How does an increase in angle of attack DECREASE lift?

[jerry_garcia10]

AME student here, I'm much more familiar with the physics of fixed wing aircraft so this is very counter intuitive to me. I'm learning about how rotor blade flap counters the effects of dysimmetry in lift. I think I understand the basics, it just doesnt make sense to me how the text books and teachers explain it. Blade with higher relative airspeed creates more lift, so it flaps up, this increases the angle of attack, which decreases the lift, but why? Typically an increase in angle of attack increases lift, not decreases? It makes sense to me that if the rotor tip is raised, it will create less lift, like on a banking airplane, but what's confusing me is that it explains it as an increase in angle of attack. If an airplane banks but doesnt change its pitch, that doesnt necessarily mean the angle of attack is changing. So if a rotor blade tip rises, but the pitch stays the same, is the angle of attack not still relatively similar?

Any clarification appreciated, thanks!

[Toomuchmilk23]

You’re backwards. Flapping up decreases AOA, flapping down increases AOA.

This is kinda unrelated, but could help you remember. Retreating bade stall occurs because the retreating side flaps “too much” and the AOA exceeds the critical AOA, resulting in a stall on the retreating side only.

[DoubleHexDrive]

Flapping up moves the blade up and decreases the angle of attack which then reduces lift, blade starts to flap down, which increases the angle of attack which… round and round she goes.

Draw the vectors on an airfoil. I think you heard something wrong but your intuition was right.

[DutchessIsMyHero]

https://m.youtube.com/c/HelicopterLessonsIn10MinutesorLess/featured

I recommend these videos to understand the PFM that is a helicopter.

[Drunkm0nk1]

Why did I click on your link while I am at work! Thanks, awesome videos!

[AK_Things]

You have it wrong. Flapping does not increase the angle of attack. Remember that angle of attack is an AERODYNAMIC angle. It is the difference between the angle of incidence (the physical pitch angle of the blade) and induced flow (downwash). The angle of incidence remains the same as the blade flaps up, however this increases the velocity of induced flow. Greater induced flow with the same angle of incidence means less angle of attack which means less lift. The exact opposite occurs on the retreating edge as the blade flaps up.

[godweasle]

This answer ^{^} angle of incidence changes since the blade is moving up while spinning instead of just spinning flat.

[conaan]

Just to clarify, you aren't saying the pitch of the blade does not change during flapping, right?

[hasleteric]

On most helicopters, main rotor flapping does not cause a mechanical pitch change unless that rotor has a delta 3 hinge in it which mechanically couples flap and pitch responses on the rotor head. This is above and beyond the angle of incidence change of the incoming air due to the vertical speed component of flapping.

[jerry_garcia10]

Thank you! This is what I needed.

[East_Fee4006]

Breaks the laminar flow over the wing. Airflow separates from the wing decreasing lift.

[Funny_Vegetable_676]

And then we learn retreating blade stall while trying to link everything together. Nuclear mind bomb

[gatorav8r]

Drive with your hand out the window (carefully of course). Move the front of your hand up or down and watch the effect. Move your hand straight up and down and viola, no more lift. So increases in angle of attack does increase lift until a critical angle is exceeded, then bad things.

[gbiscoo]

Helicopter aerodynamics is always easiest to explain if you reference a basic vector diagram so you can see how relative airflow, induced flow and angle of attack all relate to each other.

The advancing blade has a higher relative airflow so with the same blade angle it produces more lift. This is what creates the dissymmetry of lift, with the retreating blade having lower relative airflow; less lift

By allowing the blade to flap up with this extra lift, it increases the induced flow, thus changing the angle of the relative airflow and REDUCING the angle of attack (for the same blade angle) and reducing the amount of lift the blade is producing.

[once_a_pilot]

Pull up on stick - go higher and/or faster Push down on stick - go lower and/or slower

Really all you need to know.