We used to sling O-1E Bird Dogs (Think Cessna 175), Beavers, and Otters under the Chinook frequently....and the odd A-37 now and then.
A very religious experience could be had when the 4x4 timbers strapped to the top of the wings as Spoilers suffered an asymmetric loss when one of them escaped its lashing causing a really crazy kind of oscillations that started in an instant as the timber departed. Having one of the critters poke its nose up in front of the helicopter as it waved around flashing a wingtip in front of the gunners on either side got your attention in a jiffy. That was immediately followed by at the announcement of "Bombs Away! Let's go home!". When hauling airplanes we never put the Cargo Hook Switch on "Safe" and our unit SOP was for the FE who was monitoring the load continuously kept his thumb over the Jettison Button or his hand on the mechanical jettison lever...as the lever was quicker opening than the electrical release.
Sad story however, a sister Chinook Unit lost a CH-47C when an O-1 swung up under the forward rotor blades....missed them....but then swung up under the aft blades and did not miss....causing the Chinook to suffer a totally catastrophic deconstruction event at 3,000 feet above ground. I flew the recovery of the wreckage and crew which was an all day job and was a very sad chore. The cause of the accident was the Chinook crew went way too fast and despite radio calls from the Huey Recovery Crew Pilots did not slow down before the airplane collided with the rotor blades. Six crew members died in the crash.
Hueys usually were very docile sling loads but Huey Cobras without a drag chute and swivel between it and the Cobra was a completely story.
For training loads we used to have Lockheed T-33's and F-102's which rode really nice.