[NSRCA-discussion] Judging Snaps & spins II

[Matthew Frederick]

Jack, I think you touched on a VERY important aspect of snaps in regards to model aircraft. For the most part, our models have very low wing loadings when compared to their full-scale equivalents. That's the reason having a smaller wing helps an aircraft's ability to snap. This weekend I was flying with Bryan Hebert and he had both his original prototype Shinden and one of his production model Shindens. The production model has a larger wing than the prototype. As a result, they both requires totally different inputs in order to snap. The one with the smaller wing (higher wing loading) snaps much like the G-200 you described below: slam to the corners and it goes around. The one with the larger wing required a more well-defined break and different amounts of control input (i.e. more rudder) in order to achieve the autorotation. I'm sure it doesn't help that the production model (which is 100% composite) is 6 - 8 ozs lighter than the prototype. I guess my point is that I'm agreeing with your point that wing loading is a major factor of what it takes to perform a proper snap, and I think we limit ourselves when we force judges to look for a well-defined break in pitch prior to a snap. Some aircraft just don't require a discernable break in order to achieve autorotation.

Matt
  ----- Original Message ----- 
  From: Jack Keiser 
  To: NSRCA Mailing List 
  Sent: Tuesday, October 23, 2007 10:41 PM
  Subject: Re: [NSRCA-discussion] Judging Snaps & spins II


  Earl,

  Been following this thread for a while now and felt compelled to offer my two cents worth. 

  I have a little problem with your auto analogy. In the case of an abrupt steering input in a fast moving car, the forces trying to change the car's path are due to the traction of the tire against the pavement; the force trying to keep the car going in its original direction is due to the momentum of the vehicle, according to some guy named Newton. Even the smallest lateral force will produce some correspondingly small change in direction. In the real world, a lot of factors come into play: sidewall flex causing tire slip angle, suspension response, roll stiffness and so on. If traction were lost immediately, as might be the case when driving on a sheet of ice, turning the wheel sharply produces no change in direction - momentum keeps the car going straight ahead. Only after some traction is regained will the car turn.

  Now let me switch gears a bit and talk about snaps and spins. I'm not an aerodynamicist so I don't pretend to understand all the aerodynamic events that take place in a snap. But I do have quite a bit of experience in full scale aerobatics in a variety of aircraft ranging from Cessna Aerobats, Pitts bipes, Giles 200&202 to the Sukhoi 26. I have also been flying pattern since the '70's (with a few breaks here and there) and recently, IMAC, so I appreciate that aspect as well. 

  The judging criteria for snap rolls is basically the same in all three venues; particularly with respect to the pitch break. I have received zeros in all three venues when that was not clearly observed ( I can't recall any 10's even when the break was obvious).  That does not mean the airplane did not snap (auto-rotation); I just didn't do it according to the prevailing judging criteria. When you're in the aircraft, its quite obvious when auto-rotation occurs. Some aircraft, particularly the Giles 200, can roll so fast (about 540 deg/sec) that its nearly impossible from the ground to tell a full deflection aileron roll, with a little rudder thrown in, from a true snap. And you do not need the pitch break to get a true snap - just jerk the stick toward the right (or left) rear corner and kick hard right (or left) rudder simultaneously, and I guarantee you'll experience a violent snap with true auto-rotation. The G-200 is the closest thing you can imagine to a model; you can fly it with two fingers and it responds about the same as an IMAC plane.

  The other extreme is the Su-26. Its like flying a John Deere. Two hands on a stick that resembles a baseball bat. If you try the same control inputs for the snap as the Giles, it just mushes around. Here, the pitch break to load the wing seems more of a necessity. And (in a positive snap) you only pull the stick back partially,but smartly,  maybe a third of the way, then shove it quickly toward the right forward corner to unload the elevator and bring in the aileron. Full right rudder applied just as the nose pitches up. Despite the significant mass, and relatively high polar moments, the plane auto-rotates with surprising speed - not like the Giles, but pretty darn quick. To stop on heading, you need hard left rudder and some left aileron. You may stop on heading (with practice) but you will have moved to the right of your original flight path - that can be readily observed in a snap on the vertical up line.

  Why the Sukhoi and the Giles behave so differently is due to a lot of factors. I suspect one major factor is the gyroscopic effect of the extremely large prop on the Sukhoi; the gyro effects are much more noticeable - that's why it does a Lomcevak so well. The wing loading of the Sukhoi is much higher than the Giles, and so are the moments of inertia. Of course, the wing airfoil is different too. Many other differences (including fuel consumption).

  Regarding spins, hardly any full scale pilot does a true spin in competition. The entry is usually done in the classic way; stick full back to the stall break, nursing it with subtle aileron and rudder input to get it to drop off on the side you want, then as soon as the nose falls through, push the stick forward to break the stall and keep the rotation going with aileron and rudder. You're really in a tight aileron turn, both wings are flying, but it looks like a spin. Most important, you can stop on heading very accurately. The downside is that you probably lose a little more altitude. You could just hold the stick back, full rudder deflection and neutral aileron, then stop the spin with opposite rudder and stick forward but the stop is unpredictable.

  How does all this relate to models? Not sure, but I thought a different perspective might be interesting. 

  It might also be interesting to know the that the same topics - spins, snaps, judging competence, etc - receive about the same amount of gripes in each venue. No surprise there.

  Jack



  ----- Original Message ----- 
    From: Earl Haury 
    To: Discussion List, NSRCA 
    Sent: Tuesday, October 23, 2007 5:26 AM
    Subject: [NSRCA-discussion] Judging Snaps & spins II


    Posted this as a reply yesterday but it got snagged as too long with the ongoing thread attached - started a new thread.

    Jim

    Don't think that'll work very well. Let's take the analogy of a car moving in a straight line slowly - turn the steering quickly and the car will turn - changing "track". Do the same thing at high speed and the car will skid - track stays mostly the same and only attitude changes. Entering a snap is similar - establish a "skid". In both cases there will be a minimal departure from the original line until "traction" is lost and the skid occurs. Consider that the faster the pitch input the less AOA increase will be needed before rudder application. This is where we get into trouble trying to define a "break" into a snap, some visualize this as needing to be huge while, in reality, it may only be a few degrees. 

    Also - with regard to the departure from track before the "skid". F3A rules require a "separation from the flight path" and AMA rules allow it "track closely maintains the flight path". Numerous things will define the amount of "separation" including rapidity of pitch / yaw, mass of airplane, wing loading, etc. The separation may be a few inches to a couple of feet and is not to be downgraded as long as the aircraft "closely maintains" track. So - if the snap (skid) progresses more or less parallel to but slightly offset (maybe in both pitch & yaw) it's not only OK, but a pretty good indicator that the snap isn't an axial roll.

    Use the description to think through set-up and control inputs. Recognize that a properly done snap entry takes only a fraction of a second (if you have time to see exactly what's going on you'll be getting downgrades for track changes). With the proper set-up and practice a snap entry can be perfect almost every time - that just leaves the exit to deal with. Finish wings level and enjoy the 10!

    Earl


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