Source: 2020 Q1 Beartracks
Building light takes on a whole new meaning with Bob’s newest prototype. His latest design is an ultralight, intended to have electric propulsion. The fuselage is 4130 steel tube. The wings will be a fabric-covered aluminum structure. And compared to the rest of the Bearhawks, it is a whole different animal.
Part 103 in the United States allows very generous freedoms. There is no FAA requirement for pilot training, pilot certificates, or medical certificates. Anyone can make, work on, or design an ultralight. Airframes do not need to be amateur-built. There are no ADS-B requirements. In the legal sense, aircraft that operate under Part 103 are “ultralights” and not “aircraft” so they don’t need to comply with Part 91 (except for one specific point about TFRs). Part 103 is very short by regulation standards, but there is a specific list of limitations. Notably, there can only be one occupant, and the maximum empty weight is 254 pounds. The power off stall speed has to be less than 24 knots, and it can’t be capable of more than 55 knots at full power in level flight.
Airplane designers always make trade-offs depending on the mission at hand, and this is a very specific mission. Bob is looking for a local “around the patch” airplane. He has still not ironed out all of the propulsion details, since he hasn’t needed to yet, but he’s targeting a flight time of around 45 minutes. Based on what is currently available, he says that will require about 60 pounds of batteries. The target landing speed is 25 miles per hour, and with a 50 horsepower motor he’s concerned about exceeding the top end speed limits. “At 50 hp it will be capable of more speed than I want to have.”
Most ultralights on the market are high drag, so exceeding the top speed is not a concern. They have very short takeoff and landing capabilities, in part because their drag allows for steep approaches. Climb rates are usually great because of the excess thrust available at lower climbing speeds. Bob’s philosophy for this one is a little bit different. It will be very low-drag, with only a single wing strut as in his other designs. As he says, “Compared with most ultralights this thing should go through the air pretty easy. It should still land slow but will climb out fast.”
Those single wing struts are an airfoil-shaped aluminum extrusion, sourced from Carlson. They are half the weight of the struts on the Patrol, but still heavier than he’d like. While a wall thickness of .060” would make the struts strong enough, they are made to .080. He says, “You can only make extrusions so thin or they don’t extrude well. The depth and chord are good, but the wall thickness is thicker than ideal. They weigh about 4-5 pounds each. I hate to put that much weight in it, but some places you don’t have any choice. Another option would be a round tube with composite streamlining, but the extrusion is much easier and only slightly heavier.”
As of today, both wings are framed. They will be fabric covered to save weight, with a shop-laid carbon fiber skin leading edge under the fabric. Bob made a mold by bending a sheet of aluminum to the shape of the leading edge. Then he laid up two layers of carbon fiber inside the mold, so that what will become the outside of the skin will be as smooth as the aluminum mold. The skins are 4 feet long each, with three per wing. They will be bonded to the spar and ribs, with flush blind rivets. Most of the strength will come from the adhesive. Any single strut wing requires lots of torsional stiffness, and the bonding will help gain that. The weight of the spars and ribs is 17 pounds per wing. The fuselage is framed up with the primary tubes, along with the landing gear and shock struts. The bare frame was 33 pounds, which became 42 pounds with the main landing gear, and control system (stick, rudder pedals, etc). These numbers are just staggeringly low. The wings are hanging from the ceiling, with the fuselage rolling around on the wheels while it receives a few formers and stringers.
The electric powerplant is still in the works, in part because the technology is still advancing, and in part because Bob hasn’t found exactly what he wants yet. He’d like to try to find a motor and controller from an American manufacturer, but hasn’t located one yet. The motor and controller need to be closely correlated, as will the battery with a battery management system. Bob asks if anyone in the Bearhawk world can be of any assistance with narrowing down the choices, please reach out. He knows it would be no trouble to install a combustion engine in this size range, but wants to try and make the electric work. He envisions that the battery packs would be quick to swap out, through some sort of access panel. It may be one single pack, or two smaller packs, whichever is easier to handle.
As for covering, Bob says he’s been vacillating with using light weight Ceconite or Oratex. The Ceconite would be painted with a water based latex paint just to fill the weave, not the regular Polyfiber process. He’s willing to forego UV protection since it will spend all of its life hangared. He’s just wanting to fill the weave so that the air doesn’t blow through it. Oratex makes a lightweight fabric that is an appealing option, but is much more expensive.
Speaking of expense, he says, “It’s not going to be a cheap airplane. I’m not looking for the cheapest or simplest, but rather the best. It won’t be for the guy who is wanting to see how cheap he can get away with flying, but would be really nice for someone who wants real performance but doesn’t want to mess around with government regulations.”
Soon he’ll need to find or design a windshield. He’d rather find something formed with compound curves, rather than a simple bent-wrap of a flat sheet. He says the formed windscreen flows much better into the upper surface of the wing, and looks better. It would be hard to get that smooth transition out of a single sheet wrap, and not end up with a sharp corner. He called a few days ago to LP Aeroplastics but wasn’t able to get through. He’s hoping they can find something already in production, but have it made out of a thinner material. Next will come making the boot cowl.
On the topic of designing airplanes, Bob says, “It’s all interesting, constantly creating ways to get the result you want. That’s why I love designing airplanes, it’s just so much creativity involved. Not following any directions, just working it out in your brain.” He says that having an engineering education is important, especially for being able to model how structures will perform in advance. But when it comes to having fun and making great airplanes, education doesn’t help with the crucial mental 3D puzzle of how things will go together. He loves the challenge of setting out into uncharted territory, building without instructions for any road map to get to the final destination.
As for his recovery from the crash last year, he’s still wearing a special boot most of the time. He can walk around in the house without it, but still is not ready to fuel his airplanes or climb ladders. He’s comfortable flying but not yet with hand propping his airplanes. His friend Mike Meador will stop by on occasion to help get things going. He’s still getting around quite a bit in his old cars on nice days, touring the back roads of the countryside around the mountains of Virginia.
Here is Mike making airplane noises in the fuselage:
Other progress photos:
