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Aerodynamics

The aim of HLG flying is reaching long flight times out of the hand. First requirement for this are good throw heights. The second, and at weak lift conditions more important, is an excellent sink rate. When there is wind, a good cruise between the thermals is important.

So three contrary requirements to the aerodynamics can be realized. An optimization of good sink rate is difficult to combine with reasonably low drag at high throwing speeds. Lift coefficient and Reynolds number in cruise are between the two extrema and make pure two point designs unfeasable.

The cl-max must not be too low due to unavoidable circling in thermals. An increased airfoil drag is not that critical here, as the induced drag is high anyway. A not exagerated aspect ratio is beneficial to Reynolds number and does not have more induced drag at constant mass, as can be shown theoretically. But building (compartement of servos,...) and stiffness are improved significantly.

Unfortunately, the actual optimum design depends on the actual wheather conditions when it should be optimal. There is not a best plane for all situations. Rather, it makes sense seeking for a robust compromise. Simply said, the plane must work.

To achieve this the pecularities of low Reynolds number aerodynamics have to be regarded. Laminarization is not the problem, rather the opposite. Pressure drag resulting from laminar separation bubbles is the most important factor producing loss of performance. So the airfoil design is aimed at keeping pressure drag small. In the climbing phase a small drag is only reached with long lengths of laminar flow, nevertheless.

The level to surpass was already quite high in the form of the SuperGee airfoils by Mark Drela. Concerns existed with respect to the resistance to flutter in the throw. Therefore and also to compart reasonable servos completely inside the wing a greater thickness of 7.3% was oriented to. This does not necessarily degrade drag in the climb phase, as the laminar drag bucket can be extended down to lower lift coefficients.

Without camber changing flaps a DLG is without chance. So care was taken for optimizing the response to flap deflections. The important sides are smooth for the corresponding settings. At negative deflections the lower surface then features almost 100% of laminar flow. The difficulty is to maintain laminar flow on the upper surface over the flap hinge.
The other way round, for positive flap deflections the upper surface pressure distribution is smooth. The long flat bubble resting on the lower flap kink does even reduce friction drag. Flight performance and wind tunnel tests confirm the success of the approach.

The vertical tail is profiled asymmetrically in order to reduce the oscillation more efficiently. Cambered sections are also less prone to dead-band-effect.

Many people wonder about the large dihedral. It is the result of extensive prototype tests and does not decrease throwing heights. But circling is improved significantly.

Construction

As simple as possible but as sophisticated as necessary was the motto. For a high-performance DLG some effort is required.

The Aspirin wing is available in a multitude of versions. The standard competition version provides superior stiffness and strength at lowest weight. This of course requires acribic manufacture from materials with better specific mechanical properties. Rohacell is used as core material.
Based on this standard version many different measures for further increasing stiffness and robustness can be taken. CFK-D-Box, Kevlar-D-Box, full-CFK and full-Kevlar are possible. From our point of view the most reasonable is the reinforcement with carbon lattice fabric (Disser wing), as this brings the lowest additional weight.

The flaperons are silicon hinged on the lower surface. The smooth connection and air tightness ensure minimum disturbances of laminar flow over the flap combined with high robustness. The gap on the upper surface is closed with a special foil seal.

Available colors: red, orange, lilac, green, blue and black.

The fuselage is laminated from UD carbon Gelege and cured under high pressure. This provides exceptional strength. Even hard crashes are taken without damage. Standard is the equipment with two servos. No fumbling with linkages and maximum resistance to torsion of the flaperon are the reasons for that. But there is space enough to put 4 servos in if desired.
Under the wing 32 g of ballast can be added (with a different system more is possible) in order to extend the range of usability to higher wind speeds.

The tail boom is manufactured separately from UD and braided carbon fibres. This way by applying a high pressure an optimum fibre volume content and extremely low weight can be reached. It also leaves some possibility of variation of tail position. The different fiber angles are necessary to optimally resist bending and torsion loads.

Both high-end versions feature tail planes produced in negative moulds. This makes sure handling characteristics are reproducable.
The elevator is mounted removable on a small pylon. The fin is bonded to the boom with the help of a template.

Standard version of RC integration in the fuselage
Different version with SMC-14
Strong shear webs in wing and aileron made from GFRP/balsa resp. C-braid
Connection of elevator and rudder by Kevlar lines and torsion spring

A new fin of slightly enlarged size is available, also for left handers. Span was increased from 215 to 240 mm, the area to 1.68 dm².

The throwing peg is profiled and therefore less detrimental to aerodynamics. The large contact area for the fingers enables highest throws and reduces loads on the finger tips. The area of load input is reinforced by carbon cloth, of course.

Masses of the parts have to fulfill strict tolerances (+- 5%). This ensures quality and the intended flying characteristics.
Take off weight is typically 260-265 g. When building light, 250 g are possible without much effort. Not looking at the weight, you will end up at about 280 g.

The Aspirin is delivered almost ready to fly. Only the rudder linkages have to be completed and RC-components have to be installed. The following parts can be used for that:

Further information and pictures of the mould building can be found at www.agm-penig.de.

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