ANDY'S OPINIONS ON ENGINE SELECTION

Which engine should I choose?

Which engine you decide to use depends on which Challenger model you are building and what you want it to acomplish.  Rotax and Hirth are both excellent engines and each has a niche.  I sell both brands, and make the same profit margin on both, so I can give you straight unbiased opinions.  There are several big differences.  Rotax uses older technology and has not significantly changed thier 2 stroke design in many years.  While this makes for a very tested and reliable engine, it does not keep up with the power to weight ratio, TBO, and fuel economy of the Hirth.  The Hirth with optional fuel injection has eliminated the hassle and potential problems of carb jetting and has taken a big step in improving fuel economy.  They also have incorporated carbon fiber reed valves instead of piston porting further improving fuel economy and performance. The cylinders in a Hirth are Nikisil like the Rotax 912 and 914 engines, while the Rotax two strokes are still steel sleeves.  This makes for a much more durable cylinder thus enabling the Hirth to extend the TBO way beyond Rotax, and also makes a piston seizure very unlikely since the piston and cylinder are both aluminum and expand at the same rate.  The only down side to this type of cylinder is it cannot be honed.  If a piston is seized or otherwise damaged it must be replaced and is very expensive.  As a manufacturer of engines for military unmanned aireal vehicles, Hirth has pioneered other advances in the two stroke industry such as carbon pistons and heavy fuel (jet fuel or diesel fuel) engines that hopefully will soon trickle down into the recreational aircraft line.  Hirth continues to be on the cutting edge of two stroke technology. Having said all of that, no one can boast more flight hours for a two stroke engine than Rotax.  They have three current (or at least very recent) production two stroke engine models.  The 447, 503, and 582.  All three have been widely produced since at least the early 1990's.  The 503 remains by far the standard for Challenger engines.  The 447 and 503 are aircooled engines.  They are a simple design, competitively priced, and simple to repair.  They work very well on all Challenger models.  The 582 is a liquid cooled engine.  It is commonly used for clipped wing two seaters or any model on floats.  It boasts 65 HP compared to the 40 HP and 52 HP of the 447 and 503 respectively.  The liquid cooled Rotax engines are rotary valve ported.  This means they have a perpendicular shaft driven by the crankshaft that has a rotating disc on one end that covers and uncovers the ports.  There is a seperate oil reservoir for this shaft. This adds to the complexity of the engine.  Also with liquid cooling comes added weight, drag of the radiators in the airflow, and the potential of a leak and subsequent failure.  Personally I do not like the 582 as aircraft engine.  It is very expensive, heavy, adds drag, is less reliable than the air cooled models and burns a lot of fuel.  Consider that statistically in the automotive industry the number one cause of breakdowns on the road, by far, is cooling system failures.  Why would you want to use liquid cooling on an airplane?

Which model shoud I choose?

Again, the Challenger model you are building and what you want to accomplish with it should determine what engine you use. 

Part 103 legal

The first scenario is the person who wants to build a truly part 103 legal ultralight.  This is a difficult undertaking, but it can be done with a Challenger.  The 447 is the lightest Rotax that is currently available new.  It is however by most opinions too heavy to actually meet part 103 on a Challenger.  It will probably get you within 10 or 15 lbs, so it is unlikely that anyone will ever bother you about it, but nevertheless, it will be over weight.  The 2702 Hirth is a great engine, but also is too heavy for part 103. The Hirth F-33 may be your only option to actually meet part 103.  It is a single cylinder engine which incorporates a belt redrive.  It is very light, and at 28 HP is sufficient to fly a Challenger ultralight with good performance.  It is also a very competitively priced engine.

Clipped wing single seat

The single seat clipped wing special is a hot rod.  It will fly with the 447 or 503.  You can also use a Hirth 2702, 3202, or 3203.  The 503 is the most popular engine for this model and it does a great job.  However, I would suggest other options.  If you are going to do a lot of cross country flying I would recommend the 3202 for consideration.   It is a 55 HP engine, weighs the same as the 503, and is specifically designed for high torque low rpm operation resulting in excellent fuel economy.  If your objective is flat out horsepower, I would recommend the 3203 for consideration.  It also weighs the same as the 503, but delivers more horsepower than any sane person would ever want for this application.  It also gives you the option of electronic fuel injection, which is extremely desirable.

Two seat long wing

The two seat long wing will actually fly quite nicely on a 447!  Most people opt for more power though.  It is nice to have the extra power to get out of tight spots, especially when heavy, or at high density altitude.  The most common engine on this model is also the 503.  It does a very good job.  The 3202 is also a very good option for improved fuel economy and range as well as a longer TBO and the option of fuel injection.  The 3203 also is a good option at 65 HP.  This is a great engine if the airplane is equipped with floats or is operating at high density altitude, but for most applications it really is not necessary, the 3202 or 503 is plenty of power.

Two seat clipped wing

The two seat clipped wing does like to have more power when flown with two normal sized adults.  I would highly recommend the 3203 in this application.  The 503 is really marginal on power if is to be flown at gross weight.  I own this model and had a 503 on it.  I flew 150 hours on the 503, but I never really felt comfortable with the climb rate when carrying an adult passenger.  I took the 503 off and installed a 3203 fuel injected engine.  What a difference!  I did not increase the empty weight of the airplane and added 15 HP.  Now it climbs very well at gross.  The fuel burn is about the same as the 503. 

Engine Options

Oil Injection

Oil injection comes standard on the Rotax engines, but you will have to buy additional parts for it such as control cabling, tank, line, reservoir, filter, etc.  The Hirth has it as an option and costs additional.  There are pros and cons to it.  The pros are; it is better for the engine since the oil is precisely controlled according to engine operation.  This allows for less carbon build up and better lubrication.  Also it is very convent especially on cross country trips where you would have to fuel up at an airport and tank mix it.  That can be a real hassle, and it is difficult to assure a good mix, especially when it's cold out.  The downside; It is expensive initially to install, and it can fail.  While failure of the oil injection is rare, it does happen.  When it does, it usually results in a destroyed engine.  If you are lucky the engine will seize a piston before it ruins the crankshaft.  Either way you are coming down. When the oil is mixed with the fuel it is impossible to not get to the engine.  The only way to oil starve a pre-mix engine is to forget to put the oil in the gas, which does happen occasionally!

Electric Start

The advantages; Obviously convenience is the main one.  Also, the ability to restart the engine in flight is a real advantage in some situations.  The disadvantages are also obvious; Cost and weight.  You are looking at around $750 when you figure extra wiring and battery.  You will add 20-30 lbs to the empty weight of the airplane, most of that being the battery.

Electronic Fuel Injection

Not an option for Rotax engines.  For Hirth engines, I absolutely recommend it.  Other than initial cost the only disadvantage it has is that it is dependant on the electrical system. You must make sure that a failure of your wiring does not cause the shutdown of your fuel system.  I can assist you in your wiring to help prevent this.  Properly installed, it is very reliable. Regardless of the weather or altitude, your mixture will always be ideal for the engine.  The ECM monitors the crankcase temperature, ambient air temperature, barometric pressure, rpm, and throttle position.  From those parameters the ECM calculates the precise fuel mixture needed for the engine.  This means it automatically compensates for weather changes and altitude.  You do not have to adjust anything, just turn the key and forget it.  Also it gives you cleaner combustion resulting in less engine deposits, and about 10% better fuel economy which really adds up at the current price of fuel.  The performance is great, no hesitation, exhaust temp is always dead on, idle is always perfect.  It really has made carbs obsolete.