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EngineOut
8th Apr 2007, 10:08
I understand that Max range cruise speed is determined from the total drag curve and that it the point where a line drawn from the origin of the graph is tangential to the curve.

My question is, why is the line drawn from the origin of the graph? What is the significance of this point being drawn from the origin and meeting the drag graph at a tangent?

Thanks.

EO
hawk37
8th Apr 2007, 12:02
The line drawn back to the origin is just a simple way of showing the point on the drag TAS curve where you have the maximum ratio of TAS/drag. That's all.

Draw out the curve on a piece of paper. Your aircraft can fly on any point on that curve. The one you want for max range is the point that has the least slope, a tangent.

Hawk
Old Smokey
8th Apr 2007, 21:19
Ditto to hawk37's wise words, and don't forget that the origin of the tangent moves Left and Right of Zero on the TAS axis in accordance with the Wind Component, to calculate the Ground range speed. From the 0/0 origin, it applies to still air only.

Regards,

Old Smokey
wondering
9th Apr 2007, 10:59
Slightly off topic. I reckon, max range is best illustrated by the term 'Specific Range' and not the drag curve. IMHO, the drag curve concept is one of those popular things for exams and interviews. Itīs a simplification of what is really happening with powered airplanes. The point of minimum drag should give the highest value of L/D, which means best glide ratio (unpowered flight!).
But, donīt take my word. Out of 'Jet Airplane Performance':
'In any flight performance problem involving fuel consumption, the engine and the airframe have to be regarded in relation to each other. Engine performance and the aerodynamic behaviour of an airplane is determined by numerous conditions that not only influence the final results, but also affect each otherīs significance.
The partially complex correlations involved in the computation of cruise performance can be illustrated by just one term, the ''specific range''.'
SR=Still Air Distance/Fuel Quantity[NAM/kg]
or
SR=True Airspeed/Fuel Flow[NAM/h / kg/h]