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Old 17th Aug 2003, 08:02   #1 (permalink)
 
Join Date: Jul 2002
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Delta hinges

I am reading up for the JAA Type Rating Exam on the Schweizer 300CB...(to the FAA guys out there..... YEAH I KNOW!!)
I stumbled across the different Delta 3 hinges for the TR..namely positive and negative Delta hinge...? What exactly does this mean?
I know I can put negative pitch on the TR, but somehow I don't believe that is what they mean.....
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Old 17th Aug 2003, 09:13   #2 (permalink)
 
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Check your PM for a full answer....


or....go to this addy....

www.digital-fusion.ca/sbr/delta.htm

A nice discussion with diagrams of the issue.
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Old 17th Aug 2003, 09:19   #3 (permalink)

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Thumbs up Delta fringe

To: TIMTS

Hopefully this does not set me up for a lot of critical comment. The Delta or difference is the relationship between the pitch change axis of the tail rotor and the flapping axis of the tail rotor. This difference depending on the design can range from 30-degrees to 60-degrees. The delta hinge or flapping axis is offset which allows the tail rotor to rock across the flapping axis. In the process of rocking one blade will move in toward the fixed pitch link and in doing so pitch will be removed from that blade. Conversely when the blade moves away from the fixed pitch link pitch will be added to that blade. This gives the tail rotor the ability to equalize the lift across the tail rotor disc. This is similar to pitch flap coupling on the main rotor system. As a blade is moved upwards due to gusting which raises the blade from the commanded tip path the blade moves away from the pitch link and pitch will be removed from the blade allowing it to return to the commanded tip path. As the blade drops down into the commanded tip path the pitch angle of the blade will return to the commanded (Collective / Cyclic pitch setting).

Awaiting all comments.



To: SASLess

One of the illustrations in your reference shows the pitch horn / pitch link coincident with the flapping hinge. The description states that with the pitch link in this position there is no delta hinge effect when the blade is flapping. This is true if the blades were static and you lifted one blade. In actuality when you add collective pitch the two points are no longer coincident but the pitch link will rise above the flapping hinge and with the blades dynamic gusting will result in pitch flap coupling (delta hinge effect).

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Old 17th Aug 2003, 11:03   #4 (permalink)
 
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Positive delta-3 is when the delta-3 angle is between 0 and 45.
Negative delta-3 is when the delta-3 angle is between 45 and 90.

Delta-3 angle of 0. ~ Pitch angle is not changed.
Delta-3 angle of 1 to 44. ~ Pitch angle change is less than flap angle.
Delta-3 angle of 45. ~ Pitch angle change equals flap angle.
Delta-3 angle between 46 and 89. ~ Pitch angle change is greater than flap angle.

Edited to clarify explanation.

Last edited by Dave_Jackson; 17th Aug 2003 at 20:53.
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Old 17th Aug 2003, 11:47   #5 (permalink)
 
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Negative delta 3 is used to make the pitch capability of the rotor greater. It helps package more collective pitch range into the rotor head without needing that much pitch change rod motion. This is because the blade flaps and creates more pitch as it does so.
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Old 18th Aug 2003, 05:12   #6 (permalink)
 
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Delta 3

Keep going chaps. Im pretty new to Prune, but how refreshing to see so much interest in the technical stuff. Who wants to watch tv when you can get your kicks here.
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Old 24th Sep 2003, 04:53   #7 (permalink)
 
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Unhappy

Opps!! I was wrong.

The last four lines of the previous post, as quoted below, were OK. They explain the relationship (ratio) between pitch and flap.

"Delta-3 angle of 0. ~ Pitch angle is not changed.
Delta-3 angle of 1 to 44. ~ Pitch angle change is less than flap angle.
Delta-3 angle of 45. ~ Pitch angle change equals flap angle.
Delta-3 angle between 46 and 89. ~ Pitch angle change is greater than flap angle."


But, the first two lines of the post, qualifying 'positive' and 'negative', were very wrong.
_____________________

The following two quotations discuss positive and negative delta3.

[Source ~ Helicopter Theory p.239] ~ "Note that positive coupling [delta3 >0] represents negative feedback, decreasing the blade pitch for a flap increase."
[Source ~ me ] ~ This is the common usage.

[Source ~ Helicopter Theory p.850] ~ Discussion about speed stability on the tandem configuration "..... differential pitch-flap coupling (positive on the front rotor and negative on the rear rotor)."
[Source ~ me ] ~ This implies that positive delta3 will result in upward flapping causing the pitch to increase, and negative delta3 will result in upward flapping causing the pitch to decrease. In other words, if the reference flapping (or teetering) hinge is normal to the span of the blade, then rotating this hinge in the opposite direction to that of the rotor's rotation will result in negative delta3. Conversely, rotating this hinge in the same direction as that of the rotor's rotation will result in positive delta3.

Since "positive coupling [delta3 >0] represents negative feedback", it appears that any discussion of 'positive' or 'negative' must depend on whether one is talking about the direction of rotation of the delta3 angle or talking about the feedback resulting from the delta3.

To me, having upward flap increase the pitch sounds scary, since this increase in pitch will want to increase the flap even more.
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