mesh components that extend the body - these do not have physics enabled or else they drop off… Two of these (rotor and tailrotor) are used as locations to ‘add force at location’. They were targets of the addforce node but now 4.21 complains that they do not have physics enabled.
I have made the body as the target instead but now, the physics seems to work near the centre of the map (0,0,0) but as the model moves away from the centre the control gets worse and worse until it seems not to respond to inputs at all and nose dives into the ground.
Does anyone know why this might have changed. I have read what I can find about the changes moving to 4.21 and they claim that physics will work the same (some changes to your code if C++) but the ‘add force’ node is not working the same for me.
Could this be due to the way the model is constructed from several meshes or is it a physics issue? I have also read that people have had problems with 4.21 and destructible meshes - mine are not set up as destructible though.
I thought it was about time I moved to a more recent engine version but now I am not sure which way to go. Perhaps I missed a change in an earlier engine version that I should know about?
Any clues would be gratefully accepted.