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Mini Swift

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Joined
Feb 20, 2012
Messages
18
Location
Bristol, UK
As a bit of background I’m a sailplane pilot and ex paraglider pilot, occasional powered pilot and I’m an aerospace engineer (structural).

For a long time I’ve been fascinated by aircraft which fill the gap between hang gliders and gliders, but always wondered why there are so few designs that really exploit the benefits of both types of aircraft. The foot launched aircraft places some fairly tough requirements on most hang gliders and usually limits wing loadings to less than 15kg/m2. On the other hand, most modern sailplanes fly with a wing loading of 30-55kg/m2.

This leads me to my main question: What would happen if you removed the foot launched requirement from hang glider design and looked for the ultimate compromise between a modern hang glider and a modern sailplane? If you asked that question to a sailplane designer I’m pretty sure they would just end up with a lightweight sailplane with wings, a fuselage and tail, if you asked a hang glider designer I’m pretty sure you would end up with something completely different and possibly very interesting.

Modern rigid wing hang gliders like the ATOS and Swift-Lite are fascinating craft and show some real promise of what’s possible when searching for the simplest designs, but for me, the real crippling feature of these craft is their low wing loading. I wondered how far you could realistically shrink these wings if you scaled them till they achieved the same wing loadings as gliders. Given a full scale ATOS is 36kg it wouldn’t be unreasonable to assume an empty weight of 30kg which might give you an AUW of 130kg, thus an area of 4m2 would give you glider like wing loadings, that’s about a 1/3 of the wing area of an ATOS or Swift-Lite!

Now I know completely ignored real life practicalities such as taking off and landing safely but stick with me in my purely hypothetical world!

To see glider like performance the wing aspect ratio would need to match the sailplane at a value of around 20, giving you a wingspan of 9m and a mean chord of only 450mm! That’s a pretty skinny wing but not unachievable by copying the composite wing construction of modern gliders, I think the small scale and low AUW of this wing helps you a little in this respect. To give you an idea of structure weights a glider wing weighs about 80kg for 11m2 but is designed for an AUW of over 500kg including water. To picture this wing I think it would end up nearly identical in shape to the Brazilian Roberto Stickel’s Pyxis concept pictured below which has been my inspiration for a long time.

Roberto_Stickel_PYXIS.jpg

I understand the stability and control issues associated with flying wings, especially ones with such high aspect ratio but I think they are surmountable problems, I’d envisage being able to find a modern laminar flow airfoil with low value of Cm and achieve a good stability margin using moderate taper ratio of not less than 0.8, 30degrees of sweep and about 5-7degrees of twist. With such a simple and light weight craft it would be easy to trim using variable CofG or flaps, which I think would probably be essential for the high values of Cl you would need for taking off, more than likely it would use flaperons.

So..... taking off and landing! I think hang gliders are used in different ways now than they were, foot launching from a hill used to be the only game in town, then came along winching and aerotowing off wheels or a trolly which I think I’m right in saying is becoming more popular now. Once you stop foot launching then there is no real reason to have the low wing loading anymore. Faster launch speeds like gliders use should be usable for a craft like this, and even bungee launch might be possible at a hill soaring site. All landings would have to be done on wheels or a skid. The pilot would fly in the prone position, lying above the wing to keep them away from the ground during landing.

So, purely from a technical point of view, would it be possible to have such a simple craft with the following specs:

Construction Full carbon sandwich construction in female mould
Wing area 4 m2
Wing span 9 m
Aspect ratio 20
Chord 0.45 m
Empty Weight 30 kg
Pilot weight (Max) 100 kg
Max AUW 130 kg
Normal packed length 4.5 m

PERFORMANCE
Min sink less than 1 m/s
Max Glide 35ish @ 90 km/h
Stall 50-60 km/h
Wing Loading 32.5 kg/m2


Regards,
Kai
 
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