912 vs 582/618 - 4 stroke vs 2 stroke
One of the most neglected and probably most costly items on light
recreational aircraft today is the engine, especially if it is a 912 Rotax. In Canadian
funds this engine can cost over $15,000 when you include, shipping, radiator, exhaust
system, etc. While this is money that you have to spend, considered it as an investment
rather than as an expense. What kind of investment? An investment in safe fun affordable
recreational flying. Remember, if an investment is properly cared for and maintained it
should increase in value as time goes on.
Now there are two words that are used in the opening sentence
"costly" and "neglected." If we could let's deal with each separately.
Costly - is the 912 a costly engine when compared to the 582 or 618 Rotax? To figure this
out requires that the cost be broken down over the period of the engine life. The
recommended rebuild time of a 912 Rotax engine is 1,200 hours. This has been born out by
infield experience. Today several aircraft are flying on 912 engines that have over 1200
hours on them. The recommended rebuild time on a 582 and 618 Rotax engine is 300 hours.
This is the recommended rebuild time as stated in the Rotax engine manual. This time is
also born out by infield experience.
NOTE this table was done in 1997!
Engine prices, fuel prices, repair costs will have increased! Also note
the TBO on a 912 Rotax 80 HP engine has been increased to 1500 hours.
| Cost of 912.... |
$ 12,500 |
| Cost of running 912 for 1,200 hrs. |
|
| 2.5 gallon of fuel per hour at $2.50 per gallon |
$ 7500 |
| 8 spark plugs every 200 hours @ $5 each |
$ 240 |
| Oil filter and oil every 100 hours |
$ 220 |
Total cost for 1,200 hours of
operation $20,460
| Cost of 582 Rotax engine b Drive electric start |
$ 6500 |
| Cost of rebuild at 300 hrs. |
|
| Crankshaft |
$ 800 |
| Rings |
$ 125 |
| Gasket seals |
$ 125 |
| Wrist pin bearings |
$ 50 |
| Labour |
$ 250 |
| Total |
$9200 |
| Cost of rebuild at 600 hrs |
|
| Crankshaft |
$800 |
| Pistons rings |
$250 |
| Rebore |
$50 |
| Gasket seals |
$125 |
| Wrist Pin Bearings |
$50 |
| Labour |
$250 |
| Total |
$1525 |
| Cost of rebuild at 900 hrs |
|
| Crankshaft |
$800 |
| Rings |
$125 |
| Gasket seals |
$125 |
| Wrist Pin Bearings |
$50 |
| Labour |
$250 |
| Total |
$1350 |
| 4 spark plugs every 50 hours |
$ 480 |
| Fuel 4 gallons per hour (includes oil) at $3.00 |
$ 14,400 |
| Total |
$17,580 |
Total cost for 582 approximately
1,200 hours $28,305
| Cost of 618 Rotax engine E drive .... |
$7525 |
| Cost of Rebuild at 300 Hours |
|
| Crankshaft |
$800 |
| Rings |
$125 |
| Gasket seals |
$125 |
| Wrist Pin Bearings |
$50 |
| Labour |
$250 |
| Total |
$8875 |
| Cost of rebuild at 600 Hours |
|
| Crankshaft |
$800 |
| Piston Rings |
$250 |
| Rebore |
$50 |
| Gasket Seals |
$125 |
| Wrist Pin Bearings |
$50 |
| Labour |
$250 |
| Total |
$1525 |
| Cost of rebuild at 900 hours |
|
| Crankshaft |
$800 |
| Rings |
$125 |
| Gasket Seals |
$125 |
| Wrist Pin Bearings |
$50 |
| Labour |
$250 |
| Total |
$1350 |
| 4 Spark Plugs every 50 hours |
$480 |
| Fuel 5 gallons per hour (includes oil) at $3.00 |
$18,000 |
| Total cost after approximately 1200
hours |
$30,230 |
|
Cost of operating for 1200 hours is |
| 582 |
$28,305 |
| 618 |
$30,230 |
| 912 |
$20,460 |
From these figures you can see that if you are operating an
aircraft for 50 hours a year which is about average for most pilots, it would take 6 years
before a 582 or 618 would need rebuilding, the first time. The 912 would last 20 years
before it needed rebuilding.
This means that if you are using your craft in a training
environment or are flying over 200 hours a year and your aircraft can handle the extra
weight of the 912 then over the long haul it should prove more cost efficient.
However if you are using your aircraft in a training environment
you will probably be rotating your stock and it would be more cost efficient to use a two
stroke, since your initial investment is lower and engine depreciation will most likely be
born by the buyer.
So which two stroke should you use the 582 with a B box or the
618 with the C/E box.
Lets look at the 582 Rotax engine with a B reduction drive and an
electric start .
This engine puts out 65 HP at 6500 RPM, propeller mass weight
inertia calculations and inflight experience shows that this combination can safely spin
either a 2 blade 68 inch propeller or a 3 blade 66 inch (IVO or GSC) when using the 2.58
to 1 reduction ratio. When you look at most of the aircraft on the market this is about
the maximum diameter of propeller used.
However if you check out the market you will find that most of
the manufacturers are moving away from the B gear box and using either the C or E drives*.
The most popular ratio appears to be 3 to 1. Thus they are spinning a larger diameter
propeller slower for more efficiency and less noise, right? Yes and No!
Inflight and on ground sound tests have shown that the noise of
an aircraft at cruise is nearly the same whether you are using a B or C/E reduction drive.
Performance seems to differ from aircraft to aircraft.
Flying identical Kitfox aircraft, one with a 582 B Box the 2.58 1
reduction ratio using a 3 blade 66 inch IVO prop, the other with a 582 C drive with a 3 to
1 ratio with a 68 inch IVO prop I found little noticeable difference in the climb, cruise
or top speed.
On two identical Merlins' one equipped with a 582 B drive 2.58 to
1 using a 3 blade 66 in IVO and the other with a 582 C drive 3 to 1, I found the 582 B
drive equipped craft to cruise faster at lower RPM. Climb rate was also better by about
100 feet per minute.
So why would the manufacturer sell his craft with a C drive
instead of a B? The difference in the cost of the two drives is about $1,000. If you were
selling something would you rather make 25% of $900 or 25% of $1900?
Some "experts" have indicated that the C drive is a
more reliable drive and that it passes less vibration and stress back into the crankshaft.
This may be true but I have not seen any difference in the "reliability" of the
two drives. Remember you are NOT talking to someone that only flies for 50 hours per year
and just got into this business yesterday!
If there is less stress passed back into the crankshaft using the
C/E drive with its rubber coupler, why isn't the life expectancy of the engine more when a
C/E drive is used? Rotax still insists that the engine be rebuild with a new crankshaft at
300 hours whether the B or C/E drive is used!
582 vs 618
The advantage of the 618 is that it does supply 10 more
horsepower, but only 3 lbs more torque. This relates to about 2 degrees more pitch
in a three blade 66 inch prop. The 618 also revs higher with a top RPM of 6850. This added
RPM has been shown to work better on float equipped aircraft.
The disadvantage is the price is about $1,200 more, only two
exhaust systems are currently available, and there is a substantial increase in weight
mostly in the exhaust system, and if you are using the E drive the weight is put farther
forward/backward on the plane (depending on whether you have a pusher or tractor).
Next issue we will deal with "neglected" in
"Protecting your investment - proper engine installation and monitoring!"
*It is my recommendation that if you are using either the
582/618 with a C drive and electric start that you purchase the new E drive instead.
The E drive comes equipped with the electric start as part of the drive. This means that
when you start the engine you turn the engine over from the prop end. Using Rotax's
standard model electric start you are turning the engine and propeller over through the
crankshaft. Over time this could effect the crankshaft since it's components are press
fitted together.
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