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Wednesday, May 23, 2012

Soft Throttle

I've gone to a soft throttle approach with my powerful airplanes.

Some helicopter ESCs use a "soft start" to protect the gears from too much power too fast on initial start. I noticed a different, but similar issue on my hopped up 4, 5 and 6 cell brushless aircraft: ESC timing gets ahead of motor spool-up time due to the high mass and subsequent momentum of the larger bells, props, and spinner assemblies, often causing the motor to choke back to a hard idle reset.

I discovered the simple trick of setting a servo speed delay between 0.5 and 1.5 seconds on the high side of the throttle curve cured every one of these ills. There is actually no slowing of throttle response, since the physical momentum of the motor prevents it from spinning up any faster, regardless.  This idea simply matches the ESC timing to reality for reliable throttle operation.
I'm not sure why I've never seen this discussed before, I suppose most people don't like to push performance like I tend to enjoy.   I find it invaluable.  No more ESC slipping and choking when I need power the most!

Tuesday, May 15, 2012

Banana Hobby 80" B-17 Flying Fortress - Build and Flight Review

Update 5/15/12:  Today the double-throttle configuration saved the model when the #2 engine blew while initiating an outside loop.  After shutting off the inboard motors' throttle, the plane was able to land uneventfully with plenty of thrust from the outboard pair.  In a rare moment of luck, I caught it on film:

Update 5/7/12:  The Fortress continues to fly beautifully, without a hitch.  Here is a First Person Video and and external view of a typical flight.   Play both views at the same time for synchronized playback:

0:00 Taxi Out
0:42 Throttle 1 Cutoff Check (inboard motors off)
0:45 Throttle 2 Cutoff Check (outboard motors off)
0:50 No Flap Take Off
1:29 Gear Retract
2:00 Fast and low pass
2:48 Two maximum rate Aileron Rolls
3:20 Outboard motors cutoff
3:40 Two motor pass (outboard motors windmilling)
3:50 Two motor climb to falloff
3:58 Four motors on
4:03 Loop
4:23 Inverted flight
5:15 Getting dirty
6:10 Slow and dirty pass
7:45 Fast and low pass on low battery
8:15 Half Flaps
8:25 Gear/Full flaps (went to 0% first, by accident--whoa!)
8:30 Dirty Go Around for previous misconfiguration on final
9:10 Go Around for potential long landing
9:50 Full Flap, on speed landing
The flight above used three 4S batteries, a 3200 mAh flanked by two 2200 mAh for 7600 mAh total fuel capacity, positioned full forward against the firewall.  CG/balance with this battery configuration feels perfect with easy and graceful flight manners. 
2200 + 3200 + 2200 = 7600 mAh full forward
for perfect CG balance.  The specs call for only 4400 mAh
total capacity which would be very tail heavy.
Original review follows:

Spanning 79.5", this is the largest chunk of EPO I've seen in a long time.  Check that, it's the largest I've ever seen.

At $300 with free Springtime shipping, this B-17 was too interesting to pass up.   Two huge boxes arrived just three biz days after placing my order using Banana Hobby's latest free shipping promotion.

I see Banana Hobby (no affiliation) continually trashed on the internet by larger hobby store owners and employees posing as disgruntled mail order customers.   Direct from China importers like Banana clearly threaten their "rip off the customer hard" business model.  Their dishonest shenanigans are not hard to see through.  These internet con-men are typically low IQ and unable to articulate any genuine or believable concerns.

We are witnessing a long, relentless, continuously evolving revolution in the RC community that greatly benefits customers. The days of lazy fat cat RC middlemen are over.  Emerging manufacturing and communications technologies, combined with reduced worldwide shipping costs have enabled small-lot operations to offer better products, faster, and often for half or less money than the established RC retailers.

So let's have a look at the future of our hobby.  This B-17 is a relatively new, monster foamie brought to market by a nimble operation that is actually listening to what people want.  Banana continues to impress me with their rapid customer-focused product development, speed of service and more than reasonable price tags.   There is nothing more enjoyable than watching big, fat, lazy, expensive, lower quality retailers be overtaken and driven out of market share by a hard working, young entrepreneur.  Way to go Pete and Banana!


  - Wingspan: 2000mm (79 inch)
  - Length: 1400mm (57 inch)
  - Flying Weight including 2 x 2500 mAH 4S Hyperion: 143.7 oz
  - Wing Area: 955 sq inches, measured
  - Wing Loading: 21.7 oz/sqft
  - Drive System: 4x Brushless OutRunner KV750 Motors
  - Propeller: 3-Paddle
  - Speed Controller: 4x 45A Brushless ESCs
  - Dedicated 3A BEC
  - Servos: 10
  - Retract System: Servoless Retracts
  - Required Batteries: 2x 14.8V 4S 2200mah 20C Li-Polymer
  - Required Radio: 7 ch (aileron, rudder, elevator, throttle, flaps, retracts, bomb bay)

Target Wing Cube Loading
Gliders - 1 to 4
Trainers - 4 to 7
Aerobatic - 8 to 11 <-- B-17 = 8.4, fantastic for a behemoth!
Scale - 12 to 14
Racers - 15+
1 Liter water bottle inserted for scale.  This huge model already rates high on the Value Meter.
It's big
Details are immaculate, as we've come to expect from no-name Chinese manufacturers.  This model has an identical build style and shares exact components with the 63" Starmax P-51, such as translucent red servo cases on the right side.

The Starmax B-17 Flying Fortress is without a doubt the most airplane I've ever seen for the money.  Consider, the Fortress comes with:
  • Four 4-cell .15 size motors, each is visually identical to the Hobbico Select Scale Fw-190 motor which can also be run on 4-cells
  • Four 45A ESCs - although each pair is permanently wired for twin engine operation by sharing a common signal wire, making one ESC per set the master and the other one a slave
  • Stand alone 3A 5V BEC (ESCs are opto only)
  • 10 servos (2 elevator, 1 rudder, 1 tail wheel steering, 2 aileron, 2 flap, 2 bomb bay)
  • Four scale props, 2 pusher and 2 puller
  • Gorgeous scale details (windowed cockpit, scale guns and gun  turrets some with rotating or azimuth adjustable guns)
  • Three sturdy servo-less retracts with high quality scale wheels and wide rubber tires
  • Flaps preinstalled
  • Functional bomb bay doors
  • Two landing lights
  • Huge 79.5" span with terrific EPO foam build quality
That for $300 including shipping two large boxes??  I find it astounding.

Even better, the electronics are well thought out, mostly pre-installed, and flexible in operation.  Ok I admit it, I've only done a static test run and I already love this thing!

Stats from the first 4S 40C ground run:  1210 Watts, 80.08 Amps

That's some serious power, and the sound is amazing.  But unlike every other aspect of this tremendous RC offering, it doesn't fully compute.   Why 4 x 45A ESCs?

Well, one reason for the apparent overkill is the stock props are only 9".  Switching to 10" props might  really pull this bomber through the mud.  Fuselage to nacelle clearance won't allow a larger prop diameter than 10" on the inboard motors.

As shipped, the Fortress is only using 45% of its ESC capacity at full bore.  In a way it's great because it pulls like a horse without breaking a sweat, and God knows it's not going to be an easy electronic configuration to fix, so I'm happy it should never get hot and has a lot of headroom.  50+% headroom is rare in today's dollar-driven, rather than value-driven marketplace.

That said, a 2.5X safety margin is probably a bit excessive since there is a quadruple weight penalty.  Still, if there is one plane that can probably handle double+ the ESC weight, it is this one.

The electronic configuration is the most complex I've ever done, or seen.  Although no RC plane is really that hard to put together, this one has a lot of decisions that have to be made early.  Plus the shear size of the plane demands a way to break the wing down easily.

The plane is rigged to use two batteries independent of one another--essentially two, discrete, twin engine power systems linked by a common throttle signal.  The power from either battery can only reach two of the four motors.   Each battery feeds twin ESCs which are linked together by a single common signal wire, there is one ESC control lead per set of two.  The supplied stand-alone BEC is only tapped into the left battery, so the twin battery solution provides no redundancy whatsoever as shipped.  I added a Castle 10A BEC to the other side, for the purpose of not wishing I did.

The first major electric decision is the throttle/ESC configuration and spin direction for engines 1 thru 4.   There are two pusher props and two pullers, so counter rotation is the only option out of the box.

The way the plane ships, it leaves that decision entirely to the builder, the instructions never say put motor X on ESC Y spinning in Z direction.   I love that they leave it up all to you, but...  the implied way to hook up the ESCs and batteries has the potential to generate some very nasty emergencies.
Four ESCs.  Two per side, stacked on on top of another.  The symmetrical ESC-to -motor connectors allow you to easily reverse any motor's spin direction by plugging it in backwards. Wonderful!
If you simply hook up the motors on the left to the ESCs on the left to the battery on the left, and hook the right side components together the same way, all hell could break loose in short order.

Reason: if you lose one battery before the other (not hard to imagine) you'd have the worst possible thrust asymmetry.   Worse still, if you have the left two motors turning top-out (counterclockwise looking from the rear), and the right side motors also turning top-out (clockwise looking from the rear), both torque and P-factor would be additive to the full thrust asymmetry.  I doubt the plane would be land-able with one battery out in this sort-of-implied configuration.

What's the optimum configuration?

Well, the real B-17 is no help.  To keep 100% parts commonality for wartime simplicity, the 1:1 scale Flying Fortress used four co-rotating engines.  God forbid the pilot lost the two left engines.  If he lost the two right motors I'm certain it wouldn't be a walk in the park to fly, but at least torque would be your friend.  Yikes!

So after a day or two of mulling over the best twin battery peacetime RC layout, this is what I came up with.   If it doesn't work well in the field I'll post an update, but I'm really happy with the way it turned out on the ground.

Counter-rotation is a good thing since we don't have to add and fix gearboxes to reverse the spin direction, so that stays.  But, I wanted to be able to lose one battery with minimum impact.  That drove putting the inboard motors on one battery, and the outboard motors on the other to achieve single-battery thrust symmetry.  Taking that a step further, to eliminate torque in any one-battery situation dictates that the set of inboard motors must spin opposite one another, and the set of outboard motors must spin opposite one another.

As far as which motors should spin in which direction, there seems to be only one best way to do a two battery, four motor, RC set-up. Since the outboard motors naturally generate the most thrust asymmetry, they should be the ones to torque-away from their natural pull direction.  That is, the leftmost motor should spin conventionally (top-blade inward) so it torques to the left, minimizing it's natural moment-arm tendency to induce yaw to to right.

From there, everything falls out:
Figure 1.  Connecting the motors, ESCs, and batteries this way seems to be the most forgiving if any single motor fails.
As a bonus, if you run two discrete throttles in this config, you can eliminate any asymmetry from a single motor failure by pulling the affected throttle to idle and flying only on your good set of counter rotating motors.  This is a big deal in RC Land, because as glamorous as it sounds to land on three motors, if you aren't actually sitting in the plane that is extremely hard landing situation to perceive properly and successfully pull off.  Hopefully, shutting a set of motors down for practice results in a good flying airplane; I think it will, based on the copious power I'm seeing on the ground.

All that said, there is really no reason to use a discrete a two battery set-up.

I understand that each battery connector can only take so much throughput before it melts (been there; done that) and 80+ Amps through a typical 4S connector would be pushing it.  But that doesn't prevent connecting the two power systems downstream of the two battery-to-ESC connectors.  That creates some thing like this:
Wiring Diagram
Figure 2. Connecting the batteries in parallel, downstream of the connector, has advantages and disadvantages, but this is how I chose to wire my plane.

This config also enables running parallel batteries of different mAh capacities, since the voltage will always be equalized and the batteries' voltage will degrade at the same rate. Although the larger battery's connector will see proportionally more amperage throughput as it performs the heavier lifting.

Another reason to connect the power systems is to force a common system voltage, so voltage telemetry becomes relevant to all four motors.

Piled into the advantages column, P-Factor dictates that down-going prop blades have the highest true airspeed as Angle of Attack increases, meaning the high thrust point will blow the center of each flap during slow flight and landing.

The downside to this configuration is that in the event of battery failure, conceivably, one good battery  could wind up feeding all four motors and possibly melt your only good battery's connector, assuming you select full throttle.   Or, the bad battery could short-out the good battery.  Now, you might have no good batteries (though one would remain connected and presumably drive the servos).

Realistically that is not likely to happen, since the battery with the slightly higher voltage will always feed the system, equalizing both battery voltage at all times.

All considered, I chose the Figure 2 setup, and was happy I did.  As you'll see, after the maiden flight I needed more battery capacity and nose weight.  Another upside of a parallel solution is that you can combine any number of batteries of any capacity, you are not limited to two.

I wound up running a variety of three battery solutions totaling 7000-8000 mAh  to find the best CG balance with good flight time:
Battery capacities are notional to illustrate how current flow could be distributed among connectors (depicted in orange)

This config worked great.  And it still goes easy on each battery connector, with the Y-harness connector generally experiencing the highest amp flow.

I elected to run twin throttles, one for the inboard motors and one for the outboard motors, both are operated by the throttle stick. Each throttle has it's own cutoff switch, so I can shut down the motor opposite any single motor or ESC failure if the situation dictates.

The mixes required to do two throttles with individual cutoff switches was interesting with the Aurora 9. Since the throttle cut function appears to be for a single throttle, I did the dual throttle cuts with mixes. For some reason this took two mixes per throttle. The first -100% mix on a switch only cutoff the bottom half of the throttle range, the second -100% mix on the same switch worked to cutoff the rest of the throttle range. So there are four mixes required, two per throttle.

In the final config, I have engine #1 and #4 on the Throttle 1 with a cutoff Switch D (the closer switch to the throttle stick cuts the inboard engines), and Engines #2 and #3 on Throttle 2 with cutoff Switch A (the farther switch from the throttle stick cuts the outboard engines). Both are mapped to J3 which is the normal throttle function for Hitec radios (Joystick 3).

I also added a second BEC and ran a Hitec SPC line for triple-redundant receiver power, redundant servo power, and voltage telemetry.

Lucky for me, only 9 channels were required:

Ch 1 - Aileron 1
Ch 2 - Elevator
Ch 3 - Throttle 1 (two inboard counter-rotating motors)
Ch 4 - Rudder servo and Tail wheel servo
Ch 5 - Servo-less Landing Gear x 3
Ch 6 - Aileron 2
Ch 7 - Flaps x 2 servos
Ch 8 - Throttle 2 (two outboard counter-rotating motors)
Ch 9 - Bomb Bay doors

The next challenge is making it all easy to break down.  The wings are apparently designed to come off in halves, but that quickly seemed crazy.  There is so much servo and ESC wiring to re-seat, I can't imagine a three-piece build. 
Additionally, the wings are so enormous, holding the two halves together and to the fuselage with the four structural wing bolts and a center spar, without gluing it all up, seems nuts.  You basically have to glue the wing halves together to form a single piece, removable wing.   I don't see any practical alternative.  The question then becomes, can you make the wing fully detachable, or does it stay electronically tethered?  My  answer to that question is: it needs to stay tethered.

There is no practical way to hook up all the wing connections at the field, given four motors with props matched to their rotation direction,  ailerons, flaps, lights, and two bomb bay doors--it has the potential to be a real chocolate mess.
That means creating a strong, permanently connected wiring harness between wing and fuselage.  To break the plane down, you can detach the wing, rotate it 90 degrees, and shove the plane in a vehicle with 79" of longitudinal space.  An an open rear or side window, or two, might also do the trick.
As it turns out, creating two permanently connected wiring harnesses seemed best.  Each one tucks nicely into a slot provided inside both sides of the fuselage, in the front of the Bomb Bay:
Creating two permanent wiring harnesses allows the wing to drop about 6 inches so it can be rotated 90 degrees to help transport the plane.  The wing halves are Gorilla Glued together in front and behind the Bomb Bay and the wooden wing spar is glued inside the wing for strength.
Each wiring harness goes in a hole provided on the sides of the interior fuselage.  Can you imagine trying to hook up all these wires at the field?  Don't even think about it.

With all complete, the shear size of the plane is a bit of a challenge.  A fun challenge.

The Fortress weighs in at a smile-inducing 7.8 lbs.  Add two 9.6 oz 4S 2500 mAh Hyperion batteries and the B-17 flys away at 143.7 oz, a nice round 9 lbs.  Not bad for a $270-class foamie, considering more typical shipping costs.

Even better, the huge Boeing wing underpins the massive bomber with 955 inches of Wing Area, producing a stunning Wing Cube Loading (WCL) of 8.4, about the same as my Great Planes Yak-54!  Yay!

Ground testing is complete, here are the stock numbers:

1210 Watts | 80 Amps | 124 oz Empty | 143 oz w/5000 mAH 4S | 122 oz Static Thrust | 0.85 T:W

Sensing the fun behind those numbers, I loaded the B-17 up hoping for a calm day for flight test.

The most unexpected thing so far happened.   It fit in my truck without breaking it down!  I never even entertained that possibility, thinking 80" was going to be way too big.  Not!  The plane slid right in fully assembled.  Ooops!  I could have glued the whole thing up and wired it a lot more easily.  Oh well, maybe later.

Bottom line up front:  The Fortress is the most model I've ever got for the money. 
At 10 lbs wheels-up, the aluminum overcast is an Industrial Age work of art.  The value delivered is unmatched in my RC experience.
I wound up putting 29:32 minutes on the airframe (which I just realized is low because my clock only ticks with Throttle #1 above 1% and I flew a few minutes on each set of inboard/outboard motors).  But it was a maiden day, so even though the plane performed almost flawlessly, there was a lot of learning packed into that brief half hour!
First, the manual's CG is way too far aft, which I recognized during the build and partially corrected.  But even so, my maiden flight was still substantially tail heavy.  The recommended battery setup (4400 mAh total) is just too light and results in about 5 minutes of flying.   I guess they suggest small batteries to keep the perceived operating cost as low as possible.   Luckily, I brought all my 4-cell batteries and a parallel connector to the field today, and I needed them.
There are actually two mistakes with the CG in the instruction manual:  An incorrect depiction of the proper position, and in incorrect illustrated  measurement for the (wrong) desired CG point. 
The manual lists the CG at 140mm back from the leading edge at the root, but the CG dot is shown very close to the leading edge (with a 140mm double-arrow erroneously describing that distance).   The CG dot is actually shown at a point that is about 70mm behind the leading edge, which would be way too nose heavy.   But, even if the dot was correctly shown at 140mm that would be way too far back and tail heavy.
Fortunately, neither depiction is reasonable, so it was an easy set of errors to catch. Heads up!  The manufacturer uses both misinformation and disinformation in an attempt to get the most important single piece of info in the manual dead wrong.
The correct CG is, not surprisingly, traditional for a warbird, located at 20-25% of the wing chord behind the leading edge.  That works out to be around the thickest part of the airfoil (25%) to about an inch in front of that point (20%).   To achieve 25% chord in stock layout requires about 7000 mAh of 4S battery capacity in the forward part of the battery tray.  To fly a more stable, but slightly nose heavy CG at 20% chord required about 7500 mAh of honking LiPo(s) at the forward part of the battery tray, or 8000 mAh mid-tray. 
After a squirrelly, tail heavy maiden voyage due to only hauling 5000 mAh of 4S capacity, the Fortress had nothing but love to give.  It's proper-CG adjusted WCL of 9 is still phenomenal for a 10 lb bomber, and the plane has no problems (at all!) lifting the weight.  In fact, it still flies nimbly, slows down impressively, and displays utterly benign stall behavior.  
On my third flight, the slightly nose heavy Fortress flew beautifully through some extremely windy conditions (20-25 mph) before calling Knock it Off for 30 mph+ winds.  This B-17 is one amazing flyer for a lumbering beast.  It moves like Jackie Gleason on the dance floor.  What fun!
General flight impressions after the CG was fixed:
Takeoff roll was nice and straight even with some crosswind, likely due to the heavy mass and lack of torque.  I jammed the throttles to half to energize the rudder, then eased to full.  The plane had already lifted off by about 3/4ths throttle and it climbed out very strongly.  The bomber needed about 10 clicks of aileron trim, even though I had the surfaces visually aligned.  That's not surprising given four thrust angles and functional flaps all contributing to roll. 
Once trimmed up, the Fortress cruised like a bullet.  Tracking is perfect, almost too good (see my rudder authority discussion in a couple of paragraphs).  The plane clearly carries momentum, but the light wing loading feels more airy than heavy.  The amount of acceleration available is striking.
Elevator control seemed short coupled; I had too much throw dialed in.  That made the tail heavy maiden even more fun.  The elevator is very effective, especially considering it's a big ol' B-17 we are talking about.  The plane definitely asked for more nose-up in the take-off/landing trim position at slower airspeeds. 
Aileron authority was excellent considering there are four big motors in the wings.  The plane rolls on the average-to-quick side of the warbird spectrum.  Due to the throw required to spin the airplane's mass, I used 50% differential and it turned without much adverse yaw, but I'm not sure how much I really needed--more testing required.   Rolls are axial with the differential
Rudder was by far the weakest axis of control.  At one point on low rates, I tried a hammerhead stall and wound up nearly hovering!  Unlike my P-38 twin which has super effective rudders due to twin motor/twin rudder thrust vectoring, the B-17's big rudder is not blown directly by a motor, so slow airspeed means little-to-no rudder authority.  Plus there is high rotational mass, especially in yaw.  You'll want to dial in maximum rudder throw. 
Loops can be enormous.   I'm not sure how tight they can be, but my guess is: very.   On Knife Edge the B-17 lacked rudder authority and dropped slowly and controllably.   Hopefully, conjuring up 20-30% more rudder throw doesn't upset the apple cart. 
The most surprising area of the flight envelope was slow speed work and stalls.  The Fortress has a ton of nose-up authority combined with very symmetrical airfoil and wing taper--a lot like an aerobat, perhaps amazingly.  This enabled the bomber to plow through a full stall, nose level-to-high, wings level and steady.  Into a strong headwind, I was actually able to slow to zero groundspeed without drama.  The final stall break is almost Harrier-like into high AOA that remains controllable with throttle.    I kind of couldn't believe my eyes. 
What fun this plane will be, once wrung out and rigged up!
Flying an Overhead Pattern allowed me plenty time to trim back on downwind to prop the nose from continually inching lower around the final turn. I was rewarded with the airplane's friendly slow speed handling and stall behavior all the way through the flare and touchdown.   I was surprised to balloon moderately in the flare but the platform didn't budge through a landing that was a lot slower than I envisioned. 
The only cavet to wonderful pattern/landing behavior is the bomber's increased momentum, which requires more time to nudge the flight path than say, a big parkflyer.
A note on twin engine flight: shutting down two engines (given my symetrical thrust config) was no sweat and the B-17 flew very well on either pair.  Top speed and acceleration are a bit lethargic, but the plane flies just fine and bystanders probably can't tell the difference.  The Fortress actually flies on two motors more like I expected it to fly on four, which is testimony to how strong it is with all four windmills churning.
Upon taxi back and shutdown, all four ESCs were only luke warm; batteries the same.  Wow, the lumbering beast isn't breaking a sweat.   Magnificent!
I can't wait the fly the Fortress again with a few tweaks.  On the first outing, onlookers were mesmerized by the presence and beauty of this flying Art Deco sculpture.  For that matter, so was I!   I can't help thinking this B-17 could be a #1 airshow attraction with, or even without the right set-up:
As the exquisite lines, the takeoff and remarkable climb angle give way to rousing, four engine warble doppler passes, gawkers are already cheering aloud.   Then you layer high pace, curvacious aerobatics with nose and roll rates that make most Sportsters lower a jaw.  Next, you breech the topic of 3D, causing hardcore aerobats turn their heads and point.   I can imagine finishing-up with a dirty pass that nearly stops show center, pulling up hard to the vertical.    That's not only possible, it's stock.

Appearance: A+
Head-turning, Art Deco masterwork.  Great color scheme aids airborne visibility.  Beautiful scale lines make overall glory undeniable.

Airframe: A
Beyond my expectations for a four engine bomber.  Inspiring top end with docile slow speed handling.  Individual servo channels can be mixed to maximize aerodynamic response given a 9 to 10 channel radio. 

Power System: B+
Stock power aplenty.  Battery tonnage required for balance yields 10+ minute flights.   Needs careful thought during setup to mitigate failure modes.  Four ESCs but with only two signal lines.  50% headroom for prop growth.  No interior cooling as shipped.

Build Quality/Durability: C
Great EPO but it's still only 10lbs of foam.  The main spar and wing halves need to be glued up to a one-piece wing for strength.   Sturdy retracts but plywood anchors could be better glued (easy fix).  A few lighter areas in the silver paint. 

Value: A+
Top of the mark.  Enough said. 

Overall Grade: A+ 
More plane than I imagined.  Dreamy sport flier.  "Can't wait to fly" meter is pegged.
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