The Zippy Sport has been in service at Green Sky Adventures, Inc. as
a "Flying Test Bed" for over 15 years. It has flown with a wide variety
of Rotax, and a couple other brand powerplants. The Zippy's wide speed
range yields a variable load (engine load) environment that has been extremely
useful evaluating the numerous engine/gearbox/propeller combinations that
have been used.
In July of 2001, Jim Crabtree, a BD-5 builder, contacted us
with a request to perform the break-in on his Rotax 582. A tight installation,
particularly pusher configuration, like the BD-5, often makes the published
break-in nearly impossible. Neglecting the break-in is always a bad
idea, especially so with the BD-5 and it's very high landing speed ...Blasting
off into the wild blue with zero operational evaluation is a foolhardy
act. Other issues were the modified exhaust system, and function
of the special HAC (high altitude compensating) carburetors
The fact that this powerplant performed as expected during this test
does not automatically mean it will do so when installed in the BD-5.
However, these checks do eliminate some unknowns in preparation for testing
in that aircraft.
Below are some images from that day of testing.
Click to enlarge
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Here, and at the right
Removing the GSA Remanufactured
On this engine, a single compensator
regulates pressure to each carburetor float
bowl there by changing the mixture. Changes in density altitude are
sensed through a static line on one airfilter to affect the deflection
of a diaphragm in the HAC chamber. The diaphragm moves a metering
pin which varies vacuum to the float chamber(s)
The HAC will alter the mixture of both Carburetors.
In the event of a differential pressure between
the two air filters, a corresponding differential mixture will result.
If a high pressure exists at the air filter where the static pressure
for the HAC is sensed, the other carburetor and cylinder will be inadvertently
be leaned...Purhaps to the point of seizure. This lesson was learned by
direct experience method.
On dual carb HAC installations, use the single
oval filter with provision for both carbs
Once the installation was pretty
much set, Mr Crabtree was invited to proceed with the Rotax Break-In Sequence
This muffler started out as a
The inlet cone was cut in the middle and turned
180 degrees. No change in tuning would
be expected from such a modification.
Observe, though, on the muffler canister,
last (silencing) chamber was eliminated
for airframe clearance consideration. Theoretically,
this mod would affect only the sound, but a comparison test would be the
The 90° elbow pictured attached
to the inlet end, is a stock
(973 180) elbow, connected by
an "aftermarket" fastener.
|The exhaust configuration
for the Zippy Sport is a 100% stock Rotax 3-piece system, with Fisher style
muffler. On this day of testing, the subject engine was set up with
a Type B 2.58:1 Gearbox, 54 inch GSC Tech III
with standard blade profile, set to degrees pitch. The
owner's HAC carburetors with standard HAC jetting were also used. During
the early stages of the Break-in sequence, it was discovered that propeller
loading was limiting full throttle rpm to an acceptable (for purposes of
this test) 6,250±, however, EGT was very close
to the 1200°F limit, so the run was halted long enough to set
jet needles one position richer. Upon resumption of the sequence, engine
break-in was completed without exceeding any temperature or operational
Full power RPM = 6,250 ± and peak EGT of
1150°F was observed at 5,500 rpm, with jet needles set 1 groove richer
Control: Stock Rotax components as described at
Full power RPM = 6500± and peak EGT of
1075 was observed at 5,500 rpm, with jet needles set 1 groove richer than
Variable: Owner's modified muffler as described
Owner's modification was not detrimental to
performance. Indeed, performance of the stock system is in question.
We have an opinion as to the reason behind
this result. If you have read and absorbed this much, and would like to
here that opinion, please email your request.
Following completion of the break-in, the
was allowed to cool, and a general inspection
the installation was performed. Later,
a brief test flight was conducted
you would like to view some pictures from a pilots perspective, click Here
Jim and Jerry ham it up for the camera