turning left problem

[tflash]

I dummo what the problem is, but I have much difficulties turning left with the Ka-50.

 

It simply resists turning to the left, and when I trim I get a violent movement back to right. Even gentle turning to left often ends with alarm going off and my rotors ripping off. I also get into a sideslip.

 

Turning right is no problem, I do gentle stick movement, trim and that's it.

 

I guees this is a rudder calibration problem? I use a Saitek X52.

[Kwill]

When in flight you don't turn with the rudder, you do it with the stick AFAIK. I use the rudder very gently and only to line up with the target or the FARP or runway.

[Nate--IRL--]

Turn on the Joystick indicator, that'll show any calibration problem, also use the flight director mode to see if the problem disappears, if it does you are fighting the autopilot, if it doesn't it's a likely a calibration problem.

 

Also a track file would be helpful in diagnosis.

 

Nate

[tflash]

Thanks for the advice. When I do not use rudder to turn, it goes a lot better. But the joystick indicator shows my rudder is biased to the right. Is this normal? I guess on a tailrotor chopper this would be the case, but here?

 

I need precise turning adjustments since I'm praticing with rockets.

[Kwill]

If your rudder pedals are centered and you have reset trim then the indicator should stay in the middle, I don't use the rudder to turn because it happens what you say it happens, the rotors collide.

[tflash]

Indeed, I recalibrated my stick (strange, with my other games like Locon:FC I do not have this problem), and then the rudder was centered again. But it degrades during flight, I'll have to reset trim more often.

 

And you're right: turning the rudder makes the rotors collide!

 

Thanks all!

[Yskonyn]

What are you using for the rudders? The twist motion of the stick? If you I would suggest finding another way of controlling the rudder.

The twist of the stick is far too sensitive to use it as a rudder and it's far too inaccurate as well. The tendency to chance over time out of calibration is an often heard problem with the twist stick axis.

I would invest in some proper pedals. You'll need good rudder control in this sim.

 

And using rudder in flight does not always make your rotors collide, but you have to use the rudder correctly and cautiously. If you slam it one way or the other, sure you'll overstress the aircraft. Using rudder and stick can result in some pretty awesome maneuvers without smashing the rotor blades. ;)

That's a problem with the twist axis as well, because you have little room for movement you tend to overuse rudder quite easily.

Edited April 11, 2009 by Yskonyn

[GGTharos]

Post a track, it's hard to diagnose without it - maybe your're fighting the auto pilot, maybe you're flying too fast, maybe this or the other.

Maybe you're just too keep to yank and bank like you would with a fighter ;)

[Lav69]

When in flight you don't turn with the rudder, you do it with the stick AFAIK. I use the rudder very gently and only to line up with the target or the FARP or runway.

 

I mash the hell out of my rudder. :)

 

No, but really to turn left you have to close your right eye and hold your left ear lobe with your left index finger and thumb. :megalol:

[tflash]

I mash the hell out of my rudder. :)

 

No, but really to turn left you have to close your right eye and hold your left ear lobe with your left index finger and thumb. :megalol:

 

Are you sure it is the right eye? It doesn't seem to work ;)

[Lav69]

Are you sure it is the right eye? It doesn't seem to work ;)

 

yes Locutus.

[EinsteinEP]

I posted this elsewhere, but it kinda fits here, too, so...

 

 

Sorry in advance for what must seem like a dissertation, but this is what I've learned about coaxial rotor flight dynamics. It may help to have a picture or model of the Ka50 in front of you while you read this.

 

In a hover, the two rotor disks of the Ka-50 contra-rotating coaxial rotor helicopter are designed so that they produce the same amount of torque (due to drag) in opposite directions. The lower rotor actually produces less lift/drag than the upper rotor for the same relative airspeed, but since it is working in the higher airspeed downwash of the upper rotor, they match torque production at a hover setting (at sea level on a standard day, <yawn>). This designed so that minimal (ideally NO) yaw input is needed at a hover.

 

During forward flight, this difference in lift production capability results in the upper rotor generating more lift than the lower rotor. The upper rotor rotates clockwise, so you get a net counter-clockwise torque from drag, thus the left yawing tendency at airspeed.

 

 

This yawing tendency, left uncorrected, will result in uncoordinated flight which is messy and unprofessional. So, in a Kamov contra-rotating coaxial rotor helicopter, like the Ka-50, a Good Pilot, interested in maintaining steady-state coordinated level forward flight, will apply right rudder to counter the left-turning tendency of the upper rotor disk.

 

In a conventional helicopter, applying pressure to a rudder pedal increases (or decreases) the pitch on the blades of the tail rotor, increasing (or decreasing) the lift they generate. The mounting of the tail rotor is such that lift from the blades directly translates to turning moment. So, pressing a pedal directly affects turning moment (aka torque) on the chopper.

 

 

 

Unlike conventional helicopters, however, there is no tail rotor on the Ka-50. While removing moving parts from a design is usually a good thing, the function those parts served must be replaced by other parts, or else thrown out of the design. Yaw control is most certainly not an optional "feature" of a combat helicopter and the solution implemented in the Ka-50, while complicated, is quite elegant.

 

In the Ka-50, yaw torque is controlled by varying the pitch of the blades of the two rotor disks. To yaw to the right, the pitch of the blades in the upper rotor disk is decreased which decreases lift which decreases drag which decreases the counter-clockwise (i.e., left-turning) torque it produces.

 

 

 

If nothing else was done, the helicopter would yaw to the right due to the excess torque generated by the lower rotor disk, but it would also start sinking due to the loss in total lift. To counteract this, the pitch of the lower rotor blades is increased at the same time, increasing lift which increases drag which increases the clockwise (i.e., right-turning) torque it produces. The total change in lift is zero while the total change in torque is to the right.

 

 

The process happens in the opposite for yawing to the left. This is all accomplished automatically by the helicopter control systems when the pilot stomps on a rudder pedal. This technique was apparently pioneered by Mr. Kamov, by the way.

 

Unfortunately, that's not the end of the story.

 

Due to a number of factors, including the fact that rotor blades are not infinitely stiff and thus flex a little, rotor blades slope upwards away from the rotor mast when the produce lift. This causes the rotor disk, which is essentially flat while making no lift, to form a cone shape, or rotor cone.

 

 

 

In forward flight these cones are asymmetrical, even in single rotor disk helicopters, because of a phenomenon called "dissymmetry of lift". Dissymmetry of lift is caused by the fact that, on a helicopter that is moving, the rotor blades on the side of the disk that is moving in the same direction as the helicopter are moving faster through the air and thus generate more lift than the blades on the other side of the cone. More lift means more force pulling up on the blade, which means a steeper cone angle. Less lift means less force pulling up, which means a shallower cone angle.

 

 

 

 

 

From the outside it looks like the cone is leaning away from the side of the rotor disk that is headed in the helicopter's direction. This side of the cone is generating more lift than the other (hence the handy-dandy "dissymmetry of lift" moniker) which creates a banking torque on the helicopter. In the Ka-50 contra-rotating coaxial rotor, the upper rotor is producing more lift overall, so although the lower rotor is producing a banking moment in the opposite direction, there is net banking moment to the right.

 

So, in addition to the right rudder needed due to the differing torque production of the two rotor disks at airspeed, left cyclic is also needed to counter the dissymmetry of lift effect. Hence the left stick and right rudder we keep finding ourselves using to maintain straight and level coordinated forward flight. It's natural. And it's right.

 

Now for the Dark Side:

 

One of the cons of this design is that the cones of the two rotor disk system lean in opposite directions. This is because the two rotor disks spin in opposite directions. Although the upper rotor is a bit higher than the lower rotor, at a high enough airspeed, the two cones can cross, which, if you remember from Ghostbusters, is a bad thing:

 

Dr. Egon Spengler: There's something very important I forgot to tell you.

Dr. Peter Venkman: What?

Dr. Egon Spengler: Don't cross the streams. Or rotor cones.

Dr. Peter Venkman: Why?

Dr. Egon Spengler: It would be bad.

Dr. Peter Venkman: I'm a little fuzzy on the whole "good/bad" thing here. What do you mean, "bad"?

Dr. Egon Spengler: Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light. Or your rotor blades clashing at high speed and breaking off your helicopter, sending you spiraling to the ground!

Dr. Ray Stantz: Total protonic reversal! Or rotor clashing!

Dr. Peter Venkman: That's bad. Okay. All right, important safety tip. Thanks, Egon.

 

To make things worse, since the lower rotor disk is working in the downwash of the upper rotor disk, its coning angle is more severe than the angle of the upper disk's. In forward flight the top rotor cones a little to the right, and the lower rotor cones severely to the left, bringing the tips of the lower rotor closer and closer to the blades of the upper disk.

 

 

 

This is not an unanticipated aspect of the design. Mr. Kamov didn't have a maximum indicated airspeed warning system put in his attack helicopter to keep pilots from getting speeding tickets - he put it there to prevent the pilot from reaching airspeeds that cause intersecting cones. The system doesn't, however, take into account rapid increase of the collective at high airspeed (increased pitch = increased lift = increased coning = see Egon's saftey warning above). Stomping on the right rudder makes the upper rotor disk cone less, but the lower disk cones way more, further reducing the distance between the two cones. Throw in some cyclic controls in just the right direction and voila, rotor blade salad and lots of paperwork!

 

The solution is simple: avoid high airspeeds! If you have to move fast, limit climbing to very gentle rates and avoid strong cyclic or rudder inputs in any direction. If you have a need to climb, pull back on the cyclic first while holding the collective steady. The helicopter will pitch up and start climbing without significantly increasing the rotor coning angles. As airspeed bleeds off slowly introduce collective to maintain your climb rate, but keep the total collective input low until the airspeed bleeds down. There's probably a table somewhere to help real pilots figure out what sort of climb rates they can get at high airspeeds before the rotors clash, but experimentation in DCS: Black Shark suggests you can comfortably climb/maneuver at speeds up to 225 km/hr without coning problems. At 225 km/hr and above, more and more caution is needed with the cyclic, collective, and rudder inputs!

[sildude]

Excellent post Einstein :thumbup:

 

I never understood how exactly rudders work in a coaxial helicopter until now.

[HansRoaming]

Awesome post EinsteinEP, personally have never had the rotor cone intersection problem as never really stomp on anything as have always found that never works well in any aircraft.

[Sharkster64]

I dummo what the problem is, but I have much difficulties turning left with the Ka-50.

 

It simply resists turning to the left, and when I trim I get a violent movement back to right. Even gentle turning to left often ends with alarm going off and my rotors ripping off. I also get into a sideslip.

 

Turning right is no problem, I do gentle stick movement, trim and that's it.

 

I guees this is a rudder calibration problem? I use a Saitek X52.

 

The correct way to turn is to do a co-ordinated turn. To do a co-ordinated turn, you must first understand what each input does. If you are flying forward and you apply right rudder, the nose of the aircraft will turn to the right. The aircraft will still maintain the same heading however. In essence, you will be sideslipping. If you are flying forward and this time you push the cyclic to the right, the aircraft will start turning to the right with your nose slowly turning right but not pointing in the direction of flight. In essence you will now be sideslipping in the direction of your turn. This is where you apply the co-ordinated turn, (or centering the bubble). This time, when you turn to the right first push the cyclic to the right so you start your turn. Then apply right rudder to align your nose in the direction of the turn. You can watch the bubble on your SAI to make sure it is centered by applying left or right rudder to center it. By centering the bubble, you now have the nose of the helicopter pointed in the direction of flight. Hope this helps. :)

[Rangoon]

Regarding EinsteinEP's and Sharkster64's posts:

 

Talking about use of rudder in turns while flying the black shark, this brings up a nagging question which has been brewing in my mind while learning to fly this coaxial system. I fly helicopters for a living, but have never been in one with a coaxial system (until now!). Normally in helicopters, you don't use tail rotor authority to coordinate a level turn like airplanes do. The tail rotor's main job is anti-torque, so if I turn and descend or turn and climb I'll use tail rotor inputs to coordinate as a reaction of the change in power. However in the black shark I do seem to need rudder to coordinate level turns. Anyone know why this is?

 

Maybe it's also related to speed (I'm normally cruising at 100-120 knots, which is about 190-225 kph).

[sobek]

Talking about use of rudder in turns while flying the black shark, this brings up a nagging question which has been brewing in my mind while learning to fly this coaxial system. I fly helicopters for a living, but have never been in one with a coaxial system (until now!). Normally in helicopters, you don't use tail rotor authority to coordinate a level turn like airplanes do. The tail rotor's main job is anti-torque, so if I turn and descend or turn and climb I'll use tail rotor inputs to coordinate as a reaction of the change in power. However in the black shark I do seem to need rudder to coordinate level turns. Anyone know why this is?

 

I think this is due that ED implemented a incorrect AP yaw channel routine. In the real black shark, the autopilot yaw channel tries to maintain a turn, once the angular velocity reaches a certain threshhold, the DCS black shark tries to maintain the trimmed heading, so you have to fight the AP in a turn. This will be fixed with the patch, AFAIK.

[EinsteinEP]

Rangoon,

 

I have a whopping 0.5 hours of rotor time logged, but I'm amazed that you don't need any rudder to keep the ball centered when you make a turn in a chopper. Cyclic only and the chopper maintains coordinated flight throughout? Wow. That's cool! Definitely need to use the feet when flying most fixed-wing aircraft.

 

It may be that the particular heli you're flying has very high "weathervane stability", which means it has great tendency to point into the wind. Standard fixed-wing aircraft have comparatively little weathervane stability, which means they generally point nose first into the wind, but not with a whole lot of authority behind it, hence the need for supplementary pedal action in a level turn.

 

The big fan on the tail of conventional choppers probably adds a lot of weathervane stability (think of the feathers on a dart or arrow) which is why, in general, they don't need a lot, if any, pedal to maintian a coordinated level turn.

 

I'd guess that if you make a really hard turn with just the cyclic (60 deg bank, sustained), you'll see the ball bump some out of the pocket. The "return moment" is strong enough to maintain coordinated flight at low turn rates, but higher rates will cause the nose to drag behind where it "should" be, requiring some pedal to put it there.

 

It would be an interesting experiment!

 

As for why the Black Shark DOES need rudder to coordinate a turn, the Shark has reduced weathervane stability compared to the conventional chopper - on purpose! It can turn on a dime at any time (try stomping down on a pedal while cruising at 150 kts. it's cool!), a handy attribute for an attack chopper. Classic stability vs. maneuverability design tradeoff.

[Rangoon]

Thanks for the replies sobek and EinsteinEP.

 

I should clarify - to maintain a true level turn, one does need to add power since some of the vertical lift becomes horizontal. To maintain constant altitude power has to be added (because collective pitch must be raised), and therefore left pedal (with a counterclockwise main rotor). But aside from that, tail rotor is not used for coordination. There is significant weathervaning, yes, due to the shape of the helicopter (tailboom, etc.) and the vertical/horizontal stabilizers - esp. in cruise. But in a turn, you tilt the rotor disc and the fuselage basically swings out 'behind' the disc (like a pendulum) and that is what turns the helicopter. No rudder/tail rotor pitch change required if power doesn't change.

 

I've only flown three types of helicopters, but this is the case in my experience.

 

What you're saying makes sense if use of rudder in DCS is partly due to the autopilot's heading hold fighting the pilot's inputs (I know that's a topic floating around here a lot). Whether it's right/wrong/fixable it at least makes sense what's happening. And if it's wrong - glad to hear it'll likely be fixed.

 

And yes - I'm quite sure the maneuverability of the Black Shark is top notch and designed to do much more than I'm used to. Sure I can jam the pedal in cruise flight and find myself with some high crab angles, but I wouldn't feel confident in things holding together for very long at high airspeeds! :)

 

That said, even the little civilian helicopters are extremetly maneuverable compared to fixed-wing aircraft and can fly sideways at 30-40 knots all day long.

 

I think I'm still on topic - curious to see how the flight model evolves with the patch. They've brought the Black Shark to life with such fidelity that my inclination (for the first time ever in a PC helicopter sim) is to defer to the simulation, not my own experience. So any oddities I assume are true to the Ka-50 and mark down as differences before I dismiss them as inaccuracies.

Edited May 31, 2009 by Rangoon

[Jack McCoy]

I don't think it makes sense to NOT use the rudder in a turn. Depending on the ratio of helicopter mass to surface of the tail fin (airframe design), you'll more or less apply rudder. A heavy helicopter like the Ka-50 or Apache will need more rudder and/or higher speed to stay aligned with the flight path (i.e. velocity vector).

 

"Stomp/squeeze the ball" is the old adage: if ball is to the left, you need left rudder and vice-versa. This is more important in a fix-wing aircraft as the lift is generated by the wings. In a helo, flying uncoordinated is an intrinsic asset which can be put to good use. It'll only cost you more fuel as you increase drag, but then again, you don't fly cross-country with 80 degrees of "crab".

 

If you don't have to use rudder to keep a turn coordinated, it's because the helicopter is designed (or it just so happens) to weathervane just the right amount when at a specific speed, probably cruise speed.

 

In a Cessna 172 and the like, very little rudder is use to keep the turn coordinated as the engine/prop "pulls you into the turn" (and also increase the airflow over the fin/rudder). You'd use rudder to minimize adverse-yaw caused by asymetrical drag produced by the upper and lower aileron in a turn.

 

In a glider, the rudder is much more important throughout the turn.

 

As far as being delicate with the Ka-50, personally I'm more careful above 200kph. Below that, you can (gently) rough it up. :)

 

EDIT: For my fellow engineers, I mention mass to keep it simple, but it should be "First moment of inertia", which is indirectly related to mass as you know.

Edited June 1, 2009 by Jack McCoy
Precision

[Rangoon]

I'm not sure beyond the light helicopters I've flown, but if I bank the aircraft and just freeze my feet in place, trim is maintained throughout the turn as long as I'm in forward flight above (now I'm guessing - but will check during my next flight) about 40 knots or maybe less. As soon as collective (and therefore power) is changed trim is lost without input from the feet. It is common in right turns to use left pedal, in fact, because if I hold altitude I will need more power and therefore left pedal. Right turn,- left pedal to hold trim.

 

I guess whether it makes sense or not, that's been my experience. And I resisted that in my early days because I had been flying sims since the early 80s - it was habitual to engage my feet during turns (a la fixed-wing flight models and the little real-world fixed-wing flying I had done at that time) but my instructor kept scolding me. Eventually I stopped moving my feet and my turns improved dramatically. Unless I was changing power (but generally it's still less pedal movement than a comparable turn in an airplane unless the power change is drastic).

 

I'm glad DCS prompts discussions like this. And it makes me want to fly a lot more helicopter types.

 

Edit: grammar

Edited June 1, 2009 by Rangoon

[Jack McCoy]

Turning / coordinated flight

 

As soon as collective (and therefore power) is changed trim is lost without input from the feet. It is common in right turns to use left pedal, in fact, because if I hold altitude I will need more power and therefore left pedal. Right turn,- left pedal to hold trim.

Very interesting. :book:

For other people's sake: this does not reflect the Ka-50 coaxial rotors system.

 

I guess whether it makes sense or not, that's been my experience. And I resisted that in my early days because I had been flying sims since the early 80s - it was habitual to engage my feet during turns (a la fixed-wing flight models and the little real-world fixed-wing flying I had done at that time) but my instructor kept scolding me. Eventually I stopped moving my feet and my turns improved dramatically. Unless I was changing power (but generally it's still less pedal movement than a comparable turn in an airplane unless the power change is drastic).

I trust your experience.

 

I had a "too much rudder" issue when switching from glider to powered fixed-wing. :helpsmilie:

 

I've glad DCS prompts discussions like this. And it makes me want to fly a lot more helicopter types.

Yes, good thread.

 

--

Would you agree that regardless of the method to achieve it (which may be different depending on the aircraft), the GOAL is to achieve coordinated* flight in turns?

Of course, intentional side-slips to drop altitude or hold heading on final (in fixed-wing) are OK.

 

--

Notes to new flyers:

*Coordinated flight is when the longitudinal (length-wize) axis is aligned with the oncoming air (due to wind and/or movement). This usually, if not always(?), offers the the least drag possible from the fuselage.

 

To coordinate a turn, learn to "read" the movement of the ground; do not keep your eye on the "ball". Use to ball to confirm you trimmed nicely. In a Ka-50, below 50kph (and above 4m radar-alt), you can use the horizontal velocity vector on the HUD: when the line is straight up, you're coordinated.

 

For "turn pratice" flights, switch OFF the Heading Hold autopilot channel. I prefer this method to the Flight Director. Even when doing mild aerobatics, I use only Bank and Pitch Hold (Hdg and Alt OFF).

 

--

Side-thread:

I haven't flown any helos (yet :D), but I figure that a level (constant altitude) turn in a forward-moving helicopter requires zero or even forward cyclic to maintain airspeed. This is very different from airplanes! Is this your experience?

[Rangoon]

For other people's sake: this does not reflect the Ka-50 coaxial rotors system.

 

Thanks for reiterating this. It's a very important distinction.

 

 

--

Would you agree that regardless of the method to achieve it (which may be different depending on the aircraft), the GOAL is to achieve coordinated* flight in turns?

Of course, intentional side-slips to drop altitude or hold heading on final (in fixed-wing) are OK.

 

--

 

In my profession, it's either coordinated or intentionally uncoordinated (as you indicated). You're absolutely right that coordinated turns are more efficient and professional, but they also just feel better to the client on board (some non-aviators do get nervous on board and that's only aggravated by sloppy flying). But frequently with photography/videography uncoordinated flight is essential to keep the frame regardless of the ground track.

 

Oh, and in virtual combat! :) (thank you DCS) :thumbup:

 

 

--

Side-thread:

I haven't flown any helos (yet :D), but I figure that a level (constant altitude) turn in a forward-moving helicopter requires zero or even forward cyclic to maintain airspeed. This is very different from airplanes! Is this your experience?

 

I'll have to pay more attention next flight for a better reply, but you're right that it's either 100% lateral or slightly forward. My instructor always insisted lateral, but that's because I kept pulling back on the cyclic in a subconscious effort to hold altitude (I think) rather than using collective. So it seems the 'right' answer must be simply lateral cyclic, no fore/aft pitch. And it probably is right. Now I just always feel a slight forward pressure during turns. It's probably me feeling the resistance of my early misconceptions, fighting the urge to pull back. Pitch = airspeed. Collective/Power = altitude. It really does work.

Edited June 1, 2009 by Rangoon

[Topgun505]

AFAIK you generally do not encounter rotor strike unless you are going blazing fast and then make a turn and crank on the rudder hard at the same time. Go easier on the rudder and slow down a bit and that should reduce the likelihood of a rotor clash.

[Rangoon]

Today I talked with another pilot who helped me understand the real difference between traditional helicopters and fixed-wing aircraft when it comes to coordinating turns. As stated above, it's not unusual for me to bank the helicopter right but input left pedal to counter the greater torque as I pull collective pitch/increase power. Or, during a descending left turn, I'll input right pedal in response to decreased pitch/power.

 

As he explained it, fixed-wing aircraft use rudder to coordinate a turn due to adverse yaw from the aileron - more lift/drag on one side causes that wing to fall behind the other wing, which has its left/drag reduced. The rudder makes up for adverse yaw.

 

Helicopters don't suffer from adverse yaw (although we suffer from plenty of other aerodynamic phenomena instead).

 

Can the fixed-wing pilots here confirm I've got that story straight?