V Speeds

[Bill Smith]

Having a discussion the other day, we were told that if the OAT decreases then Vr decreases but V2 goes up.

I can understand why Vr would go down but why would V2 go up?

[oldebloke]

If your messing around with Flex/Reduced(assumed temp') thrust then on a cold day you up the V1 and VR(if theyr'e close together)to have the actual V1 equal the true V1..You'd get to the Indicated V1 sooner on a cool day(density)and it wouldn't equalize the Stop/Go distances...:ok:

[BOAC]

Bill - V2 - s/eng min control speed - will be higher in colder air as thrust will be higher.

EDIT: omigawd! Just returned to the thread to see what garbage I posted:confused: Blame it on posting too quickly, did not RTFQ and was trying to get Mrs B sorted for some days in NY. Sorry all and thanks M(F)S.

[Mad (Flt) Scientist]

OK, Vr-V2 speed spread will increase on a colder day, because the thrust will be higher (unless you're totally 'flat rated') which will provide more acceleration at a given weight.

Whether one or both of Vr and V2 are constant depends on what the limiting factors are for either.

Vr may be driven by V1, in which case the limiting factors on V1min may be relevant. V2 may be driven by Vmca OR Vs(Vsr). Without knowing the details for the specific conditions, it's impossible to know what the impact on Vr or V2 will be absolutely, but the relative behaviour is thrust-driven, as noted in the first para.

And V2 isn't single engine minimum control speed; that's Vmc (a, g or l, as approrpiate to the phase being considered)

[AirRabbit]

In the US I think you'll find that most of these "V" speeds are "defined," and not necessarily calculated. For what it's worth, here are a few of the "takeoff" V speed definitions:

Vmcg. (Minimum control speed on the ground): The minimum speed at which an aircraft is defined to be “controllable” (lateral excursion lower than 30 feet) with aerodynamic controls only after an engine failure on the ground.

Vef – (Engine failure speed): the speed that the critical engine is assumed to fail. It may not be less than Vmcg.

V1 (often referred to as “Decision Speed”): Must not be less than Vef plus the speed gained between when the engine failed and the pilot recognized the failure. This means that this speed is the minimum speed at which an engine failure may occur and the pilot may continue the takeoff. At the same time, V1 speed must not be greater than the speed at which a rejected takeoff can be initiated and stop the airplane within the calculated accelerate-stop distance.

Vmu (Minimum unstick speed): Minimum airspeed at which airplane can safely lift off ground and continue take-off. Because of the way this speed is determined, lift-off is not possible prior to reaching this speed.

Vr (Rotation speed): Must be greater than V1 and greater than 1.05 Vmca and may not be less than the speed that would allow reaching V2 before reaching a height of 35 feet above the takeoff surface. This speed is selected so that the rotation begins at Vr and provides that Vlo occurs at a speed greater than VMU The rotation is continued at approximately 3 degrees per second until reaching the desired pitch attitude.

Vlo (Lift-off speed): Must be greater than 1.1 Vmu with all engines, or 1.05 Vmu with engine out.

Vmca (Minimum control speed in the air): Minimum airspeed at which, when the critical engine is made inoperative, it is still possible to maintain control of the airplane and maintain stable, controlled flight. [The rudder is used to compensate for the yaw moment caused by thrust asymmetry. There is a minimum speed at which full rudder will be necessary, in order to fly a constant heading with level wings. To reduce sideslip, this speed can be reduced even more, if the aircraft is banked on the live engine’s side. The lower the speed, the greater the necessary bank angle. The speed that corresponds to a 5-degree bank angle is defined as Vmca.]

V2min (Minimum take-off safety speed): Must be greater than 1.1 Vmca and 1.2 Vs, the stalling speed in the take-off configuration.

V2 (Take-off safety speed): May not be less than V2min and may not be less than Vr plus the speed gained before reaching a height of 35 feet above the takeoff surface. [If one engine is lost before reaching V2, then the initial climb is flown at V2. If thrust is lost at a speed between V2 and V2+10, then the current speed is maintained, to ensure the most efficient climb speed. It is not necessary to increase pitch, in order to reduce the speed to V2, when a higher speed has already been reached.]

I think when you read through these definitions, you can see that V1 speed is selectable and may represent several different factors, depending on the airplane and the runway. For example, a very light airplane on a very long runway might combine to allow you to have a relatively low Vef – where you could experience an engine failure (as long as this speed is not less than Vmcg) and have enough runway to continue to accelerate with the remaining engine(s) and make a safe takeoff. At the same time, you might be able to accelerate to a relatively high Vef – where you could experience an engine failure, and have enough runway to bring the airplane to a safe stop within the confines of that runway. Here you would have two “selections” of V1, with different points along the runway and different values for the term.

AirRabbit

[Bill Smith]

AR, I don't know what you are flying but V Speeds are definitely calculated ! They are calculated carefully and tabled in flight manuals etc to be compliance with performance regulations.
They are based on numerous factors A/C weight, runway length available, OAT, PA etc
This question has been posed as it was asked on a performance exam.
It may be just that it is badly worded.

I think Mad Scientist is the closest with:

OK, Vr-V2 speed spread will increase on a colder day, because the thrust will be higher (unless you're totally 'flat rated') which will provide more acceleration at a given weight.

Thanks for the replies guys & gals

[AirRabbit]

Hey Mr. Smith:

I guess I should explain myself a bit better, sorry. Of course, I recognize that specific numbers are calculated – what I meant was that you can’t calculate anything until you have defined what it is you are calculating. And after you have calculated the appropriate numbers, you select the number you are going to use for a certain value.

What I placed in my post was the definition of the terms, in an attempt to help interested parties understand how these terms relate to one another. Take “V1” for example.

Yes, you gather the information you need, and “calculate” V1 for the particular takeoff. But what is V1? I know it can’t be less than the speed at which the pilot recognizes that an engine failure has occurred and continue the takeoff, achieve the required height above the takeoff surface, and do it within the takeoff distance. I also see that, by definition, I can’t use a V1 that is less than Vmcg. So, now I have to understand what that number is so that I don’t use a V1 less than that. And, I also have to know what the required height above the takeoff surface is.

Reading the definition of Vr, I see that this is a height of 35 feet above the takeoff surface. But I also see that I have to begin to rotate the airplane (at a particular rotation rate) so that I”ll arrive at that 35-foot height after reaching a speed called V2. OK, what is V2 speed?

After further definition reading, I find that V2 is a takeoff safety speed – but it, too, has to meet some definitions; i.e., it may not be less than V2min and may not be less than Vr plus the speed gained prior to reaching the 35-foot height. And the effort continues…. (as we know).

But, even after running around to understand and get these values to determine the smallest V1 I can use, I still have to come back to the original definition of V1 and recognize that V1 also may not be any greater than the speed at which a decision to stop can be made; and that stop has to be able to be made within something called the accelerate-stop distance. And, as I’m sure anyone here would recognize, if I carry this on we would have to know how to obtain Vmcg and Vmca and Vmu and on and on – and then we would have to relate all these numbers to each other.

And then, after all this, we would still have two numbers – and we’d have to pick the one we wanted to be V1 – unless all the factors just fell into place and they happened to be the same number (balanced field).

My point is (and was) that V speeds are as much “defined” and “selected” as they are “calculated;” and I should have made that point more clear initially.

AirRabbit
:O

[Old Smokey]

Bill Smith, there's a lot of good discussion here, but your question posed is very very type specific.

Here is ONE of many reasons why Vr may decrease whilst V2 increases on a cold day. There are others, which may have the opposite effect.

One of the more critical performance limitations to be considered in aircraft certification is the capability to accelerate from V1 (on the runway) to V2 (at 35 ft above the runway) within the Takeoff distance available (TODA). At high temperatures (and therefore lower thrust) this can be very limiting, particularly for a 2 engined aircraft with one engine inoperative. It's necessary to shorten the gap between Vr and V2 as much as possible in these circumstances to accomplish this acceleration. In the worst case, V1 will be at the highest (causing the aircraft to be accelerate-stop limited) and V2 at V2 min, yielding the lowest 2nd segment climb gradient.

As the temperature decreases, thrust increases, making the Vr to V2 acceleration less limiting, and the certifying Performance Engineer can then avail him/herself of using a lower V1 (allowing an increased accelerate-stop limit weight), and a higher V2 (allowing an increase in 2nd segment climb limit weight).

This increasing Vr to V2 delta can widen as temperature decreases, down to the temperature where the engine becomes Flat Rated, whereafter little is gained from manipulating the numbers further (a little, but not a lot).

Again, I say that it's very very type specific, some manufacturers adopt one simplified schedule of speeds, and adjust the performance penalties around that, rather than adjusting the speeds within the approved limits to gain the optimum performance.

Regards,

Old Smokey

[Bill Smith]

Thanks Smokey !

[411A]

...when 'overspeed' takeoffs are considered.

Have used this to good effect while departing ZRH runway 16 (among others), and it is indeed very useful for increased takeoff weight, providing sufficient runway length is available.
Many ways to skin the cat...legally.:ok:

[AirRabbit]

Well said, Mr. Smokey! :ok: