The conceptual design process described here starts from the design brief - essentially a broad brush mission profile - encapsulated in the following set of numbers.
At this stage in the design process it is not uncommon to only be able to provide an initial guess at the design gross weight. This may come from simply multiplying the desired payload weight with the payload fraction (payload weight divided by design gross weight) derived from looking at other aircraft designed for similar missions. In this case this “zeroth order” guess is:
In : DesignGrossWeight_kg = 15
In : GroundRun_feet = 197
In : TakeOffSpeed_KCAS =31 In : TakeOffElevation_feet = 0
The cruising altitude may be viewed in two fundamental ways. First, it may be a constraint - for example, due to regulatory requirements the aircraft may have to cruise at, say, 350 feet. It can also be viewed as a design variable, in which case you may wish to return to this point in the document and revise it as part of an iterative process of optimization / refinement.
In : CruisingAlt_feet = 400
In : CruisingSpeed_KTAS = 58.3