WillyPete/VF,
Thanks for the responses.
VF; you are of course correct. Anyone that fails to understand the performance limitations of a helicopter will find that their performance problems are very short lived! Private pilots and commercial guys alike must understand and stick to these limits, you'll get no qualms from me on that matter. However, the point I am trying to get to with this thread isn't the skill and discipline of experienced expert commercial pilots that deal daily with extreme operations of the edge of the performance curves, but rather to understand whether design traits of an aircraft can be improved to help the infrequently flying private pilot population avoid trouble given their limitations.
I think we would all agree that power gets you out of trouble in most situations in a helicopter. The problem piston pilots face is that the performance of the machines is marginal even for relatively benign operations.
If you take the Robinson R44 Raven 1 for example, the O-540-F1B5 (205hp/225hp) carburetted version. This machine can only execute a nil wind OGE hover at max gross weight on a 20C day up to
500ft! Therefore, when flying with four normal size adult passengers and 2 hours of fuel you have very limited performance. If you factor in limited pilot experience and currency surely this is a recipe for trouble?
The R44 Raven 2, uses an angled valve engine that breathes more freely and extends this performance to a 2,600ft OGE hover ceiling at MGW on a 20C day, but surely even in cool low green lands of the UK this is pretty limited performance.
How many mishaps would have been avoided if the machine could climb vertically at 10,000ft on a 20C day and hover OGE at this level?
WillyPete:
I agree that the cost, complexity and weight of turbo-normalised piston engines is somewhat of a downside. However, if this is properly taken into account during the design phase, I wonder if the overall effect would be a positive one?
For existing aircraft piston engines, turbocharging is generally used to maintain performance at altitude rather than increase power. The naturally aspirated versions of the IO-540 can make 300hp+, with the turbo'd versions offering 350hp, but weighing about 100lb more and being less fuel efficient on account of the low compression pistons. However, naturally aspirated engines performance lapse the moment you get off the ground, to around 60% at 10,000ft whereas a turbo engine will maintain its full performance to 20,000ft.
The major benefit of the 'Robinson de-rate' of the engines, is that they generally maintain a degree of performance up to a few thousand feet, older designs (and some newer ones) that have squeezed more out of the engines really struggle once away from the ground or on warmer days.
The Cabri G2 can't hover at all out of ground effect on a 20C day at MGW!
When you consider the performance penalties that naturally aspirated engines suffer at modest DA conditions, then all of a sudden the sea-level weight penalty doesn't seem so bad to me.
Happy to learn what others think?
