Constant Speed Prop vs Variable Engine Timing

On Mon, 01 Mar 2004 17:52:00 -0800, pacplyer wrote:

Great Stuff Kevin, thanks for your insight. I have a couple of points I
slightly disagree with however further down in your post. :-)

Kevin Horton wrote:

After you got yourself behind the power curve, however, for whatever
reason, I'm essentially talking about old-school guys like me who were
used to flying non-FADEC machines capable of "overboost." If you got into
trouble, because you were stupid, in say the previous generation of Boeing
products: You could always push up and call for power far in excess of
limiting max GA epr or N1, N2, EGT limits. (but maybe that's because like
you say: old eng's didn't operate so close to the surge/stall margin.)
It's unlikely the engines were going to fail like a piston or super/turbo
charged engine might. Those old buckets would warp. The blades might
creep and stretch and the engines might have to be scrapped (at say 5 mill
a copy.) But you had a better chance of clearing the trees by going all
way the to the mechanical stops (physical wire to the FCU Hydr/Mech
linkage) than you do now with a Throttle resolver / PFM/MEC/ FADEC
arrangement. The airbus test pilot may think he's called for Jesus power,
but FADEC will not let him have it.
This may have saved me a couple of times in my career flying 60's gen
aircraft overseas. You smash everything to the wall and only slightly
pull back on the engines that are "barking." (compressor stalling.) ATC
would steer you into mountains in those days in some places. (more war
stories.)

OK. I misunderstood your gripe against FADECs in the first message. I
too am not happy to have a FADEC limit how much thrust I get out of the
engine. I would much rather have some way to override it if the s*** hits
the fan. But, I don't think this would have made any difference in the
Habsheim accident. All published reports have said either that the
engines didn't respond at all, or that they were still spooling up when he
hit the trees. The only report I can find that actually quotes an N1 says
the engines were at 83% N1, which must be well below TOGA, so the engines
would have still been accelerating, and it wouldn't have mattered what rpm
was commanded.

There was a bit on an internal bun fight at Bombardier when the CRJ-700
was being designed. It has a FADEC engine, and the flight test folks were
not happy about the inability to get more thrust if needed. The engine
does have an Automatic Power Reserve (APR) that commands a thrust bump if
you have an engine failure. The powerplants engineers were persuaded to
add a heavy detent that you can push the thrust levers through to allow
you to get APR thrust with both engines running if you really need it.

The Bombardier Global Express also has a FADEC engine, but there are two
little switches behind the thrust levers that allow the crew to manually
select a back up N1 control mode. If the engine is in N1 control mode,
you are no longer limited except by the overspeed limiter, which allows
you to get much more thrust if needed.




Turbine engines run more efficiently if they are running close to the
surge line (i.e almost ready to compressor stall). But the engine has
to come closer to the surge line to accelerate. So the closer you run
to the surge line the slower acceleration you'll have.

FAR 25.119(a) requires go-around performance to be calculated using the
thrust that is available 8 seconds after a throttle slam from idle.
Manufacturers want the engine to run as efficiently as possible, but
they don't want to take a hit on the AFM go-around performance. So,
they typically design the fuel controls to allow full go-around thrust
to be reached in just less than 8 seconds from a throttle slam from
idle. I've done tests to check the acceleration on many transport
category aircraft, and the result is usually somewhere between 7 and 8
seconds, and this is the same no matter whether the engine has a FADEC
or an "old fashioned" hydro-mechanical fuel control unit.

Were the engines in flight idle? Were you guys pulling the ground sensor
breaker? Ground idle takes longer. Older High Bypass designs eg: the GE
CF6-80 series only take about six seconds from flight idle to reach GA
thrust IIRC, but still cannot over boost. But it's more like twelve
seconds in profile mode (slow spool up looking at FMS parameters.) So I
remembered it wrong. I think he tried to change alt with Level Change and
Profile mode engaged first, and when nothing much happened (norm) he
smashed the thrust levers to the wall and ate wood. But Kevin, I'll
concede the acceleration argument to you. (Older designs were even slower
(something like fifteen seconds to spool up (e.g. GE CF700's aft-fans.)
You could bust altitudes descending, if you didn't lead with the throttles
a couple thousand feet before level off.

The 8 second requirement is for an acceleration from flight idle. There
is typically some worst case condition (specific bleed configuration,
altitude and temperature) where the engine will be close to 8 seconds, and
it will be a bit better at other conditions. So I am not surprised if you
saw about 6 seconds in many cases.



So don't blame the FADEC for the A320 accident at Mulhouse-Habsheim. It
was caused by a pilot who had way too much confidence in the low-speed
protections of the FBW.

Yep, you're right. FADEC by itself didn't put him in the trees. But most
accidents have "a chain" of factors that cause the accident. If you can
break any one of the factorial links the accident would not happen. I
submit the inability to get over-boost power was just one of those links.
Another was a FBW AOA limit that cannot be temporarily sacrificed to clear
obstacles.

Fortunately the FBW prevented him from raising the nose, as then the
aircraft would have stalled, any many people would probably have died.
As it was "only" three live were lost.

Well I have to disagree with this. We train annually now to fly below
stick shaker to escape microburst wind shear ground contact on t/o. We
will go below stall speed (bugged) momentarily in ground effect will full
power to avoid contact. We don't care about airspeed. We only look at
V/S. If we didn't do this, some dry microbusts would kill us. Risking a
stall is always better than contact with hard objects. (remember impact
g-force energy goes up exponentially with speed) (besides: most jets
don't break fast, they burble and pre-buffet a bit first. After a
positive rate is obtained and we're still alive, then we fly on
intermittent stick shaker (way higher deck angles than FD/AP AOA limits)
until about 1000 ft AGL. AOA FBW autopilots never fly at speeds this low
to escape terrain to my knowledge. But I'd have to ask an A320 driver to
be sure. The other thing that bugs me about that machine is not being
able to bust into a 45 degree bank. (Another thread for that one.)

Well, the AOA limiter an the Airbus's is set very close to the stall. It
is well beyond where a stick shaker would be. The curve of lift vs AOA
tends to have a fairly flat top with modern swept wing jets, so once you
get up on top of that curve there isn't any benefit to pulling more AOA,
as you don't get any more lift. I wish there was some way to get in a FBW
Airbus sim with you. We could do two windshear recoveries - one using
full aft stick riding on the AOA limiter, and one in Direct Law, with no
AOA limiter. I'm convinced you would do better just using the AOA limiter.

For the record I think FADEC is great. Do you want it in your GA airplane?

Well, I'm a suspicious type, and I want to see some more service history
first to assure myself that they've sorted all the bugs out. So not on
my RV-8 project, but maybe on the next one.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com

Kevin Horton wrote snip

Well, the AOA limiter an the Airbus's is set very close to the stall. It
is well beyond where a stick shaker would be. The curve of lift vs AOA
tends to have a fairly flat top with modern swept wing jets, so once you
get up on top of that curve there isn't any benefit to pulling more AOA,
as you don't get any more lift. I wish there was some way to get in a FBW
Airbus sim with you. We could do two windshear recoveries - one using
full aft stick riding on the AOA limiter, and one in Direct Law, with no
AOA limiter. I'm convinced you would do better just using the AOA limiter.


I would enjoy that. You could be right.

For the record I think FADEC is great. Do you want it in your GA airplane?

Well, I'm a suspicious type, and I want to see some more service history
first to assure myself that they've sorted all the bugs out. So not on
my RV-8 project, but maybe on the next one.

Sounds very wise indeed. Great shots of your RV-8 by the way.

pac