§ 92. Real estate tax for debt service. If a city issues its obligations for any project or to raise funds to loan in aid of a project, such city may levy an ad valorem tax on real estate to the extent that revenues derived in any year from taxes levied pursuant to this chapter, together with revenues from the project or projects aided by the proceeds of any such obligations shall become insufficient to provide fully for the payment of principal and interest in that year upon obligations issued for either or both purposes.