Last one of these, I promise. That beginning bit was a roller coaster: 0:10 "dis-symmetry of lift" Good... 0:12 "Cyclic feathering and blade flapping" Good... 0:19 "Critical angle of attack" Okay... 0:27 "... gyroscopic procession..." NO! BAD! 0:29 "...and phase lag..." Okay, good, so what the hell was that about gyroscopic procession? Okay, now on to the rest of it. 1:15 Minor point: Tip-speed is fine, but if you want a good representative point for the blade as a whole, the 75% radius location is typically used. So that would mean 135 m/s +- the forward speed. Don't think anyone cares though. 3:00 Flapping is the angular movement of the blade relative to the "*shaft plane*," which is the plane normal (perpendicular) to the shaft that passes through the center of the hub. 3:09: Flapping *rate* up or down will decrease or increase the blade angle of attack, respectively (this assumes normal downward inflow and positive collective pitch). You're not wrong, but this should just be clarified. 3:30 Put another way, the rotor will automatically flap such that the blades maintain roughly equal AoA. 3:38 Ah, yes. Finally a rotor that spins the correct direction :D 4:33 Minor point: The rotation you describe is called feathering or pitch. Twist generally refers to a geometric property of the blade and is the pitch distribution along its span. Blades will dynamically twist in flight due to airloads and propeller moment (aka, the "tennis racket effect"). Again, this is more a point about terminology than anything. It's not something the layman would care about. 5:37 NOOOOOOOOOOOOOO! 5:44 No, it's not 6:17 In the case of retreating blade stall, yes, it will. Even in the case with 90 degree phase. 6:23 No, **NOT** because of gyroscopic procession. 6:30 The front half of the disk would not see more lift (Ignoring the effect of coning). The disk just tilts back. 6:43 Right, it's not a gyroscope. So why mention gyroscopic procession at all? 7:04 Yes, though phase lag is a *very* general term that can apply to any oscillating system. It's not a "broad version" of gyroscopic procession. Gyroscopes behave they way they do because of Coriolis forces. Rotor flapping occurs because centrifugal forces act as a restoring force in such a way that the rotor can be shown to behave like a linear second order spring-mass-damper system. 9:40 There will be a flapping response, to be sure, but there should be a larger roll to the right than you're letting on. Explaining why exactly is difficult to do without illustrations, but it has to do with rotor coning and hinge forces. Just know that in the case of retreating blade stall, there should be a characteristic roll to the retreating side. 10:50 The buffeting is due to to the stall as the blades go through a non-linear region of their lift curve every rotation. In a 5-bladed rotor, this will result in a 5/rev vibration. 11:40 The violent pitch up likely wasn't due so much to the retreating side stall as it was you failed to keep the stick moving forward as your speed increased. What I think happened (I could be wrong) is the descent kept you accelerating even while the rotor was flapping aft in response to the increasing speed. The entire aircraft likely built AoA, but you didn't notice. Eventually, the horizontal tail stalled, resulting in the pitch up. Retreating blade stall isn't usually a sudden dramatic thing like that. I'll notice it in the Huey if I'm in a fast descent with a lot of forward cyclic; I'll have to maintain a lot of right stick as the aircraft wants to roll to the left. That pitch up can occur in wind up turns, however, which is what you were doing towards the end. 13:17 I agree that the roll to the left is suspect, and the fact that you had to hold so much right stick during the dive is also strange. However, this is almost certainly *not* a case of copy-pasted code from the Huey. Their rotor model should take care of this stuff automatically. It could be that there are complicating and competing effects at play here. I suspect the rapid pitch up is complicating things. I would encourage you to repeat the experiment, but instead, keep the nose down by increasing forward stick as the speed increases. Then see if you get the characteristic right roll. 15:55 Again, not flapping. It's because of cyclic pitch. Anyway, that's it for me. Hope it's helpful. I quite enjoyed the series. Thank you very much for making it.
Thank you for these comments, I am learning a lot as I read them. The reason I talked about Gyroscopic Precession is that as a layman when you look up RBS you will find page after page and site after site talking about Gyroscopic Precession and not Phase Lag. You have to dig deeper to learn about that and the differences between them, so I wanted to make sure I at least attempted to explain that in the video. Thanks for the tip on the pitching up behavior, I will definitely retry this with your advice and see what happens.
@@vsTerminus I'm glad you found them useful. And thank you for the series as a whole. I never took the time to learn the Mi-8's systems in the past, so this was very useful. I tend to get on people's case about gyroscopic procession precisely because of the reason you mention; that myth is everywhere. In my opinion, it's best to first describe the rotor as a second order system, explain that it has a ~90 deg phase as a result because it is excited at or near its resonant frequency, and then acknowledge the gyroscopic procession myth and explain why it's wrong when it comes to rotor flapping. Unfortunately, like I said in one of my other comments, this doesn't lend itself well to simple explanations.
@@ErikScott128 What is the reason gyroscopic precession is a myth? I’m having trouble with this, and need help understanding, as it seems the concept of gyroscopic precession is only applied during conversations of certain aerodynamic principles, but then utterly disregarded during others. I could use some further help with understanding.
@@fmfmichael The concept of gyroscopic precession in general isn't a myth. It's a real phenomenon. It's just not really the right explanation for how articulated rotors behave. A better explanation is that the flapping motion can be shown to be equivalent to a spring-mass-damper system: the moment about the hinge due to centrifugal force is equivalent to the spring, the blade's rotational inertia is analogous to the mass, and the damping is aerodynamic (as the blade flaps up, the angle of attack is reduced, and the lift therefore decreases). If you actually do the math, you find that the natural frequency of this system is equal to the rotor rotational speed. Cyclic pitch, almost by definition, is also applied once per revolution, so this is a system excited at resonance. All resonate systems show a 90 degree phase lag; the system responds one quarter cycle later than the input. Strictly speaking this is only true for blades with no hinge offset (such as a teetering rotor). For articulated rotors with small flapping hinge offsets, the natural frequency is slightly higher than the rotor speed, and so is excited slightly below resonance. This results in a phase lag slightly below 90 degrees. The exact phase angle depends on the amount of damping, which aerodynamic in nature and thus depends on the ambient conditions (namely air density) as well as the blade aerodynamic and inertial properties.
At the 13:17 - I think it is the same phenomena as for the taildragger prop plane. When you pitch the nose down on the take off, the aircraft wants to yaw left or right, depends on the propeller rotation. Same for helicopter but the result is in the roll axis. The fast change of pitch probably caused the left roll.
What I get from this video tells me is that a "supersonic" helicopter is physically impossible because the advancing blade tip will go supersonic well before the fuselage can, resulting in a complete loss of lift on that side.
Supersonic is never possible with moving parts outside the fuselage, there were tests in the 1950s that confirmed this. One of the famous planes of that era was this project: en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech Basically, if you want to go supersonic, you have to have an powerful exhaust jetstream either generated by a rocket, ramjet or common jet engine. If you use ambient air, that air has to be sub-sonic in order to be compressed inside the engine, even if the plane moves at supersonic speeds the air has to be slowed down to sub-sonic before hitting the fan blades. That's why the MiG-21 has the famous nosecone in front of the intake, or the F-15 has the movable intake nozzle.
For some reason, this video only loads on 360p. None of the other "quality levels" works. Is there a way you can force youtube to re-process the video?
This one too? Sigh. I must have rendered these ones differently or something. Thanks for the heads up, I'll add it to the list to re-record in the future.
@@vsTerminus Yep, sorry. By the way, have you reported the issue with the reverse side retreating blade effect? Your assumption about reused Huey code sounds plausible to me.
Last one of these, I promise.
That beginning bit was a roller coaster:
0:10 "dis-symmetry of lift" Good...
0:12 "Cyclic feathering and blade flapping" Good...
0:19 "Critical angle of attack" Okay...
0:27 "... gyroscopic procession..." NO! BAD!
0:29 "...and phase lag..." Okay, good, so what the hell was that about gyroscopic procession?
Okay, now on to the rest of it.
1:15 Minor point: Tip-speed is fine, but if you want a good representative point for the blade as a whole, the 75% radius location is typically used. So that would mean 135 m/s +- the forward speed. Don't think anyone cares though.
3:00 Flapping is the angular movement of the blade relative to the "*shaft plane*," which is the plane normal (perpendicular) to the shaft that passes through the center of the hub.
3:09: Flapping *rate* up or down will decrease or increase the blade angle of attack, respectively (this assumes normal downward inflow and positive collective pitch). You're not wrong, but this should just be clarified.
3:30 Put another way, the rotor will automatically flap such that the blades maintain roughly equal AoA.
3:38 Ah, yes. Finally a rotor that spins the correct direction :D
4:33 Minor point: The rotation you describe is called feathering or pitch. Twist generally refers to a geometric property of the blade and is the pitch distribution along its span. Blades will dynamically twist in flight due to airloads and propeller moment (aka, the "tennis racket effect"). Again, this is more a point about terminology than anything. It's not something the layman would care about.
5:37 NOOOOOOOOOOOOOO!
5:44 No, it's not
6:17 In the case of retreating blade stall, yes, it will. Even in the case with 90 degree phase.
6:23 No, **NOT** because of gyroscopic procession.
6:30 The front half of the disk would not see more lift (Ignoring the effect of coning). The disk just tilts back.
6:43 Right, it's not a gyroscope. So why mention gyroscopic procession at all?
7:04 Yes, though phase lag is a *very* general term that can apply to any oscillating system. It's not a "broad version" of gyroscopic procession. Gyroscopes behave they way they do because of Coriolis forces. Rotor flapping occurs because centrifugal forces act as a restoring force in such a way that the rotor can be shown to behave like a linear second order spring-mass-damper system.
9:40 There will be a flapping response, to be sure, but there should be a larger roll to the right than you're letting on. Explaining why exactly is difficult to do without illustrations, but it has to do with rotor coning and hinge forces. Just know that in the case of retreating blade stall, there should be a characteristic roll to the retreating side.
10:50 The buffeting is due to to the stall as the blades go through a non-linear region of their lift curve every rotation. In a 5-bladed rotor, this will result in a 5/rev vibration.
11:40 The violent pitch up likely wasn't due so much to the retreating side stall as it was you failed to keep the stick moving forward as your speed increased. What I think happened (I could be wrong) is the descent kept you accelerating even while the rotor was flapping aft in response to the increasing speed. The entire aircraft likely built AoA, but you didn't notice. Eventually, the horizontal tail stalled, resulting in the pitch up. Retreating blade stall isn't usually a sudden dramatic thing like that. I'll notice it in the Huey if I'm in a fast descent with a lot of forward cyclic; I'll have to maintain a lot of right stick as the aircraft wants to roll to the left. That pitch up can occur in wind up turns, however, which is what you were doing towards the end.
13:17 I agree that the roll to the left is suspect, and the fact that you had to hold so much right stick during the dive is also strange. However, this is almost certainly *not* a case of copy-pasted code from the Huey. Their rotor model should take care of this stuff automatically. It could be that there are complicating and competing effects at play here. I suspect the rapid pitch up is complicating things. I would encourage you to repeat the experiment, but instead, keep the nose down by increasing forward stick as the speed increases. Then see if you get the characteristic right roll.
15:55 Again, not flapping. It's because of cyclic pitch.
Anyway, that's it for me. Hope it's helpful. I quite enjoyed the series. Thank you very much for making it.
Thank you for these comments, I am learning a lot as I read them.
The reason I talked about Gyroscopic Precession is that as a layman when you look up RBS you will find page after page and site after site talking about Gyroscopic Precession and not Phase Lag. You have to dig deeper to learn about that and the differences between them, so I wanted to make sure I at least attempted to explain that in the video.
Thanks for the tip on the pitching up behavior, I will definitely retry this with your advice and see what happens.
@@vsTerminus I'm glad you found them useful. And thank you for the series as a whole. I never took the time to learn the Mi-8's systems in the past, so this was very useful.
I tend to get on people's case about gyroscopic procession precisely because of the reason you mention; that myth is everywhere. In my opinion, it's best to first describe the rotor as a second order system, explain that it has a ~90 deg phase as a result because it is excited at or near its resonant frequency, and then acknowledge the gyroscopic procession myth and explain why it's wrong when it comes to rotor flapping. Unfortunately, like I said in one of my other comments, this doesn't lend itself well to simple explanations.
@@ErikScott128 What is the reason gyroscopic precession is a myth? I’m having trouble with this, and need help understanding, as it seems the concept of gyroscopic precession is only applied during conversations of certain aerodynamic principles, but then utterly disregarded during others. I could use some further help with understanding.
@@fmfmichael The concept of gyroscopic precession in general isn't a myth. It's a real phenomenon. It's just not really the right explanation for how articulated rotors behave. A better explanation is that the flapping motion can be shown to be equivalent to a spring-mass-damper system: the moment about the hinge due to centrifugal force is equivalent to the spring, the blade's rotational inertia is analogous to the mass, and the damping is aerodynamic (as the blade flaps up, the angle of attack is reduced, and the lift therefore decreases). If you actually do the math, you find that the natural frequency of this system is equal to the rotor rotational speed. Cyclic pitch, almost by definition, is also applied once per revolution, so this is a system excited at resonance. All resonate systems show a 90 degree phase lag; the system responds one quarter cycle later than the input.
Strictly speaking this is only true for blades with no hinge offset (such as a teetering rotor). For articulated rotors with small flapping hinge offsets, the natural frequency is slightly higher than the rotor speed, and so is excited slightly below resonance. This results in a phase lag slightly below 90 degrees. The exact phase angle depends on the amount of damping, which aerodynamic in nature and thus depends on the ambient conditions (namely air density) as well as the blade aerodynamic and inertial properties.
At the 13:17 - I think it is the same phenomena as for the taildragger prop plane. When you pitch the nose down on the take off, the aircraft wants to yaw left or right, depends on the propeller rotation. Same for helicopter but the result is in the roll axis. The fast change of pitch probably caused the left roll.
Great editing! Love how the presentation is organized into cohesive sections. The timestamps are an especially nice touch.
I understood this better then when my teacher taught me in my own language, well done
Amazing vid. I love that you go deep into the theory. Well done. Learned a lot.
What I get from this video tells me is that a "supersonic" helicopter is physically impossible because the advancing blade tip will go supersonic well before the fuselage can,
resulting in a complete loss of lift on that side.
Supersonic is never possible with moving parts outside the fuselage, there were tests in the 1950s that confirmed this. One of the famous planes of that era was this project: en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech
Basically, if you want to go supersonic, you have to have an powerful exhaust jetstream either generated by a rocket, ramjet or common jet engine. If you use ambient air, that air has to be sub-sonic in order to be compressed inside the engine, even if the plane moves at supersonic speeds the air has to be slowed down to sub-sonic before hitting the fan blades. That's why the MiG-21 has the famous nosecone in front of the intake, or the F-15 has the movable intake nozzle.
Great learning tool! n good explanation. thanks
Awesome video as always !
You make complex concepts crystal clear, thank you.
The only (small) negative point I'd raise is that the video is SD only.
Is dis-symmetry different from asymmetry?
dcs simulates no stress dmg?
For some reason, this video only loads on 360p. None of the other "quality levels" works. Is there a way you can force youtube to re-process the video?
Sadly no, I will have to upload a new one. That's ok, it'll give me the opportunity to improve on it.
@@vsTerminus It's 1080p for me.
ThX! Awesome!
Nice! thx
Looks like this is another one that won't play in HD.
This one too? Sigh. I must have rendered these ones differently or something. Thanks for the heads up, I'll add it to the list to re-record in the future.
@@vsTerminus Yep, sorry.
By the way, have you reported the issue with the reverse side retreating blade effect? Your assumption about reused Huey code sounds plausible to me.