A history of engine-mount systems and what is being used today
May 2007 147
Also included in this column:
• More about the ZALP
Combat engine
[[email protected]]
Control Line Combat Rich Lopez
A reinforced center block that will accept metal engine mounts.
A Fora 1/2A with slotted engine mounts that allow for balance
adjustment.
Vintage Nemesis and Spectrum models with the “old-school” nonadjustable wooden
mounts and nacelles.
I WANT TO touch on the attention to details
that could transform an ordinary model into
one that truly performs well. In past columns
I have addressed the need to trim and dewarp
each model. Some of the small details, such
as the mounting systems, can make a big
difference in an engine’s performance.
In the golden age of Combat, all designs
used wooden mounts exclusively. The Count
Clipper design of roughly 1960 was an
exception since it incorporated a Harter’s
magnesium speed pan.
Most of the early designs used a block of
balsa with maple or some other hardwood on
the top and bottom that the engine would
eventually bolt onto. The maple sticks were
referred to as the “engine bearers.”
This system was usually reinforced and
streamlined with a balsa nacelle. The nacelle
also served to hide the blind nuts that were
embedded in the bearers. This mounting
system was often solidified with fiberglass
cloth and resin. Some pilots made it a habit of
using dowels to pin the two hardwood engine
bearers to the balsa block.
Some adhesives of yesteryear were
nowhere near as good as the ones we have
now. This meant that from time to time there
were engine-mount failures, causing the
engine to fly off the model. If the mounts
were not properly fuelproofed, you could
almost bet that the glue joints would fail.
Flimsy engine mounts would produce
excessive power-robbing vibration. Other
sources of this shaking were the engines
themselves. Some were poorly designed and
improperly balanced, causing vibration even
in the best-built models.
No matter how well you balanced your
propellers, the engine would still “shake,
rattle, and roll.” Those steel pistons running
on steel cylinders sometimes produced so
much vibration that your hands would burn
after holding a model for launch. An
unbalanced propeller would only aggravate
the vibration.
Getting back to engine-mount building, I
have seen builders glue the top and bottom
bearers to the block with no reinforcement.
Sure enough, the engine would fly off after a
few flights. I have not observed this in
probably the past 25 years.
The Western Associated Modelers
organization saw the problem back then and
required the use of dowels, 6-32 nuts, bolts,
and safety cables from the bellcrank bolt to
the engine. A firm mounting block that is
solidly attached or connected to the wing
spars can make the difference between a 105
mph model and one that goes 120 mph.
Loose engine-mounting bolts can rob any
model of performance. Some pilots have a
05sig5.QXD 3/23/07 9:47 AM Page 147
regular routine of tightening the bolts before
the engine is started for the day. Even with
this checking, bolts have broken or backed
out of the mounts. On wooden-mount models
this occurrence could cause structural
damage, including a loose engine-mount unit.
The most common form of engine mounts
these days are the slotted, adjustable metal
types. They are found on Fast, FAI, 80 mph,
and 1/2A models. The adjustability factor is
invaluable to the novice and the expert pilot.
Most of the currently available Yuvenko
or Viko RTFs from Ukraine are stable yet
responsive when given control. There is
enough latitude of adjustment in metal
mounts to turn one of these world-caliber
models into a docile sport aircraft. This
would be like making a vehicle that is ready
for the racetrack into a snappy, street-legal
sports car with a few adjustments.
The bolting of the metal mounts to the
model configurations will vary. When I
started using metal mounts I had 3/16-inchdiameter
threaded aluminum spacers made
that I would press-fit into the model’s
mounting block.
Then I would use two bolts on the top
mount and two bolts on the bottom mount to
secure the mounts to the block. I needed two
4-40 screwdrivers because I tightened the top
and bottom bolts at the same time.
I never had failures with this bolting
system. I have had the metal fatigue and
break. Some of these failures may have been
caused by the poor design of the mounts
themselves.
The Cyclon mounts that came with the
PC2 and PC3 had an odd design to
accommodate a large, round backplate.
These tended to fail on a regular basis.
Bill Maywald made replacement mounts
that solved the problem. Cyclon also made
a sturdier version that solved the breakage
problem.
A different approach to attaching the
mounts is to use a long bolt and a nut to
clamp the top and bottom mount to the
model. Some pilots use nuts with nylon locks
and others use a mounting plate.
I have recently found that using the
larger size of 4-40 blind nuts with two flats
ground, sanded, or filed on opposite sides
works well and is inexpensive. The little
spikes on blind nuts can easily be broken
off with long-nose pliers and a quick backandupside down against the bottom mount.
Some models come with a hole drilled to
accept the mounting bolts, and others
incorporate the use of aluminum spacers.
The last type of metal engine mount used is
the nonadjustable. These usually have the
bottom mount holes threaded to accept the
bolt that comes in from the top. This works
well, provided all your models weigh the
same and never require slight trimming.
One thing to consider is making sure
there are no binds when the engine is
mounted. Some of this can be taken care of
by drilling the mounting holes on the engine
a bit larger than they come so there is a bit of
play. You never want to put additional stress
on the engine crankcase because of a
misaligned engine mounting block or mounts
that do not fit flush.
Since there are so many different engines
available for the F2D (FAI Combat) event,
you will often find that the crankcases are
wide on one engine and narrow on another.
If the model is designed around a widecrankcase
engine, trying to mount a narrowcrankcase
engine will be impossible without
putting severe stress on the mounts.
Special mounts that have an offset built in
are the answer. This means you have to make
them or have them made.
Mounting a wide-crankcase engine on a
model designed for a narrow-crankcase
engine requires the use of shims under the
mounts themselves. You can make the shims
from various sizes of plywood or even brass
stock.
The idea is to eliminate binds and stress
on the engine and the mounts themselves.
Try to get rid of problems you have control
over. The Murphy family is working on
plenty of new problems to keep your flying
experiences interesting.
A couple columns ago I mentioned the
testing I was doing with the ZALP Combat
.15 engines. I have some updated
information and performance data to share
with you. I am now running six of these
engines and will be using them in
competition.
The ZALP .15s are well made and
consistently come set at roughly the same
head clearance, which is approximately .006.
I usually like to start running my engines at
.010 for the initial rich break-in runs and to
test a variety of propellers. I ordered extra
head shims that are accurate at .0015 and
added a few to each engine.
All the ZALPs are running great, at less
than 18 seconds for eight laps. This puts
them all at more than 100 mph. There is only
.3 second variation from the fastest to the
slowest engine. They start well and need
little break-in time.
US F2C team member Bob Whitney is
importing this engine. You can correspond
with him via E-mail at [email protected].
Be prepared to pay $160-$170, depending on
bank and shipping charges.
Do not forget to order a head wrench
that sells for roughly $25. It will tighten
the propellers and plugs. That may seem
expensive, but this tool is well made and
is a one-time purchase, provided you
don’t lose it. MA