Common YASim Problems
By Gary "Buckaroo" Neely
These are some of the more common problems I've noticed in YASim FDMs since I began a serious study of YASim. Be careful when examining an FDM made by another developer-- what looks like a mistake may actually be intentional and based on careful study of the aircraft's actual or reported behavior by the developer. As always in my guides, these suggestions are based on what's typical for relatively modern general aviation aircraft. In general, the following items are flags that signal possible problems in a YASim FDM. If you don't have any of these problems, chances are you'll get a good flight model.
Geometry mistakes
Easily identified by importing the FDM into Blender using Melchior's importer script and comparing against the real aircraft configuration. With most FDMs it's not a problem, but it's worth the test as it can reveal a glaring mistake.
CG placement
After geometry, this is the most important thing to get right and one of the most common problem areas. It's importance is stressed on type certifications and stressed in flight manuals. It needs to be stressed in YASim. The CG needs to be in a proper range or the FDM will not feel real in flight. For most general aviation aircraft, the CG should be ahead of 36% MAC. For YASim purposes, a CG in the range of 25-30% usually gives good results.
Extreme stall values
Conventional GA aircraft airfoils tend to stall in the 15 to 18 degree range. High-lift airfoils often stall much lower. These numbers do not reflect the effects of incidence and twist, and YASim needs to allow for a degree or two of interpolation before hitting critical AoA, so the aircraft itself will likely begin to stall 3-6 degress less than this. So when I see stall AoA values set to 19 or higher for high-aspectc general aviation wings, it's worth a second look. Stall widths wider than 12 degrees are also worth a second glance, as are those less than 4. And very few people understand what stall peak does, so values other than 1.5 are usually suspicious.
Unusual Approach settings
A quick look at approach AoA can say a lot. A high-lift aircraft having a high approach AoA value signals a problem, as these aircraft typically have very nose-low approaches. Heavily cambered airfoils designed for high-lift applications can have stall alphas as low as 10 degrees, so approach AoA will be much lower, possibly even negative. Delta-wings or a very fast aircraft lacking flaps will usually have high approach AoAs, so an FDM with a low one suggests a mis-configuration. Approach speed values should be somewhere between stall and stall * 1.4, anything outside of that should be re-examined. Check that control settings are reasonable: flaps set, a throttle setting somewhere around 1/4 to 1/3, and gear down.
Unusual Cruise settings
It should be obvious, but approach and cruise speeds should not be the same. Altitude should be a typical cruise altitude. Fuel should be something reasonable after a long climb-out. Check control settings. I recommend settings optimized for maximum continuous power in level flight at the optimal altitude. I think this gives better flight results than solutions using economy cruise settings since YASim cruise determines the top end behavior of the aircraft. In general, throttle should be wide-open, props full-fine, flaps and other surfaces clean, gear up. If the aircraft has a blower setting, make sure it is set appropriately for the cruise altitude.
Wing incidence values of 0
Most general aviation aircraft will have several degrees of incidence, so it is unlikely that you will have a wing incidence of 0. Be careful though, because some GA aircraft actually do have an incidence of 0. For example, the DHC-2 Beaver despite being a high-lift configuration actually has a wing incidence near zero, which some consider to be one of its very few design flaws. Wing incidence naturally has a major effect on YASim, so try to find the right value before you do much work on your FDM. Also, remember that you cannot set horizontal stabilizer incidence-- the YASim solver sets this value.
Camber values of 0 on high-lift airfoils
It can be tough to find airfoil data, but if wing camber is 0 on an aircraft that clearly has a high-lift airfoil, it's obviously not right. Similarly, a large camber on a fast or aerobatic aircraft can also signal a problem as many of these have symmetrical or near symmetrical airfoils. Camber has a great effect on the aircraft's intended performance, so it's worth careful study.
Use of the "Effectiveness" attribute
This is commonly used on horizontal stabilizers as a cheat to get a solution when nothing else seems to work. Effectiveness alters the drag of a flight surface, so what it's doing is forcing the stabilizer to drag behind the rest of the aircraft. In most of these cases, the proper solution is to first get CG right, then examine other values for problems. Effectiveness should be used only to increase drag, not as a crutch to get solutions.