Measuring drag in flight
More than 50 years ago I remember participating in measured descent performance tests on the Twin Pioneer over Prestwick, and later on the 748 over Woodford, with all engines shut down/feathered as a means of confirming the airframe drag. I believe it was part of the continual discussion/argument between airframe manufacturer and engine manufacturer as to whose drag or power calculations were valid!
The internal quietness without engines was amazing, but so were the creaking noises from the airframe!
Fascinating times!