If you are building or planning on building a ducted fan
airplane this is why you need plenty of excess power.
As you can see the airplane is not going to take off
and climb very well unless you do. If you have enough
power to go 300 or 400 MPH then you probably have enough
power to take off as long as your wing area and gross 
weights are reasonably. 

This chart is from Perkins & Hage Airplane Performance 
Stability and  Control John Wiley & Sons.

Paul Lamar
 
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