Targetware: Flight Model Development

Targetware ACM File Guide

PART REFERENCE: Airfoil

If the airfoil data is contained in a separate .acm file, the following entries are required.

force_limit = negative, positive (measured in Newtons)

The force limit determines the structural integrity of the surface.  It can be exceeded by pulling excessive g-forces in flight or by taking damage from gun ammuntion.  Since a wing is normally split up into many parts, the force limit for each part needs to be calculated as follows:

force_limit = design mass * g-limit * 9.8 * part_area / total_area

There is a 50% safety margin built in, so that the wing may not fail right at the G-limit, unless the aircraft is experiencing buffet, either from a stall or in the transonic region.

incidence = degrees

If the exact incidence is not known for the plane being modelled, it can be set at the angle that gives the best lift/drag ratio (usually close to a lift coefficient of 0.3).

dihedral = degrees

Dihedral is what gives a wing it's V-shape when looking at it head on.  The purpose of dihedral is to help maintain coordinated flight if the aircraft finds itself sideslipping or skidding.  That is, the angle of attack will increase on the wing that is on the same side as the direction of slip.  You'll notice that in the case of the Mig-15, the wing has 'anhedral', which is really just negative dihedral.  This is because wings that are swept back give the same effect as dihedral.  So, the anhedral is designed to cancel the sweep effect.

area = square meters
chord = meters (the average distance between the leading and trailing edge of the surface)

aspect_ratio = span^2 / area

The aspect ratio largely determines how efficient the wing is at high angles of attack.  A low aspect ratio wing will make more induced drag at high angles of attack.  This value should be set to that of the whole wing or stabilizer, since using a value for the part will result in too much induced drag when the parts are added together.

critical_mach = a value between 0 and 1

This is the freestream velocity at which the local airflow over certain parts of the aircraft (airfoil, body) reaches supersonic speed.  A rise in drag will result.  Since the airspeeds corresponding to a particular mach will vary with altitude, you can use this setting to make sure that speeds are correct at all altitudes.  For example, if you've fine tuned the drag areas so that speed is correct at sea level, and you know the power vs altitude characteristics of your engine are accurate, yet your plane is too fast at high altitudes, your critical mach setting may be too high.

stall_warning = number of degrees of angle of attack before the stall occurs that buffeting is noticed

Some airfoils have better stall characteristics than others.  Some stall more gradually and some more abruptly.  If you read in the pilots notes that a plane gives a warning in the form of airframe buffet, say 5 or 10 knots before the stall, you can calculate the difference in the lift coefficient between those speeds and compare to your airfoil plots for a corresponding number of degrees warning.

You can then use the #include airfoilname.acm command to include your airfoil, which should contain the following information.

zero_alpha = the angle of attack for zero lift
zero_drag = the drag coefficient at the zero lift angle
max_alpha = the angle of attack for maximum lift coefficient
max_lift = the maximum lift coefficient
max_drag = the drag coefficient at max_alpha
min_alpha = the angle of attack for maximum negative lift coefficient (a negative angle)
min_lift = the maximum negative lift coefficient
min_drag = the drag coefficient at min_alpha

Airfoils can have control surfaces attached to them.  To do this, you must specify the number of surfaces,

num_surfaces = number

Then the control surface is defined under it's own header/section.

[Part number Surface number]
name = the name of the control surface
type = (control, flap, slat or trim)