Harold & Susan Vordos wrote:

"Mike Firth" wrote in message
...

In a pure ram jet engine, the engine (and plane) have to get up to some
speed before the jet will work.

That's not true. If you start them by compressed air, or other means, they
will keep running even when sitting stationary, although they run much
better with speed. The low pressure in the combustion chamber after
ignition causes the valve to open and draw in air and fuel, which is then
ignited and the cycle repeats. Apparently many things influence their
operating frequency, but in small models its hundreds of cycles per second,
at least from what I've read. I gather that's the source of the "buzz"
sound.

Harold

Harold, you're thinking of a pulse jet there, not a ram jet. A ram jet
uses the shock wave fron the inlet air as a barrier to the flame front
moving too far forward. No forward speed = no run at all. Pulse jets
have valves. Ram jets are pretty much open from one end to the other.

Cheers
Trevor Jones