Featured Bush Planes

The Prototype

It's been more than a year since the last update on the innovative prototype DoubleEnder. Ride along in the cockpit with designer Alec Wild as we're treated to a demo ride in this forward-thinking twin-engine bush plane.

By | October 06, 2015 12:00 am
Video

What is actually new these days in bush planes?

We have it good in the modern era of bush planes. As a student and historian of all things bush flying, I often wax philosophical about the state of technology and innovation in this particular niche of aviation. Traditionally, bush and STOL aircraft have been little more than modified certified trainers, with engine and prop upgrades, or the addition of fatter tires. The evolution of all modern bush aircraft can be traced back to the early 20th century and the origins of Cessna and Piper, and modern incarnations don't fall far from those family trees. But if one were to step outside the box-- the constraints of convention and tradition, and completely eschew the blueprint for the traditional bush plane that we all know and love— what would result?

Not possible. Or rather, not advisable. Aviation, like most applications of technology, is evolutionary and iterative. Innovations and inventions for the Super Cub (and many other bush planes) are rooted in real world experience. Most of the good stuff on the market was invented by the pilots who are out there actually landing in the highest, shortest, and roughest spots. The same guys who have broken their gear or needed a better prop are the ones who came home and invented better ones. And this is true for the inventor and pilot of the experimental "DoubleEnder" twin-engine bush plane, Alec Wild.

In my opinion, the DoubleEnder is one of the most innovative and creative designs in recent light aircraft history, and I was fortunate enough to go flying in the current prototype with Wild.

Design considerations

At this point the DoubleEnder is pretty well known, despite there being only a single flying prototype. A little over a year ago it was getting some heavy press, featured in EAA Sport Aviation magazine, AOPA.org, etc, and the video of Wild performing a "cliff dive" takeoff in Alaska went viral in social media (and it recently surfaced again last week on Reddit.) There was a lot of excitement, and rightly so. It is unlike anything the aviation world has seen, with its twin Rotax 914 push-pull engine configuration, its unobscured view over the pilot's feet from the bubble canopy, its active STOL wing design using slats on the leading edge and enormous double-slotted flaps, and its super-beefy landing gear with 35" bushwheels. Or is it? None of these features alone is anything new— the SQ aircraft have been doing things with slats and slotted flaps for years, the inline twin configuration is an age-old design, and bubble canopies have been helicopter and ultralight pilots for decades. But Wild's motivation for an improved bush plane had him asking the right questions when he dreamed this thing up, and that allowed him to draw from the wealth of innovation and invention previously accomplished by some of the great pilots and inventors who've already been here. The genius is in the painter, not the paint.

DoubleEnder's double-slotted flapsThe massive Performance STOL™ double-slotted flaps run nearly the entire length of the wing. OratexThere has only been one DoubleEnder prototype, but you may have seen it in a few different color schemes. It currently wears the paintless Oratex fabric on the wing.

What is the motivation behind such an aircraft? Prior to arriving at Wild's hangar at Aurora airport just outside of Portland, Oregon a few weeks ago, I was convinced that he was like most other STOL innovators, concerned mainly with performance. An experienced Cub pilot himself, he has wrung them out all over the globe, from backcountry Alaska to bush Africa, taking the model to the limits. In our cockpit conversation though, I was surprised to learn that his primary motivations were safety oriented. He recounted the losses of pilot friends who may have been saved by an aircraft with engine redundancy, or with a wider and more forgiving low-speed envelope.

In addition to being a more forgiving aircraft in flight, there are other advantages to the design: The high props for instance are so far off the ground that inadvertent prop strike is nearly impossible unless the aircraft ends up on its back, and as this is a prototype aircraft, Wild intimated that there are plans to modify the design to prevent it from ever ending up on its back in the first place. We'll have to wait and see what emerges from his bush plane skunkworks in that regard.

As for the landing gear, it is one area that utilizes a more conventional design, at least in the context of modern fat tired bush planes. The gear legs are extended heavy duty PA-18 design, but the bungees have been traded in for the polymer sprung/dampened AOSS system, a popular suspension system with Super Cub owners, built by Burl's Aircraft.

The use of Rotax 914 turbocharged engines as a pair is a departure from traditional Cub powerplant choices. The smaller Austrian engines boast such a weight savings though, that hanging two of them incurs little more weight penalty than a single O-360. The in-flight adjustable pitch Airmaster propeller hubs swinging Warp Drive composite blades make for an extremely versatile combination.

With this in mind, I can't help but think that aside from increased safety, or performance, or any other advantages one might feel this aircraft offers, the absolute coolest thing about it is the unobstructed view between the pilot's feet.

I was both excited and a little apprehensive to climb in with Wild and get a demo ride, and that's mostly what this story is about. If you want to learn more about the technical evolution of the DoubleEnder, check out Budd Davisson's article from the February 2014 issue of EAA Sport Aviation.

Tufts of yard for aerodynamic testingTesting and changes continue with the wing design; these tufts of yarn allow the designers to review the aerodynamic performance on video.

Flying in the DoubleEnder

Showing up at the hangar, I wasn't sure quite what to expect. Alec Wild is a busy guy and scheduling with him had proven a little difficult, but the planets finally aligned and I rolled up to find the crew at work in the hangar. Wild greeted me and gave me a brief summary of the recent developments on the wing, which was covered in little tufts of yarn for aerodynamic testing of their latest generation slat mechanism.

HangarDoubleEnder skunkworks

It is not obvious from photos or even videos, but one of the most innovative features of the DoubleEnder's wing is the spoilers that deploy from just behind the leading edge of the inside wing as the aileron nears maximum up deflection. At extremely slow speeds that require maximum control input, the aileron just isn't enough to provide the needed roll authority. The little spoiler pops up as the aileron begins to top out and provides the needed drag to get that wing down with very little airflow. It's a brilliant idea, one that is already being integrated into other custom STOL aircraft builds in the community.

Wild and one of the mechanics wheeled the aircraft out and I loaded my camera gear into the baggage area behind the rear seat, keeping my camera in hand for the in-flight footage. It's a pretty tall step to climb into this thing, but the long rail-style steps that have become so popular these days on Cub gear legs made it quite a bit easier.

I settled into the passenger seat and was pondering how easy the step was into the fuselage as the dreadful realization hit: There were no rear seat flight controls. For the safety of all involved, it was probably for the best, but it is with a heavy heart that I report that I did not get to pilot the DoubleEnder.

We taxied out to the takeoff end of the gigantic runway 35 at Aurora, called our intentions and then proceeded to takeoff from the grass in between the taxiways. The two 914's — one carbureted and one fuel injected — roared to attention, and we were climbing out at an incredible rate. How fast? I have no idea, it felt impressively steep. After all, each of the engines is a turbocharged 130hp unit by itself, so depending on blade pitch and RPM, that's potentionally 260 HP in a 2-seat aircraft. We leveled off at about 1,000 feet and headed out to our favorite local river bar spot where I planned to take some photos and maybe shoot video of a few takeoffs and landings.

Wild slowed the aircraft down considerably, and suddenly began doing aggressive dutch rolls at a rate that seemed rather fast for a Cub-style wing with some pronounced dihedral. I asked him what we were indicating on the airspeed; 35 mph. The aileron-coupled spoilers do their job well.

Forward view View from the cockpitThe view even from the rear seat is amazing, allowing for observation of the surface almost directly beneath the aircraft.

I won't go into too much detail about our play spot. Many local pilots will recognize it as their backyard though; it is a slightly curving gravel bar that is mostly non-existent in the high water months but during summer is massive, measuring roughtly 1,000 ft in length. As we neared I figured we would just be doing a few stop-and-goes there, especially given that it was already north of 80° F out, and my 200 lbs wasn't exactly helping short field performance, but as we neared the bar I noticed that Wild was setting up a little differently.

My situational awareness is often severely reduced by looking through a camera lens, and it's always an exercise in complete trust of the pilot-in-command for these filming experiences, which is why I'm very selective about who I'll fly and film with. As the scene unfolded inside the viewfinder, Alec turned up a narrow side channel of the river that, despite spending a fair amount of time in this area, I had never even noticed before. Before I knew it he put us down on a tiny gravel bar covered in massive tufts of grass that by my estimate was no longer than 200 feet, in a very narrow spot shrouded by tall trees. In calm winds, the touchdown is never quite as slow as you'd like for truly short stuff, but Wild was a master on the brakes as he wheeled it on and held the tail level with elevator, kicking a 90 degree turn as the tail dropped and coming to a stop near the end of the bar. He shut down the engines and we climbed out.

I was unloading my camera gear and setting up as the sound of an ATV approaching cut through the rustling of wind on the grass. Two local farm kids pulled up, one with a shotgun slung over his shoulder. I could see on their faces the disbelief that both 1) an airplane was sitting here and 2) one that looks more like a Mars rover than any aircraft they'd ever seen. I think they'd seen us fly over and high-tailed it to the river to check out the action. Wild answered their questions as they ogled the machine, managing the expected public relations aspect of operating the DoubleEnder like a pro.

As the ATV guys said goodbye and rode off, Alec climbed into the plane as we prepared to shoot some action footage with me on the ground. He brought the engines to life one at a time, and as I held some long tree branches back for his wing to clear, taxied out to fly a few laps.

View from the bushesThe sound of DoubleEnder's dueling propellers makes the most incredible sound as heard from the LZ.

If you've already watched the video, you know that the sound of this thing is unique (and awesome.) The twin props cycle in and out of sync at lower RPM, making that distinct WAARRR WAAARRR sound that multiengine pilots are well-acquainted with. And the high RPM 914's boosted to nearly 40" MAP whining in unison-- I just think it sounds rad, a totally unique kind of engine music.

The experience behind the lens

I had come on this flight ill-prepared as a videographer. In documentary filmmaking they call these kind of experiences "run and gun" because the opportunity to plan and control the shots and environment doesn't exist. If any of my flying experiences has deserved my whole kit of cameras and lenses, this was it, but alas due to limited time I only brought a single camera and 3 lenses. No GoPros, no fancy stabilizer rigs or sound gear. This approach is usally more amenable to STOL pilots anyway, as the extra pounds equal extra feet on the roll. I even forgot my handheld radio in the truck, so we were limited to hand signals to coordinate the shots. Basically, Alec just flew and had fun and I captured what I could.

The viewWhen your shadow doesn't quite match your self-perception...

That's fine, I like a challenge. I'm often acutely aware of the fleeting nature of these moments, and how lucky I am to be in this situation to begin with, so instead of simply watching and enjoying such a fascinating airplane in action, I found myself running back and forth along the bar to find the best shots, the coarse sand and gravel mix stealing my energy and finding its way into my shoes. At 10:00am it was really heating up out here, and I was sweating as I swapped lenses carefully to avoid contaminating the camera sensor.

After a few laps on this small circuit, Alec picked me up and transported me to another small bar just 300 feet downstream where'd I'd capture some of his water-assisted hydroplaned landings. A fairly dry year already, the water level was down and had left behind a crusted slime layer with mud just underneath, which loaded the soles of my shoes, eventually finding its way into the DoubleEnder cockpit. Sorry Alec! It was also like operating on peanut butter-- a locked brake just slides across the surface.

Beautiful in flightThe DoubleEnder in the environment where it shines: Close to the ground. Notice the pronounced wing dihedral.

To behold the DoubleEnder in flight, even taxiing on the ground, is something else. It is simply so different from watching a more conventional aircraft like a Cub or Cessna. I could see Alec's every move through the canopy as he flew the aircraft-- his feet working the pedals, his hand on the stick, the other on the throttles, and the operation of the flap lever. There's no mystery about what the pilot is doing, and likewise the pilot has an excellent angle of view side to side and top to bottom. It's not unlimited though-- the blind spot is directly overhead where the engines mount, shrouded by a composite nacelle.

The prop discs being so high off the ground, hanging on the massive nacelle overhead, were a reminder of what a serious design advantage it is for the off-airport environment in terms of clearance. Taildragger lovers have always heralded the extra prop clearance of the 3-point attitude, but nothing can best this. There's also a safety advantage to people on the ground, at least on 35" Bushwheels: At 6'1", the front prop disc was higher than the top of my head. The rear, however would still present a serious risk, so nothing changes in the way of protocol for working around running aircraft.

Big thrustThe massive thrust of the twin Rotaxes moving the surface of the shallow water.

The significance of the DoubleEnder

Why would you want an aircraft like this? For purely experiential reasons, it's enjoyable. As someone who learned to fly in open cockpit/overhead engine ultralights, I've always felt that there was something lost in the flying experience with traditional aircraft, something I could never truly convey to pilots who learned in more conventional airplanes. That could be due more to the stigma of ultralights than anything, but apply these design features to a Super Cub-type platform and it gains some legitimacy. I'm talking of course about the view over your feet. It's just awesome.

Why two engines? Performance and redundancy. Redundancy and performance. The two attributes are not really separable, save for perhaps some of the older Piper twins that had terrible single-engine performance and introduced serious adverse yaw, making the design almost a liablity rather than a safety feature. Put the engines inline though and you eliminate that yaw issue, but introduce cooling issues for the rear engine because of the obstructed airflow, case in point the Cessna 336/337 Skymaster; beautiful airplane but it was an expensive design lesson. That's not to say the DoubleEnder hasn't had some growing pains with the arrangement of the 914's overhead, but they have a distinct advantage: water-cooled heads.

As the temperature approached 90°F, we took off from the downstream gravel bar for the final time and headed back to Aurora. Enroute, Alec demonstrated the significance of a second engine by shutting down the front one, which is always slightly startling with the Rotaxes because they stop nearly instantly. But in this case that rear engine kept on screaming, potentially putting out 130 HP and doing a fine job of carrying around 2 guys on a hot day. The early Super Cubs only shipped with 135 HP powerplant, as did some of the Pacers, so for a short time on this flight we were like any other economy bush plane. It was a reminder though that this aircraft was as much about safety as it is about performance, because losing an engine on takeoff at speeds that barely register on an airspeed indicator can have deadly consequences.

How do I hydroplaning the wheels?How do I hydroplaning the wheels?

We landed back at Aurora with an approach befitting a helicopter pilot practicing autorotations. Extremely high on final, Alec dropped the massive Performance STOL flaps and we began the slow sink toward the surface. He nailed his spot on the grass between the runway and taxiway and our rollout was a mere 30 or 40 feet. This is of course mostly the technique of a superb pilot, but the aircraft is the real deal in slow flight.

Our time together was short, and I still have many questions about the design that will perhaps be answered the next time I see Alec, but I was left with the impression that the DoubleEnder was indeed the innovation I thought it was from afar; modern lightweight thrust technology well-integrated with the best design features of bush plane history.

I'm really looking forward to my demo ride in the forthcoming side-by-side DoubleEnder design for which Wild has kit manufacturing aspirations. Stay tuned.

For more information on the DoubleEnder project, vist the official website: Bushplanedesign.com

How do I hydroplaning the wheels?Autorotation practice back at home base.
Zzz

Zane Jacobson

Zane Jacobson is the founder/editor of Backcountry Pilot and currently flies a variety of rental aircraft around his home area of Portland, Oregon while continuing construction of his Bearhawk.

Website: www.backcountrypilot.org

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