Craft & Technique: Improving Aircraft Performance

evaluation of what modifications are right for you. You'll need to wade through all sorts of marketing claims, cost comparisons and, most importantly, safety and ...
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nuts & bolts

craft & technique Lookin’ for Knots Improving aircraft per formance JEFF SIMO N

S

ince the first flight of the Wright brothers, pilots have been on a never-ending mission to find new ways to go farther and faster. However, aircraft designers need to balance speed, climb, useful load, cabin space, and production cost to produce a viable aircraft. Compromises must be made. With the exception of newer aircraft such as Diamond, Columbia, and Cirrus, these compromises were made with technology from the 1950s and 1960s. Experimental aircraft use the latest technological advances but still make many compromises to appeal to a wide audience. The result is that the most common general aviation aircraft are designed to be just large enough to be comfortable, just powerful enough to keep up with the competition, and simple enough to keep production costs in line with the sales price. Pilots make the best decision possible when deciding which aircraft to buy. For some, it doesn’t take long for that age-old itch to surface, “What can I do to make this thing faster?” The good news is that an entire “performance enhancement” industry has emerged with all sorts of products to scratch the itch and squeeze out a few more knots from your plane. However, you’ll need to be careful in your evaluation of what modifications are right for you. You’ll need to wade through all sorts of marketing claims, cost comparisons and, most importantly, safety and reliability issues related to applying numerous modifications to a single aircraft. For certificated aircraft, most modifications are available via the FAA’s supplemental type certificate (STC) process. The STC process is designed to ensure that the modification is safe for the aircraft. However, the STC process

does not validate any performance numbers. It’s still up to you to determine if the modification is worth the cost. The market for experimental aircraft modifications offers both advantages and disadvantages. Advantages include greater selection, more cutting-edge technology options, and lower cost. However, without the controls of the STC process, you’ll need to be careful to ensure that the modification is safe and reliable for your aircraft. Reputation is the key. There are some reputable companies out there providing improved exhaust systems, ignition systems,

There are a number of “other companies” advertising unproven mods, which will have you working out the bugs and acting as test pilots. engines, and propellers. However, there are a number of “other companies” advertising unproven mods, which will have you working out the bugs and acting as test pilots. Caveat emptor—Let the buyer beware!

Aircraft Modifications If you want to improve the performance of your aircraft, you must either increase the efficiency of what you have or add more power. It’s that simple. Increasing efficiency: Any aircraft designer will tell you that drag reduction is the single easiest way to improve an aircraft’s performance. You get a lot more “bang for your buck” from drag reduction than adding horsepower. If you want to know why the newest line of fixedgear, composite aircraft can blow right past older retractEAA Sport Aviation

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craft & technique able aircraft, all you need to do is look at the efficiency of their respective airframes. Smooth-as-glass composite fuselage curves, laminar flow wings, and tight wheelpants all contribute to reduced airframe drag. However, less obvious advances, such as better cooling designs, can also have a dramatic effect. Whether you own a certificated or experimental aircraft makes little difference; the steps to reducing drag are the same for both:

STEP 1 CLEAN IT UP:

Chances are that your aircraft’s performance is somewhat less than the numbers in your operations manual. While it’s questionable whether any of our planes actually fly quite as fast as the original advertisements, there is a lot you can do to get close. Before you start shopping for airframe mods, it always pays to make the most of what you already have. Start with the simplest item of all: paint. Unbelievably, the speed difference between a dirty aircraft and a recently cleaned and waxed one can be as much as 5 knots! Next, examine all exterior fairings and make sure they fit as tightly as possible without any obvious gaps, cracks, or misalignment. Depending on the situation, the part can be modified to conform more closely to the aircraft, or small gaps can be filled with clear silicone. The goal is to ensure the air flows as smoothly across the airframe as possible. Don’t ignore the underside of the fuselage and wings. These oftenneglected areas are important to drag reduction. Propellers are another opportunity for efficiency improvements. All propellers are designed to exacting standards when they leave the factory, but they change a great deal in the field and become less efficient over time. Erosion is the main reason for this lost efficiency. Erosion occurs from dirt, sand, and rocks being pulled into the propeller while it’s running. This begins by wearing off the paint, but 110

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continues to gradually change the profile of the propeller as the leading edge and propeller face wear. Another way that propellers erode is through intentional, and necessary, repairs and overhauls. Every time a nick in a propeller is dressed out or the entire propeller is overhauled, material must be removed from the prop to get it back to a uniform surface. Overhauls are important. They verify the safety of the propeller, help protect it from corrosion, and return the propeller to its proper pitch setting.

STEP 2 RIG IT RIGHT: Proper rigging

is essential for top performance. If your plane is not properly rigged, your control inputs to correct things will introduce a lot of drag. Rigging requires special tools (as well as someone well-versed in their use). If you haven’t had your aircraft rigged within the last few years, the chances that it’s in spec are slim. A properly rigged aircraft should require little deflection in fixed trim

aircraft. Now do the same thing from your left wingtip. The arcs should cross exactly at the center in front of the aircraft. If they don’t, it’s time to re-evaluate that “minor damage history” note in your logbooks. Rigging of the control surfaces is next. Note that when setting the proper rigging for flaps, be sure to carefully review the manufacturer’s rigging instructions. Many aircraft flaps have a little play in them, and you need to set the position for flight, not ground conditions. If you talk to folks involved in aircraft racing, you’ll find that some aircraft flaps can be set with a slight “reflex” up that helps to reduce drag in cruise. Maule aircraft include this feature in their design to increase cruise speed, while still allowing the aircraft to be a stellar short-field performer. When you’re done with the airframe, don’t forget the engine. Aircraft engine alignment is important to ensure that the propeller is pulling the aircraft right where it should be.

There are a number of options to improve the power output of an engine. However, the simplest approach is to follow the power process from the air intake, through combustion, and out the exhaust. tabs to keep the wings level in normal flight. A small amount of rudder trim tab is okay. However, if you find that the aircraft requires significant aileron trim to fly hands-off, something is probably amiss. Your rigging check should include the fixed airframe as well as all of the control surfaces. Checking the rigging of the fixed portion of the airframe includes proper wing geometry and incidence checks. Some of these checks are simple. For example, attach a piece of string to the leading edge of your right wingtip. Bring it out to a point in front of the cowl and draw an arc on the ground at the limit of the string length in front of the

The position of the engine can usually be adjusted by means of adding or removing spacers at the engine mounts.

STEP 3 MODIFICATIONS: The available modifications for aircraft vary widely, but the simplest aircraft drag reduction modifications always involve removing things that cause drag in the first place. Antennas, steps, and “jelly jar” style rotating beacons are some of the more common items that can be improved. In one aircraft study, the relocation of three external antennas to internal positions (wingtips, rudder, etc.) led to a 5 mph cruise improvement.

Layering STCs Adding a series of modifications to an aircraft poses a challenge to

a new LoPresti cowl is installed, followed by a Power Flow exhaust

builders and aircraft mechanics. This is because it’s the installer’s

system, followed by…you get the picture. The problem is that it’s the

responsibility to ensure the new modification will safely integrate

installing mechanic’s responsibility to ensure compatibility between

with previous aircraft modifications. STC holders have only one

the STCs. This includes installation issues, safety, and reliability.

responsibility: to make sure its modification can be safely implemented

Many performance modifications are designed specifically to work

on an aircraft conforming to its original type certificate (as it left the

with stock engine configurations. Engines modified with increased

factory). Therefore, the STC holder for a cowl modification has no

compression may not work well with an electronic ignition system

responsibility to ensure that it works with a different exhaust system,

designed for the stock compression engine. Even tuned exhaust

etc. There are exceptions, but not many.

systems are designed for an expected engine configuration.

The concept of implementing STCs on previously modified aircraft

Layering STCs is not necessarily a bad thing. If carefully thought

is known as “layering STCs.” Doing it properly is becoming an

out, it can really increase the performance of the aircraft. However, it’s

increasingly complicated process.

crucial to understand what each modification is actually doing, so that

Let’s describe a real example: We start out with an aircraft that, since leaving the factory, had a different propeller installed via STC. Next,

Other modifications, such as improved cowls, sleeker windshields, wingtips that are more efficient, vortex generators, etc., can also reduce the total drag of the aircraft. Engine cooling drag accounts for a significant percentage of the total drag of an aircraft. Unfortunately, most engine cooling designs leave much to be desired. This is especially true of older certificated aircraft. Newer cowl designs employ much smaller cooling inlets and incorporate tighter baffling that does a better job of containing and directing the cooling air in to and out of the cowl. Propellers have also evolved a lot over the years with newer technologies and materials. There are a variety of propeller mods available for both certificated and experimental aircraft, and many of them can improve climb, cruise, or both. Some notable newer propeller options include MT-Propeller’s composite electric constant-speed propellers, A.M.R.& D. Inc. propeller tip mod STCs, and Hartzell’s Scimitar propellers. Adding power: Once you’ve done all you can to reduce the drag of the airframe, it’s time to add power! It’s best to start by maximizing what you already have. Be sure the engine is tuned up with the right magneto timing settings, good plugs, wires, etc. Check the engine compressions to ensure there aren’t any significant valve or ring leaks.

we can ensure that we’re helping—not hurting—the aircraft’s safety and performance.

There are a number of options to improve the power output of an engine. However, the simplest approach is to follow the power process from the air intake, through combustion, and out the exhaust. Starting at the intake, the goal is to ensure that air flows as freely as possible into the cylinder to increase the amount of fuel/air charge that makes it into the cylinder for each compression stroke. Some Mooney aircraft employ a “ram air” bypass valve that bypasses the air filter in flight, increasing the air pressure into the engine. Experimental aircraft can use a similar model and increase intake manifold pressure by 1 inch or more in cruise. One STC option for many certificated aircraft is to replace the standard air filter with Challenger’s K&N filter replacement. Aircraft cylinders can also be modified to improve power. Some O-320 Lycoming cylinders can be modified with taller pistons that increase the compression ratio from 8.5-to-1 to 10-to-1. Another popular modification is to have the cylinders “ported and polished.” Porting, polishing, balancing, and flow matching are terms used in the cylinder overhaul industry to describe the improvements that can be made to stock cylinders to increase their efficiency and power output. By smoothing the internal aerodynamics of the cylinder, the efficiency can be greatly improved. In

addition, balancing of all reciprocating engine parts reduces power losses due to vibration. Both of these engine enhancements can have a significant impact on the smoothness and power output of your engine. This process can be somewhat more of an art than a science, so it pays to go with a reputable shop that can document its actions and results. While we’re inside the cylinder, another improvement that can be made is with the ignition process. Electronic ignition systems are available that can control ignition timing to improve power. For certificated aircraft, Unison’s STC’d LASAR ignition system is the only current option widely available for many aircraft. For experimental aircraft, there are a number of options, such as Light Speed Engineering’s plasma CDI ignition system. Exiting the cylinder, we reach the exhaust system. The exhaust system is as important as any other component in the process because flow through the engine is only as good as its weakest point. Most general aviation exhaust systems leave a lot to be desired. Fortunately, some companies have stepped up to the plate to offer better options. For certificated aircraft, there’s only one place to shop: Power Flow Systems. Power Flow offers tuned exhaust systems for many aircraft that can increase engine power by up to 10 percent. For experimental EAA Sport Aviation

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craft & technique aircraft, there are many options. One of the better-known systems on the market is the Vetterman crossover exhaust system for Van’s aircraft. In addition, Aircraft Exhaust Technologies Inc. offers complete custom systems that you fit yourself, using a kit made of PVC components to model your desired system. A well-designed exhaust system can also be integrated into the exit ramp for the engine cooling air and can serve to increase the airflow out of the cowling, thereby cooling the engine. Moreover, if you make gains with cooling airflow out of the cowl, you now have the opportunity to reduce the cooling inlets. Drag is reduced and performance increases again! All of these improvements are interrelated and none works miracles alone. Together, they can make a difference, but the entire system needs to be engineered as a whole. One of the most interesting books I’ve read on the subject of improving aircraft performance is Kent Paser’s book Speed With Economy. It’s a tale of an aircraft builder’s quest for performance that resulted in a top speed increase of 57 knots, climb increase of 800 feet per minute, and economy cruise fuel consumption reduction of 50 percent. All of this was accomplished with drag reduction and engine efficiency improvements. Next time your eyes start wandering to another aircraft, do a little research into what you can do with the aircraft you already have. The answer might surprise you. If Kent can get 57 more knots out of his Mustang II experimental, I just know that there’s at least another 10 knots out there somewhere waiting for my Grumman and me! Jeff Simon is the president of Approach Aviation, a provider of educational products, tools, and supplies for aircraft owners. For more information, visit www.ApproachAviation.com or call 877-564-4457. 112

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