Bjorn’s Corner: New engine development. Part 10. Propeller, Rotor or Fan?

By Bjorn Fehrm

June 7, 2024, ©. Leeham News: We do an article series about engine development. The aim is to understand why engine development now has longer timelines than airframe development and carries larger risks of product maturity problems.

To understand why engine development has become a challenging task, we need to understand engine fundamentals and the technologies used for these fundamentals.

Following the last Corner on airframe integration, several comments were made about the definition of propeller, open rotor, and/or fan. So, we’ll explore this further.

Figure 1. Evolution of Wright Brothers propellers from 1903 to 1905. Source: wright-brothers.org

Propeller, Rotor or Fan?

The question of what to call a device that creates thrust by grabbing air with wing paddles and accelerating it backward has been discussed since the 1950s, at least.

The first device was a propeller, which started out as a two-bladed paddle-like design, Figure 1. 

More power, more blades

The propeller remained two-bladed with a fixed pitch until WW2. The first 78 mk I Spitfires had a fixed-pitch two-bladed wood propeller (Figure 2).

Figure 2. The first 78 Spitfires had a fixed pitch two-bladed propeller. Source: WW2 forum.

The next batch of Spitfires got a two-position, three-bladed propeller, which reduced takeoff run by 30% and improved climb and cruise speeds.

As the horsepower of the engines increased from 1,000hp (early Merlins) to 2,000hp (Griffons), the propellers went from two to six blades. The extra blading meant there was more aerodynamic area creating Overspeed into a more homogenous propwash.

Propellers limit the number of blades because of the complexity and, thus, the cost of the variable pitch propeller hub and the cost of more blades. However, the trade-off of propeller diameter for more blades gives lower tip speeds and more ground clearance. Propellers have generally stayed below six blades over time.

The measure used to describe how much of the swept propeller area is covered by blades is the solidity ratio (which is simply: total blade area divided by swept area).

High-speed propellers

After WW2, gas turbines gave much higher power at reduced size and complexity. Apart from jet engines these were used in turboshafts to power propellers, creating turboprop propulsion units.

The typical airliner and military turboprop aircraft flew at cruise speeds below Mach 0.6, where a normal straight propeller would have tips speeds below Mach 1.

The odd project accepted supersonic propellers to gain higher cruise speeds, like the Russian Tu-95 Bear, Figure 3.

Figure 3. Two Tu-95 Bears at a US Airbase in 1992. Source: Wikipedia.

Its four counter-rotating four-bladed propellers are well into supersonic tip speeds at the Tu-95 cruise speed of Mach 0.67, as it converts 12,000shp per engine into thrust. You can hear the propeller sound here.

The way to avoid supersonic flow on the blades at high cruise speeds was to sweep the blades. Research into high-speed turboprops was conducted by NASA in the 1970s and 1980s, where the propellers had higher solidity to transfer more power and swept blades to avoid supersonic flow, Figure 4.

Figure 4. An advanced propeller explored by NASA in the 1980s, called a Propfan. Source: NASA.

Such propellers were called Propfans to indicate that they were suitable for higher cruise speeds.

To fly larger aircraft at even higher speeds, you need more power. To harness this power without using too high a propeller diameter, a dual counterrotating Propfan was used, Figure 5. The counterrotation has the advantage of a ~5% efficiency gain as the engine is now de-swirled.

Figure 5. The Pratt&Whitney Allison Propfan engine flight tested in 1989 by NASA. Source: Wikipedia.

However, the gain in transferred power and efficiency comes at a price. The double rotor hubs, sitting at the back of the engines, fed by a complicated dual gearbox, required another name than a counterrotating turboprop, a turboprop being a rather simple engine in comparison, used for lower speeds.

What to call these engines has varied by manufacturer and teams over the years. The Pratt&Whitney-Allison team called it “Profan”, GE “Unducted Fan”, SNECMA/SAFRAN “Open Rotor”.

CFM, with GE and SAFRAN as owners, finally called it an Open Fan engine when it presented the CFM RISE.

Summary

All these engines, with different names, do the same thing. They convert shaft horsepower to thrust by creating an air Overspeed on the air passing the rotating blades of the engine.

It is up for debate whether you call this device a Propeller, Propfan, Unducted Fan, Open Rotor, or Open Fan. One could say the Propeller is a simple stage, low-solidity device, whereas all the others are high-solidity, high-speed thrust-generating devices.

What term finally prevails will probably be decided which one comes into operation. The CFM Open Fan has a good chance of being the winner.

19 Comments on “Bjorn’s Corner: New engine development. Part 10. Propeller, Rotor or Fan?

    • I was wondering about that too.Seems to have gone out of fashion

  1. I wonder if HPC bleed air could be ducted through the propeller hub and exhausted toward the trailing edge on the suction face of the propeller aerofoil to give a super-circulation lift effect? That might lead to fewer blades and/or reduced diameter with knock on effects to tip speed, max RPM and ground clearance.

    Just a thought.

  2. To me there is enough difference in the RISE or similar engines to deserve its own moniker. OR works well I think.

  3. All this discussion has forgotten a propfan engine did soley power its proposed airlifter the 4 engine Ukrainian An-70. All the other flying propfans were single test units on a flying test bed
    The Ukrainian engine maker Moto Sich and development bureau Ivchenko designed D-27 as 3 shaft axial gas turbine with 2 contra rotating prop fans
    The engine output of just under 10,000kW is above the 8200kW of the A400M D400 turbo prop.
    As it was designed for the Soviet era in Ukraine and main buyer being the later Russian Airforce it wont go into full production. Only 2 An-70 airframes built and flew on the propfan only engines.
    Noise wasnt a problem either.

    I wonder if the GE ‘inspiration’ for having the propeller blades forward came after the D-27 flew ?

    • Snecma has been a big proponent of the OR.

      Who did what when?

      I have not seen any direct reports on baselines of the D-27 and how well it ran and how quiet. Only two aircraft built and 8 engines and no movement to use it since as far as I know.

      So, Boeing comes out with a TTBW and the public is thinking, isn’t that a Cessna? Well its got props (public does not care what GE calls it, props = Prop job)

      And P&W with the GTF is not sitting still. 11 years from now they could do an all new engine let alone the fine tuning and incremental improvements to existing.

      What does CFM do with LEAP? Oh, we are doing nothing as RISE replaces it.

      So give RISE a theoretical 10% now, 11 years from now its still 10% and GTF has closed that to even.

      CFM backed itself into a corner when it went more exotic materials and heat. The crossover is a more expensive engine that does not return better fuel economy (though currently reliability is better).

      Your exotic materials run out cost wise and now what?

      P&W can work with speed ratios and use all the exotic stuff CFM used.

      P&W can NEO the A320NEO as well as NEO the MAX and it can offer the same upgraded engine for a TTBW. P&W looks at the customers, do you want a well proven design or do you want to jump into the deep end of the pool with those whirely thingies?

  4. The one thing I have not seen addressed is how much thrust is derived from the RISE (or any other open rotor?)

    A TP gets a bit maybe but you have all those awkward flows and then to get the exhaust out and away from the engine and airframe.

    WWII the side stack did their thing but I hate to think how loud it was in a cockpit with 12 (V-12 liquid cooler) straight pipes in front of you. At least the various Air Cobra’s had it behind the pilot.

      • Scott:

        I get the overall power, but how much is out the back of the engine thrust like the tube jets?

        Most of the thrust is going to be absorbed/used by the prop.

        Unlike a TP the RISE of course has a straight path out the back.

        The TP I know anything about aka the PT6 has a reverse flow engine and the exhaust stacks are at the front.

        I have not studied any other TP in depth nor have I seen any good informational material on other layouts.

  5. I think airlines are going to be very wary of new engines, designs and promised results from OEM’s.

    Given the problems with almost every new engine (P&W GTF, Rolls, GE) and the time on wing shortfalls, many may just use the tried and true – waiting for something new like an open rotor to mature a little bit.

    Is it worth risking your most expensive assets sitting around, grounded while they troubleshoot teething problems, while customers jump to the competition because you can’t deliver?

    New aircraft are really becoming a crapshoot for airlines.

    • Airlines were getting spoiled by CFM56 reliability, todays situation is more normal with a good number of initial problems, remember JT9D-7, JT8D-200, RB211-06, CF6-6, PW2000, PW4000

      • This is the big problem with replacing the current narrow bodies,most of the problems have been ironed out over 40-50 years and even replacing the last generation of engines with iterative improvements has been immensely painful for customers who have grown used to this peak of reliability.
        Can’t we just fly a little bit slower and use advanced turboprops on the shorter routes?
        It’s a few minutes added to the time of half of the average Ryanair or easyJet flight. I know, I know ,utilisation rates,etc,etc,but aviation fuel is under taxed and we can make them care much more about burning it.For anyone who doubts how effective punitive taxes can be,listen to Michael O Leary bleat about departure taxes that are almost imperceptible to the average traveller,whilst he collects his massive annual bonus

        • Grubbie:

          I have pondered the same question on slowing down, but even when it makes little if any difference (AK/Horizon) they have moved away from a TP (dropped entirely if not now soon).

          While I had no issue with the Dash 8, I can see the appeal of the E175 for public in general, its a very nice jet.

          Something I have also pondered is not just a single flight but the carry on flights. I like real examples, so the SEATAC to Bellingham flight (90 miles as I recall) then goes onto say Helena Montana.

          I think the -8 just went SEATAC/BHam and back, the E175 can do other runs (longer).

          You would have to dig into a route, what the turnaround does etc.

          I did follow one Airline flight in the MidWest that bounced around on shorter regional flights then a jump to the West Coast.

          What you can also loose is quantity of flights. AK used to have a lot of flights to BHam from SEATAC. Now I believe its two a day.

          I was never on one of those -8 flights that was not packed full.

          But I can also say the last few times down there I took the bus. The cost difference had gotten huge. So that may be a factor in some of this as well.

          There is a very good bus service up North of Seattle and sub distribution for smaller towns.

    • Frank P:

      I believe the teething issues are about behind the airlines, it was not new aircraft but the new engines.

      But the base is not just a new engine that has its questions regardless of type, but a whole new type of engine with virtually no development.

      So yea, you jump one fence and then you have another fence ahead of that and the RISE has so many fences it looks like one of those UK Courses.

      P&W is not standing still nor will be LEAP regardless of the RISE hype. CFM has to cover two bases and the hedge has to go with what brings in money.

      And that assumes the RISE is legit, not just a vehicle for CFM to get a new core and gear box so they can do a GTF.

      By which time P&W will have matured their GTF and extracting more and more SFC while keeping it reliable.

      Looking at CFM histyory, both GE and Safran have kept an interested in OR.
      So they both went down that path.

      I really don’t get it. GTF was proven, just not on that size and any smart organization would have at least kept a foot in both sides of research.

      If this does not pan out then CFM is in panic mode, they have to get into GTF. The more they focus on RISE even with a new core and gear box its still putting a whole new package together with its own development issues and risks.

      It looks a lot like RR the the V2500. RR and P&W would have made a good combination using that group as the base for the GTF.

      As it is, P&W has partnered with all the other companies in that group as part of the GTF. Its a different structure with P&W as the listed owner but MTU and the Japanese companies have their share of the program.

      With a full partnership P&W might not have made some of the mistakes they did. The V2500 was pretty successful.

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