November 4, 2013
By Jeff Simon
Once you’ve done all you can to reduce the drag of the airframe, it’s time to add power! Again, it’s best to start by maximizing what you already have. Be sure that the engine is tuned up with the right magneto timing settings, good plugs, wires, etc. Check the engine compressions to ensure that there aren’t any significant valve or ring leaks.
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” valve that bypasses the air filter in flight, increasing the air pressure into the engine by as much as one inch of manifold pressure in cruise. Even without structural mods, you can improve intake airflow. Begin by ensuring that your current air filter is clean and passing as much air through as possible. If you’re interested in improvements beyond stock, supplemental type certificates exist for many certified aircraft to replace the standard air filter with Challenger’s K&N filter replacement. This is an effective modification brought from the automotive world into aviation.
On the fuel side, GAMI offers custom-balanced fuel injector sets to greatly improve engine efficiency, also making lean-of-peak operations possible. The other side of their business, Tornado Alley Turbos, has a variety of STCs for adding turbochargers to increase power at altitude. This is especially effective in increasing maximum cruise speeds, albeit at higher altitudes.
Aircraft cylinders also can be modified to improve power. For example, some Lycoming 0-320 cylinders can be modified with taller pistons that increase the compression ratio from 8.5:1 to 10:1. 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 their 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 certified aircraft, Unison and ElectroAir offer STC’d electronic ignition systems that can control the timing and spark duration to improve efficiency and power output. For experimental aircraft, there are a number of options, including ElectroAir, Lightspeed, and E-Mag.
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 aircraft exhaust systems leave a lot to be desired. Fortunately, there are companies that have stepped up to the plate to offer better options. For certified 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. As you would expect, the options for experimental aircraft are abundant. One of the more well-known systems on the market is the Vetterman cross-over exhaust system for Van’s aircraft. In addition, Aircraft Exhaust, Inc. offers complete custom systems for experimental aircraft that you fit yourself using a kit that they ship made of PVC components for you to model what you want.
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. And, 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!
Complete engine upgrade STCs also exist which can radically transform the performance of an aircraft; some even going so far as to transform an aircraft from a 100LL-burning “piston pusher” into a Jet-A Turbine “cruise missile” for the flight levels.
All of these improvements are interrelated and no single one works miracles alone. Together, they can really 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 Ken Paser’s book Speed with Economy. It’s a tale of an experimental aircraft builder’s quest for performance that resulted in a top speed increase of 57 knots, climb increase of 800 fpm, and economy cruise fuel consumption reduction of 50 percent. All of this was accomplished with drag reduction and engine efficiency improvements.
So, next time you’re 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.
Jeff Simon is an A&P mechanic, pilot, and aircraft owner. He has spent the last 14 years promoting owner-assisted aircraft maintenance as a columnist for several major aviation publications and through his how-to DVD series: The Educated Owner. Jeff is also the creator of SocialFlight, the free mobile app and website that maps more than 5,000 aviation events. Free apps available for iPhone, iPad, and Android, and on the Web at www.SocialFlight.com.
Aircraft Power and Fuel,
If you own an aircraft produced in reasonably large numbers, you can be assured that there are companies marketing modifications to improve performance. However, it pays to be a bit skeptical when evaluating any specific speed gains promised.
If your plane is not properly rigged, the aircraft is fighting against itself in flight.
Chances are that your aircraft's performance is somewhat less than the "book" numbers in your operations manual, but there is a lot that you can do to get reasonably close. A fresh wash and wax could give you an extra 3 knots in cruise.
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