Estimating Aircraft Speed Increases
Through Additional Power

(Created 13 December 2017)

Simple Equation To Estimate Speed Increases from Additional Power

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

Equation Explained

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)