The reason the only aerobatic helicopters (if you can say that) are turbine
powered, is merely power. Where can you get 600 HP out of an engine that
weighs less than 300lbs.? We need loads of HP and turbines deliver that.
With that said, the turbine does act as a large gyro, however the rotor is
about 5x stronger than the engine. It's just that the engine mounts are not
made to take those loads. It's assumed that the helicopter will not normally
snap around like you can get in aerobatics. That's why we have to be careful
about it.
"Peter Holm" wrote in message
om...
"nametab" wrote in message
k.net...
Power reaction is not such an issue for a single-shaft engine. My engine
can
go from 10-90% torque in one second, no problem.
However, you did hit on the larger problem: Very high gyroscopic forces.
Although the spinning mass is not very large, my engine turns at
43,500rpm.
That's loads of gyro. I have to be very careful not to snap against the
engine or I might just loose it.
BTY, I fly aerobatic helicopters, so there's a larger gyroscope to deal
with, but it "flies"...
"Peter Ashwood-Smith C-GZRO" wrote in
message
om...
(cut)
There are actually a number of tubro powered aerobatic planes, think
for example of the PT-6 trainers.
Where and when do they fly?
For competition aerobatics however which includes lots of gyroscopic
forces, there are I believe concerns about the long shafts in those
engines and the huge gyroscopic forces at work. That would limit them
to sportsman stuff .. which is quite a restriction for a $1,000,000 +
airplane.
(cut)
Because you are flying such a large gyroscope, I wonder if you might
not be overestimating the gyroscopic forces produced by turboprop
engines. Because in distiction to what Peter Ashwood-Smith is saying
above, small turboprop motors only appear to have a long rotor shaft.
In reality they have two shafts mounted separately one behind the
other: The (single) rotor shaft and the power shaft (with the
reduction gears in front of the latter).
I can really speak competently only about model aircraft turboprops.
And the weight ratio of rotor shaft weight/total aircraft weight
should be at least equal if not higher in model aircraft than in real
aircraft. Besides, microturbine rotor shafts rotate at speeds
typically between 120.000 and 200.000 rpm. And in model aircraft, the
guroscopic forces from the rotor shaft are considered to be
negligible. I donīt see how this could be due to some sort of scaling
effect.
So in order to put an end to all of this speculation, I think what
would really be needed here is the testimony of a pilot who has
actually flown aerobatics with a turboprop plane.
One additional question for you out of interest: Do you believe that
aerobatic turboshaft helicopters are relatively more abundant than
aerobatic turboprop planes? And if yes, why would that be so? After
all, this appears to be contradictory since the pilot of an aerobatic
turboshaft helicopter ought to have to deal with much higher
gyroscopic forces than the pilot of an aerobatic turboprop plane.
Peter H.
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