FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1921
1921 - 0135.PDF
FEBRUARY 24, 1921 curves where the slot is opened and the higher values inpressure obtained at the leading edge of the aft plane. General ConclusionsThe record which has been given is one of progress in experimental work with the slotted plane. In general, theresults show that depending upon the slot shape, position, width, inclination, etc., an increase in lift coefficient of from40 to 60 per cent, can be obtained with one slot, and up to 200 to 300 per cent, with a multiplicity of slots. The dragcoefficient is slightly increased on the slotted plane with the slot closed, compared with an unslotted plane of similarcross section. The gap on the lower surface of the plane 1 // • i 1 1 • i 1 F/G.27. *——1 ^I ii I . Slotted aerofoil No. 42 makes but little difference to the drag, but any discontinuityon the upper surface is at once attended by a large increase in the drag coefficient. With flaps fitted to such an aerofoilthe necessary increases in lift coefficient can be obtained, so that a proper aileron control is still available. This is a abnormal pressure increase over the small area at the frontedge of the auxiliary aerofoil is followed by a very rapid pressure drop, the pressure on the main aerofoil only reachinga value of 1.3. This very steep pressure graduation immedi- ately results in " burbling," the maximum value of thepressure at 200 having fallen to 1.75. The same type of results are found with an ordinaryplane, except that the rapid rise in pressure of the leading edge would have taken place at a smaller angle. To prevent" burbling " it is therefore necessary to ensure that the angle of the auxiliary planes is always kept sufficiently small,so that a rapid increase in pressure is avoided. With a multiplicity of slots this is possible, as has alreadybeen shown in the case of the R.A.F./19 tests. It would appear that the rapid rise in pressure is due to an abnormalvelocity increase, with corresponding contraction of the live air stream, and that slightly farther back on the plane thenecessary velocity reduction cannot be effected without setting up discontinuity and the eddying effect known as" burbling." Effect on Design The increase in lift coefficient possible with the slottedaerofoil permits either of slower running speeds than at present, or, alternatively, of less power at top speed. Thefirst is self-evident; the second requires some explanation. In an aerofoil design with unslotted planes, the lift co-efficient at top speed is usually less than that at which the best value of lift/drag is obtained. The landing speed and d / 14 distinct advantage over the method of increasing the liftcoefficient by alteration of the flap angle, for with the flap at its maximum angle no aileron control is possible.The centre of pressure is slightly aft of the position at smaller angles on a plane of similar section, but unslotted.This result is evident from an examination of the pressure plottings which show that distribution of pressure on eachof the smaller constituent aerofoils, whilst similar to an ordinary aerofoil, result in the lift being more evenly dis-tributed over the plane. Causes oj '' Burbling ''If reference is made once more to Fig. 28, it will be seen that as the angle of incidence is increased, the pressure atthe leading edge increases very rapidly.. At 140 the negative suction on the upper surfaces of the plane reaches a valuepf i .2 for both auxiliary and main aerofoils. After this point is reached, the auxiliary aerofoil's pressure increases morerapidly, reaching 1.65 at 160 and 2.2 at 180. At 180 the maximum lift coefficient determine the value of the liftcoefficient at full speed, the drag at this latter speed— excluding body resistance for the moment—the horse-powerrequired to obtain this speed. With the slotted plane the reverse procedure may beadopted. The lift coefficient top speed can be chosen with reference to the best lift/drag ratio of the plane, and the slowspeed for alighting obtained by the provision of the necessary number of slots to give the required lift coefficient. Attop speed it will therefore now be possible to work at lift coefficients between .2 and .3 instead of the lower valueswhich call for the use of a section such as R.A.F./15 with low values of drag at very small angles. The trend of designwould therefore be toward the choice of sections with high maximum lift/drag ratio rather than fairly high lift/dragratios at low values of the lift coefficient. If, then, machines can be designed with their planes atnormal cruising speed, set at angles of incidence where the 135 ,.„# -
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
