V1, Vr and V2 question

[metrodriver]

scubabri, some airplanes have a V1=Vr, like the Be90. The metro's I flew had a Vr that was about 10 kts higher than V1(depending on weight). The B727 had V1=Vr under normal conditions.
V1 is depending on a lot of things, and changes dramatically when you operate from contaminated runways

 

[Hawker rider]

HI,

It scares me a little bit that there is so much un-knowledge about this subject. I even hear people say stuff WITH a multi engine rating that don't really know anything about this.

I'm just talking to my Lear co-pilot here in the hotel, and he NEVER even has heard of an minimum-unstick-speed, which is part of what your Vr is based on (Vmmu), in my opinion somebody has no business being in a multiengine airplane without a basic knowledge of this.

It is late here, so I might even get into this on a later date.

By the way V2 is your takeoff-safety-speed, not your minimum climb speed, and it won't hurt anything if you're above your v2 to keep that speed at that moment ( within reason) there is alos something as an optimizedV2 with is basically Vxse and is a speed +/_ 1.5 times your stall speed, so you would sacrifice climbperformance going back to v2.

anyway....M

 

[2000flyer]

From FlyChicaga:

Some mistake V1 for being the speed at which an engine can fail, and then you abort. Actually, you need to have made the decision to abort BEFORE V1.

This statement is factually correct. Boeing has done a video on RTO's (rejected take-offs). I don't remember the actual figure, but their study indicated that there was a 3-second delay in recognizing an engine failure and initiating an abort BEFORE reaching V1. This is also why you'll notice some AFM's will reduce V1 by a determined value on wet runways, thus giving extra time to recognize a need to about and giving you more runway left to abort in wet conditions.

From Hawker Rider:

which is part of what your Vr is based on (Vmmu)

I've never heard of Vmu used in certification process for a typical business jet. That isn't saying you are incorrect, but all the Vmu numbers I have ever seen are on large transport aircraft (Boeing's, Airbus, etc.). I can't imagine, at VR, being able to yank the yoke on a Citation Ultra and being able to ground the tail before becoming airborne. Then again, I've never tried. But I've haven't heard Cessna having to run Vmu tests prior to certification. I just checked our manuals for the 2000 and Ultra and no where in the limitations is there any reference to Vmu.

Regards,

2000Flyer

 

[aewanabe]

More on V2:
In our airplane, at typical weights we typically blow through V2 during rotation, before the mains leave the ground. With 14degrees nose-up, the speed usually settles around 170-185 knots for the initial climb. V2 comes into play as the takeoff safety speed, when you lose one at v1. Then v2 is the MINIMUM speed you would use for the second segment climb, prior to levelling off and cleaning up.

 

[TurboS7]

BTW V1,V2,VR only applies to Part 25 Aircraft. All the little birds are certified under Part 23, these fall into two categories depending on weight, one requires a climb gradient with one engine out, the other just has to maintain a certain altitude with one engine. I think flight schools confuse the issue as they want there people to think they are preparing for the big time. People need to grow up and call a spade a spade.

 

[enigma]

BTW V1,V2,VR only applies to Part 25 Aircraft. All the little birds are certified under Part 23, these fall into two categories depending on weight, one requires a climb gradient with one engine out, the other just has to maintain a certain altitude with one engine. I think flight schools confuse the issue as they want there people to think they are preparing for the big time. People need to grow up and call a spade a spade.

Turbo. We may have the start of another thread here. I wasn't aware that flight schools were teaching incorrect/confusing stuff, but I'm not surprised.

Are some schools trying to use V1, etc, when flying non-V1 airplanes? If so, what other false material/ideas are the teaching in other areas? It's been almost twenty years since I started in a college flight training program, and 15 years since I taught in said program, but at Central Texas College we certainly didn't intentionally teach anything, or use any procedures that weren't technically correct. I can see where a school might want to utilize the concept of V1 in it's multi program, but I would assume that they would also correctly explain the term and explain that while they (the school) was using the concept that they were not in an airplane that used V1. If not, no wonder we have to deal with all of these know it alls (comment pointed at no one in particular).

regards,
enigma

 

[bigsky]

I do agree the king air is not a good airplane to discuss v1 as it doesnt apply 100%. And just because you are at v1 it doesnt mean you couldnt stop on ry available. I am sure on a 10000 foot ry you could climb to 50 feet and land and then takeoff again also. HOWEVER in that case you are not talking about a balanced field in which v1 becomes much more critical.( very seldom are you on a true balanced field-unless you are ry and weight limited or if reducing thrust and are temp limited I believe you have essentially balanced the field)
There is a big difference when discussing large turbo jet aircraft(king air excluded).
If you look at the stats there have NOT been very many high speed aborts at or above v1! Jets make better airplanes than land buggies! I routinely see takeoffs at weights in excess of 600000 lbs and when we get to v1 I am thinking to myself "those airport analysis people must be smoking weed-there is no way we could think about stopping"
V1 also has 2 meaning- max speed at which an abort has to be initiated to stop on ry and (b) minimum speed at which you could lose an engine and continue takeoff and still meet all the performance and climb restrictions. Thats why it is not a good idea to mess with v1...I have heard about some pilots who have arbitrarily lowered v1 to give themselves a little cushion on stopping- however now if they decide to continue?

 

[CLCAP]

Even on a 12000 foor rwy it may not be advisable to abort after V1. Maximum brake energy limits are also taken into account when V1 is determined (high gross weight) and you may end up with no brakes doing 40 kts of the end of the rwy.

V1 is usually determined by balancing the field length. (Acc go = Acc stop distance) so that the manufacturer can show the most favorable numbers as far as the airplane is concerned.

I think the test pilots have a 2.5 sec break (they're not allowed to touch anything) after V1 and then either abort or continue in the certification process.

Also - flying faster than V2 (if you lost an engine that is!!) can have dire consequences if your departure is predicated on a climb gradient.

As far as Vmu (minimum unstick speed) goes - I thought that that was the lowest speed when the airplane would "fly" and had nothing to do with striking the tail.

 

[Snoopy58]

V1 also has 2 meaning- max speed at which an abort has to be initiated to stop on ry and (b) minimum speed at which you could lose an engine and continue takeoff and still meet all the performance and climb restrictions. Thats why it is not a good idea to mess with v1...I have heard about some pilots who have arbitrarily lowered v1 to give themselves a little cushion on stopping- however now if they decide to continue?

Let me agree with your main point & disagree with what you said...

"Dont' mess with V1." I agree. Being a test pilot without any engineering data to back you up amounts to "using the force," and that doesn't strike me as a very good idea for most of us. V1 gets determined using a LOT of factors (many of which have been touched on at some point in the discussions above), and arbitrarily mokeying with it is setting yourself up for some very uncomfortable questions from an FAA type. Or worse!

OTOH, V1 might be the max speed at which you can safely initiate an abort, and it might be the min speed at which you can lose an engine & safely continue the takeoff, and it might be both (i.e. exactly balanced field in front of you), but it might be neither.

Take a lightly loaded small jet of your choice on a 15,000' dry runway with a good headwind on a cold day. Great performance, and, as you (BigSky) pointed out, V1 isn't much of a player -- you will get to rotation speed LONG before the point you need to put on the brakes to safely stop at the end of the runway, and you'd be able to continue the takeoff on one engine long before you reach Vr. In that case, V1 is neither of the above.

In a "stop-oriented" airplane, say the C-130, you always made V1 (though we called it something else in the Air Force) as high as we could. It was either rotation speed, or refusal speed (i.e. accelerate on 4, lose one, and stop on the last brick). As long as you could fly on 3 at that speed (and you pretty much always could), you went with it. (and if you couldn't, all you could do would be reduce weight or increase thrust by going bleeds off) Unlike in fast jets, we never reduced V1 in deference to brake cooling or such things. Heck, we had lots of reverse & beefy brakes! (insert manly Tim Allen grunts here)

In a "go-oriented" airplane, say the T--38, you virtually ALWAYS had a significant gap between V1 and VR, just because the jet flew pretty well on one engine (afterburners are wonderful things), and regardless of how much runway you hed in front of you, absorbing the energy of a T-38 at it rotation speed was awfully rough on the brakes (and consequently, dangerous) if you didn't have a really stiff headwind.

Other airplanes are somewhere in the middle -- like the 737. Frequently, V1 = Vr; sometimes it is slightly lower, other times (contaminated runway) it's a LOT lower. And it can be driven lower by things other than just accel-stop distance. Anymore, there are enough factors going into the number-crunching computers that you can't always say for sure what the controlling factor is.

Usually, there is a range of speeds below refusal (accel-stop) and above decision speed (accel-go) where you -- or really, the engineers who write your takeoff data tables -- can set the V1, and you'll be able to stop at or below V1, and also be able to fly at or after V1. (And, live with the brake temperature if you stop.) For instance, if Vr is 130, you may be able to fly anytime after 110, and you can stop up until 125. Where's V1? In a Herk, we'd say 125; in a T-38 probably closer to 110 or 115. Where you put V1 in that range for a particular aircraft is a choice. Sometimes that choice is based on factors such as the knowledge that, on the one day in 1000 when you actually face to go/stop situation, you may not exactly duplicate the test pilots' performance. As one performance engineer put it, if you lose one and then cross the departure end at 34' instead of 35', nobody will care. But if you're a little slow getting on the brakes and you stop with the nose tire at 7001 down a 7000' runway, your day just got a lot worse!

So, in simple pilot terms, V1 is the speed by which you make your decision. Until V1 I can decide to stop, after V1 I need to go. At Vr I rotate. After that, if I'm flying with a problem, I want V2 at least. V2 may be predicated on Vyse, or Vmca, or other things. (for instance: Herk V2 went UP with increased thrust -- worse asymmetric thrust if you lost an outboard; 737 V2 goes DOWN with more thrust -- more thrust to balnce the greater induced drag at low speed) Prior to V1 you MAY be able to fly away safely, but it's a better decision to stop. After V1 you MAY be able to stop safely, but it's a better decision to go (if you can. If not... "hit the fence fast, or hit the fence slow" applies). In any event, as you're rolling down the runway, you need a simple decision point, not lots of factors to think about. And V1 is that easy simple decision for you. All the hard thinking has already been done by the engineers.

So I'll repeat my agreement with your main point: don't go "adjusting" V1 arbitrarily!

 

[antney]

Flychicago I'll check that out.

Also those runway analysis charts are numbers and a lot of is in theory. Wait till to you take off from Midway on a 95 degree day, full boat in the Saab, and when your calling V1, you tell me if you would have enough runway left to stop with normal braking, meaning no reverse. It's one thing to read the numbers, its another to be calling V1 and already knowing that you were taking off before you called it. You'll love seeing that saab rotating at the touchdown markings on the opposite side!
Anthony