OT Tee (oz10420)
About this Plan
OT Tee. Radio control 2m span glider.
Quote: "Neil Frost presents his two-metre glider - MkII
Mk II is a polite way of admitting that the first one flew like a dog. (Actually, this is unfair - I've seen whippets with outstretched legs glide better but fixing the bungee hook is tricky). Mk I was going to be a world beating two metre competition glider thanks to cribbing Martin Simons' ideas on low aspect ratio wings ie short and fat is not all bad. The L/D chart for his 6:1 Martinet wing shows better efficiency at higher speeds than the usual 10:1 compromise without too much damage at slow minimum sink speeds. The model needs to be kept moving well clear of the stall to avoid the severe vortex drag from the broad wing tips.
I was working towards something which would be competitive at our evening competitions where the wind is often light and bungee launches dictated a light model - preferably under 30 oz and less than 7 oz/sq ft. Now if by incorporating a slippery section and a large 11 in chord, we could get within these figures and still be competitive if the wind blew. The aptly named 'Wooden Butterfly' behaved weirdly from day one - George Stringwell joined me on the field and witnessed the first flight - we both aged visibly in the process. In order to fly with any degree of sanity, the C of G was threatening to fall off the nose and the keep it light strategy was being defeated by an up front lead collection large enough to refurbish the local church roof.
What went wrong? It was clear that the Selig 3021 section was not working at these slow speeds. I suspect the flow separation was more of a mass walkout as soon as it looked at all that wing top surface. If you read Martin's article properly, there is a clear indication that a bit of wing loading is key to the trick. No good to me - I still needed a slow flyer for good still air thermal detection and easy bungee launches.
Plan B. So let's try again - same design brief but with the benefit of hindsight, let's use a normal 8 in chord and build it light to stay within the sub 7 oz /sq ft range - this should keep it competitive on still nights but stay with the slippery section to give a good performance in a breeze. In keeping with the desire to produce a forgiving fun model, the wings are banded on which should reduce those after slope repair sessions and the design will take a moderate breeze and spin down from lift without drama. Construction is conventional and only needs simple two channel gear.
Now I always prefer to fly T-Tails since it is possible to use a flutter free elevator, the rigging is easy, they are efficient and the trim can be checked at a glance - the additional bonus is they look so pretty. The price ticket is that all that whiplash load on the base of the fin is forever causing problems. What I have tried to do here is without carcinogenic super goo and NASA space fibres, produce a very robust set of tail feathers without having to counteract the weight with three house bricks in the nose.
The lower half of the fin is doubled both sides with drilled 1/32 ply which is then carried up along the fuselage to spread the shock loads. Properly used, plywood has a natural resilience which can soak up lots of abuse - but just like ice cream, the trick is to use just enough to avoid getting overweight. If we had to design fuselages solely for flight loads and good landings, none of this would be needed - shock loads are the killer. Despite my usual indifferent arrivals and Derbyshire slope 'landings' ( slow crash), things are still looking intact. But be in no doubt, this aircraft is so relaxing to fly that there will come a time when complacency will strike..."
O T Tee, Silent Flight, December, 1995.
Direct submission to Outerzone.
Supplementary file notes
Article pages, thanks to RFJ.
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