Rotor collision

[Inrideo]

Takeoff went well, but apparently I pushed things to far trying a hard turn.

The lower rotor seemed to be off the center plane, and for a moment the chopper flew fine, but then both rotors collided and *FUN* (as they say in Dwarf Fortress) was had.

[BitMaster]

ehhh yes

 

that is what happens if you push her far too far !

 

lower collective when you do wild manoeuvres, that gives more response from there bird and the blades will collide a bit later.

 

Don't manoeuvres hard anyway above 250, for not at 300+

 

Bit

[msalama]

Yes, the Ka-50 wants to get her moving parts in a bunch if you let her. A feature, not a bug ;) Just ease up on the collective when throwing her around and she'll behave like a lady :)

[King_Hrothgar]

If you watch the rotors in external view as you increase speed, you will notice the bottom rotor lifts up on the right and the top rotor lifts up on the left. At about 300km/h, the lower rotor gets very close to the top rotor on the right side. As others have noted, when flying at high speed, you must be careful of that gap and not do anything that might cause the bottom rotor to flex up any more on the right side.

 

If you want to do hard maneuvering, keep it under 200km/h and you can toss her around as hard as you like so long as you aren't inverted. If you do want to fly inverted, additional caution must be used. And yes, you can loop, split s and immelman it.

[Eagle0110]

Also, since the yaw control is achieved with different blade angle on the two rotor(which generate different torque), a combinations of left bank and right rudder input when flying with some speed will create forces that push the rotor tip toward each other on the right side of rotors and increase the chance they hit each other. And the faster you fly the closer this control in put will bring the rotor blades.

Try not to apply left bank and right rudder at the same time when flying fast :)

Edited January 16, 2015 by Eagle0110
Additional information.

[Flagrum]

To add a bit theory to all the good advice here:

"Asymetry of lift". That is why.

 

A rotor provides different ammounts of lift when in forward flight: the side where the blades move forward generate more lift as the airflow is larger because of the addition of the speed of the airframe and the forward movement of the blade. On the other side, where the blade moves backward, the lift is less - because this time the speed of the blade is substracted from the overall forward movement.

 

Therefore, the side with the forward moving blade - where more lift is generated - bends more upwards than the other side.

 

Now, in our coax rotor helo the lower rotor disc has only so much space to bend upwards, before it intersects with the upper rotor disc (where the other side bends upwards and therefore does not "get out of the way" of the lower rotor disc).

[Eagle0110]

To add a bit theory to all the good advice here:

"Asymetry of lift". That is why.

 

A rotor provides different ammounts of lift when in forward flight: the side where the blades move forward generate more lift as the airflow is larger because of the addition of the speed of the airframe and the forward movement of the blade. On the other side, where the blade moves backward, the lift is less - because this time the speed of the blade is substracted from the overall forward movement.

 

Therefore, the side with the forward moving blade - where more lift is generated - bends more upwards than the other side.

 

Now, in our coax rotor helo the lower rotor disc has only so much space to bend upwards, before it intersects with the upper rotor disc (where the other side bends upwards and therefore does not "get out of the way" of the lower rotor disc).

 

Actually, it doesn't seem that simple, there is an effect called Gyroscopic Precession.

[Flagrum]

Actually, it doesn't seem that simple, there is an effect called Gyroscopic Precession.

... yes? But what has that to do with the problem of intersecting rotor discs?

[StrongHarm]

Also your AP might fight hard maneuvers enough to cause collision. I like to use the AP Emerg. Off before really stressing the bird.

 

I have my WH HOTAS CMS set to

PUSH = AP EMERG. OFF

FWD = HEADING AP

LEFT = PITCH AP

RIGHT = ROLL AP

BACK = ALT AP

 

This binding is convenient for heading AP as well. I prefer switching heading AP on and off when changing heading instead of messing with all trim aspects just to change flight direction.

[Bushmanni]

... yes? But what has that to do with the problem of intersecting rotor discs?

 

Asymmetry of lift causes the chopper to pitch up when speed increases which you counter by pushing cyclic forward.

 

Rotor collision is caused by lack of downwards airflow at the rear of the lower rotor. Center of the rotor doesn't push air down as there's no blades and the inner portion of the blades move slowly. This slow speed flow area get's dragged aft when chopper increases it's speed increasing the lift at the rear sector of the rotor disk much more than elsewhere. This extra lift at the rear causes the right side of the lower rotordisk to raise when speed increases (remember the precession effect).

[Suchacz]

You should read the SimHQ articles in my sig :thumbup:

[Flagrum]

Asymmetry of lift causes the chopper to pitch up when speed increases which you counter by pushing cyclic forward.

 

Rotor collision is caused by lack of downwards airflow at the rear of the lower rotor. Center of the rotor doesn't push air down as there's no blades and the inner portion of the blades move slowly. This slow speed flow area get's dragged aft when chopper increases it's speed increasing the lift at the rear sector of the rotor disk much more than elsewhere. This extra lift at the rear causes the right side of the lower rotordisk to raise when speed increases (remember the precession effect).

Why would the no-lift area get dragged aft?

 

And why would then the aft portion of the rotor disc - with the no-lift area - produce now more lift?

 

The no-lift area gets larger, but predominantly on the side of the retreating blade - that is how I understand it. But I still can not see how/if that would add to the intersection risk.

[Isegrim]

Stay below 250kph and you'll be fine.

 

No..... you can scrap your rotor even at 50 km/h when done horribly wrong.

Edited January 17, 2015 by Isegrim

[Bushmanni]

Why would the no-lift area get dragged aft?

 

And why would then the aft portion of the rotor disc - with the no-lift area - produce now more lift?

 

The no-lift area gets larger, but predominantly on the side of the retreating blade - that is how I understand it. But I still can not see how/if that would add to the intersection risk.

 

Basic thing is that force applied to the rotor causes movement about 90 degrees after the point where the force was applied. For example pulling cyclic back causes blade pitch to increase on the right side of the lower rotor and left side of the upper rotor ie. 90 degrees before front sector where the upwards tilt is desired. You can check this yourself in DCS BS. So whatever force is causing the lower rotor to tilt up the right side at high speed is applied at the aft portion of the disk.

 

Upper rotor blows down on the lower rotor which needs to increase it's blade pitch in order to compensate for it. This flow is skewed by the apparent wind resulting from forward speed just like a column of smoke is skewed by a wind. Highest lift and highest downwards flow is at the tip of the blades as they move fastest. The closer the slow center flow gets to the outer rear edge of the lower rotor disk, the more lift is generated there because of less downwards flow from the upper rotor. This lift causes lower rotor disk to tilt up about 90 degrees after the point where the lifting force is applied (because of precession). As the lower rotor spins counter-clockwise this tilt up happens at the right side of the lower rotor disk.

[Flagrum]

Basic thing is that force applied to the rotor causes movement about 90 degrees after the point where the force was applied. For example pulling cyclic back causes blade pitch to increase on the right side of the lower rotor and left side of the upper rotor ie. 90 degrees before front sector where the upwards tilt is desired. You can check this yourself in DCS BS. So whatever force is causing the lower rotor to tilt up the right side at high speed is applied at the aft portion of the disk.

 

Upper rotor blows down on the lower rotor which needs to increase it's blade pitch in order to compensate for it. This flow is skewed by the apparent wind resulting from forward speed just like a column of smoke is skewed by a wind. Highest lift and highest downwards flow is at the tip of the blades as they move fastest. The closer the slow center flow gets to the outer rear edge of the lower rotor disk, the more lift is generated there because of less downwards flow from the upper rotor. This lift causes lower rotor disk to tilt up about 90 degrees after the point where the lifting force is applied (because of precession). As the lower rotor spins counter-clockwise this tilt up happens at the right side of the lower rotor disk.

I am not yet convince that this is the determining factor. But I admit, I am far from being an expert here..

 

What I don't get:

a) the upper rotor, rotating CW, tilts up on the left side, to a similar degree as the lower rotor does on the right side (F2 view from behind). Theat area of reduced downwash can not be the cause here, as there is none for the upper rotor.

b) the area of reduced downwash extends to the side(!) of the returning blade - i.e. right hand side for the upper rotor (CW) and left hand side for the CCW spinning lower rotor. So if this area of lesser downwash of the upper rotor shifts because of the helo's movement, it will not affect the lower rotor at the 6 o'clock position, but rather somewhere around the 4-5 o'clock position

c) the upward tilt of the blades happen because of the lift they create - at the position they create it. The torque effect then, due to precession, affects the rotor with a 90° delay. But the blades are not displaced by some toque effect but only by the airflow. So it is not plausible for me that the up-tilting should occur elsewhere but where they actually generate the respective lift. Why would blades move upward when there is no air supporting them? Yes, due to torque induce banking the may move out of the horizontal plane - but that would happen because the whole helo banks.

Edited January 17, 2015 by Flagrum

[Bushmanni]

a) Lower rotor affects the upper rotor presumably by sucking air downwards. How exactly the lower rotor affects the upper rotor is beyond me but NASA paper does say that it happens but lesser degree than the upper affecting the lower.

 

b) Retreating side of the rotor disk has the same strength of down wash as the advancing side or the chopper would start to pitch up or down. This balancing act is done by the chopper pilot by pushing the cyclic forward (blade pitch increases at the retreating side and reduces at the advancing side) as speed increases.

 

c) Precession occurs because the blade has tangential speed. When you apply a force to a moving abject, it doesn't start to move directly to the direction of the force but its trajectory shifts some degree to the direction of the force. In a spinning motion this causes the highest deviation from the neutral plane to occur somewhere after where the force was applied. If the rotor would be a rigid body it would be exactly 90 degrees. In a non rigid body like helicopter rotor the exact amount of degrees when the highest point occurs depends on multiple factors like mass of the blade, spin speed of the rotor, how the blade pitch changes with its rotation angle, etc. Basically "centrifugal" force tries to center the blade in the neutral plane like gravity in a pendulum while aerodynamic force resulting from blade pitch tries to move the blade away from the neutral plane.

[Inrideo]

I like how my amusement at watching my rotors suddenly shoot past my cockpit windows in pieces has turned into an educational discussion :)

Awesome forum.

[Inrideo]

Just mentioned it to dad, and he had an interesting story about the Voyageur SAR choppers at CFB Summerside. (He worked there until it closed, and I spent a lot of time on the base there as a kid)

 

Once, when they tried to start the Voyageur up in high winds for a mission, the rotors were being bent too much and collided during the startup procedures. As a result, when preparing for missions during days with high winds they did the startup procedures inside the maintenance hangars, and rolled them out after the rotors were up to speed.

[Flagrum]

a) Lower rotor affects the upper rotor presumably by sucking air downwards. How exactly the lower rotor affects the upper rotor is beyond me but NASA paper does say that it happens but lesser degree than the upper affecting the lower.

 

b) Retreating side of the rotor disk has the same strength of down wash as the advancing side or the chopper would start to pitch up or down. This balancing act is done by the chopper pilot by pushing the cyclic forward (blade pitch increases at the retreating side and reduces at the advancing side) as speed increases.

 

c) Precession occurs because the blade has tangential speed. When you apply a force to a moving abject, it doesn't start to move directly to the direction of the force but its trajectory shifts some degree to the direction of the force. In a spinning motion this causes the highest deviation from the neutral plane to occur somewhere after where the force was applied. If the rotor would be a rigid body it would be exactly 90 degrees. In a non rigid body like helicopter rotor the exact amount of degrees when the highest point occurs depends on multiple factors like mass of the blade, spin speed of the rotor, how the blade pitch changes with its rotation angle, etc. Basically "centrifugal" force tries to center the blade in the neutral plane like gravity in a pendulum while aerodynamic force resulting from blade pitch tries to move the blade away from the neutral plane.

Helicopters are held in the air by witchcraft - this discussion proves once more that this statement is not too far away from the truth. :D I guess, I have to ponder about all this a bit... :o)

 

But it is alwayas awesome that, no matter how much you (think) you know, there is always so much more to learn! I love it.

[BitMaster]

I have to admit Bushmanni is right with his dis lift description. All forces work in conjunction and some, as mentioned, have a 90° offset with their actuating forces, a nature of the beast, known since we threw stones with a sling to hunt the Mammoth.

 

 

Base line is, even if it would not collide, soon after 300kmh you would loose ALL lift on either side that moves backward and your helicopter falls downward like a brick, period ! :)

If that won't happen fast enough, there is another nice effect bugging choppers, the sound barrier.

The forward moving blade tip must not reach the SoS or it will disintegrate.

 

Stay below 265kmh unless forced to evade quickly and save some big headaches.

 

There is even a few degrees shift angle in the swash plate, depending on what kind of linkage is used, 3 or 4 point ( 120° or 90° ) but enough of that... LoL

 

There are excellent books for chopper setups, usually RC, they sell for a few bucks and teach you all you need to know, build and set up a chopper. It by far exceeds the knowledge you need in DCS but is well worth the few hours reading...and understanding.

 

Bit

Edited January 21, 2015 by BitMaster