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 Rotorheads A haven for helicopter professionals to discuss the things that affect them

 17th Aug 2003, 09:02 #1 (permalink) Thread Starter   Join Date: Jul 2002 Location: Murica. Age: 39 Posts: 223 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.....
 17th Aug 2003, 10:13 #2 (permalink) Join Date: May 2002 Location: "Deplorable but happy as a drunken Monkey! Age: 69 Posts: 1,162 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.
 17th Aug 2003, 12:03 #4 (permalink) Join Date: Apr 2003 Location: Vancouver, BC, Canada Posts: 1,635 Positive delta-3 is when the delta-3 angle is between 0º and 45º. Negative delta-3 is when the delta-3 angle is between 45º and 90º. Delta-3 angle of 0º. ~ Pitch angle is not changed. Delta-3 angle of 1º to 44º. ~ Pitch angle change is less than flap angle. Delta-3 angle of 45º. ~ Pitch angle change equals flap angle. Delta-3 angle between 46º and 89º. ~ Pitch angle change is greater than flap angle. Edited to clarify explanation. Last edited by Dave_Jackson; 17th Aug 2003 at 21:53.
 17th Aug 2003, 12:47 #5 (permalink) Join Date: Apr 2003 Location: USA Age: 69 Posts: 3,010 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.
 18th Aug 2003, 06:12 #6 (permalink) Join Date: Feb 2003 Location: ON A HILL Posts: 373 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.
 24th Sep 2003, 05:53 #7 (permalink) Join Date: Apr 2003 Location: Vancouver, BC, Canada Posts: 1,635 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 0º. ~ Pitch angle is not changed. Delta-3 angle of 1º to 44º. ~ Pitch angle change is less than flap angle. Delta-3 angle of 45º. ~ Pitch angle change equals flap angle. Delta-3 angle between 46º and 89º. ~ 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.