Dissymmetry of Lift - Expanded

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Once again, I'm Jacob and in this video I want to expand more on what I covered in my first video Dissymmetry of Lift. More specifically, I want to break down the 5 different areas of the rotor disk that have drastically different aerodynamic characteristics. By expanding more on this, it makes explaining Retreating Blade Stall a little easier.
If you remember from my earlier video, I explained how the advancing blade will produce more lift than the retreating blade if left uncompensated. This is due to differences in airflow and how they affect the blades. But to go into detail, is can be broken down even further. The part of the blade closest to the hub on the retreating side experiences reverse flow. This means that the airflow literally flows from trailing edge to leading edge and therefore does not produce lift. This is all based on the fact that at any given blade RPM the blade tip travels faster than the hub. In this reverse flow region, the forward airflow through the disk is more than the rotational velocity.
The next region is the Negative Stall region. Simply put, a blade stalls if the resultant airflow is outside the critical angle. It creates boundary layer separation, turbulent air, and loss of lift. What makes it negative is that the airflow is impacting the airfoil above the chord line. In essence, the induced flow causes the airflow to push down on the airfoil outside the critical angle where it isn't useful.
Moving more outboard the next region is the Negative Lift region. This region. Is also Negative because the airflow impacts the airfoil above the chord line. But at this point in the blade the rotational speed combined with the induced flow causes the Resultant to be within the critical angle. This means the airfoil is no longer stalling and now creates lift. But because it is negative lift, it pushes the airfoil down. It's aerodynamically the same as if you were to impart negative pitch in the blade. These 3 regions are known collectively as the "No-Lift Areas" and they grow in size as airspeed increases. Outside of these regions is where positive lift occurs. Just like the name implies, this is the good stuff that we need to fly. The Resultant airflow is inside the critical angle and below the chord line which produces lift. For more info on how lift is created, check out my video here ( • How Lift is Created ).
With enough forward airflow, a 5th region can develop. This is a Positive Stall condition that begins to form on the retreating side. It's positive because the airflow is below the chord line and is a stall because the airflow is outside the critical angle of the airfoil. Here the airflow separates from the airfoil and lift no longer increases with increases in Angle of Attack. My next video will expanded more on this subject ( • Retreating Blade Stall... ).
Thanks for watching! Don't forget to hit like and subscribe below. Be sure to like us on Facebook as well. As always, safe flying!
If you're just getting started and want more information, pictures, and more explanations, I'd recommend reading the Rotorcraft Flying Handbook - amzn.to/2ifPlnZ. If you've already got a basic understanding, and want to further your professional helicopter education with advanced helicopter concepts, I'd recommend reading Cyclic and Collective, by Shawn Coyle - amzn.to/2ifQGLx

[@LifesLogbook]

I'm enjoying your instructional videos. I've been a fixed wing guy for a decade and the explanations you have are turning the light bulb on. Thanks for taking the time to do these.

[@ColeShortridge]

man these videos are gonna get me through my private's license training, thanks for making these

[@jeremylakenes6859]

At 4:42 could you just draw the relative wind hitting the airfoil, but draw it upside down showing “lift” then when you flip over the page you see it’s creating “lift” but it’s downward lift.

[@ChopperKool]

Well Jacob, what can I say... You really have stolen the words from my mouth! Ha ha! Simply excellent ...! And thaks for the class... Gretings from Guatemala!!!

[@antoinecamus51]

Nice one again Jacob! Looking forward to watch your next video

[@jeremylakenes6859]

Positive stall the air doesn’t seem to be too much below the chord line, but the retreating blade is so close in speed of the helicopter. Blade at tip spins at 200kts and heli goes 150. That’s only 50 knots over the airfoil.

[@ddtddt8493]

Jacob well done (this is no news) I'm posting extracts from my flight lesson. oh how I remember when it seemed I was unlearning as the hours accumulated. you videos are a great scientific explanation of the feedback my butt and back are telling me whilst flying.

[@Chilli1291]

Excellent videos. Thx u. Could you explain me what’s the reason for the vne reduction in increasing altitude.

[@christopherpeters5916]

Would that be the ideal nose pivot point?

[@zachrubino7869]

I can smell that marker from here. Great videos man!

[@EUC_Senior]

Very informative. Thanks again Jacob! Mark

[@adamhough7941]

Hi Jacob. I have cyclic and collective and principles of helicopter flight. They explain it well enough, but I have been unable to locate the five regions you speak of. Where did you find them? Thanks for your time and all of the videos-they're great.

[@helicopterlessonsin10minut10]

Adam Hough. Those regions come from the Army Fundamentals of Flight manual. Thanks for the feedback!

[@davidwallace5738]

Thank you sir! Great video and explanation! Fly safe.

[@tyto39]

Jacob thankyou for making these informative videos. it's a great help to all aviators. Can you please explain why resultant wind impacts from above Chord line in Regions B & C while the Pitch Angle is greatest at Root and Least at Tip? Shouldnt resultant airflow be impacting below chord line in all regions?

[@blake8ism]

Great breakdown!

[@iPod386]

I've never heard about B and C area. Could you explain this more?

[@sharonburns4788]

Thanks, that answers many questions.

[@mariojorgepiresdasilva9956]

Good vídeo ! 👍🏾🚁

[@victor-emmanuel7485]

I can not explain how thankful I am for you making these videos! When I eventually have the funds, I plan on doing my PPL-H. I know you are a military pilot, but maybe you have some experience with this; Which course books are the best to buy to study the theory on my own? I noticed there are a lot of titles to choose from. Any recommendation I would greatly appreciate! I’m from Belgium, but I guess the theory is the same here, as it is in the usa?

[@helicopterlessonsin10minut10]

Verdi. I glad the videos are helping out. I haven't gone through the process of a private helicopter school. I received all my training in the military and then did the my civilian equivalency ratings. If you're looking to just learn the basics in aerodynamics I'd recommend reading the Rotorcraft Flying Handbook - amzn.to/2ifPlnZ. If you've already got a basic understanding, and want to further your professional helicopter education with advanced helicopter concepts, I'd recommend reading Cyclic and Collective, by Shawn Coyle - amzn.to/2ifQGLx.

[@victor-emmanuel7485]

Helicopter Lessons In 10 Minutes or Less I will look into it, thanks a lot!

[@eshanawasthi6081]

can you suggest me any video on blade element theory. thanks --student from India

[@OmnipotentSag]

That's cool and all, but how does that play out when you are flying the heli? What does having lower lift on one side mean? I don't see any heli's flying sideways just so, so how is flight effected? What does the pilot need to think about because of this? It is a great explanation (this and the shorter video you did), but I'm just curious about applications. -=80)

[@Al-lu8fd]

Helicopter designers compensate for this by allowing blades to flap. I believe he has a video on this already, so that might be a useful one to watch if you haven’t gotten an answer on that yet.

[@matas1024]

why does it go to critical AOA at B area?

[@christopherpeters5916]

I'd turn collective loose use pedals to rotate around that nose point.

[@diegoplata5570]

again, thanks

[@christopherpeters5916]

Is that correct sir?

[@matas1024]

or else, why its differend AOA at B and C?

[@helicopterlessonsin10minut10]

matas102. B and C represent different resultant airflows in relation to the chord line. While AOA may be different at different parts of the blade in the same relative position due to blade tapering, this diagram outlines the subtle differences in how the airflow strikes the airfoil. The biggest difference between B and C is that area B has more of a vertical downwards velocity due to induced flow velocity being greater than rotational velocity. This puts the airflow outside the critical angle. Area C is the point at which rotational velocity overcomes induced flow velocity enough for the airflow to intersect the airfoil inside the critical angle. Thanks for the question. I hope this helps.

[@matas1024]

Have you covered what is blade tapering in your past videos? I just searched for good explanation of it on the internet and I got some kind of idea what's going on. Is it worth a video about it? I find it confusing without your drawings. And thanks for the answer, very informative :))

[@helicopterlessonsin10minut10]

matas102. I haven't covered blade tapering but I'll look into it as a topic for a video. Thanks for the feedback!

[@christopherpeters5916]

Sir?