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In a message dated 5/30/2001 4:09:30 PM Pacific Daylight Time, Eric Ahlstrom
at StarAerospace@aol.com writes:
<< Last time I checked, the 2001 Oshkosh race was still a few months away.
If we offset by one year, we get last year's results as the "nearly won". >>
I hate it when my keyboard does stuff like that to me. Of course, you're
right. Indexing back one year aligns the claimed results with reality, which
is the Factory won in 2000 and Engineair won in 1999.
<< Let's be totally clear about last year's Dayton to Oshkosh
race...........On the remaining leg a factory stock Lancair IVP finished
first overall at 319 mph average equipped with a full interior and powered by
a TSIO-550; all stock.>>
In the interest of the total clarity which you seek:
(1) The 2000 race was planned as a two-leg race, so that participants with
limited fuel capacity could run flat-out on BOTH legs. The cancellation of
the first leg for weather has little bearing on the point you make, except
perhaps on reliability issues.
(2) I am sure you already understand that evaluating an engine installation
solely on the basis of a race like Kittyhawk-Dayton-Oshkosh is potentially
specious reasoning. There are numerous factors other than the engine which
enter into a free-form race, not the least of which is the optimization of
the vertical course profile, which attempts to balance time lost in climb
against higher airspeeds at altitude, headwinds (even using up-to-the-minute
winds aloft data), and the optimal recovery of the altitude potential energy.
The Venture has a very low-drag airframe, and can take good advantage of fast
max-power cruise at low altitude and avoid the loss of a climb into the
flight levels. (I had the privilege of picking Jim Griswold's brain about the
clever ways he designed low drag into the Venture airframe.)
Both the factory L4 and the Venture you mentioned had an advantage: Oren and
Lee are good pilots who are apparently able to fly a straight line course,
follow a planned vertical profile, avoid inadvertent entry into a TCA (aka
"Class Bravo Airspace"), and cross Aurora on a heading somewhere between 307
(MGY to ARR) and 358 (ARR to OSH).
BTW, If you believe the TSIO-550 in the factory airplane was STOCK, then I've
got some beachfront property in which you might be interested. ( I have seen
that engine and know the guys who built it.)
(3) You attempted to marginalize the 1999 race that Engineair won. However,
if you examine the vast database, I think you'll see that the 1999 margin of
victory (in seconds) was comparable to the 2000 race. So if the 1999 win was
"...marginal at best...", then by your own criteria, the 2000 differences
were "marginal" as well.
(4) The problems you mentioned regarding the Orenda installation are
completely tangential. "Temps-in-the-red" are symptomatic of problems with
the heat transfer characteristics of the installation, which are a major
challenge in a liquid-cooled aircraft engine installation. There is a tough
tradeoff between good heat exchanger performance and cooling drag, without
even mentioning external constraints on the installation, such as a
requirement that it be completely "firewall forward". (For more data on
Orenda installations, check out our website at www.epi-eng.com)
<< As for helical gears being inferior to straight cut, every turboprop
engine
on the market uses a reduction unit. Almost all use helical gears. They are
nothing new and helical has proven it's superiority in load carrying on
everything from turboprops to the prop reduction units on aircraft carriers
(yes, the 100 rpm, hundreds of thousand ft-lb. props pushing the ship). >>
(1) Comparing the PSRU in an aircraft turboprop or in a ship to that of an
aircraft piston engine is simply another example of reaching a false
conclusion by making superficial comparisons without considering the various
technical constraints each application must solve. The design constraints of
each system you mention are quite different. It's a lot like comparing apples
to oranges.
(2) I'm not clear on your criteria for inclusion in the category of "almost
all", but the Garrett TPE-331 constitutes a large proportion of the installed
turboprop population. The TPE-331 that I'm familiar with have a two-stage
PSRU consisting of (a) a small SPUR gear on the turbine shaft driving a large
SPUR gear on an intermediate shaft which then (b) drives a planetary final
reduction which is also implemented with SPUR gears. (I don't know firsthand
what's in the PT-6 PSRU.)
The TPE-330-10, as a specific example, produces 940 Shaft HP at 1591 prop
RPM. If you do the arithmetic, you'll find that the output torque is over
3100 lb-ft. However, the turbine shaft turns at about 41,000 RPM, so the
torque the turbine produces at 940 SHP is slightly over 120 lb-ft. It is
clear that, in this design, spur gears were the choice for such varying
requirements as very high torque (in the output stage) and very high
pitchline velocities (over 40,000 FPM in the first stage).
<< However, they (Engineair) have ignored the vast database of both piston
and turbine aviation engine reduction unit design that goes back nearly to
WWI. All of the big radials used reduction units, none were designed the way
that the Engineair PSRU is. >>
and
<< As an engineer, it is not my job to invent everything myself, especially
when there is a large database of applicable hardware already available. For
an offset reduction unit on a piston engine, I would look to the V-12 engines
used in WWII. >>
(1) Thank you for the philosophical input on your approach to engineering.
I'm not sure where you obtained the necessary knowledge about the EPI PRSU
for you to make the claim that "...none were designed the way that the
Engineair (actually the "EPI") PSRU is...", but here are some facts. Drawing
upon the "vast database", the EPI PSRU borrows liberally from the PSRU
technology found in the Rolls-Royce Merlin, Griffon and Peregrine engines,
the Allison V-1710, as well as the (more current) Continental GTSIO-520, all
of which use stacked SPUR gears in the PSRU. You might also be interested to
know that the Lycoming TIGO-480, which used helicals in the PSRU, proved to
be rather fragile.
Of the "big radials" you mention, almost all P&W 985's and 1340's were direct
drive. The P&W 1830 and 2000 used single stage planetary reductions
implemented with SPUR gears; the P&W 2800 used a two-stage planetary
implemented with SPUR gears; the P&W 4360 used a multistage planetary which
used spur gears for one stage, while the other stage used helicals for the
specific purpose of implementing a torque-sensor in the gearbox. The Wright
1820 used a single-stage planetary and I'm almost certain it used spur gears.
EPI chose not to use a planetary system because we didn't want the thrustline
down low on the engine (on the crank centerline, as it is on a radial). As
you know, lowering the thrustline with respect to the aircraft CG is
destabilizing (see Perkins & Hage, Airplane Performance, Stability and Control
), and raising the engine up in the airframe to compensate has the potential
for difficult lofting.
(2) Your assertion that "...helicals have proven their superiority..." has,
again, apparently been made without consulting the "vast database".
Helicals are well-suited to a variety of applications, and their properties
can be used to great advantage. For example, the Napier Sabre (WW2
leading-edge piston engine) used helicals in the final stage of the PSRU to
implement effective load sharing between the two crankshafts. However, the
input gears to that PSRU were SPUR gears. The P&W 4360 used a helical gear in
its PSRU to implement the integral torque sensor, but the input stage uses
SPUR gears. FYI, The EPI PSRU uses one set of helicals for a special purpose.
In addition to all the PSRU's I've listed, if you examine the accessory
drives of almost all the WW2 engines, you'll find spur gears almost
exclusively, especially in the extremely-high-load application of the
supercharger drive units, having step-up ratios of 10:1 to 12:1. These
relatively small gears carried several hundred HP driving the blowers and
were subjected to unbelievable acceleration loads. For specific examples, see
the P&W 1340, 1830, 2000, 2800, and 4360; Rolls Merlin & Griffon; Wright
1820; and for a real surprise, check out the sleeve and accessory drive
system on the Bristol Hercules radial (over 25 SPUR gears).
<< As for helical gears being inferior to straight cut, .....>>
Upon a careful review of my posting on PSRU Design Issues, I don't think
you'll find any claim that helical gears are inferior to straight cut (spur)
gears. What I attempted was to refute a previous posting claiming the
superiority of helicals over spur gears, and to counter rumor and speculation
with fact. In so doing, I specifically referred to gears of similar design.
<< ...Many of them (WW2 V-12's) dealt with far more severe conditions than we
would need on a Lancair IVP sized powerplant. >>
As an engineer, I'm sure you understand the concept of scale. The torsional
excitation loadings imposed by the even-fire V-12's are actually LOWER than
those of a V-8 when proportioned (scaled) to the engine mean torque. The
other loads (mentioned in the previous posting) are proportionally similar
when all the design parameters are taken into account.
Jack Kane
EPI, Inc.
tech@epi-eng.com
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