### Simple Equation To Estimate Speed Increases from Additional Power

VN = VO * (PN / PO)1/3

Equation Explained

• VN = Velocity, New

• VO = Velocity, Old

• PN = Power, New

• PO = Power, Old

NOTES:

This equation only works reliably when the Power Difference Ratio (PN / PO) is equal to or less than 1.5 – any PDR higher than that causes an increasingly unreliable answer. This was found out through looking at known speed/power curves for the P-51D, shown below:

 Power Required Level Flight P-51D 44-15342@ 9,660 lb weight @ 10,000 feet(Local Storage of Image)(Original Image Link)(Link to Report Image is From) Speed (MPH) Horsepower Required 417.209 1682.35 402.79 1541.17 362.79 1124.7 345.11 981.17 326.5 835.294 297.67 663.5 252 459

### Example

We have a P-51H Mustang with an engine rated at 1,900 horsepower at 20,000 feet, which gives us a top speed of 451 MPH at 21,200 feet. How much more airspeed would we get if the engine was rated at 2,100 horsepower?

451 MPH * (2100 HP / 1900 HP)1/3 = 466 MPH

For this answer, the PDR is: (2100 HP / 1900 HP) = 1.1053; or under 1.5, so the answer can be considered somewhat confident.

References:
FLIGHT TEST OF THE P-51H AIRPLANE, AAF NO. 44-64182 (14 October 1946)
Technical Counselor Note #5: Aircraft Performance – Chapter #1 – How Power affects Aircraft Performance by Will Fox (PDF) (original link)

### Built-In Estimator

Original Speed
Original Power/Thrust
New Power/Thrust
New Speed
PDR Ratio (lower than 1.5 = good)