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  #11  
Old September 26th 04, 08:05 PM
Smutny
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The Australian Roulettes fly the PT-6 as well as a few other countries
I can't think of at the moment. Brazil perhaps? But one has to
remember, what aerobatic teams do, and what happens in an aerobatic
competition aren't identical.

The competition box is one size for all competitions, 1000 square
meters. Changing it would require sancitioning from the FAI. But
creating different sized boxes for different aircraft neutralizes the
spirit behind the competition.

Airshows on the other hand, boxes are determined mainly by the venue.

One of the reasons you don't see many more turbine GA aircraft is
partly due to the lack of certified small turbines. There is a
growing number of turbine homebuilts. Comp Air and Lancair being two
that offer kits designed for them. I've also seen an article on a
turbine RV-4 in Sport Aviation not too long ago. As time goes on,
cheaper and smaller turbines will probably have a greater presence in
the GA market. Don't know what the economics are though, how much
does a hot section cost compared to overhauling a piston engine? And
what does that translate to per hour costs?

No need to get hostile over all this, Holm. Turbines are not the
end-all, be-all of aviation. Jets, turboprops and pistons all have
thier strengths and weaknesses. And their appropriate application.
Aviation pistons right now are just at the top of thier game when it
comes to hardcore aerobatic aircraft. And you wouldn't put a piston
in a commuter airliner.

-j-
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  #12  
Old September 27th 04, 06:29 AM
John Clear
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In article ,
Peter Holm wrote:

As you concede yourself, when it comes to aerobatic airplanes there
exists a source of financing which generallty doesnīt exist for
private airplanes: Sponsoring. Therefore, the cost argument in itself
can only explain the relative scarcity of turboprop engines in private
aircraft, but not the near absence of turboprop engines in aerobatic
aircraft.


Airshow aerobatics have sponsorships. Competition aerobatics don't have
as much in the way of sponsorships except at the highest levels. Even
with sponsorships, there is a big difference between a $50K Lycoming
and a $500K+ PT-6.

The Turbo Raven was just a show plane, in the same category as the
jet powered Waco. Fun to watch, but totally ridiculous.


Well now, this jet powered Whacko is nothing but a piston biplane
with a turbojet strapped to its belly. And this turbojet engine has a
bent exhaust pipe - kind of like a turboprop. As long as I havenīt
seen more data on this plane or seen it fly, I wonīt believe that this
setup has much more than decorative value.


I've seen it fly, and it does some pretty impressive vertical climbs.

Some stats on the plane are available at:
http://www.franklinairshow.com/jetstats.htm

There is also video on that site, doesn't show much of the vertical
though.

John
--
John Clear - http://www.panix.com/~jac

  #13  
Old September 27th 04, 08:46 PM
nametab
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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.



  #14  
Old September 29th 04, 11:31 PM
Peter Holm
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Smutny wrote in message . ..
The Australian Roulettes fly the PT-6 as well as a few other countries
I can't think of at the moment. Brazil perhaps? But one has to
remember, what aerobatic teams do, and what happens in an aerobatic
competition aren't identical.


So it appears. Thanks for the tip about the Roulettes, but all the
images I could find of them in the web were just about formation
flight. No torque rolls etc.

I do envy John Clear a lot for having seen (live, on top!) an
aerobatic turboprop plane strut its stuff. I might never get to see
this in my life – perhaps not even on a video. And this to me is
rather disapointing, whether you understand that or not.


The competition box is one size for all competitions, 1000 square
meters. Changing it would require sancitioning from the FAI. But
creating different sized boxes for different aircraft neutralizes the
spirit behind the competition.

Airshows on the other hand, boxes are determined mainly by the venue.


The idea that the competition aerobatic box might be too small for
turboprop planes came up in this thread, presumably based on the
hypothesis that the throttle lag of an aerobatic turboprop plane
would hamper its agility of flight in comparison to a piston plane.
But as nametab has pointed out on 9-17 in this thread, and I think he
is right, is that throttle lag is not likely to be a problem for
single spool turbine motors. Small turboprops are double spool, though
not coaxial but in series (one turbine shaft and one independent power
shaft).

Therefore, the idea that the competition box might be too small for a
turboprop plane is, like next to everything else on this thread, based
on nothing but speculation. So I repeat: What would be needed here is
the testimony of a pilot who has actually flown aerobatics with a
turboprop plane.


One of the reasons you don't see many more turbine GA aircraft is
partly due to the lack of certified small turbines. There is a
growing number of turbine homebuilts. Comp Air and Lancair being two
that offer kits designed for them. I've also seen an article on a
turbine RV-4 in Sport Aviation not too long ago. As time goes on,
cheaper and smaller turbines will probably have a greater presence in
the GA market.


I donīt think that the general aviation market has a lot of relevance
for aerobatic planes. The abundance of turbine planes (not only
turboprop) in general aviation is far higher than in aerobatics.

Don't know what the economics are though, how much
does a hot section cost compared to overhauling a piston engine? And
what does that translate to per hour costs?

No need to get hostile over all this, Holm. Turbines are not the
end-all, be-all of aviation.


Of course not. There are ramjets, and perhaps some day there will be
scramjets. But when it comes to aviation, I cannot think of any
advantage of piston engines over turbine engines besides their price
and their fuel efficiency (I know of people who actually think that
the maintaineance for turbine engines costs even less than for piston
engines).

Jets, turboprops and pistons all have
thier strengths and weaknesses. And their appropriate application.
Aviation pistons right now are just at the top of thier game when it
comes to hardcore aerobatic aircraft.

(cut)

But this is exactly what I donīt understand.
I am perfectly aware of the fact that under certain circumstances a
more primitive technology can have advantages over a more advanced
technology dedicated to the same task. An example in case would for
instance be police officers patroling park and beach areas on bicycles
instead of in police cars. But whenever this happens, there exists a
cogent explanation for that circumstance. And a cogent explanation -
preferrably from someone who talks from experience - is what is
missing in this thread so far.

So why should, of all cases, aerobatics be one of those examples where
a comparatively primitive (engine) technology would have the edge over
a more sofisticated one? Why should the aim in aerobatics be to only
make the aircraft lighter, but not to make the motor stronger – and
lighter at the same time? Why should an aircraft which cannot fly a
sustained torque roll be better for aerobatics than an aircraft which
can? After all, the relative lack of power of piston planes is
responsible for the fact that a torque roll is often confused with a
tail slide.

This remains an obvious paradox – at least on this thread - which
requires something more than speculations for a convincing
explanation. And I canīt help feeling that you guys are trying to sell
me apples for oranges on this issue.


Peter
 




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