Conceptual design, design requirements, high efficiency twin

Here are set of requirements I have combined for an aircraft suitable for my use case. I have been collecting these things for quite long time now, and have changed them back and forth. However, it seems like they are becoming more stable now:

– Two engines. Rotax 914 (preferably fuel injected) or similar (912 turbo conversion). Alternate engines: HKS700T (the speed may not be achieved with the HKS option). (low power engines which run on autogas are mandatory requirement)
– Range 1000 nm with three on board (mandatory requirement)
– at least 3 places (mandatory requirement, long range flights, third seat is needed for baggage and rescue equipment)
– Designed for IFR flying (mandatory requirement)
– statically stable, dynamically stable behavior (mandatory requirement)
– gentle stall (mandatory requirement (for safety))
– Cruise speed > 200 kts @ 80% power (mandatory requirement for both range and usefulness)
– Stall speed max 55 kts (mandatory requirement, for safety)
– High altitude capable (cruise at 24000 feet) (optional requirement)
– Pressurization as an option (optional requirement)
– Lightning strike protection (mandatory requirement)
– Positive climb rate with one engine out (mandatory requirement)
– Spin recovery possible (mandatory requirement)
– Very high glide ratio and long glide range when both engines out (mandatory requirement)
– BRS system (mandatory requirement, for safety)
– Spin recovery parachute (mandatory requirement, for testing safety)
– Tri-gear possibly with RG, at least the nosegear with RG mechanism (Trigear mandatory, RG optional)
– At least normal category (mandatory requirement)
– Utility category (optional requirement)
– Reasonable cost to build a prototype

Means how to achieve this:
– Selection of efficient NLF airfoils
– By minimizing fuselage and engine pod wetted area
– By minimizing skin friction drag (smooth surface, gelcoat on top of laminate and polyurethane paint on top of gelcoat)
– By utilizing laminar flow over wings and fuselage as much as possible
– By using wing geometry that has higher effective aspect ratio than actual AR
– Turbocharged engines
– Lightweight molded composite structure manufactured from carbon fiber prepregs, foam.
– By minimizing intersections and protruding elements. As clean fuselage and wing as possible. Known limitations – double slotted flaps do require external mechanism.
– By use of double slotted flaps for high Clmax.
– By use of either T-tail or V-tail for good spin recovery.
– Large fuel tanks in engine pods
– For cost effectivity, a pair of midtime Rotax 912ULs equipped with e.g. VEMS fuel injection and Garrett turbocharger is more reasonable cost than pair of stock Rotax 914s. Downside: ease of installation is lost when the engine requires more work than usual for Rotax installations. However, fuel injection is essential for safety.
– Negative sweep on main wings
– Glass cockpit (IFR requirement)

Possible configurations to achieve this:
– Twin with tractor propellers on each wing (known limitation: the propeller causes turbulent flow behind it which increases drag over the engine pod and the wing behind the propeller arc)
– Twin with pusher propellers on each wing (known limitation: the pod on front of propeller decreases the propeller efficiency)
– Push-pull configuration with twin boom tail (known limitation: front propeller disturbs the airflow to the rear propeller and the efficiency of the rear propeller decreases)

How to verify the effectiveness of each parameter:
– Calculate basic parameters for each combination where only one parameter is altered in each.
– This concept generates several different designs and each parameter is justified if it produces verifiable benefit.
– The design points that are proven to produce positive results with large enough margin are incorporated into the design if it does not overly complicate the manufacturing.

Low hanging fruits (design points known to have been succesfull in other designs):
– Double slotted flaps with a mechanism similar to Dynaero. Proven on Dynaero MCR.
– NLF-airfoils. Proven on Cirrus, Lancair and Columbia (Cessna 350 and Cessna 400 nowadays) high performance aircraft.
– Molded composite structures. Industry standard nowadays in most new aircraft designs.
– Tractor twin. Proven on most twins around.
– Pusher twin. Proven on a Polish design called Orca.
– V-tail. Proven on Beech Bonanza and later Cirrus Jet and Eclipse Jet.
– T-tail. Proven on many aircraft designs to date
– High aspect ratio on twin engine propeller driven aircraft. Proven on Diamond DA42.
– Negative sweep (moderate) found on many dual seat gliders. Small amount of negative sweep can also be found from Diamond aircraft.
– Trapezoid on wings. Easy to manufacture from composite materials, the shape is not limited by manufacturing process.
– Prepreg composites. Proven on Cirrus, Lancair, Diamond and Columbia aircraft.
– Rotax 912 series engines. Proven to be highly reliable and simple workhorse on almost all new ultralight and LSA aircraft. HKS is slowly gaining some share, but Rotax rules so far. From personal experience I also know that the Rotax engines meet their TBO, our flying club frequently runs Rotax engines to their TBO without problem without major overhauls. Claims that TBO of Rotax is just marketing and that overhaul is required well before 1000 hours to that is simply not true, this has been proven by experience, the aircraft we used to own has flown about 1000 hours, and the Rotax is the original one and runs nicely without problems and it has been in hard use because the plane was used for the first 683 hours to train student pilots. Rotax can also run on autogas (actually the preferred fuel is autogas, not 100LL).
– Glass cockpit can be made a lot simpler than tradtional gauges. Wiring behind traditional gauges is a mess and takes a lots of handwork to accomplish. Glass cockpit wiring can be made very simple and it can be highly integrated where most of the tasks are done in software rather than mechanics.

Criteria for defining success and failure
– Minimum acceptable range is 800 nm.
– Minimum acceptable payload with full fuel is three persons with no baggage.
– Minimum acceptable cruise speed at 80% is 160…170 kts on Rotax 912/100 hp. However, when comparing to Rotax 912 twins (coming and existing), the speed is no longer in the “top class”, but rather below the top.
– Minimum acceptable glide ratio is 1:15. Target is more than that.
– Maximum acceptable stall speed is 55 kts. Design requires changes, if it is more than that.

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