Vx and Vy after an engine change

N941WR · #1 10-12-2010, 05:40 PM

Will my Vx and Vy numbers change with the new engine or will they remain the same but with higher FPM numbers?

Thanks for the help!

apkp777 · #2 10-12-2010, 06:45 PM

Vx and Vy should stay close to the same, but Vtol will be substantially improved!

az_gila · #3 10-12-2010, 07:35 PM

...if you changed the empty weight (and hence the flying weight) with the new engine?

apkp777 · #4 10-12-2010, 08:03 PM

It's a great question. I can't think off hand if I have ever seen any reference that indicates Vx and Vy vary with aircraft weight (that is a gross weight Vx and Vy vs a solo weight Vx and Vy). It's my understanding that it is related to thrust. wing design/lift, and air density (altitude). I am sure someone has a good explanation on the subject. I know the two speeds converge at the aircraft absolute ceiling which lends me to believe that Vx is not dependent on weight but Vy is?

chipf · #5 10-12-2010, 08:33 PM

Vx and Vy are defined by the total drag curve, which is - as we all remember - the sum of the parasitic and induced drag curves.
Parasitic drag won't change, it's the drag caused by moving air past the airframe at zero-lift.
Induced drag is caused by lift. More weight -> more lift -> more induced drag. So the induced drag curve shifts upward with weight, and so does the total drag curve. So the Vx and Vy will have to shift up also.

In this case, we're not talking about a lot of extra weight, so the changes won't be drastic, but will need characterizing again.

N941WR · #6 10-12-2010, 09:34 PM

Thanks, that's what I thought and since I'm not changing my GW, the prior numbers should remain the same.

Kevin Horton · #7 10-13-2010, 04:54 AM

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. I suspect that Vy will increase a bit, as the rpm at the old Vy will be lower than it was with the old engine and prop. Flying a bit faster will allow the engine rpm to increase, giving more power, allowing higher rate of climb. I wouldn't be at all surprised to see a 5 kt increase in Vy.

Vx is a bit harder to guess. I think it may decrease a knot or two, but I wouldn't put too much money on this one.

rzbill · #8 10-13-2010, 04:57 AM

Climb rate is supposed to be a simple calculation using excess HP over and above what is required to fly straight and level at stall speed.

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.

az_gila · #9 10-13-2010, 09:04 AM

Quote:

Originally Posted by rzbill

Climb rate is supposed to be a simple calculation using excess HP over and above what is required to fly straight and level at stall speed.

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.

Assuming an installation of FP props, then is would be around 70% or so of that number.

If it went form 125 HP FP to 160 HP CS, then the gain would be bigger than your number...

We need more details...

rzbill · #10 10-13-2010, 11:06 AM

Sure, if the prop efficiency improves, the delta in HP is greater so the perfomance change will be greater.

I won't get into the discussion on prop "K" factors since hard data is elusive.

The point of my post was to show how simple the math is for prediction of climb rate if you know excess HP.