Is THIS Really The Future of Jet Engines?!

[@MentourNow]

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[@RanjakarPatel]

please make video four hindi four make equalitee all humanitarians. i from rajasthan india. i speak xcelent four english but many my familee wanting four enjoi youre video. but very very difficultee four language only enlgish my dear. i wait four you remember martin luther king dreams, and I pray four you make video four all societee.

[@raananh-w2j]

For sure, new battery technology, in 5, 10, 15 years from now, will be much lighter and more energy storage, allowing very thin airplane wings (no need for fuel tanks inside them). Aircraft manufacturers should at least start to plan totally new designs for aircrafts based on electric engines (propellers?). Electric engines requires no maintenance, they are not noisy, weigh much less, and you can put many engines on a wing. Actually one possible design is that whole fuselage shape is used to push air.

[@RanjakarPatel]

@@hayleyxyz i have wife.

[@RashakantBhattachana]

@@hayleyxyz वह खुशहाल शादीशुदा है।

[@alexdhall]

​@@RanjakarPatelUh Petter doesn't speak Hindi so I doubt that's going to happen anytime soon.....

[@nigelbond4056]

I’m neither an engineer nor a pilot, just a fascinated lay person. Your detailed explanation of technicalities are so good, I understand them. Thanks Petter 🙏🏻

[@mattball2700]

As a former P&W engineer, thanks - this was fantastic!

[@MentourNow]

Thank you, too! Glad you enjoyed it!

[@marhawkman303]

@@MentourNow Yeah it's not rocket science... but.... adjacent.

[@stephenspreckley8219]

@@marhawkman303 🤣

[@danisyx5804]

ok, so how does removing the ducting make it more efficient?

[@allenklingsporn6993]

​@@danisyx5804 Think about the difference between a jet and a turbofan engine. The bypass air on the turbofan that doesn't make it to the core is what actually produces the majority of the thrust (which is why turbofans are more efficient than true jets). Now, remove the casing from turbofan. You'll have even more "bypass" air, that flows at lower velocity (increasing total thrust), and you'll also gain back some losses due to case drag. All for the same roughly the same fuel burn, allowing you to use smaller engines and less fuel for the same loads.

[@jeffhatmaker817]

Thanks for the video. My first job as an aircraft technician was on Learjet 23, 24 25 aircraft in St Louis, Missouri in 1986. The engines were GE CJ610s and burned 2,000/lbs/hr first hour fuel burn. They were straight jets (no bypass) and were deafening on takeoff. The acceleration on takeoff is legendary for a civilian aircraft. One of our aircraft was serial number number 23-010, which was the 10th Learjet made in the 60s. It was N500BF which was originally owned by Broyhill Furniture. Good memories.

[@Lovesausage269]

lol I got lost near the end and was thinking how did a furniture company get into building jet engines.

[@noah4987]

Those engines are so loud that they're not allowed to fly in the USA anymore. My A&P school has a Lear 25 that was donated to it because the installation of new engines would have been cost prohibitive to the last owner. Technology sure has come quite a long way.

[@jeffhatmaker817]

@@noah4987 That’s correct. Even in the late-80s, Lear 20-series aircraft were being sold to buyers in South America. My company sold a beautiful Lear 24D to someone in South America for $245k. Two years after my first job I transferred to Jet Corp which had Lear 35s. First hour fuel burn was 1,500/lbs/hr and much quieter and more reliable.

[@KuostA]

@@SevenCostanza What's the point of commenting this u NPC smooth brain pleb? I care.

[@gator1984atcomcast]

Distributed propulsion with electric motors should be a good option for jet engines. Jets should be used to generate electricity. Laminar flow and lighter stronger materials are better than more powerful jet engines. I fly electric flying wings with autopilots. I was an engineer on the SR71.

[@fWfwwaw]

I absolutely have no idea why KZbin recommended this to me, but I'm absolutely in love with the content. Seriously, what a great video. It literally made me want to pay attention to aircraft engines every flight I take from now on 😂

[@keithv3767]

Yeah, sometimes KZbin knows our interests better than we do. Scary but also very helpful.

[@wsshead]

As an aircraft mechanic, I really enjoyed the GTF vs. LEAP LPC/HPC/LPT/HPT break down. NOT nerdy at all, it is very interesting and the animations were excellent!

[@REOswedewagon]

<a href="#" class="seekto" data-time="497">8:17</a> I think Petter's personal mission is to find the fastest possible way to say "anyway" without losing intelligibility, and he's doing an impressive job so far.

[@MentourNow]

I’m always trying to improve!

[@jillcrowe2626]

​@@MentourNowyour English pronunciation has improved to the point that you sound like you've lived in Southern California since you were 16 years old. I worked with people from all over the world and the Swiss Germans sound like you, but their pronunciation wasn't nearly as good as yours.

[@awg6397]

Oh I remember the videos from his living room, first time I heard him say "jaw" it took me a minute to realize he was saying "yaw" Pronunciations have come such a long way since then

[@Juan-qv5nc]

@@jillcrowe2626 That's because he's Swedish: the Germanic language root plus the coolness of the Swiss, but without the mess of Danish, nor the oil of the Norweigans.

[@CptDallas]

I like his accent. It's pleasant.

[@nanotyrannus5435]

Fun thing about modern turboprop planes, you can't put oxygen in the blade strike zone. Which isn't usually a problem since even if you have a tank you usually put it elsewhere. But in the case of the A400M that limits the area able to be used for medical transport units (at least in normal operation, in wartime they would likely just ignore it if necessary) since the units have independent oxygen tanks.

[@Danstaafl]

I actually got to see the UDF of the late '80's fly back in my days as a skydiver. In fact one of our jump pilots was a GE Test Pilot at the time. It was as loud as it was efficient. We all agreed.

[@theharper1]

Was the noise due to the contra-rotating blades though?

[@hsttrek1]

Yes, the sound was like a P51 Mustang.

[@barrycox7922]

The basics: The air velocity over a fan blade is basically the vector addition of the air speed entering the fan in the direction of travel of the aircraft and the fans tangential speed. These two vectors are at rightangles, so for example (unducted fan), if the aircraft is travelling at Mach 0.7 and the fan pitch is 45 degrees then the fan velocity will be Mach 0.7 and the air velocity over the fan blade will be (square root of (0.7^2 + 0.7^2)) = 0.989949. The above assumes the fan is on a zero angle of attack and producing no thrust. When the fan is producing thrust the Mach number would exceed 1.0 and shock waves would exist reducing efficiency. Ducted fan: The duct around a fan changes the environment the fan operates in. Before the fan the duct forms a diffuser (expanding duct) this slows the air velocity entering the fan, increases the air pressure and temperature and therefore it reduces the mach number. Because the air entering the fan is at a lower Mach number the fan can do more work on the air without forming shock wave. After the fan and straightener vanes the duct forms a nozzle which accelerates the air, reduces the pressure and temperature. Unducted fan: The fan must operate with air entering at the aircrafts velocity, which means it must operate at higher Mach numbers than the ducted fan or at very high pitch angles. This means the fan cannot do as much work on the air before shock waves are formed. Very high pitch angles tend to produce more swirling of the air rather than thrust. Conclusion: To fly with an unducted fan would require the aircraft to fly slower. What is the fastest a propeller driven aircraft can fly at in level flight (much slower than a Jet). An unducted fan is only a multi-bladed propeller. How's that for geek?

[@desertstar223]

Ask your mom

[@markg7963]

While I would never have been able to articulate “why” as well as this, my intuition has been saying this inside my head, that ultimately the un ducted fan and its problems bring more problems to the table, and ultimately I wonder if the market is willing to withstand these limitations (limiting engine choices, slower speeds, etc. ) something tells me this may not all play out for the unducted fan. I can see it on shorter hop medium sized planes perhaps. But ultimately it’s a bit “niche “. Boeing for example is in a pinch already time wise, and I’m wondering if financially they can wait for this to play out. They are not in a place where they can guess right now.

[@NicholasStreet]

Exactly what I thought.

[@topethermohenes7658]

The unducted fan seems to have a bigger core than traditional turboprops. Maybe in high altitude, high mach regime, the gear would further slow down the fan while the core picks up the slack meaning more speed. In the low alt regime (below fl250) it would have the efficiency of a turboprop and above FL 250 it would be as efficient as the PW gtf

[@raybod1775]

Great geek comment. Geeks rule the world!

[@michaelcagle5938]

As an arm chair airplane enthusiast I have often wondered why propeller design hasn't kept evolving further....possibly slowed down by the advent of the jet age. I still think propellers or some evolution of an exposed airscrew still has much to offer us.

[@nufosmatic]

<a href="#" class="seekto" data-time="230">3:50</a> - I worked for a company that vended a real-time UNIX machine to P&W to support simulation of the PW4000 high-bypass turbofan targeting the 777 test flight and production. It was the first engine approved for a test flight entirely upon simulation results. The machine emulated the Hamilton Standard electronic engine control by running the FORTRAN codes that would run in the HS in a commercial machine in a separate rack. Happy to see the PWxxxx name applied to a new generation.

[@ClockworksOfGL]

I had a conversation with one of the leads of the JT9D program (ie, 1st gen 747…late 1960s) and he admitted his department- the engine control system - was responsible for the first commercial 747 flight being delayed. The control system was apparently a total Rube Goldberg setup, with hydraulics and solenoids and relays at every turn. It also didn’t help that Boeing made the mounting struts for the engines too wimpy and that lead to all sorts of trouble with engine casing flex.

[@o0SingingInTheRain0o]

As an aerodynamics and performance engineer, my CS friends are always surprised when i tell them 80% of the codes i work with are written in FORTRAN. Even currently maintained/updated programs, although thankfully the newest ones are no longer fixed format

[@NibsNiven]

@@o0SingingInTheRain0o Nothing wrong at all with Fortran. It still dominates the compiled scientific & mathematical application space (with Python ruling interpreted code). Fortran has been modernised over the decades. Fixed format has been obsolete for over 40 years though. That's some _old code_ mang!

[@boballen4665]

Another excellent video. The 777X looks a good replacement for the 747, but the various delays in certifying it mean the design is now quite old and you wonder how much more life it has in it if yet another delay in entry into service takes place. A 350neo could soon be announced. Bob Allen

[@PavelKostromitinov]

<a href="#" class="seekto" data-time="720">12:00</a> "Get a little bit nerdy"? Hooray!

[@Juan-qv5nc]

Yeah, it's cute that he kind of apologises for that when, on the contrary, that's the part of the video we look forward to.

[@mrxmry3264]

yeah, nerdy is good :-)

[@chrishb7074]

‘ De-swirl’

[@jcrawford5569]

@@Juan-qv5nc

[@jonathanbair523]

Like Weird Al says in his song "I am white and nerdy" LOL so bring on the nerdy!..... I do enjoy how Mentour gets into the nerd area, with out going to deep making it for a select few high level understanding. He gets nerdy but keeps it so just about anyone can understand.

[@TheCreditDisputeCenter]

I hope RR gets the ultrafan done, so maybe we can see a comeback of the super heavies in a 2 engine configuration, imagine a return of the Queen!

[@MentourNow]

I have a video on a 2-engine 747, based on exactly this idea, if you want to look it up!

[@tin2001]

Would a 2 engine 747 still really be a 747 though?

[@RevolverOcelot79]

@@tin2001 The airframe makes it a "747" Not the number of engines..

[@DominicMazoch]

​​@@MentourNowBut is not the 777 the Queen without the hump? In terms of carrying pax. With 2 engines.

[@w8stral]

Uh, that "queen" hump is why it ain't the queen anymore. It wasn't the 4 engines which made it fuel inefficient or costly to operate buddy.

[@sumikomei]

I absolutely love it when you get technical and "nerdy" - more of that please, I like the technical details c:

[@johnp139]

I’ve been hearing about this for FOURTY YEARS!!!

[@philipp280]

I really appreciate the technical depth of this video. The brief but insightful footage in the first half added significant value. Thanks for sharing!

[@MentourNow]

Awesome, thank you!

[@MegaKernii]

​@@MentourNow I think you nailed it, without going too nerdy for us aviation enthusiasts

[@sharoncassell5273]

I don't know if we'll ever have the ultimate perfect engine: fuel efficient quiet high thrust and not expensive all in one package.

[@Dmitry_IMHO]

In one size, suitable for all type of aircraft - from J3Cub to 787 - and reparable in the garage for low cost.

[@BlackStar2161]

Cheap, efficient, high thrust - you can only have 2 of the 3.

[@Mentaculus42]

Let us substitute “reliable” for “cheap” as cheap is not what is going to give us efficient and efficient is not negotiable.

[@ThatAviatorKyber]

Why did it say you posted this 3 days ago and the video was uploaded 3 hours ago today?

[@Synergy7Studios]

On a long enough timeline all technology will become perfect. I'm sure the guys designing radial engines never considered the existence of the the massive turbofans and their monocrytaline or composite blades.

[@TheBullethead]

As you said, this sort of engine was experimented with decades ago. There were 2 main problems, 1 real and 1 pax perception. The real issue was noise. At the time these experiments were happening back in the 80s, even the then-new and rather big high-bypass turbofans on things like DC-10s were VERY loud, right up there with most turboprops of the day with Garret engines. And yet these open fans were even louder. All this noise imposed noise-abatement regulations and take-off and procedures we're still stuck with 40 years later. Also, the noise was a symptom of vibration that accelerated fatigue on the airframe. Thus, even while fuel efficiency was upped, the downside was too much. The pax perception issue, at least in the US, was with replacing cleanly cowled jet engines with external prop blades. This was seen amongst the pax public as a retrograde step in the jet age. I mean, turboprop airliners were a thing in the US from the 50s into the 90s but were replaced by regional jets, even if less efficient, because turboprops were perceived as inherently slower, louder, less comfortable, and less "cool", etc. I imagine the same perception would prevail if open fan engines were tried today. Of course, the airlines could tell customers to suck it up. I mean, they've already done this on the vast reduction in interior space for both pax and carry-on bags since the 80s, before regional jets started flying routes previously flown by DC-9s/MD-80s and such, and the replacement of wide-bodies on domestic routes with 737s and A320s. It's like, "OK pax, we know you hate this downgrade in space but it's either that or we go bust and you're back to riding trains cross-country. Oh, but wait, you can't do that anymore because air travel killed most passenger trains. You don't want to have to drive there yourself, do you? Didn't think so. So shut up, sit down, and buckle your seatbelt."

[@baldytail]

Planes do have a weight limit,. Most of the aircraft flying now were built when people were less fat.

[@christopherg2347]

<a href="#" class="seekto" data-time="537">8:57</a> Ship design suffer pretty much the same issue with their propellers : Cavitation on the leading edges from high rotation speeds. Which is logical, as both water and air are effectively following fluid dynamics.

[@reiniernn9071]

Also easy to change with more blades (even more propellors if neccessary) to keep trust the same and slower rotation speeds (at the tips of the propellor). Only negativ side...You'll never get your ship faster than to the speed that the blades can push water...However we can also go around this....(at higher costs of course) Also better efficiency with steg turbine systems (engine upgrading? ) in a ship. Sorry. link in German language. de.wikipedia.org/wiki/Gas-und-Dampf-Kombikraftwerk

[@gustavgans8278]

@@reiniernn9071 I doubt that the GuD would be able to adjust quickliy enough to varying engine loads, as they appear on ships. Shore based powerplants can simply run at their ideal load, which isn´t achievable on ships.

[@reiniernn9071]

@@gustavgans8278 I'm sure that no turbine system can adjust quickly. But , like diesel electric trains...or submarines...it is easy to adjust power when you use that powerplant for producing the needed energy and electric engines for drivetrain...This allows for sudden changes in engine loads. I told it would be costly to do it good...but not impossible. (Electric alternator efficiency is up to 98%...an electric engine also. And if needed an engine controller...at least 95% efficiency...)

[@wesselgroenewegen59]

Indeed, but we should differentiate between the two as we can compare the flows in liquids to supersonic flows in gasses, with some caveats. Cavitation can be sort of translated into the shockwaves in gas flows, yet they differ in compressability. So it's a good but complex comparison.

[@donaldgrove1971]

This idea was abandoned 35 years ago for mainly two reasons. There is the problem of not containing a broken blade, turbofan engines have a strengthened shroud around the fan area to contain the blade. Second and most troubling is the noise. An open rotor could not meet the noise criteria.

[@foximacentauri7891]

The electric car was also abandoned 100 years ago, then 50 years ago, and then 30 years ago. And today you see loads of them. Sometimes an invention is ahead of its time and needs better technology to really be practical.

[@charlesjohnantau5199]

Yes, i remember reading about this in Popular Science in the 80's, always wondered why it never caught on.

[@ElliotNess-jg8gt]

@@charlesjohnantau5199Electric cars were originally marketed to women as the cleaner more dainty option than a oily man car with a complex engine. This was eventually manipulated by the oil industry and eventually gas powered cars became ubiquitous.

[@HSS_yt]

the combination of those truss-braced wings and RISE open fan engines give me a sort of nostalgia because of how it resembles to aircraft a century ago. neat video! this was a great watch

[@AbrahamArthemius]

Reject modern design, embrace the aviation pioneer aesthetic.

[@Alex462047]

It's funny how they go backwards in order to go forwards, isn't it? You gotta wonder.... 🤔

[@LTVoyager]

@@Alex462047It all depends on how you define forward. Is flying slower and lower really moving forward?

[@Alex462047]

@@LTVoyager I don't know, mate. To tell you the truth, I'm finding it all a bit boring, really. Slower, lower, more eco friendly. 😒 They sell us shit and try to convince us it's gold. Did anybody try making Concorde more efficient? Or was the Paris crash just their way to make flying more beige. That plane was so marvellous that you didn't even have to fly on it to be inspired by it, it was enough simply to know that it existed. I don't see anything like that these days, nothing really inspiring, despite Petter's best efforts, God bless him!

[@ZiggyMercury]

@@LTVoyager My thoughts too.

[@douro20]

The GE36 UDF demonstrator engine was based on the F404. It used the low pressure turbine of the F404 plus an additional counter-rotating stage to drive the two propellers, eliminating the need for a gearbox. It was substantially quieter and produced more thrust than the Pratt & Whitney/Allison 578DX which used a gear driven counter-rotating fan on the back of a modified Allison T701 turboshaft engine.

[@Ninthofnine1969]

Nerd alert 😂

[@gcorriveau6864]

Re old turbjets/DC8s making '...a lovely sound in my ears." ... Air Canada had a small number of DC-8-40s with RR Conway turbojets which employed Very Aggressive noise abatement techniques towards the end of their service lives. This comprised a full thrust takeoff and maximum! climb (whoopee) to 1,000 feet, followed by an immediate bunt-over and engine thrust reduction to almost idle (in the cabin it sounded like a 4 engine flameout). Then it flew a level flight segment at this low power until clear of the noise sensitive zone. It will be fascinating to see what the future holds.

[@danfuller4189]

The Conways were bypass engines. Still loud though

[@NathanXxplosion]

Were there any announcements made before take-off to let the passengers know about that? That would be a very unsettling take-off if you weren’t prepared.

[@Holland41]

@@danfuller4189 Yes, the Rolls-Royce Conway was the first turbofan.....but still loud as you say.

[@bur_n_t]

my father was a pilot for the military. wanted to fly all his life just like my great granddad, who also flew for the military. we're not close anymore- he wasn't a very good father. i've always loved technical engineering sorts of things like this. he never saw that, because he never cared about his children more than he cared about his interests. i wonder if he watches you, too. it seems like something he would do. maybe in another life, if he paid attention, i'd be working on these engine designs. i absolutely loved this video. i hope to see more like it. i wonder if my father even knows how much i love his interests.

[@geoffreyjames3107]

Me again I just saw the ATR 72 that lost no.1 on short final. Everything worked out great until they stopped and started evacuation. People were taking hand luggage, refusing to drop the 1.5M from the ATR to the ground. Passengers walking around. Passengers walking through the stopped fan blade arc. I think it’s time you do the “Surviving a plane crash” How to get off a 7700 landing. Walk to the place directed or to the edge of the run / taxi way. Plug doors (not an …. The self opening in flight model). Helping others. Keeping your belt on during the entire flight. What to do if you are down and you are only survivor . There is half your script filling in the blanks and you will have another brilliant video from Mentor Pilot.

[@kenbrown2808]

I do like the sound of a turbojet, but I don't think I'd want to listen to it full time. what I've learned about airplane engines from these videos, is that the more air you can push backwards in proportion to heat produced, the more efficient your engine is, and the biggest limitation on that is your blade efficiency falls off a cliff when the tips approach Mach 1. therefore the design goal is to have as much surface area of blade, at the optimum angle to generate as much blade loading as the structural integrity of the blade allows, as you can get; while having the overall engine structure designed to minimize air being pushed in directions other than backwards in normal operation. (of course, thrust reversers are there to do the opposite of that) I would have 2 primary questions: first, the animations showed both engines rotating in the same direction, but wouldn't it be better for the handling of the aircraft to have them counterrotating? or are the airflow and gyroscopic characteristics negligible in that size range? second, wouldn't it be a boost in efficiency and noise reduction to put a duct around the RISE engine? the other question would be if there is an engineering reason they would go with the gullwing, instead of mounting the engine above the wing? I can see design reasons and maintenance reasons why a below wing mount would be preferred; and I recall your explanation that putting the engines inside the wing root made it impractical to upgrade from turbojet engines to turbofan engines. but I'm curious if it affects the structure and stability to simply move the engine from below the wing to above.

[@camojoe83]

Put a duct around it? Like maybe a ducted turbofan?

[@iskierka8399]

Gyroscopic concerns are non-negligible, but small enough that the cost of a reversing gearbox and the asymmetrical parts production is rarely considered worth the cost. One of the few examples where it is is the V-22, because such large rotors, only half with a wing behind them, are extremely dependent on flow direction. And while ducts do reduce noise, generally, they are not actually an efficiency gain - though this depends on the specifics. For most engines they are an efficiency loss, but gain in ability to approach the sound barrier - which for engines heavily biased towards jet thrust, means a gain in the operational efficiency that can make it worthwhile. Rise is attempting to cross the gap, but that means sacrificing the duct to allow higher efficiency, and having to find other optimisations to approach the sound barrier instead.

[@camojoe83]

@@iskierka8399 lol whatever. It's a contra rotating turboprop.

[@kenbrown2808]

@iskierka8399 rather than a reversing gearbox, i would expect the entire engine to be mirrored.

[@tomarmadiyer2698]

This "summary" is awesome. I learned more from reading that than my entire 37 years alive on the same topic

[@JulietHerty]

Strong people make as many mistakes as weak people. Difference is that strong people admit their mistakes, laugh at them, learn from them. That is how they become strong.

[@FlexibleToast]

We've been hearing about unducted turbofans, ultra-high bypass turbofans, etc... for decades. It's definitely in the "I'll believe it when I see it" category for me at this point.

[@ASDasdSDsadASD-nc7lf]

Next version will be all electric with power stored in supercapacitors an recharge on descent and slowing for landing. If they slow down enough from enough height they will have more energy when they land than when they started and can immediately take off again.

[@BedrockWarriorGames]

@@ASDasdSDsadASD-nc7lf That doesn't even make sense

[@ysesq]

@@BedrockWarriorGames only because you forgot about the nuclear power plant powering it with a large extension cord.

[@armwrestlingprofessor]

​@@ASDasdSDsadASD-nc7lfPerpetual motion machines do not exist brother

[@stephhugnis]

There are 2 AN-70's in service with the Ukrainian air force powered by D-70 unducted turbofans with a pair each of contrarotating fan blades. The design dates to the 80's. It really isn't anything new. Just a compromise between a turboprop and a turbofan.

[@mikebel74]

“A lovely sound in my ears”. Music to mine. I lived under the takeoff path on most days at Dulles Airport in VA in the 1980’s. The Concorde. What a sound! A low frequency roar that would stir the intestines. But I loved it! We would always run out to see that magnificent delta winged aircraft go right over us. Even the awesome SR-71 went over. I think I creamed my jeans with that one. Wow! There were still turbojet or very low bypass powered turbofans then. Loved the sound. Still do, when I’m near a military base like Pax River in southern MD. The soundtrack to my life since 1962. Yes, I’m am old geezer. But I still get goosebumps listening to that awesome low bypass roar. Thanks for your channel. You’re the boss.

[@zagonialpar9208]

Verry verry well put together video. I love the fact that you lay out all this information in a way it's easy to understand for everybody, while still including a good number of details in it. Great stuff as always!

[@theharper1]

My wife and I were plane spotting (taking stills and video) at the local airport on the weekend, and it was really noticeable how much quieter the A220-300 Pratt & Whitney geared turbofan was compared to the CFM56 on the 737 and A320, or the CF34 on the E190. Given how loud turboprops like the Dash-8 are in comparison (at least to a passenger in flight), I'm really dubious about the propfan meeting noise abatement requirements.

[@rainerkinzinger555]

Haha! Nice!😁 never knew my comment would be part of this video 😎 <a href="#" class="seekto" data-time="394">06:34</a>

[@keyserzoom9154]

It will be interesting to see how electrification will change engine design.

[@LiamDennehy]

I had the pleasure of bumping into a Dresmliner pilot on layover a few weeks back and got to talk with them about their work in their own language thanks to the content you and similar producers make. Was fantastic to demonstrate my insight and connect with their passion for their work. Thanks!

[@arandomperson920]

I you think you mean Dreamliner cause idk what a Dresmliner is.

[@LupeCoded]

Dude is so thorough and pleasant with his presentation I dont even fast forward thru the ad spots. Very informative and fascinating program sir.

[@myth-n-m4yhem]

By the time it is certified, it will have been through so many safety checks of course I'd travel in an aircraft with them. I will trust in Petter's vetting of them down the road.

[@StefanoBorini]

MCAS says hi

[@tuank624]

@@StefanoBorini This has nothing to do with the mcas

[@krmould]

@MentorNow On the issue of engine mounting, can you do a video on the reason why above wing engine nacelles are rare? For example, the P-3 Orion uses the same engines as the C-130 Hercules. The Hercules has a high mounted wing with engine nacelles hung below the wing, while the P-3 Orion has a low mounted wing with the engine nacelles above the wing. If aircraft manufacturers are considering new designs for the CFM RISE engines, rather than build a gull wing design, why not move the nacelle from the bottom of the wing to the top of the wing? That way, the wing box can stay where it is, but the centre of the engine is moved significantly higher off the ground (allowing the larger blades).

[@iskierka8399]

One of the major reasons is maintenance, as under-wing engines are accessible on foot, with no additional height required (for most parts). Granted some of this is lost when the parts to maintain are not mounted in a large duct, and so high off the ground anyway. There will probably be evaluation of other ways to access the engine, such as designing the wing to be walkable so crew could access via the aircraft, but there is a good reason almost all aircraft tended towards the under-wing design over the past half-century.

[@marconawijn405]

Another reason is that the upper surface of the wing is much more important from an aerodynamic efficiency point of view. So in general you want to minimize disturbance over the top surface.

[@jukeseyable]

1 issue is servicing on the wing. airlines like it cheap, thus the engines need to be easily accessible, so as close to ground level as possible is prefered. the blade size on the CFM necitates that they will need more ground clearence, hence the gull wing, but they if placed above the wing maintainence crews would need to be on the wing itself, this is best avoided

[@Mentaculus42]

@@krmould The Boeing YC-14, saw that fly at an air show at NASA AMES once. That aircraft had some impressive performance. So the concept has been proven with real flying hardware. See “Coandă effect”. Unfortunately this configuration had 7% higher drag.

[@johnhaller5851]

If the mounting pylon fails, a bottom-of-the wing engine tends to start dropping at the front, so when it falls off completely, it's initially pulling itself to the ground, and not into other parts of the aircraft. An engine mounted above the wing could damage the wing and/or empennage, as the front of the engine would start to go up. While there haven't been any pylon failures since the DC-10 with improper maintenance, but I'm guessing the 777X thrust struts failing could cause an engine departure if both struts failed.

[@johngibson3837]

That gearbox is pretty much the same as a hub planetery drive in truck or in my case for the lower gears in unimog, very good way to transfer power

[@erwan44150]

Please keep it nerdy & technical because this is what the viewers of this channel are looking for here. Precise, technical and accurate everytime ! Love it so much ! 🥰😍

[@jaredgriffiths1361]

I'm so glad this guy is a trainer as he reminds me of a great teacher I had at school. He can take difficult and complex information and present it in a simple way. More than that, he clearly has a passion for what he does, which is one of the best ways to encourage the next generation. These videos are so interesting and actually make me less scared of flying, something that hundreds of flights hasn't achieved!

[@KarpKomet]

This feels like a passive aggressive response from a team of engineers who are asked repeatedly to make a engine with a higher and higher bypass ratio.

[@Alex462047]

I am not an expert on any of this, but I have followed hydrodynamic design, specifically propulsion units, more specifically submarine propulsion units (thanks, Sean Connery😂), and I have questions. The move in ship building has been away from open propeller designs to use shrouded designs, since they control pressure differential spillage over the ends of the propeller blades better. To that end, it would seem to me that similar inefficiencies would exist in open fan designs, and I wonder what issues the designers of the CFM engine have run into, regarding this question. Incidentally, I can entirely relate to high speed inefficiencies. Classic bladed ship propellers are very prone to cavitation (water boiling on the low pressure side of the blade, then that steam bubble collapses loudly as it rolls off the blade). The solution submarine designers came up with in the 1980s was a shrouded water pump, the "propellor" of which looks more like the elongated compressor blades of a combustion engine turbo or turbine in a hydroelectric power plant. The aim of the game was not to shovel the water through, but to more gently accelerate it down the slower-moving vanes, using the shroud to hold the water in the pump, which removes the low pressure issues of a classic propeller spinning at high speed, while generating even higher thrust. The principle of operation is almost crudely simple, but highly effective, very quiet and actually very energy efficient. It's only because of the legendary secrecy of the submarine community that this technology isn't better known, though it is, by now, already for a long time not a secret.

[@Mentaculus42]

Interesting observations. To get answers to your questions would require the company releasing more information.

[@Alex462047]

@@Mentaculus42 Yes, I suppose it would. The trouble is, of course, that this is new technology, and still in development, so they won't be saying much just now, I would think. We'll probably hear all about it in approximately a decade or so.

[@Rasta8889]

I think a big thing that you completely glossed over is that these engines have prop pitch control which means they'll operate more efficiently at slower speeds which means better climb performance (including single engine climb speed). To take advantage of that, you'll need planes specifically designed with those engines in mind. This is a long term evolution project, not a quick "let's wring 2% more range out of a 737" kind of thing.

[@XJapa1n09]

That video you made a year and a half ago about RISE was so good. Was waiting for this video very patiently. Thanks!

[@mack.attack]

<a href="#" class="seekto" data-time="900">15:00</a> thank you for using the phrase "downhill from here" correctly. it means it's easier/better from here yet people keep using it to mean getting worse

[@mxg75]

The fuel efficiency vs speed trade off always leads to some interesting math. As crews are usually paid by the hour, reducing flight time saves on labor costs, which can offset some higher fuel burn. (Long travel times and hourly crew wages is also why Amtrak is not competitive on long haul routes. An airline needs to pay their crew for a few hours to go from Chicago to LA; Amtrak needs to pay for two and a half days of labor for the same route.)

[@TheCatLady65]

First, transition to high speed rail. Second, fuel burn isn't just the price per litre you pay, there are external costs to the planet that need to be taken into account.

[@danharold3087]

Sounds like Amtrak will be introducing robots to reduce crew costs.

[@danharold3087]

​@@TheCatLady65 stronger individual property rights, environmental protections, and requirements for community consent can present significant challenges for high speed rail. High-speed rail projects in the US are likely to create significant legal work

[@christianellegaard7120]

On the other hand, a train can carry a couple of thousand passengers at a time, while a plane can handle a few hundred.

[@markmiller2263]

Thats my take, crew wages against millions in pounds a train can handle vs a plane that's a tough math question.

[@LiamDennehy]

That A400M with half contrsrotating props is just gorgeous.

[@ttystikkrocks1042]

I think I speak for the majority of the regulars of your channel when I say that WE WANT MORE NERDY CONTENT N

[@EVH3730]

I loved this "nerdy" video more than many others! I learned so much about the new engine technology under development in very little time. Thank you 😊

[@danmcbride6258]

I enjoy your Page and it’s very informative. On this video you spoke about new engine development. I was waiting for you to speak about the Trbofan engines in the Airbus 440 or the Turbofan engines in the C-130’s or the the Tubofan engines on some of the Antonov aircraft. With these applications, there must be advantages of using these type of engines as they develop enormous power and thrust. 🇨🇦

[@lhw.iAviation]

Would be nice to see how they can reduce the size of the core of the engine to get a higher bypass ratio

[@MentourNow]

That is one thing they Are working on

[@Mentaculus42]

AND yet when when a design increases the “COMPRESSOR RATIO” for higher efficiency the volume of the core decreases (and thus size) BUT the “ENERGY DENSITY & MAXIMUM SUSTAINED TEMPERATURE WITHIN THE HOT SECTION INCREASES”. As it is the reduction of the core size vs increased power output have been significantly diverging with each new engine generation. In addition to higher temperatures in smaller spaces, numerous efficiency losses do not scale advantageously with more compact cores (blade tip losses, etc). So as is usually the case, it is complicated and full of compromises. Generally speaking, more compact cores are already a design and reliability problem.

[@RZ-ey9jk]

Basically the GTF core is already the size of a business jet engine. For the huge RR ultrafan there were studies to use a BR725 core (Gulfstream 650) with just one additional LP turbine stage.

[@ps.2]

Just add a chain drive like the 1903 Wright Flyer. Move the engine out of the way of the fan entirely. (:

[@Tmaxar]

I’m sure you must be an excellent pilot…but I have no doubts you are one of the best, if not the best, narrator and presenter on KZbin.

[@A-Milkdromeda-Laniakea-Hominid]

Has trouble saying "ratio" which I'm sure is his Swede mother tongue peeking through. It's endearing as he's such a nice dude.

[@JonDoe-ln6nl]

Absolutely love this guy! He cured my fear of turbulence.

[@Name-ot3xw]

We made a fighter with a supersonic prop, all reports say that it was just awful to be around. Ground crew reported becoming ill from being in the vicinity.

[@hemidaytona2378]

From Wikipedia: The Republic XF-84H "Thunderscreech" was an American experimental turboprop aircraft derived from the F-84F Thunderstreak.

[@Rasta8889]

The TU-114 apparently was horribly loud for that reason.

[@kukuc96]

@@Rasta8889 The Tu-95, which is a strategic bomber the Tu-114 is based on is even louder. That also has supersonic prop tips like the Thunderscreech. But they are the fastest turboprop aircraft out there, being very nearly as fast as passenger jets.

[@MaxCarponera]

Engineering is compromise. And the more sophisticated the engineering, the trickier the compromise and balance will be. Excellent vídeo.

[@amosshaltiel9719]

I envy the way you explain the technical stuff. I lecture at a university and always examine how information is transferred to the audience. The new animations are so helpful. Although I am not from the aviation industry, I find it a very interesting subject, both technologically and economically.

[@MostlyPennyCat]

It's funny, I've grown up listening to ONLY turbo-fans. All of the 1980s and 90s common airliner ones, love them. And then there's the Bristol-Siddeley Pegasus in the beloved Harrier Jump-Jet. I love the sound of the Pegasus starting up. The core of the Pegasus was the Bristol-Siddeley Orpheus Turbojet.

[@paulqueripel3493]

Its big brother, the BS.100 used the core from an Olympus. With the equivalent of afterburners in the front ducts it had up to 35000 lbs of thrust.

[@MostlyPennyCat]

@@paulqueripel3493 Was that for the supersonic harrier? My dad spent a large portion of his career on Invincible so I grew up around harriers, great fun.

[@paulqueripel3493]

@@MostlyPennyCat yes, the P.1154.

[@roberts9095]

While we're on the topic of abbreviations, LEAP actually stands for Leading Edge Aviation Propulsion. Very nifty marketing on part of CFM.

[@roderickcampbell2105]

One needs really good marketing when your engineering is poor.

[@roberts9095]

@@roderickcampbell2105 To their credit, CFM engines have typically been more robust than the competition.

[@roderickcampbell2105]

@@roberts9095 I don't why I said that. CFM engines are fine. Something must have been on my mind. Sorry.

[@Mentaculus42]

<a href="#" class="seekto" data-time="738">12:18</a> “Gearbox strong enough to handle a lot of thrust” → “a lot of torque”. Gears handle the torque, bearings handle the unbalanced thrust. But then again, a gearbox has bearings, but those thrust bearings would be similar to the thrust bearings in a non-gearbox version of a turbofan.

[@ScottieAltmann]

If you live near an airport and don't like the noise now, better consider moving. Propfans are incredibly loud !! I worked on the NASA Gulfstream with the experimental prop spinning the Allison design at Lockheed.

[@itetecnun]

Lovely video, really well done. I love this kind of air technology videos where a little bit of theory, physics and practical engineering implications are put together. Really well done!

[@Mwhusa1957]

:30 Boeing is skeptical about it because they finally got an engineer in charge😂.

[@flymachine]

One thing I’ve seen in my lifetime which spans the launch of the A300 to the A350 is that airliners tend to remain conventional, unconventional ideas and concepts help improve the conventional but almost never make it into passenger carrying production, the most radical concept that became reality was split scimitar winglets but everything else has been developments of conventional technology and designs and I don’t think that will change, we may see radical designs in production in the distant future but we will get there in slow increments of conventional developments.

[@oadka]

Increasing pressure from legislation can change that....maybe?

[@peterjones6322]

GE90 composite fan blades were not conventional and made it onto production aircraft. Launch customer was British Airways.

[@ShadowOppsRC]

Legislation rarely fixes it and causes prices to sky rocket. New tech takes money to develop Aand time to get working. Non of which bureaucrats understand or even care to!​@oadka

[@SmileyEmoji42]

@@oadka Difficult because of the time and cost from design to mass rollout and existing fleet sizes.

[@wjhann4836]

Well - two things (I live near Germanys DLR) - they had a plane with some kind of rise design. It was dropped since it made an incredible amount of noise - like a table saw. - During a open house at DLR some scientists (in conjunction with water propulsion) were talking about benefits of that "mantle" around the prop. This leads me to the bypass engine design. What I would like to know from developers is, why they "stuck" with some 12% of bypass while they talking about much higher numbers at Rise engines.

[@7177YT]

I learned a lot I didn't know about those engines today, Thank you!

[@MajorCanada]

This is amazing! Love how we covered extensively and carefully the ins and outs as to why it may or may not be viable!

[@Buddha_the_Pug]

The graphic starting at <a href="#" class="seekto" data-time="860">14:20</a> has both engines high pressure turbine stages labeled as compressor stages.

[@UAL320]

<a href="#" class="seekto" data-time="860">14:20</a> The slide is written incorrectly, but we know what you meant 👍🏻

[@scottdickson9224]

<a href="#" class="seekto" data-time="560">9:20</a> look up the Thunderscreech. Yeaahh...

[@Holland41]

And the Fairey Rotodyne.

[@krozareq]

The math better defines what a prop is versus a turbofan. They are efficient in different speed regimes. This becomes apparent when calculating the Power_available (horsepower) which is a property of thrust (pounds) * flight velocity (ft/s). A prop aircraft uses its highest thrust typically only for takeoff, which is a fairly low speed affair (V). Thus, thrust and power are very different things in aviation. With turboprops (or piston props), our horsepower (Pa) stays roughly the same but thrust decreases dramatically as we increase velocity. Let's use: For turboprop thrust: T = (Pa_(shp)*375*eta)/V For power availability (turbofan and prop): Pa = (T*V)/325 Pa = power availability in HP (shaft hp for the first), eta = prop efficiency of 0.8, T = thrust in pounds. Feet-per-second is used here for V. For example, an ATR 72-600. A PW127M engine produces about max 3,365 shaft horsepower (shp). Props typically have an 80% efficiency. With a typical rotation (Vr) velocity of 113 KIAS, the thrust is around 5293lbs. Conversely, at 250 KIAS it's producing about 2390lbs. However, the power availability (in horsepower) is roughly the same at only about 15HP difference Now, let's use Petter's 737-800NG as an example. A CFM56-7B engine produces about 11,760 HP. At Vr, using 140 KIAS here, the thrust is about 27,300lbs at 11,760 HP. At 250 KIAS, the thrust is about 27,300lbs at 21,000 HP. That seems like an error in running the numbers as the thrust in the ATR drops significantly as V increases but power remains about the same. Whereas the 737-800, as V increases, the thrust remains the same but the power increases. *Bear in mind with aspirated piston engines, manifold pressure will drop significantly with pressure altitude. Palt losses would have to be factored in for those engines. The POH will have a graph that shows efficiency by altitude. This is because propellers lose efficiency as the air speed increases. A prop produces thrust through the difference in pressure in front and behind the "blades." As air speed increases, this difference decreases. However, the mechanical nature of the engine still produces roughly the same power. Turbofans (jets) ingest some of the incoming air pressure directly into the combustion chamber and compressors. This design allows the jet engine to increase horsepower as it's a function of both thrust and velocity. As such, jets are far more efficient at higher speed regimes, as well as higher altitudes since the ingestion hitting the compressors acts as a "blow down" turbo while the airframe experiences lower drag. As we can see with the CFM Rise, it still produces significant exhaust gasses. The RISE and other open fan designs may appear similar to something like the NK-12 engines on the TU-114 or An-22, but the design is dramatically different. They either have an open duct in the center or an open cowling between the fans and stators.

[@karlthebarbarian9875]

One of the issues with the external fan design was excessive noise if I recall. Though I might misremember, but I remember them from "future air tech" books from the 80s. If they can make it work this time it'd be interesting!

[@jonnyueland7790]

I am a CFM56 mechanic, and I am very sceptical to the Rise Engine. More sound, more likely for birdstrike, more dangerouse for ground crew and not able to adjust speed exept by tuneing Rpm. The future will be Geared TF with adustable exhaust or Hybrid El Jet. Just wait and see.

[@MooRhy]

It has pitch control

[@jonnyueland7790]

@@MooRhy Easyer to have a calibrated duct.

[@freshname]

the nerdier the better

[@DAWesome_]

I can't wait to see these next gen engines make their way into my MRO over the next few years.

[@sharoncassell5273]

In the 60s I've heard sonic booms and windows rattled but I liked the exciting fact that the plane was passing the speed of sound. When I watch your videos I rewind to reiterate some parts, never skip like I do on certain channels to get through quicker. Your team certainly have enhanced the show. Thank you. Resubbed, Shan.

[@MotoMarios]

This is layman-technical. And we want more of it. Truly nerdy would be to see the thermodynamics equations and efficiency graphs of the engines described.

[@michaelpipkin9942]

I saw your "interview" on the thing with the thing. It was good!!!

[@MentourNow]

Thank you! Glad you liked the “thing”

[@alexdhall]

Wait where was this interview on this "thing"? Interested people would like to know.....🤔

[@CAHSR2020]

@@alexdhall I'm assuming they meant the recent Veritasium video.

[@michaelpipkin9942]

@@MentourNow Veritasium. Hahaha. All joking aside, thank you for your videos.

[@markedis5902]

<a href="#" class="seekto" data-time="1169">19:29</a> What are Boeing worried about? How long have you got?

[@MiaNeagu-c8n8n]

Each of your videos is a true work of art that brings joy and inspiration!🤗🍾🌠

[@MentourNow]

Thank you! 💕💕

[@xaviersavedra711]

I like the uniwing look of the Truss Wing design. One long wing connected to the fuselage at the center and supported by pylons.

[@c182SkylaneRG]

My recollection, at the start of the video, is that something like this was tried a few decades ago, and it was abandoned for being exceptionally loud. Ducting your fans allows you to avoid blade-tip problems.

[@christianellegaard7120]

A high-bypass turbo-fan engine is, essentially, a turbine driven ducted fan. I find it odd that going back to an un-ducted fan would result in increased efficiency.

[@leonfa259]

It's more efficient since the amount of air moved is much larger. Kinetic energy (fuel use) is =.5mv² while moment while moment (thrust) is =mv so the larger the mass of air l accelerated and the less the exhaust velocity v the more efficient it gets.

[@christianellegaard7120]

@@leonfa259 That's a question of bypass ratio, not whether it is ducted or not. In general a ducted fan is more efficient since less air is lost to the sides.

[@Rasta8889]

A turbofan has a nozzle at the back for both hot and cold sides, a ducted prop does not. These do not behave the same way.

[@christianellegaard7120]

@@Rasta8889 Yes, that's pretty much the definition of the two. What's your point?

[@Rasta8889]

@@christianellegaard7120 That a high bypass turbofan is not essentially a turbine driven ducted fan like you said. One has got EPR, the other doesn't.

[@davidorth4906]

My opinion. 60 Year old pilots like Pure Jet Engines. 50 Year old pilot's accept low bypass engines. 40 Year old pilots like High -bypass engines. But ...30 Year old pilots like a Huge, Jet powered, propeller engine. But 20 year old pilots are Scared of a Cesna 172.

[@ianriggs]

Lol

[@rodneyblackwell7477]

Don't hold your breath for anything involving NASA.

[@silsed]

Sorry to be offtopic, but I would like to congratulate for the collaboration with Veritasium Channel!!! wonderful video!

[@aprilborne1406]

Actually I like the idea of open fan or we can called it propfan machine. More noise but efficient than turbofan yet higher speed compare with turboprop classic engine. Actually there are technics to reduce the decibel level, for example using more shorter blades with flexible round, so the sounds will be just buzzing instead of screeching or screaming.

[@hansolofsson6403]

<a href="#" class="seekto" data-time="882">14:42</a> i think there is a typo here " 8 high pressure compressor stages and 2 high pressure compressor stages"

[@MentourNow]

You're right!

[@stephenholland5930]

Also incorrect on the LEAP-1A.

[@camtonyray666]

Hey Petter, I hope you don't mind me for saying this but you repeated "Ratio" incorrectly. "Ratio" is pronounced ray-shee-ow in both American and British English translations.

[@ADHJkvsNgsMBbTQe]

I’m glad I’m not the only person who noticed this. It sounds like he’s saying ray-THEE-oh but I’m used to pronouncing it ray-SHEE-oh. I’m thinking maybe that’s just how some people say it.

[@CAHSR2020]

Another big one that always drives me nuts is "STAB-alized" instead of "STAY-balized."

[@mack.attack]

If you could understand someone without any trouble and you're not their English teacher, "correcting" someone's speech is so f___ing rude.

[@camtonyray666]

@@mack.attack I disagree. If I could ignore it I would but unfortunately I couldn't and that is because it's incorrect and it's something that a professional wouldn't willingly do had they known or been told about it.

[@robkom]

@@camtonyray666 Totally agree! Everything he says sounds fine to me, but when he said "ratio" it thew me off; it just sounded so out of place, like he was saying just that word with a lisp.

[@RubenvanKuik]

You know it's gonna be a huge success if Boeing is sceptical about it haha

[@PsRohrbaugh]

Replying to your reply on my last superchat regarding hybrids: you can apply TONS of torque to the outer fan blade instantly. Not only will this produce instant thrust, but it will increase the performance of the jet engine very quickly. The performance increase would be akin to an afterburner. You would build the electric motor assembly directly into the rotating parts of the jet engine. Once you had taken off, gone around, performed a terrain escape maneuver, or similar - the same coils and magnets would apply load to the engine to recharge batteries. And I say batteries - but honestly super capacitors might be a better fit. The goal is to provide a burst of power on short notice. By doing this, you can decrease the maximum thrust of the jet portion of the engine. You can optimize for cruise, since your TOGA thrust comes from the electric component. Maybe a more robust APU plays a part of this. But electric motors are the exact inverse of turbines - instant torque. It just feels like a good pairing.

[@davidorth4906]

Embraer is on it. A 300 person, 3x3 seating arrangement, 32" isle, 3 bathroom model... Only Coach class. With Huge turboprops that are jet powered. Ok. Sounds fun !!!

[@erasmus_locke]

I think a good way to put it is jet engines need a gearbox just as a car does. Without one you'd need an absolutely enormous engine to have both the torque to pull from a stop and the power to drive at highway speed.

[@MentourNow]

Well not exactly, here the gearbox is not changing the gear ratio. It is a fixed gear reduction mechanism.

[@cjmillsnun]

@@MentourNow More like that on an EV

[@Flies2FLL]

They have been talking about UDF technology for 30+ years. Yes they are efficient, but the problem has always been NOISE.

[@MentourNow]

They have solved that now

[@tomg6284]

Lol, I will believe that when I see it.

[@danieldonaldson8634]

<a href="#" class="seekto" data-time="405">6:45</a>: God knows that when even Boeing has reservations about something, we all should worry

[@That_droper]

?

[@That_droper]

Ohhhh 😂😂😂😂 ture

[@kashkaMH1]

😂😂

[@travellerworld-s6i]

Can you do a video on how Airlines and plane manufacturing companies worked the best cruising heights and best take off and landing practices?

[@Granny_Cat_Lady]

I love a bit of nerdy Mentour ... that's what I came here for ✈

[@smithandshortdogs]

Sorry not an expert but these still seem like fancy turboprops...

[@davidbastow5629]

Turbo props with fancy gearboxes, to be more precise 😂

[@camojoe83]

That's because that's what they are.

[@TurbopropMaster]

Switch to Airbus if its Boeing I'm not going

[@TyroneElectra]

The rain pelted the windshield as the darkness engulfed us.

[@jacobmoriancumer7588]

<a href="#" class="seekto" data-time="548">9:08</a> - I'm havin' flashbacks to the "Ear Banger". AKA, *The loudest frackin' plane ever made*