Form/Profile Drag

[hdaae]

According to aerodynamic theory when the airspeed increases the parasite drag increases. Easy enough to understand.
Double the speed, four times the Parasite Drag.

BUT, when it comes to Form drag(which is profiledrag which is also a parasite drag) it depends on the adverse pressure gradient. Adverse pressure gradient is a result of increase of Angle of Attack which usually is due to airspeed reduction.
So, the way I see it, with airspeed REDUCTION comes an INCREASE in formdrag?
PARASITE drag as a whole will DECREASE with Airspeed REDUCTION.

Are there any expanded Equations that explains the SPESIFIC magnitude of each type of Parasite drag?
The sum of parasite drag will be a increase in drag with increase in airspeed and vice versa, but Ive never seen any good explaination concerning formdrag, and its magnitude in both Parasite drag and Profile drag.
When Profile drag only is considered, it still follow the rules of Parasite drag concerning Airspeed, but again I wonder about the magnitude of Form drag since Profile is both skindrag (increase in drag with increase in airspeed) and formdrag (increase with decrease of airspeed..I think.....)

I would be MOST greatful for any enlightment on this subject coz its bugging the hell out of me! :confused:

Any links would do fine also if any exsists.

[Offset]

There is no automatic link between angle of attack and airspeed - AoA can of course be increased without any speed reduction and the APG will increase with it. It is important to remember that the theory will assume other variables (including in this case airspeed) remain constant. Once you start mixing real world operation with the theory, you introduce too many variables for a straight answer. For instance, if the AoA increases as the aircraft decelerates (as in your case), the lift produced will stay constant - assuming level flight. So has the APG increased or remained constant? What ever the answer is, it's obvious there's more to it!

[hdaae]

As far as answering your question about AOA increase due to airspeed reduction in straight and the effect on the Adverse Pressure Gradient you need to take a look at the reason why the Lift remains the same.

L=1/2*Rho*CL*VelocitySquared*WingSurfaceArea

If the airspeed decreases you would have to increase something else to have constant lift. Density and Surface area are considered constant, so that leaves to increase the Coefficient of lift CL. You would have to increase AOA to accomplish this.
Increase of AOA will lead to gradual separation of the airflow over the wing due to the Adverse Pressure Gradient.
Low Static Pressure were the max curvature(reduced Cross sectional area) of wing is (accelerated airflow) and High static Pressure at the trailing portion of the wing leading to decelerated airflow. When the AOA is high enough (close to CLmax) the APG is so pronounced that the airflow has reversed its flowdirection at the trailing part, separation occurs and in this portion we find Form drag.

And this is were MY question comes in, about Form/Profile drag.

Im aware this is a little beyond real world application, but Im really intrested in THEORETICAL models since Im reading for ATPL exams.
If possible Equations, which I have been unable to find anywere.

[hdaae]

I find it hard to belive that noone knows....

[bookworm]

Isn't this just a terminology thing?

A lexicographer could have a field day with drag, as there seem to be so many different conventions for what you include in what.

You can notionally divide the drag of an aerofoil ("profile drag", aka "section drag") into "skin-friction" and "form drag" (aka "pressure drag"), according to the mechanism of origin of the force. If you add an airframe, you can add in "interference drag" and end up with "parasite drag". Now make it fly and you get "induced drag" which you can add in to get "total drag" or some such.

From the point of view of the drag polar, it makes more sense to divide drag up into a component independent of Cl ("zero-lift drag"), and a component that depends, approximately quadratically, on Cl ("drag due to lift").

There's no trivial 1-1 correspondence between the two.

Skin friction is (almost) independent of Cl. As such you can pretty much depend on it increasing with speed in unaccelerated flight.

Induced drag is (almost) quadratic in Cl. As such you can pretty much depend on it decreasing with speed in unaccelerated flight.

Like total drag, pressure/form drag has a Cl-independent part (a non-lifting object has some form drag) and a Cl-dependent part (your APG), often with a complex relationship. All bets are off when it comes to how it varies with speed. It depends where you are on the drag polar.

Hope that helps.

[hdaae]

Well :) it only restates what I already knew I guess.
Ive been discussing this with a BA engineer, and basically he said that noone wants to know unless your building an aircraft.
I need to ponder a little more about his reply and read some books before I can give it a rest.

Thanks for the reply thou!