 What might look like a step backwards is actually two giant steps forward on the future path of advanced aircraft engines. Since 1941, the Lewis Research Center has developed an international reputation for its research on jet propulsion systems. However, in 1977, the researchers at Lewis began to take a new look at the propeller. Their challenge was to combine the efficiency of the propeller with the power of the turbine engine. What has evolved is ATP, the Advanced Turboprop Project. Keith Severs, manager of the NASA Advanced Turboprop Project office at the Lewis Research Center. Back after World War II, jets were the coming thing. Propelers were very efficient in those days, but they couldn't go fast. They couldn't go to high altitudes. Quite frankly, they were not jazzy like jet engines, which gave way to turbofan engines and high bypass ratio engines. When fuel was cheap, people didn't worry much about it. Ten cents a gallon, as long as they get speed, high altitude capability, which the air transport system required. Propelers just kind of withered on the vine for 20 or 30 years. The Arab oil embargo of the early 70s not only hit American consumers at the corner gas station, but was also reflected at the airline ticket counter due to the soaring price of jet fuel. In 1973, the price of a gallon of jet fuel was 12 cents and represented one quarter of the direct operating cost of the jet. By 1979, the price had risen to 63 cents a gallon and represented half of the direct operating cost. In 1981, the cost of jet fuel was over a dollar eight cents a gallon. With the rising fuel prices, a fuel-efficient propeller engine again became the object of an aeronautical propulsion research effort. The researchers' objectives were to develop a power plant that would be fuel-efficient, do .8 Mach, cruise at 35,000 feet, operate at a reduced noise level. These propellers look a lot different. They're very highly swept. They're very thin compared to old propellers. And the thing that hits you first is there are eight to ten blades on these rather than three or four as you've been accustomed to in the past. And what the sweep does in the propeller in a thinness is to reduce the drag losses at the higher tip speeds and higher Mach numbers. It also helps to reduce the source noise of the propellers. We've got a higher blade count and we've loaded these blades up much higher than old propellers so that we can get a lot more power at a lot smaller diameter. And this saves weight on the propeller, saves weight on the engine, and it also packages better on the aircraft. Earlier turboprops were limited both in horsepower and in flight speed, mainly because of the compressibility effects that occur at the propeller tips. With the recent advances in computer design technology, we are now able to optimize the blade shape to minimize the compressibility effects occurring at the blade tips, allowing the planes to fly faster up to 600 miles an hour and at higher altitudes up to 35,000 feet. As you approach the speed of sound, you run into a problem where the apparent pressure that you're trying to push through suddenly takes a quantum leap. And because the blades are very large in diameter to 13 feet in diameter, the tip speed is upwards of 800 feet per second, which is just under Mach 1 or the speed of sound. As you try to push through that Mach 1 range, you run into this compressibility factor. It's almost like running into a brick wall with the blade. The blade construction is now a composite type arrangement where you have a metallic leading edge such as aluminum and a graphite epoxy-resigned internal construction that allows the blades to be very lightweight and because they're lighter weight, minimizes the centrifugal stresses that occur when the propeller system is rotated at 12,000 rpm. The ultimate goal is to save fuel, commercial aircraft and also military aircraft where it can be applied. And this fuel saving is very dramatic compared to the fuel burn that aircraft have today, such as a 727, 737, that type of aircraft. A prop-fan-driven airplane can do the same mission at like 40 to 50 percent of the fuel that they use today. If you look at just the US fleet existing today for medium to short range aircraft, and I'm talking 727s, 737s, DC-9s, MD-80s, those aircraft in a typical year burn about 5,000 gallons of fuel. If these aircraft were equipped with prop fans, they could do the same mission they're doing today and save two to two and a half billion gallons of fuel per year. Propeller-driven airplanes have traditionally been noisy. Advanced turboprop researchers are tackling the problem of trying to build a turboprop to rival the relative quiet and smoothness of the jet. Well, we've had to look at the entire aircraft as a system. We didn't look just at sticking a new widget called a prop fan on them. We had to look at what did it do to the rest of the aircraft. And particularly we're concerned about passenger comfort. It doesn't matter how much fuel we save. If the people don't like it or they're uncomfortable, sound, vibration, then they're not going to ride it, so it won't matter. So part of our project goal is to make sure that people have the same comfort that they're used to on today's wide-body aircraft. In addition, we're concerned about community noise. These things are allowed and we have to provide the technology so that they can live within the existing far-thirty-six and our stage-free regulations and the laws that we have to abide by around airports and communities. Most of the time, we really don't even know what's running inside the cockpit. It's that quiet. And we're only 15 feet away from it. After several years of wind tunnel and static engine testing, a full flight test of the advanced propeller system was held on May 19, 1987 at Lockheed, Georgia Company in Marietta, Georgia. So far, operation has been very good. Everything is about as predicted that the engineers had predicted from their initial assessment of the program. We haven't really had any big surprises, I should say. When you pull the prop fan in and add power to it, the pilot really knows it. But there again, this airplane wasn't really designed to fly with an engine out on the left wing. But again, what our engineers predicted is pretty much what's happening with the airplane. So I'd have to say, so far it looks good. So far it looks very good. If all goes to plan, prop fans will probably be fitted on short to medium-range aircraft. We hope to have, as part of the project goal, to have the technology in hand by the end of the 1980s that industry, the engine people, the aircraft people can make marketing decisions because it involves a lot of private capital that makes or breaks their company. So the way things are proceeding right now, it looks like Boeing and McDonnell Douglas are aiming for new aircraft engines with prop fans in, say, the 1991, 92, 93 timeframe.