Page 28 - MFW June 2024

V intage

SIG

Allan Knox

By the time you read this, the vintage community will have side exhaust with standard muffler), Allowed run time is 20
met at our on line AGM and hopefully have a committee in seconds.
place for 24/25.
There is an excellent comparison between Cross Flow
Vintage flying will continue throughout the year with the and Schnuerle Port systems in Wikipedia for those of you
National Decentralised Competition ( N D C ) interested in the subject. You can find it here
accommodating virtually all Free Flight and R/C classes https://en.wikipedia.org/wiki/Schnuerle_porting
over 11 months. We encourage anyone keen on vintage
competition to participate in NDC as it can be Flown on But here are some extracts.
your home strip without the need to travel far. All you need The efficiency of the two-stroke engine depends on
to know about NDC can be found here:............................ effective exhaust scavenging and the more complete
https://www.mfnz.org/members-pilots/ndc/ Currently the replacement of the old spent charge with a fresh charge
only vintage regional rallies happen at Levin with three the better.
invitational memorial events annually then there are the
Nationals at the end of the year in Carterton. The simplest arrangement is a single transfer and single
exhaust port, opposite each other. This "Cross
Aside from competition, vintage models are ideally suited scavenging" or “Cross Flow” performs poorly, as there is
to relaxed undemanding flying with their excellent stability tendency for the Flow to pass from the inlet directly to the
and light weight inherited from their Free Flight origins. As exhaust, wasting some of the fuel mixture and also poorly
such they are perfect for fun flying and just pottering scavenging the upper part of the chamber. Before
around the sky. Best of all perhaps, they are built in the old Schnuerle porting, a deflector on top of the piston was
ways by sticking Balsa bits together to build open light used to direct the gas Flow from the transfer port upwards,
weight structures that can covered in doped fabric or in a U-shaped loop around the combustion chamber roof
modern heat shrink films. All work fine. There are lots of and then down and out through the exhaust port. Apart
kits out there too if building from plans isn’t your thing. from the gas Flow never quite following this ideal path and
tending to mix instead, this also gave a poorly shaped
Don’t forget to sign up for AVANZ, our vintage focused combustion chamber with long, thin flame paths.
regular newsletter. AVANZ has loads of pictures, plans
and news about vintage and free flight generally. Back
copies are here on the Vintage SIG page:.........................
https://www.mfnz.org/members-pilots/special-interest-groups/vintage/

Cross Flow verses modern Schnuerle Ported 2
stroke engines.
What’s the difference, how do you tell one from the other
and why does it matter to vintage fliers?

It matters because we have different engine run times for
the two types in the Duration classes. This is because
modern two stroke sport engines have markedly better
performance with their ball raced cranks and Schnuerle
ported gas flow paths so they are allowed shorter run
times than the older Cross Flow and Schnuerle Ported but
plain bearing 2 strokes.

The vintage competition rules say:
c. Two-stroke Cross-Flow glow or diesel, two stroke
Schneurle ported plain bearing glow or diesel, non-
vintage ignition,and pre-1987 four stroke. Allowed Run The piston deflector and matching head depression are
time is 25 seconds conformation that a motor is a Cross Flow design as in the
pictures of this OS Max-H 40 P circa 1970. You can also
d. Post-1986 four-stroke, and two stroke Schneurle ported clearly see the bulge up the side of the crankcase for the
ball bearing sport glow or diesel engines (front intake and induction Flow passage and the exhaust on the opposite
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