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Wednesday, September 1, 2010

Parkzone Extra 300 PnP vs. Electrifly Edge 540 ARF

Parkzone Extra 300 PnP
41" wingspan


Electrifly Edge 540 ARF

41" Wingspan
$105 + (4 servos @ $40) + (ST .10 @ $20) + ($30 ESC) - ($5 GP rebate) = $190

(subtract 20% from the Edge +ST price for Tower Hobbies standing member  discount.  Unfortunately they do not carry Parkzone stock.)

The balsa and ply Edge takes some effort to assemble, will the performance be worth the build time?

Construction Time and Ease
First things first, how do these planes compare from box to flight? Well, that's an easy answer: they don't. There is no comparison!

The PZ Extra 300 (why didn't they call it a 330?) is really an RTF packaged in PnP form. Setup is very straight forward and quick. You insert the carbon tube, slide the wings into place over the tube, then install two screws inside the fuse tack each wing-half in place. The tail section is classic PZ slide and tape, then you clip the elevator clevis to the control horn. Lastly, wire landing gear is popped into a slot on the bottom, then flimsy orange fairings are snapped on and screwed into the under body. The 4 landing gear fairing screws are fussy to install, other than that, assembly is a breeze. Let's say, 30 minutes to read through the directions as you go.
The Great Planes Edge 540 (why not call it the 580, because it doesn't have a real AEIO 580 under the hood?) is a completely different kind of build. The Edge is a balsa and ply ARF pre-covered in Monokote. It is essentially a scratch build with the heavy lifting already done for you. It is beautifully assembled and covered, to a higher quality than most could probably hope to achieve even if you wanted to spend excess time. After that, the minor construction details are unfinished and the guts need to be conceived and added. For example, you have to hinge all the control surfaces and fashion a suitable engine mount. Unlike the Xtra, as I assembled the Edge I kept thinking to myself, I hope I never crash this thing because it is a mini work of art. There is a lot of time and glue required, and even more that has already been accomplished. Drilling a few holes is also required to complete the model. Resultant model breakdown and setup is similar: half-wings slide onto a carbon rod, each tacked inside the fuse with a small screw. The box says ARF with 4-6 hours assembly time required; I say, no way. Let's say, one very full day from box to runway. If you don't like working from morning to evening without pause, two days is more like it. No small difference. In the end, you have a beautiful model with a finish that will last a long time with care.

Extra 300 construction time and ease (1-10): 10
Edge 540 construction time and ease (1-10): 5

Kit and Component Quality
The non-comparison doesn't end with time to build, there is a major quality dichotomy. The Xtra 300 is typical PZ. The foam is tough, generally attractive at a distance. The plane's clean "newness" will deteriorate over time as the foam creases and ages. The Edge 540 is a gorgeous model in every sense, the perfect paint, the glossy metallic plastic covering, the classy color scheme, the impressive laser-cut craftsmanship.

Instruction manuals are also different. The Xtra manual is basically a legal disclaimer with a page or two describing assembly that you could figure out on your own. The Edge manual requires full featured assembly instructions, as it is an advanced stage model kit, plus it has several pages of nicely detailed 3D aerobatic flying instructions.

The Xtra comes with a 15-sized motor and a minimal 30A ESC. Initial engine runs are rough and out of balance, hopefully it is only the prop. Servos seem adequate but PZ's record providing quality servos is rather poor. Every PZ foamy I've owned (let's just say, many) has developed a chattering or erratic servo, or two. These are entry level components at best.

The Edge comes without guts, but the kit price of $105 is low enough to equip it at the same or higher quality level. I chose a Super Tigre .10 engine due to the low price, superior workmanship, and extreme thrust to weight ratio of about 22:1 with a 10x5 in front of a good cowling. Seems like the perfect match for this light weight model. I expect an aircraft T:W of about 1.5:1.

Extra 300 kit and component quality (1-10): 6
Edge 540 kit and component quality (1-10): 10

Appearance and Overall Design
These two models are virtually identical in scale. The wingspans match within 1 centimeter, the Edge is has a 70 millimeter length advantage, or about 2.75". Within those parameters, the Edge has much larger control surfaces all around, and potential throws are a lot more extreme. Eyeballing it, I would say the Edge possesses roughly double the control surface area and triple the limits of travel. The following video clip shows the two planes bound to a single transmitter, with the control horns set to one hole from maximum throw in both planes:

Parkzone Extra 300 and GP Edge 540 Control Surfaces (0 min 27 sec)
Click the link above for video.
Even though the planes have the same wingspan, it's clear in a side-by-side comparison that the Edge has a significant wing area advantage. The Edge lists 353 square inches of wing area, the Extra specs are missing that stat. At 13.6 oz per square foot, the Edge 540's wing loading is medium to slightly heavy in this class, so the Extra's wing loading has to be rather high.

The Edge looks scale. The trademark Edge straight leading-edge with trailing-edge sweep, combined with the lines of the enormous rudder and hefty elevator look almost outrageous to the trained eye. The high gloss metallic blue and bright yellow color scheme make the plane seem classy, even a little conservative. The edge is pilot and phony flight instrument free, which I prefer. I figured out that there aren't little toy people who fly these things. Right?? The access hatch is located on the bottom between the gear, and it also uses a dowel and magnet system. The painted fiberglass cowling pops on/off using 8 strong rare earth magnets, a nice touch.

The Xtra doesn't look quite scale. The mid-wing is a bit high for a full scale 300 match, and the fuselage has a slightly hunched appearance. PZ fixed their usual weak magnet problem on the electronics hatch with a much longer and stronger magnet. The 300 cowl is cheap orange plastic with quite a few small screws to remove. PZ's availability of spare parts makes the cheap plastic less of an issue. There is a red helmet ninja-looking pilot, for effect.

In fairness, the Xtra is marketed as an "In-between" that sits between trainer and 3D, so I guess we shouldn't expect the same kind of absurd control surfaces and insane throws. Those are plain dangerous to expert and novice alike. The Edge looks like a purpose-built unlimited-class all-out aerobatic performer. Flight tests will reveal the effectiveness of the obvious visual differences in model accuracy and execution.

More practically, both planes sport a high-vis color scheme. The Edge chooses the scientific best base color for aircraft clarity in the sky, yellow. The Xtra goes with an extreme look, wearing a mildly fluorescing orange suit. From the bottom, Great Planes choose a high-contrast blue and white checker board, great contrast though not at all in-sync with the overall look of the plane. PZ stuck with orange for their large checkered bottom, it could more easily be mistaken for the top wing's sweeping orange diagonals. Functionally, the Edge should be easier to see and orient.

Extra 300 appearance and overall design (1-10): 7
Edge 540 appearance and overall design(1-10): 9

Static, Installed Thrust to Weight Ratio
Static Thrust-to-Weight ratio is perhaps the most important power metric of an aerobat. Thrust to weight ratio is a little more slippery than some might think. Like a car with a broken manual transmission, most aircraft with fixed pitch propellers are perpetually stuck in one gear. The prop you select is the one gear you get to use. To continue the car analogy, you can chose a low gear like 1st if you want to go up steep hills, but you won't have a high top speed. If you want to gear for top speed it's best to chose 5th gear, but you'll have trouble getting off the line.

To make matters a little more complicated, aircraft propellers are wings that commonly list two key aspects of their "gearing," diameter (wing span) and pitch (Angle of Attack). The astute reader might recognize some unaccounted-for terms from the Lift Equation, namely wing area and speed. Wing area, or in this case propeller blade area, is obviously known but not listed. Most people assume it away as a non-factor though it is linearly important. More important still is speed because the term is squared. With props, speed translates into achieved RPM where the prop tip is moving faster by a factor of PI than the prop hub, or wing root. To compensate for the speed difference, prop designers usually taper the blade area toward the tip to distribute the generation of thrust more or less evenly as a function of radius. To make calculations even more squirrelly, speed, or RPM, is limited by drag which is primarily a function of lift, and lift is the equation result we are trying to determine!

But wait, there is more, much more. Isaac Newton told us when you get hit with a moving apple, it hurts. The impact of Newtons Second Law on aircraft propellers is that a lot of thrust is lost. When moving air, pushed by the propeller, deflects off of a part of the airplane behind the prop, which is in turn connected to the prop via the engine shaft, it creates a force than tugs back on the prop. This force substantially reduces achieved thrust. A quick thought experiment reveals why: imagine an engine mounted to a flat board that prevents 100% of the air from shooting behind the engine--this engine generates no thrust--even though it may be spinning at the same RPM as an engine that isn't installed on a flat plate. This is called installation error. Any amount of thrust left over after installation error can propel an plane forward, so this is called Installed Thrust.

You'll often see thrust measurements from RC enthusiasts who mount engines on test stands. Ignore these tests and chuckle kindly at those who perform them. They are off by around 50%, depending on a given airplane's drag contour as it sits behind the motor. Since every aircraft's drag profile is completely different, there is no value whatsoever to this type of test. The results are useless for any given engine installation, and most people need to install motors to use them. It is tempting to think you might gain some general sense of engine/prop performance from test stand results, but you can't. The prop's lift contour will align to any given aircraft's drag profile completely differently for every prop and engine selected. Also, the mass of the engine will massively influence flight characteristics, and will not be reflected in the result. Like car manufactures trying to sell cheaper cars, they crash-test into "infinite mass" barriers. This hides the effect of mass when a cheaply built car is utterly obliterated in a crash by a heavier, well built car. Mass matters.

For my thrust tests, I was careful to use a useful, but certainly not perfect methodology. There is only one perfect methodology, flying the airplane, feeling it, and recording empirical data. Since I can't strap on these planes, I chose to measure and compare "Static, Installed Thrust-to-Weight Ratio." "Static" because the test is conducted at 0 airspeed. "Installed" because the engine and prop is tested as installed in the airplane that will eventually be flying behind it. To conduct the test, I simply tie each aircraft to a scale and put the pedal to the metal, firewall it, drop the hammer, give'r the gas, go max dinosaurs, WOT, put the screws to her, whichever you prefer.

For an initial test, I felt compelled to go with Parkzone's factory prop on the Xtra 300, which is a PKZ 5101 spec'd at 10.5 x 9. Later on, I might switch up the prop to generate more data. This prop is a pretty interesting choice. The diameter is small for a 15-sized motor and the pitch is very steep for an aerobat. The small diameter will lower overall drag to juice the RPM, mitigating some of the drag from the steep pitch and allowing the engine to turn in very solid static thrust. The steep pitch will achieve a very high top speed, once the prop engine is unloaded in the air and the RPM goes even higher. PZ's choice favors speed. To go back to the car analogy, it's kind of like choosing 4th gear.

I'm guessing PZ picked this prop for a few key reasons, in order of guesstimated importance: (1) high airspeed will likely be perceived as high performance from PZ's target demographic, who may not be not the most aviation savvy consumers in Toy Land. (2) it probably keeps max amps relatively low so the included ESC can stay cheap. (3) the airplane has a tapered wing with relatively high wing loading, so they probably wanted to keep it moving to avoid novice pilots tip-stalling, snap rolling, and spinning it into a dirt pancake. Ironically, if this does happen, the prop's steep gearing will make it harder to recover since low speed thrust is limited, resulting in slower acceleration out of square corners and lots of induced torque per oz of pull.

The Xtra uses the ParkZone 15BL Outrunner 950Kv, with a list price of $70. I couldn't find a weight listed for it, or any decent specs or power limits. For a rough idea, these are engine specs for E-Flite's Power 15. I'll add the correct specs here if I can find them:

Power 15
* Recommended Prop Range: 10x6�13x6.5
* Voltage: 7.4 - 14.4V
* RPM/Volt (Kv): 950
* Idle Current (Io): 2A @10V
* Continuous Current: 34A
* Maximum Burst Current: 42A (15 sec)
* Speed Control: 40-45A Brushless
* Weight: 152g (5.4 oz) without mount

The Edge is an ARF, so I got to choose both the engine and the prop. I picked the Super Tigre .10 for its light weight, strong pull, and low list price of $19.99. I chose an 11 x 5.5 prop for my one gear. The wider diameter prop and lower pitch should help generate huge low end grunt and extreme acceleration. The low pitch should allow plenty of RPM for a brisk top speed, but this Edge won't set any speed records from NY to London. Basically, I reversed PZ's priorities.

Super Tigre .10
* Recommended Prop Range: 10x7e - 11x7e
* Voltage: 7.4 - 11.1V
* RPM/Volt (Kv): 1250
* Idle Current (Io): 1.2A
* Continuous Current: 29A
* Maximum Surge Current: 33A
* Max Constant Watts: 320W
* Max Surge Watts: 370W
* Speed Control: 30A Brushless
* Weight: 63g (2.2 oz) without mount

Here are some initial results:

Extra 300 RTF weight, no battery: 28.3 oz
Edge 540 RTF weight, no battery: 26.7 oz

Extra 300 RTF weight including NRG 2200 mah 35C battery (6.8 oz): 35.1 oz
Edge 540 RTF weight including NRG 2200 mah 35C battery (6.8 oz): 33.5 oz

Interestingly, the weight difference is almost exactly the difference in engine weight if you use the Edge's stock plastic spinner (I added a solid aluminum spinner), so, sans engine, these airframes are virtually identical in both size and weight.

Extra 300 Static, Installed Thrust: 43.6 oz
Edge 540 Static, Installed Thrust: 48.9 oz

Extra 300 Static, Installed Thrust to Weight Ratio: 1.24
Edge 540 Static, Installed Thrust to Weight Ratio: 1.46

For more detailed results, please see the attached chart. Again, it is somewhat amazing how close these two are on paper. The main power difference boils down to an arbitrary selection of prop gearing. I geared the Edge for optimum static thrust, while PZ geared the Xtra for high airspeed. As such, two differences jump right out: Aircraft T:W which heavily favors of the Edge, and Pitch Speed which heavily favors the Xtra. It would probably be pretty easy to swap the results by swapping the props.

One clear loser is the 30A ESC. 30A is no where near sufficient for these motors/props and battery. My guess is that PZ will be forced to raise the ESC rating to 40A, much like they've done on other models. PZ's decision to include a weak ESC more than negates the Edge's $15 higher price tag, since the Edge is an ARF instead of a PnP, you can buy one 40A ESC instead of buying a packaged 30A with the need for a 40A.

It is worth noting again that the PZ 15BL motor is not the exact model of the stats I have quoted. Chances are, the bundled motor is of lower quality than the Power 15, placing both motors slightly in the red zone. That said, the performance of both planes is outstanding, so a lesser performing battery could easily bring everything a little closer to the green arc.

First Flight Impressions
The wind was blowing at only 10 knots for about an hour this morning, so I took the chance to put a battery through each plane. I only had two batteries, so making adjustments then re-flying wasn't a possibility.

On low rates, the Xtra went first. On takeoff roll, the plane was definitely poorly propped and under powered. The engine wailed, but not a whole lot of acceleration was evident. Liftoff took a fair amount of road/rwy in contrast to a loud motor whir. As soon as the plane broke ground, high torque, fairly low speed, and a lack of sufficient pull wanted to roll it over. I was able to counter the plane's initial move but almost immediately, the plane was calling the shots and not me. Once the plane was farther airborne, it became evident that, most likely, the CG was too far aft as the plane was only about 75% controllable. At high speed, control was vague but more like 90% predictable. It was all I could do to get the plane reasonably in trim, onto a base leg, and back down on the ground for a high speed, head-first slide into home plate. Between the moderate winds and the aft CG (I think) issue, my first Xtra 300 flight was simply a terrible flail.

I checked the CG 3" back with with a finger balance before take off, but clearly this plane needs a more precise calibration, or perhaps I just missed it. It's odd that the Edge calls for a 2.5" CG with the same proportions. Since I know the CG was reasonably close, I have to admit, I'm not sure about this plane. Next time it'll have the right prop on it and the battery pushed farther forward. I've seen enough to recommend not even trying the factory prop. Maybe a 12x6 will give more low end muscle while still preserving enough RPM to get some speed against the wing loading? Or maybe this plane would be a good candidate for the Bf-109 3-blade. The third blade should give more static thrust and the 8 pitch should keep the plane moving? I sense testing in the future.

Determined to get some decent flying in while the winds were still in check, the Edge was up next. I checked the CG by balancing 2.5" back, at the main spar, using my fingers, again. I've balanced every RC plane I've ever flown this way and I guess I'm too stubborn to believe it doesn't work. The Edge taxied forward making quite a well amplified scraping sound. The plane has no tail wheel, and the balsa and ply frame turn the plane into an amplified Monokote drum. The manual recommends inserting a metal washer in the tail skid for durability, which I did. As long as the throttle is above 20% or so, the plane actually taxis fine. I got it pointed it down the runway and eased the power in about halfway. The nose initially tracked left under torque, and without a tail wheel, any input of right rudder had to fight the tail skid's new orientation. Within a few feet, the tail picked up and the plane was easy to correct back to center line. The Edge popped skyward.

Trimming the plane in the bouncy air took a few passes, but soon it was tracking well. Elevator trim was more difficult to nail than roll and yaw. On low rates, I had too much elevator travel. I found my DX6i's trim function was lacking enough resolution, the plane either wanted to dive or climb, there wasn't a correct trim level available. I settled for a bit of climb. The plane flew beautifully considering the unstable air. Low speed flight was controllable down an airspeed that seemed like the plane should stall and falling off to a side, but no bad behavior was evident. High speed flight got a bit exciting in the pitch axis, as the plane had a tendency to "dig in" and dart up or down. I think it's a simple matter of too much throw, and possibly a CG that is a tad aft considering the lack of flight control testing and dial-in. I flew the Edge in "sport mode" for the rest of the sortie and found the plane to be a very enjoyable sportster.

Approaching to land, the plane was pretty good in the cross wind especially for its size. A little wing low and opposite rudder put it down in the middle of the road. I decided to forgo flap testing on the first flight, as the winds were not favorable for a floating approach. As the plane lowered to about 3 feet above ground, I noticed the left wheel pant had reversed in flight and was in fact upside down. Since there was no was to fix it in flight, I continued to land. Touchdown was uneventful. The left pant scrapped on its top with little friction and the plane's huge rudder wasn't about to let the plane get far off track. The Edge half-rolled, half-skidded to a nice strait stop. A minor white scrap on top of the pant was the only battle scar.

For the next flight, I'll need to reduce the elevator throw either physically or digitally. The wheel pants need to be epoxied against the flat metal gear strut, sandwiching it between the double nut on the wheel bolt doesn't seem to grab enough pant to keep them in place. I think the 11x5.5 prop is about perfect, providing tons of low end grunt of plenty of top speed. The first flight was a lot of fun, minus a few issues that I think I can iron out. I think we are going to get along just fine.

Extra 300 Static, First Flight Impression (1-10): 1
Edge 540, First Flight Impression (1-10): 8

...two week update...

Update and Conclusion
Like so many others, my Extra 300 has a serious product defect, the airfoil is semi-symmetrical instead of symmetrical , and the flatter side is installed on the top. Please do not attempt to fly this airplane until Parkzone recalls all existing stock and fixes the defect.

I rate the Parkzone Extra 300 a 0 and give it an "Avoid at all costs" recommendation.  The Extra $300 has the honor of being rated the suckiest RC plane ever. F

The Great Planes Edge 540 ARF has exceeded both my kit quality and flying expectations by a wide margin. It is an outstanding flyer. A+

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