That's funny and it reminds me of A&P school when I would ask my instructors "yeah but *_why_* does this or that happen, which *_causes_* this or that?" Finally one day they called me into the office and told me I should consider becoming an engineer instead, so I did but still stayed and got my A&P but I've thanked them for the advice ever since, I would never have had the self confidence without their suggestion! P.S. in my opinion it's because 1) some information is proprietary and 2) they're training parts changers now and not mechanics like they were in the 1950's.

It wasn't long and boring....It was educational and you cant rush learning things of this nature....been watching you for YEARS....Keep up th good work !!!

Having been an enthusiast of jet engines for decades, this is the first time that I fully understand what a variable stator vane and a bleed valve does to airflow.

I was once sitting on the tarmac in a C-130 in Baghdad waiting to go somewhere. Engines were kept idling while we loaded up and sat around for what seemed like forever in the unrelenting heat. All of a sudden there was a large bang and the entire bird shook. It did this 3 or 4 more times. I see 2 crewmembers run out of the cockpit area stick their heads out around the ramp. I asked what that was and they said it was one of the engines had a stall. We don't get the choice to get off and get on a new bird. Luckily, the rest of the trip was uneventful.

thank you. not an airplane mechanic, not a mechanic. I do fix my own bicycle. what you are illustrating is how one can develop intuitive understandings of the everyday items in our lives. this is awesome.

You are a one man university for anyone who has the desire and patience to listen to every single little step to really understand what is going on. One at a time. No nutshells allowed. Thanks.

31:15 Thankyou for the few exceptions. Much appreciated 👍 36:24 And we got to experience another kind of "stall", but it happens, and it doesn't damage anything, just keep on rollin' ❗

ignore the people saying its long and boring, its information that needs to be in context! also its cool :3 keep making the long videos, they arent boring to ppl that want to know things!

6:30- 9:30 Those are literally my exact thoughts and critics about textbooks i wrote today on a piece of paper. They fail to illustrate the topics and they leave it to us whether we can imagine it or not. No surprise that many students are overwhelmed and lost when we have to imagine a lot and correctly connect the dots without proper Visualization.

This is exactly how it was taught in my aerospace engineering propulsion courses, great video

Great follow up and great timing. I had just left a comment on your compressor stall Q&A days ago. In it I was emphasizing that the non linear increase in velocity through the compressor as we increase %rpm is what drives such dynamic AOAs. You mentioned a couple times how “this arrow grows faster than this arrow” referring to the same phenomena. Explaining why beyond just citing compressor efficiency at higher rpm’s would be interesting. The books I have such as the Jeppesen gas turbine engine book is not too specific on this. I’ll be looking for a copy of the book mentioned here as it seems like a great reference. Sometimes thinking about mass airflow and compression ratios at various rpm’s, and relating that to velocity makes my brain ache. Thanks for the videos! Really interesting stuff.

I had a stall I was here at work for at a power plant, LM6000. The unit it approx 100 yards away. It shook the building, it got everyone’s attention. They trip immediately. Then comes the borescope. 😂

i like the way you explained the air hitting the blades from different angles cause the air to mix into even more turbulence instead of be moved to the next stage as one

Slats on an aircraft wing have a similar purpose like variable guide & stator vanes. They also bend the airflow around the leading edge of a wing so the aircraft can fly angles of attack that would otherwise cause it to stall.

Thank you for your videos :)

Another excellent video!

I had a similar problem trying to find(on KZfaq) how a torque converter works. Went though a lot of over simplified junk.

The SR-71 / A-12 engine, the Pratt & Whitney J58 had to deal with compressor stall and COMPRESSOR CHOKING at different flight regimes (possibly simultaneously). The U. S. Patent Number US3344606A, title → “Recover bleed air turbojet”, goes into some detail about this. Is “compressor chocking” sorta the opposite (as in angle of attack of the airflow) compared to compressor stall and the relationship to compressor surge? 43:32 “It is always more complicated than you think!” The safest generalization to live by.

Your first video is great. And yes, I did watch video from Mentour - it was good as well, but from different perspective.

I love Mentour. I believe his "Now" channel is for less well-produced videos than his main channel (youtube algorithm crap I guess) Always prefer longer videos, even if it takes multiple watches to absorb everything.

Piston engine fans will see parallels in the phenomena of a (genuine, not Afterfire of popping rich exhaust stacks etc) "Backfire". The latest miss-hap at Reno air races involved an engine failure of the Sea Fury "Dreadnought". You can find good video of the aircraft experiencing a staccato of backfires (like JayZ says here about compresssor stalls) as the pilot pulls the aircraft high over the pattern to gain altitude. The backfire was caused by half the engine being jammed from a broken crankshaft, thus the still open inlet valves and functioning ignition system repeatedly set the whole induction system alight (that's 28 induction pipes on a 71L engine). The percussion was severe enough to buldge the intake duct. I personally have only ever seen the effect on startup of big radials, too rich priming with burning feul escaping back into the induction system whilst turning too slow I presume. The sound nearly makes me backfire to😁.

Thank you once again for the honourable mention at the end of this video. However, I've only just found time to watch the whole video, because I've been busy, as my daughter and family have been visiting from Singapore. I'll come up with an answer in due course and offer some further comments: however, I must remind you that I was responsible for compressor mechanical design for several years, but not the specification of the aerodynamics. In the meantime I'm going to have one of my pedantic rants (well, you have yours!). On your side of the pond, the terms ‘stall’ and ‘surge’ are used interchangeably, which I think can be confusing, if not misleading, particularly for those of your subscribers you are educating about jet engines and the way they work (but sometimes don’t!). For example, ‘Aircraft Gas Turbine Powerplants’ by Jeppesen (of which I have a copy) shows a diagram of a compressor characteristic: it shows what I knew throughout my career as the surge line labelled as the surge/stall line. So why do I consider that this can be misleading, particularly for those uninitiated in the ways of the jet engine ? Because an engine can continue to run with the compressor experiencing a stalled condition: more specifically, there is a phenomenon that can occur, which is known as ‘rotating stall’. To give a specific instance, I believe that the Armstrong Siddeley Sapphire 6 and the Wright J65 may have been prone to a rotating stall condition when throttled back during a landing approach. I’ve heard a story from an RAF Hawker Hunter pilot, who usually flew an Avon-powered Hunter. On one occasion, he had to do a ferry flight in a Sapphire-powered Hunter, with which he was not familiar. During the approach, the engine was making unusual (to his ear) out-of-tune musical noises. He put this down to the compressor bleed valves opening - but, unlike the Avon, the Sapphire didn’t have compressor bleed valves. I suggested to him that the noises were probably created by a rotating stall event. Of course, if such a compressor stall condition develops, it can ultimately result in an engine surge, when there is a loud ‘bang’ (which may be repeated), with the flow through the compressor reversing and flames emerging momentarily (and sometimes repeatedly) from the engine intake. There are several clips of engine surges posted on KZfaq. The one I recommend is that of a Thomsonfly B.757, which suffered a bird strike on take-off from Manchester Ringway (in England), back in 2007. There are repeated surges of the right-hand engine as it climbs out, before it is shut down. I’m also reminded of an experience I had while waiting for a flight out of JFK back to LHR many years ago. There were three loud ‘bangs’ in rapid succession as a DC-10 (I believe it was) took off. And then, some years later, I was on a B.737 coming into land at LHR. As thrust reverse was selected, there was a loud ‘bang’ and a momentary flash of flame from the right-hand CFM56 engine. As far as I’m concerned, those engines surged: I believe that it’s a far more accurate and descriptive term for such an event. However, I must stress that there is most definitely the potential for damage to the compressor blading when a surge occurs. That's why an aircraft may be taken out of service for a precautionary inspection of the affected engine. This is particularly so in more recent compressors with their higher stage loading and pressure rise per stage, as the rotor blades (and vanes) may experience very large transient deflections because of the rapid and extreme pressure variations during a surge event. If these deflections were imposed statically on the blades, then they would very probably suffer a permanent set: in other words, they would be bent. The provision of sufficient axial clearances between the blade and vane rows to allow for large surge deflections is an important part of the design of a compressor.

Amen to the steps ! Everywhere !

Way too long way too boring? Dull subjects are those we have failed. Additionally thank you for being thorough.

29:58 as I understand it from the description of the SAS flight 751 crash, the procedure for compressor stall/surge is to throttle back the engine, it the case of 751 that was prevent by an automatic throttle system and the engines already damaged by ice destroyed themselves

Thank You Agent JayZ

I still am always learning new things or a better understanding of the jet tech World and I agree that you are able to make this complex subject As if I am working on a small piston engine You do make a very talented educator which I very much appreciate, I wish to update my jet City membership which I will do when finished here, thank you for all the years you have given me Mic

OK Here is the real deal -- Agent Jay Zed, I have been a sub for many, many, many years -- in fact Graham has emailed me directaly to correct my vobaulary -- in the end, it does not matter what others say or think. This channel has always been top-notch and above board. I do miss the River Run videos but hey, things move forward. Graham, if you see this, know that you are uber cool and I honor you (Bravo Zulu) I have to say (Agent J Zed) that one of my most depressing days was when you retired. We will never be the same as no one else gives us the facts (and Graham's Approval) as you do. God Speed Agent Jay Zed.

KZfaqs algo sucks. I am subscribed to many thnigs including this channel. It hasn't recommended your channel for months> I just don't understand.

Why use both systems [to prevent surge/stall]? Probably because it’s the most practicable solution to the problem in each engine design. However, at off-design conditions, different things happen at the back end of a compressor as compared to the front end. But first, let me take you back to the origins of the R-R Avon, which started life as the AJ.65 project at Derby at the end of WWII. Stanley Hooker in his autobiography says, “that it took about seven years before the AJ.65 could get a clean bill of health. Seven years when we had made the Nene in as many months!” That is a measure of how difficult it was to get the early axial compressors right. And for those of you who think that the wartime German engines were “advanced”, think again. In 1944, their axial flow compressors were abysmal, in terms of surge margin, pressure ratio and performance. They were inferior to the single-stage centrifugal compressor in Whittle’s W.1 engine, which powered the first flight of the Gloster E.28/39 in 1941. Even into the latter part of the last century, axial compressor design was far from an exact science. I could tell you a story about the problems with a certain ten-stage compressor, with which I was involved in the late 1980’s. It had to be rescued from major problems with lack of surge margin - but I'd better not tell the story here. So back to the story of the AJ.65: its twelve-stage compressor was a disaster. It was difficult to start and would not accelerate, even though it had bleed valves. Now for those of you who think that GE pioneered variable stators, think again. The AJ.65 compressor was tested with variable stators in 1949, but the engine was made to work with just VIGVs and an improved bleed valve arrangement. So why go for the complication of all those pivots, levers and unison rings? However, the Avon was further improved and turned into a great engine, which has soldiered on for decades, with the incorporation of the proven compressor aerodynamics of the Armstrong Siddeley Sapphire. The old hands I worked with as a young designer at R-R IMD, Ansty, near Coventry (which was formerly an ASM site), back in the 1960s were always rather smug about this. The Sapphire compressor would operate throughout its speed/power range without the need for any variable vanes or bleed valves. However, as I’ve previously stated, it was probably “on the ragged edge” at part-speed with a rotating stall condition. This was indicative that further progress in terms of pressure ratio and the addition of more stages would require VSVs and/or bleed valves - or two spools, which is the direction in which P&W went in the USA. So exactly what is the problem with an axial flow compressor when it is running at low speed and way off design? I'm busy drafting my answer, so watch this space.

Hi JayZ, can you plz do a video on Hydromechanical fuel unit which would be more understandable in your version 😅

Yay!

Amazon has that book listed and available in a 1974 edition used. And only for the Amazon Prime price of $4,995.00 USD...

So exactly what are the problems with an axial flow compressor when it is running at low speed and way off design? Before I go any further, I must explain that I will continue to use the terminology of a career lifetime. Where AgentJayZ and US industry use the term angle of attack, I and UK industry use the term angle of incidence. I had a row about my terminology on this channel some years ago, but I’m sticking with it. Now let’s start by thinking about reducing engine power and speed from the design condition, where the air angles match the blade and vane inlet angles, so that the angle of incidence is ideally zero at every stage. Because of the inefficiency of the compressor off-design, the reduction in mass flow and hence the flow velocity at the intake falls off more rapidly than the reduction in compressor speed. Consequently, the velocity triangle for the entry to the first-stage blade becomes ‘flatter’, and the air onto the blade leading edge has a positive incidence, potentially with some flow separation on the convex surface. This effect would also tend to be replicated in the immediate downstream stages. Early compressors might tolerate this, but with increasing pressure ratios and more “aggressive” blading, this would result in the blade rows at the front of the compressor stalling and the compressor surging. The solution was a variable inlet guide vane (VIGV) row to reduce the angle of incidence at low power/speed and, with further increases in stage pressure rise, the use of several rows of variable stator vanes (VSVs) at the front of the compressor. In addition to reducing the angle of incidence in the front stages, the closure of the variable vanes at low speed also tends to act as a throttle reducing the mass flow and air velocities. Now let’s go on to consider what happens in the rear stages of the compressor as power/speed is reduced. A compressor annulus is typically designed with a reduction in the annulus height, front to rear, to match the reduction in volume of the compressed air at high power, to give a constant axial flow velocity through the compressor. However, at low speed, with relatively little compression, the contraction in annulus height results in an increase in axial velocity through the compressor. This increase in axial velocity changes the inlet velocity triangles in the rear stages, progressively changing the angles of incidence to the extent that they can even become negative. Again, early compressors with relatively fewer stages might tolerate this. However, with higher pressure ratios and the addition of more stages, flow separation could occur on the concave side of the aerofoil, as opposed to the convex side in the front stages. In the limit, the flow that was entering the compressor could be restricted to the extent that it couldn’t pass through the compressor outlet, and the result would be an engine that couldn’t even be started. The obvious solution to this problem was to reduce the increase in velocity, and a simple way to do this was to reduce the mass flow by bleeding off some of the air part-way down the compressor. In other words, incorporate a bleed valve arrangement. In early engines, operating at relatively low pressure ratios, the use of variable vanes or bleed valves might be a matter of choice, and the engine might work satisfactorily with either system. However, with increased pressure ratios, more work per stage, and more stages in the latest compressors, it may not be a question of a choice of using one or the other. Both systems may be needed to allow a modern compressor to start and to operate off-design at low speed/power.

Is there any situation where the turbine section could stall or is it exclusive to the compressor?

Hello JayZ, I have a question since you have lot of expirience with older american jet engines, I have worked on old P&W JT8D, and accesory drives, like pumps and starter Are held with a v-clamp, which seems a bit weak compared with being held by bolts? Also these clamps are quick disconnect style so vibrations could open the clamp? I would be grateful if you clarify it for me.

I learned recently that the propulsive power of a turbojet(no bypass air) goes up as the speed goes up, but I can't find out WHY. I was wondering if it has to do with the air behind the jet offering less and less resistance to the out flowing gasses? And if such a jet would work at maximum efficiency in space(provided you feed it all the air it needs)? I'm told that rockets are more efficient in space, so that's where my thought process is coming from.

One down side bleed valves is a loss of efficiency. Most of the work you put into compressing the air is lost when you dump it overboard. Depending on how it is ducted you can get a small amount of thrust from the bleed air dump, but not as much as you would get if the same amount of air went through the combustor/turbine/exhaust nozzle.

How can i send you my flag🇸🇦? I been watching you maybe for 4 months now, Am a student and your channel hlped me alot thanks to you, keep it going!

Where are you headed to on your next road trip?

Peace for you... I like your vedios as mechanical engineer in real world i have tiny problem with atlas compressor manual zh1800 and zh1000 can you help me to get my answers

Hello AgentJayZ, I left a comment on another video filled with engineering books for gas turbine engineers. You then responded to my comment asking me to email you, and I just wanted to confirm whether you received my email or not.

11:29 This makes no difference. When not in stalled condition, decreasing angle of attack will reduce the amount of lift force at a given airspeed, and as a result airspeed will increase until the amount of lift generated is once again sufficient to support the plane against gravity, which is (more or less) a constant. This only holds exactly for a powered aircraft in straight and level flight (where lift cancels gravity and thrust cancels drag), though an exact analysis for gliders just further strengthens the rule since if you do the math you find that airspeed in a stable glide is inversely proportional to (lift coefficient)^2 + (drag coefficient)^2 so not only will airspeed decrease with increasing angle of attack _up to_ stall, it will even continue to decrease slightly _into_ stalled condition (until the lift coefficient starts to tank hard enough that it's no longer compensated for by the increasing drag).

Thank you very much for your effort in the practical explanation. I had some many confusing questions about jet engines and I would like to discuss them with you...I would like to provide me with a WhatsApp number or email to communicate. Thank you...

the book is available at the Internet archive. KZfaq dislikes links. search there for pratt_&_whitney_the_aircraft_gas_turbine_engine_february_1958