View Full Version : Vx VS Vy

15th May 2003, 02:45
I am just studying topics regarding Vy and Vx, best rate of climb and best angle of climb.

May I ask what is the point of best angle of climb? Surely in any recovery, takeoff, go around or emergency you would always opt for the best rate of climb.

Best angle of climb...just seems absolutely pointless. I know I am wrong but I don't know why. Comments appreciated.


15th May 2003, 03:38
Imagine that you are about to lift-off and there is a very high obstacle a short distance beyond the end of the runway. In order to clear the obstacle without turning, your flight path must be at a greater angle than a line drawn from your lift-off point to the top of the obstacle. It does not really matter how long it takes to clear the obstacle, but it is essential that your climb angle is great enough to clear it. In this situation you require VX to give you best angle of climb.

If on the other hand you know that a very large bomb is going to explode somewhere on the ground near the airfield very soon, you will want to get away from the gound as quickly as possible. To do this you must fly at VY, which will give you the best rate of climb.

Best angle gives best height over a given ground distance. Best rate gives best height over a given time. But VX is slower than VY, so the best angle of climb does not give the best rate of climb.

15th May 2003, 04:00
I think I can just about manage to understand the examples you gave, but can't prevent myself from arguing that you would also clear that obstacle if you flew at Vy? Say it's a given distance from the threshold, at Vx you can clear it at a given speed. At the same speed, you are travelling that distance in the same amount of time as in Vx but you would get higher?

Cheers for your atttempt to put straight my senseless mind! It would of worked with someone else, it's just me! :sad:


Onan the Clumsy
15th May 2003, 04:03
Taken to its absurd limits...

Imagine you were Alan Tracy in Thunderbird 3 and you were about to launch from the underground bunker, up through the circular house. Any flight path other than vertical really won't do you any use and so that's what you use. This is the best angle of climb.

Meanwhile Virgil, is laughing at you over the private frequency (and all your other brothers are listening in) because he's already at two thousand feet but he's five miles down range. He's pitched for best rate of climb.

Now Virgil might laugh, but you'll get the last laugh because you get to visit Thunderbird 5 and watch your brother scrambling for his spacesuit when you pretend to open the door.

15th May 2003, 04:09
Ahh! I knew the Thunderbirds would help.

So VX is related to Altitude gained VS Ground distance travelled?

VY being altitude gained over time travelled?


Onan the Clumsy
15th May 2003, 05:21
This bit was supposed to be a separate post...

Vx might give you a climb rate of 500 fpm, but downrange of only 1 mile a minute

Vy might give you 1000 fpm, but down range of 3 miles a min.

So in two mins, Vx will only have gained you 1000' - not enough to clear a 1200 obstacle say, whereas Vy will put you at 2000, well above the obstacle,


By using Vx, you're only 2 miles down range and by using Vy, you're already 6 miles downrange.

If the 1200' obstacle is 3 miles down range, using Vx, you haven't cleared it, but you also haven't got to it yet. When you do get to it, you will have cleared it (by 300'). Using Vy, you've already blown right through it. :ouch:

25th May 2003, 01:33
Surely in any recovery, takeoff, go around or emergency you would always opt for the best rate of climb.

I think you have just made every performance engineer in every airline operations department feel unwanted :(

Takeoff planning and Regulated Takeoff Weight Charts and Special Departure Procedures/Escape Routes are all about trading Vx, Vy, acceleration in level flight etc etc to maximise uplift from a runway/aircraft/terrain combination.

To the purists please forgive the following but a V2 climb following failure after V1 is basically a Vx situation. Looking for the maximum height gain in the minimum DISTANCE (not time) to a chosen acceleration altitude.

So, yes, Vx is part of day to day flying.

16th Jun 2003, 01:57

The other consideration for obstacle clearance is the distance consumed in accelerating to each speed: for Vy you will be further down range as well as climbing at a lower angle.

In larger aircraft this same principle shows up in climbing out initially at V2. V2 is not the best anything - it is just the lowest speed at which the aircraft can demonstrate the minimum certification climb gradient - but it is the target speed at 35' over the end of the clearway and hence is the earliest point at which a reasonable efficient climb can be commenced. The difference in climb gradient between V2 and Vx is generally not sufficient to warrant delaying the climb while accelerating the extra 10-20kt to get to Vx. As for another 10-20kt to get to Vy, forget it or you might have granite acne.

Stay Alive

de Lattre
16th Jun 2003, 07:29
V2 is only for multi engine, and for aircraft belonging to the JAR/FAR 25 classification.

Basicaly the "lawyer" says that if you are flying on a commercial airline, an engine at take of may brake down, and the constructor has to proove that either the aircraft is capable of aborting take of within the runway, or continue the course acceleration to take off on the only remaining engine, climb with the minimum gradiant to avoid obstacles, upto the safety altitude, and to accelerate.

This is why we are calculating 3 different speed. Al those speed are in fact calculated by the constructor, and in most cases integrated in the Flight Managment Computer, and verified by the crew prior to take off.
Those speed are evoluting according to different factor, such as outside air temp, airport elevation, lengh of the runway, the weight of the aircraft etc...

Nevertheless, they all obey to a law, that a constructor as to be in accordance with.
Here is for you, a little explanation of various speed:

When the aircraft takes of, it is simply because several given elements meet the following formula:

Mg = 1/2PzSVpCz

It means that for a given weight and a given configuration, an aircraft will be airborn at a given incidence, called "alpha" thus to a fixed Cz. This apply for any external conditions, and at a given incidence will correspond an equivalent of speed (Ev or EAS).

Given this, here are the differents calculated speed.

1/ Vmu

It is the minimum speed for sustentation at which and over which the A/C, the aircraft, lift of ground and proceed on take off with out representing dangerous caracteristics like high pitch (tail strike) or not enought lateral control ( engine or wing tip hitting ground).

This is detemined in flight with at take off the stik completely backward, like for a high pitch take of. Vmu is determined successively with all engines running and with the critical engine shut down. So we must have:


Note that the Vmu search is calculated at high pitch, so the thrust will add to the lift.

2/ Vloff

She is called effective lift off and at this speed the lift is assured and safe.
The following HAVE to apply:

Vlof > 1.05Vmu(n-1)
Vlof > 1.1Vmu(n)

3/ Vmca

It is the minimum controlable speed with the critical engine dead. at this speed if you loose the engine, it is supposed to be possible to keep the aircraft under control with either a zero slide slick or a bank angle not greater than 5.
This speed is calculated and demonstrated in flight using the following given elements:
- All engines at maxi take off thrust and very quickly the critical engine is shut down
- Max Take Off weight at brake released
- Center of Gravity the more defavorable
- Gear up
- Strengh on the rudder not greatter than 667.2 Newtons

and Finaly we get:

Vmca < 1.2Vs Vs = stall speed

4/ V2

On the jar 25 it is called security speed at take off. At this speed, the take off is sure and secure. This speed has to be reached at least at 35 Ft above ground and maintained up to 400 Ft or the HSD ( Safety Altitude at take off).
The following have to apply:

V2 > V2 mini

V2 mini is the bigger speed of the following:
- 1.2 Vs or 1.15 Vs (aircraft with 4 turboprops)
- 1.1 Vmca

The first comes for the heavy take off weight, the second for the lower take off weight

5/ V1

Every knows this one, it is the decision speed. This speed is a LIMIT. the failure before it is an aborted take of, after we have to proceed on take off.

We have the following:

V1 = Vef + Delta V

Delta V is the increase of speed during the time where the pilot recognize the engine failure and is ready to apply the first breaking way.

We have to verify that:

Vmcg < Vef < V1 < Vr

I remind you that Vr is the rotation speed (=3/second)

6/ Vef

It is the effective failure speed. Speed at which is supposed to arrive the engine failure during take off for the retained V1. Given this we have so:

Vef > Vmcg

7/ Vmcg

It is the minimum controlable speed on ground at which when mr Murphy decide we have an engine failure, we can take the aircraft under control only in using the aerodynamics commands. The strengh applied on the rudder has to be inferior at 667.2 N.

The maximum deviation on ground is 30Ft.


some other speeds can be a limitation, in the good way or in the bad way. This speed is called the V1 brake or the Vmbe (maximum brake energy).

In case of an aborted take of, the "cinetic energy become at the brakes level Kalorific energy". this is the speed where the brakes are at their maximum absorption capacity.
You will find this mostly on 747 and other havy aircraft. This is why some times in De Gaulle (LFPG) you will see 747 going to the runway tracted by the "Tracma" when the outside temp is high and when they are at max take off. The only effect of breaking to make a turn, considering the weight of the aircraft is going to raise the wheels temp enough to not respect the following:

V1 < Vmbe

This is also why whe have on our A320 fans on brakes, in order to be abble to performe a turn over in less than half an hour.
It is also because we have carbon brakes and the raise in temp is impressive.
At Air france I don't know, but at the ex Air Inter, that was the case.

The last limitation speed is the wheel speed, but mainly it is for Concorde and old fighter like the Mirage 3.

I hope you won't be too lost in those explanation, if it is the case i will try to find example with the Thundirbirds.

If you have any questions, be my guest.

Cyrille de Lattre

16th Jun 2003, 18:32
Ah, don't you just love performance! That's why I fly an aeroplane without any.;) Just up, down, left, right and forward with differing amounts of noise.:}