Dave Martin wrote:

Lovely model!

Well, so far, I've counter-rotated the props for now till I can find out if they do in real life.

I've got the thing flying reasonably and the stall normalised at about 80-85 dirty / 100ish clean.

I've already experienced what you mention with the incidence at the tips (the twist of the wing). I'm trying to work out if I can make an average between the two that wont make Yasim throw the toys out of the pram.

I've also managed to reduce the 'dragster' runway performance a bit but it needs more work to match up things like rate-of-climb etc to the real figures.

Yasim has a "magic" solver that is sometimes sensitive to specific inputs. In the back of my head I imagine a little robot trying to climb to the highest point on the map by always going up ... but then coming to the top of a smaller hill and getting stuck.

The solver tunes the lift and drag coefficients to make the aircraft hit the numbers you specify ... so if you provide engines that are too weak, you will end up with a super "slick" model which an incredibly efficient wing ... thus it can still hit the numbers but has really slow acceleration and climb. On the other end of the spectrum, if you provide too much power, you end up with a high drag, low lift model (so you don't blow past the provide performance numbers.) This will give you great ground acceleration and probably great climb, but will still top out at whatever numbers you specify.

So once you have your basic YAsim model flying, you can tune things like rate of climb by adjusting actual engine output. You can tune roll/pitch rates by adjusting the size or effectiveness of the control surfaces.

I'm not convinced you could get a YASim model close enough in every area to get FAA level 3 certification or higher, but you can get a really fine flying model in most regimes with a bit of tweaking and understanding (at least at a simple level) how various configuration options relate to each other.

The other thing that confused me early on was how YAsim handles weight. I don't remember the rules well enough off the top of my head to summarize them here, but the solver solves at 80% fuel load I believe. This means that unless you are very careful with your fuel load and the weight the solver uses, you won't hit your performance numbers exactly ... those number only are for one particular aircraft weight. Once you figure out how to control the weight the solver uses and figure out how to configure the aircraft at that exact same weight, you do hit the performance numbers dead on.

For someone like me with zero aeroengineering background, YAsim is a *really* fun tool to play around with. After a few hours with it, I almost feel like I understand it enough to build pretty plausible numbers. When it comes to stability derivatives and aero coefficients, I'm still pretty much as clueless as the day I was born.


Curtis Olson http://www.flightgear.org/~curt HumanFIRST Program http://www.humanfirst.umn.edu/
FlightGear Project http://www.flightgear.org
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