TJQ,
If the following explanation is too messy, refer back to
FE Hoppy's first post - Eminently practical and useful
Sorry that I can't post a diagram, so try to visualise this -
In Performance Analysis, we refer to the relationship between the Drag Curve and the Thrust available curve. The speed for Maximum Endurance lies at the lowest point on the Drag Curve which is tangential to the Thrust Curve (Imagine slowly 'bumping up' the Thrust Curve until it
JUST touches the Drag Curve ..... There you have it, the point of tangency is at the speed for Maximum Endurance. Now, here comes the problem -
For
PRACTICAL purposes, we assume the Thrust curve to be a Straight Horizontal line. If we accept this as fact, VMD does indeed become the Maximum Endurance speed. It is a VERY good assumption for overall Jet performance understanding, as the Thrust Curve is
NEARLY a straight line, but isn't
At zero speed, and with a particular engine setting, Static Thrust is produced. As speed (TAS) increases, due to the THRUST EQUATION, thrust declines. As Mach Number increases, ram effect causes thrust recovery, weak at low speeds and very high at high speeds. Thus, our simplified Thrust 'Line' is actually a curve, descending at low speeds, bottoming out at about M0.5, with recovery to Static Thrust at about M0.75. Above this Mach Number, thrust will exceed Static Thrust.
Now, as most holding is done at relatively low altitudes, and therefore low Mach Numbers, the point of Tangency of the Thrust Curve to the Drag curve is typically below VMD, as BOTH polars are descending. That accounts for Best Endurance speed below VMD.
If we were to now increase our holding Altitude to that where Holding speed was approximately M0.5, the point of tangency is spot on VMD, the thrust curve at this point is horizontal, just like our assumed and simplified Thrust Line. At even higher altitudes where the holding speed was above M0.5, the point of tangency is at a point where BOTH polars are increasing, and Best Endurance speed will be ABOVE VMD.
If it's still as clear as mud, refer back to refer back to
FE Hoppy's first post, the practical solution
Regards,
Old Smokey