Has anybody built and flown the DJ Aerotech 'Roadkill-series' Fokker Triplane and how does it perform as standard?

From : Don Stackhouse

I have really mixed feelings about replying to this. On one hand, I fully admit that I'm undoubtedly biased, after all I did design this airplane, and designers often tend to feel somewhat parental about their creations. On the other hand, at this time I probably have as much experience or more with this particular airplane than anyone else on the planet, so I am qualified to comment on its quirks, of which there are just a few. Also, there are probably a few other folks on this list with our little Triplane, or who are considering getting one, so my comments could be helpful for them as well. In that light, I'll proceed...

Yes, I have, except mine was from hand-cut parts. Joe Hahn built the one in the instructions photos, and it was from laser-cut parts. BTW, for anyone who might be thinking about "scratch building" a model like this, it takes about 3 full days or so just to cut out the parts by hand, and that's after I have the full size computer printouts to use as patterns. It takes me about 3 hours to assemble the basic airframe from laser-cut parts, and I'm a slow builder.

It has a few quirks, but it's far better behaved than I expected. In flight it's delightful, although a bit of an electron hog. It takes a lot of power to drag all those wings through the air, so cruise power is about 85% on the throttle stick position. It has short wings and a lot of power (and the torque that goes with it), so I've found that it likes to have about 1/8" (3.2mm) of right aileron rigged into it to counteract the torque (NOT P-factor in this case!), with the rudder neutral. If you don't have the ailerons rigged properly, then you end up holding or trimming right aileron into it with the Tx in order to keep the wings level. Because this also results in additional right rudder (due to the coupled rudder and ailerons), the airplane ends up flying wings level, but yawed to the right. This makes gobs of drag, making it look like the plane has a power problem. Land it, rig the ailerons so that it flies level when the rudder is centered, and you'll find it's much happier.

A note about power: Every now and then we hear reports of power problems with RK models. In most cases this is the result of bad batteries. There is a wide variation in the quality of small NiMH cells out there, and some of the more common and more popular ones don't even make a good paperweight. The ones John Worth sells (his "Double Time" 110 mah NiMH cells) are among the very best we've tested, and we've recently found another brand that looks very promising so far. There are some brands that do OK at currents below about 0.5 amps, but that totally wimp out at anything higher than that. The Triplane needs at least about 1 amp or so at full throttle.

The best way to test power is to check the prop RPM at full throttle. With a full charge, you should get at least 5000 prop RPM.

The other power problem we've seen once or twice is the result of overheating the motor terminals when soldering on the leads. This can soften the plastic end bell of the motor, allowing the brushes to shift and lose some of their contact pressure with the commutator. This happened to me on the right engine of the prototype Douglas DC-3. The airplane would swerve right when the throttle was opened for takeoff. I checked the RPM's and discovered that both motors accelerated normally up to about 3200 prop RPM at about half throttle, but the left engine continued to increase to a peak of about 5600 while the right engine actually decreased a little to about 3100 RPM (the twins use a 170 mah battery, and if one motor doesn't absorb its share of the power, there is more juice left for the other motor). The brushes on the right motor were bouncing at anything above half throttle. I changed motors, and both then read 5100 RPM at full throttle.

To cut down on the heat applied to the motor terminals during soldering, I solder the capacitor to the terminals, clip the capacitor leads to about 1/4" long, then solder the leads from the ESC to the ends of the capacitor leads.

OK, so you've checked the batteries and motor and you're getting full power, and you've rigged the ailerons so that the rudder is centered when the airplane flies with wings level, even though it needs some right aileron to offset the torque. At that point its general behavior in flight is similar to the A6M2 Zero, except for the higher throttle setting in cruise. Flight times for typical sport flying with my Triplane (the red one in the photos on our website) including some dogfights with Joe's Camel usually run somewhere around 5 minutes or so, on a good quality 7-cell 110 mah NiMH battery. Roll rate is very good, and turning radius is very tight (10 ft. dia. turns are pretty normal), pretty much like you'd expect from a Triplane. The flights aren't hugely long, but for such a nimble and responsive airplane they're plenty long enough. I haven't clocked it precisely, but cruising speed seems to be a little slower than the WW II warbird RK models, probably about 10 to 11 mph. I've flown it in my back yard, in a basketball court and in a golf dome and it's very comfortable in all of those venues. It seems to handle wind and turbulence surprisingly well, up to about 8 knot winds, maybe a little more.

It does like to fly with the tail up. If you're having trouble climbing or maintaining altitude, it could be that you're flying too slowly. Get the tail up, get some speed and it gets more efficient. Despite all that wing area, it doesn't have much span (in fact it has the smallest span of any of the RK models), so airspeed helps keep the induced drag to a minimum.

So what about the Triplane's legendarily bad yaw stability, and the problems with C/G from that short nose? In this model in flight they're truly a non-issue. The profile construction means there is very little weight in the tail. There's still plenty of room in mine to move the battery further forward, and there's no nose ballast in it. As for yaw stability, the profile aft fuselage acts like extra vertical fin area, so yaw stability in flight is fine. The rudder looks tiny, but it's really quite effective in flight.

The Triplane doesn't retain energy terribly well (too many extra wings hanging out in the breeze), so you need power and/or altitude for aerobatics. It will do a fairly nice loop with the right technique, and does respectable barrel rolls, whip stalls and hammerhead turns and wingovers ("bridges" for you Brits). The Sopwith Camel retains energy a little better, but does not turn quite as tight (however, the Camel does have really nice aileron response and probably a little quicker roll rate). The two models' dogfighting strengths and weaknesses are matched about the same as their full-scale counterparts.

So what are those "quirks" I kept referring to above? Well, those show up on the ground. Remember that old saying "Takeoffs are optional but landings are manditory"? That was never truer than with the Triplane. Takeoffs are great fun if you know the right technique (which I'll tell you in a moment), but for first flights a hand launch is often a good idea. It does ROG very nicely if the surface is adequately smooth. The catch is that the delta-shaped stabilizer and the two lower wings completely blank the rudder when it's sitting on the ground in three-point attitude. To take off, first hold full forward stick, then give it full throttle. You need to get the tail up immediately so the rudder will get some airflow, and you need to accelerate as quickly as possible to further improve rudder authority. As the tail comes up it will probably want to swing to the left, so be ready with a healthy jab of right rudder if required.

The tail comes up almost as soon as you've opened the throttle, so be ready to neutralize the elevator as soon as the fuselage approaches level. The elevator is quite effective, and it is possible to nose over if the ground is rough, such as on Astro-Turf. Once the tail is up, you hold the fuselage slightly tail-low with elevator, keep it straight with rudder, and it will lift off in a very scale-like manner pretty much by itself. If you survive getting the tail up safely and the initial takeoff run stabilized, it's a piece of cake after that. Or you can wimp out and just give it a hand launch, which works fine. Just make sure you start out with full throttle. Once it's safely in flight you can experiment with backing off on the throttle, but this airplane does like lots of power.

Landings have pretty much the same challenges except in the other direction. Do not try to three point it on. Leave throttle at about 50%, fly it on with the fuselage level, and hold it level with elevator till the tail won't stay up any longer. Use rudder to keep it straight during landing roll, keep the tail up and prop blast over the tail until it's slowed down to taxi speed, then let the tail down, back off to about 1/4 throttle (NOT zero throttle!) and let it roll to a stop. Taxi very slowly. The airplane behaves very well at a slow walk, and steers surprisingly well during taxi despite the fixed tailskid. It's really quite well behaved on the ground except for that speed range between a little faster than taxi to about halfway to flying speed. On the ground, rry to either go very fast with the tail up or very slow with the tail down, and spend as little time as possible in that dicey range of speeds between those two.

If you get really good and you're on a smooth surface such as the wooden floor of a basketball court, it's possible to do circling touchdowns on one wheel, and do some 20ft. diameter pirouettes like an ice skater circling on one foot. However, you have to do everything perfectly, just the right pitch attitude, and technique. It looks spectacular if done right, but takes lots of practice and you can expect more than a few ground loops in the process of perfecting your technique.

If you do have a prang or two, this airplane seems to handle it very well. The wings and struts of the Jenny, Camel and Triplane really stiffen up the whole structure, and those short noses are good for buckling strength as well. Overall, these three are all very durable.

I really wasn't sure what to expect the first time I flew the Triplane, and I ended up being very pleasantly surprised. It does have its quirks, but nothing like I expected, and nothing that required any unusual amount of flying skill. I would not recommend it as a trainer, but any reasonably proficient sport flyer should do fine. It's overall handling is really quite delightful, its appearance in flight is quite impressive (it always gets lots of attention from the spectators at the golf dome in Ft. Wayne), and it quickly became one of my favorite indoor/backyard models. Of course I might be just a little biased... ;-)

Don Stackhouse
DJ Aerotech