There may be no need for a requirement on the first mode frequency on this wing. I don't know of one. In other structures, one reason to have a first mode requirement is because there is a vibration load that acts on the structure. For example, a pump runs at a certian frequency and has an imbalance. You don't want the support structure to have a matching modal frequency because the structure will be induced to vibrate at that frequency which can lead to fatigue failure. In that example, the structure may have a modal requirement called a "keep-out zone" that requires no modal frequencies between two values that bracket the known vibration frequency of the pump.
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