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What causes "blowback/flapback" when accelerating through transverse flow effect?

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What causes "blowback/flapback" when accelerating through transverse flow effect?

Old 10th Aug 2022, 15:44
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What causes "blowback/flapback" when accelerating through transverse flow effect?

I'm trying to get my head around some of the aerodynamics, and I have yet to find a good explanation of blowback.

I understand the slight right roll (CCW main rotor) when entering transverse flow effect, and the vibrations due to the uneven airflow and forces across the disc as we move through TFE. I'm not understanding why the nose wants to come up as we are getting through TFE and into ETL. I found a couple references to it being due to dissymmetry of lift becoming present and blade flapping starting to occur, but if that's the case, why would the change be so pronounced?

Thanks!
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Old 10th Aug 2022, 16:54
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Flight school was a long time ago, but I don't remember "blowback" being associated with Transverse Flow.

From what I recall, yes, the onset of Dissymmetry of Lift and ETL will cause the nose to want to rise, but I don't recall it being all that much. I mean, my subconscious just pushes through it anymore, so I haven't experienced it in years.
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Old 10th Aug 2022, 17:28
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Originally Posted by SunofAtom View Post
I'm trying to get my head around some of the aerodynamics, and I have yet to find a good explanation of blowback.

I understand the slight right roll (CCW main rotor) when entering transverse flow effect, and the vibrations due to the uneven airflow and forces across the disc as we move through TFE. I'm not understanding why the nose wants to come up as we are getting through TFE and into ETL. I found a couple references to it being due to dissymmetry of lift becoming present and blade flapping starting to occur, but if that's the case, why would the change be so pronounced?

Thanks!
Good, practical answers to helicopter aerodynamics....Ray Prouty.
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Old 10th Aug 2022, 19:18
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Also known as Flapback, caused by the difference in Ralative Air Flow between the advancing blade (getting more RAF) and the retreating blade (getting less lift). The difference is greatest at the 90/270 degree position, and the differences in lift make the advancing blade want to flap up and the retreating blade flap down. The dreaded "flapping to equality."

If unchecked, the nose would rise, the airspeed drop off to zero, and the aircraft then fly backwards, because that's where the disc is pointed now. The same thing happens backwards, so the tail flaps up, nose goes down, and it all starts again. However, the pendulum effect of the fuselage makes the nose kick higher at the front and then the nose kick lower at the back, so the oscillations increase each cycle until after about 3 or 4 cycles, you have crashed.

Simple fix - as you move forward and the nose wants to rise, add a little more cyclic to hold the attitude where you want it.
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Old 10th Aug 2022, 21:16
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Flapback (blowback) = differential lift caused by speed differences between advancing and retreating sides of the rotor disc.

Inflow roll (transverse flow) - differential lift caused by differences in inflow angle between front and rear of rotor disc when tilted in the direction of travel.

In a normal transition gives a nose up and roll towards the advancing side

If you change something in the lift equation - Lift = CL 1/2 rho V squared S then with a helicopter rotor you create an inequality which is compensated for by a change in rotor flapping.

In Blowback the inequality is in V squared, in Inflow roll it is in CL as the inflow angle and therefore AoA changes due to disc tilt.

Remember inequalities in lift are resolved 90 degrees out due to flapping and aerodynamic damping (very much not due to precession)
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Old 10th Aug 2022, 22:10
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Originally Posted by Robbiee View Post
Flight school was a long time ago, but I don't remember "blowback" being associated with Transverse Flow.

From what I recall, yes, the onset of Dissymmetry of Lift and ETL will cause the nose to want to rise, but I don't recall it being all that much. I mean, my subconscious just pushes through it anymore, so I haven't experienced it in years.
Yes, as a commercial pilot, I have definitely learned to push through without any thought. As a new-ish CFI, I see students wrangling the helicopter throughout the takeoff, and I wanted to really understand the aerodynamics so that I can explain it thoroughly. I’m sure the “just push forward” explanation that I was given would probably suffice, but I also enjoy this pursuit of better understanding.
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Old 10th Aug 2022, 22:11
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Originally Posted by LRP View Post
Good, practical answers to helicopter aerodynamics....Ray Prouty.

Awesome, thanks! I just ordered volume 3 of his articles.
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Old 10th Aug 2022, 22:29
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Originally Posted by Ascend Charlie View Post
Also known as Flapback, caused by the difference in Ralative Air Flow between the advancing blade (getting more RAF) and the retreating blade (getting less lift). The difference is greatest at the 90/270 degree position, and the differences in lift make the advancing blade want to flap up and the retreating blade flap down. The dreaded "flapping to equality."

If unchecked, the nose would rise, the airspeed drop off to zero, and the aircraft then fly backwards, because that's where the disc is pointed now. The same thing happens backwards, so the tail flaps up, nose goes down, and it all starts again. However, the pendulum effect of the fuselage makes the nose kick higher at the front and then the nose kick lower at the back, so the oscillations increase each cycle until after about 3 or 4 cycles, you have crashed.

Simple fix - as you move forward and the nose wants to rise, add a little more cyclic to hold the attitude where you want it.

Interesting, so it does seem to be related to the start of dissymmetry of lift. It’s surprising that it causes such a big change, but I guess it makes sense. We are moving out of a near vertical induced flow of air to a relatively undisturbed horizontal flow of air. The AoA change on the advancing side is probably quite dramatic.

I hadn’t considered how it would play out if it was left unchecked. That sounds… less than ideal.
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Old 10th Aug 2022, 22:48
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Originally Posted by [email protected] View Post
Flapback (blowback) = differential lift caused by speed differences between advancing and retreating sides of the rotor disc.

Inflow roll (transverse flow) - differential lift caused by differences in inflow angle between front and rear of rotor disc when tilted in the direction of travel.

In a normal transition gives a nose up and roll towards the advancing side

If you change something in the lift equation - Lift = CL 1/2 rho V squared S then with a helicopter rotor you create an inequality which is compensated for by a change in rotor flapping.

In Blowback the inequality is in V squared, in Inflow roll it is in CL as the inflow angle and therefore AoA changes due to disc tilt.
Excellent info, I’ll have to play around with the lift equation.

Originally Posted by [email protected] View Post

Remember inequalities in lift are resolved 90 degrees out due to flapping and aerodynamic damping (very much not due to precession)
So phase lag, not gyroscopic precession? (Another difference I’m trying to better understand.)
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Old 11th Aug 2022, 07:51
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When I was at flight school many moons ago, I was a cadet "tagged" to a fixed wing course. The fixed-wing cadets (we'll call them planks) studied "Principles of Flight Fixed-Wing" as it was a know science. For the bold, courageous, handsome rotary-wing cadets (all 2 of us), it was called "Theory of Flight Helicopters" as some of it was still an unknown science and still a theory.

To answer your question, not a clue mate, it was a long time ago. Good luck with the flying

ITI
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Old 11th Aug 2022, 08:56
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Interesting, so it does seem to be related to the start of dissymmetry of lift. It’s surprising that it causes such a big change, but I guess it makes sense. We are moving out of a near vertical induced flow of air to a relatively undisturbed horizontal flow of air. The AoA change on the advancing side is probably quite dramatic.
Ahh, now you are getting into Translational Lift, which as you correctly say, is moving into air that has had little chance to start moving downwards through the disc. And push it down you must, in order to hold the aircraft up. A lot of theory books concentrate on the Bernouilli equation without mentioning that the airflow at the back of the airfoil is headed downwards - the flow over the top doesn't "have to meet the flow underneath", and in fact it gets there well before the lower flow, hence the movement downwards, the downwash.

The term Gyroscopic Precession has used up many threads on this site. It is only a means of understanding the dynamics of the rotor system, but the rotor is NOT a gyroscope. Phase Lag is the correct term, and it is fixed by adding an Advance Angle to the inputs from the swash plate to the disc. The lag is approximately 90 degrees, but gets as low as 72 degrees on the Robinson. But if precession is the way to grasp this concept, go for it, but don't tell anybody that you believe in it. A bit like being a Flat Earther.

Regarding Dissymmetry of Lift, it only happens until the pilot pokes the cyclic forward to stop the flapback. After that the lift on both sides is exactly the same, in steady flight. The retreating blade is suffering, with the relative airflow reduced by the forward velocity, so the advancing blade has to "throw away" all that beautiful lift it gains from forward movement, to match the poor cousin on the other side.
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Old 11th Aug 2022, 09:07
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So phase lag, not gyroscopic precession? (Another difference I’m trying to better understand.)
Yes, as AC says above - phase lag

You can perform a very good demonstration of inflow roll and flapback - start in about a 10' hover into wind if there is any (best done with zero or very light wind) and with the AP disengaged if you have one.

Maintaining the collective position, initiate the forward movement with a small amount of cyclic and hold it. The aircraft will start to move forward and descend slightly - then in reasonably quick succession the nose will pitch up and the aircraft will roll towards the advancing side of the disc.

Once your students understand what the aircraft wants to do, they will better counteract it by maintaining the disc attitude during the transition.
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Old 11th Aug 2022, 10:49
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Remember inequalities in lift are resolved 90 degrees out due to flapping and aerodynamic damping (very much not due to precession)
Good old Lu Zukerman!
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Old 11th Aug 2022, 11:07
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Good old Lu Zuckerman!
Very knowledgeable man. Rocket scientist too. He wrote a book called "Finger Trouble" which makes your skin crawl with the things that people did wrong in the space race.
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Old 11th Aug 2022, 11:37
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Originally Posted by SunofAtom View Post
Interesting, so it does seem to be related to the start of dissymmetry of lift.
The advancing blade gets increased lift due to it 'seeing' higher relative airspeed. In a counter clockwise rotor this starts around the 5 o'clock position. from then on the blade will see higher lift than on the opposite (retreating) side. The max increase of lift will be on the 3 o'clock position. After that the increase will reduce (but the delta of lift force to the opposite blade will still be positive, i.e. the blade will still continue to rise, only at a lower rate). Even at the 1 o' clock position it will see higher relative airspeed and thus lift than on the opposite side. So up to beyond the 1 o' clock position there will still be higher lifting force compared to the opposite blade and will thereby lift the advancing blade. The net decrease of lift will start at 12 o'clock. Thus the apogee of the blade track will be beyond 12 o' clock. And that is not yet including inertia. Just net lift forces
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Old 11th Aug 2022, 11:58
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When I was in basic training we never heard the term 'blowback' (is that an Americanism perhaps?) and the 'nose' most certainly did not rise with flapback. The clue is in the word itself, it is the disc that chages attitude.
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Old 11th Aug 2022, 12:11
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Originally Posted by meleagertoo View Post
When I was in basic training we never heard the term 'blowback' (is that an Americanism perhaps?) and the 'nose' most certainly did not rise with flapback. The clue is in the word itself, it is the disc that chages attitude.
Behaviour of the nose might depen on whether you are sitting in a semi- rigid rotor helicopter or an articulated/rigid one. In semi rigid ones the disc is living its own -mostly independent- life. In the other two types cabin and disc are somewhat sharing their behaviour.
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Old 11th Aug 2022, 12:18
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Originally Posted by meleagertoo View Post
When I was in basic training we never heard the term 'blowback' (is that an Americanism perhaps?) and the 'nose' most certainly did not rise with flapback. The clue is in the word itself, it is the disc that chages attitude.
What normally happens when the disc attitude changes? Isn't that the whole basis for changing direction and speed?

For the original poster, you may find this interesting reading - https://assets.publishing.service.go...elicopters.pdf (hopefully won't give some readers nightmare flashbacks!)
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Old 11th Aug 2022, 13:10
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When I was in basic training we never heard the term 'blowback' (is that an Americanism perhaps?) and the 'nose' most certainly did not rise with flapback. The clue is in the word itself, it is the disc that chages attitude.
Change of attitude is what the fuselage does as a response to where the rotor is taking it.

As alluded to by Henra - a teetering rotor drags the aircraft body around underneath it, so changing the relationship between the rotor disc and the fuselage (from control input, gust of wind or aerodynamic flapping) may take slightly longer but the fuselage attitude change still happens. As 212man says - it is the whole basis for changing direction and speed.

The bigger the hinge offset from the centre of the rotor mast, the bigger the lever that the rotor has to exert force on the fuselage so the quicker the fuselage attitude change is following a rotor movement.

If your helicopter during training didn't noticeably exhibit a nose up as you increased speed, it is probably because you automatically compensated for it by moving the cyclic - that means your instructor taught you correctly.
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Old 11th Aug 2022, 15:19
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Originally Posted by SunofAtom View Post
Yes, as a commercial pilot, I have definitely learned to push through without any thought. As a new-ish CFI, I see students wrangling the helicopter throughout the takeoff, and I wanted to really understand the aerodynamics so that I can explain it thoroughly. I’m sure the “just push forward” explanation that I was given would probably suffice, but I also enjoy this pursuit of better understanding.
Actually, I was agreeing with you that its a combination of the onset of Dissymmetry of Lift and ETL.

"Just push through it" is the solution, not the explanation.
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