Intruder,
Logic suggests to me the Vy will give better obstacle clearance over trees
following an engine failure for a number of reasons.
- The aircraft will be higher - meaning it will glide further and longer - creating a larger landing footprint.
- The aircraft will have traveled further - meaning the available landing footprint will be further in front of the aircraft.
- If it is only a partial engine failure Vx and Vy will become closer together - it is far easier to decelerate from Vy to maximise performance than to accelerate from Vx.
- Whilst the aircraft will be lower in the very early part of the climb (due to loss of climb performance due to acceleration) most of that energy would be able to be converted to height anyway.
Granted prior to an engine failure climbing at Vx will increase the chances of outclimbing the trees in front. Some seaplane texts talk about a V obstacle clearance speed, which is lower than Vx. Basically the theory is that for very close in obstructions the loss of climb performance due to accelerating to Vx is greater than the loss of climb performance due to climbing at a less efficient airspeed. However I've never seen these speeds documented, and as the actual speed would vary depending on the distance and height of the obstruction as well as the normal factors that determine performance one would well and truly be entering test pilot territory experimenting.