Uneven Co-axial Rotors

[Avio]

So I bought the Apache yesterday, and was so disappointed with the yet to be tuned FM that I am back to flying my beloved Mi-8 and BS2.

In the BS2, at higher speed, the twin rotors appear to be closer together on the right side of helo when viewed from outside from the front. Why is that so, when it is supposed to be symmetrical? Is it due to gyro precession issue? Is that the cause of the helo needing much right rudder to keep the ball centered at higher speed? Or is this a bug in the modelling even?

Hope someone could help shed light on this. Thank you.

 

[Volk.]

I used to think that narrowing gap was an error, after some pilots explained the maximum upflapped point would happen at the point where the lift was generated. That turned out to be a mistake, based on errors in a more official manual. The principle of flapping is explained often enough, but not *specifically when it happens* and when it's at it's highest/lowest, which is mostly a thing for coaxial.

On the ground without wind you'll see the blades are perfectly synced, generating equal lift everywhere. At fast forward airspeeds, the flapping on the advancing rotor kicks in with the advancing airflow, and then the downflap about 180 degrees later. Which also means that the upper retreating blade has flapped maximally down (talking about the point of being furthest down, not flapping down or lift generation per se) at the same plate where the lower advancing rotors have flapped up fully.

Maybe @Yo-Yo can explain better if he has time.

 

My assumption is the left cyclic at fast speed is actually due to the uneven separation/gap at these higher speeds and thus different induced flow.

 

As for the right rudder, AFAIK if there's no wind affecting you, no you don't hold in right rudder during forward flight. The Shark should fly/point almost perfectly straight into the direction of your flight path, depending how perfectly you can trim it. At speed the slip ball will show slightly to the right - I've learnt to ignore that.

[DmitriKozlowsky]

8 hours ago, Avio said:

So I bought the Apache yesterday, and was so disappointed with the yet to be tuned FM that I am back to flying my beloved Mi-8 and BS2.

In the BS2, at higher speed, the twin rotors appear to be closer together on the right side of helo when viewed from outside from the front. Why is that so, when it is supposed to be symmetrical? Is it due to gyro precession issue? Is that the cause of the helo needing much right rudder to keep the ball centered at higher speed? Or is this a bug in the modelling even?

Hope someone could help shed light on this. Thank you.

 

I used to question this myself. Still do. But it appears to be correct behavior in fast forward flight and has to do with dyssymetry of lift in coaxial (or contrarotating) rotor system, and as lower disk is always in flow 'shadow' of upper disk. So I take as correct, as a given, and fly. I recall back in late 1980's and early 1990's either a Russian general or Western general officer saying this was a weakness, as it required combat hard maneuvering to the left when breaking to avoid attack, or response. When turning hard, lower collective as much as possible, regardless of turn direction. Rolls should be done with extreme care in such helicopters.  A Russian Air Force helicopter general bought it when put then new KA-50 into prohibited regime, which caused his rotor disks to intersect. He did not eject. There are advantages over main/tail rotor rigs. LTE is never an issue, as there is no tail rotor. Settling with power risk is reduced, but is still there. Faster Vne. Contrarotating helicopters are faster then similar rotor diameter main/tail rotor types. KA-50 in DCS can haul 280kpb and approach 300 kph, 180-but must be flow with care at that speed. Apache starts to blade stall and roll to the left at 240kph. In my odd experience with empty DCS Apache with 40% fuel. At around above 135 knots, I get uncommanded rolls to the left, that must be countered with right cyclic. With 150 knots being Apache's Vne in DCS, but I think of 140-145 knots as its effective Vne. 

For fast forward flight try lowering collecting and aft on cyclic , that may reduce disk tip distance. If you Vne chirp, drop collective immediately, either center cyclic or slight aft on cyclic to slow down. CHirp gives few seconds and 1-3 more knots of speed before rotor disks intersect.

Americans don't seem to like this layout, and have avoided it, but are now back to it with rigid rotor contra-rotating.  Americans have had better results with intermeshing rotors of Kaman Husky and Kaman K-Max.

[Avio]

Thanks much folks for your detailed replies. They make much sense indeed.

On the use of right rudder at higher speed, I find that seems necessary as otherwise the shark would start drifting slowly to the left with time, when flying straight and level with no rudder input, with the slip ball skewing quite a bit to the right, the higher the speed gets. With right rudder input, however, the shark would then fly true and straight. This observed yaw to left (if uncorrected) at higher speed, could it be because of the “pinched” rotors on the right side during high speed forward flight? Anyone noticed this?

 

[Deano87]

The reasons why the rotor disks tilt towards each other is because they rotate in opposite directions. It is not to do with the lower blade being in the flow shadow of the upper disk.

All flapping helicopter rotors will want to tilt like this in high speed forward flight because of the way the center of lift moves horizontally across the disk as the forward speed increases. The advancing blade has a higher airspeed than the retreating blade, so produces more lift, and this causes the disk to tilt. In the case of the Ka-50 the two rotors rotate in opposite directions so the side which has the advancing blade is opposite as well. Which is why at high speed the left hand side of the top rotor and the right hand side of the bottom rotor lift upwards, obviously this brings the right side of the top and bottom rotors into proximity with each other.

[Mars Exulte]

It is correct and described in detail in the manual nobody reads  It's literally one of the very first things it goes into.

Every rotor has an advancing blade and a retreating blade. Advancing blade generate slift, rereating blade loses it. This pushes the blades up on side and down on the other. The faster you go, the more pronounced the effect. This is normal and present on all helicopters, just not as glaringly obvious.

Contra rotating systems like the Ka-50 uses attempt to cancel out the cons of helicopter rotors by rotating in opposite directions, so the aircraft has roughly neutral lift/loss on both sides. However this means the blades are very close together at high speeds as one is high/low and the other is low/high, potentially causing a blade strike at extremes of the envelope.

These characteristics are one of the main limitations on helicopter forward flight, as the lift gets stronger on one side and loss greater on the other, the tendency to roll becomes impossible to prevent, all other considerations aside.

Edited October 14, 2022 by Mars Exulte

[AeriaGloria]

On 10/14/2022 at 3:56 PM, Mars Exulte said:

It is correct and described in detail in the manual nobody reads  It's literally one of the very first things it goes into.

Every rotor has an advancing blade and a retreating blade. Advancing blade generate slift, rereating blade loses it. This pushes the blades up on side and down on the other. The faster you go, the more pronounced the effect. This is normal and present on all helicopters, just not as glaringly obvious.

Contra rotating systems like the Ka-50 uses attempt to cancel out the cons of helicopter rotors by rotating in opposite directions, so the aircraft has roughly neutral lift/loss on both sides. However this means the blades are very close together at high speeds as one is high/low and the other is low/high, potentially causing a blade strike at extremes of the envelope.

These characteristics are one of the main limitations on helicopter forward flight, as the lift gets stronger on one side and loss greater on the other, the tendency to roll becomes impossible to prevent, all other considerations aside.

 

totally with you buddy. It’s the first section, also in chucks guide

 

But to answer some other questions. I believe left roll makes it worse becuase, what causes rotor clash is the right side. Well when commanding a left roll, the right side has to flap up. But the right sides of bottom and lower disks don’t flap up evenly becuase the bottom disk is creating more lift and the top has less lift!! So the bottom flaps up more, hitting the top that is flapping less. 

  Flapping is there to help even out lift. As a blade creates more lift it will flap up which decreases AOA and also lift. The flap up causes it to flap down when it produces less lift, and the flap down increases AOA and thus increasing lift. So flapping helps slow down dissymmetry of lift by using the flapping motion to decrease the lift of high lift areas and increase the lift of low lift areas, but it still can’t prevent dissymmetry of lift entirely.)
 

For right rudder it’s more simple, you have to increase lower blade pitch and decrease upper blade pitch to yaw right, which makes the lower blades flap up more and the top blades flap up less. 
 

As the blades also contact in straight and level flight becuase of flapping, it is a balance of all these three things.