Thinking experiment about a flying device having the fewest moving parts, whats the slowest cruise speed achievable using a ramjet, no matter the scale of this device. Working altitude is troposphere. What would section cut view of this engine look like?
Theoretically Ramjet should start working from 0.5 Mach (this is the speed where compressibility of fluids becomes significant). But it won't be much efficient until ramjet reaches around Mach 3. Because RamJet compresses incoming air by slowing down the air speed. More the speed of incoming air more compression can be achieved. Around Mack 3, diffuser can achieve upto 25 atm pressure (depending upon altitude it may vary). Nevertheless ramjet are historically used in tip-jet helicopters. Here is the link -> https://en.wikipedia.org/wiki/Hiller_YH-32_Hornet