Vx and Vy after an engine change

[N941WR]

Originally Posted by Kevin Horton

I'm guessing the prop will also be different, and it plays a big role too. Assuming FP props before and after, the new prop will need to be coarser pitch than the old one. ...

Yes, FP before and after. The old prop was a 68" dia x 64" pitch Catto and was a climb prop. The new prop is a 72" dia by 73" pitch Catto and should be a cruise prop.

Originally Posted by rzbill

...
Since no builder really knows that HP figure, we have a different opportunity with your engine change (aren't you moving from an O-290 to a O-320?). Making some assumptions such as 125 hp for the O-290 and 160 hp for an O-320, the HP diff is 35. Assume 85% prop eff for approx 30 hp. Convert HP to ft-lbs/min (33000 ea) equals 990000 ft-lb/min. Divide by the weight of the craft (say 1800 gross) and you get 550 ft/min.

So the difference in climb rate should be of this scale. Hope that makes you grin. Let us know if math predicts real world in this case.

Only the O-290-D2 was rated at 140 hp for takeoff and the new engine is an ECi O-360-A1A equivalent, rated by ECi at 188 hp. Add the dual electronic ignition to both engine and those hp ratings should be around 148 and 199, respectively. The GW on the -9 is 1750 lbs, which I see no need to raise.

Based on your math, I should see around 817 ft/min. Holy cr@#!!!

But I expected to see a significant increase in my ROC, so that is no surprise. Getting back to the original question, will my Vx and Vy numbers change?

[Sig600]

Vx and Vy are really just ball park numbers that correlate to angles of attack. It's a funtion of the wing shape.

All things being equal, your Vy/x should stay the same, your pitch attitude to maintain Vy/x will just be higher with more power, resulting in higher rate of climb.