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Monday, December 19, 2011

FlyZone Fokker Dr.1 Micro EP RTF - Flight Review

Update: Also see: Fixing the Flyzone Fokker Dr.1 Micro

Update:  After all that was discussed below, the little Fokker's motor shaft reduction gear completely stripped dead on the 8th flight.  I was really trying to give Flyzone the benefit of the doubt.  Now there is no doubt.  Overall grade updated to F for consistent power system failures with both the Albatros and now the Dr.1.  Video of the latest failure uploading now.

Winds in the following video are swirling 5-10 mph mostly on the nose.  You can see the plane torque hard right out of a left correction as the gearbox seizes:  Dissection of the dead gearbox confirmed the plastic motor pinion and main reduction gear had seized together.
Original review follows:
SPECS:    Top Wingspan: 14.1" (358mm)
          Length: 12" (305mm)
          Weight: 1.27oz (36g)
Drag and Drop the pic into the URL input window and
click to magnify to 100% if you can't read the 
box with the standard Blogger picture viewer.
Oh great, another radio!

The cockpit after a little black paint and gluing the 
machine guns. Flyzone should have done both.  Note the 
impression for the included, unpainted foam 
pilot figure, which I decided to omit (see first picture).
Very nice scale details!  The wheels turn independently 
which caused severe pull off to one side during 
taxi--a trivial fix is in my review.   There is only a 
tail skid which compounded the problem.
The wind mostly calmed down for an hour or two.   I thought I could squeeze in several reviews, but got stuck on this one.

First, this mini Triplane looks fantastic in the air, for a micro. The scale lines are impressive for such a tiny model. Even better, it flies a lot like a Dr.1. That's good because it adds interest and challenges the flier, but it is not great news for beginners. This plane is best suited to advanced novices or intermediate to advanced RC fliers, especially in anything but zero wind. Here is why:

Fokker Triplanes are notoriously finicky to balance, as the effect of CG movement is amplified by the tall wing stack.  Imagine that you and a friend are moving her dining room table.  You each take a side, and lift it up.  The placement of your hands matters.  If you spread your hands wide apart, the table is easier to balance.  Imagine that you try to lift the table with your hands only a few inches apart, suddenly the table's CG becomes much more important.  Now imagine stacking a few heavy boxes on the table, in a tall vertical stack at that CG point.

This is the Triplane dilemma.  The wing(s) chord gets thinner as the stack gets higher, to keep total wing area and total weight near the design goal.  It is like placing you hands closer together when you lift a table, and it is like stacking weight high on the table at the same time.  As you lift and move the table around, the situation is a lot more precarious then if you placed your hands wide apart to lift a table with nothing on it.  Now, add swirling wind.

One thing Flyzone did to help alleviate some of the Dr.1's handling challenges is sneak in a little dihedral, which helps alleviate most of the intentionally embedded roll instability inherent to the fighter's tall, squarish wing box.  Without the non-scale dihedral, a Dr.1 falls off to either side very easily, and needs to be constantly balanced with rudder and aileron (which are unfortunately not applicable to this 3 channel micro); with it, the micro baron happily putts along upright, but is not as stable in roll as some might be used to.
Some false dihedral makes the Flyzone Dr.1 a lot more beginner friendly.
Depending on your level of flying proficiency, the Triplane dilemma described above can either sound like fun or sound very painful.  I find it fun; others will not.

The fun arises because the big wing stack is the thing lifting the plane, not you.  So it feels quite effortless, because low wing loading (arising from a large total wing area) is a very good thing for nimble airplanes, as long you keep it all precariously balanced in the air.

The challenges are plentiful:
  • Finding the exact right battery position
  • Controlling pitch changes, as the wing stack tilts fore/aft
  • Controlling roll changes, as the wing stack tilts left/right
  • Landing on the narrow wheel stance without dragging a wing tip
The benefits are powerful:
  • Light, nimble handling
  • Tight turn radius
  • Very slow flight capability
  • High wing efficiency from high aspect ratio wings (a longer total wing span)
  • Short takeoffs and landings
  • Appreciation of the skilled pilots who flew these unique aircraft
 In a few words, I love it.

So what could possible be more fun than flying a Triplane around your own front yard?  Uhm, well, I have an idea... how about flying one that doesn't break down!

Within five flights of CG tuning, the main gearbox apparently failed.  It seemed like the motor shaft shifted and decoupled the motor pinion from the main reduction gear, causing initially intermittent, then a permanent loss of thrust, as the engine oddly surged faster.  The combination of motor surge and decreasing thrust in the air was strange, but familiar to me:  http://z8rc.blogspot.com/2011/08/flyzone-albatros-flight-review.html

Compounding the problem is British Sports car-like motor access.  The plane is buttoned up tight.  I started trouble shooting the still prop during high RPM with a brand new number 11 blade.  After an hour of painstaking cowl removal (if the cowl was tacked with tape instead of glued it would've been simple) it turned out, the Dr.1's eerily similar failure to the Albatros was a red herring.  The gearbox was fine.  In fact, examination of the gearbox indicated that Flyzone had improved it with a much longer motor pinion, presumably to eliminate the Albatros problem.  The thrust problem was actually a more simple, decoupled prop!  The motor shaft was spinning inside of a hollow prop hub. 

The shaft/prop connection is not threaded, it is splined.  This is a minor design flaw, because the prop hub is not solid plastic.  The metal shaft's splines can actually migrate into the hollow interior portion of the hub and lose their grip entirely.  The prop was a bit fussy to pull off while holding the shaft with small needle-nose pliers, but once removed, a dot of CA on the shaft before pushing the prop back in place was a relatively simple, 100% fix.  Uhg!

I won't blame Flyzone for spoofing me with the previously unreliable Albatros gearbox, or my wasted time and risk opening a gorgeous little tightly sealed airplane.  I should have avoided jumping to conclusions. But this less minor problem still resulted in total inflight power loss and (perfect) forced landing.

At least all went well in the maintenance hangar, and I got a few interesting pictures as a result!

Flyzone went through great pains to stuff the servos 
right up next to the main gear in the nose, all to 
make the plane way too nose heavy.
Aside from the obvious FAIL, the plane is really fun in the air.  It is extraordinarily nimble when properly balanced.  Slow flight capability is uncanny.  Loops are very challenging to keep axial due to the height of the wing stack.  And the plane needs to be trimmed well and flown as hands-off as much as possible, to keep it humming (more like screeching) along smoothly and on top of the correct CG.  No wind will help this plane more than most, I didn't have that luxury, but I am really looking forward to flying it on a no wind day.

Speaking of proper balance, Flyzone has the Dr.1 set up all wrong.  Manufacturers like to keep their planes nose heavy to be beginner friendly.  The thought is that nose heavy planes are flyable and tail heavy planes may not fly.  But with a Triplane, this is an even worse idea than usual for reasons stated above.  Flyzone really screwed the dog on the set-up.  Many-a-beginner will crash and crash again.

The nano Dr.1craves pavement with power off and the battery in the design position.  The wing stack tilts forward and so the balance does too, making the CG problem even worse (imagine the tall stack of boxes on your friend's dining room table, tilting forward as you lift, with your hands placed close together, yikes!).

By trial and error I discovered the proper battery position is with the leading edge of the battery at the leading edge of the bottom wing.  So you'll need to stick-on another piece of Velco.  Flying with the proper CG is critical with this plane.
The proper CG requires a battery position well behind the range 
possible in the plane's stock battery bay.  The plane exhibits 
alarming pitch tuck-under using the manufacturers' CG range.
This makes the battery a lot easier to attach and replace 
without breaking the delicate airframe or landing gear.
I do not recommend flying your Dr.1 with Flyzone's criminally nose heavy setup.  The plane tucks-under hard with any solid pitch down, exacerbating your stick forward input and bringing pavement up uncomfortably quickly.  Doing my CG test (see side bar) on your maiden flight will help determine if you should land and re-balance before continuing.

Interestingly, even when balanced properly the plane needs to glide more nose low than most, just to overcome all the drag it is carrying abound.  It is almost like the plane has its own drag chute.  Proving one again that lots of lift = lots of drag.  The lift/drag intensive design makes the Dr.1 naturally happier putting around than screaming into a formidable drag wall.

Another interesting but unrelated characteristic of the Triplane is how hard it is to orient from the ground.  Simply stated, there are wings everywhere.  You have to be really careful when trying to determe if the plane is banked left or right.  Take your time when flying the Dr.1 in anything but good lighting, no false moves.

Next up, a landing gear design flaw.  The large scale wheels turn independently on a shaft that also spins.  Bad idea.  Wheel axle friction differences made my plane pull uncontrollably to the right during initial takeoff.  Two tiny dots of CA on the outside wheel hubs fixed both wheels to the free axle (careful not to glue the inside hub or the axle itself might not turn), removing the possibility of any unwanted and inevitable differential braking on a notoriously finicky, narrow undercarriage. This will also stop the inevitable main wheel departure, see my Albatros review linked above.

Was this plane even tested by Flyzone?

As far as aerobatics, hmmm, well, not so much.  The tiny Dr.1 motor is both loud and slightly to moderately underpowered, even for a floater like the 3-wing.   Simple loops are strained.  Full bore climb-outs need to be measured.  Frantic audible feedback from the motor coupled with low power output lends a false sense of security.

Battery life is excellent; 10-12 minutes with the included 1S 130 mAh LiPo.

Overall, this plane might be the most fun and most frustrating micro I own.  Fun: because the airframe has so much raw capability for a slow, front yard flier, and it flies correctly once set up right.  Frustrating: because it was boofed as tuned by a typical, aerodynamically clueless toy manufacturer and it has systemic quality problems.

Appearance: A+
Gorgeous scale rendition.  Tall wing stack is surprisingly difficult to orient from the ground. 

Airframe: A-
Exceptional low speed capability.  Nimble as hell.  Must trim to fly smoothly.  Too nose heavy as shipped--easily fixable. 

Power System: C-
Underpowered.  Loud. Several reliability issues.

Build Quality/Durability: B+
Prop decoupled after five flights.  Real sub-micro servos.  Solid nose and frame.

Value:  B
Around $70 RTF after typical discounts.  Locked to a toy radio.

Overall Grade: A
Wonderfully scale lines!  Tricky flight characteristics when pushed.  Wrong CG.  
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