Bjorn’s Corner: New aircraft technologies. Part 22. Engine characteristics.

By Bjorn Fehrm

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.

Figure 1. The RISE engine from CFM. Source: CFM.

Engine characteristics

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.

Engine characteristics

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.

What it means for next-generation engines

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).

A bit of jet engine history

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.

10 Comments on “Bjorn’s Corner: New aircraft technologies. Part 22. Engine characteristics.

  1. “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.”

    The NMA thrust requirement was, I recall, about 45,000lbf which was in my opinion one of the problems with it since no existing engine modern existed in that thrust bracket. But that was for a twin-aisle, I’d think for a single aisle NMA 35-40,000lbf would be about right (I also recall that our old CEO A321 operated V2500 engines at 37,000lbf).

    Question – If a conventional turbofan has a bypass ratio limit of 20:1, what is it about an unducted fan that allows a bypass ratio of 50:1. I always thought that the bypass ratio was decided by the pressure ratio and max EGT of the core. If that was right the bypass ratio limits would be about the same. What am I missing?

    • Hi Chris,

      It’s not a core limit. The BPR of a Turbofan is a mechanical limit. The fan and fan case get to heavy above BPR 20; also, you need to gear down the fan speed to not get the blade tips above Mach 1.5 at takeoff. Otherwise, you have efficiency problems. And for that, you need a gearbox with a high ratio; today the planetary principle gives you about 4:1 with a reasonable size and mass gearbox. It limits the fan size and thus BPR as well.

      The Open Rotor BPR and thus fan size is set by your Specific Thrust spec. which has to do with the aircraft’s speed range. It sets the rotor pressure ratio and thus fan size for a certain thrust. A BPR of around 50 is the right one for a Mach 0.78 engine.

    • The IAE V2500 topped out with the V2533-A5 of 33k thrust. An UDF or prop with highly curved blade outer section should allow the normal to the l.e. component of the rotational speed come way down and hence allow a higher rpm. All this possible due to woven 3D structure carbon fiber blades. We will see as the RISE fan blade shape is finalized and the fan rpm is inputted into its Type certificate.

  2. What I think is quite interesting is the topic of blade-off containment.

    Obviously, with an open rotor, there is no blade containment.

    With a closed fan, there’s a blade-off containment pack around the fan perimeter. But if open rotor engines can get certified without one, why can’t a closed fan get certified without one? And, if a closed fan can get certified without a blade containment pack, how much efficiency gain (weight reduction) does that get?

    The point I worry about is that, if in a dash for open rotor we skip lighter closed rotor fans because of assumptions of regulatory requirements, are we perhaps missing out on a bigger efficiency gain? I know that GE are confident of their open rotor being “good”, but a shrouded fan is probably always going to be less of an aerodynamic compromise; you can make the whole blade length, especially the end section, work to a maximum extent. Whereas an open rotor is always going to have losses at the blade tip.

    • Thats what I am thinking, a small shroud should help contain or deflect a broken blade.

    • The slow turning RISE fanblades will certianly get a life limit as there is no containment as for a big prop. There might also be requirements for yearly deblading, clean, NDT inspect, reapply anti-friction coatings with moment weight check and reinstallation with a trim balance check. We will see the FAA/EASA thinking.

    • I think the problem is that the turbofan diameter is still limited, so like an undersized airplane wing that is too inefficient for the amount of lift it needs to create, the smaller diameter length of the fan produces too much drag for the amount of thrust produced (compared to a much longer, unshrouded propeller). And if you simply tried to shroud the propeller without reducing its diameter or increasing its RPM, you’d end up with an even bigger weight problem.

  3. Bjorn,

    Does the RISE engine as currently being developed include variable pitch fan blades or just (as I understand) just variable stators?

  4. In the article, it seems as if overspeed is referred to sometimes as the difference in airspeed before and after it passes through the prop/fan blades, and at other times as the final airspeed (after going through the prop or fan).

    Also, 600 knots at 35,000-feet altitude is about Mach 1.04. Is it a problem to have the final airspeed being above or near Mach 1.0?

    When Pratt & Whitney teamed with Allison in the 1980s on the latter’s 578-DX propfan engine (which had a bypass ratio of 56, BTW — higher than the target stated in this corner), they claimed that it would reduce takeoff distances by 30%.

    With the cruise engine thrust becoming more important a design parameter than takeoff thrust, it would be nice for comparison purposes if the listed engine capabilities would be rated at cruise instead of at takeoff. Because the higher-BPR open rotor engine will have a higher takeoff thrust than an updated turbofan. (Although the RISE might be rated in horsepower instead of pounds, anyway.)

  5. “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.”
    It was a historic trend for engine to get more thrust for more payload and range as aircraft technology on aerodynamic maintain relatively stable. I am wondering if payload and rang fixed as most NB aircraft in market by today and aircraft aerodynamic technology improved significantly, most likely unconventional configuration NB may cost engine thrust goes down. Just like GE9X on more efficient B777X other than GE90-115B on B777-300ER.

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