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UL SAILPLANE - Footlaunchable, foldable

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ultralajt

Well-Known Member
Joined
May 9, 2009
Messages
1,581
Location
Slovenia
I will introduce here my design of ultralight sailplane.

It is result of my years long dreaming of concept that will suit my wishes.
It is result of my flight experiences with Hang Gliding, Sailplane soaring, and evaluating of other footlaunchables, that were caught in my eye by their particular features.

So far it is only virtual, preliminary design…nothing more.

I am hoping for some help from HBA members in checking or directing me to do proper stress analysis of some cruical composite parts (tail boom and D-box skin).
As it will be UL in my country it will be just OK to calculate it to +6 and -2,5 G (safety factor of 1,5 included), same as Hang Gliders.
As you will see from explanations and drawings, my sailplane is very different in some areas from what is common at this moment in field of footlaunchable 3 axis gliders.

First of all, my sailplane will be:
1. Ultralight
2. Footlaunchable
3. Foldable for a car rooftop transportation
4. Affordable price
5. Ease of construction and assembly
6. Use of common and available materials
7. Builder friendly manufacturing

I am willing to sacrify some of the sailplane (let say »Glider«) performances in order to achieve goals written above.
That means that I will reduce awerall size of the glider. Less size, less material, less weight…

Some of preliminary measurements:

Wing span: 11 m
Wing area: 12,1 m2
Total length: 5,2 m
Expected weight: 35kg max

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Regarding the glider performances, my goals are pretty modest:

Max glide angle: ~15-20
Min sink speed: ~0,75m/s
Stall speed: < 30 km/h
Maneuvering speed ~70 km/h
VNE ~ 110 km/h

These performances are somwhere in range of modern Hang Gliders.

Why one want such »poor“ performances, equal ordinary hang Glider?
Would not be better to just fly one of these Hang Gliders?
I flew Hang Gliders for 15 years, and now as I am becaming older, wishes for true 3 axes control, comfortable hammock, wind in hair… became obvious.
And landing on the landing wheel of course.
With such »poor« performance one can still easily stay in the air for hours and even do some XC flights.

My glider will not be designed for competitions. It is hard to compete with these days best footlaunchable gliders such as SWIFT, Phantom Rigid, Atos Rigid, Archaeopteryx sailplane, Xxtherm sailplane…
They are way better in performances, but larger, heavier and also very expencive!
They are somehow »clumsy« for transporting, heavy, and vounerable at not perfect handling on the start or landing place and during transport.
At Swift one must take much care where to hold and where not to grab the wing at manipilating on the ground in order not to damage the skin.
(just read the Swift manual)


So, how it looks my glider structure?

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Wing

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Wing consist from torsion D box in front that include a spar. Torsion box skin is composite laminate.
Probably a carbon/glass laminate. It is only one skin, not sandwiched with foam. Sandwiched skin could be stronger, but not resistant to local »garbage loads« so it can be punctured easier, than equal heavy one layer skin.
These so called »garbage« loads are present at transporting, loading/unloading and manipulating at assembly/deassembly on the start/landing place. I want pretty resistant D box of the wing.
Forward of flaps and ailerons hinge line location, there is another spar in the wing. This spar has four colapsable compression struts. They are hinged on the main and rear spar. The sewn dacron covering is over the wing as and »sock«. When both wing spars are tensioned appart by compression struts, this dacron sock sail is tensioned in proper shape. As airfoil contour at upper and lower side aft from D torsion nose is straight, there is no need for any internal wing ribs.
On the outside of the aft spar, a plastic profile is attached. This profile create a short lip for the flap, and also for the aileron.
Flap will be with short lip to create as much as possible lift, and ailerons will be plain.

Wing will have same basic airfoil over entire wingspan, to simplify D box molds, and to simplify wing folding.
Only flaps will be tapered to bring some more chord to the root of the wing.

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Construction of flaps and ailerons is like a small wing. Main spar is a tube. Flap will get a styrofoam nose shape at face end of that tubular spar. Trailing edge is a triangular cross section balsa wood encaptured in carbon fabric sock. Ribs are diagonal to enhance torsion rigidity of the flap and are made from sandwich plates. Extruded styrofoam with laminated skins.
Surface will be covered with heat shrinkable Oratex UL.
Structure of the aileron is pretty the same as for flap, except nose styrofoam insert.

Wing will have strut, for lowering the stress in the spar and for less weight and more simplicity of hardware and rigging.

There is an diagonal strut inside wing at wing root, which goes diagonally from front spar near wing strut attachment, down to the rear root attachment of the rear spar. This spar will introduce horizontal forces from the wing to the fuselage pylon and of course also the torsion loads.

Tail surfaces.

Tail surfaces are of full flying type. To prevent flutter, they are partialy aerodynamic and static balanced such way, that a surface and some mass is located in front of their hinge lines. Well the speed of this glider is not high, but as it is light build, flutter shall be prevented anyway.

As all surfaces (wing and tail) are foldable, also a tail feathers are covered with dacron fabric.

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Basic structure.

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Dacron covering added. Note some similarity with Hang Gliders structure.

Main spar is tube (aluminum or composite..yet to be defined). In front of the spar, there is an extruded styrofoam insert, that create an airfoiled nose. It is covered with thin glass laminate or even simplier, just self adhesive tape. To achieve foldable surface, there are some ribs and struts hidden under the skin. Ribs or better »battens« are the same design as at Hang Gliders. They make tension between rear od the main spar and edhge of the sail at the trailing edge. Sail is sewn from dacron and attached to the spar at end points. At the root, there is an colapsable diagonal strut that is mainly responsible to accept torsion loads and asure tension in the sail. Tail tips can be left bare or covered with teardrop shaped plastic caps.

Fuselage.

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Fuselage is so called »pod and boom« design. At this moment I am thinking at »lost foam« composite sonstruction. It is made from foam parts carved out from extruded styrofoam and covered with carbon composite cloth. At points of hardware attachments, some hardpoint inserts will be placed into laminate or inside the part. Pilot compartment consist with only two side beams. In the front end these two beams are connected together. There is also a cross spar and a front landing skid.
Rudder pedals are hanged from that cross beam.
Pilot is equipped with a harness, similar to early paraglider harness, that allow inclined and also sitting position. Left and right side of harness is equipped with attaching brackets to connect it to the hang points inside each cockpit beams. Hanging front or aft, pilot can set proper C.G.
Rearward from pilot, at the underside of the beams, there is an lightweight retractable landing gear.
Here, two cockpit side beams goes together to the area, where a tail boom and wing pylon are joined. At this moment I am not sure to make tail boom deatachable for the transport or no.
Pilot controlls are situated at top of each cockpit side beams. At left hand there is a control stick with two functions. Left/right it controlls rudder when pilot has legs outsire of the cockpit (starting). Forward and back sliding it controlls flap deflection. For flap operation, also electric drive should be considered . At the right hand, another control stick is situated and it operate usual way with elevator and ailerons. Ailerons acting also like flaperons, as they travel slightly down proportional with flap deployment.
This bare structure of the fuselage is further refined with some aerodynamic fairings to lower the drag. Cockpit is covered with dacron fabric, streched over light carbon tube longerons. Aft of the wing pylon is tapered with elastic fabric.

For pilots that dont want to do footlaunch, an optional composite shell is possible, that represent a bottom side of the cockpit, with integrated pilot seat. This shell is bolted to the side cockpit beams.
It allow to make start by aerotowing, or just rolling down the slope.

Glider will be folded in 5 main components, of each will be placed in tfabric transporting bag:
- left wing with wing struts, flas and aileron
- right wing with wing struts, flas and aileron
- fuselage
- elevator
- rudder
Making so, it will be easy for the user to load relatively light components on the cartop for the transport.
Also cariying of the components from the parking lot to the assembly field will be much easier handling light components rather than carying whole glider at once.

As seen from glider 3 view drawings it is pretty small. To achieve decent low starting speed at such sparse wing area, I opted for large flaps and force ailerons to also get some flaperons movements. This is only way to achieve large enough lift coeeficient.

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What now?

I need to evaluate glider weight and calculate loads.
Then I must calculate main structural elements… wing spar, D box skin, tail boom, cockpit beams.. to set size and thicknes of elements.
I would like to get hints and suggestions in order to do proper thing…not to drift away to danger zone, neather be much conservative to fail make thing lighter enough.

I think, my goals are not too unachiveable as some of present designs are proof of some things such as:

- Wing of Rigid Wings hang Gliders are of similar construction (D-box and dacron skin) and they are foldable. Also the wing of sailplane AL-12 Alatus is proof of the concept.
- Archaeopteryx and XXTherm are proof that conventional glider layout (with tail) can be footlaunched.
- Rolling start from the slope is also possible option.

So much for now!


Mitja
 
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