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Aviation History
1934
1934 - 1398.PDF
SUPPLEMENT TO FLIGHT FEBRUARY 22, 1934 THE AIRCRAFT ENGINEER useful for determining the range within which experi- ment may usefully be conducted, the lines of the body may be extra-polated forward past the engine to com- plete a reasonable streamline, and the chord of the ring should then be set at an angle between parallel to a tangent to, and at about 4 deg., converging rearwardly to that tangent, the tangent being taken in a plane corresponding to 50 per cent, of the chord length. Fig. 10 shows the variation in drag of an engine and streamline nacelle fitted with a Townend Ring for various chord angles of the ring itself. The curves marked 1 relate to the ring when the midpoint of the chord lies in the plane of the cylinder centre line. Curves 2 and 3 relate to the same Townend Ring moved forward by successive steps each of about 20 per cent. of the chord length, this chord length in the particular case being about 48 per cent, of the engine diameter. For the position 1 it will be seen that there is a range of chord angle from —2 deg. to —6 deg. over which the drag is practically constant, and that at —8 deg. the drag starts to rise very rapidly. For positions 2 and 3 the minimum drag has increased appreciably and the flat portion of the curve has disappeared, No. 3 showing a sharply marked minimum value of drag, the curve rising steeply on either side of this minimum. The general characteristics shown for the lower set of curves without slipstream are retained by the upper set which represents the conditions with an appropriate air- screw running at conditions corresponding roughly to climbing airspeed. With very few exceptions, it has been found that the presence of slipstream does not affect the relative merits of different ring arrangements. Fore and Aft Position of Ring The curves of Fig. 10 also indicate thn nature of the effect of changing fore and aft position of a Townend Ring relatively to the body. The results relate only to one particular type of ring used on a particular form of body, and even for that case do not extend sufficiently far to prove that position 1 is the best possible. Experi- ence, however, indicates that in nearly every case a ring which is placed with its chord extending equally ahead of and behind the cylinder centre lines will give better results than one placed in any widely different position. Chord Length The chord length required to produce a given degree of constraint on the tendency of the airflow behind the engine to break away from the body depends mainly on the lift coefficient which is developed by the ring sec- tion. Sections of the types previously illustrated which have been found satisfactory apparently develop when used as Townend Rings lift coefficients of the order of 0.5 to 0.6, and with these sections a chord length of approximately 0.5 of the engine diameter is found to give the maximum reduction in drag. The flow over the nose of a body engine combination is necessarily curved and change in chord length of a ring alters the effective angle of incidence between the ring and the airflow, therefore the effect of change of chord is not a simple effect. Fig. 11 shows the measured drag of a nacelle and the engine mounted on a wing when fitted with three different rings, (a) is a ring having a chord of approximately 0.33 engine diameter, (b) is a very similar ring with a chord length of 0.52 engine diameter, while (c) has a slightly increased chord 0.525 engine diameter and is fitted with a bulbous nose ex- haust collector. The difference between (b) and (c) has little to do with chord length, but that between (o) and (b) indicates the kind of difference which attends on change of chord length. 120 no 100 90 80 60 50 50 FIG.I! COST WITH NO TOWN END Rll*5 I WINS KL 2 This figure shows also the increase in drag due to the engine nacelle when no ring is fitted, and it will be seen that with no ring this drag increases very rapidly with increasing wing lift. With any of the rings this increase in cost of engine as wing lift increases is very greatly reduced and within the range covered by the figures has completed disappeared for the best of the three rings, i.e., (c). It may be remarked that it is a general characteristic of a good Townend Ring that it maintains its effectiveness over a considerable range of conditions. Polygonal " Rings " Fig. 12 shows two Townend Rings of identical section, chord length, and chord angle, made for use w*ith the same nine-cylinder engine. Tested on a streamline nacelle, the measured drag using the polygonal ring was
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