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!

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.

Good, practical answers to helicopter aerodynamics....Ray Prouty.

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.

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)

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.

Awesome, thanks! I just ordered volume 3 of his articles.

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.

Excellent info, I’ll have to play around with the lift equation.

So phase lag, not gyroscopic precession? (Another difference I’m trying to better understand.)

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

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.

Yes, as AC says above - phase lag:ok:

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.

Good old Lu Zukerman!

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.

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

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.

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!)

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.

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.

If it helps...

The combination of inflow roll and flapback makes the disk move up or down at a point just left of the nose, but they have their greatest individual effects at different speeds. Those of inflow roll are greatest at low speeds, on leaving the hover, and decrease significantly with speed. At higher speeds, dissymmetry of lift will be the most dominant force.

As the effects of flapback are greatest at high speeds, the Tip Path Plane rises near the retreating blade when first moving from the hover, then moves round towards the front as speed increases.

As for having to push forward, one simple explanation is that, as you push the cyclic forward, the body lags behind, but, when it does catch up, it centralises the cyclic, assuming you haven't moved it. So you have to push forward to keep things going.

Phil

As an American the only time I ever heard the term "blowback" was in reference to one of the things that happens after you get into "Low-rpm Rotor Stall" in the R22 and are falling to your death.

,...and correct, the nose does not rise with it.

In order to make the helicopter rotor do what we want it to do, we have to create a dissymmetry of lift with the cyclic.

Flapback and inflow roll are the rotor flapping to equality because we have created an inequality, either in speed on one side of the disc or inflow angle between front and back.

In a transition to forward flight, if left to its own devices after an initial forward input, the rotor will pitch up and roll towards the advancing side - we prevent that happening with appropriate control inputs.

Flapback occurs throughout the flight envelope and is very noticeable when you change speed - inflow roll is most noticeable at lower speeds but still occurs when there is a difference in inflow angle between front and rear.

The rotor seeks to equalise lift, we seek to create an inequality to control the aircraft and make it go where we want.

Robbiee - the US military used the term blowback for many years to describe flapback,.and the FAA helicopter flying handbook uses exactly the same term.

Well, I've never been in the military (and they seem to have their own way of explaining things anyway which at times has conflicted with the civilian side), nor am I familiar with the term "flapback".

I'm also not familiar with the Helicopter Flying Handbook, but in my old Rotorcraft Flying Hanbook neither term is used to describe Transverse Flow Effect. If it is used elsewhere you'll have to point it out, as I only recall it ever being mentioned in the Robby POH under Low-rpm Rotor Stall.

I will admit though that my understanding of aerodynamics is on a very simple "surface level".

Well, not a nghtmare, but a huge flashback!

When I took over as CGI at Culdose in 1976, the available PoF teaching materials were "somewhat lacking in rigour" and I determined to to improve them. I gathered information from all the sources I could tap into and synthesised them into a set of class handouts. I was afraid that I might have over-stepped things a bit with the rigour, and some of the beefers of 705 Sqn were not too happy, either. When I left the job to go back to sea, I expected there would be a certain amount of watering down in the future.

In those days the AP3456 was quite a small volume and the Helicopter section quite brief, so it was largely ignored. Over the years I have come across some phrases and diagrams in various other publications that looked familiar, but I have never seen the later versions of AP3456.

Imagine my surprise, and gratification, when I followed the link and found huge chunks of the early chapters were verbatim from my handouts. Even most of the diagrams are [almost] photographic copies of my diagrams, with the labels [replicating those I] made with Rotring stencils.

There are revisions and interpolations, of course, and there may be some original errors, but it does seem to have stood the test of time pretty well. ;-)

Edited to clarify that, on closer inspection, the diagrams are not photographs. However, they are such close replicas that I was fooled at first.

As stated, in any Professional discussion of helo rotor aerodynamics the effects we are discussing occur on and around on the disc alone. What the fuselage does is entirely a secondary matter of mechanics, not aerodynamics.

The only use of the expression of 'blowback' that I know is always accompanied by a number up to five and ideally followed by a cry of 'Syph on your donk' or some such ribald dimissive.

No-one who isn't ex-Brit mil will understand that reference:)

It is only unequal during the control input. Once things stabilise, lift is equal on all of the disc.

Hey, I can post again! Apparently I'm limited to 5 posts per day, so I'll try to group responses until I'm out of probation.

My understanding was that it was gyroscopic precession when an outside force is acting on the disc, and phase lag when we are changing the disc itself through control inputs. You're saying it's always phase lag?

If I'm understanding this correctly, flapback is moment of dissymmetry of lift, and the rotor system needs a nudge from us in order to move back into symmetry. Once in forward flight, it's actually a control input that creates a moment of lift dissymmetry, after which the rotor system will re-balance?

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I do this with students currently, and while I could explain the roll toward the advancing side, I couldn't explain the nose pitching up. You guys are definitely helping me understand that part of it.

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So the large forward cyclic input during takeoff is, at least in part, due to the weight shift of the fuselage as it swings under the moving rotor disc, plus the changing angle between the fuselage and rotor disc altering our cyclic inputs?

This looks like a great read, thank you for the link!

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Very cool backstory as well!

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Another book I'll have to find, although this one seems to be long out of print.

Sun of Atom, it seems your Instructor Training was lacking a lot in aerodynamic theory - not your fault, it is the result of too many junior instructors passing on what their junior instructor told them. Chinese Whispers.

GP doesn't exist in a rotor system. A gyroscope is a flat disc fixed rigidly to a shaft. A rotor system had flexible blades attached to the shft by flapping hinges, and often lead-lag hinges, and the blades can feather as well. Too many flexy floppy bits to be called a gyroscope. But to make it easier for students to understand phase lag and lead angle, it has been likened to a gyroscope. Sadly, some sources have locked onto this part and pass it on as the truth.

The only time the disc is OUT of symmetry is the momentary change of something, be it a wind gust or a control input. Three poofteenths of a second later, the lift over the entire surface of the disc has equalised, otherwise you will get acceleration in the change of disc attitude.
Say you are in a stable hover, cyclic neutral. A puff of wind blows onto the nose. The initial part of flapback is the disc flapping away from that extra wind. The disc has "flapped to equality" and lift is again equal over the disc, but it is now pointed backwards, and the aircraft starts to move backwards. the momentum builds, the disc passes from having a breeze from the front, to now having the effective breeze coming from the back, and it flaps away from that "breeze". The fuselage lags a bit behind, so as the disc moves forward, the fuselage swings nose down, helping the disc to go a bit lower at the front. The forward motion speeds up, and again it flaps away from the wind, to go nose high. The lagging fuselage swings even further nose high, so now you have an exaggerated rearwards thrust vector. Very soon, you will crash.

To stop this, as soon as the pilot sees the attitude change from what he/she/they/whatever desire, a cyclic input is made to stop it. And in stable flight, be it a hover or forward/sideward flight, THERE IS NO DISSYMMETRY OF LIFT.

To move into forward flight, a cyclic input is made to lower the disc (and the nose) at the front. The aircraft starts to move, and with movement comes the flapback (countered by progressive forward cyclic) and inflow roll (countered by lateral cyclic). For every increase in speed, there is an initial forward cyclic to start it, and then some more to hold the attitude down against flapback. Eventually you reach the limit of cyclic movement and you cannot go any faster because you cannot prevent the subsequent flapback. The weight shift of the fuselage, as you put it, isn't a factor in this, unless there is some extreme drag associated with the fuselage.

"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."

Lu is now deceased, but I did publish his book for him. There may be one or two around on the internet. I will see if there are any lying around here.

Lucky me, I have 2 of them, signed by Lu.

I'd be interested in purchasing a copy if you have any extras.

That brought back memories from some years back on the forum! Loved Lu’s passion for trying to get to the bottom of the Robinson rotor divergence issue - I remember the thread where he even got a response from Frank Robinson himself. He certainly got some sparks flying with the debate.

I think that is probably correct in theory AC but not in practice since a helicopter never seems to be in stable flight.

In an unstabilised heilcopter, no matter how accurately you set it up in the hover or at a specific height and speed, you are always making small control inputs to prevent the aircraft going off and doing its own thing.

In a stabilised aircraft the actuators in the SAS/ASE are constantly working to do the same thing. Or in the AP in a more complex aircraft.

There are too many variables - wind gusts, turbulence, the wake from the preceding blade, the disturbance of a blade passing over the fuselage, MR/TR interaction etc etc and as many have said here, the reason a helicopter is more difficult to fly than a FW is because it is inherently unstable.

Try taking your hands and feet off the controls in an unstabilised helicopter in the hover and see how long you last :)

Instability is a bit different from an unequal distribution of lift, though. The lift is the same, until something disturbs it, be it a wind gust or the copilot losing control of his bowels. Once disturbed, the static stability tries to restore it to its original position, but the dynamic instability makes it overshoot, and away you go.

I agree but there is always something disturbing it so a steady state of equal lift is an ideal but not a practical reality.

It's just a heap of flapback similar to when you raise the collective in forward flight.

When the disc is completely through ETL, there is a massive increase in AoA all over the disc as the disc moves away from the induced flow. You can tell because not only does it pitch up, it also climbs. Since there dissymmetry of lift and L = AoA x V² , there is a more lift on advancing blade causing the nose to come up.

Sikorsky had a really good book it published on helicopter aerodynamics.....which I knew as the "Sikorsky Blue Book"....which some evil rascal thought more of than I did and stole it.

The Title is "Sikorsky Helicopter Flight Theory For Pilots And Mechanics". I highly recommend it for use by those wishing to learn more about the subject as it is written in a manner that even Pilots can understand and has drawings for the CFS CFI's to color with wax crayons.

In my quest to find one....I discovered there are three printings of that fine tome on helicopter aerodynamics.....1953, 1964 (the version I carried for years in my helmet bag) and 1994 (which according to John Dixson has a gray cover).

I have found a few at some used book sites.

They are not cheap as they are scarce.