continued from part Yesterday we asked how one should compare the lift and drag characteristics of annular wings and flat plane wings, mentioned that they are typically compared only according to their planshape area, and suggested that this can lead to some misleading conclusions.It's like this: when measuring the planshape area of an annular wing one is not measuring the true size of the wing. . .. . unlike a flat plane wing, where the planshape area captures accurately the size of the wing.For example a rectangular flat plane wing with a span of ten inches and a chord length of 4 inches has a total area of 40 square inches, whereas a circular annuular wing with the same span and chord length has a total wing area of 128 square inches and a half pipe annular wing of the same diameter has an area of 64 square inches.This becomes interesting when we see that the official word on annular wings is that they produce less induced drag, but more skin friction drag. Then the conclusion is drawn that because skin friction drag increases with speed while induced drag reduces with speed that therefore the annular wing is only suitable for low speeds.That is the erroneous conclusion. . . .. . . it is erroneous because they compare wings of two different sizes. . . . and bigger wings have exactly the same properties which they describe: Less induced drag at low speeds and higher skin friction drag at high speeds.Now a bigger wing also produces more lift, so if my contention is correct then we should expect that the annular wing being compared should also produce more lift than the flat plane wing, simply because it is bigger than the flat plane wing which it is being compared with.Is this the case? Does an annular wing of a given span and chord lengthproduce more lift than its flat plane equivalent ? Yes it does, but this fact has been carefully hidden in the literature on the subject.Ok, so how is it that this has happened ? What has been done is that during comparisons the angle of attack of the annular wing is lowerd until it produces the same amount of lift as the flat plane wing. At the same angle of attack as the flat plane wing,the annular wing would produce more lift, so they simply lower the angle of attack in order to hide this. Since annular wings operate at lower angles of attack than flat plane wings anyway, this trickery is effectively hidden .There are several consequences of this lowering of the angle of attack of the annular wing during comparison:1) The fact that the annular wing is simply bigger, is hidden.2) The induced drag of the annular wing is lowered, this would be the case with a larger flat plane wing also ( when the angle of attack is lowered so is the induced drag and the lift.So in order to make comparison with a flat plane wing in this case we need to choose an annular wing with a smaller diameter and the same surface area. Doing so will mean that the annular wing has the same skin friction drag at high speeds as the flat plane wing, and thus will have no disadvantage at higher speeds.One might argue that in terms of lift the annular wing is effectively only as big as its planshape area, so therefore it is correct that it is compared on that basis. That is not correct however, as the evidence shows that the annular wing produces more lift than its planshape area suggests. . . .. . the total wing area contributes to lift because the wing is an enclosed tube.
A related subject which we will deal with tomorrow is that of fin size generally ( not specifically annular wing size, but all fins ) : At a first glance it might appear that small fins are better for high speed and large fins are better at low speed, due to the fact that large fins produce more skin friction drag and that this increases with speed, whereas at low speed they have less induced drag . .. . . and since induced drag diminishes as speed increases the induced drag advantage is lost at high speed . The answer is ' Yes for aircraft but not necessarily in the case of surfboards'. . . . . . . .. ...
A related subject which we will deal with tomorrow is that of fin size generally ( not specifically annular wing size, but all fins ) : At a first glance it might appear that small fins are better for high speed and large fins are better at low speed, due to the fact that large fins produce more skin friction drag and that this increases with speed, whereas at low speed they have less induced drag . .. . . and since induced drag diminishes as speed increases the induced drag advantage is lost at high speed . The answer is ' Yes for aircraft but not necessarily in the case of surfboards'. . . . . . . .. ...
