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Thursday, April 19, 2012

Select Scale Focke Wulf FW-190 EP RxR Flight Review

Update:  Here is the Fw-190 with a Power 10 running on 3S (4S is not a Power 10 option).  It achieves about the same speed as the stock airplane on 4S.  It might even be a hair faster, but I'm still not 100% happy with an Fw-190 that isn't the powerhouse it should be:

I've also installed servoless retacts, which was not easy.  I used the same $10 retracts I put into my J-Power P-38.  The stock retract are very hard to remove. I had to slice them down the middle with a Dremel metal-cutting wheel, then slide the front half back and the back half forward to remove the base from the foam wing.  Next I had to cut the gear hinge rod in half to similarly remove it.  The cut carbon fiber reinforcing rods that extend back into the wing from the gear hinge point.  To install the servoless units, I had to cut a complete hole in the wing from top to bottom, then glue them in place and re-cover the top.  

The final product is worth the effort.  The servo-driven stock retracts sag too much in the air and collapse under any firm landing.

Update:  70 mph obtained with a 4S and 10x7 prop on the stock motor.

Update: 4S tests added.

Original article follows:

The Fw-190 is a classic warbird with some of the most sinister lines of all time.  Perhaps the most recognizable feature are its long legs and wide stance.  Hobbico came very close to replicating Kurt Tank's masterwork, this is a beautifully executed foamie.

This is the 2.4GHz radio controlled brushless electric powered RXR (Receiver Ready) Focke Wulf FW-190 from Flyzone by Hobbico.  This version includes motor ESC and servos.

 **Additional Technical Info Will Be Added When It Becomes Available.**

      - Construction: AeroCell foam
      - Motor: Brushless
      - Servos: Five micro (1-ailerons 1-flaps 1-landing gear 1-elevator
      - 1-rudder)
      - ESC: 30A
      - Landing Gear: Retractable
      - Propeller: 10x6 3-blade
      - Spinner

      - RTF Focke Wulf FW-190 with five Servos 30A ESC
      - Propeller Spinner and Instruction Manual

      - Radio: 6-channel
      - Battery: 11.1V 1800mAh LiPo
      - Battery Charger
      - Tools and field equipment

      - Length: 39" (990mm)
      - Wingspan: 44.5" (1130mm)
      - Wing Area: 349 sq in (22.5 sq dm)
      - Weight: 2.25 - 2.5 lb (1020 - 1134 g)
      - Wing Loading: 17 oz/sq ft (52 g sq/dm)
      - Center of Gravity (CG): 3" (76mm) from the leading edge
Control Throws: Low Rate, High Rate
      - Elevator Up/Down: 5/16" (8mm), 7/16" (11mm)
      - Rudder Right/Left: 3/4" (19mm), 1" (25.4mm)
      - Ailerons Up/Down: 3/8" (10mm), 1/2" (13mm)
      - Flap: Down 1/2" (13mm)

On the Ground:

This kit comes as an RTF and RxR, so one would expect little-to-no assembly.  The airframe ships in three pieces:  Wing, Fuse, and Horizontal Stab.
The Aerocel foam used in this build is among the best there is:  hard but pliable, lightweight, and very smooth. This is my second Select Scale model, both are stronger and better looking better than Horizon Hobbies Z foam.  For example, the paint doesn't usually peel off with tape application and removal.
The paint job is scale, so I can't critique the utility--it is designed to be hard to see from the ground.   The yellow nose should be a nice visual cue in the air.

The lines of the Fw-190 can't be beat, the plane is menacing from every angle.  At the same time, it looks athletic and muscular.  The stubby nose and resulting short CG moment indicate a monster motor for the size of the airframe.  Hobbico stuffed the ESC up in the nose, next to a reasonably chunky motor to keep the balance right:
The prop mounts on a collete-style adapter, which first goes though a hole in a supplied plastic plastic engine bay cooling fan.  Neat.  The engine's side louvers are functional, they vent the engine bay to outside air, as shown in wide angle shot below:
The Wulf's paint job is tremendous, with nice details and presumably authentic German warnings printed on the plane mostly everywhere you look.

Inspecting and assembling the plane is full of pleasant surprises:

First, the plane is listed as RxR, basically an RTF without a battery, charger or receiver.  Of all the types of purchases, I enjoy RxR/PnP the most, because I like the latitude to use my preferred radio system and I have too many LiPos already.  This plane has a very high degree of in-the-box completion, but it isn't quiiiite RTF, even if it had a Rx.  You'll have to slide the tail plane in and glue it, then install the prop-fan, collet, propeller and spinner.  The ESC wires are on the short side to get the Rx in easily.

The second great surprise was the inclusion of split flaps and retracts.  I actually didn't notice that this plane had either of them when I bought it.  That's a very good thing, because the flap mechanisms were both broken out of the box.  The gear works ok, but the included servo needs a significant total travel increase, on my set up, in order to fully lock down and to fully lock up.  The gear seem to stress the servo a lot.
The flap problem is troubling, since a quick inspection reveals that the flap pushrods turn an interior bellcrank which then extends the flaps.  The bell-cranks are located inside the foam wing, which is pre-glued together, and both of them separated from their glued base and were free-floating out of the box.   The only way to fix them would be to cut an access hole in the bottom of the wing, unless there is a trick I'm missing.  I can't imagine that Hobbico isn't fully away of this quality/design defect.  The good news is that the flaps are rather small and their partial function is a bit asymmetric, so the fix is easy: glue them up and save the servo weight. 

But wait, that's not all... there was a third surprise.  The second time I plugged in a battery to finish up testing the radio function, there was kind of a funny smell.  Sort of... electrical.  Hmmm, that looks like whitish-gray smoke coming from the cowl.  For a moment, I thought the battery might be on fire and thought twice about grabing the plug to unhook it.  But after a quick focus, the source of the smoke now rapidly intensifying, was the interior of the ESC.  Luckily, I was able to yank the battery plug before any flames appeared.  The ESC smoked for another 30 secs, a quick touch revealed intense heat.  Ooops!  That's not supposed to happen.

Five minutes later a trusty old 30A from my parts bin was installed and ready to go.  I used the opportunity to move the ESC backward, just in front of the wing leading edge, to help with a very nose-heavy CG when using a 2000mAh battery.

Speaking of CG, as assembled with an 1800mAh battery (recommended) in the vertical battery compartment, the plane balances about an inch in front of the prescribed CG location.  Moving the battery back, almost to the leading edge, puts you in business. 

Overall, the plane looks terrific and is a quick build, around 30 minutes as designed.  But I spent a few hours anyway, figuring out, then fixing several serious quality defects.  The flaps and gear didn't have enough servo travel as designed, which also takes a little time to work out.  The flaps are no big deal, you may wish to seal them up anyway.  The gear is another story.  Unless you increase servo travel by 25% or so, the gear will either collapse, hang out when retracted, or both.  This could be a trick depending on your choice of radio.

In the air:

The 190 is a typical WWII warbird in that the control surfaces and throws are reasonably sized, if not small. I decided to set up my rates biased toward maximum, with 125% being in highest rate switch position. I'm glad I did that.

Taxing out was uneventful, the plane clearly has good pull. A nice positive run up to full power for takeoff brought the rudder to life and the plane lifted off the ground after a 25 foot roll, with a minor toque tug to the left. The Focke Wulf has enough power to climb out 60-70 degrees nose high without much fuss, but it is no 3D machine. The plane needed quite a bit of nose down elevator to achieve a solid speed level cruise; flush elevator seemed to be set up for the forward CG, as shipped, not the instruction manual's recommended CG which is how I set the plane up.

Even with the more aft CG (the flight manual position), the 190 tracked beautifully in the air once trimmed up. After the straight tracking, the most interesting characteristic is a lack of motor noise--this plane is exceptionally quiet. Part of the quiet nature of the plane is good strong power system, only running at about 30% throttle for metered flying.

Unlike most RC warbirds, this one has storng slow flight ability. Bringing the throttle back to just 25% brought the Fw to a nice medium-high alpha attitude in level flight. The plane looked so relaxed and silent, I wasn't 100% sure the motor was running. The Fw tools around slowly without a whole lot of fuss, but you have to stay on top of the yaw axis with solid rudder corrections. Getting slow, even at low power settings, torques the nose left, so you need preemptive right rudder. Failure to keep the fuse aligned with airflow results in a mild snap roll to a nose-down spin.  At any point in that chain of events, a spin-prevent (relax back pressure to slight forward stick, along with a positive application of opposite rudder) brings enough rudder to bear to stop any auto-rotation.
Flight control authority is normal for a WWII style aircraft.  There is enough rudder authority to upset the apple cart with full application, but nowhere near enough to sustain knife edge flight even though the power system is plenty strong.  Aileron throw is very good, but the ailerons aren't big enough to rapidly roll the plane; the achieved roll rate at 125% servo movement is reasonably spirited, but not excellent.  Adverse yaw is significant due to the moment arm of the high throw ailerons located so far from the fuselage.  Elevator authority is more than adequate, and the wing supports impressively tight turns.  With full throw on the elevator, demanding too much pitch results in a sharp, uncommanded roll as one side of the wing quickly  decides enough is enough.

The plane has plenty of power with the stock motor and prop running on 3S.  Torque from the viscous 3-blade needs right rudder whenever the plane is slow, to include initial takeoff, which requires about half of the available right rudder or the plane wants to veer to the left side of the runway.   I visually aligned the elevator with the horizontal stab, which turned out to be unnecessary once airborne.  The plane wanted to climb very strongly at first, so the stock setup was probably closer to the final solution.  I also visually aligned the rudder with the vertical stab, that will not counter the left torque in cruise--try about 1/8th inch of right rudder deflection when the stick is neutral for straight line flight at 50% throttle.

Once the plane accelerates to a comfortable cruise, left yaw is easily countered with a dose of right rudder trim.  As discussed above, without a mixing radio and a lot of experimentation, the Fw-190 needs a canted rudder to track straight and level, which means the plane will gently spiral to the right in a hands-off glide--including the end-game flare and landing.  The only way to eliminate that tendency, while still tracking straight during cruise is with a Throttle>Rudder mix.  I'll post the mix that works for me as so as I have time to iron it out.

Top speed is good, but not exactly rousing.  The draggy 3-blade holds the plane back in the zip dept.  I'll move my Parkzone Fw-190 2-blade spinner over and retest the plane with a few 2-blades...

Ok, here are the numbers we are looking at using an average battery:

Select Scale Focke Wulf 190
3S 25C
Stock 10x6x3-blade
3S 25C
APC 10x7
3S 25C
MAS 10x7
3S 25C
GWS 10x8x4-blade
3S 25C
MAS 10x7x3-blade
4S 25C
Stock 10x6x3-blade @ 55% 
Throttle Limited
4S 25C
APC 10x7
@ 100%  Throttle

Note: the first 4S solution choked (screeched and wound down) above 57% throttle, so I limited the 4S 10x6x3 thrust curve to 55% by computer radio.  It is unclear at this point if the ESC or the motor is the problem, but this partial solution still added 150W to the previously most powerful solution.  More testing required to see if the engine choke is an ESC, Motor, RPM, Amp, prop drag/torque or a thrust limit. 

A second test with an APC 10x7 registered almost 11K RPM at 100% throttle, and a pitch speed of over 70 mph.  This indicates that prop drag/torque may have been be the limiting factor on the 3 blade test.

These preliminary results were obtained with a Hobby King 40-50A ESC driving the stock motor.

Max Amps
Avg Throttle
Battery mAH
Flight Time

Hobbico doesn't publish the motor's specs, so it is unclear exactly how far we can push the power solution.  The base prop works fine and presents a reasonable compromise between performance and looks, so there is no need to change.  Only one 3-blader is supplied, so it is a good thing that if/when one landing gear collapses the other main gear standing alone is long enough to keep the prop from striking the ground (the landing gear had a tendency to collapse until I increased servo travel and adjusted the travel end points.

If you want longer flight times and faster passes, the MAS props look like the best way to go. For absolute performance, the MAS 10x7 was the most effective with 33% more speed and the most efficient with 36% longer flight times.  In the "scale-look" category the MAS 10x7x3-blade was the best choice tested, with 24% more speed and 7% longer flight times.

This plane flies very nicely and is more stable than most warbirds, due to the built-in wing dihedral and long tail.  It is comparable to, if not easier than a PZ T-28 Trojan in ease of flying, but a better turning airplane at the limit.

The setup ships with a far forward CG, but even with the CG moved to the manual's recommendation, the plane dives strongly in idle.  The CG could slide a little farther aft still, for ultimate maneuverability.  Even so, loops can get impressively tight and rolls are naturally axial, minus noticeable adverse yaw tug on the rising wing.  The adverse yaw tend to help keep the nose from falling as much when the wings are slicing along the gravity vector, so it is not all bad.  Still, an aileron roll will drop the nose by 5-10 degrees per 360 without mid-roll rudder and some down-elevator application when inverted.

The gear up position looks more realistic as the plane effortlessly rolls into strafe, terrorize, and lay waste to the good people of the world.    Surprisingly, lowering the gear at pattern speed doesn't generate much of a pitch change, though the speed clearly bleeds.

Landing is easy once you become accustomed to the sensitive but fully predicable elevator--nothing a little D/R and Expo can't smooth right out.  Three point touchdowns quickly become the rule rather than the exception, and the plane rolls out very straight as long as you avoid the temptation to over-correct.

The landing gear is quite complex, and includes spring piston shock absorbers and plastic extension guidance hardware.  And that created my final maiden voyage surprise: after I taxiied over and picked up the plane, a main wheel feel off.  Hmmm, probably not the best plan to use a axle nut that one wheel naturally spins to loosen.  Nut lost.  Replacement size hard to find. 

Another problem with the gear is the plastic elbows providing directional guidance for the mains.  The plastic is so flimsy that the wheels tend to wobble.  Another easy fix, super glue the spring pistons so nothing moves.  Shock absorbers are overrated, anyway.

Appearance: A+
Nice scale rendition.  What's not to love?

Airframe: A
Nice foam.  Easy assembly.  Strong pliable foam and surface.

Power System: B+
Not a bad stock motor so far.  Battery strap is useless.

Build Quality/Durability: F
Great airframe.  Crap components.  Dangerous ESC malfunction.

Value:  B+
Better than more expensive Parkzone flyers in this class.

Overall Grade:  D+
Super foamie, minus a little issue with spontaneous combustion.

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