NACA Report No. 662 - Design of NACA Cowling for Radial Air-cooled Engines was issued by the United States National Advisory Committee for Aeronautics in 1939.
Contents
Summary
The information on the propeller-cowling-nacelle combinations, presented in NACA Report No. 592, NACA Report No. 593, NACA Report No. 596, and NACA Technical Note No. 620 is applied to the practical design of NACA cowlings. The main emphasis is placed on the method of obtaining the dimensions of the cowling; consequently, the physical functioning of each part of the cowling is treated briefly. A practical method of designing cowlings and some examples are presented.
When the radial air-cooled engine was first introduced, the engine cylinders were cooled by exposing them to the air stream. In 1929 the NACA reported the results of some tests in which the cylinders were enclosed by a sheet-metal ring or cowling, which became known as the NACA cowling. This cowling reduced the drag of the radial engine to less than 20% of its original value and gave sufficient cooling for flight operation. In order to improve the cooling available with this cowling, deflectors or baffles were used to guide the airflow close to the cylinders. With the combination of baffles and cowling, a large gain over the exposed engine in both cooling and drag was realized. At that stage of development, cut-and-try methods were largely used in cowling design. Often a supposed improvement in design resulted in a decrease in performance and cooling. In 1935 NACA mounted a comprehensive investigation of the cowling and cooling problem, the general purpose of which was to furnish data on the physical functioning of the propeller-nacelle-cowling unit under varied flight conditions. The information obtained embodies the detailed principles of operation. If a complete understanding of the cowling and cooling problem is obtained, determining the installation dimensions becomes simple. Since an airplane designer has neither the time nor the opportunity to acquire a detailed knowledge of every part of the airplane, he wants a simple method of obtaining the optimum cowling dimensions, and some of the more important reasons for selecting these dimensions. It is the purpose of this report to present such a method and to illustrate the method by working a few examples.
The design of a cowling may be divided into two parts, the nose section, and the exit slot. Each part may be considered separately because the functions of each part are separate and distinct. The nose section, or leading edge of the cowling, must have an opening in the center to allow cooling air to enter the engine compartment and be of such shape that it will smoothly divide the air entering the cowling from the air going around the outside. The exit slot retrims the cooling air to the main air stream and the area controls the quantity of cooling airflow.
The complement of a good cowling design is a good baffle design. A brief discussion of baffle design and dimensions will therefore be given to complete the design analysis.
From flow visualizations (using smoke) around the cowling's leading edge, three salient points may be taken:
- the direction of the airflow directly in front of the cowling is almost radial
- the percentage of the main air stream that enters the cowling is very small
- the air velocity inside the nose of the cowling is low.
These conditions indicate that the nose contour must meet the local radial air flow and have a sufficient radius of curvature to allow the flow to follow the shape smoothly and efficiently until it is flowing parallel to the main air stream; and the shape of the inside of the cowling or of anything located inside the nose of the cowling is unimportant, for the velocity is low in this region. The shape of the inside of the nose being unimportant; the only necessary dimensions for the nose section design are for the outside contour. Well-designed nose sections must therefore have a change in angular direction of approximately 90°. The curvature is determined by the length in which this angular change takes place and is governed by the distance between the engine rocker boxes and the trailing edge of the propeller. The two designs given in figure 2 are the best contours for their particular dimensions and cover the normal variation of length as encountered in practice. Either design may be used with almost identical results at speeds below 350 miles per hour. Above that speed, nose 1 is recommended, as the maximum local velocity produced by this cowling is less than that for nose 2 and, if the local velocities exceed the velocity of sound, the drag of the cowling will be multiplied many times.
The important factors in the design of an efficient exit slot are the shape and the area of the exit passage. The shape determines the efficiency of the slot; and the area, the pressure available for cooling the engine. The exit passage should be smooth, with a gradually diminishing area so that the cooling air will have a maximum speed at the exit, and should be of such shape as to give this air a direction parallel to the direction of the outside flow. For maximum efficiency in mixing the two air streams, the streamlines of the outside flow should be straight as they pass the exit passage. An example of a good exit passage is given in figure 2.
Conclusions
The coordinates for two nose shapes that can be applied to most cowling designs are given. A method of obtaining the dimensions of the exit of a cowling is presented. An evaluation of the increment of drag associated with the flow of cooling air through the engine is given. An evaluation of the increment of drag associated with the addition of an engine cowling to the nose of a streamlined fuselage is given.