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HomeAircraftHelicopterHelicopter Main Discussion › Why have different pitch curves in different flight modes?
02-04-2010 09:18 PM  8 years agoPost 21
djrformcar

rrApprentice

Las Vegas, Nevada USA

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If that were true, then how much negative pitch would you need to get 0 lift?

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02-05-2010 06:20 PM  8 years agoPost 22
dhc8guru

rrApprentice

Fort Worth, Texas

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I remember(from 20 years ago during my flight instructor training)that in "theory" you are always producing lift with a symetrical airfoil because both surfaces are curved and identical which have a lift vector that at 0 degrees are equal and positive but in opposite direction of each other therefore canceling each other out.
So any degree in a direction that favors one or the other begins to give the opposite surface more lift than the other.

For example:
If your heli needs 4° positive pitch to hover upright, it will need
-4° to hover inverted.
Ever fly an rc airplane that has a semi-symetrical wing inverted? It requires almost twice the pitch while inverted as upright.

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02-05-2010 10:05 PM  8 years agoPost 23
JasonJ

rrKey Veteran

North Idaho

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Later I read that symetrical blades still develope lift at 0° pitch. Lesson learned with no damage.
It isn't that the rotor is producing lift at 0 degrees by itself. It's that a rotor disc requires less pitch to create lift when the helicopter is in directional flight compared to hovering. Wind passing across the rotor disc of a hovering helicopter creates the same lift as if the helicopter was moving. That is why the helicopter balloons in wind gusts. If it is windy enough and the circumstances right, 0 degrees pitch can still produce lift.

I almost lost a helicopter when I was first learning due to not having any negative pitch. It got windy and the helicopter just went up and up over a tree. I had never been more than about a few feet up prior to that. I basically had to just wait it out and slowly get it back down as the wind calmed. Scared the crap out of me. After that day, I went to full pitch range in all modes and only flew in Idle Up. I never developed the chop the throttle instinct because of that.

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02-06-2010 01:44 AM  8 years agoPost 24
Coolrunnin

rrVeteran

Manchester U.K.

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I'm wondering if there is some confusion here between 0 degrees of pitch (measured relative to the rest of the mechanics of the rotorhead) and angle of attack.

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02-06-2010 04:15 AM  8 years agoPost 25
GimbalFan (RIP)

rrProfessor

Big Coppitt Key, FL

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I'm wondering if there is some confusion here between 0 degrees of pitch (measured relative to the rest of the mechanics of the rotorhead) and angle of attack.
Me too.

Symmetrical blades spinning at zero pitch (zero AoA) can't possibly produce lift -- but a heli with symmetrical blades actually can maintain autorotation rpm at zero pitch if the AoA of the rotor disc relative to airflow as it moves forward and downward is at nearly exactly the ideal angle, usually around 45°.

This doesn't mean the blades are producing lift though.

op-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-t

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02-06-2010 03:05 PM  8 years agoPost 26
Nelson J

rrApprentice

Canada

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I like not having a jump/change in pitch when I switch modes/hold.
If the pitch is different (less negative) on N mode up to half stick, you would only see the jump if changing from N to 1 while on the ground when you haven't reached middle stick, or if you are flying up high, and go into a auto-rotation with the throttle stick on low position, that is if your auto (hold) curve has less pitch (-) then your flying (3D) curve as well.
If the curves are the same where you hover, and you switch modes/hold while hovering, there will be no jumping up or down.

You can change to less negative (-1 or -2) and less positive (+6.5,+7 or +8) on N mode, a slighter curve ( almost straight) for hovering, like the F3C guys do, still trying to keep the hovering point the same on all modes, to avoid the jumping when changing modes while hovering. Doing this you have to change you throttle curve if you don't have a governor, or your engine will be screaming at full throttle.

((For F3C I think it'll always jump a little, they hover at much lower throttle))

If you want to give it a try, have the pitch curve on your radio screen, take a quick peek at radio while hovering to see what point of the pitch you hovering, then try not to change it there.

Bit by Bit

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HomeAircraftHelicopterHelicopter Main Discussion › Why have different pitch curves in different flight modes?
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