July 21, 2023, ©. Leeham News: Developments in engines and airframe technologies require that future airliners are flown differently to maximize the technology benefit.
After looking at the consequences of new developments for the airframe, we now analyze what engine developments mean for how they will be sized and flown.
We have described the advances in engine technology that can be expected to be ready for the next generation of airliners. The most interesting engine size bracket is the one that shall propel the aircraft that will replace today’s Single Aisle generation, the Heart of the Market airliner.
With the Heart of the Market gradually moving up in capacity, the engines have to follow. Today’s Single Aisle engines cover thrust from 23,000lbf to 33,000lbf. We can expect the new generation of engines to cover 25,000lbf to 35,000lbf with a stretch to something like 37,000lbf to 40,000lbf in the future.
We have seen that a very wide wing, like a Truss Braced Wing, reduces the dominant drag at takeoff, Induced drag. The engine’s takeoff performance is dictated by the “One Engine Inoperative” (OEI) case. It means the aircraft shall be able to continue a climb after rotation on only one engine (for a two-engine aircraft) and be able to go around and land.
With wide wings enabled by folding wingtip technology, the required low-speed thrust is reduced. Classicly, the engine size was dictated by the required thrust at Top Of Climb (TOC) or the OEI situation.
As engines are designed with increased ByPass Ratiois (BPR) of up to 20:1 for Turbofans or 50:1 for Open Rotor engines, engine size is increasingly decided by cruise thrust requirements. To understand why need to understand how engines work and how they are affected by altitude and speed.
Engines generate thrust by grabbing air that passes the aircraft and accelerating it to exit the engine at a higher speed than it entered. We call the increase in airspeed “Overspeed.” Engine people call it “Specific Thrust.”
Thrust is then = Air massflow * Overspeed.
Engine designers start by deciding on the Overspeed (Specific thrust) an engine shall have as it sets the engine’s characteristics. Then they increase the Air massflow of the engine until they have the required thrust.
They do this as the efficiency of the thrust generation depends on the Overspeed. High Overspeed = low propulsive efficiency, i.e., we must invest a lot of core horsepower to drive the fan/rotor that generates the Overspeed and thus thrust. Low Overspeed = high propulsive efficiency, i.e., we get a high thrust with less invested core horsepower.
So far, so good. But engines with a low Overspeed, like a Turboprop engine or Open Rotor, lose thrust fast with speed (their thrust lapse is high). It’s because if the aircraft goes from 200kts to 300kts and the Overspeed is 400 kts, our Overspeed difference shrinks from 200kts to 100 kts, or by 50%, and by it, the thrust is halved. If the engine’s Overspeed was 600kts, the loss would be 17% of thrust.
So while a high BPR (which is the way to achieve a low Overspeed) of, say, 20:1 of the next generation Turbofans or 50:1 of an Open Rotor is good for engine efficiency and thus fuel consumption, it poses sizing challenges at the cruise speeds of Mach 0.78 at 35,000ft required of the next generation Heart of the Market airliners.
The sizing challenge is increased by the Air massflow side of the thrust being affected by altitude. At sea level, air density is 1.2kg/m3, but at 35,000ft, it’s only 0.4 kg/m3, a third of the sea level density. It means the thrust of engines, regardless of Overspeed differences, declines by a factor of three. Helping the thrust decline with altitude is that the dominant drag, parasitic drag, also declines with lower air density.
The combined effects of altitude thrust decline and a higher thrust lapse with speed for next-generation engines means these can be sized by cruise thrust requirements rather than Top Of Climb (TOC) thrust or Takeoff OEI thrust.
There are no direct negatives of this change in the sizing of the engines, but rather a positive. There will be plenty of thrust for takeoff, enabling shorter runway airports to come into play. And climb speed can be adjusted to keep the required climb thrust to reach the altitude where cruise will be efficient (ref. last weeks Corner).
With the above, we can understand the problems of the first generation of airliners that used straight jet engines. While their high Overspeed meant that they could propel airliners like Boeing 707 or Douglas DC-8 to high cruise speeds, their low Air mass flow meant that One Engine Out, OEI, thrust was lacking.
Hence, four-engine configurations in the first generations and the requirements for long runways. Also, extra tricks like water injection and JATO rockets were used for military takeoff assistance.
There were even some aircraft projects that installed a couple of turboprop engines to help with takeoff and initial climb. We now know why.
The straight jet’s low Air massflow and high Overspeed characteristics made aircraft takeoff design challenging.