Piston Airplane Cruise Performance

Journal of Aviation/Aerospace Education & Research Volume 4 Number 1 JAAER Fall 1993 Article 6 Fall 1993 Piston Airplane Cruise Performance Melville R. Byington Jr. Follow this and additional works at: https://commons.erau.edu/jaaer Scholarly Commons Citation Byington, M. R. (1993). Piston Airplane Cruise Performance. Journal of Aviation/Aerospace Education & Research, 4(1). https://doi.org/10.15394/jaaer.1993.1115 This Article is brought to you for free and open access by the Journals at Scholarly Commons. It has been accepted for inclusion in Journal of Aviation/Aerospace Education & Research by an authorized administrator of Scholarly Commons. For more information, please contact . Byington: Piston Airplane Cruise Performance PISTON AIRPLANE CRUISE PERFORMANCE Melville R. Byington, Jr. Ability to achieve efficient range and endurance performance can mean the difference between an uneventful flight and one which ends in anxiety or even tragedy. Beyond the economics of fuel costs, the presence of unexpectedly strong headwinds, navigational error, or deteriorating weather may test the pilot's cruise management capability. The prudent pilot will be prepared by thoroughly understanding the principles underlying cruise performance. Federal Aviation Regulations Part 61 requires Commercial Pilot applicants to have received instruction in maximum performance takeoffs, landings, climbs, and descents. Conspicuously absent is any requirement for instruction in maximum performance cruise, where the vast majority of flight actually occurs. Although the Commercial Pilot requires 50 hours of cross-country flights, there is no requirement that understanding of the principles involved be achieved. The Flight Training Handbook (1980) devotes three pages to the effects of variables, but provides no practical guidance. Advanced performance texts employ calculus techniques to derive theoretical results of little practical use to pilots. No questions or instruction on optimum cruise planning are found in Commercial Pilot study guides. In summary, the Commercial Pilot is neither required nor encouraged to gain practical competence in efficient cruise planning and management. Planning and executing efficient cruise profiles require logical integration of five variables. These are power, altitude, speed, weight, and wind. Whether the objective is saving time, fuel, or both, interdependence among the variables must be appreciated. Although the subject is complex, it can be approached logically. First, theory will be explored, then several representative airplane examples used to test the theory and examine the many tradeoffs. Procedures to minimize the adverse effects of headwinds will be presented. The following procedures provide logical alternatives which enhance safety and operating economy. The goal is a set of cruise optimization steps which can be applied before and during flight. Although a substantial level of detail is provided, it is not necessary to follow every theoretical and mathematical detail in order to apply the fundamental concepts. Aviation educators and flight instructors are the keys to propagating the required knowledge to the piston-pilot population. SYNOPSIS OF CONTENTS 1. Optimum calibrated airspeeds (CAS) for both maximum range and maximum endurance vary with weight, but each is conducted at a specific angle of attack (AOA) independent of weight. At constant AOA, optimum speeds are proportional to the square root of weight. Therefore, maintaining efficient range or endurance flight requires progressive power and speed reductions as fuel is burned. 2. Maximum endurance (time aloft) corresponds to minimum fuel flow (FF) and engine power output Page 14 Published by Scholarly Commons, 1993 required to maintain altitude. The power required for maximum endurance flight is very low, typically about 30% of rated power. For endurance, the lower the altitude the better. 3. a. Neglecting wind effects and fuel burned during climb and descent to and from cruising altitude, available maximum range is independent of altitude. b. Maximum range CAS and AOA are constant for a given weight, independent of altitude. However, true airspeed (TAS) and power required increase with altitude as density ratio (actual density compared to JAAER, Fall 1993 1 Journal of Aviation/Aerospace Education & Research, Vol. 4, No. 1 [1993], Art. 6 Piston Airplane Cruise Performance standard sea level density) decreases. The ratio between TAS and CAS is the reciprocal of the square root of the density ratio. This ratio is termed "SMOE" (which derives from ~tandard means Qf ~valuation). Table 4 contains SMOE versus density altitude in abbreviated form, but in practice SMOE normally is calculated using an analog or digital flight computer. A useful thumbrule is that (for constant CAS) SMOE and TAS increase approximately 1.5% per 1,000 feet. c. Maximum range TAS, FF, and power required all increase with altitude in direct proportion to SMOE. The key conclusion is that maximum available specific range (miles per gallon or pound of fuel) is independent of altitude. d. The common, but mistaken, belief that piston airplane maximum range improves with altitude is based either on constant power or constant TAS, neither of which provides maximum-range flight conditions. 4. Tradeoffs between speed and range (for constant weight and altitude) are linked by complex but generic relationships best interpreted graphically. See Figures 4 and 5. Moderate speed increases are possible with minimum range sacrifice. Consistent with jet transport practice, the "long-range cruise" condition is defined as that speed above maximum-range speed which corresponds to a 1% range sacrifice. Piston airplanes can fly 7% above maximum-range speed and achieve 99% of their absolute maximum range. S. Theory was compared with performance data for nine representative airplane models, as derived from their pilot operating handbook (POH) data. Deviations from theoretical performance relationships were minor and plausible. 6. In the presence of significant headwind or tailwind components, the optimum (no wind) maximum-range airspeed requires adjustment. Based on empirical data, simple and practical headwind/tailwind rules of thumb were developed. 7. Analysis of a particular airplane's cruise performance is keyed to the determination of its maximum range CAS (at standard weight). Unfortunately, this speed will not be found (explicitly) in the POH. However, four methods for estimating an airplane's maximum-range CAS (and lAS) are offered. JAAER, Fall 1993 https://commons.erau.edu/jaaer/vol4/iss1/6 DOI: https://doi.org/10.15394/jaaer.1993.1115 These are: a. listings of 10 models' characteristics (Table 2), b. Kershner's rules of thumb (1985), c. a method derived from POH performance data, and d. a method based on a quick, simple flight test. 8. Detailed flight-planning steps are provided for two common, baseline mission profil (...truncated)