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PERFORMANCE Interference between a Tug REFERENCES TO LITERATURE (1) Durand, W. F. Aerodynamic Theory, Vol. 1[. (2) Huls, L. L. Th., The Use of Gliders in Civil Air Transport, and a Glider AIRCRAFT ENGINEERING, Vol. XVIII, p . 199, June 1946. term W sin γ represents a correction which has an By L. L. Th. Huls, A.F.R.Ae.S. appreciable effect. In Fig. 1 the dotted line indi cates the corrected values of C , assuming the glider to have flown at a height of about 30 ft. Results of Flight Tests made in the Netherlands to Determine the above the tug. The value of y also has to be taken into account when calculating the angles of attack Influence of the Wake of the Tug upon the Glider's Performance of the glider. During the flight tests referred to in this note the mean values of y ranged from 0∙ 3 deg. at C =0∙ 2 to 1∙0 deg. at C =0∙7 . L L WHE N considering the dynamics of a The effect of the downwash of the tug has been This deviation between the direction of flight combination of a tug and a glider one is accounted for in the following way. The system and the local airstream induces an apparent apt to neglect the interference effects of vortices, shed by the tug, in its simplest form increase in the drag of the glider, which necessi between the two aircraft, especially when the can be represented by one horseshoe-shaped tates a larger tow force than is determined from towing cable has a length of several hundred feet. vortex, consisting of the circulation around the elementary considerations. That the disturbances caused by the tug have an wing and the two trailing wing tip vortices. From The method of calculation, indicated above, appreciable influence on the forces acting on the sample calculations it appears that at a consider can serve to give an impression of the magnitude glider was demonstrated by the results of flight able distance behind a wing the contribution of of the interference between a tug and a glider. It tests recently carried out by the Nationaal the circulation to the downwash is negligible cannot give more than a first approximation Luchtvaartlaboratorium (National Aeronautical when compared with that of the wing tip vortices. because in the derivation of the formulae the Research Institute), Amsterdam, Holland. According to Ref. 1 the downwash angle induced viscosity of the air has not been taken into In these flight tests measurements were made on by the two trailing wing tip vortices at a con account. The actual values of the downwash a German 'Gövier' two-seater sailplane in order siderable distance behind an aerofoil can be angles will be less than those calculated because to determine its aerodynamic characteristics for calculated from the formula: at the position of the glider the viscosity of the comparison with the results of wind tunnel tests, air will have reduced the strength of the wing tip as part of a research programme for collecting vortices shed by the tug. data on the reduction of coefficients obtained From these tests it became clear that the from wind tunnel tests to full-scale conditions. measurement of tow forces is not very well Readings from specially calibrated instruments The vertical distance z has to be measured from suited to determine the drag of a glider because in the sail plane were taken by means of camera the centres of the wing tip vortices, which have of the difficulty in accurately accounting for the recordings, in free flight as well as during aero been displaced downwards owing to their mutual interference effects. The problem of accurately plane-tows. A Stinson 'Vigilant' was used as a interference. In the case under consideration this measuring the height of the glider relative to the tug and was connected to the sailplane by a steel downward displacement amounted to some 10 ft. tug, which has an appreciable effect on the value cable about 320 feet long. A special tow-hook at CL=0∙ 7 and to 3 ft. at CL=0∙ 2 at the position of the downwash angle, and the difficulty in with built-in dynamometer was constructed for of the glider. accounting for the viscosity of the air are the main the 'Govier', giving indications of the magnitude From equation (1) it appears that the down- obstacles to using this method. T and the direction β of the tow force. wash angle varies with the distance y from the The marked increase of the tow force, due to the From the flight test data the lift and drag plane of symmetry of the tug. Assuming the glider above mentioned phenomenon, should be taken coefficients of the sailplane were calculated and it to be flying in the same plane of symmetry, the into account when considering the economics of appeared that there was a marked discrepancy distribution of the value of y along the span of transport gliders. It adds another to the dis between the values of CD calculated from the the glider can be calculated and, taking into advantages, mentioned by the author in his data collected in towed flight and those obtained account the chord distribution of the glider, a recent article on this form of air transport (Ref. 2). from free flight measurements, as is shown in the mean value of γ can be determined. Notation accompanying diagram, Fig. 1. When, however, From Fig. 2 it follows that b =semi-span of the tug. the effect of the downwash of the tug had been D= T cos(β–θ+γ)–W sin γ (2) C , C =lift and drag coefficients of the glider. taken into account, the results showed good The influence of y on the lift force as well as on L D agreement. L, D=lift and drag of the glider. the term T cos(β–θ+γ) is negligible, but the T=tow force. V=speed of flight. w=induced vertical velocity. W=weight of the glider. Wt=weight of the tug. y=horizontal distance from the plane of symmetry of the tug. z=vertical distance above the centres of the wing tip vortices of the tug. β =angle between the tow force and the chord of the glider. γ=downwash angle induced by the tug, at the position of the glider. θ=angle between the chord of the glider and the horizontal. Aircraft Engineering
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Dec 1, 1948
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