Vx and Vy and commercial a/c
 

Hi

I been wondering quiet a while now about what and how Vx and Vy is used on a commercial a/c.

I am flying ATR and really can't find anything about it in the FCOM's or AFM and the performance books I have been reading tells me that Vx is Vmd on the thrust curve or max excess thrust. Vy is Vmp or max excess power. But how do I find them on my ATR? Do we call them something different on a "big" aeroplane (CS-25)?

Anyone who can give me a hint or maybe have some good links to where I can read more about this?

Thanks

Best Regards Jic.

V2...........

V2 is not the equivalent of either Vy or Vx...

They're still called Vx and Vy, but seldom used because transport aircraft work to scheduled performance, rather than best.

G

V2??? No!!!
 

JIC, you have it all basically correct, if one assumes thrust is constant (which it may be close to true for an old turbo jet, not that close for a turboprop). Usually the flight manual or performance manual publishes Vy, speed for maximum rate of climb (all engines). Typically though, one climbs at a higher speed, computed to minimize the cost of the flight.
Vx (all engines) is rarely published, and I've never heard anyone use it although I can imagine some scenarios where it would be useful. If the published holding speeds are in fact Vmd, and not some higher speed to aid in aircraft handling, then I'd say this is the Vx speed for that weight.
What one may be able to determine is Vx for one engine inop. Typically, Vfs, the final segment climb speed, is chosen to be the maximum angle climb speed for one engine inop, to allow the best climb gradient.
How do you find Vx and Vy on the ATR? Some elaborate flight tests would get you the answers.
Vx and Vy mean the same for big or small aircraft, regardless of the power plant. Keep googling, and reading.

Fair enough. My "V2" was a bit flippant. In the three jets I have flown, none of the documents refers to Vx and Vy. Hawk sounds on the mark re Vx being (close to) the holding speed.

In the engine-out case, what is the difference between Vx and V2? Don't both achieve the best possible gradient for the configuration?

On the 777, the FMC provides the Vx, which is usually around 240-270 kts. Vy is Vref30+ 140 (which is also Vcl + 60).

We use them in the standard way- to meet a lateral requirement (cross X at or above Y) we use Vx, to meet a time requirement (expedite trough X leve) we use Vy.

V2 IS the best speed to fly for obstacle clearence after takeoff, as the acceleration segment to get to Vx would rob to much distance, which I think is what Genghis was alluding to.

JIC
 

ATR fcom gives best glide speed for obtruction, both engine out as Vmhb0. So this should be the Vx. this is best L/D spd

As Vy is generally more than Vx, Vy will be greater. i guess we'll have to check climb rates at diff speed from fcom.

Another interesting formula is

roc(ft/min) = Pavailable - Preqd/ 33000

hope it helps;)

Are the Vy and Vx values displayed in your FMS or are they implied? Not knowing your exact Vrefs, it seems your "Vx" and "Vy" are almost the same?

I could be wrong, but I believe that, on an airbus, Vx = green dot, Vy = climb at cost index 0.

On the 737 NG,the best Rate of climb will be same as ECO CLB at cost index 0....interesting:ooh:

In airbus family.....
Vx=green dot
Vy=climb speed with CI=0.
Michelda

Bloggs, Bloggs, Bloggs
 

http://www.pprune.org/tech-log/90019...tml#post896930

note the date...:{ :{

Stay Alive,

Bloggsy,

Vx displayed on the CLB page of FMC, headed "Best Angle".

Vy is a "canned" speed, meaning a fixed value over Vref30. No actual figure is displayed, but you get Vref30 from the APP REF page and, if you aren't too tired, add 140kts to it (I usually need the calculator function of my Ipod!!)

In that particular configuration they will probably be very close if not the same. However by definition they are two different speeds and need not be identical. V2 speed is designed to provide maximum obstacle clearance at take-off with one engine out, it will probably not be the speed that will give you maximum angle of climb in the long run, which is Vx (and probably a higher speed than V2 I would think).

I do find it strange how many larger aircraft do not have published Vx and Vy speed, as they are so useful to have knowledge of. With all the other flight tests those aircraft go through I can't imagine it would be difficult to determine Vx or Vy, so why not just add it in the books!?

can you imagine the deck angle/pitch attitude for Vx with all engines running?

15 to 20 degrees would be alot...but imagine you might get close to 23-30 degrees.

anyway...the whole idea might be : you takeoff headed in the direction away from your destination...you want to climb as quickly as possible/angle to get to a place where ATC will allow you to turn towards destination...it doesn't make sense to fly quickly farther away

so you climb at min clean maneuvering

or you leave some flaps out and you climb at min speed for that configuration.


so, nowyou are headed towards your destination and you want to get up high to take advantage of better fuel flow or tail winds...but some ssuper smart guy figured out that the best way to fly is to fly at cruise climb speed as the best compromise for unknowns.

so Vx and Vy are nice to know...but why use them?

V2 speed is designed to provide maximum obstacle clearance at take-off with one engine out

I don't think so.

The OEM's interest is to sell aircraft and one measure of considerable importance is field length requirement which, in turn, is critically dependent on speed (V2).

The OEM will be looking for the minimum V2 consistent with Design Standards .. ie, driven by Vs or Vmca, although there may be some Type reason to go a bit faster.

Where does this leave us so far as obstacle clearance is concerned ?

(a) for close in obstacles, min V2 will give the best obstacle clearance as you can't go slower and any increased V2 will compromise the clearance.

(b) for distance obstacles, min V2 definitely is NOT the way to go - refer to the use of improved performance/overspeed V2 takeoff schedules.

Figuring Vx and Vy equivalents is straight forward but they are light aircraft considerations, not heavy .. the OEM/operator's ops eng folk will give you information regarding best climb rates.

I am confused, doesn't this just confirm what I wrote?

I would appreciate if you cared to elaborate a little on this, I am not sure if I understand:

Thanks

Probably a touch of wordiology. I am looking at V2 being driven by field length considerations (limited by Design Standards). The reference to field length is to emphasise that. Probably not worth fussing too much over ..

There's a slight difference.

Twin Otter figures (at max weight): V2: 80kias. Vx flap 10 (T/O config) is 75kias. So you would climb a tad quicker, but on actual engine failure on takeoff we use V2 and stick the ASI needle at 80. And since the manoeuvre allowances are +/-5kias, the POI doesn't like it when we go below 80kias on engine failure trainings :E

Another thing to point out of interest perhaps, it's that our SOP states that if we are climbing at Vx flap 10 (75kias) we should accelerate to V2+5 (85kias) before flap retraction. The SOP also states that our climb speed on normal circumstances is Vy flap 0 which is 100kias (is there a Vy flap 10?).

By the way, which is the equivalent of, or the closest speed to, best L/D ratio on aircraft like the Twin Otter?