Contributed by: Ken Isaac | Published: October 2007 | Views: 50807 |  Email this Article
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RCU Review - OS 75 AX 2-Stroke Engine
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- Mounting
lugs are the same dimensions as a 61 FX
- Larger
head for more cooling
- Rear
angled needle valve
- Easy
starting
- Reliable
low idle
- Pulling
power
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- The
engine is "thirstier" than it's 60 sized counterpart.
- Larger
prop size may not fit on existing setups
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The
current trend in engines is one that would make Tim
Taylor grunt with pride: "More Power". As
pilots push their planes to ever increasing limits in
aerobatic performance they want engines that deliver
more power in smaller sizes. Recently many manufacturers
have started offering engines that are just that - bigger
engines in smaller sized packages. It's easy to understand
why this appeals to so many pilots out there - the ability
to have the power of a larger engine without the penalty
in weight and size of bigger engines. The latest offering
from OS is one that is sure to make a lot of pilots
sit up and take notice. Their newest engine on the market
is the OS .75 AX ABL 2-stroke glow engine. OS states
that it is "The new power source for 60 sized aircraft."
It is a .75 cu displacement engine that has the same
mounting footprint as the earlier OS .61 FX engine.
A
lot of the planes I fly are 60 sized aircraft because
I like the ease of transport and fuel economy of flying
this size of plane. So when I was asked if I would like
to take a look at the new .75 AX it was a pretty easy
decision for me to make. One of my "throw around"
planes is a RCM plans built Kaos 60 with an OS .61
FX which has seen a lot of airtime over the years. Since
I am very comfortable flying this plane I thought it
would make the perfect test bed for this engine. Since
the 75 AX is billed as a "60 sized" engine
I wanted to use a plane with a 60 already in it. This
would let me truly test that the 75 AX could be simply
"swapped out" with the old 60. It would also
let me make an accurate comparison with the power difference
between the 60 and the 75 AX.
So
with a hearty Tim Taylor "arrrr, arrrr, arrrr...
More Power" grunt let's take a closer look at this
engine........
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OS .75 AX ABL
2-Stroke
- Fits
the same mount and bolt pattern as the 61FX.
- The
angular head design looks great and significantly
increases surface area for better cooling.
- Includes
the new "Power Box" muffler, designed with
more volume for increased power while still being
very compact.
- A
diagonally-placed needle means no remote needle valve
is needed.
Specifications
SPECIFICATIONS:
Price: $194.97
Displacement: 0.75 cu in (12.3 cc)
Bore: 1.02 in (25.8 mm)
Stroke: 0.93 in (23.5 mm)
Practical rpm: 2,000-16,000
Output: 2.4 hp @ 15,000 rpm
Weight: 19.4 oz (550 g)
Includes: glow plug, E-3010 muffler
Recommended Props: 14x6, 14x8, 15x7 |
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The Engine
The
OS 75 arrives packaged in a plain blue box that is so
familiar to any OS user. Opening the box reveals that
the engine and all parts are well packaged and protected
against damage. The contents of the box are the engine,
the Power Box muffler, an OS #8 glow plug, and a small
plastic bag containing the muffler bolts and a needle
valve extension. One word of note here. Make sure you
look for the glow plug in the box. I found the plug
at the very bottom of the box and didn't actually find
it until after mounting the engine on the plane. A first
look at the engine will reveal that it is a bit different
than most will be used to. OS has increased the size
of the head and angled it forward in order to give the
engine more surface area. This helps keep the engine
cooler while running. Another change that will be noticed
right away is the muffler. The muffler is now rectangular
shaped as opposed to the old round mufflers on previous
OS engines. This is the new Power Box design that OS
has come out with. OS states that this muffler will
help quiet the engine and still increase the power output.
One other thing of note is the use of a jamb nut for
mounting the propeller. Jamb nuts are usually found
on 4-stroke engines, and are rarely seen on 2-strokes.
But it is nice to see that OS has included this with
this engine as it will help keep the prop on during
flight.
One
of the big "selling" points of the OS .75
AX is that it will mount in the same space as an OS
.61 FX. Pictured above I have put the new .75 next to
an older .61 FX. Sitting side by side with the .61 it's
easy to see that the lower part of the engine body is
pretty much the same and that the mounting holes are
indeed identical. That's where the similarities end.
Comparing the two engines the .75 stands out with a
larger carburetor, as well as more head area to accommodate
the larger piston. Also increased is the amount of cooling
fins on the .75.
Another
notable change with the new engine is the location and
position of the needle valve. The .61 FX had a remote
needle valve setup which placed the needle valve at
the rear of the engine. The .75 AX has moved the needle
valve back up to the carburetor, but has slanted the
needle valve back away from the propeller. This allows
for easy adjustment of the needle valve with the engine
running while keeping delicate fingers back away from
the spinning propeller. Also included is a length of
braided steel wire and a small knurled knob. This is
included to use an extension to the needle valve.
The Manual
OS
has always provided excellent manuals for their engines,
and the manual that comes with the .75 AX is no exception.
The manual is a 4" x 6" 45 page document that
is well written and easy to understand. After the mandatory
safety warnings the manual starts off by explaining
the basics of the engine, including talking about the
various parts of the engine. The manual does a great
job of explaining how to properly operate the OS 75
AX as well giving a complete break down of all the parts
of the engine. Also included with the manual is a sheet
of color decals that can be used to liven up the looks
of your plane.
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NOTE:
When choosing a fuel for the OS 75
AX keep in mind that this is an ABL engine and the fuel should
be suited as such. OS recommends that for non-ringed engines
such as the 75 AX a fuel with some castor oil should be used,
such as
Morgan Fuels Omega blend fuel. Morgan fuels state that the
Omega blend is specifically formulated for ABL/ABC engine
applications. For engines such as the 75 AX the castor oil in
the fuel provides extra protection from a lean engine run
where a 100% synthetic lubrication would not provide as much
protection.
To
break-in the OS 75 AX I mounted it in a test stand instead
of in a plane. While setting up the engine to break-in
I noticed something that is normally not seen on a 2-stroke
engine; a double nut mounting system. Normally these
are used on 4-stroke engines to keep the propeller from
being thrown in the event of an engine backfire. I imagine
with the extra power of the OS 75 they chose to use
a double nut for the same reasons. I used a Master Airscrew
14x8 sport prop for the break-in, which is also the
same prop that I used when the engine was mounted in
the plane.
With the tank filled I opened the needle
valve 2-1/2 turns as directed by the instruction manual.
To prime the engine the throttle was opened fully, the
carburetor was opened fully, and the engine hand cranked
until fuel was pulled from the tank into the carb. With
the throttle set back to idle position the glow driver
was placed on the glow plug and the engine was flipped
with a chicken stick. I was pleasantly surprised when
the engine fired up on the second flip of the stick
and idled smoothly. To break-in the engine the manual
for the OS 75 AX calls for alternating the engine between
a lean "2-stroke" setting for 10 seconds to
a rich "4-stroke" setting for 10 seconds,
and continuing this throughout the first tank of fuel.
This procedure was followed for the first tank of fuel
with nothing of interest to report. The engine alternated
between those mixture settings with no problems. Although
the manual says that the engine is ready for flight
after the first tank of fuel I chose to go ahead and
run a second tank of fuel through the engine in the
same manner as the first. The only reason I did this
was because I wanted to open the engine up and lean
it out in order to get a good tach reading while the
engine was still on the test stand.
After
a second break-in tank it was time to see what the engine
could do. I fueled it back up and restarted the engine.
After getting warmed up I opened the throttle up and
leaned it out to the point where it would be when flying
the plane. Swinging a Master Airscrew 14x6 prop the
engine tach'ed out at 9,100 rpm. With the high-speed
end checked I dropped the throttle back to an idle setting
and the 75 AX settled in and idled a very smooth 3,000
rpm. I let it idle at that setting for about a minute
to get it "loaded up" and then quickly opened
the throttle back to full. The engine quickly went to
full throttle without missing a beat. Satisfied that
the engine was going to be a reliable power source for
my plane it was time to take if off of the test stand
and get it in a plane to see what it could do in the
air.
One
major point of note that I noticed while breaking in
the engine was the new configuration of the needle valve.
Instead of a remote needle valve the 75 AX has the needle
valve mounted on the carburetor, but the needle valve
is angled back at a 45° angle. This allowed for adjusting
the needle valve while keeping the hand away from the
spinning propeller. I've never been a huge fan of the
remote needle valve setups found in the FX line of engines,
so this new layout is a huge hit in my book. I really
like the way this engine is set up.
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The
plane I chose for the OS 75 AX was my Kaos 60 that was
built from RCM plans. I chose to use this plane for
two reasons. First, this is a 60 size plane which I
had an OS 61 FX on. Since the 75 AX is advertised to
have the same footprint as the 61 FX it should drop
right into the same engine mount that is already on
the plane. Second, I have a lot of airtime with this
plane and I'm extremely comfortable flying it. Because
of this I felt that I would be able to notice any differences
in power and handling that the 75 had over the 61. For
these reasons I felt that this would a great choice
to evaluate the OS 75 AX.
Swapping
out the 75 in the plane was really anti-climactic. Mounting
it into the plane was really as simple as removing the
mounting bolts from the 61, putting the 75 AX in place
on the motor mount, and bolting it in place. The engine
did indeed fit exactly into the existing engine mount,
with the mounting bolts matching up exactly. The throttle
control arm was also in exactly the same place, and
I was able to hook up the throttle control arm with
no adjustments at all. The only thing that did not match
right up was the needle valve. With the 61 I had a cutout
in the side of the plane at the rear of the engine.
With the needle valve mounted on the carburetor on the
75 AX I needed to make a new cutout for it in the fuselage
side. But that was it. I was able to remove the 61 FX
and install the 75 AX in its place in about 20 minutes
total time. It was a very simple task to perform.
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With
the engine in place I ran into the downside of the new
engine. A couple of small issues presented themselves
that had to be addressed, but nothing too difficult.
First was the balance of the plane. The weight of the
75 AX is 3 oz. heavier than the 61 FX. Because of the
weight difference I checked the balance of the plane
with the new engine on it. The plane was nose heavy
because of the new engine. To balance the plane required
2 oz. of stick on weights applied to the tail of the
plane. The overall weight of the plane with the new
engine and tail weights was now 6 pounds 12 oz. The
second problem I had was the propeller. The old 61 FX
used a 12x8 propeller, and for the new 75 I had a 14x8
to go in its place. This was a problem because of ground
clearance. With the new prop I only had about ¾"
of clearance between the prop and the ground. I didn't
make any changes to the plane at that time because I
wasn't sure if I would keep the engine on the plane
or not. Since the plane had a tricycle gear on it I
felt that the nose gear would protect the prop for the
most part.
With
the engine in place on the plane it was time to head
to the flying field to see how it would perform.
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At
the field I couldn't wait to get the OS 75 AX into the
air to see how it performed. I fueled the plane and
a couple flips of the chicken stick brought the engine
to life. I ran it up to full throttle to make sure it
was still tuned. Since it's still a new engine I ran
it rich enough to see a little bit of smoke in the exhaust.
The normal checks of pointing the plane's nose up to
the sky and then down to the ground showed that the
engine was running well and ready to fly. I taxied out
and pointed the nose of the plane into the wind, started
the flight timer, and then moved the throttle stick
forward. With the throttle at about 3/4 of the stick travel
the plane lifted off the ground and was quickly gaining
airspeed. I pulled the throttle back to about ½ and
circled the field twice to make sure the plane balance
was indeed correct. When I was satisfied I decided it
was time to see what she could do now. I leveled out
across the field and punched the throttle forward. When
the engine was opened up I pulled the elevator stick
back and pointed the nose upward. With the plane climbing
upwards there were two words on my lips at that time.
"HOLY COW!!"
The new engine pulled the plane vertically with no problems
at all and only stopped climbing when I cut the throttle
back. It really was quite impressive the difference
it made. So now that I knew what kind of power I had
I decided to play with it a little bit. As I said earlier,
I have lots of experience on my Kaos and it's one of
my favorite planes to fly because of the capabilities
it has in the air (anybody that has ever flown a Kaos
will know exactly what I am talking about here). But
with the 75 AX on the nose of this bird it was like
flying a new plane. All maneuvers were performed with
more authority as the engine had no problems pulling
the plane around the sky. In fact, I was able to perform
one maneuver that was very difficult for me before -
a vertical 8. With the 61 FX on the plane I barely had
enough power to pull the outside loop at the top of
this maneuver, but with the 75 on the plane it pulled
through the top of this maneuver with no lack of power
at all. It definitely left me with a huge smile on my
face to see it pull it around the sky. With the playing
done I came around on a landing approach. I intentionally
left it in an idle for a long period during the landing
approach in order to see what it would do if I needed
to go around. As I came in I "waved off" the
landing and powered up to go around. The engine responded
by quickly moving to full throttle without missing a
beat. With the timer running down I decided it was time
to stop playing and bring the plane in. As I came in
and touched down the heard the prop hit the runway as
the nose touched down, so I guess that nosewheel didn't
protect it as much as I had hoped it would.
I
flew 4 more flights after the first and I got a pretty
good feel for the engine. There are a couple of downsides
that I need to mention here. The first is fuel consumption.
With my 61 FX 11-13 minute flights are normal,
but with the 75 I had trouble making it to 11 minutes
before I was out of fuel. I made a deadstick landing
twice because I ran out of fuel in the air. So it's
quite obvious that the 75 is definitely more thirsty
than the 61 FX was. The second problem is one that I
already mentioned: prop strikes. I had more of them
throughout the morning as I simply did not have enough
clearance with the larger prop on it. Neither of these
problems is difficult to overcome as a larger tank would
easily fit in the fuselage of the plane, and larger
wheels would fix the prop clearance issues. I mention
these simply so those planning to upgrade to this engine
can properly plan for it.
After
5 great flights with this new engine I packed up and
left the field with a huge grin on my face. The OS 75
AX really made for some great flying. It made an already
fun flying plane an absolute blast to have in the air.
It's definitely a keeper and will get plenty of flying
time in the future.
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Simply
put the new OS 75 AX is a "stump puller."
The 75 AX is a powerhouse of an engine packed into a
compact 60-ish size case. The OS 75 AX will add new
life and power to an existing 60 sized plane, or it
can make a new plane wow people as the engine pulls
the plane around the sky. The OS 75 is easy to start,
has a very reliable idle, and a top end that packs a
lot of power. I would highly recommend this engine to
both beginners as well as experienced pilots. I've got
a feeling that the OS 75 AX will soon become the standard
by which other engines in this size class are judged.
OS certainly has a winner with this new engine. Whether
you are trying to add some "zip" to an existing
60 sized plane, or powering a new 60 sized aerobat,
the OS 75 AX should be the engine that you look at to
take care of your needs!
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The comments, observations and conclusions made in this review are solely with respect to the particular item the editor reviewed and may not apply generally to similar products by the manufacturer. We cannot be responsible for any manufacturer defects in workmanship or other deficiencies in products like the one featured in the review. |
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