Engine Selection – Archive

Engine Selection

Which engine to use depends on a lot of factors such as the airframe, the gross weight, the intended mission, etc… There are many different engines that have been tried on aircraft. Some were made for aircraft, some were made for other purposes. In the experimental aircraft world we have the freedom to use any engine we would like, no matter how crazy! The purpose of this article is to discuss the options that are currently available and in common use with a good safety record. To choose a correct engine, first we must look at the intended mission. All of the aircraft we sell are used exclusively for recreation, with a possible exception for flight instruction. Aircraft that are used for recreation generally do not see a lot of hours. The average is around 50 hours per year. In reality, at 50 hours per year, a 2000+ hour TBO (Time Before Overhaul) really is not a significant benefit to us. The years will do more harm to the engine than the wear will. Also, fuel burn really is not all that relevant of an issue at 50 hours per year. A difference of 1 gallon per hour would burn 50 gallons per year more. At current gas prices, that amounts to less than $100. In comparison to the upfront cost of the engine and the cost of maintenance that really is not very significant. In our application, some of the factors that are very important to us, are the weight of the engine, and not only the power, but the thrust it is capable of producing. Another key factor for most of us, honestly, is price. Who can afford a $25,000 Lycoming to put on a $15,000 airframe to fly 50 hours per year? The cost per hour would be staggering. When we consider these factors, what we will find is that a two stroke engine really is perfect for our airframes.

A two stroke engine has an incredible power to weight ratio compared to a four stroke. That is a very important factor for our type of airframes. If you add 25 lbs to your engine, you take away 25 lbs from your useful load. If you have a two seat airplane with a 500 lb useful load, you really can’t afford to loose 25 lbs of it or you won’t be able to take an adult with you. Not only do we want it to be light weight, but we want it to make lots of POWER! After all, we are flying for fun, and power is fun! A two stroke makes a whole lot of power for it’s size and weight. However, the way that it makes so much power is by producing a lot of rpm. Horsepower is a measurement of torque (the turning force of the crankshaft) multiplied by RPM. (the speed at which the crankshaft is turning, measured in Revolutions Per Minute.) A two stroke has less torque and more RPM, compared to a four stroke. For an aircraft, we not only need an engine to produce lots of power, but we have to convert that power into thrust, which is what propels the aircraft forward. If we simply put a prop on a two stroke, it will not make very much thrust because it does not have very much torque, therefore cannot swing a very big propeller. But we have a solution for that. It is called a Propeller Speed Reduction Unit, or PRSU. In other words, we use a belt drive or a gearbox to act as a transmission to spin the prop slower, but with much more torque. Kind of like shifting to a lower gear in your car. Now we can use whatever ratio will work best for our airframe. We might use a very deep reduction like 3.5:1, which turns the prop one time for every 3.5 revolutions of the crankshaft, on a slow aircraft to give it maximum low speed thrust with a big propeller. Or on a faster airplane, we might use a 2.5:1 ratio so that the prop does not unload as much as the aircraft picks up speed. So now we can custom tune our propeller RPM to the needs of our airframe and engine to maximize our performance and efficiency. This gives us a very high performance, light weight engine package.

Now let’s discuss the economics of a two stroke engine. While a two stroke engine does have a much shorter TBO, it is far less money to purchase up front, and far less money to overhaul. Again, this is perfect for our type of flying. An engine that is overhauled more frequently, but at much less expense results in more reliability. At 50 hours per year, 20 years down the road we would have 1000 hours. A 2000 hour TBO four stroke would have another 1000 hours to go. However you would have an engine that had not been torn down for 20 years! To tear it down would cost well over $10,000, quite a deterrent to doing an overhaul, so it will probably keep flying like a ticking time bomb. But in the case of a two stroke, take a Hirth for example, lets say for preventative maintenance we do a full teardown, inspection, and re-seal every 7 years, we would have spent about $650 on it twice, so about $1300, and now at 1000 hours we are going to do a full overhaul, new crank, pistons and all wear parts. We would be looking at about $3000 and we have an engine that is like new and ready for another 1000 hours. Now we have an engine that we spent far less on upfront, was never more than 7 years from a tear down inspection and re-seal, and can be overhauled completely for a fraction of the cost. That makes a lot more sense in my opinion.

While there are a few four stroke engines that are being used in our type of aircraft, I really think that a two stroke is much better suited to this application. The main two models of four stroke engines that are popular in our aircraft are the Rotax 912, and the HKS 700. I will gladly sell and install those engines for those who want them, but for the purpose of this article I am going to focus on the two strokes as that is the overwhelming majority of what we sell. First, we will discuss each engine model, then we will consider each airframe and what is the best match for each.


Rotax General information

Header_Rotax_LogoRotax, a division of Bombardier, has been building two stroke and four stroke engines for many years for many different markets. They are very well known in the sled and jet ski markets. In the early 80’s when the ultralight movement was taking off, many people found the the Rotax engines worked well for this new application. In time, Rotax began marketing to this new industry. In the early days the failure rate was very high, and slogans like “friends don’t let friends fly two strokes” were coined. In time people learned that if you take a new Rotax engine and tear it down and straighten the crank and check the balance and tolerances and adjust as necessary that you could make a fairly reliable engine out of it. Rotax took notice, and created a separate line of dedicated aircraft versions of many of their engines paying careful attention to quality control.  Soon they even rolled out their first dedicated aircraft engine, the 532 at 65 HP, including a dedicated geared PRSU, the A gearbox, that bolted right onto the crankcase. This really propelled them forward in the aviation market. Soon they had a “provision” version of the 377, 447, and 503 as well. The 503 still experienced a fairly high failure rate of PTO end main bearings and soon the 503 was redesigned to include larger PTO bearings as well as dual electronic ignition, and a new style provision 8 crankcase. They also came out with the B gearbox for the provision 8 crankcase, followed by the C gearbox which incorporated a rubber dampening coupler instead of direct drive and larger gears to accommodate more horsepower, larger heavier props and deeper reduction ratios. Later the E gearbox was introduced, being similar to the C gearbox with the addition of electric start integrated into the gearbox. In 1990, the first 582 model 90 aircraft engine hit the market, replacing the 532. With a larger bore and dual electronic ignition it was nice improvement over the 532 and increased reliability. In 1999 the 582 model 99 was introduced, which made a few reliability improvements. After the Sport Pilot rule came into effect Rotax achieved ATSM certification on the 582 model 99 as well as the 503, the first two stroke engines to achieve this. However, not long after that the 503 was discontinued, and the 582 remains the only ATSM certified two stroke, at least for now. Prior to the discontinuing of the 503 and 447, Rotax enjoyed about 80% of the ultralight and ultralight type aircraft market. With the 582 being the only remaining two stroke they currently produce, it is inevitable that Rotax will see an ever decreasing market share. Rotax also produces the 912/914 series four stroke engines. The four stroke engines sell at approximately 10 times the volume of the 582, thanks to the Sport Pilot rule, as well as military contracts for them powering UAV’s. There may come a day when Rotax exits the two stroke aviation market completely. Only time will tell.

Warranty: The warranty on a Rotax 582 is 18 months from the date of purchase.  The warranty does not consider the hours of operation, only date of purchase.  (Keep in mine when ordering an engine to take delivery of it as late as possible in the building process so your warranty does not expire sitting on the shelf.)

Rotax 377/447Rotax447

The Rotax 377, 35 HP engine was later replaced by the 447, 40 HP engine. The main difference is the 447 is a larger bore. Later the 447 was changed from points to Ducati ignition for the aircraft line. The 377 and 447 were both used in the sled market as well, and some of those engines have found their way into airplanes too. The engines that have Denso ignition were originally sled engines. The sled version of these engines work just fine though if properly jetted and operated. The 447 engine was discontinued around 2006, and Rotax has committed to providing parts for 10 years. It will soon become difficult to get certain parts for the 447. While the 447 was a good engine, it probably is not practical to install one on a new airframe. It would be better to start with a current production engine so that parts will be available for the long haul. The 447 filled an important niche in the aviation market and will be missed.

Rotax 503Rotax503

The Rotax 503, 52 HP engine was probably the most popular engine in the two seat ultralight type aircraft for over 25 years. It also started out as a sled engine that was converted for aviation, but in the early 90’s Rotax came out with a dedicated aviation version that featured dual Ducati ignition, Bing carbs, a stronger crankshaft, and the new provision 8 which allowed the use of the B, C, and E gearboxes. This made the 503 even more reliable. The 503 enjoyed a long production run and was very popular, but in 2009 the 503 met the same fate as the 447 as Rotax decided to cease production. As the story goes, the tooling for the 503 was worn out and Rotax could not justify retooling for such a small volume of engines sold. While parts will still be available for a few more years, they will eventually dry up. The 503 is a well proven engine and is very reliable when operated and maintained properly. While it may be feasible to find a core and overhaul it for use on a new airframe, keep in mind that it will become harder to find parts. You may want to buy an extra crank shaft and pistons and put them in your closet for that future overhaul.

Rotax 582Rotax582

The Rotax 582 65 HP is the only two stroke aviation engine that Rotax currently produces. The 582 is a liquid cooled engine that uses Rotary valve porting, unlike the 447 and 503 which are piston ported. The 582 model 90 came out in 1990 and is often referred to as a “grey head”. In 1999 Rotax introduced the model 99, also called the “blue head”. The blue head incorporated several upgrades. First, the rubber lip water pump seal that had been prone to failure was replaced with a ceramic seal that is much more reliable. Also, a two stage thermostat and internal coolant bypass was employed to help prevent cold seizures. A cold seizure occurs when an engine that has not been properly warmed up and is subjected to take off power. The engine heats up quickly until the thermostat opens quickly and allows cold coolant to rush in which cools the cylinders rapidly and contract until they are smaller than the pistons and a seizure occurs. This scenario has occurred many times in the 582. The model 99 was designed to help prevent this, but is has not eliminated it. However, if operated properly this would not be an issue. I do not consider the 582 to be as reliable of an engine as the 503. The 582 seems to have a higher crankshaft failure rate. There are several theories as to why that is, but it is hard to say for sure. There are also some failures related to the rotary valve seals and gear train. But most issues with the 582 tend to involve the cooling system as installed on the airframe. I am a strong believer that airplanes should use air cooled engines if at all possible. There are several OEM’s that are currently using the 582 in kit aircraft as well as SLSA aircraft. Quicksilver for example uses the 582 in the Sport 2S and SE models, as well as the GT500 and soon the GT400. While the 582 would not be my first choice, many of the installation related issues have been worked out, and by using Quicksilver’s installation design without modification, many problems can be avoided. In summary, the 582 is an acceptable engine if installed and operated properly, and is in current production. Therefore, it can be purchased new and parts availability will be good for many years to come. However, in nearly every application where the 582 can be used, the Hirth 3202 or 3203 would be a better choice. The Hirth 3203 at 65 HP is equal to the 582 in performance, but without the problems of liquid cooling, and coming in around 25 lbs lighter on the complete installation than a 582. Not only that, but the 3203 is considerably less expensive than a 582 when including all of the necessary components to install it.

Hirth Engines general information:

Hirth Logo medium

Hirth has been in the engine manufacturing business since before World War 2. They made engines for the Luftwaffe during the war. After the war they were no longer allowed to make aircraft engines. To stay in business they began making two stroke engines for non aircraft purposes. Eventually, as the ultralight aircraft industry began to take off they began to make engines for that market, and soon the UAV market as well. To this day, Hirth manufactures many models of two stroke engines for aviation, both manned and unmanned, but they still have no interest in the four stroke market. Hirth uses many modern designs, one of which is Nikasil cylinders. Nikasil has several advantages over the old steel sleeve design that Rotax uses. First of all, Nikasil wears far better than steel sleeves and will out last a steel sleeve many times over. Another huge advantage of using Nikasil in a two stroke is that it expands at the same rate as the piston making a piston seizure far less likely.  Another advance in two stroke technology pioneered by Hirth is  carbon fiber reed valves in place of piston porting or rotary valve porting. Reed valve porting is far more efficient than other forms of porting. It increases engine torque and reduces fuel burn. In the past, reed valves were made of stainless steel and would eventually fatigue and crack or break off. With the advent of carbon fiber reed valves, that is no longer an issue. We now can have all of the advantages of reed valves without compromising reliability. Another modern design that Hirth has developed for the two stroke aviation market is electronic fuel injection. Many of the problems that two stroke engines suffer from are related to improper jetting and prop loading. The fuel injection eliminates those issues by using an ECM to continually monitor the operating parameters of the engine and adjusting the fuel rate accordingly. The fuel injection also compensates for altitude and weather changes to keep the engine operating at peak performance in all conditions. Not only does fuel injection help the performance, but it also keeps the engine cleaner, reduces fuel burn by about 10% and eliminates the possibility of carb ice. Other Hirth features include 4130 chrome moly crankshafts with a three year factory warranty, viton crank seals, and more. Because of these modern design innovations Hirth is able to rate their engines at a 1000-1200 hour TBO. But keep in mind that it is still necessary to do a teardown, de-carbon and inspection at 500 hours. All of these Hirth engines are available with Electronic Fuel Injection (EFI) or carbs. Oil injection is also available on all of these engines except the 3002/3003 models. However, I personally do not recommend oil injection. While it is conveinent, it is a mechanical device that can fail. It also does require the pilot to monitor the oil usage and make sure it is working properly. Also, it is my suspicion that oil injected engines have a higher failure rate and do not last as long as pre-mix engines. I do not have hard data to prove that, so it is just an opinion. So far in nearly 10 years of selling Hirth engines, the only one that has failed was an oil injection failure (although that may have been an installation error).

Warranty:  The Hirth factory warranty is one year from the date of purchase for the entire engine.  The crankshaft is warranted for 3 years from date of purchase.  For new Hirth engines purchased from Heavenbound Aviation we offer a limited warranty for an additional 4 years, giving you a total of 5 years of warranty.  Our limited warranty covers all labor and parts at our cost for any failure, not including operator error, abuse, or routine maintenance.  We do this for two reasons.  First, there seems to be a skepticism about Hirth engines and their reliability among the Rotax crowd.  I will put my money on Hirth engines.  Secondly, if there is a failure in any engine I sell, I want to know about it, and to be the one to find out what failed and why so we can prevent it in the future.  Often, when an engine fails in the experimental world, it simply gets repaired without finding the original cause.  That means that it may happen again to that engine, or to someone else.  At Heavenbound Aviation, our goal is to make ALL engines as reliable as absolutely possible.  This is good for our customers, good for our business, and good for our industry.

Hirth F33F33-pic

The Hirth F33 is a single cylinder, 28 HP engine that is uniquely suited to the part 103 ultralight category. Weighing only 40 lbs, including the belt reduction drive, it is in most cases the only engine that can actually make weight on a Part 103 ultralight. Using all of the same technology that the larger Hirth engines use, even including dual ignition, this is a high quality aircraft engine from the drawing board up. It can even be equipped with electronic fuel injection if desired. In many cases the F33 can even make ultralight weight with electric start! The F33 is the only engine I am aware of currently that can make ultralight weight on a Challenger. It is also used on the Aerolite 103 to make Part 103. The F33 is a very versatile, good performing and reliable engine.

Hirth 27022702 pic

The Hirth 2702, 40 HP is the smallest inline twin that Hirth produces. The 2702 is a very reliable engine, and economical engine. It produces its HP at a lower rpm and with higher torque than most two stroke engines with it’s peak power being produced at 5500 RPM. The engine is very robustly built for a 40 HP engine, using many of the same components as the 65 HP 3203. Like all Hirth aviation engines it includes dual electronic ignition. The 2702 is a great replacement for the 447 in many applications. It’s only disadvantage compared to a 447, is that it is significantly heavier, around 10 lbs. In an application that requires a gearbox, the 2702 is fitted with the G50 gearbox. While the G50 gearbox is a excellent gearbox and rated for up to 100 HP, it does increase the weight even more. For an aircraft that does not need to meet part 103, it is an excellent option. However, when considering all of the options, for a little more money and nearly the same weight you could opt for a 3202 at 55 HP.

Hirth F23F23-picthumb

The Hirth F23, 50 HP engine is an incredible design. It is a horizontally opposed twin cylinder, free air cooled engine. This engine is a boxer engine with both cylinders firing at the same time. This makes an exceptionally well balanced and smooth running engine. The real beauty of this engine is that it puts out an honest 50 HP and weighs in at only 71 lbs complete including the belt redrive! That is 15 lbs lighter than a 447 with B box, and 10 more horsepower! The F23 should be the most popular engine in the single seat and light two seat market. The only real challenge for the F23 is the installation. Since most airframes were designed around inline twin cylinder engines, it can be challenging to work out a good installation. To further complicate things, it uses dual exhaust. But once the installation is worked out, it is an extremely reliable and durable engine. It is also a very economical engine. We have already completed an F23
installation on a Kolb Firefly, and will soon be working on a Challenger installation. There is an installation available for the Aerolite 103, and one is being developed for a F23 KolbQuicksilver as well. It appears that the F23 will also be the first of the Hirth line to achieve ATSM certification for Special Light Sport Aircraft.   Skycraft is working with Hirth to get the F23 certified for their SD-1 aircraft.  You can see those flying on youtube.  The F23 is also gaining popularity in the paragliding and powered parachute world.  I expect the F23 will be one of the most popular light sport engines in the future.

Hirth 3202

3203-picThe Hirth 3202, 55 HP engine is a low rpm, high torque engine, making it’s peak power at 5500 RPM. It is a very powerful engine that works very well on many two seat light sport aircraft. This is an ideal engine for most of the Challenger line of aircraft as well as many of the Quicksilver models. The 3202 makes an excellent replacement for the 503. It will install easily in nearly any application that the 503 was used in, and will out perform it with no weight gain. Compared to the 503, the 3202 is the same weight, or if comparing both with electric start, the 3202 is little lighter. The 3202 offers a much higher TBO than the 503, and will burn less fuel, especially with fuel injection. The 3202 often looses out to the 3203 however, because for a little more money and no weight gain, it is possible to have an additional 15 HP.

Hirth 3203

The Hirth 3203, 65 HP, is nearly identical to the 3202, but is ported and tuned differently to make peak horsepower at 6500 RPM rather than 5500. This gives it a very respectable power to weight ratio. The 3203 is a direct competitor of the Rotax 582 in many applications. Since it is an air cooled engine it does not suffer from the downfalls of liquid cooling. Also, because it is air cooled it is far lighter installed than a 582. Not only that, but it costs less than a 582 when all of the installation components are factored in, and is far less complex to install. Really, the only installation where a liquid cooled engine has an advantage is when the engine is located in a tight cowl that cannot get enough airflow to the engine. That is not an issue in any of the airframes we sell however, and therefore an air cooled engine is superior in every way for our application. Honestly, the only valid reason I can come up with for using a liquid cooled engine on a ChallIMG_9930enger or GT Quicksilver would be for better cabin heat. But I think a good snow suit would be a much better option when considering all the negative tradeoffs.  Another category where the 3203 trumps the 582 is fuel burn. The 3203, especially with fuel injection, will use noticeably less fuel. The 3202 and 3203 are my favorite engines for most Challenger models, as well as two seat Quicksilver models and the GT400.

Hirth 3502/3503

350X-picThe Hirth 3502, 60 HP and the 3503, 70 HP are liquid cooled versions of the 3202 and 3203. They share the same crankshaft, pistons, and the same bore and stroke. They were created primarily to be a drop in replacement for the 582. They use the same bolt pattern on the crankcase and can therefore use the same mount, as well as the same radiator system as a 582. Unlike the 582, these engines have all of the modern advances Hirth engines are know for such as reed valve porting, nikisil cylinders, viton seals, and electronic fuel injection if desired. While I would choose a 35 series engine over a 582, I would still not recommend it for our application in most cases. The 3203 is simply a better choice and more cost effective. However, if someone has an existing aircraft with a dead or dying 582, this would be a simple change out and might make a lot of sense.

Hirth 3702/3703

3701-pic3The Hirth 3702, 84 HP, and 3703, 100 HP engines are three cylinder inline water cooled engines. These engines certainly have a place in the light sport aircraft world, allowing a lot of power to be put into a narrow cowl, and with an amazing power to weight ratio. In the airframes that we sell, the only one that could possibly make use of this engine would be a GT500, but to my knowledge it has never been done. The 3002/3003 engine would probably be a better choice for that application.

Hirth 3002/3003

300X picThe Hirth 3002, 80 HP, and 3003, 110 HP engines are unique in the industry. The are a four cylinder horizontally opposed air cooled engine. The 3002 is tuned for high torque, low rpm, and the 3003 is tuned for high horsepower. These engines are a great alternative to the Rotax 912 series engines. The installed weight of a 3002/3003 engine typically comes in at about 45 lbs less than a Rotax 912.  The thrust from the 3003 will leave a 912 in the dust.  Not only that, but it also comes in at about half the cost! The fuel burn of the 3002/3003 will be slightly more than a 912, but not significantly. But the trade off is that you get exceptionally better performance. With a 1000 hour TBO, much less up front cost, and similar fuel burn, the overall cost of operation is much less. Should you happen to fly enough hours to reach TBO, you will find that for around $4000 you can have a full overhaul completed with new pistons, crank and all. If you hit TBO in a 912 you are going to find that you might as well ebay the engine and buy a new one. It is cost prohibitive to overhaul. You can expect to spend $15,000 or more to reposer with a new 912.  Along the way, you will spend a small fortune keeping up with the AD’s and maintenance of the 912, with parts prices that are off the chart and sometimes not readily available.  Keep in mind also that when comparing the cost of a 912 to other options, the price of the 912 does not include a lot of necessary parts for the installation such as exhaust, radiator, oil cooler, etc…

Kawasaki 340/440

One other engine that bears mention here is the Kawasaki 340 and 440. These engines are very similar with the bore being the only significant difference. For our application these engines should be in used in the fan cooled configuration. While these engines were never designed for aircraft use, and are not marketed for that application by Kawasaki for liability reasons, they are fairly common in the ultralight market. Although I do not have extensive experience with these engines, we do sell and install them occasionally. They are currently available brand new, and parts are readily available. Probably the biggest attraction to these engineKawasaki 440s is the low cost. A brand new 440, fan cooled with all the necessary components for installation will cost less than $3000. (not including a reduction drive) The Kawasaki engines use piston porting, but they do use Nikisil cylinders. The best application for these engines is single seat Quicksilvers, as well as the Aerolite 103.

Engine Options

Oil injection:

Many of the two stroke engines listed here have the oil injection option available. It comes standard on the 582, and can be added on to most Hirth models with the exception of the 3002/3003. Personally, I do not recommend oil injection. While it is very convenient, it is a mechanical device that can fail. Once it is mixed, pre mix is fool proof. Another concern I have with injection is that the oil may not be distributed as well through out the engine, particularly the crank bearings since it is not being carried by the fuel.  I have a suspicion that because of this, oil injected engines may have a higher failure rate than premix engines, particularly the crankshaft bearings.  This seems to be the case, but there is no hard data to prove it either way, and is therefore just an opinion.  At any rate, oil injection is not an ‘install it and forget it’ option. It needs to be carefully installed and monitored constantly to make sure it is throwing the correct ratio.  So far in nearly 10 years of selling Hirth engines, the only one that has failed was an oil injection failure (although that may have been an installation error). I fly mostly from my home airfield and I use a 30 gallon fuel caddy to premix in, then simply pump mixed fuel into the airplane as needed. I fill the fuel caddy with straight gas from a bulk tank, then mix the oil into it. This system works very well as I can shake the fuel caddy to make sure that it is mixed properly, but never have to fool with cans and funnels. A fuel caddy can be purchased on Ebay for under $200. If using a Hirth engine with BlueMax oil, and you need to tank mix at an airport, the best method is to thin the oil 50/50 with gas first, mixing it thoroughly, then pour in the tank. (I mix it in the bottle at 50/50 then simply double the ratio.) BlueMax is a heavier oil and will go to the bottom of the tank if not diluted, especially in cold weather. Once mixed it will not separate.

Fuel Injection:

All Hirth engines have the option to use Electronic Fuel Injection (EFI) instead of carbs. EFI has some wonderful benefits. With EFI the fuel mixture is always perfect. It compensates for prop load, weather changes, altitude, etc… This translates into far less operator induced failures. People who do not understand two stroke engines and tuning, really do not need to with fuel injection. You can just turn the key and go fly. With EFI the engine can be run leaner without compromising safety which results in lower fuel burn and a cleaner engine. EFI also eliminates the possibility of carb ice. Since there is no carb, and no venturi which cools the air, there is no icing. There are two disadvantages to fuel injection. First, of course, is cost. It adds about $1500-3000 to the cost of the engine, depending on model. Second, it is an electronic device with several necessary components. As with anything electronic, it can fail. Although it is very reliable, and rarely fails, carbs are more reliable, barring operator error. Choosing fuel injection is a personal preference and may depend on the area you fly as well.  If you fly from low elevation to high elevation or altitudes, EFI has a very significant advantage of precisely leaning the mixture.  If reliability is absolutely paramount for you, I would probably  recommend carbs, but only if you are willing to really educate yourself on the operation of carbs and two stroke engine tuning.   One of the most common causes of engine failure in two strokes is improper prop loading and carb tuning.  If you are not very mechanical and just want something you cannot mess up and never have to “tweak”, the fuel injection is a great option. We would be glad to help you install it to make sure that everything is setup properly.

Carbon Fiber Air Guide:

Hirth inline engines have the option to upgrade to a carbon fiber air guide instead of the original steel model. This option accomplishes two things. First, it reduces weight. Secondly, it is much more durable. The steel air guide is more likely to crack over time, which I have seen happen several times. (especially on Challengers due to the tall redrive increasing torsional vibration). The cost to upgrade on a new engine is only about $60, and well worth it in all applications in my opinion. I would like to see Hirth make that standard on all of their fan cooled engines.


Challenger 1 long wing, Part 103

IMG_0167The Challenger 1 long wing can make part 103 and be flown as a legal ultralight, however that narrows the engine selection to only one current production engine, the Hirth F33. The F33 should be used with the installation package that we sell with it.  That includes the engine mount and exhaust mount along with belt reduction unit for it. This will make the installation very easy and you will have a reliable and proven setup. While it is possible to use fuel injection on the F33, it is quite expensive, and will increase the weight somewhat, and is generally not recommended for part 103. It is possible to make part 103 with electric start on this engine. Using a lithium battery to save weight, the starter motor itself weighs very little. As far as part 103, if you wanted to be technical, if the battery has quick disconnects it does not have to be considered part of the airframe for weight purposes since is it not permanently mounted. Generally we recommend the Powerfin E model three blade prop for this application.

Challenger 1 long wing, not Part 103

If you are building a Challenger 1 long wing, but are not concerned about making Part 103, an engine you should seriously consider is the Hirth 2702, 40 HP. The 2702 is fairly heavy for a 40 HP engine, weighing around 10 lbs more than a 447. But if you can live with that, there are many advantages. The 2702 is a very robust engine. Because it is built on the same design as the 55 and 65 HP models, it uses many of the same components including the crankshaft. This means that it is way overbuilt for a 40 HP engine. Unlike a 447, it has truly redundant dual electronic ignition with timing advance. Another feature of this engine is that it is a high torque, low rpm engine, developing it’s peak power at around 5500 RPM. This makes it quieter and more fuel efficient. The people who have these engines love them. So far the highest time Hirth engine I have heard of is a 2702 engine at well over 2000 hours without an overhaul. Other options for this airframe include a 3202, 55 HP. It is essentially the same weight as a 2702, or Rotax 503. It really is overkill for this airframe, but it will work just fine. One more option might be the F23. The F23 puts out 50 HP and will weigh in more than 20 lbs lighter than a 2702, or 3202. It is an extremely reliable engine and very smooth. However, this engine to date has never been installed on a Challenger. I have installed one on a Kolb, and hope to use that design for a base to design a Challenger installation, but have not done it yet. If you are interested in an F23 on a Challenger, please contact me and I will see if I can bump that project up the priority list.

Challenger 1 CWS

This airframe is a little hotrod! There are several engine options that will work nicely here. The 2702, 40 HP is perfectly adequate to make this a fun airplane. It is quiet, reliable, fuel efficient and inexpensive. However, more power is fun. A 3202 would also make an ideal engine for this application. Essentially the same weight as the 2702, it cranks out 55 HP. The 3202 has plenty of power to take you up like rocket and with the high torque, low RPM design, it will cruise at a very low throttle setting and sip fuel. I would probably not recommend a 3203 on this airframe, it is really more power than this airframe was designed for. Another option for this airframe as well would be the F23 once the installation design is completed. It would be plenty of power and would be much lighter.

Challenger 2 Long Wing

Challenger on floatsThe Hirth 3202 is a perfect match for this airframe.  At 55 HP it will out perform a 503 and is plenty of power.  It is a low RPM, high torque engine, operating very smooth and quieter than 3203, while sipping fuel.  It has incredible torque.  We run our 3203’s at about 12 degrees pitch on a 3 blade 60” square tip warp drive.  The 3202 has to have more than 14 degrees to hold it back!   I have been very impressed with this engine.   If you are flying at very high elevations, or on floats, the 3203, 65 HP might be a better option.  Without any weight penalty you will get another 10 HP.  Since it runs up to 6500, it will make a little more noise than a 3202, and will burn a little more fuel.  But if power is what you are after, this is the engine you need.  While you could also use a 582 in this application, I think you will find that when comparing it to a 3203, the 3203 wins hands down.  The 3203 is much simpler to install, weighs 25 lbs less, has less drag, has a higher TBO, is more reliable and about $2000 less expensive than the 582.  There just is no reason to use a 582 unless Rotax is the only engine in your vocabulary.   If you are a die hard Rotax fan, I would suggest finding a good used 503 DCDI and overhauling it rather than a new 582.  If you can live with 50 HP, it is a much better engine for this application.    Another option for this airframe as well may be the Hirth F23 once we are able to get an installation setup for the Challenger airframe.  It would be very well suited to the two seat long wing, with the same horsepower as the 503 and much less weight.  For those folks migrating in from the GA world that only speak four stroke, it is possible to install an HKS 700E on a Challenger long wing.  It has been done and an inHKS Challengerstallation package is available.   There are several of them flying.  However, the performance of the HKS, rated at 60 HP, is equivalent to a 503, but the weight is about 50 lbs heavier than a 503.  If you are willing to give up an extra $10,000, 50 lbs of useful load and several hundred feet per minute of climb, you could be flying a four stroke.  The HKS is a very reliable engine and I have nothing negative to say about it, other than it is not all that impressive of a performer in this application.

Challenger 2 CWS
Andy's Challenger on grassThe clipped wing Challenger 2 is my personal favorite Challenger model. It performs very well, with a high roll rate, and higher wind capability. With a heavier wing loading it also rides out the bumps better and cruises a little quicker. However, as with any higher wing loading airframe, it needs a little more power than the long wing version. For the two seat clipped wing I highly recommend the 3203. The key to having a two seat clipped wing that performs well is keeping it light and having plenty of power. I originally had a 503 on my clipped wing and it weighed in at 444 lbs. I could get a 240 lb guy up on a cold day, but it made me very uneasy. the climb rate was 2-300 fpm. On a warm day I would not even attempt it. After a couple years of that, I installed a 3203. The increased power made that airplane a much better trainer. Now I can put a 240 lb guy in it on a cool day and see 500-600 fpm climb. Several years later I sold that airplane and restored it for the new owner. We added some instruments to the panel, upgraded seat cushions, Mike Harrison brakes a gap cover and some other odds and ends and now she weighs in at 434! That is with electric start and a lead acid battery. My new Challenger 2 CWS with 3203, and all the above options weighs just 412. You could use a 582 on this airframe, but you will never get close to these weights, and you will have additional drag. The 3203 is the best option for this airframe. If you are not convinced, I would like to invite you to come fly in mine. I think you will agree.

Quicksliver Aircraft

Sprint/Sport single seat

Sport 1 flyingThe quicksilver Sprint and Sport models are unique in the industry in that they use a reduction drive built into the airframe. Originally designed in the late 1970’s and early 1980’s the two stroke aircraft engine had not been developed. Forced to use snowmobile engines Quicksilver designed a shaft driven belt reduction drive that is directly attached to the root tube of the airframe. This design is still in use today, and is included in the kit. Because of this feature, the Quicksilver Sport and Sprint can use what we call a “non provision” engine. That simply means that the engine does not have a provision for a gearbox. There are a lot of non provision engines on the market very cheap. The Rotax 377, 447 and 503 were all made in a non provision version for the non aircraft market, and can be bought for a few hundred dollars. One of these engines, professionally overhauled, can be a very inexpensive and reliable engine for use in a Quicksilver. This makes the Quicksilver very attractive for those aspiring aviators that are on a tight budget. A complete airframe including a propeller can be had for just over $10K. A used, overhauled non provision engine can often be purchased, overhauled, and ready for use for less than $1000. It is actually possible to be in the air and flying in a brand new quicksilver for about $11K. That is unique in this industry, and an exciting option for many people. There are also other options on these airframes. For those who would like to stay with a new engine, the Hirth 2702 makes a great option. Although it probably is not possible to make part 103 with this engine, the reality is that with a Quicksilver, no one will ever question it as an ultralight. It is also possible to use a 3202 in this application, but it really is not necessary, the 2702 is plenty of power. Currently, Quicksilver is experimenting with the use of the F23 on the Sprint and Sport models. While this engine will be an excellent candidate for this airframe, the installation still has some bugs to work out. One other noteworthy option for this airframe is a Kawasaki 340 or 440 fan cooled engine. This engine can be purchased brand new for under $3000 with all the necessary accessories, and can be purchased used or overhauled for considerably less. The Kawasaki is easy to install on the Quicksilver and has plenty of power. I do not have a lot of personal experience with Kawasaki, but they seem to have a significant following and good reviews. We will be installing one on a Sprint in February of 2015, so feel free to inquire on the progress of this project. Also, note that these models come with a mount and propeller for either a 447 or 503 unless directed otherwise. For more details about a specific engine installation on these models, please contact me.

Sprint 2, Sport 2S

S2SaThe Sprint 2 and Sport 2 models, although they look very similar to the Sprint 1 and Sport 1 models, are very different concerning the engine installation. With the two seat models the engine is no longer mounted inverted under the root tube, but is mounted upright on top of the root tube. The engine is also moved aft considerably. Instead of using the original shaft drive and belt reduction, the two seat models now uses a modern gearbox reduction. From the Quicksilver factory, the 582 is the preferred engine and if ordering an airframe complete with engine, that is the only option. In this model, the 582 does have an attractive feature, and that is simply that you can order a complete aircraft from one source and simply bolt it together and go fly. As far as a liquid cooled installation goes, this is one of the better ones. Quicksilver has done an excellent job keeping it light and compact. Also, when purchasing it through Quicksilver it is surprisingly price competitive. They have the bugs worked out, and if the manual is followed closely and it is operated properly it should be pretty reliable. However, if I were to build one, I would still opt for the 3203. It is very simple installation, less weight, longer TBO, lower fuel burn… it’s just a better engine. It is also possible to use the 3503 Hirth in this application. It will bolt right on the same mount as the 582. The weight is essentially the same. I would consider the 3503 to be a better engine than a 582, having 5 additional horsepower, and very low fuel burn, and much higher TBO. However, my recommendation would still be to stay with a 3203. Save the weight, hassle and the extra money. It’s not worth it for 5 horsepower. In either case, with this airframe and it’s ability to swing a large prop, I would  recommend a gear ratio of around 3.5 and a large 3 blade warp drive square tip prop.


GT 503 installThe GT400 was designed with the Rotax 447 and 503 in mind. These engines work very well on this airframe and are certainly an option if one can be found and overhauled. Quicksilver is currently working on a 582 installation for this airframe, but I would not recommend that. The first engine that I would suggest for the GT400 is the 2702. At 40 HP it is a good match for the GT400. If you would like more power, the 3202 would be an excellent option. Being an inline engine these two would be very easy to install. Perhaps a better option that has not been done yet will be the F23. At 50 HP it is the perfect match to this airframe, and at a great weight savings over the 503 or the inline Hirth engines. If you would like a GT400 with an F23 engine, please contact me, we would be very interested in designing an installation for this application.


GT500fWhen considering an engine for the GT500, 65 HP should be the starting point. The 582 is one of the engines that can be purchased with the airframe right from Quicksilver. Also from Quicksilver, the Rotax 912 is available in either 80 HP or 100 HP. Many people like the option of having a four stroke engine, and so Quicksilver has provided that as an option. From a sales standpoint, it is much easier to simply provide for the customer what they already decided they want, rather than trying to show them that there is a better option. For the GT500, it is not only much less expensive to fly with a two stroke, but it also performs much better without all the added weight. As a Rotax dealer, I would love to sell you a $20,000 engine for your Quicksilver, and if you want one I certainly will. However, you will have a better flying airplane for a lot less money if you go with a two stroke. While the 582 is the factory option for the GT500, I would recommend the 3203 as a better alternative. The cost will be the same or less. The weight will be 25 lbs less, and you will have a better engine. Furthermore, it is much simpler to install and maintain than a liquid cooled engine. If you really want to have more horsepower on your GT500, there are several more options in the Hirth line up. The 3702, 84 HP and 3703, 100 HP are options. However, since they are liquid cooled they would not be my first choice. If you want more power than a 3203, I would recommend moving up to a 3002, 80 HP, or a 3003, 105 HP. These engines are four cylinder, horizontally opposed, fan cooled engines. They mount similar to a 912, will provide more thrust than a 912 and save around 40 lbs of weight. They are much less expensive and easier to install and operate than a 912. The TBO is 1000 hours, half of the TBO of a 912, and over three times the TBO of the 582. However, the cost of the overhaul of the 3002/3003 is much less than half the cost to overhaul a 912. It is not likely that you will fly either engine enough hours to reach TBO unless you use it for a flight school or some other commercial enterprise. If I were building a GT500 for recreational flying, I would put a 3203 on it.

Aerolite 103

IMG_6334The Aerolite 103 is a true ultralight, and therefore weight must be a primary consideration.  While many people have built the Aerolite with a 503, it will not make part 103.  However, there is no structural limitation that would prevent the use of a 503, or similar weight or power engine.  To make ultralight weight, the Aerolite can use the Hirth F33 or the Kawasaki 340/440, Rotax 447, or Hirth F23.  The F33, 447 and Kawasaki all have installation packages available directly from the Aerolite factory.  Aerolite is currently working on a factory installation for the F23.  However, it has been successfully installed on clones of the Aerolite 103 and worked well, so it should not be difficult.  However, there is doubt if it can truly make part 103.   One other option would be a Hirth inline engine.  The 2702 would be an excellent engine for this airframe, however it is too heavy to make part 103.   There is yet one more option for the Aerolite 103.  It is possible to order it from the factory with a Briggs and Stratton four stroke engine.  I personally don’t think Briggs ever made a very good lawnmower engine, so putting one on an airplane does not seem like a good idea to me.  However, it seems to be working out fairly well.  Time will tell.  The Briggs engine is 22 HP and reportedly the Aerolite will climb at 600 fpm with it.  If I were building an Aerolite 103, I would probably opt for the F23 because I really like that engine from a design standpoint.  Also, it is the highest performance engine that will (almost) make part 103 weight.  (it might also break out of the top speed of part 103 without a throttle stop though).



  1. How expensive is a Hirth 3202 with fuel injection

  2. Excellent article that explained all the questions I’ve had about two stroke engines as well as the differences between the various engine models. I am currently looking at purchasing a 103 Air Bike and from reading your article, will probably go with a Hirth F33 with EFI or a Kawasaki 440 with EFI from Heaven bound aviation. Thanks again for all the great info.

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