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

[cw4ogden]

28 minutes ago, Volk. said:

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.

I don't hold an aerodynamics degree, I'm just a fellow enthusiast with a different experience to draw on.  My hope is I am not coming off as pretentious or as being a true expert.  I enjoy the back and forth, and love a good rabbit hole so please take all speculations with a grain of salt.

 

Transverse flow is a transient phenomenon unless you remain in that airspeed regime where the front portion is inducing flow to the back.  Stay at 35 knots you will observe the moment in time captured by the picture in the illustration above.  I'd guess the Ka-50 experiences it on the lower rotor longer into the forward flight regime, but still not a factor at velocities we are considering for the blade clearance question.  Transverse flow in a 47 actually pivots you a bit about the pitch axis because instead of front half clean air, back half dirty phase lag fotor tilt, etc, in a 47 it becomes more front rotor clean back rotor dirty and the butt drops a bit, none of which you notice unless you turn off the AFCS, 

 

I think one area that's mixing things up is the difference between transient states and equilibrium states, and thinking of individual blades versus a rotor disk.  Coning aside, I can't up flap in the front without flapping down in the back.  Even when not aligned with the rotor head, i.e during blade flapping, the disk is still aligned with itself.  It is still operating on a semi 2 dimensional plane.  You can't flap up without an equal flap down 180 degrees later.  

 

 

 

 

The moment the torques aren't in equilibrium there is a yaw moment created.  But either the pilot or the autopilot corrects by I believe a counter input of collective on each head, don't quote me on the lefty righty-ness of it but you start to fly forward, a force happens of your choosing that causes a differential torque between heads, you rebalance the yaw moment with pedal by an asking a little more power from whichever head for the direction you are starting to yaw around, and a little less from the other.

 

Barring pilot correction you will reach another, different equilibrium which just means you are flying out of trim, and beyond what we are trying to navigate I think.  An uncorrected minor yaw in a traditional helicopter should eventually get weathervaned nearly of existence in progressively faster forward flight, that would be the other equilibrium.

 

 

 

35 minutes ago, Volk. said:

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.

 

It's not a coincidence.  The lift you feel instantaneously is that you don't fall into a left(?) roll.  The force we are compensating for being a loss of lift potential on the left side.   The affect being a tilt to the rotor system in the form of blowback.  

 

The upflap and affect you feel at the 90 degree over the nose, the blowback, is the maximum displacement of the blades from the upward force imposed upon it by hitting faster oncoming air on the advancing side.  The 90 degree point, also being the point by definition where it is no longer the advancing blade.  So the cause of the force is also petering out progressively and ceases to be a factor at the 90 point. 

 

Either the blade flaps up and you feel the resulting tilt 90 degrees later, as the flapping mechanism corrects, or you push forward thereby lowering the pitch on the right, increasing it on the left which imparts its own phase lag adjusting forces in the opposite direction, re-righting of the disk.  But if the lift, itself were happening 90 degrees in the direction of rotation, you'd still have the roll we are correcting to begn with, because you'd still have a dissymitery of lift.

[Rogue Trooper]

When flying it without APs and observing how the air frame yaws so easily around the rotor mast makes total sense to me.

The KA-50 modelling feels totally right to me.

 

 

 

Edited December 17, 2020 by Rogue Trooper

[Volk.]

If one compares the speed of turns to footage of the real thing, then the turning (yaw & bank) do seem right. Putting off that Heading AP is great for quick yaw turns in hover and setting the new heading without retrimming causing a potential bucking. For the way it flies, outside of the rotor intersection bit which this thread is partly about - not meant to be clickbait, just a discussion on why - and *maybe* why the Shark can occasionally go into a full spin in stronger wind (ie. weather vane into wind, but then keeps spinning instead of streamlining into the wind even if one slowed it from overshooting the oncoming wind direction) I have no issues with the way it flies. The printed takeoff weight limit number precludes doing vertical take-offs with Vikhrs + rocket pods loaded, but I've seen footage of Sharks doing a nose-down vertical take-off, Shkval practically scraping the ground from very little forward speed fully loaded. So yeah, how it flies, appears right to my layman's eye. It's just the forward flight stick position and the location of the smallest rotor separation that I'm curious about.

 

@cw4ogden I'm totally enjoying the back & forth as well. I *think* we're saying the same thing atm in term of how it behaves. Question though - do you think that if one had to trim cyclic forward left for fast forward flight so lift were equal, that the rotor disc would be tilted down at 11 o'clock and then raised up high up back and to the right? Would it be mostly that or more some airflow thing on the lower rotor.

[cw4ogden]

9 hours ago, Volk. said:

do you think that if one had to trim cyclic forward left for fast forward flight so lift were equal, that the rotor disc would be tilted down at 11 o'clock and then raised up high up back and to the right? Would it be mostly that or more some airflow thing on the lower rotor.

The forward is a given for forward flight, so let's just consider the left.  I'm unsure if you are talking about a single rotor here, or still Ka-50?

 

If we are still referring to the Ka-50, I think were it an airflow thru the rotor thing, you would see the phenomenon be highest at X on the forward flight envelope, and diminish as your speed increases.  The cleanest air the lower rotor gets is at high speeds I would guess.  

 

In a single rotor helicopter that slight left cyclic requirement could be the result of most designs incorporating a few degrees of tilt into the main rotor mast to offset tail rotor thrust at a hover.  In level flight that angle will create a slightly non-vertical thrust vector requiring a few degrees of tilt in the other direction, all but a trained eye might not notice.

 

In the Ka-50 I'm fairly convinced it is because the retreating blades get pushed further into RBS than in a single rotor helo.  I'd like to look up the rotor rpm and compute some tip velocites and see how they compare to say a UH-60.  

 

In the sim for me, about 280kph or 150 knots is where you are just flirting with disaster.  That's just not as fast as I assumed it would be having never done the conversion to knots.  I thought the Ka-50 was hitting 170ish knots in forward flight, but if it's more like 150.  But 150 is fast for a helicopter.  I don't think my previous assessment that the retreating side is generating no lift can be accurate if we are in the 150 knot range unless the rotor rpm is significantly lower than U.S. birds.

 

But I think the overall explanation may still work.  What does a single rotor helicopter do in RBS?  It should want to roll left with an accompanying secondary pitch up caused by the rolling forces and precession.  What does the Ka-50 want to do?  It want's to roll into itself, basically.  

 

"Initial retreating blade stall symptoms include vibration, nose-up pitching, and a rolling tendency toward the side opposite the advancing side."

 

Each side is just rolling towards the retreating blade, maybe?  

 

 

 

[Bushmanni]

 

tld;dr  At high speed there's less down wash at the rear section of the lower rotor or even upwash that causes the lower disk to rise at right side. The downwash of the upper disk gets skewed rearwards due to forward motion of the helicopter and the reduced downwash of the center of the upper disk gradually gets pulled more aft.

[Volk.]

@Bushmanni very interesting. I didn't see that post before, but I think the title buried it amidst a sea of general posts of 'whycome I die at 250kph'.

Do you maybe have the title of the first NASA article you linked? The link's dead and the pdf title doesn't make it apparent what to search for.

 

I'm strongly leaning towards it must be a downwash thing. But I'd imagine the downwash from the upper rotor, possibly slanted further back from the high speed would increase the induced flow of that "pushed" air, and that downdraft would essentially reduce angle of attack (relative wind now 'moving down' as it were through the blades) and thus lift. If this were to hit the lower rotor at the back that would mean a reduced lift there...

 

Is what you're suggesting that the air flowing through the centre of the upper rotor, which now hasn't been accelerated by the upper rotor (which would normally be centred on the lower disk at near hover, or otherwise hit nearer to the blade root in forward flight) now hits near the blade tip, causing the lower rotor to get 'fresh' air without induced flow at the back resulting in lift/upflap (which takes effect only on the right 70-90deg later) not compensated for be feathering? But if that were the case they that would generate additional lift on the right, creating dissymetry and requiring right cyclic to retain level forward flight (which at least in the sim it doesn't)...

[Bushmanni]

The NASA article is the same as posted earlier in this thread.

 

Flapping takes care of the moment created by the force causing the rower rotor to tilt so you don't experience roll to the left. You still end up having the lower rotor disk tilted to the left which causes slight drift to the left unless you compensate with slight bank to the right. If you try to fly very carefully as straight as you can and wings level, you will notice that the Shark doesn't want to fly directly forwards at high speed but wants to drift to the left a bit.

Edited December 31, 2020 by Bushmanni

[Volk.]

18 hours ago, Bushmanni said:

If you try to fly very carefully as straight as you can and wings level, you will notice that the Shark doesn't want to fly directly forwards at high speed but wants to drift to the left a bit.

 

That's one of the things, at faster forward airspeeds, keeping level requires a little (~15-20%?) LEFT cyclic in the sim, not right.

[Bushmanni]

Top rotor has much longer moment of arm and hence it's the dominant rotor for rolling motion. In forward flight it's leaning to the right which explains why the rolling moments are in balance even if the lower rotor is much more tilted to the left. But it's a good question why the top rotor would be leaning to the right. I have no clue at the moment. Because of its spin direction, transverse flow should make it tilt to the left. There would have to be downwash for some reason at the front portion of the disk or upwash at the rear portion.