YASIM Approach and Cruise Settings
By Gary "Buckaroo" Neely
These two elements define the limits of the aircraft behavior. Approach settings determine the necessary low-speed, high angle of attack behavior. Cruise settings determine the high-speed, low angle of attack behavior. The solver must find a common elevator incidence solution that allows both behaviors. The primary function of approach and cruise settings is to determine the spectrum used to test the horizontal stabilizer's ability to balance pitching moments. I discuss this in more detail in YASim Solutions. Here I focus on the configuration of approach and cruise element values.
Approach Settings
Let's look at the approach settings first. Here is an example approach configuration:<approach speed="80" aoa="5.8" fuel="0.2"> <control-setting axis="/controls/engines/engine[0]/throttle" value="0.3"/> <control-setting axis="/controls/engines/engine[0]/mixture" value="1.0"/> <control-setting axis="/controls/engines/engine[0]/propeller-pitch" value="1.0"/> <control-setting axis="/controls/flight/flaps" value="1"/> <control-setting axis="/controls/gear/gear-down" value="1"/> <solve-weight idx="0" weight="350"/> <solve-weight idx="1" weight="350"/> </approach>
The primary approach attributes are speed, aoa (angle of attack) and fuel. You should always provide values for these attributes. The approach element can also have sub-elements consisting of control settings and solve weights. These are optional, but all useful approach elements will have control settings.
Approach AoA sets the bottom end of the aircraft's performance. It's really about configuring where the aircraft begins to stall and as such it will have a great effect on stall performance. For example, an aircraft with an approach angle of attack that is much lower than the wing's stall angle will need a lot of elevator authority to pull the plane into a stall attitude or beyond. More about this later under Solutions. Let's look at the approach attributes first.
speed
A good guess at approach speed for general aviation aircraft is find the stall speed of the aircraft when configured for landing (flaps and slats deployed, gear down, aka Vs0) and multiply it by 1.3. For example, if you know that the plane will stall at 55 knots, try an approach speed of 72 knots. Another method to set approach speed is to use Vs0 as the initial trial approach speed value, fly the results, and alter the approach speed in the FDM until the aircraft achieves a stall at roughly the plane's correct stall speed. Be sure that you do this with the aircraft configured for the proper load. An airliner with a full load of passengers might stall at 110 knots, but its stall speed will be much lower when unladen.
aoa
Setting a good approach angle of attack requires knowing something about the aircraft's intended design characteristics. Aircraft designed for STOL or high-lift cargo operations will often have very low approach angles because they use high-camber wings that generate a lot of lift even at an AoA of 0. This is particularly true when coupled with powerful flaps that change the wing's camber. Some high-lift aircraft like the DHC-6 Twin Otter will even approach at a negative angle of attack. At the other end of the spectrum are the delta-winged aircraft. With low-aspect delta wings, no flaps, and often a lifting-body effect, they tend to approach at high angles of attack, 15 degrees or more.
For general aviation aircraft using flaps, a good guess at an approach AoA is about 1/2 the stall AoA. For aircraft without flaps, try using the actual stall angle for approach. When judging the stall AoA, consider the entire aircraft, not the wing itself. For example, the wing might stall at 15 degrees, but a two degree washout will reduce this to an average of 14 degrees, and a wing incidence of 3 degrees will reduce it again to 11 degrees. In such a case, with an aircraft equipped with flaps you might try using an approach aoa of 4 or 6 degrees. If the aircraft has no flaps, try using an approach AoA of 10 or 11.
Sometimes you can find a good approach AoA by studying photos or videos of the aircraft on approach. Make sure that you take the aircraft's glidepath into account. A typical glideslope is 3 degrees, so if the aircraft appears to be approaching at an AoA of 3 degrees with respect to the ground, it's true approach AoA would be 6 degrees.
If you know the glide ratio of the aircraft and the nose angle with respect to the horizon when gliding, you could get a reasonable angle of attack by calculating: arctan(glide ratio) + nose angle.
Approach angle of attack is a key value for tuning a YASim FDM. Often a configuration using a certain aoa value will not solve, but changing the value slightly will give a solution. I have more to say on this here: YASim Solution Troubleshooting: Singularities
fuel
This is the fraction of fuel remaining in the tanks, 0-1 and is used to contribute fuel weight to total aicraft weight. A value of 1 indicates the solver calculations should be performed assuming full tanks, and 0.1 indicates the calculations should assume the tanks contain 10% of their maximum fuel capacity. For most situations, this value should be small, leaving enough fuel in the tanks for a comfortable reserve. The default value is 0.2. The exact value is not critical, but aircraft weight will have a significant effect on solver results.
glide-angle
There is a fourth and rarely-used attribute, glide-angle. This is a relatively late addition to YASim and is typically used for unpowered aircraft. I don't have experience with glide-angle. When I do, I'll expand this guide.
Control Settings for Approach
In addition to the speed, aoa, and fuel attributes, you will want to tell YASim what control settings should be used for "approach". Commonly used settings and recommended values are:v
full flaps (1)
full slats (1)
gear down (1)
throttle to some reasonable low-mid value (approximately 0.3)
props set to full-fine (max RPM) (1)
mixture full rich (1)
elevator trim if known
The controls and settings will vary with each aircraft. Obviously some aircraft lack flaps or slats, some will have fixed gear that are always down, some will have blower controls, etc. If using fixed landing gear, tell YASim that the gear should be deployed just as if the gear were retractable.
If you know that an aircraft will approach with a certain amount of elevator trim, you can and should specify that control in your approach setting:
<control-setting axis="/controls/flight/elevator-trim" value="-0.2"/>
You cannot set a control for elevator for approach and cruise, only elevator-trim. Elevator control is the province of the YASim solver. See YASim Solutions for more information.
Control settings are critical for getting good results. Double-check your control settings, since a mistake here can have significant effects on the solution.
Solve-Weights for Approach
These settings determine run-time distribution of weight. Typically they are used to give weights for the pilot and crew, the cargo, and sometimes ballast. Simple FDMs having a single pilot and no cargo might not have solve-weights. This also means there would be no provisions for customizing weight for a given flight. When working with the solver, you need to tell YASim what weights will be used to generate the solution. My recommendation is to use a typical common-case load.
For more information on setting up and configuring weights in your YASim model, see: YASim Weight and Balance.
Cruise Settings
Before getting into the specifics of cruise settings, we need to consider what "cruise" means in this context. Nowhere in documentation or code does YASim provide a definition for cruise, so I've had to come up with my own interpretation. "Cruise" is a somewhat nebulous term that often refers to an aircraft's flight between the climb and the descent phases. A cruise configuration might maximize enduranace, or maximize range, or minimize flight-times between two points. Cruise might also refer to a state where the aircraft is in equilibrium, lift equals weight, thrust equals drag, and the aircraft is moving at a constant speed and a constant altitude. Conditions and requirements may vary, and requirements might even change after the aircraft is designed. Many airliners were designed during a time when fuel was relatively cheap and emphasis was on endurance or flight-time, but now the emphasis is often on fuel economy so cruise requirements are not the same.
I have two schools of thought for configuring the YASim "cruise" element:
1. "cruise" as equilibrium at maximum power/performance
2. "cruise" as equilibrium at best economy/range/endurance
Both strategies involve differences in throttle, propeller pitch, and speed settings. Both should assume a constant speed in level flight at a fixed altitude.
Using method 1, throttle is always wide-open, propellers full-fine (max RPM), and speed is set to the maximum speed the aircraft is known to be able to maintain in level flight at that altitude. Method 1 allows the aircraft to meet top-end performance numbers, but other solution results may be forced into more extreme values. Drag Coefficients and Lift Ratios will be more separated, and stabilizer/elevator requirements are greater, meaning less authority without greatly increasing elevator lift.
In method 2, settings are based on a typical economy cruise configuration as suggested in a pilot manual. Throttle might be something like 70%, propellers set to a reduced RPM, and speed will be some economy cruise speed. It's generally easier to get good YASim solutions using method 2, probably because the configuration is less extreme. Drag Coefficients and Lift Ratios tend to be closer together, and stabilizer/elevator requirements are less demanding. The disadvantage of method 2 is the aircraft will not likely reach the aircraft's maximum power speed.
Method 1 gives me better general results, though it is more difficult to tune. When combined with the right approach settings, using maximum power settings brackets the aircraft's two flight performance extremes: max cruise and near-stall. Using method 1, the FDM developer determines the two extremes of flight and the pilot determines the economy settings in flight.
I can't tell you which method to use or which method is more correct. I prefer method 1, but only one thing matters: the end result. If a particular strategy gives a model that comes close to matching real aircraft behavior and the plane "feels" right, then don't be concerned with what is "correct" for YASim. Use what works.
Now let's look at the cruise settings. Here is an example cruise configuration:
<cruise speed="205" alt="8000" fuel="0.7"> <control-setting axis="/controls/engines/engine[0]/throttle" value="1"/> <control-setting axis="/controls/engines/engine[0]/mixture" value="0.65"/> <control-setting axis="/controls/engines/engine[0]/propeller-pitch" value="1"/> <control-setting axis="/controls/flight/flaps" value="0"/> <control-setting axis="/controls/flight/elevator-trim" value="0.15"/> <control-setting axis="/controls/gear/gear-down" value="0"/> <solve-weight idx="0" weight="350"/> <solve-weight idx="1" weight="350"/> </cruise>
Cruise settings are similar to approach, with a few important differences. The primary cruise attributes are speed, alt (altitude) and fuel. You should always provide values for these attributes. The cruise element can also have sub-elements consisting of control settings and solve weights. These are optional, but all useful cruise elements will have control settings.
speed
If using method 1, set speed to the maximum possible at full power in level flight at a known cruise altitude. If using method 2, you'll need to do deeper research to determine a typical airspeed for your choice of cruise criteria. Sometimes pilot handbooks contain usable reference material. Make certain speed is expressed in KTAS, not KIAS.
altitude
The maximum reasonable cruise altitude, or one for which you have good numbers.
fuel
Similar in function to the value used for approach. The default is 0.5. Choose a value that allows for a reasonable burn of fuel to reach a typical cruise altitude.
If you don't have a realistic value available, try this: Find the total weight of the aircraft after accounting for solve-weights and deduct this from the maximum take-off weight. Use the difference to calculate a maximum fuel load. After getting a flyable solution, fly the aircraft from the ground to cruise altitude and note the fuel remaining. Use this to get your fuel fraction (remaining/initial) and rework your FDM with this value.
glide-angle
See my notes under approach settings.
Control Settings for Cruise
In addition to the speed, alt, and fuel attributes, you will want to tell YASim what control settings should be used for "cruise". Commonly used settings and recommended values are:
flaps retracted (0)
slats retracted (0)
gear up (0)
throttle maximum (if using method 1) (1)
props set to full-fine (max RPM) (if using method 1) (1)
mixture depends on altitude
blowers engaged to the proper level, probably maximum (1)
afterburner engaged if present
elevator trim if known
If you have fixed-gear, then you will want to specify the down or deployed position (1). If using method 1, maximum power, set throttle to 1 and props to full-fine (1). If using method 2, throttle and propeller might be nearer 0.7. YASim mixture setting doesn't work quite like a real mixture control, so a value is hard to define. For GA aircraft, try something like 0.6 at cruise. Mixture doesn't have much effect on the FDM solution.
If your aircraft uses a certain amount of elevator trim at your selected cruise setting, you can and should specify an elevator-trim control in your cruise setting. Conventional elevator aircraft designed for efficient cruise will tend to have little or no trim at cruise. Remember that you cannot set a control setting for the elevator, only elevator-trim.
Solve-Weights for Cruise
Set these to the same values as your approach solve-weights.