Question about Vx vs. Vy

[turbojet28]

Hey,

As I sit around just thinking about aerodynamics, I have seemed to see something that I haven't before. Exuse me, however, because I am just thinking this question through as I ask it. I probably have enough knowledge of aerodynamics to figure this out, but it isn't coming together quite enough. My question is this: If Vx gives you the greatest altitude gained in a given distance and Vy gives you the greatest gain in altitude over time , why aren't they the same? Wouldn't you gain more altitude in a given time period if you hold Vx? I may be making assumptions that aren't true (such as Vx always giving a greater ROC than Vy). Any help would be appreciated! Thanks!

 

[Mavmb]

I think it has to do perhaps with momentum and drag. Think of it this way. If you suddenly jerk the nose upwards what will happen? You will get a sudden increase in altitude from the momentum, but after that short boost drag will take over and the airplane will start falling again. And it could also simply be that Vy produces less induced drag over a period of time than Vx does.

 

[DanTheMan]

with apologies to SkyGuyEd for ripping this off of his website. (great site Ed

)
Here is a great animation that shows the difference.

Vx vs. Vy

 

[Hootie]

I thought I was going to come in here and whoop up , but that animation is pretty tough to beat. On another note I was looking at the forces acting on a turn illustration....is there a mathmatical equation for figuring how many pounds of horizontal lift are being produced?

 

[B767Driver]

[ QUOTE ]
Hey,

As I sit around just thinking about aerodynamics, I have seemed to see something that I haven't before. Exuse me, however, because I am just thinking this question through as I ask it. I probably have enough knowledge of aerodynamics to figure this out, but it isn't coming together quite enough. My question is this: If Vx gives you the greatest altitude gained in a given distance and Vy gives you the greatest gain in altitude over time , why aren't they the same? Wouldn't you gain more altitude in a given time period if you hold Vx? I may be making assumptions that aren't true (such as Vx always giving a greater ROC than Vy). Any help would be appreciated! Thanks!

[/ QUOTE ]


You've probably heard that Vx is predicated upon excess thrust and Vy is predicated upon excess horsepower.

Vx, the angle of climb, is very simply the difference of Thrust minus Drag. When Thrust - Drag = Zero the inclination of the flight path is zero or the airplane is level. When Thrust minus Drag is high...the inclination of the flight path is high allowing for maximum climb angles. In propeller driven airplanes, maximum thrust is found at low airplane velocities...so the max angle of climb speed will be found very near to the stall speed.

Vy, the rate of climb, is simply determined by relationship of Power Available minus Power Required. When Power Avail - Power Required = Zero, the aircraft can no longer climb. This is commonly called the absolute ceiling. Maximum Rate of Climb will be found where there is the greatest difference in Power Avail and Power Req.

Commonly, the Vx is found at L/Dmax. Vy, on a Power Required vs Available chart, is found tangent to the Power Available curve.


So, basically, Vx will be found at a slower airspeed to produce the max allowable thrust. Although exhibiting a greater climb angle, Vx sacrifices forward velocity and will not permit the best forward climb rate.

 

[E_Dawg]

[ QUOTE ]
with apologies to SkyGuyEd for ripping this off of his website. (great site Ed

)
Here is a great animation that shows the difference.

Vx vs. Vy

[/ QUOTE ]

Hey thanks

Actually I just threw a link up on my site to the UND site, who made the animation. If you like that one, there are more at http://www.aero.und.edu/multimedia/

 

[xpanterica]

Brutally off topic, I know...

DanTheMan, you're an engineer at Cal Poly, a pilot, a Kings fan, and a member of both LGK and Jet Careers... that's scary!!!! Those all describe me as well.