V1, Vr, V2.....again

[minitour]

Okies...I know this has been addressed a lot recently, but humor me...


So you're rolling down the runway and you've determined (using performance charts, FMS, your FO's forehead, whatever) that V1 is A knots, Vr is B knots and V2 is C knots.

So you hit V1 and your airplane is operating normal so (obviously) you're going. You now are at Vr so you rotate and accellerate to V2.

The way I understand it is like this.

V1 is decision speed - you decide wether or not you can safely takeoff. This could be affected by gross weight, accel. stop/accel. go distance, density altitude, runway length, wind speed and direction, presence of a stopway...(anything else?). Basically you figure out before you even start up what your V1 is.

How am I doing so far?

Vr is the speed which you rotate (duh) - I have no clue what this is based on which is where my questions will come from later...

V2 is takeoff safety speed - From what I understand this is the speed which (should an engine failure occur after takeoff) you need to accellerate to so that you can safely enter the pattern (shoot an approach - whatever) and set 'er down.

Okay...so here's the question(s)

1. I know sometimes they are the same, but why isn't V1 always equal to Vr? Like...if this is the speed you have to decide wether or not you can takeoff, shouldn't you be rotating at this time?

I guess I realize that if you had a 40,000' runway, V1 would equal (or even exceed - which by definition it couldn't since if you've rotated you've already decided, right?) Vr because you would have enormously more time to decide wether or not you would have to rotate or not and thus would be airborn well before you absolutely had to decide to abort the takeoff if you lost an engine. (and I realize that the example is distorted with a 7+ mile runway).

But my confusion stems from the opposite example. Say you had a 1,000' runway and you had to bust out a max performance short field takeoff. Your V1 would be amazingly shorter to give you adequite chance to stop the aricraft before the end of the runway, correct?

In this example, you advance the power, release the brakes, accellerate down the runway and reach V1 (at some point) and decide you can continue the takeoff. Well on a 1,000' runway (again extremely distorted from real world to make a point) you would reach V1 significantly before you could rotate, correct? Or does the fact that it is a max performance takeoff reduce the rotate speed so much as to close the gap?

2. What exactly does happen if you lose an engine between V1 and Vr? Obviously you're going airborn (since you've blown by V1), but does that slow the accelleration down enough to make you sweat waiting for Vr to show up? How much shorter does it make the runway seem? Enough to make you nervous or is that also factored into the preflight planning of runway length, gross weight, wind speed and direction, etc.?

3. Exactly what does go into figuring out Vr as opposed to V1?

I think thats it for now. Thanks for the help!

-mini

 

[Bus Driver]

Good Questions. I'm not an "Okie" but I'll take a stab at some of this.
Keep in mind that we're making general statements regarding balanced field length and every situation is different.

VMCG is also a limiting factor in determining V1. This comes into play with your micro 1000' runway. You can't have a V1 below VMCG.
On the high end, you still have to have enough runway left to stop at V1. A lot
of airplanes will take less runway to accelerate to Vr from V1 with an engine failed than to stop.

Vr is associated with V2. The idea is to be able to do a normal rotation at Vr with an engine failed and arrive at V2. But you are correct that you cannot have a Vr less that V1.

To answer your last question. In the old days on the 1121 Jet commode they used to say that you had enough time for coffee and a cigarette between V1 and Vr.

 

[Donsa320]

Minitour

You will get many wordy answers to your question.
You have read way too much into the concept.
But remember...Vr is only a turbine term. No piston powered airplane has a Vr speed simply because they do not have the excess power available for it. A turbine powered airplane does and so you can rotate to the take-off attitude at Vr before V2 and expect to accelerate to V2 in the air. In fact you can get to 35ft agl altitude as you reach V2 in the certifying process for the airplane.
Piston powered airplanes, however, stay on the ground until V2. And yes, V1 and V2 can be the same. It is, on the Wright powered DC-3...81 kts is both V1 and V2.
I hope that clears things somewhat for you.

 

[kilroy]

Citation 500 v1 and vr are always within like 1 knot so no difference on that.The plane I fly now its always about 10 knots different v1 to vr .. on a hot day you can be 5000 feet down the runway beforte you hit vr.
Regardless of runway lenght v- speeds or calculated for the given weight ,temp and runway condition.

 

[IFlyGC]

Mini,

Your mostly right in everything you've said above, but this may help.

V1 - Max speed you can be at before you abort and still be able to stay on the runway. Yes, on a 40000000ft runway V1 would be after VR and on a 4000ft runway, it would be well before VR. It includes things like brake temp, weight, runway slope and contamination. Most performance manuals will make V1=VR when V1 is actually greater than just because it makes things less confusing.

VR - Kinda self explanatory, but remember, it's not the speed the aircraft becomes airborne, just the speed you START the rotation.

V2 - This is similar to Vyse (aka "blue line") on a piston twin. Turbine aircraft have to meet certain climb gradients depending on terrain. (I believe it's 200'/NM but don't quote me on that) They also have to be able to cross the opposite end of the runway threshold at 35' AGL. V2 is the speed which will give you the most forward speed, but at the same time allow the aircraft to meet the climb gradients.

Hope this helps

 

[EagleRJ]

All mostly correct replies so far. Another fact to consider is that the charts determining V1 assume that the decision to take off or abort is made by that point. The regs assume that the pilot will take two seconds between failure of the critical engine (Vef) and initiation of abort action (brakes, pulling power to idle, deploying speed brakes). There have been numerous accidents where a malfunction occured after V1, even by just a few knots, and the decision was made to abort. On a runway-limited takeoff, this guarantees an overrun.

Vr is required to be greater than V1 per Part 25. It must also be at least 105% of Vmc, and must allow accelleration to V2 by 35 feet above the runway. Rotation must also occur at a speed greater than the aircraft's Vmu, which is determined during flight testing. Vmu is the lowest speed at which the aircraft can physically lift off the runway. Some aircraft can be rotated at a slower speed than Vmu, but drag increases to the point that acceleration stops, which is obviously not a good thing!

V1 and Vr are normally very close to eachother. In all of the aircraft I have flown and flown in the cockpit of, they are either the same or within five or six knots of eachother. In some aircraft, they are very different, though. For the Concorde, for instance, decision speed was normally around 140 knots, but rotation wasn't until 195 knots! The 400,000 pound aircraft could no longer stop on the runway at a speed similar to other aircraft of the same weight, but that delta wing couldn't start to work until a much higher speed.

 

[avbug]

But remember...Vr is only a turbine term. No piston powered airplane has a Vr speed simply because they do not have the excess power available for it.

Wrong. Many aircraft, especially older piston airplanes, utilize a published Vr number.

Having a piston engine doesn't mean that the airplane lacks power or performance, incidentally. Further, a rotation speed has nothing to do with power, or performance. It's a takeoff number; it's the appropriate time to raise the nose off the runway, or peel the entire airplane off the runway in the case of a conventional gear airplane, for a given weight, and density altitude.

V1 is not a decision speed; the decision is made before reaching V1. It's a cutoff point; V1 is used to establish accelerate-stop distance, accelerate-go-distance, or balanced field length. All of those speeds presume that the decision to abort or go has been made prior to reaching V1. It's often called decision speed, but it's not. Further, the number if very often massaged and manipulated for the purposes of takeoff distance. V1 is about takeoff and abort distances.

Vr is nothing more than the speed at which applying the rotation or liftoff is initiated. It must be equal to or greater than V1. It's also a speed often used in Part 23 aircraft as well as Part 25, and also in airplanes much earlier than Part 23.

V2 is a certification speed used to meet second segment climb gradient criteria. It is not blue line. It is not Vx. It is not Vy. It is a target airspeed for establishing second segment climb only, which is the period between landing gear retraction and 400' above the takeoff elevation (or clear of obstacles).

The airplane is accelerated to a minimum flap retraction speed at that point, and then to it's Vf, or it's final climb speed after clearing obstacles and getting cleaned up for the climb.

 

[TonyC]

V1 is not a decision speed; the decision is made before reaching V1.

In your case, when is the decision made?

 

[IFlyGC]

I suppose depending on how you read into it, V1 isn't a "decision" speed....it's just a number. You've already decided that if the kaka hits the fan prior to V1 then you'll abort and if it's after V1 then you'll go. V1 just lets you know which side of the airport fence you'll stop. Doesn't really matter what you call it, the meaning is the same.

 

[Donsa320]

Avbug

Please direct me to a piston powered transport category airplane that has a Vr in its certification. And I do not mean some made up figure from some flight department.

Thanks

D.C.