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Old 07-06-2009, 08:13 PM   #1 (permalink)
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Default does any 1:1 helicopter offer negative pitch?

I was thinking this morning that a unmanned military heli with full negative pitch function, 3 gyros, a smart computer and an awesome power to weight ratio, could dodge pretty much all incoming fire, short of lasers, by simply moving out of the way.
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Old 07-06-2009, 08:18 PM   #2 (permalink)
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Not that I'm aware of. I believe even the Red Bull helicopter stops at 0 pitch. The risk is just too high, and the forces much greater.
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Old 07-06-2009, 08:32 PM   #3 (permalink)
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thanks for that, I watched the video of it doing loops etc, then when searching for discussions on negative pitch a pilot of a BO-105 posted that it offered 2 degrees of negative pitch. So then I looked up the Boelkow bo-105 and saw .. yep .. it is the red bull heli. And the circle was complete.
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Old 07-06-2009, 08:37 PM   #4 (permalink)
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Yep its an awesome heli, but I don't believe it offers -2 degrees. Unless it has changed he was using 0 at the top of those loops. Skiddz can maybe elabarote if I am incorrect.
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Old 07-06-2009, 08:55 PM   #5 (permalink)
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quote I found was this:

Quote:

We used to operate BO105's, and I seem to remember we had one which had two degrees of negative pitch. It's a long time ago, and I've a memory like a whatsit, but there were certainly no special procedures for applying negative pitch.

My own experience of negative g, again in a BO105, was while enjoying a flight over the Western Isles. I flew into a terrific downdraught, and became aware of a sudden vast increase in pressure on the shoulder harness, my feet flying off the pedals, clouds of dust rising from the floor, oil pressure warning lights coming on, with the pressures indicating zero, and all the while the passenger pushing up on the cockpit roof as hard as he could, barely managing to suppress a scream.

It probably didn't last more than a second or so, but I'm sure it shortened my life by a few years. My thought at the time was, "Of all the helicopter types in the world, thank God I'm in a Boelkow."
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Old 07-06-2009, 09:32 PM   #6 (permalink)
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The vast majority of Helis don't run symmetrical airfoils on the rotor blades, so negative pitch won't be as beneficial as you may think.

Technically, it's more than possible. However, the statement "awesome power to weight ratio" is the tricky part. It's a very difficult engineering problem to develop a power source for full scale helicopters that would give it a comparable power to weight ratio that we find in RC helis. Chasing power to weight ratios is a game of diminishing returns......

Still, it would more than cool of course!!!
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Old 07-06-2009, 10:10 PM   #7 (permalink)
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nyc863

Yes all full scale ships have negative pitch capability. That is how we start an autorotation.
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Old 07-06-2009, 10:44 PM   #8 (permalink)
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I don't know about that... I know quite a few helis have some negative twist to the blades and to enter an auto, you DO stuff the collective all the way down, but in every heli I've ever flown, ya gotta pull some pitch back in to keep from overspeeding the mains.. I don't know if the grip actually goes into the negative... I'll have to ask at the local Bell/Schweizer/Robinson/MD/Eurocopter service center next time I'm over there...
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Old 07-06-2009, 10:55 PM   #9 (permalink)
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Skiddz

Quote:
I don't know if the grip actually goes into the negative
Mine does for sure as I am the one who sets it that way by manufacturers specs. 1.5 degrees negative. Blade engagement is done at about 2 inches of collective to keep from driving the ship into the tarmac. Auto rotate is started at full collective bottoming and then raised to keep the rotor speed within range.
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Old 07-06-2009, 11:02 PM   #10 (permalink)
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I suspect significant negative pitch could result in a boom strike depending on the design. Full scale blades appear to have much more flex than the model blades. I can't envision a full scale helicopter ever hovering inverted.
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Old 07-07-2009, 12:54 AM   #11 (permalink)
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Some of the carrier based helis have negative pitch to secure the heli on the deck after landing, before the groundcrew can tie it down. Super Lynx 300 comes to mind.
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Old 07-07-2009, 02:00 AM   #12 (permalink)
 
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Quote:
Originally Posted by Vinger View Post
Some of the carrier based helis have negative pitch to secure the heli on the deck after landing, before the groundcrew can tie it down. Super Lynx 300 comes to mind.
+1, beat me to it...
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Old 07-07-2009, 02:39 AM   #13 (permalink)
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The Westland Wasp also had negative pitch. We used it to help hold the helicopter on the rolling deck until the chains/strops were fitted.

Can't remember how many degrees but it definitely made the acft squat down.
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Old 07-07-2009, 05:15 AM   #14 (permalink)
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But the amount of neg is limited due to blade flex.

The problem is, that blades are so long, that they can't be that rigid. I am sure that a 500mm - 800 mm section of a full scale blade is VERY stiff, but when you take 6 - 10 METERS of blade, it is flexible.

As for dodging anything, that has to do with the speed and G available of what is coming after you. If you time it right, it would help, but realize most anti-aircraft missles use a proximity fuse, so they explode a ways from the target and the shrapnel cloud expands like a shotgun blast.
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Old 07-07-2009, 08:48 AM   #15 (permalink)
 
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Only Superman can dodge a speeding bullet.... think the rule nowadays is if they can see you your dead.... or is that infantry, I dun know not military myself think if you could "doge" leap out of the way the g-forces would be the real issue... yes they can make a machine quick enough but would anyone be able to ride it.
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Old 07-07-2009, 08:57 AM   #16 (permalink)
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Ummm my only question is how can they auto-rotate without some negative? Not sure that is possible.... I don't care if the negative is from blade flex or what but I know the pitch must be some negative as that is how an auto works to keep the head speed up.

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Old 07-07-2009, 09:18 AM   #17 (permalink)
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Quote:
Originally Posted by Finless View Post
Ummm my only question is how can they auto-rotate without some negative? Not sure that is possible.... I don't care if the negative is from blade flex or what but I know the pitch must be some negative as that is how an auto works to keep the head speed up.

Bob

The "requirement" for negative pitch is a common mis-perception among model helicopter pilots. Full scale does not require negative collective pitch to sustain an auto. If so, a gyrocopter could never sustain flight.

See here
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Old 07-07-2009, 09:21 AM   #18 (permalink)
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interesting link, thanks.

edit: in a vertical auto-rotation at zero pitch, on a real heli, are the blades going to stay spinning at full speed while also generating lift to counter acceleration downwards? doesn't the relative wind work to slow them to a stop pretty quick?

nevermind I see the diagram now.. even at zero or small positive pitch, part of the rotor drives the rotation, the other part just creates lift.. and the two are in equilibrium.. (as long as it is falling, or in the case of the gyro copter, being pushed along).
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Old 07-07-2009, 10:00 AM   #19 (permalink)
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I doubt that the first inverted hover of a full size will be a military or commercial heli. They do not have a need to develop the technology.

I think it will be an experimental/home built where the weights and dimensions are smaller.
Most of the home built helies have fully symmetrical blades that are rated for right or left hand rotation, this means the they are stressed to handle upright or inverted flight. Now all it needs is someone with enough money and balls to build it and do it.
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Old 07-07-2009, 10:03 AM   #20 (permalink)
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Quote:
Originally Posted by nyc863 View Post
interesting link, thanks.

edit: in a vertical auto-rotation at zero pitch, on a real heli, are the blades going to stay spinning at full speed while also generating lift to counter acceleration downwards? doesn't the relative wind work to slow them to a stop pretty quick?
Real helis don't do vertical autos, that's the reason they have a height/velocity chart. While they can do vertical autos from a low hover when they do that they are using the energy stored in the rotor to get the machine down.

this link:

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

has a lot of good information regarding the aerodynamics during an autorotation, as you can see negative pitch is not required.
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