- Boeing and Airbus has both
upgraded the engines to their 737 and Airbus A320 families
during the last decade. The new engines that they fitted
are a whopping 14 to 15% more efficient than the older ones
but now the CFM RISE engine promises to completely eclipse
this performance and strangely, we almost had this type
of engine three decades ago. It's a really fascinating
story so stay tuned. (playful chime) In a recent video I did,
I discussed the surprising announcement that came from Boeing
that they're not going to launch or even start designing a new airliner until after the end of this decade. Now, obviously, there's a lot of details
about what this really means and how it's affected
by Boeing's position relative to Airbus. And if you wanna check that video out, I can link to it up here. But one important factor
in Boeing's strategy is likely the expectations
of a new technological leap or a step change in engine efficiency. As I said in the beginning,
the CFM RISE is a central part in this but to understand why,
we have to talk a little bit about how jet engines
have already been evolving over time. Now if you compare the jet engines
of older aircraft with today's, you will see that they are
constantly getting larger in diameter and that's because for a jet engine
to be able to generate more thrust, it has to either push
the air backwards faster or push a larger air mass
at the same speed. The latter option: more air, same speed, is the more efficient option
and this is why turbofan engines have been getting bigger
and bigger by time because that have enabled them
to increase their bypass ratio, meaning that they use a bigger fan which can push more air around
or bypass the engine core. However, a problem with making
these engines larger and larger is that they are also getting heavier. Even so, modern high bypass turbofans are still more efficient
than the older designs despite the weight penalty
that comes with encasing the whole thing in
a bigger and bigger nacelle. But there are other ways
for a jet engine to push a really large air mass backwards. One option that the engine makers
and aircraft manufacturers were exploring in the 1980s
was the propfan also known as an unducted fan or an open rotor. In their simplest form,
open rotors works like turbofams except that the fan
is not encased in an nacelle. In order to have the necessary efficiency, thrust and a manageable diameter,
these early open rotors had two counter-rotating fan discs. These fans had a variable pitch,
unlike the fan blades on the turbofans that we have today and that increased their efficiency further. A number of companies
worked on these designs but the ones that were developed the most in the 1980s were the Pratt & Whitney Allison 578 Delta X-ray and the General Electric GE36. The Pratt & Whitney Allison variant
built on previous research that Allison had done on a propfan design for NASA. General Electric also based its design on NASA research and actually had
the French engine manufacturer, Snecma, as a minority partner on the project. General Electric and Snecma were also partners
in the CFM56 joint venture. And today, Snecma is known as Safran. And, of course,
the CFM joint venture is still ongoing. Now, the reason Pratt and Whitney
and General Electric were developing these propfans was because
one of them would be chosen to power the Boeing 7J7, Boeing's planned replacement for the 727. Now, of course, technically,
the 757 was the 727 replacement but it was substantially bigger
and had a longer range than the 727 did. And this was appreciated, obviously,
by many older 727 customers but not everyone wanted
to upsize to the 757 so Boeing was developing the 7J7
to be a bit smaller than the 757-200. Of course, today, we know
that this didn't happen. Boeing instead developed the 737NG
by enlarging the 737 to replace the 727 and to compete with the then
upcoming Airbus A320 family. But it's worth remembering
that the development work on the 7J7 was quite far ahead when it got scrapped. Boeing never launched the project formally but it discussed it with a number
of different airlines who wanted to replace their 727s, DC-9s
and even their earlier 737s. The 7J7 would make use
of Space Age materials and would have an advanced
cabin layout and avionics. And, by the way, Boeing
also circulated pictures of a fly-by-wire cockpit
with sticks instead of yokes. So what actually happened
to this aircraft then? Well, it turns out that what
actually killed it was the engines. In a way, open fan engines
were a bit too far ahead of their time, needing much more
development and innovation. The Pratt & Whitney Allison
578 Delta X-ray was equipped with a gearbox to drive
the counter-rotating fans and there were some concerns
about the reliability of those gearboxes at the time. Boeing eventually chose the GE36, partially because it didn't
have that gearbox. But excessive noise was a huge problem which affected both of these designs. These engines were apparently
even noisier than the low bypass turbofans that the MD-80
and the 727 used back then. Now some believe that the noise problem was a bit exaggerated
but it's worth remembering here that today's CFM LEAP
and Pratt & Whitney geared turbofans are much quieter than those old turbofans who the open fan engines
were compared with. So today, they would definitely
be considered as really noisy. And, finally, another factor
that didn't help these engines or the Boeing 7J7, for that matter,
was the fact that the oil prices dropped in the late 1980s. And with that, airlines became
less interested in the idea of a new, noisy plane that would be more efficient but used radically
new unproven technology. So Boeing shelved its plans indefinitely. At the same time, McDonald Douglas
were also considering an MD80 derivative, fitted with either of these engines
but the airlines just weren't interested. So why are we discussing this now then? What has changed since
those early designs that would make the CFM RISE
not only possible but also desirable? Well, I'll tell you all about that
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Now back to the video. Obviously, fuel efficiency
is really back on everyone's focus today. Boeing has come out and said
that its next airliner would need to be at least 20% more efficient in order to be truly competitive. The prospects for truly
great efficiency improvements in open fan engines were obvious
even before the 1980s which is why their development continued
even after Boeing and McDonald Douglas shelved their plans
in the late 1980s and 90s. And during those years of development, the engineers learned a lot
about the technologies that could make
open fan designs really possible. And actually, this research
was a process that worked great both ways. For example, General Electric,
worked on composite fan blades for its old GE36 unducted fan
that came in really handy for the GE90 engines that would later
power the Boeing 777. And the same goes
for the CFM LEAP engines for the 737 MAX and the Airbus A320neo. But then there was the question of noise. And immediately, this is where
the French engine manufacturer, SAFRAN, came with some fantastic news. In 2017 and 2019, SAFRAN
built and tested a new open rotor concept. This engine had a similar configuration
as the GE36 unducted fan but using a gearbox to drive
the two counter-rotating fans. Over the years, the gearboxes
have become much more reliable but we'll come back to that soon. The point is that,
thanks to various improvements in design and materials, Safran reported that the noise levels of this engine are the same as those
on the very latest CFM LEAP engines. If this is correct,
that removes probably the biggest objection to this design. But it turns out that was
even more great news to come. I should mention here the excellent work that Bjorn Fehrm at Leeham News has done in explaining the purpose
behind this evolution in design. If you are interested in this,
you have to check out the links in the description below to his work. It is absolutely awesome. Now it turns out that General Electric
who was already working together with Safran on the 2017
open rotor project had found a way to simplify the design. They had figured out a way
to go from a pair of counter-rotating fans to only one single fan
with a second set of non-rotating, variable-incidence stator vanes behind it. That design has now become
the basis of the CFM RISE which stands for Revolutionary Innovation
for Sustainable Engines CFM revealed this project to the public back in June 2021 but it had actually
been in the works since 2019. And it really was a continuation
of Safran's and General Electric's previous efforts. The move to a single rotating-fan from the two counter-rotating fans
on all of the previous projects only became possible
thanks to new advances in computing and fluid dynamics technology that engineers back
in the day could only dream of. And again, development
around lightweight but strong fan blades was also crucial here. Having a single rotating fan means that a much
simpler gearbox is needed, the kind of proven gearbox design that turboprops
have been using for years. And this new layout could also help
reduce noise levels even further. According to CFM, the RISE design
will meet all current and near future noise level regulations. When it comes to efficiency,
expectations around these engines are set really high. The highest bypass ratio
of a modern turbofan in a single aisle aircraft today
is around 12:1. But the design of the CFM RISE could allow bypass
ratios as high as 20:1. Together with weight savings
and other advancements that are part of the CFM RISE project,
CFM believes that a 20% improvement in efficiency compared
to the best engines that we have on the market today is actually a realistic target. And it goes without saying
that it is this 20% improvement that makes this technology
so critical for the future plans of both Boeing and Airbus. CFM plans to have an engine ready
for ground tests around the middle of this decade, that's just a couple
of years away from now. And then flight tests will follow
in the second half of the decade. Last summer, Airbus announced
that it will work together with CFM by adapting an Airbus A380 test aircraft to complete these flight tests. The CFM RISE will replace one
of the four turbofan engines under the wing which will definitely
be a really interesting sight to see. It's worth noting here that Airbus
is already preparing an Airbus A380 for testing with hydrogen-burning turbofans and hydrogen fuel cells
and there's quite a lot of work needed to add a hydrogen fuel system onto an existing aircraft
for testing like that. And this is relevant here
because CFM also wants to test the RISE engine running on hydrogen. So having an Airbus A380
that can carry this fuel will come in quite handy
and do you know what else is quite handy? Clicking like and subscribe
to this channel so you don't miss any of my future videos. CFM also wants the RISE engine to operate on 100% sustainable aviation fuel, which is the goal for all current
and new engines in the next decade. But there are a number of things
that CFM will have to show to both aircraft manufacturers
and to the aviation authorities before these engines will be allowed to enter service. Obviously, the first thing
they're gonna have to do is to validate the efficiency of that engine at the promised
cruise speed of approximately Mach 0.8. In order to be able to fly that fast,
CFM actually had to reduce the diameter of the RISE engine slightly from that
of their previous designs. A bigger fan disc pushes
more air backwards but the tips of the fan blades
also moves faster which can be a problem the closer we get
to the speed of sound. The smaller overall diameter
will also help the aircraft designers a lot when it comes to the engine installation
process and options. Previous open fan designs
required the engines to be installed at the very rear of the aircraft
like on the MD-80 or the DC-9 which is why Boeing went
with this design for the 7J7. But CFM claims that the RISE engine
will be suitable to fit under the wing of both high wing
and low wing airliner designs. Now there are also a lot of questions
of a blade-out which is what it's called when a fan blade breaks
for whatever reason. The fan blades of a conventional turbofan are encased in such a way
that they will be contained in case of a blade failure
like if the engine would hit a large bird, for example. CFM will now have to prove
that this is not a problem for this new design. Of course, manufacturers
of conventional turboprop aircraft are already doing this by reinforcing the fuselage alongside the propellers to be able to stop a blade
that might have escaped. And we actually have a great example
of how that might look like because during testing in the late 1980s, there was actually additional reinforcement for the vertical stabilizer
of the Boeing 727 when it flew with the GE36 instead of one
of its normal engines. Newer fan blades will be lighter
which helps, plus these fans will be spinning slower
than the turbofans that we have today but even so, we will have to wait
and see what kind of protections aircraft with these kind of engines
will need to have. It's likely that these protections
will be similar to what we see on today's turboprops
and obviously, additional reinforcements
for the fuselage will add more weight. So who is going to be the first one
to incorporate this new engine into one of its designs, do you think? Well, frankly it's highly unlikely
that either Boeing or Airbus will let the other push ahead
with a design like this without coming up
with a competitor of their own. This technology would
be impossible to pass up. So the answer to that question
is probably both of them. And also, let's not forget Embraer who have recently announced
work on a new turboprop with a very familiar-looking engine layout. Finally, what is actually the difference between an open fan engine
and turboprops, especially in the case of the CFM RISE with its single-fan design? Well, the difference here lies
in their potential speed. Turboprops generally cruise
at speeds of around Mach 0.55 or so but the open fan designs
will work at speeds around Mach 0.8, with some having pushed
even 0.85 in testing. Now that's very close to the speeds
of today's turbopan aircraft which I am operating,
the 737, for example. But it's also worth pointing out here that the speed difference
is not a definition. In reality, turboprops have also
been evolving slowly towards the open fan layouts
that we've seen in this video. Newer turboprop aircraft have propellers with more
and more fan blades allowing for a higher
and higher disc loading which in turn allows
more and more speed. And also, to be perfectly honest here,
the quickest turboprop of all time is not particularly new, because that will be the
Soviet Tupolev 95 bomber with four turboprop engines
using counter rotating props. It could cruise faster than Mach 0.8 but the tips of its propellers were supersonic
which made it incredibly noisy. We don't know what the flight crews
thought of that aircraft but there are some stories of fighter pilots who intercepted these planes
but couldn't stay close to them for long because of the sheer noise. In any case, I am really looking
forward to see what will come of these CFM RISE engines
and what they will be able to do. Now check out this video next
which I think you're really gonna enjoy or this playlist. If you wanna support the work
that we do here on the channel then consider becoming part
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and I'll see you next time, bye bye.
