Flac by Richard Gerrard

A Class II (or IV as it now transpires) -- 3 Axis all composite glider


  1. General
    1. Pre-amble
    2. Rational
    3. Design objective
  2. Design Concept
  3.  Performance spec
  4.  Structure
  5.  Fuz. Pod&boom description
  6.  Wing description 

1 General

1.1 Pre-amble

In England there are few inland hill gliding sites of more than 200-300 feet above the surrounding ground. There are coastal cliff sites where more stable wind conditions exist but again few are more than low hundreds of feet above sea level. In England the weather patterns are often variable and unstable, operating from small hill sites can present handling difficulties due to gust and direction changes during critical launch phase. Current self-launch hang gliders are I would guess, close to the limit of their design development. I can't see where significant performance gains are to be achieved. Performance at present is around 15:1 glide ratio and 24K minimum sink, for a structural weight of about 90 Lb.(including accessories).

 After long discussions with a friend, (this friend has for many years carried out repairs on 'glass' gliders and also made composite components for the racing car fraternity in Carbon/Honeycomb, carbon/Nomex), the base parameters of the concept were conceived. An estimate of achievable weight and a minimum acceptable performance specification was made. I believe that it is not unrealistic to set a design objective to produce a composite machine of 10 -12 Square metre wing area and below 30 Kg.(close to 60 LB), airframe weight, with a glide ratio better than 25:1.

To enable a better design estimate (quess), test ‘coupons’ of Kevlar skinned divinicell panels, wound tubular components and Uni-directional carbon spar sections, have been made. These components have assisted in a ‘spreadsheet’ estimate of airframe weight.

1.2 Rational

When considering the span and proportions, much consideration was given to the size/weight/mass one (average i.e. NON- Rugby Prop-Forward), person could reasonably cope with in rigging/derigging and ground handling, I concluded that a span of 10 Metres net (32 feet), would be the maximum for safe operation in our prevailing conditions. I also considered that it was unrealistic to attempt to design a machine able to be launched or landed in zero wind speeds and meet the above weight/span figures, so a wind speed of 10 m.p.h. plus a forward impetus of 7 m.p.h. (15 Knots) was designed for. This then requires a more complex wing (flapped) to extend the low speed end. The wing loading of over 10 Kg./Sq. Mtr.(2.3 Lb./Sq.Ft.) gives better gust and handling performance reserves in the landing critical phase. The prototype is to incorporate 'plug-on' wing-tip extensions to extend the lower wind speed launch capability, adding a square metre to each wing.

1.3 Design objective

1.3.1 To design a self (foot) launchable glider of smallest size that can be handled and foot launched by one person. To do this a machine of the highest possible aerodynamic efficiency (for its size), and low weight is essential. Therefore the most efficient use of composite structures is needed.

1.3.2 For the pilot to be enclosed within a rigid structure and in order to keep the airframe as compact (therefore light),as possible the pilot is carried between two spars in a mid-wing position. On the C/G.

A 'self-launched' rigid structure, three axis control glider. Designed for single person operation. i.e. transport to a hill-site, rig the machine, launch unaided, land and retrieve, de-rig and depart, with say the aid of a ground trolley only.

2 Design Concept

2.1 A 'Pod and Boom' fuselage Glider of composite construction. 10.5 Mtr span / 10 Sq. Mtr. net (104 Sq. Ft.) wing area(2 piece wing). A de-mountable, 'Vee' tail and stub underfin. A nose nacelle with 'under-carriage door' for landing/take-off and closes to seal with the pod for normal flight, providing an enclosed cockpit.

2.2 The pilot gains access through the top of the nacelle. Attachment to the airframe is by means of a 'Trapeze Harness' similar to the sail-boat variety but the harness is fitted with two lockable shackles which are attached to each Hip position. These in turn clip on to two short (3 in.) 'Sail-track slides', The slides are adjustable and lockable to allow for major trim change of differing pilot weight

3 Performance

  • Calculated minimum sink rate is 0.5 Mtr/sec.
  • Stall Clean 18 Kts
  • Stall Flapped 15 Kts
  • VNE Not Calc.

4 Structure Limitations

  • Normal maximum flight load + or - 4.4g
  • Ultimate failure load + or - 8.8g
  • Airframe weight 60lb. (28 Kg.) is projected
  • Maximum all up weight 220lb.
  • Wing Area 10 Sq. Mtr.(104 Sq. Ft.)
  • Wing loading 10 Kg/Sq. Mtr.(2.2 lb./Sq.Ft.)
  • Span 10.5 Mtr (32 feet) gross
  • Length overall 3.7 metres
  • Stabaliser 1.5 Sq. Mtr.

5 Fuselage description

A forward pod section consisting of three bulkheads (carbon/Aerex/carbon), integrated into a load-bearing skin augmented by two carbon ‘D’ section ribs projecting forward to support an enclosing nacelle, encloses the pilot.

A tubular boom of 2.36 metre length and 85 mm diameter (nominal) attaches to the pod via the two rear-most bulkheads. The boom loads are transmitted into the pod structure via the two rear bulkheads, the bulkheads being held by two root-ribs and a composite foam-sandwich load bearing skin of Divinicell/Carbon. 

The space frame is held rigid by a Kevlar skin and in addition by the two ribs passing through the forward two bulkheads and extending forward to provide a foot rest and mounting for the nacelle.

The wing torsional and pitching stresses are reacted through the pod via two rectangular 'carry through' tubes(carbon), these are tied in to the nacelle skin at the wing-root with two 'U' section carbon lay-ups within the wing-root, forming a 'box' section. The bending loads are transmitted via the spar root extensions into the opposite root rib re-enforcement as is current (glider), practice.

Pod and Boom

Dimensions taken from wooden mock-up to accommodate RJG.

Pod overall length 1.85 Mtrs.

Boom 2.36 Mtrs. - 2 Mtrs extending beyond Pod/Wing Trailing Edge

6 Wing Description

The wing is a two spar cantilever with the spar root extensions passing through the nascelle root rib and carry through box into the opposite wing root rib interlocking, this requires a locking pin for each spar and two locating pins to react the torsional stresses into the pod.

Each wing is assembled using a ‘build-in-the-mould’ technique, similar to the Glassair methods described in recent ‘Sport Aviation’ . (see diag.). The Spar Caps are wound from wetted roving in the channels of the upper and lower sandwich panels. The outer kevlar skin is then laid from spar-cap to spar-cap, covering the molding joints.

6.1 The wing has split flap/ailerons, the inner 2 metres of each panel are parallel chord flaps of 320mm chord, giving and area of 0.65 Sq. Mtr. each. The outer section(3 Mtr.), has tapered ailerons of 0.518 Sq. Mtr.

The wing has no Wash-out but the contol system is intended to provide 'droop' flap+ailerons for take-off thereby giving some wash-out.
The control system wiil incorporate an 'Airbrake' mode for final landing stage, where ailerons will reflex up and flaps will deploy downward, to beyond 450 .