Wyvern concept vehicle
Developed as a private project since early 2014, the Wyvern is a concept vehicle designed to test out new ideas in the lightweight flying sector.
The original design brief was to produce an aircraft to be flown under microlight regulations which could also be used as road-legal transport with minimal conversion. The wing should be carried with the vehicle when on the road, allowing fully independent fly/drive touring. Originally to be flown under a flexwing, it was thought a paraglider wing would also be an option for future development.
All previous flexwing trikes have used a traditional wheel layout of two wheels at the rear, with steering provided by a single nosewheel. This simplifies the steering mechanism whilst also providing maximum clearance for the rear-mounted pusher propeller.
However for road use a traditional trike wheel layout is relatively unstable during cornering. Having had previous experience developing three-wheeled road vehicles, designer Mick Broom very quickly opted for the reverse trike (or “tadpole”) layout. This gives two widely-spaced front wheels providing full Ackermann steering, with a single rear wheel positioned behind the prop arc for stability. A single rear wheel also greatly simplifies any power drive as a differential is no longer needed.
As this had never been done before, the first stage was to produce a prototype aircraft to discover any problems with the layout. This was built as a side project over some years and first flew in May 2019.
Built with a minimal budget, the Wyvern prototype uses a number of spare components from other aircraft. The test wing was a Chaser, giving a hands-off trim around 50-60mph. The engine is a Hirth F23 lightweight flat twin, producing 50Hp which is arguably a bit excessive. The first stage frame was made from welded steel, although carbon tubing has been investigated for future weight-saving.
The front wheels are steered in the conventional push-left-go-right flexwing system using the foot pedals. These also articulate to give throttle and brake control. Adjustable independent suspension is present on all three wheels, with an anti-roll system at the front. Braking is currently supplied by discs on the front wheels.
The engine is positioned low in the frame to keep the centre of gravity as low as possible. A belt drives the propeller, which is now far enough rearward to clear the trailing edge of the wing. This allows for a much larger diameter propeller than would normally be possible. This in turn produces far greater thrust with lower noise output.
The pylon is triangulated to remove the need for a view-obscuring front strut. This has the benefit of being easily adjusted to accommodate different wings for future testing. It is also designed to allow hinging the wing basebar down to supports on the main trike frame after removing only a single pin. This aids in rigging, as well as allowing any future topless wings to be folded on the trike in minutes for storage or road use.
On the ground the benefits of having two steering wheels are immediately apparent. The trike has much more directional control, and is amazing fun to just drive around, even without the wing attached! The trike is designed to hang slightly nose-up as with other flexwings to ensure that the rear wheel touches the ground first during landing. This allows cross-wind landings to the same extent as any other trike. Having a single rear wheel positioned so far back allows for plenty of suspension travel, as well as vastly improving stability during takeoff when used with a paraglider wing.
Acceleration was extremely quick with the large propeller fitted, and at no point during the flight testing was full throttle ever used. One downside of having such a large propeller was a very noticeable torque reaction which caused the trike to swing sideways after takeoff. It is thought that this effect can be mitigated in the future with careful adjustment of the thrust line. In all other respects there were no substantial differences to the flight characteristics compared with a standard trike.
The next stage would be to provide power to the rear wheel for road use. An additional benefit would be reducing the takeoff roll even further.
An electric motor in line with the rear suspension mount would appear to be the best way to do this. The existing internal combustion engine could be down-sized and provide a generator to charge batteries which could be positioned below the pilot seat. The IC engine could also drive the prop (possibly in addition to another electric motor) via a modified drive belt. Both motors and IC engine would be controlled by the existing foot throttle using a fly-by-wire system currently under development.
We would be very interested to hear from anyone with experience of these sorts of systems. The Wyvern is currently being developed on a non-commercial basis as a hobby project, but any ideas or advice would be very welcome. Please get in touch on the email below. For more technical information and details of help required click here.