The land clear requirement for helicopters is abated because properly handled, they can land without the engine at low speed and in a small space. The raw physics dictate that in a fixed wing you'll arrive at 50mph or so.
The Robinson books quote a maximum glide ratio of 4:1 for the 22 and 4.7:1 for the 44. You're unlikely to be able to achieve this maximum from 1000ft though as it requires a particular combination of speed and rotor rpm which will take time and height to establish. I think from 1000ft you'd be lucky to get much more than half a mile. Your fixed wing would almost certainly travel further, but potentially do more damage when it arrived.
The river would be an option in the event of engine failure, which is why single engine commercial air transport work over the Thames requires floats.
Hope that helps.
Last edited by puntosaurus; 10th Jan 2013 at 09:52.