Uncontrollable yaw in autorotation

[Richman]

Hi folks

 

Today I was training a bit autorotations with the Huey and the Shark. But the latter tends to yaw about 90° to the right as soon as the speed falls below 150-100kph, altough my collective is full down, and my pedals are full left.

 

Yaw control seems to recover as soon as I pull collective.

In normal flight, all control responses are correct.

 

It seems quite a peculiar behavour as I cant find any mentioning of this in the manuals or else where in the forums or the net. Additionally, the big tail fin of the Shark should stabilize it at these speeds.

 

Am I experiencing some rare control input misalignment, is this a game bug, or a real intentional beheavor I'm just surprised by?

 

Thanks!

Richy

[Nate--IRL--]

Have a think about how yaw is produced in the Ka-50. If the collective is down, you have no yaw control.

 

Nate

[Shadowowweosa]

according to the Flight instructions, to preserve a sufficient stock of path management, the ka-50 is necessary to keep in autorotation revolutions of the ROTOR within (not less than 80, and no more than 97).

[yodosha]

http://forums.eagle.ru/showpost.php?p=1618085&postcount=80

http://forums.eagle.ru/showpost.php?p=1622928&postcount=82

The UH-1 doesn't do that because its tail rotor is powered by the main rotor.

Control returns when you raise the collective (for the shark) because you start to use the kinetic energy of the main rotor to power itself. (trade RPMs for lifts, lifts mean drags, drags mean yaws)

[Bushmanni]

You don't have to floor the collective all the way to autorotate. Keep it high enough that the chopper can be controlled and you should be able to make a fine landing. The strange behavior of Ka-50 in autorotation is most likely pretty close to how it should be. Remember that the yaw force in Ka-50 is produced by torque differential of the rotors that is transferred to the fuselage through the gearbox. In powered flight the lower rotor blades have a slightly larger angle of attack to offset the downwash from the upper rotor to balance the torques of the rotors. When in autorotation the larger angle of attack of the lower rotor makes it have less torque (to the direction of the spin) than the upper rotor. As the upper rotor spins clockwise and has more torque in the spin direction the fuselage will spin clockwise also unless corrected with pedals.

 

Ka-50 can momentarily turn 90 degrees sideways (at least towards right) at 200km/h speed with full pedal depression. While I was testing this maneuver I also found out that Ka-50 can also fly sideways very very fast. I think my top speed was pretty close to 200km/h but I'm not exactly sure of the numbers other than that it was close to 200km/h. The tail is short and the fin isn't really big and it isn't that powerful either in order to enable the previously mentioned unmatched yawing and sideways flying performance.

[Richman]

Thanks for the inputs!

 

I was aware that the Shark controls its yaw by a torque difference on the two rotors. But I expected this to work also in autorotation.

 

Thanks for the explanations! Now I understand the reason! :)

[ShuRugal]

Please excuse my disagreement, but in an autorotation, what is producing torque that needs to be counteracted? This yawing behaviour is definitely new since last time i played (in january) and i am nearly certain it is incorrect.

 

For the rotors to produce a torque on the body of the helicopter, something in the helicopter must be either attempting to accelerate (power) or decelerate (brake) the rotors. That is what torque is: rotational force being applied on one object by another.

In auto-rotation, only two forces act on the rotor: drag and their own inertia (if you wish to consider that a 'force'). The only force from the helicopter to the rotors would be friction in the drive train which (unless the pilot engages the rotor brake) should be damn near nonexistent.

 

Even if the freewheeling unit fails to operate and allows the main tranny to remain connected to the rotors, this should not be capable of producing enough drag on the shaft to torque the body of the helicopter.

 

And, no, raising collective in auto-rotation does -not- torque the helicopter: it increases drag on the rotor blades, which are only being powered by airflow and their own angular momentum. There is no force being supplied from the helicopter to keep the blades turning against the drag from the air.

 

Now, it -is- true, for the same reason, that the main yaw control will be ineffective during an auto: Since the rotors are no longer producing a torque through the shaft, then there can be no differential torque between them to achieve yaw control, and the helicopter will windmill into the relative wind.

Edited October 5, 2013 by ShuRugal

[sobek]

You do leave out the operation of the coaxial gearbox. Think about how the rotors are coupled in their rotation. If one rotor has increased drag, the angular momentum of the other rotor will result in increased torque on the first rotor. However, since they both turn in opposite directions and the gear providing the coupling is fixed to the airframe, this effect will result in a net torque on the airframe in the direction of the rotor having less drag.

 

Edit: Essentially the gearbox works sort of similar to a differential gear in a car. If you fix one wheel so it can not move anymore and then turn the other wheel with enough force, the whole car will start to flip around the axle(IRL the drive train will possibly break or the motor will start to turn, but it is a thought experiment so bear with me).

 

The point is, the torque mechanism in Kamovs designs increases the AoA of one rotor while decreasing that of the other. While during autorotation, the net torque is inversed compared to powered flight (i won't go into the details, there is an intricate design that inverts the controls during autorotation so the pilot won't have to rethink), it still works.

Edited October 6, 2013 by sobek

[ShuRugal]

forgive me if i am rambling a bit, still chewing on the physics of this.

 

i had not considered that the rotors remained connected through the gearbox. I was thinking the freewheeling unit disengaged before the gearbox. Having the rotors linked to each other so that they always match speed, regardless of relative drag, would produce a torque force.

 

but then i still don't see why there would be a difference in drag on the two disks in the absence of pedal inputs. if both rotors are matched in RPM and collective setting, they should have the same drag.

 

The only thing i can think of that would cause a difference here would be the effect one rotor has on the air flowing towards the other. The lower rotor being in the downwash of the upper would reduce its effective AoA, but a minor pedal input should correct this, otherwise the bird would not be flyable under power either.

 

What is so special about autorotation in this bird that creates more torque, instead of less?

[sobek]

What is so special about autorotation in this bird that creates more torque, instead of less?

 

I haven't tried autorotation lately but as i said, switching from powered to unpowered flight reverses the torque. The lower rotor does have significantly more blade angle to achieve the same AoA as the upper rotor, as the air it normally hits is already moving downwards. I suspect that this might be the cause of the inbalance during autorotation. I'll also try it myself if there is anything untoward happening.

 

Edit: I tried and i found nothing untoward, there was a pronounced but not uncontrollable tendency to yaw right upon flaring that can be perfectly explained by the rotors blade angles not being balanced for unpowered flight.

Edited October 6, 2013 by sobek