MIT published as freeware the panel method CFD code called Athena Vortex Lattice for analyzing the geometry of aircraft. I input the Goat4 geometry into the program to do a little investigating. I used a 300lb takeoff weight, a 29% root chord CG location (the wheel location), an 11.7% ClarkY smoothed airfoil (from UIUC's database), and tried hard to match the wing/tail angles to Sandlin's drawings.
The incorrect tip taper isn't a huge issue considering the tips aren't loaded heavily and quite frankly there is enough other guesswork right now the tips are of the least concern. I haven't added cabanes or the nose or a pilot either.
As a start, I'm using 30 mph (44 ft/s) as a cruise condition and assuming sea level atmospheric properties. AVL gives the neutral point at 40.6% root chord for an 11.7% static margin and predicts a 0.2 deg (down) elevator to trim. Speeding up to 45 mph (66 ft/s) requires 4.1 deg (down) to trim and slowing down to 25 mph (36.7 ft/s) requires -1.1 deg (up) to trim. Based on watching the in-flight Goat videos, I'm seeing more up trim for typical cruise, so this CG location may be a bit far aft. Sandlin recommended balancing nose-heavier than this 29% location anyway,
Moving the CG position up two inches to 15.4 in aft of the LE (25.7% root chord) and re-running the 30 mph case yields -2.1 deg (up) elevator, 45 mph case yields 3.1 deg (down) elevator, and 25 mph case yields -4.4 deg (up). That's just for trim. There is some +-25 deg or more of total elevator travel for maneuvering.
I don't have anything good right now to compare these numbers to, but this is a starting point. More number crunching later...
4 comments:
Dear Sir,
I'm a brazilian ultralight and hanglider pilot and live in Ceara State a good place to gliding flights.
I'm looking for an ultralight sailplane that could have a low acquisition and operatinal cost. If you can help please write me:
meu e-mail constantinofrate@gmail.com
Hello Constantino,
If by acquisition costs you mean inexpensive to construct, you're asking about the right vehicle. If you're asking to find one to simply purchase, this isn't the right design. Mike Sandlin has freely published his drawings on his website (http://m-sandlin.info/) for home-builders to use as a guide. To my knowledge, the closest commercial offering was the Super Floater, which is some years out of production.
Need more information about what you're after to give a better answer.
Thanks for the readership!
Dan
Thank you very much for your reply! I think that there is only a solution to have a low acquisition and operational cost sailplane. The solution is to build an Yando Goat.
Thanks.
Hello Dan,
My name is Bruno and i'm a Aerospace Engineering Student in Brazil, first of all your blog is great, and secondly I think that your analysis is very accurate! Well I'll try to write my thesis using AVL but to input all the parameters for the several aircraft configurations I planned to do it would take me many hours, I was wondering if you can share with me the AVL Editor. The AVL GUI version would help me a lot in my work.
Please reply if it's possible.
my email is bmelchiori@gmail.com
Best Regards.
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