Expert: Paul Soderman - 4/20/2011

Question
QUESTION: Dear sir
I am wondering how a contour graph of Specific excess power on axes of velocity and height in MTOW & Max thrust condition.
I found equations but don't know how to apply to graph.
What are Ps=0, Ps=5, Ps=10 etc and do I just choose those values randomly??
Also, which points are stall speed, service ceiling, never exceed speed on the graph?

ANSWER: Jay
Without seeing your graphs and equations I can't answer your questions specifically. Usually, Ps is the specific excess power of an aircraft and is a function of the airspeed, net thrust, drag and weight.  Ps = Vt(Tn-D)/W.  So, you don't choose those values randomly, you calculate them for the flight condition under consideration.  A positive Ps means the aircraft is gaining energy and can climb.  At the service ceiling, excess thrust is zero and the aircraft can no longer climb. There are many ways to illustrate climb performance, so I can't answer your questions about points on the graph.
Paul

---------- FOLLOW-UP ----------

QUESTION: Dear sir

I appreciate for your reply but as you said, here I attach the graph.
How do I construct a graph like this graph?

Answer
OK - I can't see the legends very well but it looks like altitude plotted versus aircraft velocity for various contours of specific excess power. So, for example at a given velocity the power plant delivers less and less power with increased altitude so the climb ability, represented by Ps, goes down.  When Ps = 0, no more climb is possible.  That happens at the maximum altitude or service ceiling. I'm not sure how to find stall speed and never exceed speed, but I assume when the aircraft can no longer climb it is near stall speed at that altitude and velocity.  Never exceed speed is usually a structural requirement.

To construct a graph like this you have to find or calculate the parameters in the specific excess power equation I gave before - namely net thrust, drag and weight for a given altitude and velocity.  Drag and thrust vary with altitude (density) and velocity.
Paul