You need some sort of functional prototype (i.e. something that actually, in some way, does what you want it to do). Nobody will invest without one. You said you're too low on funds to have one built, so my best advice would be to spend some time learning the skills needed to build it yourself. You could for instance take free online electronics courses that MIT provides: https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-002-circuits-and-electronics-spring-2007/ or buy books, watch youtube videos, etc.

I've built my own hardware prototypes for two of my own startups, and also helped build prototypes for others through Clarity. Let me know if you'd like advice more tailored to your specific idea (e.g. the kinds of microcontrollers and motor controllers that might work, etc.)

best,

Lee

Yes, in order to explain your idea a model will be helpful, which you can also take photos of to support your description online.
Consider using drone parts to build a cheap small scale functioning prototype.
Drawings can help, whether engineering or illustrations (what ever is cheaper to start) and support your description online.
Understand the trade off involved for VC money - how much equity and control you are willing to lose in exchange for this.
Figure out how much money would be required, even if only estimating the parts list at first, and what you need to be selling to determine if it's a potentially profitable venture before spending time on it.
How much are cargo drones costing? What can you learn from these?
How can you validate whether you've identified a buying market for this?