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AVIATION CONSUMER INTERCOOLER COMPARISON

Shedding some light on what they are supposed to do and whether they are worth the expense.

A Two-Part Series

By Douglas S. Ritter

 PART I

Intercooling may be one of the most misunderstood subjects in aviation today. Sure, the basic concept seems easy to grasp: lower the induction air temp and get better engine performance. But some important stuff escapes most of us.

In part, that's because even the intercooler experts disagree about the details; and with intercooling, the devil is in the details. Before you can decide whether intercooling is right for you and before you can select the system that is best for you (when you have a choice), you need at least a cursory introduction to the subject. And to get the most out of this equipment once installed and avoid disappointment also requires more intimate knowledge.

Antiques

Big Temperature Hike

Because they are so far down the efficiency curve, they generate a large temperature increase in the compressed air they produce. In fact, they do more than is theoretically necessary. At lower altitudes there is a relatively small rise in induction air temperature because the turbo, in most installations, isn't doing much work. It isn't compressing the air a great deal because it has to overcome only whatever small pressure losses are there due to intake design and related losses. Of course, in a fixed wastegate installation such as the Mooney 231 and Piper Turbo Arrow, Dakota and Seneca, the turbo is compressing air no matter what. But it still doesn't work quite as hard because the air is naturally denser.

At higher altitudes, compressors must work harder to maintain manifold pressure. As you go even higher, it must work harder and harder to maintain manifold pressure because the ambient air is less dense, and the ratio between the ambient intake air pressure and the manifold pressure is increased. Harder work and more compression equals more heat.

The high temperature of the compressor discharge air causes numerous problems. Some are pretty obvious, some are not. First off, the high intake air temperature causes an increase in cylinder head temperatures. Different engines and different installations may be affected more than others. During climbs and at higher altitudes the increase becomes significant because the cooling air flow over the engines is reduced, and on many turbocharged (non-intercooled) aircraft these high CHTs limit the altitude and climb capabilities of the aircraft.

Inherently Poor Cooling

In other words, their performance is cooling limited. Since most modern aircraft powerplants have inherently poor cooling and there is only so much that can be done with cowl/nacelle and baffle design, some other approach must be found to cool the engine. One common method is to increase the fuel flow to the cylinders. The extra fuel helps cool things down, at the expense of increased specific fuel consumption, and sometimes decreased lubrication and increased friction as the excess fuel washes oil from the cylinder walls

The high temperatures cause another problem. Care must be taken to prevent detonation (rapid uncontrolled burning). The detonation margin drops significantly in proportion to increased engine temperatures. In fact, for engine certification the FAA requires a 12-percent increase in fuel flow from that required at the detonation limit - just to be on the safe side. Hugh Evans, engineering guru at Merlyn Products, explains that "detonation is not an all-or-nothing situation. It is a gradual process" of increasing severity as engine temps rise.

Evans claims some engines, like those on the Aerostar 601P, operate normally in a state of low-grade detonation. On some aircraft it is the detonation limit that restricts altitude. If you reduce the temperature of the inlet air, then the detonation limit is raised, sometimes dramatically. This not only offers an increased safety margin, it also allows more aggressive leaning and lower specific fuel consumption.

Critical Altitude

Finally, hot air is of course less dense than cool air. Hot, low-density air has a negative effect on power that is readily apparent when you use carb heat and the RPMs drop. Lowering the temperature of the inlet air results in denser air, and that can make a big difference in performance. On some aircraft where critical altitude is relatively low, one of the immediate benefits from supplying cooler intake air is an increase in critical altitude. This can become an important factor when operating in above-standard day temperatures.

The obvious solution to all these problems is to lower the temperature of the compressed inlet air. We do that by inserting an intercooler between the turbo compressor that compresses and heats the air, and the engine inlet where it goes to work. The intercooler is, essentially, a radiator. In aircraft we use air for intercooling, though automobiles may use water.

Efficiency Levels

When intercooler manufacturers talk about "efficiency," they are referring to how effective the intercooler is at transferring heat and thereby lowering the temperature of the hot air. A 100% efficient intercooler would lower the temperature of the hot induction air to the same temperature as the cooling air flowing through the other side of the intercooler. From a practical standpoint, efficiencies in the 70% range are the norm.

One of the complications involved in designing an intercooler is setting the size and dimensions of the unit as well as the spacing of the passages in the core. This nearly always involves a series of compromises. Space is generally very limited under the cowl, and its shape is rarely ideal. Sometimes it is necessary to use two separate units because there simply isn't a single space big enough.

Having sorted out a location and determined the available space, the next step is to determine the size and spacing of the passages in the core. Typically the core is symmetrical, with the same spacing on both sides. This is the least expensive way to do it. When the simple arrangement doesn't yield a big enough temperature drop and low enough inlet temperatures, then the engineer may try increasing fin count to help increase heat transfer. In some cores the fin spacing on both sides may be different, inevitably boosting the cost of the unit.

Back Pressure

However, if you restrict the airflow too much by narrowing the passages on the compressor side, then you build up back pressure. Too much back pressure creates extra work for the turbo as it tries to overcome the blockage, generating more heat for the intercooler to deal with. This is one of the most difficult compromises during the design process. It is often less a matter of engineering than black art. Each change creates other changes that may or may not be beneficial, depending on the installation. The cooling air side is the most challenging. Most designers believe some back pressure is necessary to ensure the air flow through the core. Depending on the velocity of the air, you can expect some pressure drop from one side of the core to the other. However, in some installations where the geometry and core size work out, the designer relies more on ram air effect, and there is little or no pressure drop across the core.

A key factor is the airspeed through the core. The air needs to be in contact with the metal long enough to transfer heat. Too fast a flow for the core design and not enough heat will transfer. Too slow an airflow can cause problems with stagnation and drag. Trying to move too much air through a fairly open core can also cause problems because the ratio of airflow moved to heat transferred can get very small. Given the limits in exhausting cooling air in most installations, that can create more drag. But like everything else, it depends on how the rest of the design works.

The Drag Factor

There is also another factor to consider here. It is often argued that an intercooler inherently produces more drag because it puts an additional object in the airstream. While some installations probably do contribute some small amount of drag, that isn't necessarily so. With careful design, the net aerodynamic effect of an intercooler installation does not have to extract a drag penalty, or it may be negligible.

On some installations the cowling and baffling is extensively reworked, and the entire engine cooling geometry is rearranged so that overall drag is reduced to a considerable degree. The Turboplus installation on the Piper Lance/Saratoga is the best example. Evans also claims that proper design of the cooling air intake, core and exhaust can cancel out any drag. He argues that a good design will result in an increase in the velocity of the air exiting the core, compensating for any drag.

Since, the core supplies heat to the air, this causes it to expand. Direct the exhaust to the rear, and the theory is that you have a kind of very low-powered jet engine.

The layout of the installation naturally has a significant effect on the drag quotient. Both ease of installation and cost are also related to whether the cooling air is ducted from inside or outside the engine nacelle. On most installations a duct of some sort is added to the cowling to collect ambient air for cooling. In most cases this will require some metal or fiberglass work and repainting. Depending on the condition of the aircraft and how well you want the paint to match, this can vary from a few hundred dollars on up.

Inside or Outside Air

Pop-Off Valves

The location of the "pop-off" valve is another installation feature to keep an eye on. This pressure relief valve prevents damage from overboost caused by faulty controller or very cold day on automatic wastegates, or from a heavy hand on turbos with fixed wastegates. For fixed wastegate installations such as the Mooney 231 and Piper Turbo Arrow, Dakota and Seneca, this is pretty critical because it is so easy to overboost, particularly on takeoff. For the rest, with automatic controllers, it is a much less critical issue.

The position of the pop-off valve is a pet peeve of Genevro, who faults positioning the valve downstream from the intercooler. Let's look at what can happen. Say, for example, the engine is originally limited to 41 inches. The overboost valve is set at 42 inches. Then you install an intercooler. The engine now produces maximum power at, say 38 inches because of the lower inlet air temps. You can see that the engine could be overboosted unless the relief valve is located where it sees the hot side of the inlet air before the intercooler and the pressure drop across the core.

Airflow Systems, Riley International (on the "T" series singles) and Merlyn Products (on the Aerostar) put their pop-off valves before the intercooler. The rest install them after.

Extra Weight Penalty?

Setting Power

Since (inter)cooling the air changes its density, the kit supplier has to give the pilot a way to set power. With the exception of TurboPlus, all supply new power charts approved as supplements to the POH or Flight Manuals, or they're certified to use the original power charts with no limitations. TurboPlus, with one exception, requires the pilot to use a differential temperature gauge and calculate the power using a formula. More on this later.

Setting power with an intercooled engine has been the subject of considerable controversy, involving the method used by the pilot to set power. Developers generally use two different ways to demonstrate to the FAA that the modification does not increase power above the original certification.

The simple method that many use (Turboplus and American for example) is to rely on the formula for standard temperature compensation from normally aspirated engine power charts. This reduces MAP by a percentage based on a reduction in OAT (derived from the formula that X = square root of the temperature ratio). This is used to arrive at power settings with the lower inlet air temps which are computed to be equivalent to the original power settings. It's pretty straightforward, but not necessarily accurate.

Jim Cristy of American Aviation and Aerostar defends their use, saying that it is the easiest and simplest method, and as long as the FAA approves, he doesn't see the need to go to extra trouble searching for alternative methods.

Harmless, but Penalty-Prone

Evans suggests that these computations are, at best, merely a poor estimate of what happens and don't necessarily relate to modern turbocharged engines. He says, "the FAA hasn't argued with it because it is very conservative." The good news is that because these figures are generally conservative you aren't likely to hurt anything using them. The bad news is that by being conservative, you may pay a penalty. Your actual power may be lower than you compute. This will vary from installation to installation, and in at least one installation there are claims it results in increased power.

Whether it really makes a big difference is open to debate. Personally, we prefer an alternative for the simple reason that experience suggests that formulas like these lack precision. They also weren't intended to be used for this purpose.

The alternative is to actually measure horsepower in flight and use these measurement to arrive at new power settings. Most developers, like Evans, who use this method rely on a torque meter installed between the prop and the engine. Using these torque meters properly is not easy, because they are temperamental and cause other problems. Many people don't really like them, but use them for lack of an alternative.

Genevro disliked them enough to come up with another alternative. He designed a pressure transducer which fits into a spark plug and measures the actual pressures in the cylinder. From this you can calculate where you are on the power curve. The disadvantage is the need to establish a baseline from which to measure. This requires testing a number of unmodified aircraft. But, once you have that baseline, it can accurately let you know what the engine is doing.

Max Power Limits

Another controversy revolves around the maximum power limit. One side says this is the power available as installed in the certified aircraft. Due to installation losses created by induction systems, exhaust and other factors, this often results in less power than the level to which the engine was certified on the factory's test stand. On some engines there is no effect, or the loss of only a few horsepower. On others it can be as high as five percent. The other side argues that maximum power is the certified limit of the engine on the test stand.

It's pretty easy to see where all this leads. When you are pulling maximum power for takeoff, the few extra horses you can now get because of the intercooler can make a difference that looks good in the advertised claims. We don't feel this is a critical issue, except for those who feel they have been stuck by the FAA with certifying it one way while others have managed to get by certifying it another way. We agree it would be nice if the agency were consistent in its interpretation. You can make an argument for either side.

From a practical standpoint the concern is whether you could damage the engine or cause other problems because you are pulling a bit more power. We don't believe there is really much to worry about, unless the overboost valve is after the intercooler, and then there is some slight possibility for serious excessive overboosting under some circumstances, particularly in colder conditions. In most cases the power boost is relatively small and generally only used for very short periods on takeoff. Jack Riley, Jr. of RAM Aircraft Corp. disagrees. He contends that in his experience some intercoolers installations increase power so much that the engine is more likely to sustain damage. If you are concerned, just be careful to abide by the original power limits and don't aim for increased takeoff performance.

Danger of Overstress?

This brings us to another side of the same question that is often debated and which has recently received a fair bit of press. Do some intercooling installations cause you to produce so much more power as to overstress the engine? RAM Aircraft recently warned its operators: "There is a limit to the amount of horsepower a turbocharged airplane engine is certified to develop, and that limit can be exceeded by the addition of extra intercooling." As we see it, the operative word here is "can." If the pilot ignores the new power setting charts, that certainly can happen.

But the new charts are predicated on developing equivalent horsepower. So, 65% with intercooling ought to be the same as 65% without intercooling or 65% with additional intercooling. The problem occurs when the new charts are not derived from actual testing, but from computations using the temperature compensation formula or some modification of this.

As we noted, generally this should be a conservative figure, but RAM claims that in the case of American it is not. American does not empirically test its mods. RAM claims that their actual inflight testing with a torque meter shows that pilots are pulling more power than they think they are when using American's charts on RAM's conversions. If that were the case, then there could be problems, as RAM alleges.

American argues that they derived their power charts from approved data, and they feel RAM is just blowing smoke. Be that as it may, from the data derived from those tests, RAM has developed its own FAA approved supplement for the AFM which allows for the increased power they observed with the American intercoolers installed.

Bottom Line

There is considerable disagreement between the experts we spoke with about this controversy and we will be looking into this further. We think the bottom line for this particular situation is that, for now at least, owners of American intercooled RAM aircraft would be prudent to rely on RAM's new charts, at least until more light is shed on this subject.

We feel this is a peculiar situation, and even though we prefer empirically derived charts, generally, if you fly according to the certified power charts, then you shouldn't have a problem. That is the crux of the matter. If you fly with the original power settings and the new intercooler is doing its job, then you will probably go faster because you are now flying at a higher power setting. Also, be careful not to do things that would cause you to exceed the rated power on which the charts are based. For example, if the POH or AFM says to use full rich on takeoff, don't lean to max power because then you are going to pull more power and possibly damage the engine. Just don't do it.

But that leads to yet another controversy. You are not required to abide by the power charts or Turboplus' conversion factors. The FAA is really concerned only about maximum power. It doesn't get involved in the question of intermediate cruise power setting. That is the purview of the engine manufacturers. So, there is not much to stop you from flying at the original MAP and fuel flows and going faster - in some cases, going lots faster. But, as we have seen, what you are really doing is pulling more power - in some cases, lots more power. We don't think ignoring the approved power setting procedures in an effort to go faster is a particularly bright idea. Consider that the engine is certified to last only 150 hours at maximum power. Now, do you really want to go faster that bad?

More Bad than Good?

The difficulty with determining whether or not an intercooler is doing its job or is itself creating a problem is highlighted by a letter we received from one owner of a B36TC Bonanza. He complained that the intercooler did not lower his high temps, but instead raised them further, making the problem worse. He went on to lambaste the intercooler manufacturer and finished by noting that his problems seemed to be solved after he installed a new Continental reman, removed the intercooler and installed new silicone rubber baffles.

Well, the B-36TC has gained a reputation for high-temp problems that are difficult to diagnose and which often go away when the engine is overhauled or exchanged. On this model, especially, the problems sometimes can be traced to things like poorly fitted baffling or a bad or poorly overhauled controller. And when we looked into this particular situation, we discovered that the customer had never complained to the intercooler manufacturer during the five years the intercooler had been installed. The kit manufacturer reported that in a few other similar instances which were brought to their attention they were able to locate the source of the problem, which did not involve the intercooler, and solve it.

There are a couple of lessons here for all of us. If you feel that you are having a problem with the intercooler or because of it, contact the supplier. You paid big bucks for this thing; if it doesn't work, make sure they know about it and fix it. Odds are they have run into a similar problem before and will be able to help you correct it. There are lots of reasons for overheating problems on turbocharged engines. Some are pretty obvious and some are downright difficult to ferret out. Sometimes installing the intercooler without thoroughly examining all the other myriad possibilities. An intercooler won't fix something that isn't working right to begin with.

Extravagant Claims

Over the years, some intercooler promoters have made some outrageous claims. Some may still stretch the truth a bit, but by and large they are a bit more realistic, and the focus is on the benefits of a cooler running engine. Experts debate whether claims of more speed or better climb performance are true or false. The letters we have received generally confirm noticeable improvements in climb performance. This is especially true for hot day takeoffs and climbouts.

But we'd be skeptical of claims for dramatic speed increases, and our feedback on this point is pretty consistent. Many do note that they can get full cruise power at maximum altitudes - something they often could not prior to the installation, and that would certainly yield more speed. Finally, many reported reduced fuel flows. This makes sense if the cooler temps allow less excess fuel wasted trying to keep temps down.

Primary Justification

From our viewpoint, the primary reason to purchase an intercooler is to lower temperatures and potentially prolong engine life. We feel that's reason enough to purchase one. Reduced fuel flows and any higher speeds are an added benefit when they occur. Non-intercooled turbocharged engines have a deserved reputation for not making TBO. That doesn't mean they cannot. It just means that most don't.

With a little luck any of these engines might make TBO provided the operator is willing to baby the engine or, at least, be extremely careful operating them. That often entails higher fuel flows, lower climb rate, decreased cruise settings and other compromises. These negate many of the reasons for flying a turbocharged aircraft in the first place.

An intercooler can often restore the performance that should be there under all operating conditions and allow the engine to deliver that performance, and perhaps longer life, at the same time. That doesn't mean you can abuse the engine, only that you don't have to baby it so much that you lose much of the utility. These are reasons that contemporary aircraft designs now come from the factory with intercoolers. The factories and engine manufacturers finally realized that the extra weight and expense were more than worth it.

Should You Go For It?

We're convinced that an intercooler can be a beneficial addition to your turbocharged aircraft. However, unless money is no object, there are other considerations. The type of aircraft and the kind of flying you do can have a lot to do with your decision.

While it has always seemed illogical to us to spend the money for a high-flying, expensive to maintain, turbocharged aircraft and then just fly around at lower altitudes to avoid using oxygen, some owners do just that. If that is how you fly and if you aren't likely to change your habits, intercooling probably doesn't deliver enough benefits to be worth the cost. The only exception might be if you fly a lot in the desert where density altitude considerations during the summer can effectively put you at altitude way before you really get there. Those who fly hot-running aircraft like the Piper Turbo Arrow and Lance/Saratoga, Mooney 231 or Cessna P210, especially at high altitudes, are the very best candidates for intercooling.

Owners of pressurized aircraft also have obvious benefits to reap. For these categories of aircraft, these mods, used as they were intended to be, may pay for themselves in fuel savings and longer engine life. For these aircraft we strongly recommend intercooling.

Some turbocharged singles seem to get along okay, even without intercooling. They may be marginal at times, but they don't have serious problems under most conditions if you are a conscientious pilot. The T-210 and Beech Bonanza come to mind. These aircraft will certainly benefit from intercooling, but they don't need it as badly, and the value isn't quite as clear. If we flew one of these types, we would probably go ahead and do it unless there were mitigating circumstances.

Like many aftermarket mods, the initial expense of intercooling isn't always returned in full at resale. If you are planning on selling anytime soon, new intercooler installation probably isn't a very good idea. On the other hand, if you are buying, an aircraft with intercooling already installed, it may be a good value, all other things being equal.

Now that we've taken a look at the technical side of intercoolers and discussed the practical aspects, we can look at what is available out there on the market. Six companies offer kits that cover almost all the common turbocharged light aircraft that were not originally equipped with an intercooler.

In a few instances one modifier has developed more effective intercoolers for aircraft already intercooled by the factory. The most popular aircraft have kits available from more than one source and in some cases there are significant differences between them. In the second part coming in the June 1/15 issue we'll analyze and compare them all and offer our recommendations.

PART II

As we noted last month in our introductory article, a fairly good selection of companies will happily hang a bunch of tin mongery in your airplane - for a handsome price - claiming zephyr cooling and a long, happy life for your turbocharged piston engine.

In a few instances, more effective intercoolers have been developed for aircraft already intercooled by the factory. Kits are available for the most popular aircraft from more than one source, and in some cases there are significant differences between them. Making a selection for you aircraft can induce shopper's overload.

After inspecting the plumbing, debating the installation ingenuity and flying the aircraft, we offer these recommendations.

Airflow Systems offers three intercooler kits. Those for the Piper Turbo Arrow and Turbo Dakota were first on the market in 1987. Their kit for the Mooney 231 received its SSTC in 1989.

The differences from the competition are pretty obvious. First off, Airflow is $600 cheaper on the purchase price, and the installation time is less than one half, which translates to an even bigger savings. For the 231, another advantage is that it requires no painting. Also, they install the pressure relief valve ahead of the intercooler. We agree with their contention that this is important, especially on these aircraft. Finally, Airflow's power tables were developed empirically (with actual test, not just paperwork estimates), using their proprietary BMEP gauge.

We flew the Mooney installation and were impressed by its performance and low workload. With power charts provided by Airflow, setting accurate power is simple compared to the Turboplus installation.