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Helicopter Air Files and Configuration - Part 2

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Often, helicopter developers for microsoft flight simulator will blame the simulator software for shortcomings relating to the behavior of the helicopter during flight. While there certainly are some ligitimate limitations, many of the problems are actually the result of the configuration and flight dynamics chosen by the developer. Either out of poor judgement, or lack of awareness of the potential to change the settings.

In this article we will be focusing on some of the flight dynamics settings that relate to the "cruising" behaviors of helicopters in FS2004 and FSX. This discussion will be based on helicopters that are derived from the default Bell 206.

This article will make frequent references to AirEd.exe, a program used to read and adjust the binary .air file for aircraft in flight simulator. You will need this software in order to understand much of what will be discussed in this article.

It can be downloaded here.

When it comes to 206 based helicopters in flight simulator, the air file is responsible for about 80% of the flight behavior that we can adjust. Only a small amount of settings are in the aircraft.cfg file, and those are covered in part 1 of these articles.

The following is what AirEd.exe should look like when you open something like the Bell_206B_JetRanger.air file.

Example of AirEd.exe Interface


Next to each small gray box there is a number. Those are referred to as sections. Keep them in mind as they may occassionally be referred to throughout this discussion.

Now of course these articles assume that you have become pretty familiar with microsoft flight simulator, and that you have no problem finding files, editing text files, copying files, and making backups of anything important you are going to touch. If you are not comfortable with how to do these things, it would best for you to get up to speed with those types of skills first.

The keys to adjusting flight dynamics effectively are:

1. Making backup copies of adjusted files incrementally so that you can revert to your last GOOD state should you hose something up, and know what you changed.

2. Always try to change the least number of settings at a time, REMEMBER the change, and test the results. So that you can accurately revert the setting should you not be pleased with the outcome. This is especially important for certain parameters that can have effects on other parameters.

I have spent a great deal of time learning how certain adjustments present themselves. I do not know everything, but I have developed a fairly decent understanding of what can be adjusted and what can not. For the remainder of this article, I am going to avoid lengthy discussion regarding each parameter. Instead, I'll try to simply guide you to which parameters control which behaviors, without boring you with all of the background details that might have lead up to that conclusion. So even after reading this article, there will be plenty for you experiment with, and you'll have a lot of room for your own opinions on what works best for certain changes.


Adjusting Cruise Airspeed

In flight simulator helicopters there are basically two components to airspeed. The amount of thrust being generated by the rotor configuration, and the amount of drag being produced by the front of the helicopter as it goes through the air. The rotor configuration is an entire article in itself, so for now, it is important to know how to adjust the drag. This will allow you to fine tune the airspeed produced in level flight during cruise. Many helicopters developed for MSFS have strange cruise speeds, usually the result of simply using the settings in the default 206. Which even for the 206, produces less airspeed than it should.

The following comes from the default 206 .air file


Section 1404 - Helicopter Fuselage Area and Drag
Frontal Surface Area(sq. ft.)=29.010000
Frontal Surface Drag Coefficient=0.400000


As often is the case in the flight simulator air files, we have two numbers here working together to produce a result. The first sets the square footage of the frontal area of the helicopter fuselage. The sim will use this in its mathematical equations regarding its helicopter physics. However, the drag coefficient behaves as a multiplier of this area. A reduction in square footage, as well as a reduction of the drag coefficient will produce greater airspeeds at equivalent power settings. For example a properly tuned Bell 206 might look like:


Frontal Surface Area(sq. ft.)=8.010000
Frontal Surface Drag Coefficient=0.3500000


This gives the default 206 a boost in airspeed numbers, that allow it to cruise at about 110kts with fairly sane torque settings for that speed. As you can see, adjusting the airspeed is fairly straight forward. Just keep in mind, that like all flight dynamics adjustments, that large changes to these values will start to create other behaviors that must be dealt with. When it comes to increasing airspeed, some side effects to address will include:

1. Yaw waggle in cruise flight
2. Inability to slow down in a realistic fashion

Not being able to slow the helicopter down is just a nasty trade-off of reducing the frontal drag and coefficient to VERY low levels. There may be nothing that can be done about that. However, reasonable airspeed values of between 110 - 200 kts can normally be achieved without screwing things up too badly. Which is fairly reasonable for most helicopters.

We will discuss yaw waggle (tail wagging) further in another section.


Adjusting Cruise Attitude

In helicopters like the default 206, the nose must be lowered in order to create forward airspeed. The more you desire the rotor thrust to move you along, the more the helicopter must be tilted foward. This is normally not a problem when playing around in the hover, but for long flights this can become a serious pain for simulator pilots. Without the ability to trim the cyclic in flight simulator, the pilot has no choice than to constantly apply forward pressure to his joystick in order to keep his aircraft in level cruise flight at the desired airspeed and power settings.

The good news, is that this can be helped.

The following comes from the default 206 .air file


Section 1401 - Helicopter Horizontal Stabilizer
*Horizontal Stabilizer Lift Coefficient=0.0000


As this horizontal stabilizer moves through the air it provides pitch stability, but does not really produce any force in an up or down direction. By increasing its lift potential, we can have it produce increasing upward pressure as the aircraft increases its airspeed. Making for a very nice way to "trim" out some of the forward cyclic needed at common cruise speeds. Typically this should be adjusted so that at the most common cruise airspeed and power settings, the aircraft holds a steady pitch on its own, or has a very slight nose-up pitch tendancy.

For example, the default 206 can be tuned using the following values. Allowing for a fairly comfortable cruise attitude at around 110-115kts and around 78% torque. (depending on a few environmental factors).


Section 1401 - Helicopter Horizontal Stabilizer
*Horizontal Stabilizer Lift Coefficient=0.450000


The concept here is pretty simple. As the pilot increases airspeed to reach his desired cruise speed, the horizontal stabilizer helps him out by providing a little lift which helps keep some downward pressure on the nose of the aircraft. The lift increases as the airspeed increases, which helps things stay fairly balanced out. Some real-world helicopters do precisely the same things, however other real-world helicopters have far more sophisticated systems involving automatically adjusting horizontal stabilizers, and trim systems.

Here's a picture to help break up the boringness of flight dynamics discussion.

Hovercontrol 412 Flight from KMSO up to Glacier Park.



Adjusting Cruise Yaw Trim

Another nasty trait inherited from the default 206 is the tendancy to produce yaw as airspeed increases. Requiring a constant twisting motion on most joysticks in order to keep a coordinated cruise attitude. Typically, real helicopters have a vertical stabilizer (or several) that are pitched in such a way that as airspeed increases as a result of increased torque, the stabilizer helps to cancel out the yaw effect that the torque would produce. This is typically balanced at around the airspeeds that would be most common for that aircraft. That way, at the most common cruise speeds, the pilot and passengers are the most comfortable, and the pilot does not have to constantly fight to keep the aircraft in straight, coordinated flight.

We can do the same thing in MSFS.

The first thing to understand is that the default 206 comes with a bug in this area. So the first step is always to address this bug before making any other changes. The vertical stabilizer on the default 206 includes a slight off-center angle of incidence (like trim). This causes the helicopter to yaw increasingly as the airspeed increases. This angle of incidence is actually realistic to the actual aircraft, however, since torque is not adequately modeled in the simulator this vertical stabilizer has nothing to counter-balance. The result is that it produces yaw all the time, with nothing to push back against it. The good news is that this can be corrected:

The following comes from the default 206 .air file


Section 1401 - Helicopter Vertical Stabilizer
*Horizontal Stabilizer Lift Coefficient=-0.3490000


Before making any other changes to the vertical stabilizer this should almost always be changed to:


*Vertical Stabilizer Angle of Incidence (rad)=0.0000


Now that that is out of the way. You can feel free to trim the yaw on the helicopter as you please. For most helicopters, the need for yaw trim is minimal. For example the default 206 is very happy with just the change above to straighten things out. However on more advanced flight dynamics configuration (for example, the HC 412) there are some change to rotor configuration and other areas that produce some unwanted yaw pressure in cruise. Which causes an aircraft that is either always slipping, or flying uncorridinated, which burns up airspeed and fuel efficiency, and is generally annoying to a serious simulator pilot.

So without too much confusion, the yaw trim of the helicopter can be adjusted in much the same way that we adjusted the pitch trim. By increasing or decreasing the amount of lift produced by the stabilizer.


Section 1401 - Helicopter Vertical Stabilizer
*Vertical Stabilizer Lift Coefficient=0.00000


Try setting numbers such ast 1.0 or -1.0 and observe the affect this has on flight. By using this parameter you can help "true up" the aircraft to fly more coordinated. Which will make for a more natural and comfortabl flying helicopters, as well as give you back some knots of airspeed, because if the helicopter isn't flying "true" then it is producing more drag, which is eating away at your maximum airspeed.

You might have asked when we adjusted the angle of incidence parameter what the difference is between that and the lift coefficient settings. Basically, they produce about the same behavior. One changes the pitch of the stabilizer (angle of incidence) and the other changes the amount of lift being generated by the stabilizer. I have found that the lift coefficient produces a more natural and subtle force, while the angle of incidence changes tend to be a lot more noticable and somewhat artificial. I have also found that the lift coefficient responds very naturally as the airspeed increases, where as the effect of angle of incidence tends to get exponentially more dramatic as the airspeed increases. Feel free to experiment, and form your own opinion.

Here's another picture to fight off the boredom.

Hovercontrol 412 getting serviced before next flight.



Adjusting/Correcting Tail Waggle

As you increase the cruise airspeed potential for your helicopter by adjusting the drag coefficients as explained above you will no doubt get to a point where the aircraft develops either a slight or severe "waggle" in flight. This is EXTREMELY annoying to flight simulator pilots. Most real world helicopters would never have this behavior, and it cause immediate problems such as dizziness and nausea. Generally, if your simulation helicopter has this behavior, the chances of people truly enjoying long flights in your helicopter are slim.

In addition increasing airspeed potential, increasing the overall responsiveness of the helicopter controls will also lead to waggle behaviors. So you may want to make note of this discussion for future reference, as we won't be discussing control responsiveness in this article.

Basically, 3 configuration items work in conjuction to control the tendancy to waggle in your aircraft as airspeed increases.


Aircraft.cfg:
empty_weight_yaw_MOI = 10000

Aircraft.cfg:
low_realism_stability_scale = 1.0, 1.0, 1.0
(the third number, which is yaw)

.air file:
Section 1401 - Helicopter Vertical Stabilizer
*Vertical Stabilizer Surface Area (Sq. Ft.)=9.650000


The values shown above are basically the default values in the 206 for MSFS. Typically, the waggle will be addressed by some mixture of increasing the Yaw MOI (moment of inertia), the Yaw Stability, and the square footage size of the Vertical Stabilizer.

Going overboard with any of these items will produce counter-productive side effects. I'll try to briefly describe the primary side effect of each.

Side Effects

MOI Increases:
Yaw will be very heavy to achieve. Meaning it will be very difficult to yaw in the hover, and once a yaw (pedal turn) is underway, it will want to continue yawing too much. (It won't decay/stop naturally as it should).

Yaw Stability Increases:
Will begin to create an artificial feel on the yaw access. It won't have a natural physical motion, but instead it will feel like the yaw is on rails, or rigid. Beginner users won't notice, but experienced users will.

Vertical Stab Size Increase:
Will produce strange yaw limitations at lower airspeeds and hovers, as well as creating anti-torque control limitations at medium airspeeds (20-40kts). Bigger Stab size is definitely good for cruising, but you can only go so big. In addition, having the stab too big can cause strange "slipping" effects at higher airspeeds that are difficult to trim out. The vertical stabilizer should be somewhere in the range of 8-35 square feet. Anything higher than that, and you'll probably get increasingly unhappy with the flying.


Conclusion

I hope there has been a few helpful points of information in this article for you. At a minimum, it should get you thinking in the right direction, give you some ideas to play with, and begun to introduce you to some of the interactions of these parameters in microsoft flight simulator.

In the next article, we'll probably be discussing rotor configuration. Which is a lot of fun. Yee haw.

Here's another picture to remind you how fun everything is:

Sweet FS2004 screenshot of EC120.












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