In short, because the extra mechanical complexity isn't worth it.
A brief description of the systems:
When rotating a prop. has a centrifugal twisting moment (CTM) trying to twist the blades towards a flatter angle ie fine pitch. It also has an aerodynamic twisting moment (ATM) trying to twist the blade(s) to coarse pitch. Unless overridden by some mechanism or other the CTM overcomes the ATM leading to an overall tendency to move to fine pitch.
In a s/e VP/CSU setup this is advantageous. Oil pressure fed to the prop hub is used to oppose this movement to force the the blades to a coarser pitch. If anything should happen to the oil control or governing mechanism then at least available power will not be limited by a severely constrained RPM.
If there's an engine failure then the only choice in a single is 'down'. Having a feathering prop only changes the situation by 'how steep'. There isn't an option to try to continue flight**. With or without feathering the only option is to choose a nearby paddock.
** Unless it's a 'mild' partial engine failure in which case fine pitch is the most desirable anyway.
In a multi setup there is an option to try to continue if an engine fails. The limitation is that any continued flight ability is marginal. In this case it's worth the extra complexity & weight.
To achieve this the overwhelming CTM effect needs to be overidden so that if worst comes to worst the blades would move towards coarse, ultimately ending up feathered.
The system commonly used in light twins uses forces produces by some combination of:
* gas pressure in the prop hub
* angled weights on the blade root
* strong springs in the prop hub
These all act in concert with the previously mentioned ATM to cause the blades to tend towards coarse pitch. Oil pressure is now used to drive the blades to fine pitch.
In the event of a need to feather then removal of oil pressure such as when the engine stops leaves the blades with nothing opposing their being shoved towards feather.
There's catch though: What to do when you want to shut down after flight? The loss of oil pressure as you stop the engine would leave it with a feathered prop. Not good for the next engine start.
So, to counter this there must be latches in the hub that activate to prevent the blades going into feather below a certain RPM - typically 800 or so. Now the whole shebang needs yet another control to prevent the feathering latches getting in the way when you wish to feather. In light multis this is usually integral to the prop RPM lever, although it could also be a separate switch/knob/button/lever etc.
All this adds cost & complexity. In a single it's just not worth it for the sake of maybe choosing a paddock a bit further on.
Last edited by Tinstaafl; 20th July 2004 at 18:41.