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. This is especially true when installing several modifications at once. It’s unlikely that adding five different modifications, each promising the magical “3 knots” will result in a 15-knot speed increase for your airplane. A wise owner once told me that the rule of thumb is actually guided simply by cash: Spend $1,000, get one knot of speed. While this may seem humorous, I’ve actually found it to be surprisingly accurate over the course of my experience in aviation.
The available modifications for aircraft vary widely, but the simplest aircraft drag reduction modifications always involve removing the 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-knot cruise improvement. While it may not be possible to relocate all of the antennas inside the aircraft structure, there are many options to replace them with low-drag variants.
Other modifications, such as improved cowls, sleeker windshields, more efficient wing tips, vortex generators, etc., can reduce the total drag of the aircraft significantly. However, they are generally much more expensive than improving on the drag caused by antennas and lights.
Engine cooling drag accounts for a significant percentage of the total drag of an aircraft. This is especially true of older certified 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 into and out of the cowl. Unfortunately, options for improved cooling airflow (beyond simple repairs to the existing baffles, etc.) generally require major surgery. LoPresti Speed Merchants have spent much of their energy focused on improvements in this area. The late Roy LoPresti was a brilliant aeronautical engineer involved in the design of Mooneys and Grummans, and focused quite a bit on cowl design and cooling efficiency. Once he turned his attention to the retrofit market, many different aircraft benefitted.
Propellers have also evolved over the years with newer technologies and materials. There are a variety of propeller supplemental type certificates (STCs) available for both certified and experimental aircraft, and many of them can improve climb, cruise, or both. Some notable propeller options include Hartzell Top Prop, scimitar, Q-tip, McCauley Blackmac, MT’s composite electric constant-speed propellers, and AMR&D propeller tip mod STCs.
There are a variety of places to go to check out the available modifications for your aircraft, including Knots 2U (Beechcraft, Cessna, Piper, etc.), Lopresti Speed Merchants (Beechcraft, Cessna, Mooney, Piper, etc.), Lasar Kits (Mooney), and Beryl D’Shannon (Beechcraft mods). That said, your first stop should always be with your aircraft’s type club. The collective knowledge of the members of an owner’s group is an invaluable way to find out what works and what doesn’t.
Once you’ve tackled efficiency improvements, the last step is simple: more power. Efficiency may be more effective, but it can’t compare to the adrenaline rush of pure horsepower!
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.