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Control Authority
#11
I don't think you have to fully understand the system you are flying.....if you are a well informed pilot, that keeps the craft within it's envelope. I sometimes think ignorance is bliss when it comes to the dynamic operation of helicopters. Things don't necessarily work the way instinct would dictate. I've spent literally hours studying the dynamics of helicopter flight. It is quite interesting. I spent much of the time hands on with model r/c choppers to just see all of the dynamics in motion.

I believe if you know the controls and understand the limitations.....you are good. If you dig too deep, the dynamics can actually be a little scary. Things just don't work "aerodynamically". That is the biggest catch of all. There are more forces involved...to produce helicopter flight.

Unfortunately for me....I have very little pilot viewpoint to cross reference to the flight dynamics. Yea...I can fly a model helo....that is totally different. I have also crashed quite a few of the models as well. The opposing flight toward the pilot can be very overwhelming. Too much to think about to keep the thing stable.

But thanks to all of the people who helped answer my original question of mast bumping (hammering). I feel well informed about the craft being much like other rigid head types.

James L
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#12
Nice discussion guys! and on Christmas day no less! Good way to spend Christmas day, learning about choppers. It is actually quite interesting to learn about all that is going on in a helicopter rotor system during forward flight. A lot more than meets the eye.
You can actually do loops etc in any chopper so long as you keep a positive load on the rotor system so that the fuselage doesn't try to flip back down under the rotor (when inverted) faster than the rotor can respond. A rigid rotor is much more tolerant of inverted loading since it doesn't just depend on aerodynamic forces to keep the rotor in line. That is why RC choppers can fly upside down. A rigid rotor however is much heavier since it handles extremely high stress loads. It is also much more difficult to make it operate smoothly so it is rare to find one on a full sized chopper.
By the way, I've never encountered mast bumping while in flight and have done some fairly radical control movements to test things out.

Merry Christmas all. I thank God for sending his Son today many years ago.
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#13
Hope everyone got what they wanted for Christmas......how ever I didn't see a Mosquito stuffed in my stocking.

Thanks for the information about your flight experiences about mast bumping.

James
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#14
You can snap roll em' get em' upside down. But Never get the rotor in a Negative G situation!!!!
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#15
Here is a fine example of not unloading your rotor. You guys probably seen it is been around for a couple of years.



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#16
Being new to Heli's I'm doing a lot of reading and found this informative US Army training film on Mast Bumping.



I think the Mosquito is classed as a semi-rigid rotor as opposed to a free teetering rotor so perhaps Mast Bumping is not a concern?

Guess I'll have to read more about those distinctions next.
Ray
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#17
I've yet to hear of a Mosquito encountering mast bumping. I've certainly never experienced it myself. Mast bumping is caused mostly by a slow moving twin blade rotor such as on the UH-1 because of its large diameter. That leads to a very slow rotor response to input from either the pilot or the rotor shaft movement. So any sudden movements of the airframe requires a longer time for the rotor to react, allowing the rotor shaft greater angular misalignment with the rotor. In extreme cases this results in the shaft bumping into the rotor stops before aerodynamic forces can move the rotor where the controls are telling it to go. Because our rotor is so much smaller and spinning so much faster than the UH-1 it reacts much more quickly to control inputs, so the rotor shaft never has time to reach the stops even in sudden movements. The extreme of this is small RC choppers in 3D flying where the movements are incredibly fast but the rotors stay intact because they are spinning so fast.
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#18
I appreciate that explanation, that is reassuring to know that the Mosquito rotor wont separate from the shaft due to poor pilot control/technique.

I had never heard of this before and was a bit disconcerting to know that this could even happen in some helicopters.
Ray
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#19
(02-13-2017, 04:17 AM)mosquito Wrote: ...Mast bumping is caused mostly by...

Hey John,
Did you watch the old vintage Army film?  Let me perhaps humbly make some points that you may not have picked up on.

Muhammed Ali hits his dangling punching bag...BAM! It hits the round disk above it.

Any 2 blade semi-rigid helicopter dangles beneath the single point of support on the CL of the mast and even with the teter bearings.
This is just like the punching-bag`s universal joint.

Suppose the punching bag is at mounted inside the Vomit Comet and at almost 0G when its dangling.  A feather could knock it hard enough to smack the disk.

Now...The Mosquito fuselage is the bag, the Mosquito teter stops are the wooden disk, and the sideward thrust of the tail rotor is Ali`s punch.  

If a Mosquito is subjected to near zero-G, (as in a climb followed by pushover) the thrust of the tail rotor swings the fuselage about its roll axis and it can hit the teter stops.  It does this because the line of force of tail rotor thrust not perfectly through the helicopter`s CG so a roll force is created.  The rotor assembly spinning fast makes it more rigid in space and really gives the fuselage a stable target to hit.

The correct recovery is to load the rotor to 1+ G and only then, add lateral cyclic to stop roll. At 1+G Gravity helps keep tail rotor thrust from rolling the fuselage.

Is that how you see it?
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#20
The zero G condition removes the force couple between the weight of the fuselage and the upward thrust of the rotor which keeps the aircraft properly aligned.  This allows the tail rotor thrust to push the fuselage out of angular alignment with the rotor and hit the stops.  However it is still the slow moving rotor that doesn't allow the rotor to react quickly enough to the movement of the fuselage (and hence the rotor shaft) to keep the shaft from hitting the stops.  Even in a zero g condition the rotor will still try to follow the rotor shaft angle.  The question is how quickly it can do this.  Small fast spinning rotors can do this quickly, like an RC helicopter doing 3D flight (which encounter zero g constantly but are also helped by a head that is closer to being rigid).  Larger slow moving rotors can't so the shaft hits the stops.

 All that being said there is no reason to tempt fate and attempt zero G maneuvers.  We just happen to have a slightly larger safety margin over a larger machine, but it still could happen in the right conditions.
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