achieving optimal performance

[Guest skyfox49]

If thrust equals drag, and weight equals lift, in an unaccelerated flight, why is there maximum drag on a low speed flight??

 

Not to ask a dumb question,.., What exactly is 'unaccelerated flight'??

 

 

[markendee]

Because of increased angle of attack

 

 

[Guest pelorus32]

If thrust equals drag, and weight equals lift, in an unaccelerated flight, why is there maximum drag on a low speed flight??Not to ask a dumb question,.., What exactly is 'unaccelerated flight'??

Unaccelerated flight is steady state flight. Acceleration is the process of changing state - that might not be the physics book definition but I think it will work. So for example going from 60 knots to 80 knots is acceleration and in that case thrust needs to exceed drag. Going from 80 knots to 60 knots is negative acceleration and thrust needs to be less than drag (assuming steady height).

 

The other part of your question is a little more complex.

 

Let's look at the lift equation:

 

Lift = CL × ½rV² × S newtons

 

This bit: ½rV²

 

 

 

Is about the dynamic forces generating lift and notice that velocity squared is the velocity contribution. S is effectively a constant - it's your wing area. The funny looking p or r is rho and it's the air density.

 

The next bit is CL

 

This is the coefficient of lift and that's mainly about angle of attack.

 

So if we have a constant requirement for enough lift to hold up a 600kg a/c we can get it from increasing the CL whilst reducing V or increasing V whilst reducing CL. We are trading one source of lift in the lift equation for another.

 

So in high speed flight we generally have a low angle of attack and in low speed flight a high angle of attack. The high angle of attack generates lots of induced drag whilst high speed flight generates lots of parasite drag. The reality is that in slow speed flight you are at one of the two points of the drag curve where drag is high - the other is high speed flight.

 

Hope that answers your question.

 

Regards

 

Mike

 

 

[vk3auu]

Skyfox49.

 

Have a look at http://selair.selkirk.bc.ca/aerodynamics1/Drag/Page9.html

 

and you will see what the drag curve looks like and how it is calculated.

 

David

 

 

[Guest disperse]

Oww ! that hurt my brain ......:confused:

 

 

[vk3auu]

http://en.wikipedia.org/wiki/Induced_drag

 

Also have a look at the graph at the bottom of the page.

 

David

 

 

[Guest skyfox49]

thanks fellas!