Compensation for Dissymmetry of Lift in Helicopters

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In this lesson I go over how Cyclic Feathering and Blade Flapping compensate for dissymmetry of lift. As a quick summary, dissymmetry of lift is the difference in lift that exists between the advancing and retreating side of the rotor disk. If this were allowed to continue without compensation, there would be no way to maintain forward flight. The advancing side would continue to produce more lift than the retreating side and this would produce a constant rolling motion to the left. But luckily helicopters are designed to compensate for this using blade flapping and cyclic feathering. These allow the pilot to reduce some of the lift of the advancing blade while simultaneously increasing the lift of the retreating blade.
Blade Flapping occurs when the advancing blade flaps up in its plane of rotation while the retreating blade flaps down. This up flap velocity results in a downward flow of air (also known as induced flow) over the airfoil. This shrinks the angle between the resultant relative wind and the chord line thereby reducing the angle of attack in the blade. Simply put, this reduces lift generated from the advancing blade. The reverse is true for the retreating blade. The retreating blade has a down flap velocity resulting in an upwards flow of air on the airfoil. This increases the angle between the chord line and the resultant relative wind thereby increasing the angle of attack in the blade. This increase in angle of attack results in increased lift being produced by the retreating blade.
Cyclic feathering occurs when the pilot introduces a control input using the cyclic. In the case of forward flight, the pilot would want more lift generated on the aft portion of the rotor disc than on the forward portion of the rotor disk. Due to gyroscopic precession this input must be made 90° prior in the axis of rotation. This results as an increase in pitch at the 9 o'clock position and decrease in pitch at the 3 o'clock position. This results in a greater angle of attack in the retreating blade and a lesser angle of attack and the advancing blade. Thus, this is offsetting dissymmetry of lift.
If you haven't seen my other videos explaining disssymmetry of lift or gyroscopic precession I would recommend watching these videos first. The links are below.
Dissymmetry of Lift: • Dissymmetry of Lift in...
Gyroscopic Precession: • Gyroscopic Precession ...
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

[@johnbruner5820]

Rewatching this three years into my engineering degree and I am making a lot more connections than before

[@ChristopherOlmstead]

I’m at Rucker now going through aerodynamics academics and this helped out a ton. Thank you for your clear visual explanations in your videos!

[@Sloth55Chunk]

A good way someone explained to me once was that The problem is Dissymmetry of Lift, the solution is flapping, and the limitation is blade stall

[@TheGman0808]

I’m studying for the SIFT using the FAA rotocraft handbook you mentioned in your first vid. This is helping me grasp the concept of what the book is tryin to explain

[@andrewmoran627]

Super clear explanation of this topic! Of course the answer is blade flapping, cyclic feathering.... but actually seeing it drawn out really helps to understand what’s happening above your head! Thanks a ton!

[@karzem1]

Please please, make more of this kind of video please 🙏, you can’t imagine how helpful it is

[@christopherleach3907]

Just started my Commercial Rotor add-on and my instructor recommended your videos. Coming from the fixed-wing world, blade flapping was hurting my head - I thought the blade was twisting or something to change pitch but you explained that it's a change in relative wind due to the upwards or downwards motion of the blade. Totally clicked for me! My only comment would be, what you're calling "lifting area", shouldn't that be called Angle of Attack?

[@mac13mike]

I love your video's, I was just reading the helicopter flying handbook and was struggling understanding these topics. You laid all this info out so well making it much easier to grasp. Thank you! Do you plan on making more video's?

[@helicopterlessonsin10minut10]

Mike McElroy. Thanks! I just moved and plan on making more once I get a work space set up.

[@luckyhoerner7909]

awesome, well explained, love how you show the lift factor on the blades, my instructor never taught me shit. I am a two year pilot, taking my CFI in May, and I will be using you teaching tecniques, let me know what I owe,,,,,hahahah "LUCKY'

[@helicopterlessonsin10minut10]

Lucky. I'm glad you enjoy them. Tips/donations are always appreciated and go towards building the channel. The link can be found on "About" section of the channel. Best luck to you on your CFI!

[@999lindsay]

You must of had a good teacher, as you explained that well.

[@rbrudder]

Amazingly simple and understandable illustration

[@ritzbitz2000]

You rock. These videos are so helpful!

[@carnevalebr]

Congratulations for your excellent video! 👏🏻 And what about the blowback tendency? In spite of providing compensation for dissimetry of lift, flapping movement generates another effect in the helicopter flight: blowback. Due to phase lag (or gyroscopic precession), the advancing blade (more lift) will have maximum up displacement in the forward section. Similarly, maximum down displacement will occur in the aft section of rotor disk. As a consequence, the helicopter tends to pitch up.

[@rmckalinijabo7689]

These are really amazing videos with great content on rotorcrafts aerodynamics! Thank u🙏

[@ddtddt8493]

Hi Jacob, if your schedule allows iI would be interested to further develop on the concepts of the various angles relative to lift. A more in depth analysis of the pitch angle, the AoA, relative wind and so on. In short how an airfoil generates lift. Thanks for the clarity of the explanations

[@helicopterlessonsin10minut10]

DDT DDT. Sure thing. I'll add it to the list.

[@nunupoem7715]

Thank you for your videos. They are extremely helpful but to the point. I ❤it!

[@davidwallace5738]

Sir, I recently read an article concerning dissymmetry of lift which stated flapping only compensates for part of the effect. Right cyclic trim assists the pilot by actually moving the position of the cyclic control to the right. I would appreciate your comments on right cyclic trim and how it works. Again, thanks for your effort and expertise in helping us understand these complex subjects.

[@helicopterlessonsin10minut10]

David Wallace. Both flapping and cyclic feathering help to offset Dissymmetry of Lift. I believe right cyclic trimming is actually to compensate for Translating Tendency. In some helicopters the rotor mast is tilted slightly to the left to overcome Translating Tendency during hovering maneuvers. But once in forward flight you this rigging overcompensates for the effect. So some helicopters incorporate a Right Cyclic Trim to alleviate the need for the pilot to hold in right cyclic the entire time. But this is still a form of cyclic feathering because it affects pitch on just a portion of the rotor disk. I'd be interested to read the article. Good question though. I hope this helps.

[@davidwallace5738]

Helicopter Lessons In 10 Minutes or Less . Thank you sir for your prompt reply. The article I read concerning cyclic trim is entitled "Our good friend cyclic right trim." I discovered this article on Google while researching the subject in relation to the R22. The article, which is more or less a forum, does address cyclic trim for translating tendency. However, the right cyclic trim in the R22 is only used at higher air speed. Could this indicate as speed and dissymmetry increase the pilot, at least in the R22, may have yo help out a little with a little right cyclic? Thank you again for your efforts and expertise. I have no one else to talk to about helicopters and your lessons are greatly appreciated and anticipated.

[@helicopterlessonsin10minut10]

David Wallace. I've never flown the R22. But in the helicopters I have flown I've never noticed a need for an increase in right cyclic at higher air speeds. While not claiming to be an expert in the R22 my assumption is that there is some type of control rigging or the transmission is mounted slightly offset to compensate for Translating Tendency. At higher air speeds this rigging could cause a left roll that would need to be compensated for with right cyclic. I'd recommend looking into the operator's manual to see if it might have an explanation for this.

[@davidwallace5738]

Helicopter Lessons In 10 Minutes or Less Thanks for your reply. I'll see if I can find a POH on line and see what it has to say. Fly safe, sir.

[@iCanHazTwentyLetters]

@@helicopterlessonsin10minut10 "But this is still a form of cyclic feathering because it affects pitch on just a portion of the rotor disk." Isnt that what the cyclic does in the first place, through the mechanism of the swash plate?

[@ReactanceIsFutile]

Fantastic videos, and I'm so grateful someone who is keenly knowledgeable and interested in helicopters is addressing the lift equation and lift dynamics regarding helicopter air-foil dynamics as opposed to airplane propeller dynamics. _thank you *so* much_. Really, really helpful. One clarification and a question if you have a moment please-- AoA then, is really the lift angle the blade uses to provide lift above and beyond what lift it gets from IF? Is there an equation for calculating how much lift is derived from IF alone?

[@helicopterlessonsin10minut10]

ReactanceIsFutile. The induced flow is not giving any sort of lift. The angle of Attack is the usable angle where lift is produced. In helicopters the angle of Incidence must be enough to overcome induced flow and produce an angle of Attack. I hope that helps answer your question.

[@ReactanceIsFutile]

Okay, I get that now, thank you.

[@RandomLeech37]

Around the 1 minute mark you say the difference of lift produced by the advancing and retreating side will cause a constant roll to the left. But in reality this would manifest 90 degrees later causing the nose of the aircraft to tip up, correct? Which is what we refer to as blowback and must be fought against with forward cyclic.

[@Miko-yu9rs]

It looks like the video is taken in germany ✌🏽 have you been flying there?

[@helicopterlessonsin10minut10]

Yep. Great place to fly.

[@P222RL]

Tkx for the video , so theoretically if we can make a solid rigid blade and have a rotor speed that can stand for 1000rpm like those RC heli, what will this happen in a 1:1 chopper ? 🙏🏼🙏🏼🙏🏼🙏🏼🙏🏼

[@tomaszwota1465]

Cyclic feathering is simple enough to understand, but the first part of this video seems to be packed with a lot of terms that are left largely unexplained. Resultant? Upflow? Downflow? Downflap? Upflap? All this could use a some explanation. Specifically, I think, the detail that it's not the position higher or lower in the given instant that matters - but the movement to reach that max flap position -- which is the thing that causes the change of resultant relative wind angle. I know I was mentally stuck with it until it was pointed out to me. :)

[@widestella5081]

Hi, Jacob. Fantastic video, fantastic knowledge....thanks a lot. Can you explain transient torque in conjunction with combat maneuvering flight?

[@helicopterlessonsin10minut10]

I’ve got that topic added to the list when I’m able to make some videos again. Thanks for the feedback.

[@Sazairo865]

Sooooo easy to understand, thanks a lot!

[@grant22vids]

Your videos really help put it all together. Thanks for that. I have one thing eluding me.........can we see flapping? Or is it an aerodynamic term (like aoa) that we can’t really see? For example, does a blade actually flap? Does it bend? Or is it strictly aero? Thx

[@helicopterlessonsin10minut10]

grant22vids. The most pronounced way you experience flapping is during your takeoffs when you experience blowback. While taking off the advancing blade begins to flap up. This manifests 90 degrees later where the front half of the rotor disk tilts up. The helicopter has now compensated for the dissymmetry of lift. However, to keep accelerating forward you have to apply forward cyclic. By doing this you are creating more lift on the rear half of the rotor disk in order to accelerate. This has now caused a dissymmetry in lift to reoccurring (but in a good way) because the aft portion is now being used to accelerate the helicopter. So in forward flight you’ll actually see the aft half of the disk flapping up. This is the easiest flapping to see visually. I hope this helps.

[@titaniumdiveknife]

You're like a magician in Vegas: so succinct.

[@robertharris9055]

Sir, the lift equation L = 1/2 C. pv ( squared). S is for fixed wing aircraft. On helicopters the lift varies with the square of radius, also taKing into account of the area.Could you give a full explanation on this.

[@haricharan4872]

Thank you for the video, during the cyclic feathering does the blade actually pitches down on the advancing side to reduce the lift (or )does its angle of incidence kept unchanged and only the angle of incidence of advancing side increased to compensate lift. ? Please reply, Thanks in Advance

[@finnroou9710]

Could you explain why in this video you explain how there is more lift in the aft portion during forward flight but in the video on transverse effect and translational lift the fore portion as more lift than the aft portion?

[@MissoPOV]

Underrated

[@jakobkonradsen1248]

So cyclic feathering counter acts the dissymmetry of lift automatically when flying forward?

[@carloolivier1940]

You said the dissymmetry of lift will cause an unwanted roll but wouldn't it cause an unwanted pitch due to gyroscopic precession?

[@benoitalford9823]

Can you post some videos of basic aerodynamics of quadcopters

[@kadenwilliamson1244]

so, when i move the cyclic forward, does the rotor disk tilt a little bit to the left to compensate for dissymmetry of lift?

[@garrettwilson4347]

Thank you👌

[@majbach1968]

Hi. I just learned about your channel today and am watching your videos in order. I hold a commercial fixed wing licence. As much as I am enjoying your lessons so far, I was a bit confused about "flapping". The clarification I need is about how this works. From what you have described, it would seem this is a naturally occurring phenomenon and not a design characteristic or pilot input. Is that correct? Are you stating that the forward movement of a blade into the headwind gives it lift such that it bends upwards AND that upward bending movement actually decreases the effective angle of attack? And, that the reduced airflow on the retreating blade reduces lift enough such that the blade spring backs down to its natural position and that movement effectively reduces the AoA? That kind of blows my mind. I would not have thought that the relative speeds - ie, blade rotation vs fwd movement would be that close. So this brings me to my two questions: If aerodynamics are as I have described above, wouldn't that mean that faster forward movement would result in drastically greater deflection, i.e. 'flapping'? That must be brutally stressful on the blade. The second question has to do with the AoA at the 12 o'clock and 6 o'clock positions. I am assuming that the relative airflow is the same at these two locations and thus so is the lift generated. At this point, wouldn't the desire for the blade to return to it's neutral position (no flapping) be the same? But the blade in the 12 o'clock position would be bending downward, increasing it's lift and the blade at the 6 o'clock is springing upwards thereby decreasing lift. Wouldn't this result in a pitch up moment? Thanks for reading through all that.

[@taltruda]

Great videos, I’m really enjoying them. Someone else asked this below, but I’m not sure it got answered to my satisfaction.. at the very beginning of the video, you mention that the retreating blade has less lift, causing a left roll, but would gyroscopic precession manifest that into a nose up reaction? Thank you.

[@helicopterlessonsin10minut10]

taltruda. I wondered the same thing and I've dug around everywhere for why it is so. All of the manuals and references point to a roll to the retreating side without much explanation. My best guess is that the left roll manifests here just like it does in the case of retreating blade stall, only less severe. When the retreating blade stalls, a pitching up and rolling left occurs due to the differences in drag and lift in the rotor system. This may not be the best answer but I'll continue to research it. Thanks for the feedback.

[@Tomelpuerro]

hi why we have on the retraiting blade an upflow of the air? can you make a picture where i can see the direction of the air? best regards from austria😂👌

[@Snowcat89onXbox360]

What I still don’t understand is why the helicopter wants to roll left as speed increases?? I thought due to gyroscopic precession the effect would be felt 90 degrees later causing an even nose up force, not a left roll. In the R22 you can clearly tell the aircraft want to roll hard left, and there is trim specifically for that when cruising.

[@ueberalleberge]

Hi Jacob, a question to the feathering, you sayfor example Pitch increased left and decreased right, but is that not turning around as the rotor spins? I mean, is that a flexible thing always changing with energy spin?

[@jaceboix1461]

solid maybe

[@tajinderpal9020]

Thank you for this useful vedio sir Sir can you please elaborate how the retarding blade produce more lift

[@brandonfranklin4533]

Does blade flapping require pilot input or is it a purely mechanical system that’s compensating automaticity?

[@helicopterlessonsin10minut10]

It’s automatic

[@noahkuhn1336]

Can dissymmetry of lift happen in all rotor blade systems? Rigid, semi, fully a, 2, 3, 4, 5 blade systems?

[@spillysock-spock9929]

Is blade flap controlled by mechanical components or the relative flow of air itself?

[@helicopterlessonsin10minut10]

The blades flap due to aerodynamic forces.

[@jetfaker6666]

is it true that dissymmetry causes a roll to the left? Wouldn't gyroscopic effects make it equivalent to a pitch up?

[@mansouralbarrak2984]

Hello jacob , hope you doing good , can you talk or provide me about ch-47 aerodynamics , like in cyclic feathering when you move the cyclic forward what will happen to the aft rotor if I fly chinook

[@Ab-ml1kd]

Please add a video explaining autorotation.

[@helicopterlessonsin10minut10]

Atin Bisht. Sure thing. It's in the plans.

[@Ab-ml1kd]

Helicopter Lessons In 10 Minutes or Less man Ur videos are awesome ....will be requiring a whole lot of them in future..thanks.

[@matthewgruber7982]

also in the cyclic pitch, is it really that the force lags 90 degrees, or is it that the added force at the left by the cyclic induces flapping where the highest position of the blades will be 90 degrees later (and equal height 180 degrees later, lowest height 270 degrees later)? and then your rotor lift vector is tilted forwards, and most of the weight of the helicopter is not spinning, so it all pitches forwards? I am not sure how the gyroscope effect works here since, the rotor is spinning and cyclic induces forces in it, then it flaps, then the rotor puts moments on the fuselage which pitch or roll it, and then you've got a spinning disc that's pitching or rolling and some gyroscopic stuff has gotta happen there too, feeding back in , complicated!! But I can see from your explanation that, if a helicopter is moving forwards fast, and the cyclic is neutral, the flapping will move the rotor disc back and the helicopter will slow down and pitch up (not roll), neat! Also, if the helicopter wants to stay moving forwards at a stable speed, the cyclic will be positioned forwards a bit to not flap the rotor rearwards against the oncoming wind, to aim it forwards a bit instead

[@BHCJ11]

Not sure whether you still keep an eye out for comments, but at the very end you said "increased AoA, increase in lift" and this is where the HFH has me confused as well. On pg 2-13 it says "When AoA is increased...there's more lift" (so a direct relationship), but then on 2-19 it says as velocity increases, more lift is generated, and the AoA decreases on the advancing blade (an inverse relationship between lift and AoA) (and the opposite of what you said at the end). So which is it?

[@tomaszwota1465]

Both, I think. I'm not an expert at anything, though. So, as speed increases you've got more ETL, therefore less blade AoA is needed to keep you at the same altitude with higher air speed? (With enough speed you'll get into retreating blade stall but we're not taking that into effect here, right?)

[@murato8711]

Do flapping and feathering happen automatically or with the pilot input?

[@helicopterlessonsin10minut10]

The rotor does it automatically due to the aerodynamic forces

[@ankitagarwal3399]

Can you please explain how the direction of resultant relative wind changes

[@helicopterlessonsin10minut10]

I break that down completely in my “Forces Acting on an Airfoil” video. Short answer: Resultant relative wind is affected by Rotational Relative Wind and Induced flow. These all affect the Angle of Attack in the blade.

[@grantshort4896]

You forgot about leade and lag, (sweeping of the blade) or were you restricting your lesson to semi-rigid Bell rotor heads.

[@diegoplata5570]

thank.MR

[@compilationsgalaxy3112]

Sir how does it is producing lift in cyclic aft position

[@PauliEphresh]

Super helpful!! Are you still in Germany?

[@sherekcio3651]

This is the only video I can't understand and don't see the connections. Especially no why the resultant relative wind is drawn where it is

[@helicopterlessonsin10minut10]

It’s outlined further in my other videos like Parts of an Airfoil.

[@them8tysibulba]

Where was that intro footage shot? the landscape looks very German

[@helicopterlessonsin10minut10]

Yep. Good ole Deutschland.

[@SaitekFreak999]

Concerning your statement at 1:00, wouldn't it cause constant nose up pitching due to gyroscopic presession?

[@helicopterlessonsin10minut10]

Andromedarr. I think you're confusing aerodynamic forces with control rigging as it pertains to Gyroscopic Precession. For a left roll a pilot makes a left cyclic input. This increases the pitch at the tail so that an increase in lift occurs 90 degrees later at the 3 o'clock position. Increasing lift at the 3 o'clock position will cause a left roll regardless of what causes it.

[@gaberatata7156]

I don't think you answered his question clear enough. Yes an increase in pitch at the tail would cause lift on the right side,,,thus rolling it left.....but here the lift is on the right so 90 degrees later shoulder be the lift no?

[@Bob_just_Bob]

You really should acknowledge for the newbies' sake that dissymmetry of lift, if left unchecked, would not produce a roll to the right in all cases. Too many students will be confused by this. You are generalizing when there are still a huge number of helicopters that they may fly one day who's rotor systems turn in the opposite direction. Get in one of those and dissymmetry of lift's tendency is exactly the opposite. Better if you say the roll would result to the side of the retreating blades.

[@Wolfhound_81]

Don't really get it at kzbin.info/www/bejne/mabPc5WsqN-Fps0 where does that downward flow of air (and upward on the retreating blade) come from, what's producing it? When I hear "flapping up" I would expect the blade increasing aoa to RotRelWind, resulting in even more lift.

[@helicopterlessonsin10minut10]

Eteokles81. The upflow/downflow from flapping occurs because the blade is moving up or down vertically relative to the mast. This affects how the resultant airflow impacts the airfoil. It's not necessarily causing any difference in pitch in the blade as it pertains to Angle of Incidence. As the blade flaps up, there is more downward flow of air (induced flow) than normal which reduces Angle of Attack. I hope this helps. Thanks for the feedback!

[@Wolfhound_81]

@@helicopterlessonsin10minut10 Yep, I got a similar answer from some guys in my flight sim group, thanks.

[@indyjons321]

so the AOA is steeper on the retreating side, right?

[@helicopterlessonsin10minut10]

indyjons321. The angle of incidence is higher in order to equal out the angle of attack on each side.

[@jamesberlo4298]

Ya, Question where do you move the Cyclic? Right / Forward?

[@helicopterlessonsin10minut10]

James Berlo. Forward Cyclic compensates for this Dissymmetry of Lift in forward flight

[@jamesberlo4298]

Thanks.

[@troyb78]

So are all cyclic inputs considered cyclic feathering?

[@helicopterlessonsin10minut10]

Troy Asher. Generally speaking Cyclic feathering is synonymous with cyclic inputs. The only exception would be when you "center" or "neutralize" the cyclic from a displaced position. This cyclic input would actually take away the feathering in the blades.

[@ricerob]

you left out lead and lag as Compensation for Dissymmetry of Lift in Helicopters

[@austincoleman8074]

What's the intro music?

[@helicopterlessonsin10minut10]

It’s just standard music clips from the Splice app.

[@Hockeysktr17]

A little confused - if the effect of feathering is not taking place for 90* later, how does that help compensate for the retreating side's lack of lift?

[@helicopterlessonsin10minut10]

Hockeysktr17. With Dissymmetry of Lift, there is more Lift being created on the right side of the rotor disk than the left. Because of Gyroscopic Precession, the effect is manifested 90 degrees later as more Lift on the front half of of the disk and less lift on the back half. By putting in forward Cyclic (Cyclic feathering) you are increasing the Angle of Attack on the retreating side of the disk. This manifests 90 degrees later as increased Lift on the aft portion of the disk and corrects the Dissymmetry of Lift caused by forward flight.

[@NJ-Tech]

So when you want to slow down fairly quickly from speed, you're actually relying on the flapping compensation much more, right? Because the opposite is then true of your example here, with the advancing side needing more pitch to tilt the rotor back, and the retreating side needing less to come to a stop? Don't you also trim the disc (cyclic) for the cruise anyway, as part of compensating for dissymmetry of lift? Otherwise i'm assuming a very tired right hand eventually. Great videos keep them coming!

[@helicopterlessonsin10minut10]

NJ Tech. Im not sure I completely understand your question. But when you want to slowdown quickly you would typically reduce the collective and apply aft cyclic. This would reduce the pitch on all blades (collectively) and increase lift on the right side of the disk (cyclic feathering). The advancing blade would continue to flap in conjunction with the increase in pitch applied. In the case you mentioned, yes, the advancing blade would potentially flap more than it was when established in forward flight. On your second question about trimming the disk, it all depends on if the helicopter has some kind of trimming build in either mechanically or electronically. This trimming is simply a set amount of cyclic feathering meant to hold an airspeed or attitude and reduce pilot workload. Thanks for the input!

[@NJ-Tech]

yea you got where i was going, and many thanks for the reply! All making more sense now :)

[@luckyhoerner7909]

if I'm not mistaken, weather you are using cyclic to turn, or slow, in forward motion your blades are still flapping for dissymmetry, and the feathering to turn slow, ect and assist blade flapping. unless you come to a hover or out of ETL. Great question, please correct me if Im wrong

[@supercat380]

Great presentation, but please try to increase the volume of your voice to make it more audible

[@darrencolegold3642]

Try increasing the volume of your computer, video is fine

[@reedofwater]

I still don't get it...

[@stevejh69]

There is NO resultant relative wind! It is relative Airflow.

[@obsoleteprofessor2034]

My God, there's inequality all around me.

[@robertharris9055]

Sir, the lift equation L = 1/2 C. pv ( squared). S is for fixed wing aircraft. On helicopters the lift varies with the square of radius, also taKing into account of the area.Could you give a full explanation on this.