Safety problem with counter rotating blades ?

[golfsierra2]

Despite it was discussed a while ago (in 2004) on another forum, I would like to know whether the claimed safety problem (counter rotating blades colliding under specific negative G maneuvers) is real and if this will be modeled by the DCS:BS flight model ?

 

Posted by Showtime 100

"I think the Ka-50 lost a lot of points with the military after the crash which killed general Vorobyov. Just like the other crash of this type, the helo went down because the counter-rotating rotor blades actually hit each other thanks to negative G maneuvring."

 

Source:

http://forum.keypublishing.co.uk/archive/index.php?t-35140.html

[-sulan-]

It does'nt happen if you keep it within normal flight parameters, and it is modeled in the sim. :D

 

Does'nt maneuvering in negetive G's screw up most helicopters?

[golfsierra2]

Common single rotor helos can hit their own tail or the AAR probe under negative G, but how can the blades of two counter rotating rotors mounted onto the same axis possible ever hit each other ? Under negative or positive G , they all should bent the same amount because the upper and lower blades should be affected the same way, right ?

[CAT_101st]

What you need to know is that when the top blaed is going noes to tail it is at slack lift and on the botom going tail to nose it is on a lift pass. So If you put enough lift on the botom blades they will lift into the top blads. Just think of The AH-64. As it travel foward it has less left on one side than the other. The faster the helo goes the more lift it has on one side and less on the other. Thes is what limets the speed of helo mostly.

[CAT_101st]

Common single rotor helos can hit their own tail or the AAR probe under negative G, but how can the blades of two counter rotating rotors mounted onto the same axis possible ever hit each other ? Under negative or positive G , they all should bent the same amount because the upper and lower blades should be affected the same way, right ?

 

http://www.youtube.com/watch?v=c2fSjHghubc

[GGTharos]

You don't need -ve G's to kill yourself. Flying at high speed (though 'normal') and putting in right rudder will do in you in beautifully.

[Esac_mirmidon]

I think this accident you had commented was because he exceed the safety parameters by far.

[-sulan-]

I think this accident you had commented was because he exceed the safety parameters by far.

 

Yep that's right, he did, I read...

 

So the faster you fly the closer the rotors get to each other? since "forward moving" blades has more lift than the retreating blades?

Then when applying right rudder the rotors come even closer to each other since it yaw's by differentiating the AoA between the rotors.

Does yawing to the left at high speed put the rotors noticably farther apart? Am I confused? :huh:

[GGTharos]

I don't see why you think you're confused, you got it right :)

 

Yep that's right, he did, I read...

 

So the faster you fly the closer the rotors get to each other? since "forward moving" blades has more lift than the retreating blades?

Then when applying right rudder the rotors come even closer to each other since it yaw's by differentiating the AoA between the rotors.

Does yawing to the left at high speed put the rotors noticably farther apart? Am I confused? :huh:

[GTengineer]

sulan is right though, the retreating blade flaps up if I remember correctly (it has been many years since I took my rotorcraft class). In traditional helos in the USA convention (counter clockwise rotation looking from above as the F-15 in my avatar) this occurs on the 9 o'clock position. The opposite rotating blade would flap up at the other 3 o'clock position. I don't know much about the KA-50 as I am not a rotohead but the blade dynamics and technology being used to control them must be nothing short of amazing.

Edited August 31, 2008 by GTengineer

[CAT_101st]

A+ Sulan

[GGTharos]

I would say down, actually, since the lift is generated on the -other- side of the rotor disc.

 

sulan is right though, the retreating blade flaps up if I remember correctly

[-sulan-]

Coolings! :)

What other kind of flight modes/maneuvering can cause blade intersection?

[GTengineer]

I would say down, actually, since the lift is generated on the -other- side of the rotor disc.

 

Nah, pretty sure it flaps up on 9 o'clock. I have it in my book but here is a quick online reference. This happens because there is a 90 degree phase angle between aero forces and flapping as explained in the bold comment below.

 

 

 

The above graphic shows that the higher angle of attack at the front of the rotor will cause the blade to flap up over the left side of the helicopter. The lower angle of attack over the rear of the rotor will cause the blade to flap down over the right side. The rotor will thus be tilted a little to the right. the sideward tilt of the rotor is increased at low forward speeds when the induced velocities are large, because the inflow not only approaches the rear of the rotor but, additionally, is bent downward. this increases the angle of attack differences.

 

http://www.dynamicflight.com/aerodynamics/flapping/

 

:)

Edited August 31, 2008 by GTengineer

[GGTharos]

Aha! Thanks GTEngineer - definitely an interesting little piece of information :)

[GGTharos]

Any hard maneuvering at high speed (hauling the stick back hard) can make it happen - in addition you could cause Pilot Induced Oscillations that could result in the same (wild up/down pitching of the nose) ... it's not as exciting as it sounds, it doesn't last long, and your blades might intersect during it ;)

 

Coolings! :)

What other kind of flight modes/maneuvering can cause blade intersection?

[hitman]

Nah, pretty sure it flaps up on 9 o'clock. I have it in my book but here is a quick online reference. This happens because there is a 90 degree phase angle between aero forces and flapping as explained in the bold comment below.

 

 

 

 

 

http://www.dynamicflight.com/aerodynamics/flapping/

 

:)

Go a little bit more in-depth here:

The advancing blade is going faster than the retreating blade in forward flight. Flying faster than the helicopters speed limit will stall the retreating blades aerofoil. You have lift and thrust on the advancing blades, you have gravity and drag on the retreating...unlike in aircraft, half of the helicopter will stall. In an aircraft, stall strips are installed at the wing root to make the fuselage stall first. You cant control that in helicopters as the retreating blade stalls (left side usually) while the advancing blade is creating lift.

 

Best example of that would the the CH-47 that tried to land on a ship, then promptly fell into the ocean. Thats not what happened there, but its the idea. (what really happened was the landing gear caught on the safety fence, ground effect pushed the advancing blades up, retreating blades didnt have any ground effect, thus stalled). So to (kind of) answer the topic of this thread, contra-rotating rotors are more difficult to stall than regular aircraft and are somewhat safer.

Edited August 31, 2008 by hitman_214th

[AlphaOneSix]

You cant control that in helicopters as the retreating blade stalls (left side usually) while the advancing blade is creating lift.

 

Only American helicopters, I think, as they rotate counter-clockwise (viewed from above). European and Russian helicopters rotate the other direction. So on most helicopters (internationally), the right side will stall first.

 

Best example of that would the the CH-47 that tried to land on a ship, then promptly fell into the ocean. Thats not what happened there, but its the idea. (what really happened was the landing gear caught on the safety fence, ground effect pushed the advancing blades up, retreating blades didnt have any ground effect, thus stalled).

 

That is not AT ALL what happened to that CH-46 (not a -47, but minor detail).

[hitman]

That is not AT ALL what happened to that CH-46 (not a -47, but minor detail).

Ok, so my instructor taught me wrong. Care to tell me what happened here? Cuz from what Ive seen and what the instructor has seen, the rotor disks had only half of the disk hitting the ground effect, while the other half was hanging out off the side of the ship...and you can CLEARLY see the main gear get caught up in the safety net.

Edited August 31, 2008 by hitman_214th

[AlphaOneSix]

Ok, so my instructor taught me wrong.

 

I agree.

 

Cuz from what Ive seen and what the instructor has seen, the rotor disks had only half of the disk hitting the ground effect, while the other half was hanging out off the side of the ship...

 

I agree.

 

and you can CLEARLY see the main gear get caught up in the safety net.

 

I agree.

 

Care to tell me what happened here?

 

The main gear get caught in the safety net, the pilot pulls pitch to get away from the ship. The main gear are caught on the safety net, but nothing is holding down the front of the aircraft, so the front of the aircraft continues to rise while the rear of the aircraft cannot. The pilot continues to pull power and the problem worsens until the aircraft almost flips over backwards.

 

Helicopters hover just fine out of ground effect. Ospreys take off from carriers routinely with one rotor disk out over the edge of the ship. Ground effect had no impact on this incident. Chinooks and Sea Knights do pinnacle landings routinely where one disk is in ground effect and the other is not. Single rotor helicopters routinely perform maneuvers where only half of the rotor disk is in ground effect.

[hitman]

The main gear get caught in the safety net, the pilot pulls pitch to get away from the ship. The main gear are caught on the safety net, but nothing is holding down the front of the aircraft, so the front of the aircraft continues to rise while the rear of the aircraft cannot. The pilot continues to pull power and the problem worsens until the aircraft almost flips over backwards.

Looks like it rolled over to the side to me.

http://www.spike.com/video/helicopter-crashes/2898734?cid=YSSP

 

I understand what your saying, but from the video, thats exactly what it looks like: left side stalled, left side hit water first.

[Yellonet]

That had clearly nothing to do with stalling of any side of the rotor, it's just as Alpha says, pilot just putting in more and more power while the back of the chopper is stuck, the rotor got turned perpendicular to the ground(sea), and with the rotor like that there's no lift.

[AlphaOneSix]

Looks like it rolled over to the side to me.

http://www.spike.com/video/helicopter-crashes/2898734?cid=YSSP

 

I understand what your saying, but from the video, thats exactly what it looks like: left side stalled, left side hit water first.

 

Again, you're baiting me. I will leave it like this, retreating lade stall and ground effect had nothing to do with this accident. I will be glad to explain in more detail via private messages, if you'd like.

[hitman]

Im not baiting you, Im trying to learn something new here. Seriously, the instructor I had wasnt all there. The class felt cheated out of $2200 a piece so we're trying to get him canned. Im just looking for better information. Like I said, I see what you are talking about. Bad angle from the video, the obvious is still there. If you say that rotor stall wasnt part of it, then Ill take your word from it. Im not going to take the word of someone that spend 6 months in the field after he graduated, just to be an instructor, you know...over someone thats been in the field for more than 6 years.

[ThunderChief]

What you have to consider is that the retreating blade will stall when its at 3 or 9 o'clock so there is no lift at this position. Vice versa on opposite there will be maximum lift.

The upward or downward force will start to move the blade at this position

 

BUT

 

the resulting flapping will reach its maximum up to 90° later, at 6 or 12 o'clock

 

this is becaus force will cause motion, and motion will then change position. That causes a phase shift of up to 90° (for 2 blade rotors with hubs that allow full flapping like on Bell 206).

Other rotor hubs are more riggid and thus the phase shift will be lower.