ORIGINAL: Ed Smith
The object of the exercise is to provide a flyable airplane for the first few flights. From the simple diagram I posted to the complicated computer programs there is something missing. They provide positions for STATIC balance only. They fail to consider engine incidence and offset, if any. Wing and tail incidence, if any. The lifting component of wing and tail airfoils. Airspeed. So simple is best.
An airplane is not a static object. After the first few flights the REAL balancing and trimming of the model starts. And that is a whole different subject. I know where my Pylon Racers balance. It still takes a dozen flights or so to get a specific racer just right. And all I am doing is flying left hand circles.
Does anybody think that by sticking numbers in boxes one can balance and trim a full house Precision Aerobatic airplane?
Ed S
Lot's of people have those same opinions. Let's look at a closer comparison between the application and trying to draw up a 25% MAC location.
In actual fact, the geistware program is simple to use. It's quite a bit easier than getting out pencil and paper and drawing a scale drawing of a wing. I can take my yardstick off the wall in a lot less time than it takes to find pencil and paper and a place to draw. Furthermore, drawing a scale wing requires you make measurements. And then you have to figure out the scale to use on your paper. Heck, take the 9 measurements and plug 'em into the application and let it worry about figuring everything out. Nah, in fact figuring out the scale of your drawing is way harder than typing in 9 measurements.
It is also designed to compute the CG for
flying the airplane. That's why it considers the effect the horizontal tail has on the stability of the airplane in flight.
And yes, sticking numbers in boxes will give an absolutely reliable and accurate CG range for establishing a safe starting point for your model's first flight. The object of the exercise is to provide a reliable CG for the first few flights, and it does that with ease. Anyone who can measure a model airplane with a yardstick can get a quick and reliable range. It's not sold as magic at all. It's purpose is to provide a safe starting point. As for Precision Aerobatic models, it'll do far better than considering less than half the things that affect pitch stability. The formula it uses is good enough for fullscale users, so there's almost a 100% probability it works as well for our models.
Aircraft designers like Lockheed use it's formulas. Airlines use the formulas in their operations 24/7. Of course, those don't use Geistware because they don't need to. Their computer systems are accessed directly. And they don't measure with a yardstick.
What is magic about the geistware application is that it does all the computation and we don't have to do
any of it. All we need is a yardstick and the time to make 9 measurements. Do that and you get a reliable CG location for flying that considers wing sweep, taper, area, tail size, moment, and a bunch of other things that do affect CG location, instead of a rough guess that won't really be very good unless everything about the airplane is average, no wing sweep, tail size average and not farther back or closer than average, tail planform average like wing planform has to be average. And you wind up with a number for the CG that isn't any more reliable than if you simply placed the CG at the wing spar. After all, spar locations are as good as anything if you're just betting on the averages. There are a number of good reasons to use the formulas the aircraft industry and airline industry developed. And it's easier to do.