Three no lift areas

[baobab72]

Hello folks, i was watching a video on youtube about dissemetry of lift. Although i did grasp the concept the lady instructor mentioned the three no lift areas on the retreating blade, referring to them as negative lift, reverse flow and negative stall.
What was she talking about?
Many thanks

Baobab72

[army_av8r]

oh boy, lets see if i can explain this without confusing you more... so, the reason these areas exist is because the aircraft has forward velocity. lets assume we are traveling at 100 knots, this 100kts of airflow is traveling across the rotor disk, and at the inboard portion of the disk(near the hub) it is actually faster than the speed of the blade itself. i assume you know that for a given RPM the tip is traveling "faster" than the hub. from inboard to outboard, it goes, Reverse Flow, Negative Stall, Negative Lift. the reverse flow section is pretty easy to comprehend, the airflow is out running the blade and the air passes from the trailing edge to the leading edge. negative stall occurs at the point on the blade where the blade is traveling slightly faster than the relative airflow. but since the majority of the air is being pulled down through the disk(induced flow) the air is hitting the blade on top. exceeding the critical angle, but because it is hitting on top it is a "negative" stall. as you move farther out on the blade, the rotational velocity of the blade continues to increase to the point where the wind is hitting closer to the front of the blade. it is still hitting the blade above the chord, but it is no longer stalled. this now produces lift but the lift is "negative", or down. the faster you travel in any direction the farther these regions will spread on the retreating side. it is all about where the total relative wind strikes the blade and the velocity of that relative wind. hope that makes it clearer and not the other way around.

[baobab72]

Hi av8r
first of all many thanks for having taken the time to answer me.
your explanation is very clear, just wanna make sure i did get this right:
1 reverse flow: due to the greater velocity of the airflow induced by the forward motion of the aircraft - relative airspeed - with respect to the relative low rotational velocity of the airfoils closer to the hub, the overall effect is that the wind if flowing from the trailing edge to the leading edge, opposite the direction the airfoil is designed to be effective, resulting in a negative airflow and no lift/thrust production.
2 negative stall: as the distance of the airfoil from the hub increases so does its rotational velocity up to a point where it becomes slightly higher than the relative airspeed which unlike the rotational velocity remains constant throughout the rotor disk, reversing the reverse flow as the air is hitting the airfoil onto the leading edge, however due to the predominant vertical component of the induced flow, the induced angle is very large resulting in a steep resultant relative wind, exceeding the critical aoa resulting in a stall.
3 negative lift: as the distance from the hub increases even further, the rotational velocity of the airfoil increases as well up to a point where the horizontal component of the resultant relative wind is such to decrease the aoa below the stall, however due to the fact that the resultant relatve wind is still above the chord, the resultant lift is oriented downwards, thus generating a negative lift.

Let me know if i have got this right.

Many thanks

baobab72

[army_av8r]

I think you have it! just remember that negative stall produces an upside down stall due to the large induced flow velocity hitting the top of the blade. I think you know that, I just didnt see it in the post. all of this makes much more sense when you use an airfoil diagram to supplement the words!

[baobab72]

Thanks to you!

Baobab72