Soaring in Thailand
Three months of ideal weather in northern Thailand (1,2) gave me the opportunity to test and develop a number of ideas in building, thermal soaring and gliding, power to weight ratios ,flying wings and hybrid designslink*. I learnt a lot by building and flying alone and with Thai friends(3), and from generous responses on rcgroups! Still mornings and evenings and plenty of thermals in the day make for rapid progress. After the wings I called Sylph(4) * I wanted to see if one could get over the compromises involved in wings. Of course one can’t, but changing the compromises is always worth a model or two. One comes back more or less to the same place but sometimes with improvements and certainly with more understanding.
The Arc series* were the one result, and ended in some hybrid designs that are very enjoyable to fly. One has an astonishing glide.*(5) In still weather it floats on and on and turns with very little bank and no apparent loss of height! It does not parachute at all, which I have had with other gliders at very low speeds. There is a lot to learn from this one and I still don’t have enough technical grounding. Also, there has not been much research on very low Re numbers. When better equipped I will post a video of this. This design also appears to contradict the rule that high aspect ratio is essential for gliding at low Re. More versions are necessary to fully establish this.
It is useful to make a version to disprove an idea. I built one “Arc”*(6) with a symmetrical section to compare the glide with a plane using a cambered section. It does not float as well, but of course has more manoeuvrability and behaves like a genteel hot-liner, and thermals readily.
So I arrived at a design that has a swept, curved wing, large (3 D!) *ELEVONS, and a small tail assembly*(7) that is all fixed, on a 4mm carbon tube. They weigh around 150 to 220gms and take a day to build.
From these models it seems that a simple flat- bottomed section as thin as structure and strength permit gives a good glide*(8) Certainly a purpose designed aerofoil would give better results .I have tried to achieve more complicated sections but several factors make it difficult: building by hand, a curving plan-form combined with taper and hollow structure. In addition the time required (for me ) to research this would at least quarter the number of test models built! .I have found with the Arc design that only a very small amount washout twist (achieved by sanding in the thickness, 3mm plus 3mm) and very little reflex, with the stabilizing tail*, are necessary. That is obvious, but what is not is that these fly only on elevons, with bags of control! No tip- stall or spin, right to the ground.
.There appears to be a threshold of weight under which its win, win, win! With a19 gm motor one can use a 10 amp esc ,a 360 mah battery ,5 gm servos and my rx’s weigh about 4gms (9).* The low inertia at the centre means that a 3mm carbon tube gives the wing enough strength . The wing is still quite flexible . This seems to be no problem even with some wind . I have this set up on planes up to 1,80 m. Most of these can go vertical” infinitely,”using a gws sf 7060 prop. These are built using a 3mm depron top and bottom layer with balsa ribs 5 cm apart (10) *which are cut and sanded in pairs by hand. For more rigidity I use diluted white wood glue to stick on various grades of tissue paper or fine mulberry ( saa) paper.
I have devised a way of testing a sketched design in a few minutes. These little chuck gliders (11)* have taught me a lot, e.g .that moving the cg 1mm makes a real difference to the glide or taking a sliver off top or bottom. Tested indoors one quickly sees the quality of the glide, and what it promises for a larger version.
So back to a pure” wing” form and a bat design(12,13) * which I wanted to fly in the evenings with the real bats .These are both successful and the smaller can out-manoeuvre the bats ! The larger has a good glide and gets around the sky quickly, finds and turns well in thermals. So this led me to think that a hybrid /compromise model (not very large) would be worth trying, that made up for less penetration and float in the glide by agility and penetration (with a small motor and battery) In other words a “go-getter” that would give me lots of time with the power off, rising and doing acrobatics in thermals, based on the above power set-up;
From this came two sizes of the Skua *(14,15) .Both float far better than I expected (with some reflex elevon); turn any radius and thermal easily. They are excellent just above the ground and easy to build. I get up to 21 mins on the 360 mah 2s with no thermal assistance. This design covered with paper and with a heavier battery would cope with more wind. All my planes and wings are pullers, except my FPV wings. Pullers are quiet and with some motors virtually silent.
All really lightweight gliders are good for showing what the air is actually doing. For instance, between 9 am and 5pm in warm weather the air is very active, in All directions , even when one can hardly feel wind This is where one sees what birds and insects live in and exploit all the time .The plane can drop several metres or jump the same in no time, very exciting, and easy to control with large elevons .I thoroughly enjoy “surfing” this, rising with some power, or on the bubble lift ,soaring in thermals, then looping and rolling in the glide. In real thermals I frequently have to hold these planes in a steep dive to avoid losing them. Then going home, smug, to recharge my toffee- sized batteries(16)! Finally, these planes require only a little space to land, without airbrakes or flaps, although full up elevon will make them parachute.
I suggest that anyone wishing to have the same excitement as I have, use my shapes perhaps, for inspiration and build a lot, using the simplest flat- bottomed aerofoil. Make as much of it as changeable as possible ,then make a polished version.* We are intimidated by the erudite discussions of aerofoils, lift and drag coefficients etc, etc, necessary for winning world championships and achieving doctorates perhaps, but they should not make us feel that only 1000Euro , 3 metre models will glide and thermal well!
I have always liked Asian kites, particularly Chinese and Japanese. Some of the planes I am making now remind me of them. I enjoy sketching the idea, making a chucky and going on to the full size in a day or so. Then cutting the balsa and shaping the wing, and covering with paper. I don’t want to be too precious about them; one moves on and they get damaged. I am sorry I can’t decorate them with the speed and vigour of the eastern kite makers!
Back home and snowing on March the 3rd! A chance to rebuild the planes left behind in Chiangmai and improve them perhaps
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