difference between VY and VX

[vikingdk]

Apart from best angle(vx), best rate of climb(vy), and the fact that you use vx for clearing obstacles, what is vy for?

[john_tullamarine]

aahh ... any time you are on one in a lightie with a load and desirous of either not going down (if you are lucky) or going up (if you are very lucky) ... ?

[Piltdown Man]

If ATC say "Give me your best rate" and the resultant deck angle is comensurate with that request, what speed would you fly at? Vy maybe?

[Old Smokey]

vikingdk,

.......best rate of climb(vy), ................, what is vy for?

Methinks thay you answered your own question within your question.

Regards,

Old Smokey

[DB6]

For going up quickest, rather than steepest?

[kaepa]

Climb at Maximum Gradient enables a given altitude to be achieved over the shortest distance.

while

Climbing at the maximum rate of climb speed enables a given altitude to be reached in the shortest time.





addtional to my message ,

Vy (best rate of climb speed ) is always little higher than Vx ( best climb gradient speed)




regards ,
kaepa

[Pegasus77]

Best angle is used to clear obstacles, usually shortly after TO. You are not always clearing obstacles, therefore in other cases best rate (=shortest time) climbspeed to your altitude could be preferable.

Cheers,
P77

[Old Smokey]

To take it one step further from Pegasus77, once clear of obstacles, Vy is definately the preferred option in terms of time, fuel, and cost. You will reach Cruise Altitude faster, and therefore use less fuel to get there, and, as Vy is a higher speed than Vx, cover a greater distance in that lesser time. Overall sector time will therefore be reduced, and fuel used will be less, both saving cost.

I presume that we're talking piston engines here, and a third benefit of climbing at the higher Vy speed will be reduced Cylinder Head Temperatures. That's a cost savings in the long run too.

Regards,

Old Smokey

[parris50]

Also, if the engine goes bang, you will have the advantage of more height, giving you more time to choose a field.

[Piltdown Man]

Vy is not a good speed if you want a tea from from the workers down the back! A deck angle of more than 5 degrees or so makes their lives (and then yours) more difficult than it needs to be.

[Flying Fluffer]

Think old smokey might be getting on a bit. best rate will get you there in shortest amount of time and best angle in shortest distance.

[readbackcorrect]

So with rspect to obstacle clearance, how come we a re taught to fly blue line as opposed to the red radial? Light twins wise?

[-IBLB-]

For a light twin, blue line is Vyse, best rate of climb single engine, alot of times close or similar to Vy.

The first red radial line is Vmc, the important twin thing. Has nothing to do with climb capabilities, or Vx/Vy etc.


On most light twins it is possible to climb out on Vx
There is a safety aspect to it though. Climbing with Vx gives you alot higher pitch, and a speed usually close to Vmc, or Vs.
So if you would loose one engine, you have to react alot quicker getting the nose down, which has to travel more down too, then for instance when you would be climbing at Vy(se) or higher.


and on the pedantic note, Vx and Vy contrary to stated above, can be the same. This will be the case at your absolute ceiling. (and by your, i mean the airplane's)

-IBLB-

[Old Smokey]

Flying Fluffer,

Your quote -

best rate will get you there in shortest amount of time

My quote -

Vy is definately the preferred option in terms of time, fuel, and cost. You will reach Cruise Altitude faster, and therefore use less fuel to get there

Ummm.....Isn't that what I said?

Regards,

Old Smokey

[SB4200]

A more theoretical question...

Why is it that maximum excess power determines Vy and maximum excess thrust determines Vx?

[Goose2]

Because power is for time and thrust for angle....?

[Onan the Clumsy]

what is vy for?

Flying jumpers

[Old Smokey]

This is perhaps a MUCH over-simplified explanation, but here goes.

Essentially all aircraft performance depends upon the 4 basic vectors, Weight, Drag, Lift, and Thrust. These may all be resolved geometrically, which, when there is an excess of thrust, yield a climb angle. Following on from this, all climb performance, when related to the 4 basic vectors indicate that all climb performance is at a particular angle, thus, the amount of excess thrust determines the climb angle.

If we want to take it to the next step, and find the rate of climb, then we must examine the rate of doing work, and,

Power is the Rate of doing work.
and also,
Power = Force (Thrust) X Velocity (TAS).

Therefore, to find the best RATE of climb, we must examine the best excess RATE of doing work, i.e. excess Power.

Regards,

Old Smokey