**Towplane and glider pilots** must thoroughly understand aircraft performance during takeoff and tow; to ensure that the operation can be safely conducted as planned, and to understand what options will be available in the event of an emergency or abnormal condition. In this instance, "performance" generally refers to the takeoff distance, angle of climb, and rate of climb. "Better performance" is understood to mean shortened takeoff distance, increased angle of climb, and increased rate of climb. The condition/configuration of the towplane, glider, environmental conditions, and pilot technique all influence aircraft performance during these phases of flight. ## Towplane ### Make/Model Obviously, different make/model towplanes will have different performance capabilities. However, even within the same make/model series, different variants can have significantly different performance capabilities. For example, Piper Pawnees were manufactured with various engines throughout the production run, from the 150hp original model (PA-25-150) to the most powerful 375hp Brave model (PA-36-375). Generally, more powerful towplanes (and towplanes with higher power/weight ratios) offer better performance during takeoff and tow. The equipment installed on a particular aircraft can also profoundly affect performance. For example, the Piper Pawnee type certificate specifies multiple propellers that can be installed; the PA-25-235 can have an 84 x 44 or 84 x 52 propeller installed, each optimized for a specific airspeed/RPM combination. ### Weight and Balance For a given aircraft, variation in weight and balance is usually determined by fuel quantity and pilot weight. While fuel and occupant loading generally account for a smaller proportion of total weight than in other aircraft types, the extremes of full/minimum fuel and very heavy/light weight pilots could account for a ~10% variation in total weight. Increasing weight decreases all aspects of towplane performance. Like all conventional fixed-wing aircraft, center-of-gravity location matters - moving the CG forward must be balanced by increasing the tail-down force of the horizontal stabilizer and elevator, which increases net down-forces, wing loading, and induced drag, thereby decreasing performance. ### Configuration Some towplane designs feature configurable high-lift devices, such as wing flaps. The aircraft manufacturer may impose limitations on flap setting for takeoff and towing operations, or selecting takeoff configuration may be at the discretion of the tow pilot. Flap extension increases both lift and drag - in general, partial flap extension greatly increases lift and slightly increases drag compared to a clean configuration, while full flap extension slightly increases lift and greatly increases drag compared to a partial flap configuration. As such, full flaps are generally limited to landing operations, while partial flap extension may be advantageous during takeoff and climb. ## Glider ### Make/Model Obviously, different make/model gliders will have different performance on tow. From a design standpoint, glider weight, drag, and allowable tow speeds will have the greatest effect on performance. Heavier designs, and designs with more drag will reduce performance, while allowable towing speeds may require the towplane to climb at a suboptimal speed, also reducing performance. ### Weight and Balance For a given aircraft, variation in weight and balance is usually determined by occupant and ballast weight. For a two-seat trainer, the difference between maximum occupant/ballast weight and a minimum weight solo pilot can account for more than a 20% variation in total weight. Like the towplane, increasing glider weight decreases performance during takeoff and tow, as does moving the CG forward. ## Environment ### Density Altitude The ambient air temperature and pressure determine the density of the air, often described in terms of **density altitude**. Higher temperatures, higher airport elevations, and lower pressures result in less dense air (*higher* density altitude); leading to substantial decreases in all aspects of performance. Additionally, towplanes with naturally aspirated engines will experience a slow decrease in power as altitude increases during climbs. ### Wind Ideally, takeoff and initial climb are planned *into* the wind - at a given airspeed, a stronger headwind results in a lower groundspeed, leading to a shorter takeoff distance and steeper angle of climb. Calm winds, or a tailwind, will increase groundspeed relative to airspeed, and lengthen the takeoff distance and shallow the angle of climb. ### Runway Surface Runway surface construction, composition, and conditions, can significantly affect takeoff performance. An upsloping runway will increase takeoff distance, while a downsloping runway will decrease takeoff distance. Hard surfaced runways, such as asphalt or concrete, offer the least rolling resistance and best takeoff performance, while soft surfaces such as grass or dirt often have irregular surfaces and greater rolling resistance that degrade takeoff performance. Day-to-day variation in runway conditions, such as grass height, soil compactness, moisture or puddles, and bumps and ruts in the runway surface have varying degrees of influence on rolling resistance and surface irregularity. Be sure to thank your airport grounds maintenance crews! ## Pilot Technique ### Airspeed With all other factors remaining constant, the towplane can optimize climb performance in one of two ways: - Maximum angle of climb will result at the airspeed where the towplane produces the most excess **thrust** - Maximum rate of climb will result at the airspeed where the towplane produces the most excess **power** In most cases, the towplane manufacturer does not provide performance data for these situations, so a speed is selected that provides the towplane with an acceptable margin above stall and adequate engine cooling, and does not exceed the glider's maximum aerotow speed. Many operators establish baseline tow speeds that work well for their fleet of towplanes and gliders; glider owners should be familiar with the performance and limitations of their aircraft, and discuss specific needs and concerns with the tow pilot prior to launch. Additionally, environmental factors such as turbulence may necessitate using a higher than optimal airspeed on tow, to ensure both pilots retain adequate control authority and margin from a stall. ### Tow Position ![[GFH tow positions.png]] ![[PHAK pitch stability.png]] In general, [[Maintaining Tow Positions#Low Tow Position]] is more efficient than [[Maintaining Tow Positions#High Tow Position]]. In the low tow position, tow rope tension provides a downward acting force on the tail of the towplane, reducing the aerodynamic tail-down force usually supplied by the horizontal tail surfaces, and improving the towplane's aerodynamic efficiency. While the low tow position is typically reserved for special circumstances, this also means that an excessively high glider position in high tow will require an **increased** aerodynamic tail-down force by the towplane, and a corresponding **decrease** in aerodynamic efficiency - not to mention the more immediate concern of the glider causing a towplane upset! In addition, the sideslip experienced by both aircraft during extreme lateral position movements will decrease aerodynamic efficiency and reduce performance. ## References - FAA, *Type Certificate Data Sheet 2A8*, https://drs.faa.gov/browse/excelExternalWindow/32C3CC4AE2153FBB862587130048EF78.0001 ### by Kevin Sakson