motorgliders as towplanes

On 19 Mar, 19:48, Martin Gregorie
wrote:

There is a question in the UK Bronze badge written paper about the
proportion of lift provided by the top and bottom surfaces of a wing
that's just as wrong. The so-called "correct" answer is 70/30, but as a
wing is a device for imparting momentum to an air mass its a meaningless
question.

Oh no. Not that one.

At the surface of the wing, it exerts a force on the air mass. A long
distance away (typically 2 chord lengths) it's a momentum change. In
between the effect of the wing is a pressure change /and/ a momentum
change.

Overall, the integrated pressure across the top surface is about 70%
of the total lift force, and the intergrated pressure across the
bottom surface is about 30% of the total lift.

Significance? Irregularities on the top surface will reduce lift by
more than the same irregularities on the bottom surface. Hence top-
surface-only airbrakes: they are more effective there than underneath,
because they destroy more lift, necessitating a bigger and draggier
change of AoA to compensate.

Ian

On Fri, 20 Mar 2009 02:39:17 -0700, The Real Doctor wrote:

On 19 Mar, 19:48, Martin Gregorie
wrote:

There is a question in the UK Bronze badge written paper about the
proportion of lift provided by the top and bottom surfaces of a wing
that's just as wrong. The so-called "correct" answer is 70/30, but as a
wing is a device for imparting momentum to an air mass its a
meaningless question.

Oh no. Not that one.

At the surface of the wing, it exerts a force on the air mass. A long
distance away (typically 2 chord lengths) it's a momentum change. In
between the effect of the wing is a pressure change /and/ a momentum
change.

Overall, the integrated pressure across the top surface is about 70% of
the total lift force, and the intergrated pressure across the bottom
surface is about 30% of the total lift.

Significance? Irregularities on the top surface will reduce lift by more
than the same irregularities on the bottom surface. Hence top-
surface-only airbrakes: they are more effective there than underneath,
because they destroy more lift, necessitating a bigger and draggier
change of AoA to compensate.

True enough, but the point I was trying (badly) to make is that the lift
is due to the whole wing section and shouldn't be apportioned to the two
surfaces as a number taught to neophytes. Apart from anything else this
breaks down when you consider the pressure distribution across the top
surface. Why not also teach an arbitrary percentage of lift generated by
the LE suction spike at high Cl?


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |