Larry Scott wrote:

>Bill Grant in Wings of the Eagle-Vietnam helicopter
>pilot tour of duty told about setting his C model Huey for a hot start
>whenever they needed to shut down but expected to be leaving in a hurry.

Well...

There's a difference between starting an already-hot engine and a "hot-start"
(which is bad). I haven't read Bill Grant's book, and have never flown a UH-1,
so I'm not aware of anything you can do to help the engine out when you have to
start it up very soon after shutting it down. Turbines DO become heat-soaked
after shut-down, and some can be quite difficult (if not impossible) to start.
Especially if you've parked with the wind blowing up the tailpipe as we've
discussed before. And maybe that's what Bill was talking about, because
there's generally not much "improvising" you can do during the starting
procedure.

Bart already detailed the starting procedures for most turbines (and they're
all pretty much alike). It helps to have at least a working knowledge of how
these engines work. There are really only two varieties, free- and fixed-shaft
turbines.

In a fixed-shaft engine, the power turbine, compressor and transmission are
linked solidly by a shaft (except for the freewheeling unit, of course). It
works okay...most of the time. However, if a heavy load on the main rotor
(like a sudden application of pitch) drags down the N2 (power turbine) rpm, it
also therefore drags down the compressor rpm. Not good. The governor will
call for more fuel, and that's the only way to get the rpm to come back up.
Once you reach the max fuel-flow limit, look out!

In a free-turbine engine, the output from PT (power turbine) is connected to
the transmission but NOT the compressor. However, there is another turbine
stage *behind* (airflow-wise) the power turbine. It is THIS turbine section
that is connected by shaft, forward to the compressor. What this means is, if
the MR rpm gets bogged-down by that aforementioned large application of
collective pitch, the compressor rpm can actually increase speed to get the N2
rpm to come back up.

There aren't too many fixed-shaft turbines around anymore. Free turbines work
better in helicopters, where the output shaft rpm must be held constant to a
fairly tight tolerance over a wide range of power demands.

Clear as mud so far?

As for in-flight operation, things are simpler yet more compicated.

Generally, the best part of flying a turbine is that you don't have to worry
about NR (main rotor rpm)...much. The N2 (power turbine, or PT) governor holds
the MR rpm right where you set it throughout the whole range of collective
pitch travel. Modern turbines have reasonable power reserves and simply don't
bleed-off rpm at high power settings like first-generation turbines used to
(like the B- or D-model UH-1, so I'm told). You mountain or sling-load pilots
feel free to jump in and disagree here. I've spent my whole career at sea
level.

Sometimes, if the governor is a little "loose," they might tend to overspeed
upon power reductions, but you quickly learn to compensate for this in your
flying.

In flight, depending on the OAT and the altitude you're operating at, there are
three gauges to watch: Torque (self-explanatory), TOT (turbine outlet temp), or
N1 (gas generator/compressor rpm). In a fixed-turbine engine, there would not
be separate gauges for N1 and N2, because they would be always proportionate.

Most modern turbines run fairly cool at low altitudes, so torque will be your
limiting value most of the time. The torque gauge is PRIMARY for setting the
power in *most* helicopters these days. There is no reason, however, why you
couldn't set a particular TOT on each flight as long as you don't exceed the
maximum torque value. Your choice. At PHI, we limit our 206-series to 710 or
730 degrees (Centigrade) for cruise, depending on model. In an L-1 with an
old, weak C-28, you could sometimes find the engine bumping up against 730
degrees at 80% torque on a hot day. So you might have to ease off on the power
a little. In a case like that though, I would suspect that the engine might
not pass a true power check (as opposed to the vague "trend-checks" that we do
daily) and would explore further.

The higher you go, the thinner the air (less molecules for the blades to slap
against), so the TOT will be higher for a given torque value and you may well
reach the temp limit before "torquing-out." In extreme cases, you might find
the N1 (compressor or gas generator) reaching its upper rpm limit. Although
"N1-topping" isn't all THAT common, if it happens you can either get a
compressor-stall or the MR rpm will droop.

So those are the extremes. In general, at low to mid altitudes, with moderate
temperatures and anything below max gross weight, you'll find a turbine MUCH
easier and more simple to operate than a recip (no matter what the Robinson
guys tell you).

HOWEVER! (And you knew there had to be one, eh?)

The downside of turbines is that when they quit, they don't usually give you
much warning. They either just blow up or sign-off. They don't "talk" to you
like a recip will. Ohhhh, if you're sharp you might notice a degradation of
power over time (which is why we do daily "trend-checks"). Or, the engine
might start "chipping" (illuminating the chip lights which are connected to
sensors that detect metal particles in the oil). PHI has some ridiculous
policy on engine chips: three in a row in a short time and the engine gets
changed. Other operators are not nearly so conservative.

There is a feeling in the industry that turbine-powered helicopters are
"safer," or "more reliable" than recips. But you know, there really aren't a
whole lot of five-to-seven place piston-engine helicopters out there to compare
against turbines. In reality, I'm not so sure. But let's not get into that.

Pilots planning to transition into turbines *MUST* learn the basics of how the
engines work, and the considerations of operating them in different conditions.
They're not all that complicated- just different from recips.

Happy flying!

rec.aviation.rotorcraft ~ August 3, 2002

Peter Gottliebwrote
> How about a turbo-electric aircraft? A high efficiency turbine drives a
> high frequency alternator which charges a relatively small battery. Then
an
> variable frequency 3 phase inverter drives one or more motors. You could
> size the battery such that if the turbine fails or you run out of fuel you
> would have, say, 15 minutes of powered flying left. The turbine and
> batteries could be placed where needed for CG considerations, and the
> turbine could be run at its most efficient speed, or idled, as needed.
>

What an interesting idea.

Small turbines that might be used on light recreational helicopters are
either single shaft and/or unreliable. Your idea should definitely increase
the safety. In addition, some of the additional weight required for
conversions from gas to electricity to thrust is offset by using the
turbine, which is lighter than the reciprocating engine.

Dave J

There are 3 different T62s The T62-2A1 is the best candidate. It is 95hp and about 100 pounds. It has an output shaft speed of 6000 rpm and burns about 12 gallons per hour. The T62-16 is about 80hp has an output speed of 8000 rpm and the same fuel burn "roughly". The T62-32 is much different It is 160hp and about 170 pounds. It has an output speed of 6000 rpm and a fuel burn of about 15-17 gallons per hour.
Then if you want to get wild you can have the T62-32 burner can put on the T62-2A1 gearbox. You get 150hp and about 100 pounds engine weight. It costs a lot more money though and is not really neccesary.
You can get a used -2A1 for about $4000 and one completely overhauled and set up with all accessories for $12,500. I have a T62-32 overhauled and ballanced with some new bearings ready to go for $5,300 my cost, no profit. Look at the pictures on Avon Aero. That is where it came from.
Clayton

I forgot to mention all of these engines need a gearbox to reverse output shaft direction of rotation. There is a nice unit by Apex that does that along with reducing the rpm to 2850. Others have used a Rotax gearbox. It appears to be very strong. They use it in the T62-32 "Jet Exec", A turbine Rotorway Exec kit.

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