Light Sport Repairman Maintenance Course Schedule

Transcription

1 Light Sport Repairman Maintenance Course Schedule The Repairman Maintenance Rating allows you to perform the maintenance, the annual condition inspection, and the 100 hour inspections (required on aircraft used for hire) on Special Light Sport Aircraft and Experimental Light Sport Aircraft and charge for your services. The course is 3 weeks long. September 6- September 24, 2010 Oshkosh, WI October 18- November 5, 2010 For a list of Repairman Inspection Courses visit our website at 20 Light Aviation Edition August 2010

2 August 2010 Light Aviation Edition 21

3 e in Two-Stroke Engines by Brian and Carol Carpenter When engines fail, they have an annoying and potentially tragic tendency to do so at the most inopportune time. Of course inopportune could be any time other then when sitting on the ramp. Michael W. Brown Detonation is one cause of two stroke aircraft engine failure. In this article we will identify what detonation is, identify reasons why it may occur, what it looks like, and steps you can take to prevent it. Because of the potential for detonation to cause a catastrophic failure it is critical to understand the process. Not only can this type of a failure kill a budget in a heartbeat, but an aircraft engine failure of any kind has consequences of even greater importance, especially if it fails at one of those inopportune times. Understanding the combustion process is the first step in identifying what detonation is and how it can affect your engine. In a normal combustion process the fuel-air mixture is ignited at a specific point before the piston has reached top dead center on the compression stroke. As the fuel is ignited it propagates a flame from the original spark. Initially, this is a very small flame kernel. As the fuel burns, the size of the flame front tincreases exponentially, increasing the pressures within the combustion chamber to provide a smooth even push on the piston throughout the power stroke. Initially the pressures created within the combustion chamber continue to increase until they have reached their peak pressure. This should occur approximately 14 after top dead center (TDC). At this point, as a result of the burning fuel, the piston downward travel increases the volume within the combustion chamber at a rate greater than the pressure increase. It is important to note that fuel burns in a very smooth and orderly fashion, out and away from each spark plug up to the cylinder walls and does not explode. When grafted as in figure 1, you can see a very smooth pressure change within the combustion chamber throughout the compression and power stroke. Fig. 1 Normal Combustion Now let s define detonation. Detonation, unlike a normal engine combustion process, is a condition where the fuel and air inside of the combustion chamber spontaneously combusts or explodes rather than burning at a very specific rate. This spontaneous combustion occurs as a result of having a combination of excessive heat and pressure. In the case of detonation, the fuel within the combustion chamber simply lacks sufficient octane to withstand the extreme heat and pressure. The destructive force that can be present within the combustion chamber is a very powerful. Catastrophic failure of internal engine components can occur in just seconds. (Detonation should not be confused with pre-ignition which we will not cover here.) 22 Light Aviation Edition Augustu 2010

4 The Hirth piston from a single place helicopter shown in Figure 2, is an example of catastrophic damage caused by detonation. The operator had experienced the same scenario on three previous occasions with almost identical failure modes. In each case, the detonation occurred as a result of reducing collective on the helicopter in order to initiate a descent.when the collective was reduced, the load imposed on the engine was reduced causing the exhaust gas temperature (EGT) to increase dramatically (more about this later), and in each case initiate detonation and subsequent engine failure. The total time frame from detonation initiation to catastrophic engine failure was less than 20 seconds. Fig. 2 Hirth Piston: Catastrophic Detonation Detonation by definition is this spontaneous combustion explosion that occurs after the fuel air mixture has been ignited by the sparkplugs at the correct time. As illustrated in Figure 3, we see the ignition occurs at 25 before top dead center (BTDC). This explosion within the combustion chamber begins at a point some where after normal ignition and usually as we approach TDC. This causes a very sharp spike in the cylinder pressures. Figure 3 Detonation Detonation is the equivalent of taking a sledgehammer to the top of the piston. It provides a tremendous amount of force for a very short period of time which gets distributed throughout the crankshaft, piston, connecting rods, cylinder heads, etc. This energy is transferred throughout the components in such a short period of time that little additional force is left to push down on the piston through the rest of the power stroke. As a result, the engine produces less power. This sharp pressure spike within the combustion chamber can often be identified by a knocking or pinging noise. Those who have driven an older car are familiar with this iconic sound. Due to the high noise level associated with the two-stroke engines it is very difficult, if not impossible, to hear this pinging or knocking noise when detonation is actually occurring. It is important to note that not all detonation will result in destruction to an engine. There are unlimited degrees of detonation. One engine may run for many hours with some degree of detonation, yet with relatively minor or no damage at all occurring to the engine, while another engine experiences catastrophic failure. The pistons in Figure 4 are off of a Moyes Dragonfly with a Rotax 582 that had been used for towing hang gliders. While the piston crown on the left does not have any detonation damage, the piston on the right has been suffering from mild detonation for a considerable period of time, yet the engine continued to run without failure. Fig 4. Rotax 582 Piston Comparison The engine installation included a six bladed Ivo prop (actually two back-toback three blade propellers) which offered the tremendous amount of static thrust needed when towing hang gliders. However the amount of load imposed on the engine is significant. Even with the blades set at the lowest pitch setting, the large moment of inertia and the excessive load on the crankshaft cause continuous overloading of the engine throughout all flight regimes. This results in an increase in cylinder pressures and temperatures. It also reduces the ability of the piston to accelerate away from top dead center reducing combustion chamber pressures. If the flame front travel within the combustion chamber rises at a rate faster than the retreating piston, the amount of pressures can build within the combustion chamber facilitating detonation. The reason that we use a fuel with a higher octane rating is to slow down the flame front travel so that the pressures remain within a manageable level. This is why a Rotax 912 (80 hp) uses regular gas and a Rotax 912 S. (100 hp) uses premium gas. The only practical difference between these engines is the higher compression ratio. With a higher compression ratio, the cylinder pressures build at a greater rate than with the lower compression ratio cylinders and therefore need to retard the rate at which the fuel is burned to keep the cylinder pressures under the point at which detonation would occur. As we discussed earlier, there is a certain point at which the combination of temperature and pressure become too great allowing the initiation of detonation. In the case of the piston in Figure 4, we suspect the detonation is occurring relatively close to typical maximum peak pressure while the piston is retreating on its downward stroke. The ability to create extremely high and damaging pressures is reduced because of the retreating piston reducing cylinder pressures. However as you can see, the constant pounding of detonation has left a large divot in the top of this piston. With continual exposure to this pounding, over time, the top of the piston will eventually blow through. Looking at the underside of the same, shown in Figure 5, we see that the metal has split and is literally hours away from blowing a hole in the top of the piston. Additionally, we can see a crack propagating towards the piston port hole. The amount of carbon build up found on the bottom side of the piston is another indication that the piston has had high combus- August 2010 Light Aviation Edition 23

5 tion chamber pressures. Although it is normal to see a small amount of carbon when using a standard petroleum-based two cycle oil, the amount shown in Figure 5 is an indication that temperatures in this area of the piston have been excessive. Even under harsh conditions, as in the case of this piston, the amount of carbon build up will be significantly less when using a synthetic oil. Notice also that the surrounding portions of the piston have no carbon build up. The reason for this is based on the Rotax 582 s water cooling, the ability to dissipate heat, and the lower operating temperatures ( F) of the cylinders. Under normal circumstances with proper combustion temperatures and pressures there will be no carbon build up on the bottom of a piston even when using a petroleum-based oil. Fig. 5 Rotax 582 bottom of dented piston This piston had been installed for 740 hours. It had never before been removed for maintenance or to undergo a de-carboning procedure. Also the engine was still running fine when the damage was discovered during a routine 100 hour inspection by LSRM/ A&P Michael Zidziunas of Plant City, Florida. While looking through the exhaust port with the exhaust removed, Mike noticed this anomaly and was able to intervene before any catastrophic failure occurred. It is interesting to see that a piston suffering from this amount of detonation can continue to run without catastrophic failure (typical Rotax). We have seen this condition on the Rotax 582 in over a dozen instances, where detonation has been occurring, without engine failure. On the other hand, we ve seen just as many Rotax 582 engine failures from this same condition. Figure 6 is an example of a Rotax 582 piston operating in the same conditions that did not fare as well. In this case this engine had been suffering detonation for a shorter period of time, but the forces were strong enough that the piston fractured into several pieces and resulted in a subsequent engine failure. Fig. 6 Rotax 582 cracked piston After examining the fractured piston, we noticed that the dimensions of the piston and cylinder indicated that there was excessive wear and determined that a previous engine repair helped to contribute to the failure. The forces placed on the top of the piston are distributed throughout the rest of the piston and into the piston skirt allowing the metal to expand (due to excessive clearances) to the point of fracture. On the top of the piston in Figure 7 we see a small flat section at the very crown indicating detonation and also a crack emanating from the center of the piston outward through the piston port hole and down the length of the piston skirt. This is a typical fracture point since this is the part of the piston with the least amount of material making it the weakest point. Notice that the aluminum has reached the melting point around the periphery of the piston dome, once again, a sign of detonation. Another sign that combustion chambers have reached excessive temperatures are indicated by the orange oxide film formed on the top of the piston. These oxides are a result of two-stroke oil additives reaching excessive temperatures. In this case, Pennzoil air cooled two-stroke oil used in this engine leaves orange colored calcium oxide residue. Fig. 7 Rotax 582 cracked Pistons top showing signs of detonation. Unlike the water-cooled Rotax 582, with its lower operating temperatures, we often see different telltale signs of detonation on the Rotax 503. The piston in Figure 8, from a single ignition Rotax 503, has suffered from detonation and subsequent engine failure. This piston dates back to the mideighties when we were operating two Phantom ultralights with Rotax 503s. One of the aircraft had a single carburetor, 48 hp engine, while the other aircraft had a dual carburetor setup producing 52 hp. During an off field landing with the single carburetor Phantom, a rock had damage the propeller slightly. So we replaced the prop with one we had in stock for the 52 hp Phantom. This propeller was pitched 2 more than the propeller installed on the 48 hp version. We thought, What s the worst that could happen? We just wouldn t get as much horsepower. Even during the climb out everything seemed normal except for a reduced rpm (due to the increase pitch on the new wooden propeller). Then after the crosswind turn, when we leveled off for pattern altitude and reduced power back into the mid-range... Bang! Bang!! Bang!!! followed by silence... and an emergency landing. We had forced the engine to work too hard and that fine line between normal operation and detonation had been crossed. When the throttle was reduced into the mid range the problems ensued. The mixture is normally set up considerably leaner in this range than at full throttle where there is excess fuel for additional cooling. When the throttle position is reduce the mixture leans out in order to provide better fuel economy. The additional load and the increased cylinder head temperatures were enough to induce the detonation. Luckily, there was very little damage done to the engine and after minor repairs and reinstalling the original propeller, we were back in the air and continued to fly that airplane for several years with no other anomalies. We have seen similar cases resulting from flying with old fuel. Since the advent of unleaded fuel, the stability of the fuel and its quality degrade 24 Light Aviation Edition August 2010

6 much more rapidly than in the old days when we had Tetra-Ethyl lead as our primary anti-detonation additive. It is not uncommon for pilots, who having previously set up their engines in an overloaded configuration, to let their aircraft sit for a few months and then take off with old fuel on board and experience almost identical symptoms as Brian had in the Phantom. Fig.8 Rotax 503 detonation Remember, the amount of detonation can vary greatly. If there is something anomalous going on with your two-stroke engine it will almost always leave some indication that can be identified by pulling the exhaust and taking a look at the piston, cylinder walls, cylinder head, and piston rings. With proper training this examination of the exhaust port becomes the single most important part of an inspection on a twostroke engine. Detonation can show up in several different ways. Probably the most pronounced and significant is mechanical failure, broken pistons, cracked pistons, connecting rod failure, broken or burnt sparkplug electrodes, cracked sparkplug insulators, connecting rod bearing failures, cracked heads, cracks cylinders and broken ring lands. This type of detonation is relatively severe and normally results in a failure within a matter of seconds on a two-stroke engine. With mild forms of detonation, we have an opportunity to intervene prior to catastrophic failure. When inspecting the top of the piston dome a symptom of detonation may show up as abrasion or pitting. This may be present on the piston and or cylinder head, often showing up on the outer perimeters of the piston. Yet another continuous symptom that can occur from relatively mild detonation is deformation of the piston crown. This may show up as a dent or even a small flat spot on the crown of the piston. Here, early intervention is critical as it can be a symptom of a greater problem. Once we understand what detonation is, we can see the correlation to other problems that lead to a catastrophic failure. Understanding and early intervention may prevent your engine from failure and one of those engine out landings that always occur at the most inopportune time. Remember that an engine that is suffering from detonation will have a tendency to overheat. And conversely, like the chicken and the egg, an engine that is overheating will have a tendency to detonate. Setting up an engine and operating the engine properly is the most significant thing you can do to prevent detonation or any other type of failure for that matter. On the Rotax engines, refer to all of the wonderful documentation available to you for free online at or com. Rotax engines that are set up and operated correctly do not fail! Below is a list of some of the most common areas related to detonation problems. Proper Engine Loading: This subject is very complex and requires a real understanding of the ramification for gear box, prop diameter, number of blades, pitch, type of blade plan form, and airframe combination. This is a six hour lecture in our repairman maintenance classes. But for the sake of this article let s summarize: Over-loading of the engine causes the cylinder head temperatures to go up and the exhaust gas temperatures to go down. Underloading the engine causes the cylinder head temperatures to go down and the exhaust gas temperatures to go up. Fuel: Use fresh fuel, automotive fuel has a shorter shelf life than avgas. Automotive fuel should not be used after 30 days. Do not mix your two stoke oil with the fuel until just before you plan to use it. Use the proper fuel / oil ratio, following the manufactures recommendations. Additional oil leans out the mixture, lowers the octane, and increases carbon buildup. Carburetion: All of the engines (hundreds) that we have set up over the years were able to run properly with stock jet settings. If you are using anything other than stock settings, the problem is probably a lack of knowledge about one of the other systems. These engines work as a complete package. Changing one variable can affect several other systems. All systems have to be set up and operated correctly. Flying, without question, is one of the most indelible experiences. We can personally attest to the importance of flying in our lives. The cost of flying is unfortunately one of the variables that keep many from enjoying the pleasures of flight. This, by its nature, tends to make the option of flying behind a low cost two stroke engine a necessity for many. The two stroke engines have a varied track record. In our experience their reliability is beyond reproach. Most Rotax engine problems are created by the owner/ operator of the equipment. Once we provide proper training, the results are starkly different. In our area, two stroke engine failures have virtually been eliminated. We have been using 2 stroke engines in our business for over 20 years and have over 4,000 flight hour flying behind them... successfully. It s incumbent upon all of us to learn about the machinery we intend to operate. LEARN about the craft you are about to command. For More Information: info@rainbowaviation.com Toll-Free: FLY-LSA or August 2010 Light Aviation Edition 25