Question about autorotation for helicopter professionals

[Echo38]

I know that with model helicopters it is indeed quite possible to stop the rotors in autorotation, and even start them again, given sufficient altitude. But then, the rotors on a model have considerably less mass, considerably lower moment arms, and considerably higher ranges of collective pitch.

As I recall, one of Pirke's earlier posts in the thread said that the rotors on a real helicopter would bend upwards until they broke, if they were motionless during flight. He said that they need centrifugal force to keep them from doing so, I.I.R.C. Of course, an R.C. helicopter wouldn't need to worry about this, because the ratios are all different with a scale model.

[Bushmanni]

The rotor blades don't continue to turn during autorotation because of blade inertia, they continue to move because the total aerodynamic force points forward of vertical. Also, during autorotation, all of the airflow comes from beneath the rotor, and travels up through the rotors. It is impossible for vortices to "stick" to the rotor during an autorotation do to this upward flow.

 

During vortex ring state, the downwash flies right back up through the rotors, but is then pulled back down through the rotors. This does not occur during autorotation.

 

What I could gather from a Navy helicopter aerodynamics course book is that during normal autorotation descent the rotor is strictly speaking in a VRS condition (airflow both up and down through the rotor disk) as pure autorotation state would produce ever increasing RPM and destroy the rotor. Outer portion of the disk is producing lift and drag (driven portion of the rotor) while mid portion is pulling the blades forward (driving portion) and inner portion is stalled. By balancing the driven and driving portions of the rotor by adjusting blade pitch, RPM and rate of descent a pilot can establish a steady autorotation. From a perspective of aviator the normal autorotation descent state is called autorotation as the aircraft behaves like it's in autorotation instead of VRS while technically it's in VRS. Helicopter can enter a stable straight vertical autorotation descent but according to H-V diagram (aka. Dead man's curve) you can't land from it but need to transition to forward flight autorotation to land. The requirements for VRS is some induced velocity, descent rate of 0.7-1.25 of induced velocity to descent with vortices and low airspeed. There's nothing to keep you from entering a VRS from autorotation if you do it wrong as rotor inertia is clearly enough to produce lift ie. induced velocity. This VRS would be short lived though as RPM would decrease rapidly leading to increasing descent rate which would bring the rotor back to autorotation if not stalled first.

[Bushmanni]

On a second thought the slow speed danger portion of the H-V diagram must be due to rotor having insufficient energy to arrest descent from hovering descent, not due to VRS. But that doesn't mean that you can't get into VRS in autorotation. Other thing I should emphasize is that VRS during autorotation is going to be very short lived and encountered only in situations you shouldn't get in the first place if you are flying properly. As such getting into VRS from autorotation is purely an academic issue. I also tried how DCS Ka-50 handles in hover autorotation descent and what you know, it will enter and maintain proper VRS for about 5 seconds before rotor RPM drops too low to produce enough lift for VRS.

[Mikoyan89]

Anyone knows (maybe the developers) why, since the version 1.2.0 ,when you’re in autorotation,suddenly the helicopter start to yaw to the right? It occurs generally when you're below a given ias (150-160 km/h) and you’re falling with more than 10 m/s….is a phenomenon exacerbated by pulling the cyclic …..even if not too aggressively.It seems that is related to the relative wind direction,in fact every time the air moves trough the rotors from below,in autorotation or when in powered flight,there’s a chance to see this phenomenon.

(It's preceded by a rapid increase of rotor RPM).

I know that coaxials copters have directional problems when you don’t have the torque of the rotors and when you have not enough airspeed the rudder become uneffective,but why this problem exists even with no wind (or with wind but coming from the front),and why this yawing motion is always toward the right?

[Bushmanni]

It could be because the lower rotor has higher blade pitch angle in normal flight and in autorotation the lower one therefore isn't "driven" as much as the upper, ie. the force making the upper rotor to keep turning is greater than the one for the lower. The moments acting on the rotors are not in balance and because the gearbox forces the rotors to turn at the same speed the difference in torque is transferred to the gearbox and from there to the fuselage. The upper rotor turns "to the right" so if more "driving" torque acts on it that to the lower it will make the fuselage to turn to the right. Reason for the difference in pitch angles in normal flight is because the lower rotor is in the downwash of the upper and hence has higher induced velocity overall that must be compensated with higher blade pitch angle.

[Mikoyan89]

thanks for the reply,but if the upper rotor (that rotates clockwise seen from above) has more torque and this torque is tranferred to the gearbox,the fuselage should react to this yawing towards the left,isn't it?And then,if a rotor in autorotation state has really enough torque to let the fuselage to react ,why in a single rotor helicopter this doesn't happen?

[Bushmanni]

The forces that turn (in general all motions of flying helicopter are caused by aerodynamic forces, here we focus on the yaw axis) co-axial chopper are aerodynamic forces acting on the rotor blades. In normal flight the forces are drag forces that oppose the turning of the blades. Hence the fuselage turns to the opposite direction in regards the rotor that has more force (drag) acting on it. In autorotation the aerodynamic forces acting on rotors are driving them, ie. they act in the same direction as to where the rotors spin. In autorotation the fuselage turns in the same direction as the rotor that has more force driving it ie. has lower pitch angle. Basically in any situation the fuselage turns to the direction where the net torque caused by aerodynamic forces acting on rotors is acting. The aerodynamic forces can either oppose or drive the rotation and hence act in the opposite or same direction of the rotation respectively.

 

In helicopters the rotor (or rotors) applies torque to the fuselage as long as there's some kind of mechanism in fuselage applying torque to the rotor. This is according to the newtons third law of motion. When a body exerts a force on another body it exerts back with a opposite force that has equal magnitude. In autorotating single rotor helicopters there's no mechanism in fuselage acting on the rotor, ie. it's "freewheeling". If you are precise the main rotor bearing have some friction and tail rotor shaft is applying a small torque to the main rotor due to the drag of the tail rotor but these are cancelled by the thrust of the tail rotor. In co-axial helicopters the gearbox has a mechanism that forces both rotors to turn with the same speed in regards to the fuselage. If one of the rotors has more torque the gear linkage will transfer some of the torque (through the fuselage) to the other rotor to balance the torque and make them accelerate or slow down at the same rate.

 

Imagine you have a very small (like 30cm long) ka-50 that you hold from the upper rotor. Then apply a torque to the lower rotor ie. you turn the lower rotor with your hand. How the fuselage would turn to keep the rotors spinning with same speed in regard to the fuselage but in opposite directions? If you turn both rotors slowly with same speed but then start to spin the other faster, how will the fuselage move now to keep the both rotors turning with same speed in regards to fuselage?

 

I hope I managed to be understandable and didn't just get you more confused.

[Bucic]

In autorotation the aerodynamic forces acting on rotors are driving them, ie. they act in the same direction as to where the rotors spin. In autorotation the fuselage turns in the same direction as the rotor that has more force driving it ie. has lower pitch angle. Basically in any situation the fuselage turns to the direction where the net torque caused by aerodynamic forces acting on rotors is acting. The aerodynamic forces can either oppose or drive the rotation and hence act in the opposite or same direction of the rotation respectively.

I'm not sure you got it right. Please refer to the updated UM (500+pages), pages 4-11 - 4-14

http://www.digitalcombatsimulator.com/en/downloads/documentation/dcs_bs_manual_en/

[Bushmanni]

It seems to me I haven't told anything contrary to the BS manual. Anyway I base my analysis on laws of physics and US Navy helicopter aerodynamics course book. I assume what you wanted me to consider is that blades have three regions ie. driving, driven and stalled regions instead of being uniformly in one of these states. For the purposes of understanding how the fuselage experiences different turning moments in normal flight and in autorotation I felt this detail was unnecessary as only the net force acting on blades or rotor is important for understanding the phenomena. I have explained the three regions of the autorotating blade previously in this thread. Was this what you meant?

[AlphaOneSix]

In powered flight, the fuselage will yaw in the direction of rotation opposite of the most loaded rotor (i.e. the one generating the most lift).

 

For single-rotor configurations, this is always the same rotor, since there is just one. For coaxial rotor helicopters, this effect is exactly what is used to create a yawing moment without the use of a tail rotor.

 

In non-powered flight, the fuselage will yaw in the SAME direction of rotation as the most loaded rotor. It should be noted however, that the amount of yaw moment generated in non-powered flight is a lot less than that generated during powered flight.

[Watari]

As a non aircraft technican i treid to follow as much as possible and its very interesting no doubt.

But overall now can we say that our black shark behaviour is too forgiving at recovering the rpm/stability in autorotation compare to real ka50??

 

Would be a real ka50 much harder to control than our virtual one?

 

I have watched this video from this dude whos flying a "real" simulator of Mi-8 or something and he made a good job?

 

Maybe we are really close to "reality"!

[Griffin]

But overall now can we say that our black shark behaviour is too forgiving at recovering the rpm/stability in autorotation compare to real ka50??

According to helicopter professionals here it is too forgiving. AFAIK none of them had flown a coaxial helicopter but it should not be that different. This is propably a limitation of the game engine as Yo-Yo has checked the values many times and corrected them too.

Would be a real ka50 much harder to control than our virtual one? ... Maybe we are really close to "reality"!

Many professionals have also said that this is the best helicopter flight model ever made. Apart from autorotation, I don't believe the real deal is any harder but it is of course different in a way that you would have to fly a bit to get used to.

 

It will be interesting to see how Belsimtek's UH-1H behaves in autorotation and in general. Hopefully their flight model will be equally praised in the sim community and thus pave the way for more helicopters. For example people who like flying helicopters in FSX/X-Plane would find that DCS is a better helicopter sim platform, spread the reputation and soon perhaps some of the developers would come here. Right now it's not easy to jump to a new platform when you have your Dodo/Nemeth etc helicopters in your preferred sim. But this is offtopic already...