Noob question to you, black magic to me...

[Belphe]

Hi All,

 

I have noticed that when I choose a direction and start to fly straight forward, my bottom rotor rolls to the left lifting the right side of the platter... The top rotor remains leveled. Is this because I MIGHT :music_whistling: have forgotten to reset/switch off the Auto Heading Autopilot and the onboard nav computer tries to face my Helo towards the old heading???

 

PS. Does your Black Shark hover about 2-3 inches above the ground when parked??? :huh:

 

Thanks!

[Namenlos Ein]

You talk about that? It is called 'realism'.

 

 

Hovering on parking is a minor graphic bug.

[Belphe]

Thank you for explaining it to me thoroughly...

[skypirate]

As well from the scheme you can see why you have to avoid high G maneuvers at high speed...

 

The first time I did that the chopper blades collided and I end up in the bushes.... burning... :joystick:

[Namenlos Ein]

And be careful with the right (or starboard?) pedal on high speeds. By the way, AI-controlled Ka-50 don't have such problem.

Edited January 15, 2009 by Namenlos Ein

[Belphe]

Guys, I know all that! :) I know pressing the right pedal could cause the blades to hit each other - it's simple! To create a yaw force to the right the bottom rotor has to create more lift (the bottom rotor blades' angle of attack increases). If the speeds are great it is very likely the blades will create something that, at least in polish is called an umbrella effect - they will bend up... and collide with the upper rotor! Such thing will not happen when yawing to the left as there is nothing the top rotor would hit...

 

My question however is why, when looking from behind, is the RIGHT side of the LOWER rotor blades higher/closer to hitting the upper rotor than the left side??? As I wrote in my previous post - all this is happening while travelling forward.

 

Thanks

Edited January 14, 2009 by Belphe

[Nate--IRL--]

The left side is travelling faster the the right side in the air so it lifts up more, simple. One side goes in the direction of flight (faster air) and the other moves towards the back (slower air)

 

Nate

 

Edit:- this is of course is reversed for the other rotor

[sobek]

My question however is why, when looking from behind, is the RIGHT side of the LOWER rotor blades higher/closer to hitting the upper rotor than the left side??? As I wrote in my previous post - all this is happening while travelling forward.

 

Thanks

 

It's called flapping, a means to counter dissymmetry of lift by using a fully articulated rotor head. [1] and [2] might be helpful reading. Dr. Google was also of the helpful kind [3] ;)

 

[1] http://en.wikipedia.org/wiki/Helicopter_rotor

[2] http://en.wikipedia.org/wiki/Dissymmetry_of_lift

[3] http://www.google.at/search?hl=dei&q=blade+flapping&btnG=Suche&meta=

Edited January 14, 2009 by sobek

[Namenlos Ein]

[Belphe]

Obviously!!! :doh: Thanks a lot for making it clear to me! :thumbup: I totally forgot about the fact that the blades will travel at different speeds when on the left and when on the right side of the helicopter when flying forward...:music_whistling: I guess you learn every day! :smilewink: Thank God we have this sim to illustrate phenomenae such as this one! :pilotfly:

 

However... :huh:

... why isn't the upper rotor plate tilted in the opposite side (right side higher, looking from the rear)? :cry: I mean, the whole thing should affect the top rotor as well, only mirrored, right??? :helpsmilie:

 

 

I would have thought it should look like in drawing B but it is more like the A drawing...

Edited January 14, 2009 by Belphe

[Namenlos Ein]

However... :huh:

... why isn't the upper rotor plate tilted in the opposite side (right side higher, looking from the rear)? :cry: I mean, the whole thing should affect the top rotor as well, only mirrored, right??? :helpsmilie:

 

[ATTACH]23711[/ATTACH]

 

I would have thought it should look like in drawing B but it is more like the A drawing...

Because lower rotor operates in air flow from upper rotor it tilted greater then upper one. Take a close look on the upper rotor and you find it tilted to the right.

Edited January 15, 2009 by Namenlos Ein

[EinsteinEP]

I've been noticing this phenomenon, too, but the thing that gets my curiosity is that it doesn't happen to my Ka-50 wingman traveling at the same speed as me.

 

Is full rotor dynamics not modeled for the AI, or do they know some nifty control technique where they can maintain straight and level flight with symmetric upper and lower coaxial rotor coning?

[sobek]

Is full rotor dynamics not modeled for the AI, or do they know some nifty control technique where they can maintain straight and level flight with symmetric upper and lower coaxial rotor coning?

 

It's not modelled, because of resource conservation and restrictions of AI flight (they would crash way more often)

[Evilducky]

It's not modelled, because of resource conservation and restrictions of AI flight (they would crash way more often)

 

Maybe that would stop them from crashing into you :lol:

[sobek]

Maybe that would stop them from crashing into you :lol:

 

Only once in nearly 3 months of flying have i encountered a crash with my wingie and that was due to me doing some very abrupt and uncoordinated maneuvering. I'm starting to think that the people who complain about their wingies are the same that complain about catastrophic in flight blade failure :huh:

[EinsteinEP]

Einstein's Opinion on Contra-Rotating Coaxial Rotor yaw and bank torque characteristi

 

Sorry in advance for what must seem like a dissertation, but here's what I've learned so far. Corrections and/or feedback would be most appreciated:

[hint: it may help to have a model or drawing of a helicopter in front of you while reading this]

 

In a hover, the two rotor disks of the Ka50 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 is per design. Whether this is accomplished by differing pitch angles of the blades of each rotor disk, or by using differing airfoils or differing total rotor sizes, I don't know.

 

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 a uncoordinated flight which is messy and unprofessional so the Good Pilot needs to get the pointed nose where he's going.

 

Which starts us on the path to our shared experiences.

 

In the Ka50, 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. If nothing else was done, the helicopter would yaw 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, simultaneously, the pitch of the lower rotor pitch is increased, increasing lift which increases drag which increases the clockwise (i.e., right-turning) torque. Total lift change is zero while total change in torque is to the right. The opposite process happens for yawing to the left. This is all accomplished automatically by the helicopter control systems when you stomp on a rudder pedal. This technique was apparently pioneered by Mr. Kamov, by the way.

 

So, in order to maintain steady-state coordinated level flight in a Kamov contra-rotating coaxial rotor helicopter, like the Ka-50, you need to apply right rudder.

 

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 pretty much flat while producing no lift) to form a cone shape, or rotor cone. In forward flight these cones are assymmetrical, even in single rotor disk helicopters, becuase of the "dissymmetry of lift". Basically, "dissymmetry of lift" is caused by the fact that 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 making a steeper cone angle while the blades on the other side of the disk are moving slower through the air making less lift and making a shallower cone angle. From the outside it looks like the cone is leaning away from the side of the rotor 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, due to the higher lift production of the upper rotor (as mentioned eariler), this results in a 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 maintain steady-state coordinated level flight. Hence the left stick right rudder we keep finding ourselves using. It's natural. And it's right.

 

Now for the Dark Side:

 

One of the cons of this design that we've all been experiencing is that the cones of the two rotor disks lean in opposite directions and, 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!

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! If you look closely enough in forward flight you'll see the top rotor does cone a little towards the right, just not as severe as the lower disk's cone to the left.

 

This is not an unanticipated aspect of the design. Mr. Kamov didn't have a MAX IAS warning system put in the chopper to keep pilots from getting speeding tickets - it's there to prevent intersecting cones. The system doesn't, however, seem to take into account rapid climbs at high speed using the collective (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, increasing the likely hood of a strike. Throw in some cyclic controls in just the right direction and voila, rotor blade salad!

 

My tip: at high airspeeds, limit climbing to very gentle rates and avoid strong cyclic or rudder inputs in any direction. If you will need to climb, pull back on the cyclic first while holding the collective steady. The helicopter will pitch up and start climbing without increasing the rotor coning angles. As your airpseed bleeds off slowly introduce collective to increase your climb rate. There's probably a table somewhere to help pilots figure out what sort of climb rates they can get at high airspeeds before the rotors clash, but I find I can comfortably climb/maneuver at 200-225 km/hr and save the higher speeds for straight and level cruises.

Edited July 10, 2009 by EinsteinEP

[Acedy]

Very nice post! One thing I'd like to add is that the bank tendency is also caused by the different leverage of the upper and lower rotors, not only by differences in dissymetry of lift. Even if the dissymmetries of lift compensated each other the chopper would still bank because the banking moment the upper rotor exerts on the CG is greater than the countering moment of the lower rotor, as the distance (i.e. lever arm) of the upper rotor to the CG is greater.

[EinsteinEP]

Even if the dissymmetries of lift compensated each other the chopper would still bank because the banking moment the upper rotor exerts on the CG is greater than the countering moment of the lower rotor, as the distance (i.e. lever arm) of the upper rotor to the CG is greater.

Thanks, Acedy. I remember reading this factoid, but didn't understand the source of the banking moment. If the banking moment doesn't come from dissymmetry of lift, where does it come from?

[Acedy]

Oh, sorry I was a bit unclear, actually dissymmetry of lift is the cause of the banking moment, but there are two factors that you have to distinguish:

 

a)

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, due to the higher lift production of the upper rotor (as mentioned eariler), this results in a net banking moment to the right.

 

So, just for the sake of the argument, let's assume for a moment that the lift that one side of a rotor produces is twice the lift the other side creates, and the overall lift the upper rotor produces is also twice the lift the lower rotor creates. Furthermore both rotors should be attached to the CG, so there is no mast acting as an intermediate lever, only the direct leverage effect that each side of a rotor has on the CG. Say the left side of the upper rotor creates lift of 20, then the right creates 10, total is 30. Hence the lower rotor produces 15, the right side of the lower rotor creates 10 and the left 5. Now total lift of the left side is 25 and 20 of the right. This will cause the chopper to bank right, just like a single rotor machine.

 

b) Lets assume now that both rotors produce the same amount of lift, but the upper rotor is attached to the mast at twice the distance of the lower rotor. Say the lift each rotor produces is 30 and the distance of the lower is 1, then the distance of the upper is 2. Now the absolute dissymmetries compensate each other, as the left side of the upper rotor creates lift of 20 and the right of 10, and vice versa on the lower. In situation a) this would cause a total banking moment of 0. However, since the lift forces on the sides of each rotor are not equal and the rotors are not attached to the CG, they exert a lever effect on the CG according to Archimedes' law (say the blades would be 100% rigid etc). The banking moment of the lower rotor would be 1*10 = 10 to the left, the banking moment of the upper rotor would be 2*10 = 20 to the right, so we have a total banking moment of 10 to the right.

 

This is simplified of course, but I hope it shows the point. :)

Edited March 5, 2009 by Acedy

[Belphe]

Amazing! Amazing times three!!!

 

1) Phisics + "modern" technology never stop to amaze me!

2) Your kowledge and the way all of you guys "translate it to english"... very much appreciated!

3) This software - it is like the most advanced simulation I have ever heard of! A masterpiece!

 

Standing ovation!

[EinsteinEP]

b) ... The banking moment of the lower rotor would be 1*10 = 10 to the left' date=' the banking moment of the upper rotor would be 2*10 = 20 to the right, so we have a total banking moment of 10 to the right.[/quote']

Acedy,

 

Thanks for the clear explanation. I know now what you mean. I have to disagree, however.

 

First off, a nit, Archimedes' Law is about buoyancy: "any object floating upon or submerged in a fluid is buoyed upward by a force equal to the weight of the displaced fluid." I think you mean leverage, which Archimedes is also known for. Leverage is measured in units of force-distance (like foot-pounds) and is calculated by a very simple forumla: M = F * d, where M is the measurement of the leverage, called "moment", F is the magnitude of force being applied and d is the perpendicular distance from the axis to the line of force.

 

But that's not the real sticky point.

 

Moments are summed together by simple addition, not as a function of distance from the reference point. In the event of your case b), the moments at each disk are equal and opposite, so the total moment at the center of gravity (CG) of the chopper is zero, no matter how high up the rotor disks are.

 

 

Of course, this is only for the fictional case b). In the real Ka-50 in forward flight the two moments are NOT equal in forward flight so there is a definite banking moment, as described in an earlier post.

 

[note: In reality, lift doesn't act like point forces like I've shown here. Lift is a result of the application of pressure to a surface so the force is spread out over the entire rotor blade. For this example, however, we can assume that the lift acts at a single point along the blade without ill effect. There are some cases where this assumption will bite you, however, so be cautious!]

[sobek]

But that's not the real sticky point.

 

Moments are summed together by simple addition, not as a function of distance from the reference point. In the event of your case b), the moments at each disk are equal and opposite, so the total moment at the center of gravity (CG) of the chopper is zero, no matter how high up the rotor disks are.

 

The force on the discs may be equal (ideally), but their distance from the center of gravity is not, thus those to equal forces result in different moments, thus there is a total banking moment applied to the CG. You took wrong distances for calculating the moments, you must not only take into account the perpendicular projected distance from the center of gravity.

Edited March 5, 2009 by sobek

[EinsteinEP]

sobek,

 

I 100% agree with you. In real flight in the real helicopter, there is a significant banking moment due to dissymmetry of lift, thus my lengthy post earlier.

 

The post above was responding to Acedy's fictional case b) which he posted earlier, which assumed, for discussion, that the two moments were identical. He was claiming that even if the two moments were identical (aka case b), there would still be a banking moment on the helicopter, which I don't believe is true.

 

Of course' date=' this is only for the fictional case b). In the real Ka-50 in forward flight the two moments are NOT equal in forward flight so there is a definite banking moment, as described in an earlier post.[/quote']

Edited March 5, 2009 by EinsteinEP

[sobek]

sobek,

 

I 100% agree with you. I was responding to Acedy's fictional case b) which he posted earlier, which assumed the two moments were identical.

 

Uhm duh :doh:, sorry i kind of misread your post :)

[mvsgas]

"Dissymmetry of lift"

"Coriolis effect"

"Settling with power"

"Magnus effect"

"Bernoulli's principle"

"Translating Tendency"

"gyroscopic precession"

"Ground Resonance"

"ground effect"

"effective translational lift"

"transverse Flow"

Etc

:book::doh::joystick::fear::dunno::shocking:

Thank God I never pursue my aerospace engineering degree and opted for ignorant, poorly educated mechanic. So confuse and intrigue :D