Jan 2002
Green Sky Adventures, Inc.    of North Central Florida
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Micro Mong 
Zippy Sport
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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 582

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, the
last (silencing) chamber  was eliminated 
for airframe clearance consideration. Theoretically, this mod would affect only the sound, but a comparison test would be the proof.
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 Prop 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 limit.
Exhaust Test
  • Control: Stock Rotax components as described at left.
Full power RPM = 6,250 ± and peak EGT of 1150°F was observed at 5,500 rpm, with jet needles set 1 groove richer than stock.
  • Variable: Owner's modified muffler as described above
Full power RPM = 6500± and peak EGT of 1075 was observed at 5,500 rpm, with jet needles set 1 groove richer than stock.

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 engine
was allowed to cool, and a general inspection of
the installation was performed.  Later, a brief test flight was conducted

If you would like to view some pictures from a pilots perspective, click Here

Jim and Jerry ham it up for the camera

Now Order Plans for Zippy Sport or Micro Mong.  on line
 $135 ppd in U.S. add $15 Outside U.S
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