Even after graduating flight school and reading TC 3-04.4 over and over Compressibility still wasn’t clicking for me until I watched this video. Thanks for your help!
Great job!! I'm a professional helicopter pilot with 17 years of experience and I found your explanation to be extremely well done. I am in midst of taking a refresher course for a new job and came across the term "advancing blade compressibility". I had actually never heard of it before, we were only taught about retreating blade stall when I was in flight school. I figured it probably had to do something with crossing the speed of sound but I wanted to learn more so I googled it and came across your video. I've done some really high altitude work in Nepal and we had to be very careful to not cross VNE when descending from extremely high altitudes. At closer to sea level it would be just about impossible to reach VNE in an AS350 but coming down from 20,000 ft that VNE is reduced to around 100 kts or maybe even lower and the aircraft really wants to dive and accelerate as you descend. Anyway great job, keep up the good work!
I just went for my introductory flight Saturday for my birthday and loved every second of it. I always knew I wanted to fly (aspired to be an Apache pilot), and always figured i would get my PPL sometime, but now that I've gotten a little taste of the flying life I plan on making a career out of it. I just can't stop thinking about getting back in the air, and these videos are an awesome tool to help me learn before I've even started flight school. Thank you for making these, and I really hope that you are able to come back and make more soon!
I just want to say thank you for all the information and being a teacher really. I myself took the SIFT and passed. Basically it was your videos and a little bit of self teaching from books that allowed me to pass. THANK YOU!!
Hey Jacob, your content is extremely helpful, I appreciate it. *update* I just passed the SIFT and your videos definitely helped me with understanding the concepts better!
Recently started using my G.I. benefits to start flight training. Discovered your channel last night right as I started getting into aero in the ground training and binge watched most of your videos... Walked into my second flight this morning and surprised my instructor understanding ETL, LTE, translating tendency etc. seemingly out of nowhere. MUCH APPRECIATED. Also gonna give you a shout out in my college helicopter fundamentals class tomorrow, should help the other students as we start moving into aero.
Loving your videos mate, really helpful. I'm doing all my pilot training theory via distance learning and these are definitely helping in the absence of an instructor. From a former US Army Blackhawk CE now doing pilot training in Australia (long story) thank you truly for these.
This was a very interesting one! I didn’t know about this aspect of low-temperature flying even though I’ve mostly flown in Russia and of course that involved flying in winter quite a few times. Not extreme but it’s still a good thing to be aware of.
Would it be possible for you to make a video on the AH 66 Cheyenne project and provide some insight on when, if ever, we might expect the Army to obtain a helicopter with the capabilities of the prototypes which are now sitting in mothballs for more than 20 years.
Hey Jacob, your videos are really informative and helpful, can you please do a video on Figure of Merit or if you may suggest from where can i get information about it
Hello! I am a civilian who is looking forward to becoming a U.S Army helicopter pilot. I am studying with my books and your videos which help a lot. I was hoping you could make a video with hints and tips or what I could expect on the current test called SIFT ( Selection Instrument for Flight Training ).
Which condition would you be likely to first encounter at high speed normal operating RPM flight retreating blade stall of advancing blade compressibility effects? Are there facors like temperature that could get you into compressibility before reatreting blade stall?
Love your videos! I have some questions about coaxial-rotor and tandem-rotor helicopters and gyro stability. As a lay person, I always thought that helicopters had the handy feature of being naturally stable because they were a giant gyro that wanted to stay level, just like a child’s toy gyro/spinning top. However, what would happen if we redesigned that child’s toy and put in TWO gyros spinning in OPPOSITE directions around the same axis? It would not stand up. Likewise, a bicycle stands up when moving because of the gyro effect of its wheels. However, if we could magically make one wheel spin in the opposite direction, the bike would be unridable. So it seems to me that the counter-rotating rotors of coaxial helicopters and tandem helicopters must take away the stabilising feature of a single rotor. Why is this not mentioned in the “advantages & disadvantages of coaxial helicopters” lists that I see on the web? Furthermore, how do the coaxial helicopters deal with gyroscopic precession? ---
mrjweate they are naturally stable on the roll and pitch axis, no helicopter is naturally stable on the yaw axis due to Newton’s 3rd law so it requires a counter torque rotor system. A child’s gyro toy spins as a whole itself, a helicopters rotor spins separate from its fuselage causing the fuselage to turn in the opposite direction, again due to Newton’s 3rd law. I think you are confused about the whole bike thing. If a bike had its wheels turning in opposite directions, of course it would be unridable but that has nothing to do with a tandem rotor system, because a tandem rotor system maintains foward flight because of the pitch angle of the blades, nothing to do with how a bikes wheels work. It has opposite turning rotors to balance the torque of eachother. I hope that answered your question, I’m not a genius like this guy but I know a fair share lol
Based on this, would it be correct to conclude that the actual Max Vne of a given helicopter would be the value where retreating blade stall and advancing blade compressibility are equal? I am basing this on: Vne kt value tends to increase with a decrease in both temperature & altitude, and that blade compressibility value increases with an increase in temperature (also assuming altitude as that has a direct correlation to Mach numbed & kt value associated). In other words, the fastest indicated airspeed I could fly would be at low altitudes on a slightly above freezing day. Thoughts?
JustaGthing. Your operators manual bases VNE off of either Retreating Blade Stall, Advancing Blade Compressibility, or structural damage (whichever is lowest). It just so happens that when it's colder it's based on ABC and RBS when it's hotter. With that in mind, I would agree that the absolute latest onset of any of these limiting factors would probably be low altitudes at cooler temperatures. But every aircraft is different. Keep in mind rotor RPM and blade design affect these significantly. Great question!
Question, why did you reduce the Mach speed by 10% and not the required airspeed to hit Mach by 10%? I am not an engineer, just curious. It seems that if airspeed around an airfoil increases by 10% it would reduce the required speed to get to Mach 1. If this were the case it would need to be around 78 knots and not 65. Right? Please help me understand your math. Thanks. I really did enjoy the explanation, just curious about the math.
Im heading to basic in 16 days and after that wocs then flight school. I wanted to know your opinion on the different airframes and why you chose the 64 over the 60 or 47. Im not set on a specific airframe yet as I know selection can be a toss up, but I just wanted to know your opinion. Thanks -Kagan
Kagan Smith. I get this question a lot and it really just boils down to personality and what you want to do. 60’s and 47’s carry people and equipment around, conduct air assaults, humanitarian relief, etc. The 64 has the hunter-killer mission. In BOLC you’ll learn all about each of the mission sets. Ultimately, pick the aircraft based on the mission that you want to do.
The V-22 Osprey rotor diameter is 38' so that means the rotor rpm can go faster since the circumference is smaller (and there is no retreating rotor blade in forward flight). The cruise speed is 270 knots. It seems like it has most of the advantages of a normal helicopter without the limited speed. The only downside is the cost.
Indeed, never thought about that! But in forward flight the Osprey in my eyes looks more like a propeller driven aircraft than a helicopter, P-Factor is applied here too, though it's not a big deal I'd say.
Tom Mendenhall. I don’t think the V-22 worries about compressibility or retreating blade stall because, just like you said, it has no advancing blade in forward flight. This really applies typical helicopter designs.
So, theoretically, if you can match the retreating stall speed with the advancing comp speed at specific airspeeds and temp, you're going straight down with no pitching. That's not a challenge, BTW.
Interesting. I wonder if copters with pusher props slow down the main lift rotors at high speed to avoid the imbalances? Slows down the main rotor so much that it acts more like a non rotating wing? First look: World’s fastest vertical take-off chopper: th-cam.com/video/Db0g4Wlcw8A/w-d-xo.html
keghn feem. Great question! I haven’t gotten into the engineering behind helos with pusher props to see how compressibility or retreating blade stall is compensated. Some use coaxial design though which should help offset it. Thanks for the feedback!
@@helicopterlessonsin10minut10 I have been thinking about this and other helicopter aerodynamics for years. The mysteries of rotor craft are fascinating. Like the blade on gyro copters. The blunt leading edge prefers to move into the wind when i would thing the sharp trailing edge would. And the effect on rotating blades that are 90 degrees out of phase. Wow. Really like you channel!
Yo I just got selected thanks to some of this stuff. Please come back. You have the skills.
Even after graduating flight school and reading TC 3-04.4 over and over Compressibility still wasn’t clicking for me until I watched this video. Thanks for your help!
Great job!! I'm a professional helicopter pilot with 17 years of experience and I found your explanation to be extremely well done. I am in midst of taking a refresher course for a new job and came across the term "advancing blade compressibility". I had actually never heard of it before, we were only taught about retreating blade stall when I was in flight school. I figured it probably had to do something with crossing the speed of sound but I wanted to learn more so I googled it and came across your video.
I've done some really high altitude work in Nepal and we had to be very careful to not cross VNE when descending from extremely high altitudes. At closer to sea level it would be just about impossible to reach VNE in an AS350 but coming down from 20,000 ft that VNE is reduced to around 100 kts or maybe even lower and the aircraft really wants to dive and accelerate as you descend.
Anyway great job, keep up the good work!
I just went for my introductory flight Saturday for my birthday and loved every second of it. I always knew I wanted to fly (aspired to be an Apache pilot), and always figured i would get my PPL sometime, but now that I've gotten a little taste of the flying life I plan on making a career out of it. I just can't stop thinking about getting back in the air, and these videos are an awesome tool to help me learn before I've even started flight school.
Thank you for making these, and I really hope that you are able to come back and make more soon!
I just want to say thank you for all the information and being a teacher really. I myself took the SIFT and passed. Basically it was your videos and a little bit of self teaching from books that allowed me to pass. THANK YOU!!
Excellent explanation, free of complicated physics calculations and formulas. Great for the instructor's teaching 'toolbox'.
Hey Jacob, your content is extremely helpful, I appreciate it.
*update* I just passed the SIFT and your videos definitely helped me with understanding the concepts better!
Recently started using my G.I. benefits to start flight training. Discovered your channel last night right as I started getting into aero in the ground training and binge watched most of your videos... Walked into my second flight this morning and surprised my instructor understanding ETL, LTE, translating tendency etc. seemingly out of nowhere. MUCH APPRECIATED. Also gonna give you a shout out in my college helicopter fundamentals class tomorrow, should help the other students as we start moving into aero.
logono7. Awesome! Glad the videos helped out. Thanks for the shout out!
These videos have been very helpful. I’m using these to help study for the SIFT. Thanks for all these videos.
Loving your videos mate, really helpful. I'm doing all my pilot training theory via distance learning and these are definitely helping in the absence of an instructor. From a former US Army Blackhawk CE now doing pilot training in Australia (long story) thank you truly for these.
Excellent as always!
Very good vídeo ! 👍🏾🚁
What a great series.
Thank you chief, videos complete and ebook purchased! Next comes the SIFT and re-taking the ASVAB!
Bro, I used your videos to help me pass my civilian CFI (IP). Thanks for laying it out.
Circumference is DXpie, area is R2Xpie. I enjoy your videos, wish they were metric though.
Very interesting Jacob. As always...well explained.
Amazing lesson, thank you.
This was a very interesting one! I didn’t know about this aspect of low-temperature flying even though I’ve mostly flown in Russia and of course that involved flying in winter quite a few times. Not extreme but it’s still a good thing to be aware of.
I just bought your book! I had life kinda knock me out of the loop for a bit but I'm hitting the books hard again. Gonna start with yours!
Does the book cover PPL H theory or principle of flight? Love the videos, looking to purchase it
Would it be possible for you to make a video on the AH 66 Cheyenne project and provide some insight on when, if ever, we might expect the Army to obtain a helicopter with the capabilities of the prototypes which are now sitting in mothballs for more than 20 years.
Hey Jacob, your videos are really informative and helpful, can you please do a video on Figure of Merit or if you may suggest from where can i get information about it
Thanks again sir. Fly safe.
That was very interesting.👍 Jacob, have you considered doing a video on weight and balance basics? Thank you for your time!
Thanks! It’s on my list.
Top shelf work here, thank you for this.
please make a video about reverse flow on the rotor
Hello! I am a civilian who is looking forward to becoming a U.S Army helicopter pilot. I am studying with my books and your videos which help a lot. I was hoping you could make a video with hints and tips or what I could expect on the current test called SIFT ( Selection Instrument for Flight Training ).
Incredible video Jacob
I’d Like to recommend a video about ground affects over different types of surfaces such as fields and water compared to pavement.
He touched on this in the video of IGE and OGE hover.
What's your opinion on the X-2? Supposedly top speed is 300 mph. What about the S-97?
I have an question if main rotator 5000rpm then tail rotator also contain same rpm. Plsease explain that and I am from India
Could you make a video about how the gauges work like the attitude indicators and such?
I’ll add it to the list!
Hi Jacob, can you please make a video explaining horizontal & vertical stabilizers?
I’ll add it to the list. Thanks for the feedback.
Thank you sir. Great video!
I like how you mentioned the engineers
Which condition would you be likely to first encounter at high speed normal operating RPM flight retreating blade stall of advancing blade compressibility effects? Are there facors like temperature that could get you into compressibility before reatreting blade stall?
or
Love your videos!
I have some questions about coaxial-rotor and tandem-rotor helicopters and gyro stability.
As a lay person, I always thought that helicopters had the handy feature of being naturally stable because they were a giant gyro that wanted to stay level, just like a child’s toy gyro/spinning top.
However, what would happen if we redesigned that child’s toy and put in TWO gyros spinning in OPPOSITE directions around the same axis? It would not stand up. Likewise, a bicycle stands up when moving because of the gyro effect of its wheels. However, if we could magically make one wheel spin in the opposite direction, the bike would be unridable.
So it seems to me that the counter-rotating rotors of coaxial helicopters and tandem helicopters must take away the stabilising feature of a single rotor.
Why is this not mentioned in the “advantages & disadvantages of coaxial helicopters” lists that I see on the web?
Furthermore, how do the coaxial helicopters deal with gyroscopic precession?
---
mrjweate they are naturally stable on the roll and pitch axis, no helicopter is naturally stable on the yaw axis due to Newton’s 3rd law so it requires a counter torque rotor system. A child’s gyro toy spins as a whole itself, a helicopters rotor spins separate from its fuselage causing the fuselage to turn in the opposite direction, again due to Newton’s 3rd law.
I think you are confused about the whole bike thing. If a bike had its wheels turning in opposite directions, of course it would be unridable but that has nothing to do with a tandem rotor system, because a tandem rotor system maintains foward flight because of the pitch angle of the blades, nothing to do with how a bikes wheels work. It has opposite turning rotors to balance the torque of eachother.
I hope that answered your question, I’m not a genius like this guy but I know a fair share lol
Based on this, would it be correct to conclude that the actual Max Vne of a given helicopter would be the value where retreating blade stall and advancing blade compressibility are equal?
I am basing this on: Vne kt value tends to increase with a decrease in both temperature & altitude, and that blade compressibility value increases with an increase in temperature (also assuming altitude as that has a direct correlation to Mach numbed & kt value associated).
In other words, the fastest indicated airspeed I could fly would be at low altitudes on a slightly above freezing day.
Thoughts?
JustaGthing. Your operators manual bases VNE off of either Retreating Blade Stall, Advancing Blade Compressibility, or structural damage (whichever is lowest). It just so happens that when it's colder it's based on ABC and RBS when it's hotter. With that in mind, I would agree that the absolute latest onset of any of these limiting factors would probably be low altitudes at cooler temperatures. But every aircraft is different. Keep in mind rotor RPM and blade design affect these significantly. Great question!
Question, why did you reduce the Mach speed by 10% and not the required airspeed to hit Mach by 10%? I am not an engineer, just curious. It seems that if airspeed around an airfoil increases by 10% it would reduce the required speed to get to Mach 1. If this were the case it would need to be around 78 knots and not 65. Right? Please help me understand your math. Thanks. I really did enjoy the explanation, just curious about the math.
What do you think of the X-2 ? Supposedly it achieves 300 mph.
Thanks again. Fly safe!
great videos! thanks allot
When is a new video coming out??
Im heading to basic in 16 days and after that wocs then flight school. I wanted to know your opinion on the different airframes and why you chose the 64 over the 60 or 47.
Im not set on a specific airframe yet as I know selection can be a toss up, but I just wanted to know your opinion.
Thanks
-Kagan
Kagan Smith. I get this question a lot and it really just boils down to personality and what you want to do. 60’s and 47’s carry people and equipment around, conduct air assaults, humanitarian relief, etc. The 64 has the hunter-killer mission. In BOLC you’ll learn all about each of the mission sets. Ultimately, pick the aircraft based on the mission that you want to do.
Thank you sir 👏👍
Do you see any noticeable increase in performance when flying the Apache in the winter? Due to the lower temperatures? Nice video as always!
Vitor Moura. Absolutely. Engines and airfoils love the cool air and can give you better performance compared to hotter environments.
Mountainous weather / wind videos coming soon?
Maybe not soon. But eventually. Thanks for your patience.
Good info! 🚁💨 🍻
Learned Something new!
The V-22 Osprey rotor diameter is 38' so that means the rotor rpm can go faster since the circumference is smaller (and there is no retreating rotor blade in forward flight). The cruise speed is 270 knots. It seems like it has most of the advantages of a normal helicopter without the limited speed. The only downside is the cost.
Indeed, never thought about that! But in forward flight the Osprey in my eyes looks more like a propeller driven aircraft than a helicopter, P-Factor is applied here too, though it's not a big deal I'd say.
Tom Mendenhall. I don’t think the V-22 worries about compressibility or retreating blade stall because, just like you said, it has no advancing blade in forward flight. This really applies typical helicopter designs.
So, theoretically, if you can match the retreating stall speed with the advancing comp speed at specific airspeeds and temp, you're going straight down with no pitching.
That's not a challenge, BTW.
Interesting.
I wonder if copters with pusher props slow down the main lift rotors at high speed to avoid the
imbalances?
Slows down the main rotor so much that it acts more like a non rotating wing?
First look: World’s fastest vertical take-off chopper:
th-cam.com/video/Db0g4Wlcw8A/w-d-xo.html
keghn feem. Great question! I haven’t gotten into the engineering behind helos with pusher props to see how compressibility or retreating blade stall is compensated. Some use coaxial design though which should help offset it. Thanks for the feedback!
@@helicopterlessonsin10minut10 I have been thinking about this and
other helicopter aerodynamics for years.
The mysteries of rotor craft are fascinating. Like the blade on gyro copters.
The blunt leading edge prefers to move into the wind when i would thing
the sharp trailing edge would. And the effect on rotating blades that are 90
degrees out of phase. Wow.
Really like you channel!
The X-2's blades are made from carbon fiber. I wonder...... Carbon fiber doesnt flex at all, how would this do with blade compressiblity?
Haven’t seen a new upload in a while. Hope your safe brother.
Sorry about that. I’ve been bogged down with competing requirements. I’ll try to get one out soon.
😀
Your intro video here would make a great Zoom background :)
Compressibility wasn't a problem for Airwolf.
"I'm flying a helicopter, why am I taking about the speed of sound?"
Airwolf, obviously, duh.
Sheesh! Some people....
😊