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Buying Decisions

TWO STROKE VS FOUR STROKE ENGINES

by Terri Sipantzi

Most of the planes in the Light-Light Sport Aircraft category (LLSA – these are LSAs that used to be considered “fat ultralights” and most people still think of as ultralights even though they are FAA registered and certificated LSA) used to be available with 2-stroke engine options only. However, since 2003 we’ve seen a steady migration from 2-stroke (primarily the Rotax 503 and 582) to 4-stroke powerplants (principally the Rotax 912).

Given the significant cost difference between a 2 and 4-stroke engine (the 4-strokes runs at least $10,000 more) why aren’t more people buying 2-strokes? After all, the Rotax 2-strokes (models 503 and 582) are proven engines with great reputations. Even in the sour economy of the last couple of years the only planes we’ve sold here at Precision Windsports are the much more expensive 4-strokes. Let’s take a look at why more of today’s buyers gravitate to the 4-stroke and whether there should still be a market for the 2-stroke (I believe the answer to the second question is yes and my reasons are at the end of this article).

TODAY’S BUYER

Instead of looking at the engines I’m going to start by looking at the buyers. My wife and I started Precision Windsports in 2003 near the end of the ultralight era (the Sport Pilot/Light Sport Aircraft rule became official September 2004). Rotax 912 powered trikes (the official name for trikes is weight-shift control aircraft or WSC) were just coming on the scene and buyers were pretty excited about them. Even so the market in those early days was still dominated by 2-strokes (at least 75% of the sales at the 2004 Sun ‘N Fun and AirVenture shows of well over 30 aircraft were 2-stroke sales).

What we noticed was this. Most of the 2-stroke buyers in those days still had their feet in the ultralight world and they considered the Rotax 2-strokes awfully good engines (which they are). While many thought the 4-stroke was a nifty addition to the fleet they seemed to feel that it was just more engine than necessary and they certainly weren’t interested in spending an extra $10,000 for it.

Since then we have noticed a significant change in customer attitudes. More than 60% of our new students are actually rated pilots. These folks have been flying 4-stroke powered aircraft and most are not comfortable with a 2-stroke. Since flying trikes is a recreational activity, and it is hard to recreate if you are uncomfortable, these GA (general aviation) pilots who become trike pilots are willing to hand over extra money to get a 4-stroke powered trike. These characterizations are generalizations, of course. We have sold 2-stroke aircraft to both GA and commercial pilots (two of whom are airline captains and one who also likes flying powered-paragliders), but the vast majority of GA pilots want a 4-stroke.

The 40% of our students who are not already pilots seem to have caught the 4-stroke bug as well. They’ve heard that 4-strokes are more reliable and that’s what they want. These customers also buy ballistic recovery systems (BRS or emergency parachutes) to have installed on their aircraft as well. They want to have as much fun as they can have in an aircraft that is going to take care of them. I guess I fall into this group myself. I like flying a 4-stroke aircraft with a BRS. And I came to trikes as a GA pilot.

2/4-STROKE COMPARISONS

Now let’s take a look at the engines themselves starting with the cost. As I’ve said before, the price of entry into the 4-stroke flying club is at least $10,000 more than for 2-strokes. Usually more because in addition to the engine the 4-strokes tend to come with a lot more bells and whistles (faster wings, glass instrument panels, better shocks, hydraulic disc-brakes, more comfortable seats, wings with in-flight adjustable trim, etc.).

For comparative purposes I’ve listed the latest Rotax engine prices I could find on the internet below:

• 2-Strokes

• 503 dual carb (50 hp): $5813-$7225 (price varies based on gearbox selected)

• 582 dual carb (64 hp): $7600-$9302 (price varies based on gearbox selected)

• 4-Strokes

• 912 (81 hp): $19,470

• 912S (100 hp): $21, 644

• 914 Turbo (114 hp): $35,792

As I said several times already, the price to join the 4-stroke club is at least $10,000 extra dollars. What do you get for all those extra dollars? Well, first of all there is more to the cost of ownership than the initial purchase. Over time 4-strokes will pay for themselves.

Some examples of where 4-stroke owners save money are:

Longevity – All of the LSA 4-strokes now have TBOs (time before overhaul) limits of 2000 hours and I believe they are all rated to 15 years. This means that if you are flying an LSA rated as a Special Light Sport you don’t have to overhaul the engine until you’ve fl own it for 2000 hours or 15 years, whichever comes first. Most trike pilots will reach 15 years before 2000 hours. A 2-stroke times out at six years or 300 hours, whichever comes first. That is a BIG difference. That means a 2-stroke will have to be rebuilt 6.66 times for every 4-stroke rebuild.

Operating Costs– 4 strokes tend to be more fuel efficient and do not burn oil whereas 2-strokes burn oil that is either mixed in the gas or injected directly into the engine. 4-strokes have a completely separate oil system for lubricating and cooling the engine. 2-strokes, on the other hand, require oil in the fuel for lubrication and cooling. So the 2-stroke owner is paying more per hour of flying for the extra fuel and oil burn.

Fuel Mixture Control– in some ways this is related to operating costs. The Rotax 4-stroke engines come with fuel/air mixture adjusting carburetors. These Bing carburetors automatically adjust the ratio of fuel and air to compensate for the altitude. This means better performance and better efficiency. The 2-strokes don’t do that – the higher you go the richer they run – that’s wasted fuel and poorer performance.

Maintenance – 4-strokes require a lot less maintenance. For all practical purposes 4-strokes require preventive maintenance every 100 hours unless something breaks. 2-stroke engines require maintenance at 25, 50, and 100 hour intervals. Something else nice about the 4-strokes, courtesy of the separate oil system, is that they store better and are less vulnerable to rusty bearings. On a 4-stroke the bottom of the engine is completely isolated from the atmosphere and soaked in oil. On 2-strokes both top and bottom sections of the engine are exposed to the atmosphere and since the oil is mixed with gas the oil can evaporate away leaving metal parts unprotected. Because trike engines spend a lot of time sitting idle, this is a valuable characteristic of a 4-stroke.

Reliability – while I don’t have statistics to back this up, it is widely understood that 4-strokes are more reliable than 2-strokes. There may be one or two 2-stroke fanatics that might argue this point but in all my years of flying, driving, and boating, I have yet to run into one of those fanatics. We just all know that 4-strokes are more reliable. And it is beyond the range of this article to go into the whys and wherefores. While reliability is sort of important in a weed whacker it is very important in an aircraft. When I fl y a 2-stroke (yes, I do fl y them because I think the Rotax 2-strokes are very reliable) I tend to fl y a little closer to emergency landing fields than I do in my 4-stroke.

Failure characteristics– this is related to reliability. Unless the engine blows up, 4-strokes tend to die gradually. They complain before quitting. That “complaining” may be the difference between getting back to an airport versus landing in a field or the trees. 2-strokes generally have two modes of operation – great and not at all. There doesn’t seem to be an in-between mode. A 2-stroke is less likely to warn you before it stops running. Of course there are exceptions to this generalization but the generalization still stands.

AIRBORNE’S INTRODUCES A 4-STROKE ULTRALIGHT (PART 103) TRIKE

An interesting development in this area is being introduced by AirBorne in their new Part 103 ultralight trike being unveiled this spring. AirBorne has built their trike around the Bailey 200cc 4-stroke engine. To the best of my knowledge there is only one other Part 103 aircraft flying a 4-stroke and it is also using a Bailey engine. It will be interesting to see how this little trike fares in a market that has been dominated by 2-strokes since the beginning. I’m anxious to try it out.

IS THE 2-STROKE STILL A VIABLE LSA ENGINE?

So with all that I’ve said, the question that begs answering is whether or not there is still a market for 2-strokes in the LSA world. I think that for trikes and powered-parachutes the answer will be yes for a number of reasons. First, the entry cost is so much cheaper. When the economy was rolling buyers were willing to spring for the 4-strokes. At the same time (2004-2008) there were a lot of good quality, used 2-strokes available on the market being sold by ultralight pilots who did not want to transition to Sport Pilot. Now that the economy is tighter and the used market has largely dried up I think we will see a renewed demand for the less expensive 2-strokes (and true Part 103 trikes).

While there is a difference in reliability between the engine types it can be largely mitigated with proper 2-stroke maintenance and storage procedures. I have friends with thousands of hours in 2-strokes who have never had an engine failure. These guys take very good care of their engines. On the other hand I have read lots of stories of 4-stroke failures, so they are not immune to problems (though many of these stories end with the planes landing at an airport because the engine told them it wasn’t feeling well – I’ve made two of these landings myself since I started flying in the late 70’s).

While I can’t speak for all buyers, if my decision was fl y a 2-stroke or don’t fl y at all, I’d be flying a 2-stroke right now. While I love the Rotax 912, I think the Rotax 503 and 582 are great engines and I have no problems climbing into an airplane powered by one of these well proven engines. If you want to fl y but just can’t justify the price of a 912, don’t ground yourself when the 2-strokes offer a safe and affordable way to aviate. Just be sure that you take good care of them.

ABOUT THE AUTHOR

Terri Sipantzi is a Sport Pilot Instructor and Examiner as well as a Light Sport Aircraft Repairman and DAR. Terri & Beth Sipantzi own and operate Precision Windsports, Inc. (www.PrecisionWindsports.com). Precision Windsports is a full-time AirBorne dealership, providing aircraft sales and support in conjunction with concentrated flight training. They are centrally located in Lynchburg, VA and are responsible for eastern US sales.

  

March 2011 Light Aviation Edition

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