newsletter for customers and enthusiasts 01 | 2018

cult — emotions — freedom — dreams

Reiner Stemme is developing and building his new high-tech aviation product RS10.e elfin at the airfield Schoenhagen

(EDAZ), south of Berlin, under the company RS.aero GmbH. The glide ratio he is aiming at will be in the 50:1 area, thus

qualifying for the FAI two-seater category (20 meters wing span). elfin will be powered by an electric engine for full

self-launch capacity and additional powered flight of one hour’s duration. With an MTOW of 900 kg the side-by-side

two seater will be certified as an EASA CS-22 motor glider. For longer range touring, a RangeExtender can be clipped

under the wing to provide power for flights of up to 1,000 km at a speed of 185 km/h. The RangeExtender is a single

under-wing pod, housing a power generator and a fuel tank. The development and production for prototype 001 and

002 in Q4/2018 is on track, with certification and deliveries to begin in early 2019.

A – development process

1. AerodynAmic design

elfin represents the logical next step forward into a new era of soaring and recreational aviation, based on recent

and continuing advances in the development of electric propulsion systems . To optimize our application of this

Fig. 1 – optimization of the aerodynamic characteristics with modern CFD simulation (VSAero)

new propulsion technology, however, we need to provide the best possible airframe. Here are some of the concepts

underlying our approach to this new design:

A modern sailplane is a complex combination of various disciplines, but all of these must be based on rigorous

aerodynamic design optimized for modern high-performance soaring. To advance the state of the art, we must

begin with thorough consideration of basic principles. In the case of elfin, this has meant performing extensive CFD

(Computational Fluid Dynamics) analysis of the basic aircraft geometry using VSAero software.

The results have been impressive. Please refer to the fuselage overview in Fig. 1 (on previous page).

The extensive laminar flow over the wetted surface is of special benefit for the side-by-side cockpit arrangement.

Special emphasis was put on the wing – fuselage junction, long recognized as a source of intersection drag with

significant potential for reduction, and now, after spending months including consultations with the program

designers, this has been achieved.

Near the fuselage, the ideal elliptical lift distribution of sailplane wings is disturbed, increasing the induced

drag. Therefore, elfin was designed from the outset with a high-wing configuration. This configuration is, in

general, aerodynamically superior to the mid-wing configuration often used in sailplane design. With a high-wing

configuration the lift reduction due to the fuselage is minimized.

Furthermore, a positive twist was applied to the wing root section of elfin, to compensate for the loss of lift in the

fuselage region, resulting in a nearly elliptical lift distribution and minimizing induced drag.

The flow at a wing-fuselage junction is generally turbulent. Thus, in addition to the wing root twist, we designed a new

turbulent root airfoil specially for elfin. It is optimized to reduce friction and pressure drag in the root area. Several

iterations of the root twist and of the airfoil were performed to ensure a separation-free root junction. In addition,

adequate fillets between wing and fuselage of the elfin prevent the so-called diffuser effect. Without proper fillets the

intersection of wing and fuselage forms a nozzle resulting in a pressure peak on the lower side of

the wing just behind its stagnation point, followed by a steep adverse pressure gradient which triggers separation.

Needless to say, elfin also features all the other qualities of high-performance sailplane design:

• Five different airfoils are used in the wing to maximize the performance of the elfin. All are derived from the

main airfoil RS10-143, optimized at each spanwise section for the specific chord length and Reynolds number.

• The airfoils feature a well-balanced and steady lift coefficient slope, without a premature lift plateau or local

maxima as is characteristic for some other airfoils.

• The shape of the elfin fuselage was optimized to ensure laminar flow over the greatest possible area in order

to minimize drag.

• The fuselage is tilted nose down compared to the wing in order to match the incoming streamlines.

• The fuselage contraction ratio is well adjusted to reduce the wetted surface area and avoid separation.

• Winglets were optimized to ensure maximum cross-country performance.

2. e-system

Extensive testing is continuing on the propulsion system delivered by Siemens AG Corporate Technology (CT NTF AI).

We were able to gain important experience and had a closer look at the system’s general behavior under load, the

battery capacity and discharge properties, and static thrust of the entire system. Overall, we were able to reproduce

the system characteristics defined by Siemens.

The new SIEMENS 70 kW (93 hp) “doughnut” motor, incorporating a central opening for the nose cone sliding support

tube, has been successfully tested. Powerplant weight and operating characteristics have been confirmed to meet our

demands.

3. FuselAge

The fuselage is built as a single unit, using the benefit of placing the light 70 kW (93 hp) electric motor (less than

30 kg) inside the forward segment instead of at mid-fuselage, and driving the propeller directly without the need for

a reduction gearbox. The structure is built up from a prepreg sandwich shell, supported by four modified hat-section

stringers from the rear to the nose bulkhead.

This structural arrangement provides the targeted weight reduction of 25% compared to former designs, which is

required to compensate for the weight of the battery. Cockpit crashworthiness received special attention, a task

made even more difficult by the requirement, for ease of entry and exit, that the cockpit sill be no higher than 96 cm

(38 inches) from the ground. This value was finally determined, after considerable wrangling, to respect the needs of

older pilots (e.g., “the Boss”—Dr. Stemme himself).

In weighing performance against ergonomics for tall (and wide) pilots and tandem vs side by side configuration, we

decided on the latter, to accommodate pilots of up to 2 m (6 ft 7”), with a width of 0.6 m (2 ft) between the cockpit

side wall and the centerline above the low center console. The engineering of lightweight and easy-to-operate seat

adjustments for pilots between 1.6 m (5 ft 3”) and 2 m (6 ft 7”) tall is still in progress. Tests for ergonomics have been

made with our mockup to match reality prior to commencing CAD design of the cockpit.

The forward-hinged one-piece canopy incorporates an emergency jettison mechanism and integrated gas springs for

easy opening. A “Roeger hook” is included to ensure clean canopy separation in an emergency. Opening and closing

handles and emergency releases are on both sides of the cockpit.

The propulsion system battery is divided into two units. Half of its capacity is located in the rear part of the fuselage

for center of gravity considerations, while the other half, each assembled from relatively light sub-elements for a total

weight of about 50 kg (110 lbs), is placed behind the rear cockpit wall. Easy access is provided for both. The forward

battery, close to the aircraft center of gravity, can be removed for competition soaring purposes to allow use of up to

180 kg of water ballast.

The electric motor is integrated into the front fuselage bulkhead, carefully mounted with vibration damping elements

to eliminate even minimal vibration and noise from an already very smooth-running powerplant. Just behind it, below

the covers of the foot spaces, are the engine power controls and small coolant radiator (dissipating less than 5 kW).

The cooling inlet and outlet are automatically actuated.

Fig. 2 – unique side-by-side crashworthy cockpit design

4. lAnding geAr

Landing gear design is not an aircraft engineer’s favorite task, but a typical source of trouble later on if not done

correctly from the outset. We investigated known and unknown designs and finally are proud of a simple single axle

solution with minimum door sizes and a number of further benefits:

1. Weight reduced by >25% compared to existing designs

2. No interference or overlap between left and right side; independent electric activation of each side of main landing

gear

3. The track of 1.20 m (47”) width is excellent for this aircraft class. A significant reduction of outer wing weight to

< 30kg each provides easier taxiing and gives more safety at take-off and landing by a reduced moment of inertia

around the vertical and longitudinal axes.

4. Electric actuators are not specially designed, but common off-the-shelf parts

5. Main wheel size is selected for soft grass fields (5.00-5)

6. High energy absorption of the elastomerically damped suspension struts exceeds the requirements of CS-22

7. Emergency landing with gear up (recommended for landing out on rough unprepared surfaces) is supported by

two strong stringers in the center of fuselage

8. In case it happens – damage to the airframe primary structure would be the final step. All elements that would

be damanged prior to the primary structure of the airframe are easily replaceable and will be kept in inventory.

But of course – this will not happen: Happy landings are part of the sales price.

9. The tailwheel is steerable, the „Moritz“ model from Tost, outer diameter 210mm (8.25").

5. rAnge extender

We are evaluating different options for the type of engine to drive the generator (diameter no more than 380 NM,

specific fuel consumption below 0.3 kg/kWh, high speed ( >6,000 RPM), weight less than 30 kg (66 lbs), TBO 1,000 hrs, ...).

The decision will be made soon.

Fig. 4 – RangeExtender mounted on one wing

Fig. 3 – Innovative landing gear design

In accordance with customer requests, we will upgrade the generator output to the 50 kW range for continuous cruise

performance of at least 215 km/h (115 KTAS) at FL100 for 6 hours.

In a departure from the original concept, rather than being integral to the Range Extender, its fuel will be carried in

an internal wing tank with 90 kg (198 lbs) capacity (installed in the opposite wing for optimal lateral balance). It thus

appears feasible to limit the weight of the Range Extender itself to the range of 60 kg (132 lbs), enabling easy ground

handling (similar to wheelbarrow) by one person, and to include an integrated electric hoist for one-person mounting

on, or dismounting from, the wing in a few minutes.

New Year get-together and opening of RS.aero facility on the airfield Schönhagen EDAZ.

Contact:

REINER STEMME.aero GmbH

CEO Dr. Reiner Stemmeinfo@reinerstemme.aero+49. (0)33731.777778Flugplatz Halle C2 14959 Trebbin, OT Schönhagen Germanywww.reinerstemme.aero

Contact partner Europe:

ZEDIA-AVIATION GMBH

Jan Zesewitzinfo@zedia-aviation.com+49. (0)172.4422527Karl-Liebknecht-Str. 30/30a16816 Neuruppin Germanywww.zedia-aviation.com

Contact partner USA:

RS.AERO USA PARTNER

Alexander Papenberg a.papenberg@reinerstemme.aero+1. (310) 614 - 0796 1140 Wall Street #477La Jolla, CA 92038 USA

The next elfin news will focus on:

• Weight reduction program results

• RangeExtender design

• Flight handles & power control

• AERO 2018

b – compAny newsDr. Stemme has sold the majority (51%) of his former company, REINER STEMME UTILITY AIR SYSTEMS (RS-UAS), to a

Qatar investment group. That company will continue the so-called Q01 surveillance program for the Emirate of Qatar.

The elfin program, formerly carried out within RS-UAS, will now be continued by REINER STEMME.aero GmbH,

owned by Dr. Stemme and aviation investors. The former engineering team is already on board, so there has been no

interruption in the program to get elfin airborne in 2018 and start customer deliveries in 2019.

The company will of, course, continue its long-standing and very successful relationship with aircraft manufacturer

WKK Andrzej Papiorek in Bielsko Biala (PL) – who have already built more than 200 S10 airframes – as well as with our

other well known business partners.

The company also enjoys a very favorable relationship with the entire German soaring and sailplane design

community – not least with the Akafliegs, whose members dominate the company team.

The airfield environment of Schönhagen EDAZ fits the task. Known companies like AQUILA are nearby. It’s about 45min

from the center of Berlin by car. A comfortable train connection is Trebbin station.

Soon you will see our new website, as well as all our other marketing material produced by the highly engaged and

professional M8 Design Agency of Priska Wollein and Matthias Fischer. They, by the way, have owned an S10 for many

years, are flight instructors and longtime members of the SOG (the international STEMME OWNERS GROUP).

Director of marketing is Edmund Mästele, who has been working alongside Reiner Stemme for many years in this role

and who is now additionally tasked with the setup and management of the administration and infrastructure of the

company to support the engineers and technical staff.

For sales in Europe we have engaged Jan Zesewitz with his ZEDIA-AVIATION company. He comes to us with a vast sales

experience for both large and light aircraft over many years – and he’s particularly close to all of us because of his

continued successful soaring career in a famous high performance glider.

The USA market will be served by Alexander Papenberg, again well known in the community, and there is very

welcome support by fellow glider pilots, who know us, our aviation history and our unbroken enthusiasm.

Remember: the next innovation always comes from us!

With best wishes for the gliding season,

Yours, Reiner Stemme