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Blade Strike


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Can anyone comment on what is the main cause for Blade Strike on the twin rotor KA50. I don’t seem to be able to pin point it to one single mistake I am making that causes this happen at the most inconvenient times.:helpsmilie:

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Overspeeding, applying lots of rudder at high speed, hard manuvering while pulling a lot of pitch...

[sIGPIC][/sIGPIC]

"Great minds think alike; idiots seldom differ.":pilotfly:

i5 3750K@4.3Ghz, MSI Z77A GD55, 8GB DDR3, Palit GTX 670, 24" Benq@1920*1080, X52 Pro, Win 7 64bit.

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I assumed the rotor strikes occur when trying to put too much control at high speeds... (of course the most inconvenient times:) )

 

It appears that the lower rotor tilt gets bigger the faster the helo goes thus making the blade strike possible only at the high speed flight.

 

BOTTOM LINE: The faster you go, more soft and precise with the controls be.... (wow that sounds like Yoda hahaha)

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Regards!







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I don't think High Power/altitude and abrupt pedal input causes blade strike because in anyone of these case the two rotors remains parallel with eachother.

 

Not true. As the upper and lower blades rotate in different directions they generate lift on different sides of their respective rotor disks. As a result of blades tendancy to bend upwards on the side of the rotor disk where they are generating more lift the rotor disks will appear to flex symetrically but definitely not in parallel. If they flexed in parallel the blades would never collide.

[sIGPIC][/sIGPIC]

"Great minds think alike; idiots seldom differ.":pilotfly:

i5 3750K@4.3Ghz, MSI Z77A GD55, 8GB DDR3, Palit GTX 670, 24" Benq@1920*1080, X52 Pro, Win 7 64bit.

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As the upper and lower blades rotate in different directions they generate lift on different sides

With the cyclic centered , the lift generated is simmetrical, the air just "twist" while going down but the force on the main shaft is simmetrical, different direction simply cancel the yaw effect caused by a single rotor..

 

As a result of blades tendancy to bend upwards on the side of the rotor disk where they are generating more lift the rotor disks will appear to flex symetrically but definitely not in parallel.

I don't think so. can you prove it ? or doyou have some direct experience with this statement?

 

If they flexed in parallel the blades would never collide.

infact they don't collide unless you move the stick aggressively..

Robbie.

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The reason for the blades rising is the result of dysymytry of lift. It doesn't matter what size the helicopter, this is something that is going to happen irregardless. As the aircraft picks up forward airspeed, three main regions shift in size on the rotor blade. You have the no lift region which is inboard nearest the hub, the lifting portion of the blade, and then the stall region at the tip. As airpseed increase, the no lift and stall regions increase effectively shrinking the lifting area, resulting in the advancing blade rising and the retreating blade dipping. The faster you go, the more pronounced this effect. Now, this is happening to both rotor discs at the same time as a result of Blade Flapping. Now, throw in cyclic feathering (the means of gaining directional control of the aircraft) you're further changing the pitch of the blades through use of the cyclic to generate greater lift on one side and roll/pitch the aircraft. At a high power setting, high speed, so i'd definately say an aggressive right hand turn (the input occurs at the nose on the swashplate, hits its highest point on the left side to result in the roll to the right) would result in a blade strike. Aerodynamically it's supported with a clockwise rotating rotor system (which I believe the lower rotor is if I remember right)

 

Brad

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The reason for the blades rising is the result of dysymytry of lift. It doesn't matter what size the helicopter, this is something that is going to happen irregardless. As the aircraft picks up forward airspeed, three main regions shift in size on the rotor blade. You have the no lift region which is inboard nearest the hub, the lifting portion of the blade, and then the stall region at the tip. As airpseed increase, the no lift and stall regions increase effectively shrinking the lifting area, resulting in the advancing blade rising and the retreating blade dipping. The faster you go, the more pronounced this effect. Now, this is happening to both rotor discs at the same time as a result of Blade Flapping. Now, throw in cyclic feathering (the means of gaining directional control of the aircraft) you're further changing the pitch of the blades through use of the cyclic to generate greater lift on one side and roll/pitch the aircraft. At a high power setting, high speed, so i'd definately say an aggressive right hand turn (the input occurs at the nose on the swashplate, hits its highest point on the left side to result in the roll to the right) would result in a blade strike. Aerodynamically it's supported with a clockwise rotating rotor system (which I believe the lower rotor is if I remember right)

 

Brad

Thanks Brad for sharing your knoledge with us,

if we have no airspeed , just hovering, and put the stick full right aggressively we still have blade strike right? so what is the purpose of having so much throw in the cyclic?

 

Thanks

Robbie.

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Applying the same logic it stands to reason that you could. But, there are different factors in a hover vs. forward flight, so my first thought would be its not that likely to happen in a hover as it is in directional flight. And, the reason for having the control input range is so that you do not limit the aircrafts maneuverability. It's why pilots spend so much time in training on their given airframes, learning the nuances of the aircraft so they can effecitively fly it in any particular envelope without to much worry.

 

Brad

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