Quite honestly if your horses are well trained then they rarely take notice of choppers.
I have had them land in the field and although the horses were in close paddocks and not in the same field, they were all at the nearest fence to investigate.
It is much the same with balloons. Horses have no predators from the air so should not be bothered by them!
I was actually leading in five young unbroken horses one evening when a hot air balloon, flame blazing, came very low directly over us. (We were hidden from his sight by trees) Only one horse started to pull away from me. A quick jerk on his halter and the words "You DARE!" and he thought better of it and stayed still.
My horses accept that if I say it is OK then it is and will trust me that nothing nasty will happen.
These are all TBs some have raced and others will be.