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Wednesday, September 1, 2010

Parkzone T-28 Motor Upgrade Comparo


The T-28 is a great trainer, but let’s face it, after a few weeks some might find it a little wanting in the Thrust:Weight category. The stock plane comes with a peppy, but fairly hefty Parkzone 480 motor that generates about 23 oz of installed thrust using the stock prop. That’s about 0.74 T:W in a stock 30 oz plane, ok, but not always enough to reliably yank beginning pilots out of a square corner. Another problem is the weight of the 480. It causes pretty fast glides and touchdowns and slow pattern turns require a careful balance between stall speed and approach speed.

For this discussion, I’ve considered three upgrade motors--the E-Flite Power 10, the Turnigy 35-36C, and the Super Tigre .10. I also provide some numbers and commentary on the stock Parkzone 480. I graded each motor in commonly desired areas for improvement, and provided some commentary on how easy they are to install and how they change the T-28’s flight characteristics. Brand loyalty seems to tug on emotion for a few people who are married to brand XYZ for some reason, so I made certain to eliminate subjectivity in my scoring method. Personally, I don’t care at all about any of the brands, but I do want a smart choice that works. As you will see, any of these engines will thoroughly transform your T-28, so if you are glued to a certain brand, by all means, choose that one.

My scoring system was simple: each engine received the exact number of points as a percentage of the maximum score for each category. For example, if the maximum category performance was a 9, and another engine managed only 8, the high score received 100% of the available points the lower performer received 89%. This removed guess work from scoring. I was careful to standardize the trial conditions, and I have documented each test with live video, already available on YouTube.

My weighting system is 25% price, 75% performance. I under-weighted price from my personal viewpoint to cater to performance-oriented hobbyists, I think it might be more like 50/50. To determine the engine’s price, I tried to pick the lowest total outlay. In my case, that meant picking two engines up locally, with representative sales tax applied, and internationally mail ordering the other. Two of the motors required new ESCs and other required equipment. I added accessories to the price only if they were absolutely required.

The battery used in the test is a mid-price, very high performance NRG 35C 2200 mAh that delivers more than 12V under load. Since it was the same for each motor, I didn't count the cost, but it could make an outstanding upgrade in its own right, as you will see shortly. Since I had a couple of new APC 10x7 props, and this is a great all around size for the T-28, that's what I decided to use. Testing lots of props wasn't the point of this test, we all know a 10x10 won't give any sustained vertical on any of these engines, in exchange for higher top speed, etc.

Two quick notes before we dive in: first, my E-Flite EFLC3115 charger tends to pack a little more juice into batteries than most, though not enough to trip the battery overcharge PCB at 12.90. Others charging to 12.6 or lower might see an oz or two less thrust, but will get more battery cycles in exchange. I also taped-over my Firewall Louvers Mod for the ground tests--note the red tape on the firewall--this sacrifices 2 to 3 oz thrust at these power levels, but results reflect a stock setup.

These findings probably surprised me the most. Since I am results-oriented, I put the conclusion first and added my subjective flight impressions, none of which were scored, in the text that follows.

Conclusion:

All three engine upgrades feel virtually identical in the air, under full power. I challenge anyone to tell the difference in a blind test; it would be extremely difficult for me after 30 years of flying everything from recreational to high performance gliders, to vintage and aerobatic biplanes, to unlimited competition aerobats, to prop and jet military trainers, to the greatest fighters the world has ever known (all those 1:1 scale) to quite a bit of RC stuff. The only really remarkable difference between all of these T-28 upgrades occurs once the throttle comes back. I found that surprising.

Another big surprise was how well the stock motor performed once connected to an top-end battery and a prop better matched to the T-28’s 5.25” wide radial cowling. Plug the PZ 480 + 35C + 10x7E numbers into my Final Standings spreadsheet, and the stock motor actually places 2nd with a score of 85% (giving it 100’s on price and ease of installation and a 55 mph pitch speed). Upgrading to a 35C battery and adding a 10x7 or 11x7 (lower top speed, more vertical) is an excellent upgrade path and should be given very strong consideration by those looking to spend basically nothing, assuming you’d buy these things anyway, to achieve a greater than 1:1 T:W with their T-28.


#1 Super Tigre .10--“Cheap thrills; fewer spills.”

If you value low cost and easy, this little motor has the added appeal of not requiring a new ESC when conservatively propped with high rate battery. $25 and done. To illustrate better, my first Trojan’s 30A stock ESC and 15C 1800 mAh battery now live in an Art Tech Pitts S2-B with an ST .10 turning a 10x5 prop. The 30A is almost 10A's too conservative for this setup but the Special really screams (easier to fly than the T-28, btw).

The real beauty of this engine upgrade is a significantly lighter nose which produces airy glides and more docile pattern turns. Greasing a slow landing with high AoA, makes me wonder if the T-28 wasn’t designed from the ground up with a lighter engine in mind. Watch out for puffs of wind when you are out of airspeed and ideas, before a tire-dancing touchdown.

After the light nose, I think the next best attribute of an ST upgrade is acceleration. The motor is noticeably more abrupt than the higher mass options, and in a different class all together than PZ's stock package. Trainers should be easy to fly, but when you get into trouble you want the airplane to stand on its tail and haul skyward. Less inertia, lower moments, and very high T:W wrenches the plane up out and away from square corners.

This T-28 has an unnatural nose rate, you'll definitely want to dogfight single-circle. If it turned a little tighter, you might shoot yourself down. So eager is the T-28 nose that I dialed-in maximum elevator deflection just to see what would happen. With up-elevator forced against the T-28’s physical limit (set by the rudder pass through), the plane loops in what is best described as a reverse Lomcevok, or inside tumble, in separated rather than laminar fashion. The second tumble becomes much more interesting, as it breaks any which way but loose.

Spins with the gear up are a continuous blur without much dumbbell effect to fight the wrap, and the plane exits immediately upon neutralizing the controls. Perfect. Continuous aileron rolls stay mostly uncoupled, spinning off four to six from 10 degrees nose high through 10 degrees nose low. With the CG set reasonably aft, this T-28 is nicely aerobatic, but not loosey goosey and is easier to land than stock. Take off procedure is Elevator- Full Up, Throttle - Max. The tail scrapes hard every time, but it sure is fun to see a takeoff roll of 6-18 inches on a hot day.

One odd flight characteristic of this upgrade is the lack of required trim. Unlike some planes, the T-28’s thrust line pulls hard downward, and with a lighter hood the amount of compensation under power is about nailed. This T-28 is as close to trim-free as any airplane I’ve flown, full scale or RC, with the exception of the F-16 which auto-trims for 1g. A joy to fly.

Parkzone T-28 Super Tigre .10 35C (1 min 28 sec)

#2 E-Flite Power 10--“When Money is No Object and Landings are Into a Headwind”

Depending on how you weight price, the Power 10 can fall to a more distant second, or even third place. This engine pumps out the highest T:W, but it takes longer to lift off than the ST, then after a brief period of déjà vu its eagerness to rip toward blue trails off as the heavy nose begins to teeter.

Flying is pure fun as long as the engine is pumping. My retractable gear, “Streak Trojan” dropped like a rock without the Power 10 on high. Aerobatics are effortless and lines are smooth. As heavy as this engine is, the first two aileron rolls track better than stock before drooping toward mother Earth. Gear-up speed is impressive, but not noticeably different than the other two. That probably has something to do with attempting to pull a heavier tree stump through the right boundary of the VN Diagram.

When power rolls to half at the perch and the gear comes down, so does the airplane, demanding a lot of up trim and the re-application of power to arrest the sink rate. Flatter, power-on approaches are probably easier for beginning hobbyists than trying to judge a steep fast glide, but this engine almost forces a loud final followed by a “chop-the-power” method of spot landing. Experienced pilots might feel vulnerable flying power-on finals, aside from begging to get shot down, any engine hiccup and you are walking the rest of the way home, or in the case of RC flying, swinging roles to CSAR on foot.

Unfortunately, I only had the opportunity for one T&G before the carefree ease of substituting this engine for the PZ 480 turned sour. A brash clanking noise made itself known as the gear fell and locked, making me wonder if the nose gear had pushed the cowling forward into the prop. Given the expense of this T-28, I thought I better get it down quick with min power, though I needed some power to make the runway and the engine delivered. Landing was uneventful. As I taxied back, I realized that keeping the power low was a rare moment of good judgment. Looks like a giant aluminum motor housing isn’t such a great heatsink after all. The heat of the Power 10 absorbing a 35C dump melted the T-28’s nylon motor mount like a BLU-97 through a T-72. The mishap occurred about 5 minutes into an aggressive flight, despite enhanced airflow from a cylinder-less cowling and a louvered firewall. See attached photos for your scare of the day.

The Power 10 runs way too hot on this battery for a simple PZ 480 style installation—bummer.

Parkzone T-28 Power 10 35C (0 min 58 sec)


#3 Turnigy 35-36C--“A Big Steed That Doesn’t Want to Be Mounted”

I like to build stuff, I’m a born tinkerer. But sometimes projects fight back too hard. The thing I’ll remember most about this engine after it is gone to ebay isn’t the strong thrust, the upgrade price, waiting on shipping, the amp drain or even the lead nose, rather, the ordeal of mounting it.

This engine has a pretty weird design. The prop mounts directly to the outrunner bell using an interesting adapter that mounts a second shaft via 4 bolts into the engine casing. You don’t mount the prop directly on the steel shaft like most motors. It works fine if (a) you have a hole in the firewall to stash the main shaft which sticks out the back (and the T-28 does), and (b) if the cowling mouth is a very short distance from the firewall. It is (b) that creates a T-28 installation dilemma: you can either reverse the Turnigy shaft by application of lethal force, void your brand new engine’s warranty and possibly bend it before you have a chance to turn it on, or, you can fashion a very unique firewall spacer.

I waited long enough for international shipping, so I chose the second method, but only because I underestimated the difficulty of the task. The engine doesn’t come with sufficient mounting hardware to tackle a T-28 installation. The ST .10 doesn’t come with T-28 spacers either, but the Turnigy problem is much more complex because the engine casing itself is drilled and tapped very close to the shaft. That means you must use the inner holes of the included “X” bracket to screw into the engine, and the outer holes to link the engine to the firewall. The front circle of the T-28’s nylon engine mount isn’t large enough to attach the Turnigy spacers. Fortunately, my Power 10 came with a small bonanza of extra mounting hardware, so I borrowed its aluminum X bracket to match up with the Turnigy X bracket’s outer holes and spacers, then I drilled inner holes to match it back to the T-28 mount, again, by stealing some extra Power 10 nuts and bolts. Assembly order is tricky—Turnigy mount to engine, Power 10 bracket to T-28 mount, bolt through Turnigy bracket, slide on spacer, slide on washer, then loosely attach to Power 10 bracket, then the nut. The bolts had to be long enough to stay loose while assembling all four spacers, but short enough to end between the two circles of T-28 nylon mount, and only after completely assembled: tightened down. After tightening, I snipped off the extra bolt length to save weight, though I know I’ll never be able to reassemble this mount without new bolts. If that sounds like an ordeal, it was nothing compared to fitting it up by trial and error. I think reversing the shaft would also require a custom spacer arrangement.

When all was done, 3.74 oz had ballooned to 5.1 oz, with two X brackets, a redundant steel motor shaft, 12 bolts (4 to mount the second shaft, 8 to build the spacer), plus 4 engine mounting screws to attach the engine. Worst of all, the T-28’s nylon engine mount couldn’t handle the heavy hanging weight, there is a loud clacking resonance at around half throttle. I’ve read that this is common with Turnigy installations, so 35-36C users should probably invest in an aluminum T-28-Turnigy motor mount (ebay). The big chunk of aluminum will be heavier still, but certainly safer. With a solid metal mount, a new problem becomes how to air-cool the higher-amp ESC, so I recommend doing my Firewall Louver mod when using this engine for better ESC cooling and to squeeze out more installed thrust to counter the heavy weight of the motor and mount.

In the air, the Turnigy T-28 had good initial vertical climb, but the nose wanted to side off at lower altitude than the others. Aerobatics have plenty of punch, and the looping maneuvers are easier to make symmetrical with sweeping back halves. The plane dives like a P-40 separating from a Zero. Flat top speed seems exponentially drag limited, and feels exactly the same as the other two.

In Aero 101 we teach that that L/D)max, or the single airspeed that yields min airframe drag, max excess thrust, best climb angle, best glide range, and maximum flight time, occurs at a faster airspeed as weight increases, but max glide distance doesn’t change. In theory, all else equal, the airplane that takes on weight (airfoil constant) will glide faster but reach the same point on the ground. This engine proves without doubt that rule of thumb doesn’t hold up when you double the weight of only the nose, requiring double down elevator force to hold down the far end of the CG seesaw as you glide. That adds even more effective weight, dropping altitude quickly in a glide without as much forward travel.

Pattern turns are best carved at a steady clip, not engine-off floaters to keep control authority above significantly increased stall speed. Landings are fast, but the good news is that wind is a little less of a factor at the higher speed and the engine weight still allows the nose gear to rise a bit above the mains as the plane scoots along. Flaps might be in order, but also become self justifying as they add a couple more ounces.

This engine nicely illustrates the basic aeronautical engineering case for obsessive weight reduction. Every ounce saved, saves up to six (rule of thumb) more ounces as less lift is required, thus creating less drag, thus requiring less thrust, less fuel, reduced complexity, and even reducing scale of the airframe itself. While every ounce added demands several more ounces to scale everything up similarly.

On a side note, since I was paying for international shipping of my Quanum telemetry system (died on second use), Pitts S2B, and the 35-36, I decided to throw in a Turnigy 3000 mAh 40C LiPo. I hooked that big momma up to my 35-36 to see if it made any difference. Bang! 46 oz of thrust, not a bad boost. Interestingly, when I plugged the 3000’s 40C numbers into my xls, T:W actually dropped. This big battery tips my postal scale at a whopping 10.1 oz, about a third of the T-28’s total weight with the stock battery installed. It adds more than 40% of the weight of the aircraft, pre-battery. I guess I could have checked that before ordering—so I have another fabulous paperweight for the time being.

Parkzone T-28 Turnigy 35-36C 35C (0 min 54 sec)
If anyone is interested in a single-flight Turnigy 35-36 and/or Power 10, PM me. Realize the Turnigy run was overstressed with a 10x7 prop pulling 36.1A and 419Ws, above its maximum allowable burst of 35A and 400W. I do not recommend using a prop this aggressive on the Turnigy 35-36C with a strong battery.

Casualties are always higher in training than combat. During this test:
1 E-Flite 40A ESC smoked by their own Power 10 on first use—don’t know why
1 battery charger out of commission after charging Turnigy 3000 mAh LiPo ...and charging ...and charging.
1 T-28 engine mount melted by the Power 10, lucky I didn’t lose the plane
1 Quanum Telemetry system, unexplained failure after two uses—bummer, I liked it



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