The S-72 from the 1970s can be seen as a sort of spiritual predecessor to today’s Future Vertical Lift program. It was all about creating a vehicle that could take off and land vertically like a normal helicopter but could also fly fast and far like a plane. But while the FVL’s top contenders are logical new versions of existing aircraft, the S-72 Rotor Systems Research Aircraft was a plane and a helicopter duct-taped together and filled with explosives.
Sounds fun, right?
The RSRA was a joint effort by the Army and NASA, and the Sikorsky Aircraft company was the primary developer. Sikorsky is the company behind the new SB-1 Defiant, and they’ve long searched to push the envelope when it comes to helicopter design.
You can actually see some elements of the SB-1 Defiant in the S-72. The S-72 was basically another Sikorsky helicopter, the S-67, with wings and turbofans added. It packed two TF34 engines, the same things that keep the A-10 in the air. Strapped to an S-72 with stubby wings, these engines could send the aircraft through the sky at 345 mph.
But the S-72 was also a helicopter with five rotor blades, so it could take off and land vertically. These blades were not propelled by the TF34s, though. Nope, those were powered by the original two T-58 turboshafts from the S-67.
But that’s a lot of engines to strap to one small aircraft. What if something goes wrong? What if you need to suddenly escape?
Well, that’s where your ejector seats come in. Yup, you could rocket yourself out of this bad boy in an explosive chair. And if you’re thinking, “Wait, didn’t you just say there are five rotor blades spinning above the pilots?” Then, congratulations on paying that much attention! But also, don’t worry, because Sikorsky strapped explosives to those rotor blades, and they would blow off before the pilots flew out.
It’s all pretty cool, if not exactly practical. The program eventually fell apart for the normal reasons. It was simply too expensive and technologically advanced for its time. It did fly multiple times, but always in either a full helicopter configuration or full jet configuration with the rotor blade removed. It never flew at 345 mph with that rotor blade flapping in the wind.
Without getting too heavy into the physics in an article written for you to read on the bus or in bed or whatever, there’s a very real reason that high-speed flight with rotor blades spinning up top is tough. Helicopters have what are called advancing and retreating blades. The advancing blade is the one moving in the same direction as the aircraft, and the retreating blade is the one spinning to the rear.
The advancing blade would generate a lot more lift because it’s moving so much faster through the air, and this effect is increased the faster the helicopter is flying. Engineers have a few ways of getting around this problem, but it gets trickier the faster the helicopter flies. That’s part of why Chinooks top out at about 200 mph in level flight. So, a helicopter flying 345 mph would face a huge problem with “dissymmetry of lift.”
The S-72’s method around this was a system that would stop the blades and hold them in place as part of the X-Wing concept. Basically, the aircraft would’ve gotten a new rotor blade with four large blades instead of its normal five. When the aircraft was flying as a jet, the rotor blades would be locked in position and would generate lift like traditional wings. One S-72 was modified as the X-Wing version, but it never flew.
Meanwhile, if the SB-1 Defiant lives up to its design promises, Sikorsky thinks it will fly at almost 290 mph. If so, it will be the fastest production helicopter and still be 55 mph slower than the S-72 that preceded it.