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- Thread starter Aeroguy82
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Well there are few things you need to clear up before anyone can answer your question.

-Weight is not in kg. The unit kg is mass.

-Weight/Mass (N/kg) does not have the units of velocity. It has the units of acceleration.

-Maximum velocity is not achieved when Thrust = Drag. This condition is known as cruise. Cruise is when you are not accelerating and are at a constant Velocity.

-Cd is a coefficient and dimensionless. Setting Fd = Cd yields nothing useful.

Now to answer your last question, you can find cruise velocity if you other know things, such as cruise at angle of attack or cruise CL.

-Weight is not in kg. The unit kg is mass.

-Weight/Mass (N/kg) does not have the units of velocity. It has the units of acceleration.

-Maximum velocity is not achieved when Thrust = Drag. This condition is known as cruise. Cruise is when you are not accelerating and are at a constant Velocity.

-Cd is a coefficient and dimensionless. Setting Fd = Cd yields nothing useful.

Now to answer your last question, you can find cruise velocity if you other know things, such as cruise at angle of attack or cruise CL.

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Alright (these match up to your responses):

-I realize kg is mass and am frankly embarrassed by this rudimentary mistake.

-I also realize force/mass=acceleration and I meant at a given time as acceleration*time=velocity.This was originally brash and illogical thinking regardless, and meant to demonstrate my futility in finding a solution. I don't have any idea what was going through mind.

-Inside my parameters (the aircraft has constant unwavering propulsion), I believe cruise to be my max velocity as this is the point at which the plane stops accelerating and the velocity is at it's highest. So, for all intensive purposes, are these phrases not interchangeable? Correct me if I'm wrong here.

-If I suggested I was setting Cd to Fd (I don't see where) I apologize as this was not properly conveyed. I was merely suggesting setting Fa equal to Fd. My concern was that both Fd and Cd are dependent upon velocity, so there is no way to calculate Fd given all other variables. What it boils down to is this: how can you find Fd at a specific velocity without testing if Cd changes with velocity. Is the only way to find Fd by testing at every different velocity you need, or is there a way to figure this out after only testing for Fd at one or two velocities? How can conclusive Cd values be predicted?

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To find Cd for form drag is fairly easy. You just need to know the pressure distribution on the surface. The difficulty depends on the shape. You can generally find this value for most airfoils and most simple shapes in books such as "Theory of Wing Sections" or probably online somewhere.

Viscous drag is much more difficult and depends on many factors. For that you need to know boundary layer profiles all over the surface. Those are not simple to find. Viscous drag generally contributes around 20% of the drag on an airliner, so ignoring it would just give you a VERY rough estimate.

Why do I say this? The drag equation you cite is much more complicated

Than it looks.your best bet is trying to find Cd published somewhere.

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