Ka-50 rotor dynamics - might need a coax expert - but is it correct?

[Volk.]

Just to preface this - I'm not a pilot, and I'm still going through FAA guides and such learning these topics.

Now guides/learning material I've found is a bit scarcer on coaxial rotors, but from my understanding there are 2 things weird with the Ka-50 - the rotors getting that close in fast forward flight and the cyclic needing forward left input to counter a right bank in fast forward flight. Again this is based more on conventional tail-rotor flight, but correct me where I'm wrong:

 

#1

If the Shark is nearing Vne in forward flight (assume no wind), then you'll see from external view from behind the lower rotor is flapping up to it's max on the right. This rotor travels anti-clockwise, so that's the advancing side. Meanwhile the top rotor is at it's lowest on the right (it goes clockwise, so retreating blades). Flapping happens naturally from the increased speed (ie. dyssemetry of lift) and increased pitch from e.g. cyclic feathering. For this dissymetry of lift, the blade has max upflap velocity, and thus lower angle of attack/lift on the advancing side, i.e. the top rotor at the 9 o-clock/left and the lower rotor at the 3 o-clock. However that's not where the blade's at it's highest, but just the max upflap velocity, hence the decreased lift. The maximum upflap, or where the blade's flapped up the highest vertically is 90 degrees later, which would be the forward for both rotors. So in fast forward flight, the rotors shouldn't be that close but in fact but should be tilted up in front and thus remain relatively equally spaced.

ps. I'm not saying the rotors will never clash, or that ~250kph shouldn't be the Vne, just that they shouldn't be clashing on the right like they do at present in the sim.

 

#2

Once you hit transverse flow, you need to (for a counter-clockwise single-rotor) push forward cyclic and left to counter the nose-up+right bank. In the Shark, you need left-cyclic as well. However, I'd think with the coax rotor, those forces would either be equalised so almost no cyclic correction needed for any banking, or that perhaps the upper rotor (which is tilted into the wind) is getting more air and dominant, which from it's clockwise rotation would lead to blowback pitching you up and banking left, requiring right cyclic instead.

 

pps. The slight left-cyclic input currently required in forward speed would logically create more pitch / upflap on the right hand side if I'm correct, but I'd think that would be less than the lift created from the Vne forward airspeed and it should also result in the top rotor also flapping up by a similar amount on the right.

[Yo-Yo]

11 hours ago, Volk. said:

Just to preface this - I'm not a pilot, and I'm still going through FAA guides and such learning these topics.

Now guides/learning material I've found is a bit scarcer on coaxial rotors, but from my understanding there are 2 things weird with the Ka-50 - the rotors getting that close in fast forward flight and the cyclic needing forward left input to counter a right bank in fast forward flight. Again this is based more on conventional tail-rotor flight, but correct me where I'm wrong:

 

#1

If the Shark is nearing Vne in forward flight (assume no wind), then you'll see from external view from behind the lower rotor is flapping up to it's max on the right. This rotor travels anti-clockwise, so that's the advancing side. Meanwhile the top rotor is at it's lowest on the right (it goes clockwise, so retreating blades). Flapping happens naturally from the increased speed (ie. dyssemetry of lift) and increased pitch from e.g. cyclic feathering. For this dissymetry of lift, the blade has max upflap velocity, and thus lower angle of attack/lift on the advancing side, i.e. the top rotor at the 9 o-clock/left and the lower rotor at the 3 o-clock. However that's not where the blade's at it's highest, but just the max upflap velocity, hence the decreased lift. The maximum upflap, or where the blade's flapped up the highest vertically is 90 degrees later, which would be the forward for both rotors. So in fast forward flight, the rotors shouldn't be that close but in fact but should be tilted up in front and thus remain relatively equally spaced.

ps. I'm not saying the rotors will never clash, or that ~250kph shouldn't be the Vne, just that they shouldn't be clashing on the right like they do at present in the sim.

 

#2

Once you hit transverse flow, you need to (for a counter-clockwise single-rotor) push forward cyclic and left to counter the nose-up+right bank. In the Shark, you need left-cyclic as well. However, I'd think with the coax rotor, those forces would either be equalised so almost no cyclic correction needed for any banking, or that perhaps the upper rotor (which is tilted into the wind) is getting more air and dominant, which from it's clockwise rotation would lead to blowback pitching you up and banking left, requiring right cyclic instead.

 

pps. The slight left-cyclic input currently required in forward speed would logically create more pitch / upflap on the right hand side if I'm correct, but I'd think that would be less than the lift created from the Vne forward airspeed and it should also result in the top rotor also flapping up by a similar amount on the right.

It's not that simple thing as you consider it. It's a bit more complicated matter. Please try to find a good source about blade flapping mechanics. The key words this source is to have are "90 degree lag or phase shift" and "flapping compensator phase shift".

[Volk.]

Hi, thanks for the reply. ps kudos on the flight model being what it is already.

I've pasted an attachment on the flapping. This is for conventional tail-rotored helicopters, but the basics of flapping should hold for coaxial designs.

I do understand the Ka-50 has a fully articulated system, so the lead-lag hinge might influence the usual 90 degree gyroscopic precession to be slightly less than 90 degree.

 

Flight dynamics info like this on coaxials is a bit hard to find - typically there are more scientific papers exploring mathematical models or purely stating "it's more efficient in a hover" than diagrams like the below. There's also a few less English-speaking pilots around to find. So might be like you say there are more complex interactions with coax that I'm not getting.

 

[Yo-Yo]

As the rotor disk has AoA the flapping is not exactly as the simplificated picture shows - exactly 90 degrees.  Regarding Kamov's bureau measurements and our calculations/model the blades proximity at high speed is around 70 degree azimuth (top view, zero to the tail, counterclockwise).

Three factors work: advanced/retarded blades, rotor AoA (side inclination flapping) and flapping compensator linkage reducing pitch with flapping.

[Volk.]

Thanks for that reply. I'm clearly not quite following yet.

Unfortunately most footage of the Ka-50, Ka-52 or just any coaxial rotor is normally either slow flight, acrobatics far from Vne or simple not at an angle where one can make out the separation between the rotors. I think I've seen footage of a Ka-52 with rotors closer on the right, possibly slightly just behind right, but I've also seen footage of a cockpit view showing a Ka-50 going 250kph with the rotor discs seemingly equally separated.

 

For cyclic feathering - it's easily demonstrable on the ground. Attached 3 pics, showing max right cylic on the ground at Auto Throttle, with collective so it won't start taking off (or coning really). While it's not that easy to see the cyclic feathering pitch changes itself (could be hard to see or may just take too much CPU out of your sim for things people will never see), but one can see the result of those pitch changes - from the rear one can easily see both rotor cones have tilted to the right. The lower (counter-clockwise) rotor increases pitch in front to produce the extra lift on the left (from gyroscopic precession) and as result of that increased pitch flaps up fully on the left - somewhere near the 9 o-clock position. In the third shot I tried to frame it - though it seems to be max upflap/vertical flapping height just past the 90 degree mark instead of just in front of it, but that may just be a wrong screen capture - so it's not an issue. The Top rotor pitches up at the rear and flaps up fully on by the time the blade reaches the left. This way lift is produced on the left to bank right (if I had enough collective pitch and RPM for flight) and both rotor cones seem to match their tilt and separation.

 

If I pitched the cyclic forward on the ground (not pictured), then both cones tilt down on front and up at the back, which again makes sense. The max downflap visually appears to be about 1 o clock for the lower rotors (that would fit with the 70 degree phase lag from the hinge mechanics/position) and the top rotor about 11 o-clock.

And yes, that was fun - don't think I've ever marvelled at the rotor disc tilting just while on the ground before - nice modeling there.

 

In fast forward flight, where there's pitch down, you'd also have coning, so both rotors would angle a little more up all round from the lift. Also dissymmetry of lift happens when the advancing side of each rotor gets more lift and starts flapping up. With that dissymmetry alone, ignoring coning from lift and flapping cyclic feathering momentarily, you'd get the lower rotor getting more lift as it advances on the right, starts flapping up to reduce the dissymetry and has it's full upflap near the front, possibly the 1 o-clock position, and maximum downflap probably 7 o-clock.

Meanwhile the top rotor would have more lift on the left, culminating in the blades starting to flap up as it gains airspeed on the left and flapping upwards to it's limits in also in front, maybe 11 o-clock.

In both tilting the disc with the cyclic and with dissymetry of lift, the discs stay equally distant from one another.

 

I do understand that the Ka-50 has a natural speed limit - it will experience retreating blade stall and the advancing blades breaking the sound barrier at one point, and there are just natural stress limits for various components. And I do also understand that it will have rotor intersections in certain violent maneuvers. Especially with low RPM and stall the blades would flap more erratically and greatly increase the risk of intersection.

 

The bit I'm not understanding is that with both flapping from cyclic input (in this case forward & tilting the cone/disc down at the nose) and from the dissymmetry of lift (tilting the cone up at the nose in my theory), should keep the blades apart. Possibly it might even experience a pitch up moment from the dissymetry before rotor intersection.

 

What force/factor is missing from the above that makes the rotors grow closer to one another on the right, possibly the 4 o-clock position (70 degrees from tail counter-clockwise) during fast forward flight? I know there's a slight left cyclic needed to keep it level in fast forward flight, which I'm not quite sure why that exist (transverse flow?) but should again result in both rotor cones tilted slightly down on the left and up on the right with equal separation.

[Cheetah7798]

Here's a video of 2 Ka-50s pelting it on a straight. (Best I could find).

 

Attached is an image I randomly pulled from one of the fly-bys. In the image you can clearly see the underside of the upper rotor, and the upper side of the leading lower rotor (blurred shade just right of rotor mast), despite the downward angle of the viewer.

 

I'm no expert on any of this, mind you. This is just what I've observed and thought it relevant to the tread.

 

Edit: On second viewing, that blurred shape is probably on the close (retreating) side.

 

Edited December 9, 2020 by Cheetah7798

[cw4ogden]

 

On 12/5/2020 at 5:28 PM, Volk. said:

However that's not where the blade's at it's highest, but just the max upflap velocity, hence the decreased lift. The maximum upflap, or where the blade's flapped up the highest vertically is 90 degrees later, which would be the forward for both rotors. So in fast forward flight, the rotors shouldn't be that close but in fact but should be tilted up in front and thus remain relatively equally spaced.

 

 

As a former CH-47 guy, we used to spend hours on discussions like this, rarely finding definitive answers.  Tandem rotor aerodynamics is a bit of a unicorn. 

 

This guy has a fair explanation, but I am not sure I am sold.  

 

If I had to take a guess, I'd say you are confusing maximum upflap with minimum separation.  Yes, the maximum upflap occurs over the nose from blowback, but the difference between minimum / maximum angle of incidence between the two blades would be highest over the right side.   Throw in some induced flow from the top rotor system and I believe a higher base angle of incidence for the lower rotor system to compensate, and the lower rotor becomes the weak link in the chain, it has to play catch up with the upper, therefore the upper rotor at high speeds is generating more of the lift and the lower is just keeping things level?

 

https://www.simhq.com/_air13/air_427a.html

 

 

 

 

 

 

 

 

 

 

 

 

 

[Volk.]

The tandem ones are also very interesting. I'm definitely wanting to understand all the flight dynamics, at least the stuff that affects the pilot, not necessarily all the crazy maths and possible forces. I'm very tempted to do a vid at some point about tandem (and specifically the Chinook, given it's one of the most popular variants). Though that has it's own complications, e.g. cyclic sideways tilts the rotor cones like one would expect in tail rotor/coax, but tilting cyclic forward/back leads to different inputs in the front and rear rotors - so different control input and reaction there, nevermind all the dynamics of the airflow between the two rotors.

 

Attached below from the Chinook A/B familirization manual - for forward flight the upflap & downflap is definitely in front & rear - which I'd expect from the Shark as well. It also makes sense that the max angle of incidence/pitch is at the 3 & 9 respectively (or close to those angles), where the flapping velocity (but not height) is at it's maximum. It'd follow that the max coning/flapping is at the 12 (or close to it depending on dampeners and offset of the hinges etc). In my mind the upflap and separation are linked, so yes, that is a point of confusion for me. I'd think maximum angle of incidence/pitch in combination with airspeed is what generates more lift (90-ish degrees later) and thus the flapping up (culminating 90-ish degrees later) , which finally leads to the rotor intersection for a coax.

 

I did see Einstein (that SimHQ links) article before - useful stuff. But for the upper rotor generating more lift at speed, would indicate more torque from the upper clockwise rotor and thus that the Shark's fuselage would naturally turn to the left and need a right cyclic input to counter it (which should also tilt the cone to lower on the right and raise on the right), not a left one. If the top rotor's lift in fast forward flight weren't dominant, then the left/right cyclic to counter transverse flow banking should be offset and simply require forward stick.

Edited December 10, 2020 by Volk.

[cw4ogden]

Volk,

 

I am going to have to read and process your points, but I wanted to throw a few more thoughts out there.

 

It's all too easy to get mixed up trying to figure out even a single rotor with the phase lag etc.  

 

I don't have a definitive answer for sure.  We used to get the Boeing people on the phone and rarely was there ever a resolution to the more complicated questions.

 

Here are a few other things to consider:

 

1.  When flapping happens, it happens automatically as you probably understand.  If you can picture a fully articulated rotor head, (semi-rigid is a different ball of wax), if the rotor head is angled at say 7 degrees nose low to the horizon, when you accelerate let's say with a very small amount of fixed forward cyclic:  the blade flapping means the flapping hinges are being utilized and the blade tip path plane is therefore not aligned with the rotor hub, by however many degrees.

 

But as that aerodynamic force takes affect, the pilot, by placing the cyclic further forward eliminates the blade flapping.  Blade flapping is more a transient aspect of dissymmetry of lift.  The point being you can chuck out a lot of the blade flapping blowback stuff because it is theoretical, you compensate for it as the pilot.  In a single rotor helicopter I don't think it is possible to not compensate for it, you either push forward or the bird's new pitch angle returns everything to equilibrium regards flapping.

 

In a chinook you can actually put the longitudinal cyclic trim switch in manual and just let flapping handle the dissymmetry of lift, but you get two rotor systems flying at different angles than the rotor hubs.  It's why we have an airspeed limit for failed longitudinal cyclic trim of around 100 knots.   You are approaching the limit where blade flapping isn't enough alone to counter-act dissymmetry of lift without severe aerodynamic stresses on the heads in general.

 

To sum that one up: flapping is a transient mechanism.

 

 

 

2.  High speed forward flight in a helicopter is limited by retreating blade stall and or what the army terms blade compressibility (supersonic).  Of the two the compressibility is more dangerous because if you look at the pitch moment it induces, it will nose you down further.  It is self aggravating.

 

Retreating blade stall is somewhat self correcting.  But either way, at the upper end of the forward flight spectrum, you are talking about one half of the disk barely breaking a sweat and the other half, only a small portion is still actually flying.  The rest are in various stages of stall or even providing negative lift.

 

Even if induced flow isn't a factor at high speeds, the lower rotor's higher inherent angle of incidence means it will stall sooner at the same pitch angles.

 

My suspicion is the right side minimum clearance is because of the cyclic input countering the blade flapping coupled with the lower rotor either in dirty wind, or achieving critical AOA and stalling sooner.  

 

Retreating blade stall is a progressive phenomenon that works it's way outward on the blade.  It happens in all but the slowest of forward flight.  But the emergency or limitation that is retreating blade stall is when the phenomenon envelops so much of the blade no amount of flapping or feathering can keep the rotor head and aircraft hanging below it level.  

 

At high forward speeds, all but the very tips of the retreating blade is meaningless or contributing negatively to the equation.

 

To summarize: I don't think blade flapping is the primary mechanism for what you are trying to understand.  It is a phenomenon for sure, but it is eliminated by proper cyclic feathering.  

 

 

I toyed with the idea of putting all this stuff in a book and selling it to flight students.  I went through flight school in 1999 and taught in 2006 and there was no tandem rotor aerodynamics class taught in 99, period.  We just got the single rotor stuff and a guy with two more years experience than you telling you whatever garbage he heard.  

 

It boggled my mind how many wrong explanations there were for phenomenon that bird exhibits.  I owe what I know to discussions like this with fellow aviators and just nugging through the physics then testing it out.  Or you just have an epiphany one day and realize the reason the bird pitches down AFCS off when you pull thrust (collective) has nothing to do with whatever lie your AQC instructor told you and everything to do with the fact the forward and aft heads sit at slightly different angles and therefore equal amounts of collective produce a more vertical lift vector on the back rotor.

 

There is an audience for this stuff, but it is likely small.

Edited December 10, 2020 by cw4ogden
To correct AFCS pitch transient direction. Pulling pitch creates a nose down moment in the CH47

[cw4ogden]

4 hours ago, Volk. said:

In my mind the upflap and separation are linked, so yes, that is a point of confusion for me. I'd think maximum angle of incidence/pitch in combination with airspeed is what generates more lift (90-ish degrees later) and thus the flapping up (culminating 90-ish degrees later) , which finally leads to the rotor intersection for a coax.

 

The generation of lift is equal across the rotor plane.  That is the flapping in action.  If and when it stops being so that you will have pitch and or roll moments.

 

What is clear is maximum upflap / downflap and minimum upflap downflap where they meet and why they don't parallel is modeled on the abeam points and that they converge.

 

We know convergence is based in reality because there are documenting blade collision fatalities and the phenomenon was associated with high speed, minus any airshow type exceeding a normal flight envelope accidents.

 

What we don't know is whether or not it is modeled accurately, or someone making the flight model applied a faulty explanation of the aerodynamics, specifically blew phase lag regarding blowback at high speeds.  

 

My gut says we just don't understand the correct phenomenon.

 

 

Edited December 10, 2020 by cw4ogden
clarity grammar

[cw4ogden]

Haven't read it yet, but I'll bet there's an answer in here.

 

https://ntrs.nasa.gov/api/citations/19970015550/downloads/19970015550.pdf

[cw4ogden]

Found this as well.  It looks like it is modelled correctly.  I just wish I could explain why.  

 

It is conceivable what's going on at high speed is the retreating side of both blades are straight up stalled from root to tip and only the advancing sides are providing lift.  The resulting configuration arriving from each side's advancing blade trying to keep the aircraft from rolling.  I wish I had a definitive answer.

[Volk.]

That article is quite fascinating - haven't seen that one before. I did stumble across those Langley tests on the flow of coax vs single before. It mentions : "Figure 17 shows the blade  separation for the Ka-32 (presumably from flight test). At low advance ratios, the minimum distance occurs around _ = 270 ° (_5), and around _ = 90 ° at higher speeds (_2)." That copy paste fmoves the azimuthal position of rotor blade symbols, but that might indicate the smaller separation is indeed near the right. I don't think it ever goes into why - just that the consensus is it's more efficient at hover and lower/medium speeds.

I wasn't clear earlier - but the lift is equal across the disc in normal forward flight regimes (away from Vne), because of the flapping, and that the flapping happens 'naturally' or rather automatically because of the extra lift that would be generated.

I did make me wonder though if this separation distance angle is somehow related to at fast speeds - that flow from the top rotor hits the rear of the lower rotor which then takes effect on the right

I've seen that second diagram on the Ka-50 as well, just in Russian. Yeah, no explanations given. The 2nd and third images make sense in terms of equal separation (for forward flight or sideslip, not a drastic turn of course). But why it would contract at that position going fast... might be RBS...

[cw4ogden]

I think the answer will elude us until that epiphany moment or a Ka-50 expert tells us the simple answer and we collectively go "duh".

 

The good news being, your question is partially answered in that it appears, by all accounts to be accurately modeled, if maybe over-modeled.  

 

I have yet to make a post in the MI-8 forum because I have no first hand experience, but if vortex ring state was as dangerous as it is in specifically that module, there would be a lot of dead Russian pilots.  It's a thing, yes.  But you have to be a bit of an show off, be unfamiliar or operating at high density altitude conditions to get into it.  

 

The 47 gets a mushing like phenomenon that I'm convinced is aft rotor system getting into a vortex ring state on rapid decelerations or even a moderately fast downwind approach.  If you are lucky you get told to put chinook on its side a bit if you have to emergency decelerate, but it was told to me as aerodynamic breaking with the hidden VRS danger being lost probably between Vietnam era and when I went through.  

 

I felt it once, and it scared the shit out of me.  

 

I would demonstrate the upper edge of VRS at a very high altitude hover for students, just so you could feel it.  

 

It is remarkably easy to get out of, if altitude is not a factor because you can dump the thrust and drop below the VRS portion of the velocity height curve, the autorotative upflow of air breaking the state, but you need I'd guess 200 feet minimum, and that's only a guess as the lowest safe altitude that's a viable recovery technique.  And you still need to get forward or for 47's we go for sideward momentum, otherwise you will just wind up in the same flight regime, but from the bottom up.

 

 

 

 

[Volk.]

11 minutes ago, cw4ogden said:

I think the answer will elude us until that epiphany moment or a Ka-50 expert tells us the simple answer and we collectively go "duh".

Most likely. And Hopefully. That said it doesn't even appear to be default training / knowledge for operational Ka-32 pilots - just you've got your limitations, don't exceed them.

 

On the Mi-8 - far as I know it's the most produced helo, so would be more than just Russians if it crashed that often.

 

I still have to troll through a lot of lore before I can get to grips with the Ch-47, but I imagine most of it's quirks come from making the rotors dissimular in lift with pitch, maybe some dissymetry of lift coming from the rotors not being right above one another and causing a yawing motion and lastly vortices from the front rotor disturbing the other. I'd imagine a pitch-up brake the wash from the front, which has had time to form vortices rather than just 'turbulent', might mess with the back end and then also cause torque issues.

[Ranma13]

I can't follow the entire conversation because it's getting a little too deep into charts and the miniature of physics, but here's how I understand it. When a force is applied on a spinning object, the reaction to the applied force is 90 degrees later in the rotation from when the force is applied. Smarter Every Day made a video about this, and the effect is called phase lag:

 

 

In the video at 4:36, he applies an upwards force (a tilt), which manifests 90 degrees later on the bicycle wheel.

 

As the rotor spins around, when the blade is moving into the wind, the airspeed over the blade is increased, increasing lift, and when the blade is moving away from the wind, the airspeed is decreased, decreasing lift. If the blade maintained the same angle of attack throughout the entire rotation, this would cause one side of the aircraft to experience more lift than the other side, which will roll the helicopter. This is called dissymmetry of lift.

 

To counteract this, two things happen. First, the blade pitch angle is constantly adjusted throughout the rotation. It's decreased on the advancing side (to reduce lift) and increased on the retreating side (to increase lift) to balance it out on both sides. You can see this effect in this slow motion video:

 

 

Second, the blades are attached via a hinge to the mast (the hinge where the blue and orange pieces are connected):

 

 

This allows the blade to move up and down, called flapping, to reduce the angle of attack when the blade is rotating into the wind (the upflap velocity is subtracted), and increase the angle of attack when it's moving away from the wind (the downflap velocity is added):

 

 

This effect happens naturally. Imagine that you're driving down the freeway and you stick your hand out of the window, fingers facing forward, and you tilt your palm up. The wind will blow against your palm and move your hand upwards. Then if you reverse your hand so that your fingers are pointing backwards, but still tilt your palm up, the wind will blow against the back of your hand and push it downwards.

 

You can see the flapping effect in this slow motion video of a Huey taking off:

 

 

Both of these things will balance the lift on both sides of the aircraft, preventing it from rolling due to a difference in lift:

 

 

Note that when the helicopter's on the ground, the lift is the same on both sides because there's no forward movement and therefore no advancing or retreating side of the disk, so the angle of attack doesn't need to change (figure a):

 

 

Understanding this, we can also understand what the cyclic does. When there's a difference in the amount of lift on one side of the helicopter versus the other, the helicopter will tilt towards the direction with less lift. The cyclic controls the symmetry of lift by adjusting the blade pitch angle to produce unequal lift, which tilts the helicopter in the desired direction. This tilt is needed to move the helicopter; the rotor blades generate a lift force directly upwards, so in order to move, the lift vector also needs to tilt forward so that there's a forward velocity component (thrust) that moves the helicopter forward:

 

 

So to try and answer what I think your questions are:

 

1. On the ground, the lift on both sides of the rotor is the same and the angle of attack doesn't change, so there's no flapping and the blades will have the same tilt on both sides. This is why when you push the cyclic forward on the ground, the blades appear to tilt forward as well. The blade itself is not flapping, it's the entire aircraft that's tilting forward, similar to swinging a ball on string in a circle, then tilting your entire arm forward.

 

2. When flying forward, the lift on both sides of the rotor is asymmetrical. The blade pitch angle changes and the blade flaps to balance out the lift, causing the rotors to squeeze together (because they're spinning in opposite directions, one flaps up while the other flaps down). They squeeze together on the right side because that's where the wind is pushing the strongest against the blades.

 

3. The top rotor flaps less than the bottom rotor because it's pulling in clean air from above, whereas the bottom rotor is pulling in the downwash from the top rotor. This causes the bottom rotor to generate less lift than the top rotor, which means it also generates less torque. To counteract this, the bottom rotor's blade pitch angle is set higher than the top one to generate more lift, which also causes it to flap more (i.e. the hand out of the car is tilted up more, causing the wind to push harder against the palm). This equalizes the torque between the top and bottom blades.

 

4. While the two rotors cancel out each other's torque, negating the need for a tail rotor, it's not 100% equal. This is why you still need to apply slightly left cyclic for level flight, because the left side is producing slightly more lift than the right.

Edited December 12, 2020 by Ranma13

[Volk.]

Thanks for the extensive reply Ranma13. I might have to disagree on a few finer points though - hope these make sense:

 

On 12/12/2020 at 3:04 AM, Ranma13 said:

1. On the ground, the lift on both sides of the rotor is the same and the angle of attack doesn't change, so there's no flapping and the blades will have the same tilt on both sides. This is why when you push the cyclic forward on the ground, the blades appear to tilt forward as well. The blade itself is not flapping, it's the entire aircraft that's tilting forward, similar to swinging a ball on string in a circle, then tilting your entire arm forward.

This might just be weird terminology, but no. The shaft, mast head (in fully articulated system) and aircraft is not going to tilt. It begins to tilt as result of the lift generated and the centre of gravity being offset then swings it. The tilting cone is not something the helicopter or your controls did directly either - it's the increased pitch from you wanting to generate lift in a direction that causes the blades to bend or "flap" near the root that makes it look like the rotor system is tilting (again fully articulate system - not talking UH1 here). There is flapping, purely from cyclic feathering as well, even when there's not dissymetry of lift from fast forward airspeed or upward coning from a lot of lift being generated. I don't think the ball-string analogy works - there it angles because you are angling the way it rotates, whereas most helo designs can't willingly angle the mast/hub directly.

 

On 12/12/2020 at 3:04 AM, Ranma13 said:

2. When flying forward, the lift on both sides of the rotor is asymmetrical. The blade pitch angle changes and the blade flaps to balance out the lift, causing the rotors to squeeze together (because they're spinning in opposite directions, one flaps up while the other flaps down). They squeeze together on the right side because that's where the wind is pushing the strongest against the blades.

Dissymetry of lift, or rather the faster incoming airspeed would hit the lower blades the most at the 3 o'clock. That's when, in part to counter this dissymetry, the blades would be flapping up the fastest - ie they're still moving upwards at that time (or somewhere near it). As the blade passes through the 3 o'clock and being perpendicular to the incoming air, it would generate less lift, and the blade's still flapping up, but the speed at which it's doing so is reducing. It then typically reaches the maximum up-flap somewhere to the front of the helo. So the increased lift generated, and the place where you can see it flap up isn't synced with when the lift is the most intense (on the advancing side in fast forward flight), but rather 'delayed' by almost 90 degrees.

Same reason that one the ground, moving the cyclic forward makes the disc appear to flap down, as the lower rotor increases pitch on the left to produce more lift at the back, making it appear to flap upwards fully near the back (see #3 below for more). If the being flapped up fully was an instantaneous reaction to the increased pitch/lift, then it would be appearing to rise on the left with forward cyclic on the ground.

 

On 12/12/2020 at 3:04 AM, Ranma13 said:

3. The top rotor flaps less than the bottom rotor because it's pulling in clean air from above, whereas the bottom rotor is pulling in the downwash from the top rotor. This causes the bottom rotor to generate less lift than the top rotor, which means it also generates less torque. To counteract this, the bottom rotor's blade pitch angle is set higher than the top one to generate more lift, which also causes it to flap more (i.e. the hand out of the car is tilted up more, causing the wind to push harder against the palm). This equalizes the torque between the top and bottom blades.

The ingestion of turbulent air by the lower rotor probably is part of the reason. But to my understanding the amount of flap is determined by the amount of lift generated - in simple terms the dissymetry of lift + cyclic pitch. So reduced lift from turbulent air would mean less lift - and if it did dynamically counter that by pulling more pitch, that would theoretically increase until it achieved enough lift and the flapping would be normal again. The flapping should otherwise only maybe be more pronounced if the blades were slowing down, but given both rotors are interlinked, then the entire system would be slowing down if rotor RPM was an issue. Additionally, this seems to be trying to generate more lift on the rear side, which then culminates in the full flap-up near the right, instead of more lift on the right culminating on the front.

 

On 12/12/2020 at 3:04 AM, Ranma13 said:

4. While the two rotors cancel out each other's torque, negating the need for a tail rotor, it's not 100% equal. This is why you still need to apply slightly left cyclic for level flight, because the left side is producing slightly more lift than the right.

If the top rotor's lift is greater, and it's torque more, you'd need to do the opposite. The top rotor moves clockwise, so it's torque alone would result in the fuselage turning left. To counter that you'd need right cyclic.

 

I'm starting to suspect (at least as far as my other physics might be wrong), that it has to be related to the airflow coming off the top rotor and specifically interacting on the lower rotor at the tail end, or close to it, which then causes the extreme flap on the near 3 o'clock. Maybe a condensed flow of air hitting specifically the lower rotor at the back cause the unexpected max upflap? If it were RBS or Advanced tip compressibility, then given the rotor speeds are linked, surely it would affect both rotors equally?

[Ranma13]

On 12/12/2020 at 9:10 PM, Volk. said:

This might just be weird terminology, but no. The shaft, mast head (in fully articulated system) and aircraft is not going to tilt. It begins to tilt as result of the lift generated and the centre of gravity being offset then swings it.

 

You're right about this, my mistake.

 

On 12/12/2020 at 9:10 PM, Volk. said:

There is flapping, purely from cyclic feathering as well, even when there's not dissymetry of lift from fast forward airspeed or upward coning from a lot of lift being generated

 

You're right, what I meant to say is that the flapping angle between the two sides stay the same when on the ground (and with no wind) because there's no difference in the airspeed that hits the blade on either side.

 

On 12/12/2020 at 9:10 PM, Volk. said:

But to my understanding the amount of flap is determined by the amount of lift generated - in simple terms the dissymetry of lift + cyclic pitch.

 

Sort of; the amount of flap is determined by the blade angle (remember the hand-outside-the-car analogy, the more steeply tilted your palm, the stronger the air pushes it upwards), but adjusting the blade angle also changes the angle of attack, and thus the amount of lift generated. What you said is not wrong, but the flapping amount is not directly tied with the amount of lift, but rather the blade angle.

 

On 12/12/2020 at 9:10 PM, Volk. said:

Dissymetry of lift, or rather the faster incoming airspeed would hit the lower blades the most at the 3 o'clock. That's when, in part to counter this dissymetry, the blades would be flapping up the fastest - ie they're still moving upwards at that time (or somewhere near it). As the blade passes through the 3 o'clock and being perpendicular to the incoming air, it would generate less lift, and the blade's still flapping up, but the speed at which it's doing so is reducing. It then typically reaches the maximum up-flap somewhere to the front of the helo.

 

You're correct that at the 3 o'clock position, the wind blowing directly against the blade would be the fastest. However, once past this point, the blade won't continue to flap upwards and will instead start to drop. Going back to the hand-outside-the-car analogy, imagine that you loosen your arm muscles while the car is going fast so that your arm will drop naturally due to gravity. When the car is going fast, the air will push against your hand and lift it up, but as the car slows down, so does the pushing force against your hand, and it will start to drop.

 

On 12/12/2020 at 9:10 PM, Volk. said:

If the top rotor's lift is greater, and it's torque more, you'd need to do the opposite. The top rotor moves clockwise, so it's torque alone would result in the fuselage turning left. To counter that you'd need right cyclic.

 

Going back to the article I linked earlier, there's two things happening. Without any corrections, the top rotor creates more torque. Because it needs less angle of attack to achieve the same amount of lift as the bottom rotor, it has less drag and therefore spins ever so slightly faster, creating slightly more torque. As you stated, the top rotor moves clockwise, so the torque force is anti-clockwise, causing the aircraft to yaw left. To counter that, you need to input right pedal.

 

At the same time, the advancing side of the blade generates more lift as the wind speed increases, while the retreating side generates less lift. Blade flapping and feathering counters this to equalize the lift on both sides, but as speed increases, it's less and less able to compensate for this, leading to an imbalance in lift. Because the top rotor generates more lift than the bottom one, the imbalance of the top rotor is larger and overrides the imbalance of the bottom rotor, and since the top rotor generates more lift on the left side of the helicopter, this causes the helicopter to roll right. To counter that, you need to input left cyclic. You can test this by keeping the cyclic centered for roll and only change its pitch; at low speeds the aircraft will stay level, but the faster you fly forward, the more the aircraft will start to roll to the right.

Edited December 14, 2020 by Ranma13

[Volk.]

 

22 hours ago, Ranma13 said:

but the flapping amount is not directly tied with the amount of lift, but rather the blade angle

That specific bit I don't think is fully correct. The blade is 'pushed up' by the amount of air. If it has full pitch but it's not biting into any air then there's no reason for it to flap up, only settle down. It's because the higher pitch leads to increased lift that it goes up. For an advancing blade that's been trimmed for forward flight, ie. it's flatter pitch, it still wants to flap up even though it has minimal pitch because of that lift. I guess ideally you're of course trimmed so cyclic feathering compensates for any dissymetry in combination of flapping so that the flapping doesn't appear excessive.

22 hours ago, Ranma13 said:

It sounds like you're treating flapping as having momentum or that the lift is a force that will continually push the blade upwards

The first part of that sentence is exactly it. It's springy. The reduced angle of attack - the very thing helping with the dissymetry of lift is the motion of traveling upwards, not the state of being fully up. As it hits the relative wind harder, the advancing side is pushed up, like your arm-out-the-window analogy. It's because the blade's moving upwards that the lift is reduced, not because it happens to be angle up (although I'm sure there's some small effect of that as well). Once that increased lift reduces as it moves past the 3/9 position (whatever is advancing), that upwards movement doesn't immediately stop dead - but it stops adding to that momentum meaning that upwards flapping slows down until it reaches the apex, and then travels back down again. My physics is sucky in terms of telling you that the downwards motion is more from gravity, centrifugal force, springyness/torsion of the flapping hinge/elastomeric bearing etc. But essentially the up-down motion is slowest at the point when the blade's about to flap highest or lowest, and fastets midway around the 3/9 o'clock. Now because of gyroscopic precession or it being a second order system excited at resonance, that max upflap is not at the 3/9 but delayed as that momentum wanes off and the blade flaps up fully or back down - which is typically 90 degrees later, or somewhat less in fully articulate systems. Check out that image again in my 2nd post - you'll find more like it on guides for single rotors.

What I can't explain is the upflap on the coax stuff at speed.

22 hours ago, Ranma13 said:

since the top rotor generates more lift on the left side of the helicopter

Before I saw your reply I was literally writing a reply offline saying that might be the case. ie. agreeing that it's not the a torque difference like in EinsteinP's article, but rather specifically the advancing side of the top rotor...but that actually also doesn't make sense - that increased lift from the top rotor's advancing side wouldn't manifest immediately on the left - it would manifest ~90 degrees later only, ie. more at the rear. Unless that 70degree angle mentioned earlier means the top rotor advancing lift kicks in mostly at the back (ie. pitch forward), but that some of it still applies a lift a little on the left as well, which would produce right bank and need left cyclic.

I can't comment on the right rudder...it might make sense but usually it's not needed. Then again I don't recall flying the Shark at 250kph with AP channels off, so maybe the Heading Hold channel is able to compensate for that element.

 

I have seen some research papers (in addition to cw4ogden's) since posting suggesting on Kamovs the separation is narrower on the 3-ish position, but they never go into why. Might be induced flow related of the air fed into the lower rotor from the top at the rear left. Or maybe a really hard stall on the 9 position leading to reactive flap up on the right. Dunno.

Edited December 15, 2020 by Volk.

[iFoxRomeo]

On 12/10/2020 at 4:38 PM, cw4ogden said:

Haven't read it yet, but I'll bet there's an answer in here.

 

https://ntrs.nasa.gov/api/citations/19970015550/downloads/19970015550.pdf

Thanks for that!

 

On 12/10/2020 at 7:32 PM, cw4ogden said:

...

I have yet to make a post in the MI-8 forum because I have no first hand experience, but if vortex ring state was as dangerous as it is in specifically that module, there would be a lot of dead Russian pilots.  It's a thing, yes.  But you have to be a bit of an show off, be unfamiliar or operating at high density altitude conditions to get into it.  

..

It's not VRS at first. Check the Ng during the approach. The DCS Mi8 has no anticipator connected to the collective(don't know if the real Mi8 has) and only reacts to rpm droop. And the spool up time is high. Basically many DCS Mi8 pilots fail to keep the Ng high during the final approach and when they need the power, it's not there and takes too long to build up. But then they are already in the elevator downwards. Settling with power is way too often called VRS.

 

 

 

 

 

 

Thanks Volk to bring this up. It's long ago since I thought about rotor dynamics.

 

 

Fox

[Volk.]

Sigh. Turns out this stuff is even more complex actually - by that I mean more correct rotor dynamics and flapping even. Was planning on making a vid anyway on rotor dynamics, first in conventional helos then coax, but now I'm even more convinced that's necessary given my own misunderstandings from the 'usual' source guides.

But my (personal) running theory is that the separation possibly is narrower rear/rear-right for coax, because of the forward-left cyclic (not 'flapping from dissymetry'), if any only if the net lift overall is dominant on the advancing side of the top rotor which then needs to be countered in fast forward flight.

[cw4ogden]

On 12/14/2020 at 5:52 AM, iFoxRomeo said:

Thanks for that!

 

It's not VRS at first. Check the Ng during the approach. The DCS Mi8 has no anticipator connected to the collective(don't know if the real Mi8 has) and only reacts to rpm droop. And the spool up time is high. Basically many DCS Mi8 pilots fail to keep the Ng high during the final approach and when they need the power, it's not there and takes too long to build up. But then they are already in the elevator downwards. Settling with power is way too often called VRS.

 

 

 

 

 

 

Thanks Volk to bring this up. It's long ago since I thought about rotor dynamics.

 

 

Fox

 

There is / was widespread disagreement on the terminology.  Settling with power is vortex ring state per U.S. Army doctrine when I left, they are the same thing officially on a check ride, right up until the moment the IP goes off the record to have the conversation we are having.  Why they call VRS settling with power I don't know.  Maybe they just mish-mashed the two phenomenon at some point.  We eventually and informally settled on the term settling with insufficient power to describe the phenomenon you are referring to.  

 

They are absolutely two entirely different things, though; one being an aerodynamic phenomenon, the other being a matter of excess downward inertia with not enough excess power available to arrest the descent.

 

I feel the VRS is over-modeled, but it very well could be accurate.  I'm suspicious, but never flew it to be able to say definitively.  My evidence being the lack of a historical trend of VRS accidents in the MI-8.  If it was as dangerous as VRS is in the MI-8 module, my supposition is there'd be a long list of VRS accidents.  To me it feels, even at sea level, that you need the kind of caution that would be required only to operate at much higher density altitude conditions, or under much higher load.  It is difficult to get an unladen helicopter into settling with power by accident.  The MI-8 may have a quirk that makes this accurate, but it "feels" overdone.

 

 

 

@volk

 

 

Per the ka-50 other discussion:

 

I am leaning heavily towards the explanation being a combination of the retreating blades completely unloading in the upper regions of the forward flight envelope, coupled with the 

lower rotors induced flow and inherent higher angle of incidence.

 

I think what's happening is at high speeds is the two retreating sides are producing no lift or even negative lift.  The resultant blade cones seen arise directly from the fact the two advancing lift vectors are competing to keep the aircraft level.  In addition to blade collision, I would suspect the torsion force created on the masthead when the retreating blades unload is also an engineering consideration.  

 

The lower rotor is holding the right half up, the upper the left, and the resulting configuration at high speeds is just the lift vector required to keep things level.

 

It is important to keep in mind a properly feathered rotor system is not experiencing any flapping due to dissymmetry of lift.  Blade flapping is a transient phenomenon with regards to dissymmetry of lift.  

 

Also, I've read a couple times your supposition the lower rotor should experience a lower torque because it is generating less lift.  I don't know that that is correct.  Higher induced flow means more induced drag and therefore the lower rotor still generating less overall lift with equal torque applied.

 

 

Edited December 15, 2020 by cw4ogden

[cw4ogden]

On 12/14/2020 at 2:15 AM, Volk. said:

that increased lift from the top rotor's advancing side wouldn't manifest immediately on the left - it would manifest ~90 degrees later only

This appears to be mixing up lift with maximum upflap.  Lift across a rotor disk is at equilibrium, whether accomplished by flapping or feathering.  

 

The lift forces manifest instantaneously, the resulting up or downflap as a result of those forces occurs at the 70-90 degrees phase lag in the direction of rotation 70-90 being depending on blade count. 

 

Phase lag itself could use an explanation.  Most people can wrap their heads around it but never understand why it occurs:

 

When a force displaces an object, it will continue in motion until another force acts upon it.  In our case the object is the advancing blade.  

 

In a nutshell, it happens exactly as newton would predict.  The advancing blade has an upward force imparted upon it at the 3 o'clock position.  It will continue traveling along that path upward until another force acts upon it, in this case the centripetal forces.  

 

The classic aerodynamics texts use an untethered ball passing by a blast of air.  We wouldn't expect the ball's maximum displacement from the jet of air to occur at the point of air impact.  It wouldn't jump immediately and instantaneously, at the time the force is imparted, it begins to move in the direction it was pushed.

 

This is the same in a rotor system with the difference being at 90 degrees, it just can't go up anymore because it is tethered and has to come down.  The ball is stopped by surface and air friction when it loses all momentum.  A rotor goes up until it just can't go up anymore because centripetal forces pull it down.  The 90 degree point is just the most a rotating body can displace up or down before being pulled in the other direction by centripetal force.

 

 

Edited December 15, 2020 by cw4ogden
readability

[cw4ogden]

On 12/13/2020 at 12:10 AM, Volk. said:

On 12/11/2020 at 6:04 PM, Ranma13 said:

4. While the two rotors cancel out each other's torque, negating the need for a tail rotor, it's not 100% equal. This is why you still need to apply slightly left cyclic for level flight, because the left side is producing slightly more lift than the right.

If the top rotor's lift is greater, and it's torque more, you'd need to do the opposite. The top rotor moves clockwise, so it's torque alone would result in the fuselage turning left. To counter that you'd need right cyclic.

 

Shouldn't any torque moments not being equal should result in a yaw moment?  If the upper system began to require require less torque due to transition to forward flight, if I understand the rigging correctly, that would generate a movement about the X axis that would be countered by either increasing or decreasing collective pitch on the other rotor system, i.e. pedal correction?  I'm asking, not dart throwing.

 

The CH-47 does not exhibit this phenomenon where one system tilts right, the other left  at high speeds.  The point being if it were a matter of downflow from the upper system pushing the lower system around, or induced flow and unequal torque moments, I think we would similar on the 47.  The coaxial design can really push beyond traditional retreating blade stall limits because one half of each disk, just isn't flying anymore, but until the mast snaps or the blades collide it just doesn't matter.

 

The 47's design can't replicate this flight envelope without imparting immense twisting torsion forces because of lever principle.  You can't get half of your lift up from from the front right and half from the left in the back without an immense twist, that would manifest as a roll as the two heads fight to keep things level and one can't keep up.

 

I think whatever the specific aerodynamic combination of factors that explains it, the result is the aircraft wants to roll right due to the upper system being more efficient.  The limit you are approaching is a left cyclic limit, which you can see by executing similarly loaded left and right banks at high speeds.  For me it takes very little left cyclic at high speeds to make the blades collide. 

 

They could have made the separation between rotors further, to eek out a few more knots, but I'd be willing to bet the other sides of this physics problem is snapping the rotor mast.  The blades hit first, but space them apart much more, and you'd go faster right up until the point you snap off the top rotor system due to bending forces.

Edited December 15, 2020 by cw4ogden

[Volk.]

3 hours ago, cw4ogden said:

properly feathered rotor system is not experiencing any flapping due to dissymmetry of lift

Yeah, that's what I just learned yesterday. Or at least when trimmed for stable level flight. I don't believe that's commonly understood.

 

3 hours ago, cw4ogden said:

I've read a couple times your supposition the lower rotor should experience a lower torque because it is generating less lift

In my mind the lift produced = torque produced. ie. in a conventional tail-rotor helo more pitch the main rotor pulls with normal RPMs, the more the tail rotor has to follow suit. Same with coax, in that normally the lower rotor is always pitched slightly more than upper rotor. My (rampant) speculation is that maybe in some circumstances the torque isn't always balanced by the system, thus the right bank that needs to be countered. Another possibility maybe transverse flow having the top rotor generate more lift on right than left (cw rotor), that needs left cyclic, but I've also heard transverse flow should disappear at some point in picking up speed, so shouldn't be a factor at 250kph. I'm welcome to correction - I just hope that correction makes sense.

 

3 hours ago, cw4ogden said:

This appears to be mixing up lift with maximum upflap.  Lift across a rotor disk is at equilibrium, whether accomplished by flapping or feathering.

I was trying to justify that the disc is in equilibrium because of the feathering from the slightly left cyclic input. That without it the disc might be producing more lift on the left from the top cw rotor producing more lift at that regime despite other measures like flapping.

The way I was explained, is that lift is generated immediately, but the affects are only felt 90 degrees late from gyroscopic precession/phase lag. In a coincidental, but not quite related note, the maximum upflap is also 90-ish degrees later from second order system excited by resonance (measure in frequency). ie. if you push forward cyclic, it increases blade pitch on the left side (cww rotor) and decreases it on the right. That left side lift only kicks in 90-ish degrees later at the tail, thus tilting you forward in conjunction with the reduced lift at the fore.

Simultaneously that increased lift on the right makes the blade flap up, which culimates 90deg later (again at the rear).

It could be less that 90 in a fully articulated system.

 

I know there's also a bunch of math American scientists (and others - couldn't find too many of the Kamov papers with detail) about the ideal separation, and how the airflow develops or funnels beneath the top rotor, so that's probably also a factor in their distance.

 

I was wondering if the Ch-47 would start twisting in RBS...good to know.

Edited December 15, 2020 by Volk.