What is the coefficient of drag on a flat surface?

[EelAnes]

What is the coefficient of drag on a flat surface?
Note:

• The body is in free fall
• The object has 2 wings planform (it's a paper helicopter), which is flat
• The object's wing spin in a circular motion (anti-clockwise) during free fall

 

[CWatters]

In this case the wings are rotating as the helicopter falls. So the wings are moving at an angle to the air flow. Eg they are not actually moving through the air "flat".

So do you really want to know the drag coefficient for a flat surface falling "flat" or at an angle?

Edit: if you are trying to work out the vertical forces acting on the helicopter there is a much easier way than trying to calculate the lift or drag. Presumably it is falling at a constant velocity. So what does that say about the net force acting on it?

 

[EelAnes]

CWatters First of all, I would like to know the drag force at an angle. Secondly, I'm calculating the drag force for an experiment. My experiment is about how does increase the surface area of the wing can affect the time it drops and hence the air resistance.

 

[CWatters]

Its going to be very difficult to calculate the drag forces from first principles using drag coefficients etc. Mainly because you are unlikely to know the angle of attack or velocity of the air. Its also not constant and will vary along the length of the blade. Too difficult for me.

In this kind of experiment its probably the vertical lift force you want to know anyway. At terminal velocity this equals the weight if the object.

 

[EelAnes]

Its going to be very difficult to calculate the drag forces from first principles using drag coefficients etc. Mainly because you are unlikely to know the angle of attack or velocity of the air. Its also not constant and will vary along the length of the blade. Too difficult for me.

In this kind of experiment its probably the vertical lift force you want to know anyway. At terminal velocity this equals the weight if the object.

Okay but how should I determine the terminal velocity without knowing the coefficient at firt?

 

[FactChecker]

You have it backward. Like most experiments, you should measure the results. That means that you should measure the terminal velocity. From that, you may be able to calculate estimates of aerodynamic coefficients, but that may be very difficult or impossible. Unless you can control the orientation and behavior, the flight condition might change a lot during each experiment.

 

[CWatters]

+1

There is data on the coefficient of lift and drag for flat plate airfoils but I don't think it will help you.

https://www.grc.nasa.gov/www/k-12/airplane/kiteincl.html

Rather than trying to calculate the lift you know its equal to weight so just use weighing scales.

 

[David Lewis]

In this kind of experiment it's probably the vertical lift force you want to know anyway.

The flat plate airfoil will develop lift, but the lift won't be vertical. The autogiro is rotating, so direction of the relative wind over the rotor blade will be inclined.

For simplicity usually the remote freestream is the only air velocity we worry about. In that case, it's only necessary to know the total aerodynamic force (which, at terminal velocity, is equal and opposite to the weight). If you can measure the terminal velocity, then it's easy to deduce the aerodynamic force coefficient.

I predict adding camber to the airfoil will significantly increase the force coefficient for a given blade area.