Feb 14, 2013, 06:18 PM
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Blenheim, NZ
Joined Dec 2007
955 Posts
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Alchemist
Hi, thanks for the reply. Alchemist is still flying but has a 5 servo wing now and longer carbon tips. Not as pretty as the swept up originals but now I can use 4 surfaces for roll and 3 for brake with some clever mixes in trusty JR 9X2. The note below was written to Mark Drela back when I built the model in 03. Full ofenthusiasm for my new toy! It is not really competitive these days as it is a bit heavy but still gets flown regularly off Aero tow and the slope. I had a great 57 minute thermal flight with it at our Aero Tow invitational recently. Longest flight of the day.
Allan
To: Mark Drela (E-mail)
Subject: Alchemist
Hi Mark,
you once said"let me know how it goes" so I thought I would put together some stuff on my new thermal saorer, the Alchemist. The answer is "It goes bloody well thanks!" You inspired much of the design, specifically, it uses AG 24 and the HT sections for the tail. Its bagged using experience built up from making DHLGs like the Kahu.
Xfoil has given the data for the performance side and helped with flap schedules. Thanks heaps for all of this.
The airplane design goal was to create a strong stiff machine capable of performing well in our windy environment and good enough to get the most out of an F3B winch or F3J tow. The name comes from the idea of making something valueable out of cheap materials. I set a budget of $300USD for the airframe. It uses 5 servos not 6 to save a bit more.
Construction is straight forward. The wing is 3 piece with a carbon spar designed per a Joe Wurt's spread sht. It uses a carbon tube for the web and uni carbon caps bonded on in a jig. The joiners are 5/8 round Carbon to fit the tube. As the wing thins, the web tube gives way to foam and glass shear web and the last 18 inches of the wing just has 3 oz uni surface laid caps over "Kaiser Stringers" like Kahu. I used Kevlar rather than Carbon for wing skins to save cost but still get good torsion. The tip panels are 3oz on the bias and the centre is 5oz bias. The flap is knuckled and the ailerons use the Icon sliding seal idea, all are skin hinged by scoring the Kevlar.
Fuselarge is a glass and carbon lost foam pod with a fishing rod boom. Fin is bagged 3oz Kevlar on the bias with 3 oz uni vertical Carbon to take bending. Tail Plane is 1oz kevlar biased over foam with suface carbon caps like HLG but with 1/64 ply web full depth and local hard points for attachments.
The layout is Tee Tail as you can see. Why? well I like the look of a tee and Vees need to be too big for the same stability, particularly in yaw. I wanted good fly itself stability and rock solid launch stability with a very rearward hook so tail volumes are generous and there are 5 degs. of polyhedral on the outer panels. I used Blaine Rawdons "Plane Geometry" to do the configuration and stability stuff. I love this little program. It's tremendous value.
Weight? You will not like this! 82 ozs. Remember though our conditions demand good penetration and our landings are all FAI 100 point Tape. I can ballast up too with a fuselage tube system. Servos are JR digitals, I love them!
So how does it go? Really well from my biased point of view. I have been flying a locally designed Saphire style 6 servo model up til now so my point of reference is below the mouldeds. All my club mates use Erasers, Cobras, E3s, Icons, Carachos and the likes so I have tough competition in terms of airframe performance. The good news is that the Alchemist seems capable of matching them with in a a few percent I would estimate. I won the first club duration contest I entered and that has just been followed by a real baptism of fire, a two day F3B world Champs NZ team trial. I'm not a contender but I was keen to put the model up against the countries best over the 3 tasks and see how it rated. I certainly didn't win but was very heartened by the comparative performance. The Lolo showed a best launch of 271 meters (889 feet). Of the 4 distance slots I won 2, drew 1 and lost 1 by 2 laps. The glide slopes seems at least as good as the mouldies at reasonably high cruise speed. My best distance was 21 laps.
Speed was a little different. I'm not really good at this being basically a thermal flier these days and I don't practise. I have won Nationla F3B titles but that was back in the early 80s when the technology was quite different. Enough excuses! I managed a best of 23 seconds but believe the model would do sub 20 with the right pilot. Either way she is slippery and supports the section polar comparison with that old classic, RG15 where Xfoil shows AG24 to be at least as good at low Cls. On the duration side, I really love this model, it flies so easily and is a real pussy cat. It is seems impossible to tip stall it so far and the brakes are strong because of the broad 28% of Chord flap. The relatively long ailerons give good authority under brakes. In the contest I dropped a lot of landing points through poor skills but still placed near the top in duration slots because of the generally poor air that saw some people down very very quickly.
I'm very pleased I spent all those hours in the workshop. Thanks heaps for your contribution Mark.
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