It's a trade off for sure. As for the flaps and slats it doesn't surprise me that there was not much to be gained from there use. Flaps and slats versus a "flap built in secton" like the S1233 is only a method of extending lift into a wider speed range. But in our model cases we don't need a wide speed range for most applications. Especially in this one. A single camber foild that will work over a relativley narrow range is just fine. What about fuel load changes you ask? Simple, as the load goes down you throttle back to extend the run time. This slows the model speed and thus the lift coeficient remain fairly constant and the airfoild keep working at it's best L/D.

In Using a smaller engine for less load than the SAE comps call for suggests you probably don't want ALL lift at the expense of a heavy drag load. And add to that the realities of building an accurate airfoild as I suggested in one of my (probably too many) posts above and it starts to look like the "porridge that's just right" may be a better option for this situation. Using Profili I see the Eppler 423 has 7% camber. That's a lot for a model that's going to be expected to fly with minimal power at a much more moderate loading than the SAE models. I'd probably think it would be better to cut back to 4 to 5 % camber for less drag as AQ500 is suggesting as a happy medium.

I'm sure there's a formula that can help. I know there are some lift coefficient calculators out there on the web. A few realisitc numbers to punch in and find out what the Cl will be should point to some suitable airfoils.

So just how much does 2 1/2 quarts of fuel weigh anyway?.....