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.

 

[GravityHater]

Is there ever a published Vr that is less than a published V1?

I think if Vr is ever less than V1, they are listed as the same in the charts because there is no meaning to V1 after you have rotated; there is no "takeoff decision point" to be made after you have already taken off.

Are there turbojet airplanes where you would set it back down on the rwy after an engine failure??

 

[minitour]

Is there ever a published Vr that is less than a published V1?

I think if Vr is ever less than V1, they are listed as the same in the charts because there is no meaning to V1 after you have rotated; there is no "takeoff decision point" to be made after you have already taken off.

Are there turbojet airplanes where you would set it back down on the rwy after an engine failure??

Like I said above, theoretically if you had a long enough runway (I said 40,000') your V1 speed could and would be higher than Vr. However, by definition, V1 can not be higher than Vr (as we both said). So you're right, if V1 is higher than Vr, they are the same.

-mini

PS
Thanks to everyone for sheading some light on the subject...not sure I still understand, but I've got some time to go before I have to understand

Thanks again

 

[IFlyGC]

You can set anything back down on the runway if it's long enough. It's more of a question of why would you want to? It WILL fly with an engine failiure.....they had to prove that during the certification process. It's always safer to fly an airplane than it is to try and drive it at silly speeds on a runway.

If you DID decide to bring the good engine back, flare and land, you've then got to worry about being over max landing weight, over the speed limit on the tires, brake fade issues, runway conditions, etc......would you REALLY want to consider all of this in a split second?

It's like most things in aviation.....you can do it, but would you really want to?

 

[Lead Sled]

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.

I love the way these threads evolve. You light airplane drivers need to make sure you aren't confusing some things here. Remember, light (under 12,500 lb) aircraft are not certified to the same rules or criteria as the large (12,500 lb +) and turbine-powered aircraft.

Part 23 airplanes (light aircraft) have no guarantees when it comes to takeoff performance. Period. Large, large turbojet-powered aircraft do. Although some manufacturers publish accelerate/stop and accelerate/go for their light aircraft, these charts really don't pertain to the discussion. (The charts for the large aircraft provide for a certain minimum level of performance, the charts for light aircraft only provide for the aircraft to become airborne.)

When flying light aircraft you have the option of delaying your decision to fly right up until you rotate. (In certain instances, even afterwards - for example, leaving the gear down on a light retractable until there is no more usable runway in front of you.) Although you might assume that in this case "V1" = Vr, this is an entirely different thing - you're trying to compare apples and oranges. Light aircraft have no published V1 speeds. When a pilot of a light aircraft "decides" to fly is not calculated from data.

Don't feel bad. 99% of professional pilots, myself included, don't fully understand this topic. We know it well enough to function in day-to-day operations, but misconceptions abound. For example, if I were to ask most pilots to define V1 I would expect the majority to answer that it was the takeoff decision speed. Wrong answer.

By definition, V1 means the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance. V1 also means the minimum speed in the takeoff, following a failure of the critical engine at VEF, at which the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance.

The point is that if V1 is the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance, then the decision to abort or continue the takeoff must be made at some point prior to reaching V1 in order to allow for pilot reaction time.

It wasn't all that long ago that the FAA changed their definition of V1, prior to that, it was the takeoff decision speed. From what I understand, it was done to, just as has been said, keep you from going off the end. (This was a real, in some cases, inevitable possibility if the crew aborted at or very near to V1 on a short runway.) US operators need to know what "delay" is built into their charts. The G200 (which was certified about 4 years ago) has, as I remember, 7 or 8 seconds built in. This should be plenty of time. A Lear 35 (which was certified sometime during the late Jurassic), for example, had only about 3 - which probably wasn't enough. At my last Lear recurrent (several years ago), the folks at FlightSafety were suggesting that the crews start calling V1 about 5 knots early.

There are several differences between what the FAA considers adequate and what the aviation authorities in other countries feel is necessary when it comes to aircraft certification - both in the equipment that must be installed on the aircraft and the charts and data that is included in the AFM, etc. and even in some cases, operational techniques. For example, our V1 charts assume a dry runway. If it's wet or has ice and/or snow on it we're still good to go, but it's up to us to decide how much if any "fudge factor" we want to add. I've seen the Canadian charts for our airplane, they list the correction factors. There are also some differences on operational techniques, for example, to comply with the Canadian performance charts, a Lear 35 pilot would technically have to taxi into position, hold the brakes, set takeoff power and let it stabilize for 30 to 45 seconds prior to releasing the brakes. Yeah, right (eh? ). There is no way anyone here (or for that matter, in Canada) is going to do that.

I guess the point I'm trying to make it this stuff is not as simple or straight forward as it might seem.



As far as V1 never being greater than VR - that's correct and also very logical. To keep things simple, let's use the current JAR and previous FAA definition of V1 - Takeoff Decision Speed.

VR is simply the speed at which the pilot initiates rotation; in other words, the speed at which the pilot begins raising the nose. The airplane will liftoff when it achieves a sufficient angle of attack.

VLOF is merely the speed at which the airplane becomes airborne.

V2 (Takeoff Safety Speed) is the speed that you will have at 35' agl - in the event of an engine failure at or before V1. You maintain that speed until you have 400' agl and are clear of any immediate obstacles. (However, if you manage to have a speed faster that V2 you wouldn't want to allow it to bleed off - you would maintain whatever speed you had at that point.) Once you are clear of the obstacles you enter the 3rd segment or acceleration phase of the where the aircraft is allowed to accelerate VFS or Final Segment Takeoff Speed. As the aircraft is accelerating it is cleaned up appropriately as the various speeds are achieved. Under normal operations (all engines operating) V2 is of little interest to the crew - the airplane is accelerating like the proverbial "raped ape" - and it blows through V2 in very short order.

For any given flap setting/takeoff configuration VR, V2, and VFS (or what ever name the particular manufacturer uses) is a function of aircraft weight.

Now for the question as to why VR is never less than V1...

Like I said, it's really very logical. In Part 25 transport category aircraft (airliners and bizjets), you can make the decision to fly before you rotate (V1 < VR) or when you rotate (V1=VR), but never after you've rotated (V1 > VR). Once you've rotated, you're going flying. Period. You've made your decision and you are committed. Could you possibly stop after accellerating past V1? Perhaps, but if you do, you're really risking some potientially very serious problems. Statistically, there are few things more dangerous, in large aircraft, than high speed aborts. You are much, much safer taking almost any conceivable problem into the air.

Hope this helps.

'Sled

 

[IFlyGC]

Lead, we can all copy and paste from part 1 of the FAR's....the fact of the matter is what you posted above is the same (in theory) as everyone else as posted.

Oh, and before you start calling people "light airplane drivers" (which is no bad thing might I add....we've all been there) you might want to check who your talking to. At least 5 of us in this thread are typed in something and that's just those of us who choose to declare it in our profiles!

Why do people always have this "your wrong, I'm right" attitude on here....not just you Lead, just kinda sick of seeing it on the forums lately.

Rant over...........

 

[minitour]

Okay...this thread has brought up a few more questions. Thanks in advance for helping me out here.

Vef - The speed at which the critical engine is said to have failed

Is this a specific number listed somewhere or is it just, you roll down the runway and the critical engine quits and you go, "hey, that makes ___kts Vef!"

Also, if the plane doesn't have a critical engine, do you still have a Vef?

Vr - Doesn't every plane have a Vr? A speed which you begin to rotate? Like in a 152 you start to raise the nose at 50kts...when the plane has a sufficient AOA, it lifts off (Vlof). I would think that would make 50kts Vr in a 152 right?

Vlof - Is this also a known speed based on Vr and weight and such or is it another "oh hey we lifted off at ___ knots so Vlof is ___kts!"

Vyse - Why is it that we shoot for Vyse if we lose one rather than (if there is one - I'm guessing) Vxse? Wouldn't you want to gain the most altitude in the least space? Avoid obstacles? stuff like that? Or is it too close to Vmc?

Vmc - Is this just simply the speed at which you can't put in enough rudder and aileron to stop the plane from rolling or does something worse happen?

Vmu - So this is the speed which the control surfaces become "effective"? So if you pull back at this speed, the nose would come up, but not rotate (eh...the plane wouldn't get off the ground?) ? Is this because the AOA would have to be increased due to the low airspeed and induced drag would be too much? Or is it a different concept altogether?

Induced "airflow/lift" - Read something somewhere about twins having an "induced airflow/lift" factor regarding the engines. Something about the slipstream from the prop going over the wing and creating more lift??? How does this work? Is it pretty simple or is it more complex than that?

Vmc conditions - Okay, so when you do a Vmc demo, you use all sorts of flap/gear/prop configurations...what would happen if you couldn't feather the prop? I would assume that performance would be severely affected...how bad does it get?

Okay...I think that's enough "nerd time" for tonight.

Thanks folkz

-mini

 

[Lead Sled]

Lead, we can all copy and paste from part 1 of the FAR's....the fact of the matter is what you posted above is the same (in theory) as everyone else as posted.

Oh, and before you start calling people "light airplane drivers" (which is no bad thing might I add....we've all been there) you might want to check who your talking to. At least 5 of us in this thread are typed in something and that's just those of us who choose to declare it in our profiles!

Why do people always have this "your wrong, I'm right" attitude on here....not just you Lead, just kinda sick of seeing it on the forums lately.

Rant over...........

I didn't mean to come across as condescending. From reading the posts, it seemed to me that there was still some misunderstanding and I was just trying to state it yet another way. Sorry if I offended you.

Signed,

Captain Obvious

 

[IFlyGC]

Vef - Just the speed at which engine failure occured. It does not have to necessarily be the critical engine although it is usually assumed that it is during calculations.

Yes, every airplane has a Vr......if it didn't it would be a tricycle ;-)

Vlof....can't say I've ever used that one to know the correct definition.

Vyse - Same reason you pitch for Vy in your single engine cessna. It gives the most efficient climb based on available power. There IS a Vxse and you'd use that just as you would in a single.....when you need to gain altitude in a hurry.

Vmc - Your mostly correct, but up to 5 degrees of bank using the aileron is allowed... The rest must be the rudder. (and to be technical, it's maintain heading, not prevent rolling forces) As for "will something worse happen" that all depends on the aircraft. Some will roll over, some will flat spin, it all depends on the aircraft, CG and what it feels like doing at the time.......just don't be there to find out. During training you'll intentionally Vmc roll the aircraft a little (usually 20 degrees or so in a demo) just to get a feel of it.

Vmu - Minimum unstick speed - The minimum speed at which you can get the aircraft physically off the ground. Doesn't really have any purpose in day to day operations unless your a test pilot crunching numbers.

Induced airflow - The prop is nothing more than a big fan moving air from the front of the prop backwards. This extra air will pass over the wings generating more lift....it's that simple.

As for Vmc conditions, when they certify the aircraft they try to reproduce the worst case conditions possible. What are they?

1) Standard day at sea level - This ensures the working engine is giving the maximum available power

2) Critical engine failed, its prop windmilling and max power on the other engine - This causes the most yawing tendancy to counteract.

3) Aft CG - This makes the effective arm of the rudder the smallest, reducing the ability of the rudder to yaw the aircraft.

4) Gear down - This is the only thing helping you. The gear acts like a keel on a boat trying to stop the yaw.

5) Out of ground effect - In ground effect you have extra support under the wings assisting you. Being out of ground effect causes the wings to work harder.

I think there's a couple more to add to the list but it's midnight and the old grey matter is getting ready to hit the hay.

hope this helps.......

 

[IFlyGC]

no probs Lead, just tired and cranky tonight. You'll have to admit, there's been a lot of "your a d*ck, your wrong" on here lately.

 

[minitour]

no probs Lead, just tired and cranky tonight. You'll have to admit, there's been a lot of "your a d*ck, your wrong" on here lately.

No there hasn't...you don't know what you're talking about

Seriously though...Thanks for the help.

-mini