How do propellers throttle?

[magnemoe]

6 minutes ago, mikegarrison said:

I think maybe you missed the whole point. @KerikBalmwas not talking about cost, but about blade element theory. http://www.aerodynamics4students.com/propulsion/blade-element-propeller-theory.php

I will point back to my first post in this topic and say again that the main reason why most propeller-driven airplanes use constant-speed variable-pitch propellers is because propeller tip speed is a very important design parameter and it is easier/better to design a pitch adjustment mechanism than it is to try to design for a wide range of tip speeds.

He pointed out that for an slow plane the propeller tip is not very affected by airspeed, on an fast plane going at half the speed of sound it becomes critical, it could not reach this speeds with an fixed propeller. 

[K^2]

It's not forward speed, but transverse. In other words, even if you don't go fast, building prop for range of RPMs is hard. Right now, a near-constant RPM prop is cheap enough to be worth the fuel savings.

[Meecrob]

19 hours ago, Dragon01 said:

I admit, I am coming over from fighters, where "jamming inputs to the limits" is something you do quite often, especially in modern FBW birds.

No,  you aren't trained in how to fly fighter jets and jamming inputs to the limits is not how you fly them. The reason I picked this quote out is not to be a jerk, but to point out that flying smoothly and having the lowest possible AoA and thus the lowest induced drag is how you fly them. Speed is life/the cobra maneuver is an airshow trick. Games do not have enough fidelity to go down into the weeds on the physics behind them/gamers for the most part want to hop in and fly, not sit down for ground school. KSP is a good example. If I said "I'm coming over from Apollo-style rockets" an astute observer might tell me "but KSP doesn't even have N-body physics." Tell me, do any of these flight sims teach you coordinated turns, sideslips/forward slips, the power curve and slow flight? Hell, do any of them teach you to fly VFR by looking out the windscreen instead of at your instruments? If not, I would suggest that they are just games and taken as pieces of entertainment, they are great, but there is a reason not one single flight school on this planet uses commercially available "flight sim" games as a training aid. Same way NASA does not make its astronauts play KSP: its fun, but not what its really like flying a spacecraft.

[WestAir]

6 minutes ago, Meecrob said:

No,  you aren't trained in how to fly fighter jets and jamming inputs to the limits is not how you fly them. The reason I picked this quote out is not to be a jerk, but to point out that flying smoothly and having the lowest possible AoA and thus the lowest induced drag is how you fly them. Speed is life/the cobra maneuver is an airshow trick. Games do not have enough fidelity to go down into the weeds on the physics behind them/gamers for the most part want to hop in and fly, not sit down for ground school. KSP is a good example. If I said "I'm coming over from Apollo-style rockets" an astute observer might tell me "but KSP doesn't even have N-body physics." Tell me, do any of these flight sims teach you coordinated turns, sideslips/forward slips, the power curve and slow flight? Hell, do any of them teach you to fly VFR by looking out the windscreen instead of at your instruments? If not, I would suggest that they are just games and taken as pieces of entertainment, they are great, but there is a reason not one single flight school on this planet uses commercially available "flight sim" games as a training aid. Same way NASA does not make its astronauts play KSP: its fun, but not what its really like flying a spacecraft.

SImmers often get caught up in what's simulated, and many don't really consider what isn't. If you're building a sim you want to sell, flaunting how true to life your sim is is a financially intelligent move. The downside is people believe you.

About a year ago Train Sim World came out with a DLC featuring the Railroad I work at. It looked gorgeous, but in terms of realism? It hit maybe the 20% mark if I'm being generous. And that's a train sim - on rails!

[Guest]

2 hours ago, Meecrob said:

Speed is life/the cobra maneuver is an airshow trick. 

Wrong on the former count. Energy is life. If speed was life, then MiG-25 and F-15s would rule the skies, and I think we (this excludes any Eaglejet and Foxbat drivers out there ) can agree that's not the case. Speed is useless when you're blazing past your opponent with 100+ knots overtake and end up getting a guns shot up your tailpipe. What really matters is the ability to retain energy, and the ability to point your nose. Break turns are a very important part of offensive and defensive BFM. In the Viper specifically, one well-timed pull to the stops will put anything short of a MiG-29 or another Viper right in your sights. Of course, the trick is knowing when exactly to turn, and in which direction. Rolling is also usually done with full aileron, roll rate is another important area the Viper excels at, (any kind of scissors against this little jet is tantamount to suicide). As for the throttle, there's nothing wrong with firewalling it into full AB on takeoff, or whenever you think you're not going fast enough. It's all computer-controlled anyway, compressor stalls are for older jets.  

As for the cobra, it is an airshow trick, but nobody said it wasn't. It's not something a Viper can do, so it can't be that important, anyway.  The only possible use I see is for guns defense, and by the time the fight gets there you'd be too low and slow to attempt it, anyway (besides why are you in guns def in a fighter that can do the cobra? Oh, I know, you're fighting an F-16 and he decided to be a showoff with the gun  ). 

2 hours ago, Meecrob said:

Games do not have enough fidelity to go down into the weeds on the physics behind them/gamers for the most part want to hop in and fly, not sit down for ground school.

Do have a look at Falcon BMS, then (in case all the F-16 jokes above weren't enough of a hint). It does take some getting used to, including quite a bit of "ground school". There are three huge, detailed manuals, and your early flights will be done with the sim on one monitor, and manual on the other, pausing every other moment to read up on what you're supposed to do. There are real F-16 pilots on the dev team, and it's been consistently said that if you can do it in the real Viper, you can do it in BMS, and it usually works the other way around as well. No, it's not easy to get into, but very rewarding once you do. About the only thing it doesn't do is electronic warfare, because it's all classified.

So yeah, it very much does have this kind of fidelity. Right down to including realistic ATC procedures, something usually completely neglected in combat sims. 

2 hours ago, Meecrob said:

Tell me, do any of these flight sims teach you coordinated turns, sideslips/forward slips, the power curve and slow flight? Hell, do any of them teach you to fly VFR by looking out the windscreen instead of at your instruments? 

All of the ones I fly do. X-plane is, for that matter, FAA certified as a training tool (well, the pro version is, but the only difference is in legal stuff and some hardware support). If it doesn't have those things, then it's not a simulator, plain and simple. You can do a forward slip even in Il-2. Granted, VFR is somewhat neglected in the flightsim world, but you can very much do it, X-plane has a lot of quite gorgeous scenery to navigate by. 

Edited July 15, 2020 by Guest

[K^2]

1 hour ago, Dragon01 said:

Speed is useless when you're blazing past your opponent with 100+ knots overtake and end up getting a guns shot up your tailpipe.

Again, you are learning something from a game that has not even resemblance to reality at this point. Never mind gun range, if you are within visual range of an enemy fighter, you are both doing it horribly wrong. The first person to pick up the enemy on the radar lets the missiles loose, then turns around and runs. Because no matter how superior your stealth is, nobody's going to miss the launch, and once they see it, they can bring you up on active and get a lock to fire off a response before they turn tail and run. This is how modern BVR combat works and why speed is so important. Your survival depends on you outrunning the range on incoming missiles. This is the only game in town for modern air superiority, unless you massively outclass the enemy. In later case, you just shoot everything from beyond detection range, and your piloting skills just need to be good enough not to total the plane on landing.

1 hour ago, Dragon01 said:

So yeah, it very much does have this kind of fidelity.

You have several people with actual flight experience and background in either making the planes, maintaining them, or in my case, writing sims for them, that are telling you that this just isn't the case. The fact that the sim has complexity does not mean that it's the same kind of complexity as on a real aircraft. Just because they did a good job replicating start-up procedure does not mean the plane actually responds to any of the inputs in a way consistent with physics. As I've told you, MSFS and X-Plane straight up do not simulate fighter aerodynamics out of the box, and I have not personally seen a single modded aircraft that fixes that.

1 hour ago, Dragon01 said:

X-plane is, for that matter, FAA certified as a training tool

So is this thing. Do you know how much aerodynamic simulation it does? Zero. FAA has very lax definitions for what constitutes a training tool, because all it's meant to do is teach you how to read instruments.

[mikegarrison]

26 minutes ago, K^2 said:

Again, you are learning something from a game that has not even resemblance to reality at this point. Never mind gun range, if you are within visual range of an enemy fighter, you are both doing it horribly wrong. The first person to pick up the enemy on the radar lets the missiles loose, then turns around and runs. Because no matter how superior your stealth is, nobody's going to miss the launch, and once they see it, they can bring you up on active and get a lock to fire off a response before they turn tail and run. This is how modern BVR combat works and why speed is so important. Your survival depends on you outrunning the range on incoming missiles. This is the only game in town for modern air superiority, unless you massively outclass the enemy. In later case, you just shoot everything from beyond detection range, and your piloting skills just need to be good enough not to total the plane on landing.

Beyond visual range sounds great, but things are rarely so clear cut that they can actually do that. Just go back to the 1960s when the thought was that guns were obsolete, and yet the Vietnam war showed otherwise. 60 years later we're still building guns into fighters. The US designed a whole fighter around the AIM-54 Phoenix missile and then never successfully used it in combat, ever. (Iran did.)

I agree with you that coming in here and talking a lot about how airplanes fly because of experience with simulators is pretty questionable, but that whole "fighters just need to be fast and fire missiles" thing is 60-70 years out of date. All fighters today are designed for maneuverability as being more important than absolute speed. The current thinking is that stealth is even more important than both.

[Guest]

42 minutes ago, K^2 said:

Again, you are learning something from a game that has not even resemblance to reality at this point. Never mind gun range, if you are within visual range of an enemy fighter, you are both doing it horribly wrong.

You'll be surprised. First of all, unless you define "following the ROE" as "doing it horribly wrong", combat might well start in WWR. You're making the same mistake US generals made in Vietnam. Yes, today you usually have an AWACS at hand, but it won't always have the print on the contacts. If you just pickle off a Slammer at a random bogey that doesn't reply on IFF, you're doing it so horribly wrong that it's an actual war crime. BMS doesn't actually haul you off to court martial unless you actually hit a friendly doing that, but IRL, that's what would happen. You never launch at an unconfirmed target, unless things have gone so far south that ROE is "shoot anything that enters the AO". Forgetting that little detail was what resulted in two dedicated interceptors (MiG-21 and F-4) duking it out in dogfights over Vietnam. 

Second, if you turn and run the moment you pickle off your Slammers/R-77s, you'll lose them. You need to wait until pitbull (that is, missile goes active), and after that, you may have to keep supporting it to get through enemy jamming. BVR is a science, and if you want to hear how real Viper pilots do it, come over to BMS forum, but in short, real BVR engagements are more of a high-speed, high stakes game of chicken. Charging through to the merge can be a viable tactic in some situations, and even if nobody decides to do that, each "short skate" (the technical term for "pitbull, then run") will get progressively shorter, until either one side has had enough, someone runs out of missiles (which may prompt an attempt at going to the merge), or someone gets shot down. Also, both sides will have each other on radar long before they're in range, unless latest-gen stealth fighters are involved (which we can't simulate, anyway, because the data is classified). 

42 minutes ago, K^2 said:

As I've told you, MSFS and X-Plane straight up do not simulate fighter aerodynamics out of the box, and I have not personally seen a single modded aircraft that fixes that.

Read again what I said. BMS and DCS. In X-plane (also Flightgear) I fly GA and airliners, which is what it is for. I fly fighters in BMS and DCS. In fact, I found the F-4 that comes with X-plane rather underwhelming. I've had several actual F-16 (and at least one Hornet) pilots involved in making the former tell me exactly how BMS compares to the real thing, and why some of the things are done the way the are. Falcon BMS is a sim that really does it right.

Edited July 15, 2020 by Guest

[WestAir]

As an aside, since you brought up DCS, my dormmate in college flies the F18 with the Navy. Sunday we were talking about Sims and I suggested we both play DCS as it has an F18.

I showed him a video that impressed me and he responded: "Haha, I wish that video wasn't a hornet, I can help but see the errors in the systems"

On the topic of study level sims, I did find the Leonardo MD-80 addon for FSX really good, but even if you learned it inside and out you still wouldn't know 10% of what you would need to do the real job.

Sims are not a replacement for reality. I think we'll have fully autonomous airliners and fighters before sims reach even 50% accuracy. Sorry.

[mikegarrison]

9 minutes ago, WestAir said:

Sims are not a replacement for reality. I think we'll have fully autonomous airliners and fighters before sims reach even 50% accuracy. Sorry.

You speak, of course, of the kind of sims that one can buy for their home.

A full-up actual simulator is a different beast entirely, but we're talking $10M or so for them.

[WestAir]

10 minutes ago, mikegarrison said:

You speak, of course, of the kind of sims that one can buy for their home.

A full-up actual simulator is a different beast entirely, but we're talking $10M or so for them.

My last time in a sim was in 2016, and most times I was just worried about passing more than nitpicking accuracy. I imagine systems wise they were nearly identical, but as for literally everything else...

I imagine some drivers school somewhere has an amazing driving sim. But can you imagine it to be the same as real life?

[mikegarrison]

4 minutes ago, WestAir said:

I imagine some drivers school somewhere has an amazing driving sim. But can you imagine it to be the same as real life?

Why would they? It's cheaper to drive a car than to simulate one.

Some race teams do have sims, because renting a track and doing testing is quite expensive. But of course they can't simulate the physical stresses on the driver.

[K^2]

1 hour ago, mikegarrison said:

Just go back to the 1960s when the thought was that guns were obsolete, and yet the Vietnam war showed otherwise. 60 years later we're still building guns into fighters.

Which is also the last time US was involved in air combat with an air force it didn't hopelessly outclass, and the seekers of the 60s are no comparison to even the AMRAAM. I think, Desert Storm and use of US planes by IAF are much better indicators of the trend, where visual range encounters are avoided, and being forced into one indicates that somebody screwed up. Both of these have been rather one-sided, however, so it's still far from perfect example.

As for emphasis on maneuverability, I would argue that it's for the same reason that US has been focusing on F/A aircraft over straight up air superiority. Which also goes back to the fact that there just hasn't been a real air superiority fight in a very long time. If all your aircraft are designed to be used in CAS roles to at least some degree, and that's where you're suffering most of your losses, that's kind of going to alter the design goals. F-22 is a way better superiority fighter than F-35, but we are still building the later, because it was built to be a better support aircraft. But we aren't talking about a dogfight at this point. We are talking about operating in what's presumed to be a secured air space and dealing with whatever might pop up there unexpectedly.

I also agree that stealth trumps everything else, but only while you have undeniable superiority on that. If you're flying an F-22 and enemy is flying an old Mirage, yeah, your job's done. The enemy Mirage is going to be a pile of burning wreckage, and you might get to practice a strafing run with your air cannon, because there is nothing else to do in the air. That's just not the kind of fight you're going to get against anyone with remotely adequate stealth of their own. And I'm not saying it has to be on par with F-22 in every way. If Russia gets Su-57 to mass production, it's stealthy enough for you to be well within detection range once you open your missile bays by the time you can get a passive lock. And from there, it really is a game of who can run away better.

Are we likely to get a sort of a fight where any of this is going to matter? Probably not unless airmen get into a really bad fight with marines, because they are the only two air forces with comparable strength and technology that aren't likely to just nuke each other. But if you want to keep only getting into the kind of fights where you hopelessly outclass your enemy, you have to have a modern air superiority force, one which is designed to fight by what might very well remain to be entirely theoretical doctrine. And I'll be happy if that doesn't change in my lifetime.

12 minutes ago, WestAir said:

I imagine some drivers school somewhere has an amazing driving sim. But can you imagine it to be the same as real life?

I actually know someone who worked on an extremely realistic driving sim for car manufacturers'. They didn't simulate the car physics, though. They just loaded the car on dynamo and simulated the city around them. The whole set up was ludicrously expensive. I doubt any driving school would consider paying for something like this.

Edited July 15, 2020 by K^2

[WestAir]

8 minutes ago, mikegarrison said:

Why would they? It's cheaper to drive a car than to simulate one.

Some race teams do have sims, because renting a track and doing testing is quite expensive. But of course they can't simulate the physical stresses on the driver.

You're right on your point. I only compared a driving sim to make it easier to visualize why even $10,000,000 sims are not true to life experiences. 

The reason sims list all the things they try to model, and not all the things they do not, is because one is a shorter list.

[Guest]

3 minutes ago, K^2 said:

I also agree that stealth trumps everything else, but only while you have undeniable superiority on that. If you're flying an F-22 and enemy is flying an old Mirage, yeah, your job's done. The enemy Mirage is going to be a pile of burning wreckage, and you might get to practice a strafing run with your air cannon, because there is nothing else to do in the air. That's just not the kind of fight you're going to get against anyone with remotely adequate stealth of their own. And I'm not saying it has to be on par with F-22 in every way. If Russia gets Su-57 to mass production, it's stealthy enough for you to be well within detection range once you open your missile bays by the time you can get a passive lock. And from there, it really is a game of who can run away better.

Do note, if both fighters are passive, the fight will be in WWR (if they can even find each other, that is, which is not exactly easy even with IRST). There are no passive BVR missiles save for those that home in on jammers and radars (and these are trivially defeated by turning the system in question off), and if you're close enough for a radar missile to go pitbull after launch, then more likely than not you've got tally already. The "visual range" in air combat is in fact quite long, aircraft can be seen and identified from several miles away. In fact, if both opponents are going for stealth, it might well end up back in Vietnam, with a guns and IR missile dogfight, albeit this time with missiles that actually work most of the time. As it happens, the most stealthy planes of US, Russia and China are all pretty good dogfighters, to say the least, so it might be that a few other people came to the same conclusion. That said, it's all speculation, and these stealth fighters aren't very numerous, anyway. The USAF roster is still mostly Vipers and Eagles, and Russia's and China's Flankers and Fulcrums, so I'd say experiences from Falcon BMS are going to hold for a while, especially seeing as every aforementioned country is also building more of the aforementioned aircraft, just with spiffy glass cockpits and souped-up avionics. 

40 minutes ago, WestAir said:

I showed him a video that impressed me and he responded: "Haha, I wish that video wasn't a hornet, I can help but see the errors in the systems"

Yeah, Hornet in particular has some kinks to iron out, to say the least (no way they're going to get it out of EA this year). That's why I'm mostly talking about BMS, DCS is notoriously uneven in quality. Some modules are top-notch, some less so, and there's a lot of early access stuff in varying stages of completion. BMS does one plane, the F-16C, and it does it really well. 

[K^2]

12 minutes ago, Dragon01 said:

and if you're close enough for a radar missile to go pitbull after launch, then more likely than not you've got tally already. The "visual range" in air combat is in fact quite long, aircraft can be seen and identified from several miles away. In fact, if both opponents are going for stealth, it might well end up back in Vietnam, with a guns and IR missile dogfight

Except, to fire guns or fire heat seekers, you have to open bays/covers, which in visual range guarantees an opening for an active missile lock, which from that range, as you've put it, a tally. So what do you suggest should be the next step in this? Pilots fly within sight range and show each other obscene gestures through the glass? Biggest innovation will be a magnifying lens in the canopy so that you can flip enemy the bird from greater distance?

Exposing your own weapons from anything but the most extreme reaches of opponents' weapons range is suicide if you have two modern stealth fighters going at each other. This is absolutely identical to situation with two non-stealth fighters, except the non-stealth fighters don't have the options of just trying to sneak away. And if all you have is two stealth aircraft in the air looking for each other, then absolutely nothing will happen. They just can't engage each other. Fortunately (or unfortunately, depending on how you look at it), each side is likely to commit more than one aircraft to combat, only some of them will be stealth, and far from all are going to be any sort of combat aircraft. Or aircraft at all, actually. You have ground stations, AWACs, and satellites in addition to networked data from radars of basically every fighter in the air. So long as they have modern enough equipment. And a modern stealth fighter is going to use this wealth of information for passive detection. It's no longer just radar. And yes, you can launch based on this. And yes, with a little bit of luck, from way, way beyond visual range.

The idea that stealth aircraft are somehow leading us back to era of dogfights cannot be further from the truth. The very way the stealth aircraft are built makes engaging in dogfight unthinkable, because you immediately lose your stealth advantage. And if that's your ace in the hole, you're probably going to get killed.

[magnemoe]

1 hour ago, mikegarrison said:

Why would they? It's cheaper to drive a car than to simulate one.

Some race teams do have sims, because renting a track and doing testing is quite expensive. But of course they can't simulate the physical stresses on the driver.

True, and most of the ones making race cars have their private tracks anyway, also used to testing sports cars who they also tend to sell. 
Aircraft simulators has two benefits, first is that fighter planes and larger passenger and caro planes are very expensive both to run and to risk, and you can simulate various fail states and train on them in an simulator. 
And as you say it don't work so well for something like an dogfight but it works for most other stuff. 

None of this is very relevant for cars, if an car has an problem it has the benefit of just braking and stopping beside the road. 
Now for machine learning an running it all on software has benefits and you can use it to sett up scenarios for training the AI on rare but dangerous events. 

[K^2]

4 hours ago, magnemoe said:

Now for machine learning an running it all on software has benefits and you can use it to sett up scenarios for training the AI on rare but dangerous events. 

Absolutely. A healthy chunk of engineers in self-driving companies I'm familiar with are actually working on the simulation side of things. I don't think I've ever seen one without a dedicated sim team. It just makes a whole lot of sense to train and test your AI on a synth world if at all possible. Keeping in mind limitations @WestAir has brought up, of course. But the argument there is that you want your AI to be capable of generalizing. If you can't train it on a good simulation and have it work in the real world, then it probably won't be able to generalize enough to catch all possible edge cases in the real world anyways, and that just isn't safe.

[KerikBalm]

14 hours ago, magnemoe said:

He pointed out that for an slow plane the propeller tip is not very affected by airspeed, on an fast plane going at half the speed of sound it becomes critical, it could not reach this speeds with an fixed propeller. 

Yes, but what you said, and what @mikegarrison pointed out, is that you implied cost was a factor.

Its not really a cost factor.You can have a pretty expensive sailplane, that still uses a fixed pitch prop. 

https://en.wikipedia.org/wiki/Schleicher_ASH_30

https://en.wikipedia.org/wiki/Schleicher_ASH_25

Spoiler

Its more of a matter of the ratio of the blade tip speed to the planes airspeed when under power, and the mach number of the blade tip.

Consider a prop with a fixed pitch of 5 degrees (low angle, because for low values of x sin (x) approximately = x), rotating at 5, 50, or 500 rpm.

When stationary in still air, the blade's AoA will be 5 degrees regardless of the rpm. The plane doesn't have to get much forward speed at all (depends on blade diameter) before it has 0 AoA at 5 rpm - yet at 50 rpm it would still have an AoA of 4.5 degrees, and at 500 rpm its AoA would be 4.95 degrees. Go 10x faster, and it again has zero rpm at 50 rpm, but still 4.5 degrees at 500 rpm.

Obviously, there is a limit to how high you can make your RPM (massive centrifugal forces), and how high you can make your tip velocity (sure, you can try to make a supersonic propellor, but there are massive drawbacks: https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech - and even that one used variable pitch)

If your plane flies slow (regardless of cost), then its not too hard to keep blade AoA within an acceptable range while also keeping RPM low enough and blade tip speed low enough.

Of course, it gets even more complex when you consider the whole blade, not just the tip, because the radial velocity differs along the length of the blade... but lets not get into that.

13 hours ago, Meecrob said:

Tell me, do any of these flight sims teach you coordinated turns, sideslips/forward slips, the power curve and slow flight? Hell, do any of them teach you to fly VFR by looking out the windscreen instead of at your instruments?

Yes, many do, and have been doing so since at least the late 90's.

I'll point out that even Arma 3 (which is a "Mill-sim" infantry focused game) has a fixed-wing flight model that rewards mostly smooth flying. Their (fictional) jets will stall before completing a loop if you just yank back as hard as you can, but will retain energy much better if you do a smoother loop. Most of the flying in that game is done smoothly, with hard inputs done during attack runs against maneuvering targets, or at the last moment when you realize you haven't got your approach right and you're about to crash into the carrier deck.

Anyway, the discussion about fighter tactics is now quite far off topic.

 

[mrfox]

I've always wonder how turbofans - effectively a fixed pitch ducted fan - deals with this blade AOA/aircraft speed mismatch - I know the intake cowlings mitigates this somewhat by reducing intake velocity, and variable stator vanes alters the downstream pressure, but I was wondering if anyone can provide or knows of a detailed treatment with numbers they can share.

Regarding sim fidelity - there is a reason NASA still sends canidates on T-38 rides. Operating a real machine teaches something called "mechanical sympathy" - and you learn quickly that "jamming inputs to the limits" is not good operating practice for a real life machine, nor the real life operator.

"Slow is Smooth - and Smooth is Fast"

Edited July 17, 2020 by mrfox

[WestAir]

2 hours ago, mrfox said:

I've always wonder how turbofans - effectively a fixed pitch ducted fan - deals with this blade AOA/aircraft speed mismatch - I know the intake cowlings mitigates this somewhat by reducing intake velocity, and variable stator vanes alters the downstream pressure, but I was wondering if anyone can provide or knows of a detailed treatment with numbers they can share.

I misread your comment and replied with a video that didn't really help. Edited.

Edited July 17, 2020 by WestAir
Misread

[mikegarrison]

1 hour ago, mrfox said:

I've always wonder how turbofans - effectively a fixed pitch ducted fan - deals with this blade AOA/aircraft speed mismatch - I know the intake cowlings mitigates this somewhat by reducing intake velocity, and variable stator vanes alters the downstream pressure, but I was wondering if anyone can provide or knows of a detailed treatment with numbers they can share.

Turbofans can be thought of as something approximating an intermediate between a propeller and a jet.

Each individual blade can do less work, because there are more blades. And the fact that the next blade is close helps with the pressure gradient along the blade chord, so you don't have to worry so much about stalling the airfoil like you do with a propeller. Also, you can treat a fan blade as more of a 2D wing because the tip flow is nearly constrained by the duct. There are still 3D flow effects, but not to the same extent as on an unducted propeller.

2 minutes ago, WestAir said:

 

Note that this is for the compressor, not the fan. There are no IGVs for the fan. There are OGVs, which primarily exist to deswirl the fan air. Swirl is useless for propulsion, so it's valuable to straighten it out.

This is also why some propeller-driven airplanes use counter-rotating propellers.

[WestAir]

39 minutes ago, mikegarrison said:

Note that this is for the compressor, not the fan.

I had that video up for maybe 90 seconds before I realized that Mr.Fox already mentioned VIGVs and edited it out.

You must have been a quick shooter in your past life.

Edited July 17, 2020 by WestAir
Spelling

[mrfox]

4 hours ago, mikegarrison said:

Turbofans can be thought of as something approximating an intermediate between a propeller and a jet.

Each individual blade can do less work, because there are more blades. And the fact that the next blade is close helps with the pressure gradient along the blade chord, so you don't have to worry so much about stalling the airfoil like you do with a propeller. Also, you can treat a fan blade as more of a 2D wing because the tip flow is nearly constrained by the duct. There are still 3D flow effects, but not to the same extent as on an unducted propeller.

 

Thanks Mike. How much performance do you think is being given up with the fixed blades setup vs a variable pitch setup - mechanical complexity non-withstanding?

[mikegarrison]

1 hour ago, mrfox said:

Thanks Mike. How much performance do you think is being given up with the fixed blades setup vs a variable pitch setup - mechanical complexity non-withstanding?

Here's a claim of up to 14%. I'm not aware of any that are actually flying.

https://www.asme.org/topics-resources/content/turbine-fuel-efficiency-fitting-a-pitch

Normally a commercial design would be optimized for cruise, and then the design would be worked until it was acceptable for high thrust and low thrust too.

I'll point out that most of the current work has gone into geared turbofans that slow down the fan. Those are flying, for instance on the A320 NEO and A220 (C-Series). No gearboxes have been successfully implemented for larger engines, though.

Edited July 18, 2020 by mikegarrison