snap displacement

[tony at radiosouthrc.com]

Earl:

Really good description!  I am working now on my own snaps to improve the initial elevator "break".  I suggest to everyone else to take the time to think over how you do a snap.  This next year, I will be looking much harder for a break prior to rotation...  If not, you will know that you earned that zero!  LOL

Tony Stillman
Radio South
3702 N. Pace Blvd.
Pensacola, FL 32505
1-800-962-7802
www.radiosouthrc.com
  ----- Original Message ----- 
  From: Earl Haury 
  To: Discussion List, NSRCA 
  Sent: Wednesday, December 29, 2004 11:43 AM
  Subject: Fw: snap displacement



  ----- Original Message ----- 
  From: Earl Haury 
  To: tony at radiosouthrc.com 
  Sent: Wednesday, December 29, 2004 10:22 AM
  Subject: Re: snap displacement


  A snap is a complicated maneuver that's performed in a blink of an eye. The words of the rules are static and may be studied at leisure. Perfect resolution of either is not easy! I perceive confusion between "break" and "offset", the rules state that "the CG should ideally follow the geometric path of the maneuver". I suggest that everyone interested in flying and/or judging snaps carefully read the maneuver description in the rule book(s).

  OK - so why the weasel word "ideally" in the rules? The answer provides the opportunity for us to improve snaps! The clue is in the statement "initiated by a rapid stall of the wing induced by a change in pitch attitude". Unfortunately, no matter how rapidly the pitch attitude is changed, the wing will generate some lift until it reaches critical AOA and stalls. This brief moment of lift moves the aircraft from the original track, providing the "separation". The separation is NOT the "break", but it is a consequence.

  How come some snaps jump all over the place, both in pitch and yaw directions, and others don't? Set-up and execution.

  First - set-up: The faster the pitch attitude change the better. Fast strong servos are a must. A secret - we've always considered setting up linkage to maximize servo travel for best resolution - servos are so good now that we can change the linkage to reduce servo travel and therefore transit time for snaps. Another secret - if you have expo in the setup for snaps the break will be slower and more offset will occur. Travel amounts will vary by airplane - have enough to clearly display snap characteristics and no more. BTW, upline / downline snaps require more pitch change to achieve critical AOA as the wing isn't lifting aircraft weight.

  Second - execution: Use the snap switch for engine kill - forget it for snaps. Stuffing both sticks into the corners doesn't get it either. Rapidly apply elevator first to achieve a pitch break and stall, follow very closely with rudder (rudder application before the stall results in sideways offset) to achieve the snap while reducing the elevator to minimize pitch progression (the wing is stalled - not the elevator) for precise exits. Oh - aileron application at the same time as rudder will handle the wing rotation inertial issues. Exit by simply releasing. Control input timing takes practice and will likely be different with the same airplane for snaps in different attitudes (horizontal - vertical - etc.) 

  Quique has been mentioned in this thread, as have the skills of some judges. I'll offer that Quique has idealized his snaps so that they minimize disruption of maneuver geometry. On a horizontal line and/or center maneuvers the break is clear in pitch and there is very little offset, the tail cones as the aircraft rotates on the CG, and the pitch attitude clearly is returned to the original attitude at the snap finishes. Guys -that ain't a zero and I don't score it as such! On the other hand - the same snap on a turn-around vertical presents so that the pitch break is not visible - even though I think I know what's going on, if I don't see the break I will and have zeroed those. 

  What I see a whole lot more of are snaps that are pitched / yawed slowly so that the track changes 10 or 15 degrees (along with similar roll) before the stall occurs - hmm - down to a seven or so, then the snap rotation is stopped an equal amount early so that the airplane can be flown to the finish. These things are easily a 4 or less! Unfortunately, some of the perpetrators of these abominations are the first to criticize those who have done the work to figure it out.

  I hope some of this info is useful. Be critical of your snaps and study how set-up and technique effects the outcome. A video camera is very useful for study of snaps (amazing how many are really barrel rolls), use slo-mo for critical examination. A data logger is also useful as it logs elevator input - altitude - speed changes. 

  Earl
    ----- Original Message ----- 
    From: tony at radiosouthrc.com 
    To: Discussion at Nsrca.Org 
    Sent: Wednesday, December 29, 2004 8:41 AM
    Subject: snap displacement


    I have seen many full-scale pilots do snaps without any noticeable displacement.  I have also seen these same pilots do poor ones that did displace.  I think you must downgrade it some (1 point per 15 deg).

    Tony Stillman
    Radio South
    3702 N. Pace Blvd.
    Pensacola, FL 32505
    1-800-962-7802
    www.radiosouthrc.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.f3a.us/pipermail/nsrca-discussion/attachments/20041229/1c39274e/attachment.html