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Saturday, June 16, 2012

Quick Take: electric flight Magazine


Z8RC Quick Take
Killing time at the airport, I was shocked to see an electric flight Magazine in a small newstand - they never carry RC mags in airports - so of course I bought it for the plane ride.

A paragraph into the cover article, How to do a "Cuban-8 with a Twist" I almost vomited on the poor soul sitting in front of me.  The technical quality of the "how to" was so abysmal, so completely wrong, and just plain humorous (in a bad way) that I had to blog this correction for fear that someone might actually listen to the ground-pounders who write for electric flight and become aerodynamically retarded forever.  You could do irreversible damage to your cortex if you read this ridiculous magazine article.  Run away!

A long time ago, when I was instructing in the T-38A now C, a foreign exchange IP was sitting at the desk next to me introducing a loop to his US stud.  He picked up a Talon on a stick, carefully leveled in the air, made direct eye contact with his SP and began, "To doa a loopa, you musta pool pool pool pool pool pool pool pool pool (making the wooden model climb up and over the top) pool pool pool pool pool pool pool pool pool pool pool pool pool pool pool pool pool .  Youa understanda?"

I caught the students eye and we almost laughed our way into the legal office to clear up an international incident. The quality of instruction in the ef magazine article isn't any better.  It might even be worse!

So, I don't claim to be an aerobatic expert.  No way, I only have a few thousand hours of pure aerobatic time in 1:1 scale which not nearly enough to master the craft or even breach beginner status, 20,000+ hours of upside-down time would make one a lot more solid.    And so I'm not about to start off this article with something as arrogant as "When I first started aerobatic competition..."  That's for dorks without a clue.  6 words into the article, the dude lost all credibility.

The first section is called, "First Things First"

Here, the guy talks about his Carbon Z Yak.  Seriously?  You "compete" with a mediocre-to-crap airplane like a CZ Yak?  Then he talks about setting up "Flight Modes" which, he explains, means combining muti-axis dual rates on one switch.  Huh?  Obviously from the BNF foamie plug, he's a Spektrum user.  Aside from never having used a passable radio, to the point of not even knowing what the term "Flight Conditions" means, what does this mini advertising binge have to do with C8 execution?  I guess CZ Yaks are piling up in the warehouse, again.

Next is In the Air

More wasted real estate talking about how to help limit the severity of tip stalls, which is not a typical problem but a big problem in the brick CZ Yak-54. 

Finally we get to the "Overview of the Reverse Cuban-8 with half rolls"

Ok, so what kind of C8 doesn't have "half rolls?" again?  Sheeze.  Let's humor the poor guy and agree there is such a thing as a well defined reverse C8 and move on.

The first thing he tells you to do is climb to 400 for a 50" model.  I hate fascists.  Who cares how high you start when any good aerobat can go over the top in more, or less, than 400 feet?  The only real difference driven by altitude changes is how far away your offset-aimpoints will be on the down-lines, but more on that later.

Next he says, when the airplane is 50 feet in front of you (huh?) push over to a 45 degree line and set 40% throttle (...what motor, battery and prop are we using again?  ...and what's my throttle curve?).  Then fly a "short line and perform a half roll with the airplane directly in front of you.  Fly another short line equal to the first.  Increase throttle and do a 3/4 outside loop then establish another 45 degree line and decrease throttle."

No instruction whatsoever.  He even breaks it down by repeating the above steps under their own subheadings, but only adds a few adjectives to the cut and paste, like "slowly" decrease the throttle and a "gentle" outside loop.  Sheeze, make the pain stop.

Let's start with correcting some of the wrong info...
  • The rolls of a C8 have to be reverse direction, not in the same direction as shown in the mag's fold-out diagram.  A C8 with two rolls in the same direction is thrown out.  
Now, in this "competition" flyer's defense, most RC judges are every bit as cluedo as he is, if not more so, so you can probably get away with same-direction C8's til you run out of room in your AMA-sponsored trophy case. The reason the rolls must be opposite direction is obviously to set a constant difficulty airplane agnostic skills test, especially with torque in play.

 If only it were so easy to "perform a half roll" (rolla rolla rolla rolla rolla rolla rolla...).

When you roll an aerobat, you cause more lift on wing and less on the other.  The increase in lift on the up-going wing (the one that is increasing in altitude; aircraft orientation doesn't matter) results in more drag too, so it gets tugged backward (adverse yaw).  That yaw must be anticipated and corrected with a rudder blend that keeps the aircraft's longitudinal axis aligned with your intended flight path from roll initiation to roll completion.

The difficulty is that when the aircraft passes the point where it transitions through 90 degrees of roll, and goes from erect to inverted flight, you'll need to swap the required rudder direction while adjusting the rudder-blend needed to guide the aircraft's flight path through Knife Edge.  In other words, countering adverse yaw takes pro-aileron rudder when erect, and opposite-aileron rudder when inverted, blended at a rate adjusted to keep the nose from dropping as you transition through the 90 degree bank points.

Because of this, you should unload the airplane prior to rolling to minimize the G and thus lift being created. Unloaded (closer to 0G on the airplane) rolls are much more axial.  

Add constant left (pilot perspective for clockwise rotating engine) torque and the required left rudder inputs generally go away or decrease, the center position requires some right rudder, and the required right-most rudder inputs are amplified.  You must roll in opposite directions on the two leaves of the 8 to make aileron/rudder coordination equally challenging for all aircraft configs and motor types.
  • The author says it's important to do the maneuver at a constant airspeed.  That way you "can count out" distances to make them equal.  
Huh?  We aren't inside the plane, you can see the distances from outside the plane and make them equal.  And this is not a constant speed maneuver.  In addition to the roll/yaw coordination challenge above, the aircraft must stay on an exact 45 degree down-line, even as it inevitably accelerates down hill.

What does 45 degrees look like?   If you are in the plane, put you a$$ on the horizon or check canopy attitude lines on the wire guides.  What does it feel like?  Maybe 0.7g; correspondingly light in the seat.   But the RC community needs neither technique, because we have the luxury of viewing the plane externally and eyeballing the angles to make them look just right, as judged.

Once established on the downhill 45, the plane will pick up speed.  It's ok to throttle back as long as you can still get over the top, but most full scale aerobats would struggle over the top from a standing start.  As the plane speeds up, the absolute best way to set the 45 line in stone is to pick a stationary aimpoint on the ground and drive your eyes straight to it, regardless of changes in aircraft attitude.

As the plane goes faster, the wings will want to lift and cause your aimpoint on the ground to walk toward the horizon, so you have to drive pitch (thus angle of attack) lower, whether erect or inverted as you progress toward the next loop entry point, to keep that aimpoint from drifting.  When transitioning through 90 degrees of roll, apply rudder as required to stick the ground aimpoint dead stationary.

So there is pitch coordination required on the 45 lines, too.

If you start the maneuver high, your two 45 degree down line aimpoints will be far outside the center reference of the looping portions of the C8, and harder to gauge.  As you bring it down lower, the two 45 line aimpoints will become geographically closer together.    They should be equidistant from show center.
  • He says the half rolls should be accomplished using the same roll rate.
No.  That's wrong.  The 45 degree down line rolls should start and stop and the same points in the sky for symmetry between leaves from God's point of view.  If there is wind, that requires varying your rolls rates.

...more soon...


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