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Saturday, October 23, 2010

Fixing Flyzone's Select Scale Super Cub

Updated in yellow.

A flurry of Super Cub's have hit the market, it's an embarrassment of riches.  Flyzone's entry, their Select Scale Super Cub is one of the best looking Cubs on the shelf, and it comes in a PNP and RTF version.  How much more could one ask?

Well.... a lot, as it turns out.

This Cub looks so good and is so reasonably priced, I guess something had to give.  Flyzone has managed to embed all sorts of nasty vices in a simple Cub, and that's really hard to do.  They must have people who specialize in complicating the handling of simple planes.  I know, I know, every review of this plane is 100% raves.  Well, the plane isn't a great flier; it actually flies rather poorly in stock form.  Those websites only want to take your money to pump their paid sponsors; time for a dose of reality: 

Photo shows various upgrades and fixes, including:
motor/prop upgrade, LED mod w/wiring in the wing leading
edge and landing lights visable, gear and empennage bracing, 
and increased Dihedral.  I use clear mailing tape for strength
on all leading edges and to create "handles" at the wing root 
to avoid dirtying the foam over time.

The good news is that Flyzone's numerous mis-steps are, for the most part, fixable.  It is a basic Cub underneath, after all.  Here how to fix Flyzone's list of mistakes:
First, a list of the largest flaws:
  1. There is essentially no dihedral.  Perhaps this is true to scale, but the scale Cub is designed to be strong and cheap, first and foremost.  There is no reason to duplicate a straight wing in an RC trainer, when you can have massively improved handling by sneaking in a little wing angle--unless--your engineers don't understand basic aerodynamic trades, and Flyzone's clearly do not.  There is slight negative stability, meaning, if you set the Cub out of level, it will slowly increase, not decrease both pitch and roll until it hits the ground.  This holds true through out the entire flight envelope.  That's crazy--for a Cub.
  2. There is not enough geometric or aerodynamic washout, impairing slow flight handling.  See #8 for a related vice. 
  3. Empennage construction is too weak; the alignment of the horizontal stabilizer "floats" in flight, inducing slight, unpredictable roll with solid pitch changes.
  4. The Cub's wingspan is too wide for it's length, and/or, the size of the vertical stabilizer is ineffective.  Adverse yaw is excessive.
  5. There is erroneous reverse-differential aileron movement baked into the internal hardware mechanism, exacerbating the adverse yaw problem.
  6. The gear suspension is excellent in function, but the top spring wires are too weak to handle a medium bounce.
  7. The Cub is significantly underpowered with a full battery charge, and becomes severely underpowered at half battery life.  The plane struggles to get over-the-top with a fresh battery, falling off to one side unless the entry is down-hill.  Takeoff is tricky unless your extended runway center-line is  free of obstacles.  
  8. Traffic pattern stall tenancy is to snap into a tight, 90 degree nose-low, rapid spin.  The worst full stall handling I've ever seen.  
  9. The paint is inconsistent in color.  The cowling is a non-matching pale yellow, while the ailerons are a the warmest yellow on the plane and they don't match the wing color.  The fuselage changes color from nose to tail.
  10. The standard wing fit is loose, possibly adding to some of the Cub's squirrelly yaw behavior in the air.

    Fixes so far:
    1. Insert a nut between the metal wing strut and wing surface to increase dihedral.  The increased static stress on the strut seems reasonable, and will likely disappear in the air.  I used #4 1/2" wood screws through 1/4" nuts and washers.  Do not over-tighten.
    2. Insert both a nut and a washer between the rear struts and wings.
    3. Add carbon fiber bracing to the tail section (shown).  I penetrated the foam by about 1/8" then anchored with a tiny drop of Gorilla Glue.
    4. I recently added a clear plastic vertical stabilizer extension in front of the tail wheel, sticking down, F-16 style.  I'll provide a picture and flight report if it helps enough.
    5. Mix approx. 10% rudder in the direction of aileron movement. This will make adverse yaw worse whenever you are flying inverted, but that overall trade is probably worthwhile.
    6. Add carbon fiber bracing (shown).
    7. Change to a 9x3.8 prop for a little more pull.  Use higher C rate batteries.  Switch to a 400 or .10 size motor.  The Super Tigre 400 ($20) is a drop-in replacement using the Cub's own plastic mounting bracket.  The Super Tigre .10 ($23) also drops in with 1/4 inch tube spacers (since you can't use the included plastic mount adapter) but the bottom two mounts must be zip tied (still a stronger mount than the foam nose that holds it all together).  The ST 400 saves weight and adds about 50% more power given a large enough prop diameter to translate it's lower kV into more thrust (like a 9" to 10" diameter slo fly prop).  The ST .10 is the same weight as the stock motor and adds about 300% more power.  If you keep the cheap, low gauge ESC, make sure you don't pro-up the upgraded motors beyond 18 Amps.  The ST .10 only pulls 14 Amps max with a 9x3.8 (compare to 54 Amps with the Bf 109 3-blade) with less than 50% throttle required for spirited flight, and battery life is as good as stock (about 12 minutes from a 1350 mAh 3-cell).  With a .10 motor and the small 9x3.8 prop, Thrust-to-Weight ratio is only slightly less than 1:1.  To handle the increased thrust without excessive climb and yaw under power use 2 washers under the top right engine mount (model's pilot perspective) to angle the trust vector down, and use 3 washers on the left mount to create a (very) slight right thrust vector.  The prop shaft should still fit through the hole in the cowl, barely.  See the 1st pic below.
    8. Fix #2 helps.
    9. No fix required.
    10. With the wing removed, add two strips of very thin foam weather stripping to the top of each side of the cabin area, to seal the wing/fuse attachment lines.  I also dripped a few drops Gorilla glue down around the base of the small pole that holds the wing mount bracket in place, so its held more securely by the foam around it.  100% fix.

    The Cub is easy to retrofit with ultra-bright Radio Shack 4-pin square LEDs.  These LEDs operate on 2.3V to 5V.  They are almost too bright to view comfortably at night with one LED wired in series for each Lipo battery cell, and they run hot enough to harden the Cub's "aerocell" foam after a few minutes of stationary use, so consider running 4 in series (12.6V/4 = 3.15V; falling to about 2.5V per LED when the battery is empty).

    My Super Cub (shown) has 6 LEDs, 3 wired in series on each side of the plane (the colored Navigation Lights are double-LEDs).  The Nav Lights face inward, toward the fuselage, lighting the Cub up like a billboard at night, and reducing the blinding effect when the plane is viewed from the side.  You can see the LED on/off micro toggle (also Radio Shack) in the photo, above.

    Z8RC's rating for Flyzone's Select Scale Super Cub: B-.  Great looks, good price, no power (easily fixable +$20), aerodynamic design incompetence (mostly fixable).  I hope to get this plane to an A- flyer with the listed fixes--I still have to finish up some testing.

    UPDATE:  The fixes outlined above bring the FZSSSC to A level handling.  Stability is noticably enhanced, and the plane is eager to float effortlessly behind it's 1 oz  lighter nose (1 oz saved by the motor, another by removing the factory ballast between the cowl and fuselage side walls).  Increased thrust is welcome in all flight regimes, but most helpful is instant activation of the smallish rudder during the takeoff roll, following quickly by very strong climb-out capability.

    The Cub's new found dihedral and wash out is especially successful, allowing the plane to finally become docile and agreeable about where you point it.  One thing you might notice is a need to increase up-elevator a bit more aggressively as the plane slows near touchdown speed.  That is because the wing tips are now at a lower angle of attack than the wing roots, decreasing tip lift, meaning, the stall will now develop at the root first, reducing any tendancy to drop a wing hard.  A side effect is less overall lift at any given incidence angle, meaning you might need to raise the nose a bit more as the plane settles into a now more symmetrical stall condition.

    I added a clear plastic shark fin under the empennage, in front of the nose wheel to set her even straighter.  Great improvement in the willingness to glide-- the Cub takes a few dozen feet (less than before) to chin-up from a TP stall, hands and power off, but once it settles into a glide the nose stays up and the speed peels away until it appears darn near motionless - luv the improvement!  Now that's a Super Cub! 

    (could use one more washer per strut!)

    I'm currently using one nut + 1 washer in the front and 2 nuts in the back.  Here is a picture of the clear plastic vertical stabilizer extension, it helps a lot but if I had to do it over again I'd make it 2about 20% larger.

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