Flying and Judging Snaps
with Dave Lockhart:
I think snaps are precision maneuvers. I don't think luck with snaps is any different that luck with spin entry/exits or stall turn entry/exits. All are difficult to consistently perform without downgrade (especially the snaps), and that is generally reflected in the KFactor
The reason stall turns are often performed conservatively is simple - a zero for a flopped stall turn is death - the scoring is usually so close at the top, that a single zero will take a pilot out of the running for the 1,000. I would say at the top levels, most pilots are 90+ % to complete a tight radius stall turn (the one that appears to rotate on the CG, and is not subject to a downgrade). The same pilots are 99.9% to complete the slightly open radius stall turn that will get the minor downgrade. If we as a pattern community want to see less conservative stall turns, all we need to do is increase the point deduction for the slightly open radius stall turn - the risk has to equal the reward, so to speak.
Two points (my opinions of course) -
1) If it is
indeed a true snap (and not a twinkle roll or barrel roll), then it does add to
the capabilities of the pilot.
2) Until we can achieve a level of
consistency in judging snaps, they detract from the event.
Now, the long part, including some of the "secrets".
Airplane design - In general, in the early/mid 1990s, most designs migrated towards increased pitch stability and damping (which is often but not always synonymous with long tail moment). This was great for groove/lock in corners and lines, but not good snaps - and perhaps not a bad bias considering the relatively low number of snaps in the pattern. The increased pitch stability made it more difficult to setup a plane to do good snaps (and spins). The same designs also generally migrated towards increased yaw stability (actually good for snaps, bad for spins, others will not agree). My personal setups used as much as 35 degrees of elevator to achieve consistent stall entries to snaps and spins (and all of the elevator remained during the spin, but not during the snap) and 40 degrees of rudder in spins (ele/rud only spins). In recent years, the number of snaps in patterns have increased, and the newer designs have shifted towards less pitch stability (and less yaw stability, but that change is a response to the combined rolling/looping and rolling circle maneuvers).
Airplane setup - This is critical with any design, and when a snap is attempted, a couple things can happen - - a good snap. - a twinkle roll (not a snap). - a barrel roll (not a snap).
The majority of the setups I see are not sufficient to consistently perform nice snaps. Consistently meaning the setup is not very tolerant of differing weather conditions (hot vs cold, snapping in/out of relative wind), pilot techniques, or entry airspeeds. I have on more than one occasion been asked to check a setup for snaps when the owner of the plane was having problems with snaps. In some instances, I've been able to consistently snap a plane with no changes to the setup, but improvements to the setup made it easier for the owner to consistently snap the plane. The below descriptions are generalizations based on my experience flying a variety of planes and observing a variety of planes and pilots from a variety of perspectives (amazing how much differently snaps look from different perspectives). And, the below does not take into account the effects of wind on track and offer relative descriptions of control throws (exact control throws are influenced by design, weight, pilot technique, etc). The most critical setup element is pitch - if enough pitch authority is not available, the break can not be achieved - the adjustment to make is more elevator throw, or a more rearward CG (and if neither work, the design has too much stability in pitch to snap properly).
- The "twinkle" roll - The plane that has too little elevator, a little bit of rudder, and a boatload of aileron. Very common. The plane has little or no displacement from track (or the path is an extremely tight barrel) during the "twinkle" and returns to the original track with zero displacement or deviation in track. The rotation rate is probably equivalent to a full stick deflection aileron roll (might be marginally higher due to a slight asymmetry in lift between wing panels caused by the minor amount of rudder used). The tail follows the nose exactly and no "cone" is inscribed. The advantage of flying a "twinkle" roll is that there is very little if any loss in track, the entry is non-critical, airspeed loss in the "twinkle" is minimal, and the only variable the pilot needs to concentrate on is getting the wings level at exit. As with the stall turn, it is a risk/reward thing - why risk doing a real snap if the "twinkle" is getting scored? The other benefit of the setup for a "twinkle" is that less elevator throw is used, and that makes the plane smoother in all other maneuvers.
- The "barrel" roll - this one can be easy to see, or hard to distinguish from a snap, and the setup that yields this result is often variable. A very tight diameter barrel roll is hard to distinguish from a good snap because it will exhibit a pitching element on entry and the plane will displace from track in pitch and yaw during the snap, but will generally return to track for full snaps (1, 2, 3....rotations) and generally remain displaced of the track (in pitch and yaw) by the diameter of the barrel for 1/2 snaps (.5, 1.5, 2.5 ....rotations). Very little if any cone inscription is visible after the entry to the barrel. The large diameter barrel has the same characteristics of tight diameter barrel, but easy to see, simply because of the large diameter alone. Any change in the control inputs during the "barrel" are generally very easy to see - because the plane is not stalled and responds readily to any change in surface deflection. Insufficient elevator to initiate the break is generally the cause of barrels. Adding aileron usually increases the rotation rate, and often reduces the diameter. Adding rudder may reduce the diameter of barrel, increase the rotation rate, give the appearance of the inscription of a cone (but it is only in yaw, and not in pitch, and the airplane is not in a stalled condition), and may also add the look of a "cartwheel". The benefit of the "barrel" setup is that less control throw is used (smoother in other maneuvers), the rotation rate is slow (easier to hit exit) and the track is unchanged (but might be displaced).
- The "good" snap - I hope we know what it is in writing by now (actually seeing it takes practice), so just some notes. Theoretically, the break can occur without displacement in pitch - but this never happens in practice (angular acceleration, mass, and other physics properties are real.........blame Newton). A good clean break (very fast elev servo and fingers) will reduce the amount of track deviation and displacement in both pitch and yaw. The rotation rate of the snap will be marginally effected (if at all) by low / high rate aileron - ailerons don't work very well when the plane is stalled.
Piloting Technique and more setup - To me, a good setup for snaps has a bigger "sweet" spot - the exact entry airspeed and exact timing of the control inputs can vary and still yield a good snap. And the good setups "feel" the same whether snapping left or right, positive or negative. I fly with triple rates - normal flying is rud on high, elev and ail on medium. For spins, everything is on high. For snaps, I use a variety of rates setup to make the variety of snaps feel balanced and I attempt to get the snaps from different perspectives in different maneuvers to "present" the same. It takes a lot of time to setup. The technique for all types (different rotations in different attitudes) of snaps is similar (lead with elevator, then rudder, then aileron) but the timing of each input usually varies a bit on both the addition and removal of the inputs. And each specific snap is usually a little different - I "feel" the plane approaching the entry to the snap and try to gauge exact airspeed, wind conditions, evaluate what the track is and if I want to change the track, etc.....and then I make very small changes to the addition of the inputs to get the desired effect (ie, I might use more elevator lead to change the track in pitch if the plane was diving slightly in pitch track prior to the snap, or lower than usual on airspeed). During the snap itself, I watch the progression of the snap and decide if I got the entry I wanted (expected), and if I need to make any changes on the timing of the removal of control inputs on the exit of the snap (ie, maybe hold the same rudder or immediately go to opposite rudder on the exit to immediately correct a deviation in yaw track). The more I fly, the better I can make these adjustments because I seem to see things better, the snaps seem to take longer, my fingers seem to move faster, and I better know what the mental image of the plane should be at any instant during the snap.
"Tells" and "cheats" One of the easiest snaps to judge is the 45 downline snap (airplane is upright and executing a single positive snap). The airplane is front and center, and the break in pitch is easy to observe - and it is actually easy to confirm. Odds are, if you don't see a break, you will see the track of the line steepen as the airplane passes through 90 degrees of rotation - because the rudder is now "bottom" rudder (and the plane is still flying, or if snapping, the break was in yaw, not pitch), and the track of the plane will be steeper than 45 degrees at the finish of the snap. Kinda of odd to see a 45 downline get steeper after a positive snap?? Big tell, and pretty common to see. I see the same thing on 1.5 snaps on a 45 downline. The same thing also happens on 45 uplines when the line substantially flattens out after the snap, but with an added variable - the flattening may be partly due to reduced airspeed and gravity (but the plane will show the initial loss in track at the 90 degree point in rotation).
Exits - exits are generally judged more critically than entrances - for snaps, rolls, and radii. A "cheat" to help with the exit of a snap is to take out elev and rud before the snap is complete - such that all the pilot is doing is completing a roll to level flight, and completing a roll is easier than completing a snap - the tell is a substantial change in roll rate (rotational inertia exists and for our models, the heavy wing monster that appreciably demonstrates this is rare). This type of cheat is also employed on spin exists and on the exit of part and full rolls. The roll rate is slightly reduced as the rolling element nears completion to make it easy to nail wings level. Most judges will downgrade for the wings not being level, fewer will catch the change in roll rate. The same thing applies to corner radii - they are often softer close to the exit to make it easier to hit the line. The reason the cheats are employed is because downgrades are not being applied - "no whistle, no foul".
How do some guys maintain distance during the entire flight (including stall maneuvers) when flying in a crosswind? They don't. They are just sneaky at hiding the wind drift, and make corrections / allowances where they are the hardest to detect. Humans having pretty poor depth perception at 150 meters also helps. This past fall, I had a trusted observer stand in the field underneath my flight track and was told that the flight track was about 145 meters, +/- 3 meters (a bit more variation after spins). And the observer was surprised to see how visible the rudder corrections were - as the same observer saw far fewer rudder corrections when viewing similar flights as my caller.
Prior to spins and stall turns, you will usually see a very small "cheat" into the wind, and then a little drift with the wind allowed immediately after the stall maneuver, followed by a small "cheat" into the wind to return to the original track. A single 15 degree change in track is easy to see and downgrade. Very aggressive compensation for wind (difference between attitude and track) while technically correct rarely scores well. Three 5 degree changes in track are harder to see and harder to downgrade, and present nicer (even if not as correct technically).
With snaps - to hide the lateral displacement and yaw track change in a snap, the "cheat" is normally opposite the direction of the snap prior to the snap entry - 5 degrees nose in before a snap and 5 degrees nose out after the snap is harder to detect than simply being 10 degrees off after the snap. And of late, there is no doubt in my mind that being 10 degrees off before the snap will earn little or no deduction if the exit of the snap is nailed. All deviations in roll, pitch, and yaw track should be equally downgraded whether before or after the snap - it is a goal that needs to be achieved. The change in track during a snap can often be hidden (or mitigated) to a large extent by wind - this is simply smart piloting by choosing to snap into (usually) the wind. And in the instance of a snap at the end of the box, where the displacement or loss of heading in yaw is particularly easy to see, a more pronounced and aggressive break in pitch can be used to minimize the deviation in yaw, as the deviation in pitch is harder to see (and thus harder to downgrade).
Regards,
Dave Lockhart
This article was published in the Kfactor in July 2005 by Don Ramsey.
Let’s talk a little about snaps, everyone’s favorite
subject. Here’s what the AMA
regulations say:
Snaps—A Snap roll is a simultaneous, rapid autorotation in the pitch, yaw and
roll axes of flight in a stalled wing attitude. The following criteria apply:
Seems to be some mutually exclusive elements in the description but lets look a little closer. The first sentence says a snap is a simultaneous, rapid autorotation in the pitch, yaw and roll axes. That’s the snap not the initiation of the snap. Under 1. there is the wording “(initiated by a rapid stall of the wing induced by a change in pitch attitude)” and a little further down “A snap that does not show a break and stall to initiate the snap, but does enter a stalled attitude during the maneuver is severely downgraded.”
As a judge what if you don’t see the initial break but the autorotation does have the classic cone of the nose and tail? Is it a zero? No, not according to the last sentence in the prior paragraph. It is severely downgraded! Surprised? What if the aircraft comes out of the stalled condition before the required rotations? This should be downgraded using the 1 point per 15 degree rule.
Of course there is always the question of displacement from the track. You can say all you want about flying full scale, IMAC or any other discipline but here’s what our current rules say about track, “while the track closely maintains the flight path.” That’s from item one under snaps above. Also, reread item 3 in the definition. There’s the “constant flight path through the maneuver.” again.
Here is the Judges Guide from the FAI regulation on snaps.
5B.4.3.5. SNAP-ROLLS A snap-roll (or rudder roll) is a rapid autorotative roll where the model aircraft is in a stalled attitude.
Snap-rolls have the same judging criteria as axial rolls as far as start and stop of the rotation, and constant flight path through the manoeuvre is concerned. (Displacement again)
At the start of a snap-roll, the fuselage attitude must show a definite break and separation from the flight path, before the rotation is started, since the model aircraft is supposed to be in a stalled condition throughout the manoeuvre, If the stall/break does not occur and the model aircraft barrel-rolls around, the manoeuvre must be zeroed. Similarly, axial rolls disguised as snap-rolls must be zeroed.
Snap-rolls can be flown both positive and negative, and the same criteria apply. If the model aircraft returns to an unstalled condition during the snap-roll, the manoeuvre is severely downgraded.
Seems that it is saying exactly the same thing as AMA except one critical element is missing. It is not stated that it is only a downgrade if the snap is not initiated with a pitch break. The last world championship judges scored snaps a zero if they were not initiated with a pitch break. Guess that’s the way it is but it can certainly be inferred that this should only be a downgrade.