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Why must VR (Rotation Speed) be set so precisely? I mean why can't they just say:

Rotate the Aircraft at 120 to 150 knots

And then, the pilot thinks:

OK, I have a full plane and half full tanks, I think I will rotate at around 135 knots

And then rotates at around that speed?

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    $\begingroup$ Because, as with anything in the world of aviation, safety is the main priority. $\endgroup$ – J. Hougaard Jul 29 '16 at 6:39
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    $\begingroup$ @J.Hougaard And what makes the plane safer when you rotate at a specific speed. Thats what I want to know. $\endgroup$ – Noah Krasser Jul 29 '16 at 7:10
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    $\begingroup$ The short, simple answer is that the calculated Vr speed gives guarantees. $\endgroup$ – Simon Jul 29 '16 at 8:22
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    $\begingroup$ To prevent tail strike? If Vr is too low, then the angle of attack will need to be higher to create the take-off required lift and the tail will touch the runway. There are many occurrences of such miscalculated Vr (e.g. this one /// or /// this one) resulting in tail strikes. $\endgroup$ – mins Jul 29 '16 at 8:36
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This answer is written for air transport category aircraft.

Introduction

During take-off there are three operationally significant speeds that ensure a safe take-off:

  • V1 - the take-off decision speed
  • VR - the rotation speed
  • V2 - the take-off safety speed

In addition there are three technically important speeds:

  • VMU the minimum unstick speed
  • VMCG the minimum controllable speed on the ground in case of an engine failure
  • VMCA the minimum controllable speed airborne in case of an engine failure

V1 is the take-off decision speed. Below V1, take-off can aborted safely without overrunning the end of the runway. Above V1 the aircraft is unlikely to be able to stop before the end of the runway so in case of an engine failure it is safer to get airborne. V1 needs to exceed VMCG to ensure the aircraft doesn't veer off the runway when accelerating to take off with a single engine.

VR is the rotation speed. At this speed the aircraft should be rotated and will get airborne soon after. The rotation speed ensures that, in the case of an engine failure, lift-off is possible and V2 is reached at 35 feet at the latest. At the end of the rotation manoeuvre, the aircraft should be airborne. Otherwise the tail will hit the ground. Therefore, VR will need to exceed VMU.

V2 is the minimum speed that needs to be maintained up to acceleration altitude, in the event of an engine failure after V1. Flight at V2 ensures that the minimum required climb gradient is achieved, and that the aircraft is controllable. V2 speed is always greater than VMCA to ensure the aircraft is controllable in flight if an engine failure should happen.


Answer

What will happen if we just pick a VR that "feels good"?

If VRfeel is lower than VMU, we will end up with a tail strike. Even if the rotation would stop just before the tail strike, the increase in aerodynamic drag (lift is generated) will cause the take-off run to be longer than needed, risking not being able to climb over objects at the end of the runway. In addition, if the aircraft has an engine failure during rotation, it may not be able to reach V2 and could become uncontrollable and crash.

When VRfeel is too high, the aircraft risks exceeding the maximum safe tire speed. There is also a risk of not reaching VRfeel before the end of the runway.


References

Airbus operations briefing note on takeoff and departure operations; understanding takeoff speeds

Boeing article: exceeding tire speed rating during takeoff

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    $\begingroup$ Wow, great answer, can you tell more about the "safe tire speed"? $\endgroup$ – Noah Krasser Jul 29 '16 at 10:56
  • $\begingroup$ @NoahFisher see the linked article from Boeing. Basically it is the maximum speed that the tire is rated for. $\endgroup$ – DeltaLima Jul 29 '16 at 11:01
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    $\begingroup$ You say that this was "written for air transport category aircraft". Is there any real significant difference for any category of aircraft? Physics works pretty much the same no matter the class of aircraft, it's just that an F-16 will reach Vr faster than an A320 which will (may?) get there sooner than a C150, right? Also, +1 for "Vrfeel"! $\endgroup$ – FreeMan Jul 29 '16 at 13:56
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    $\begingroup$ @FreeMan in a C150 we don't have V1, Vr and V2; since the aircraft can easily be stopped unless you're taking off on super short runways. On multi-engine aircrafts there is Vmu. Since C150 is single-engine it doesn't have one. $\endgroup$ – kevin Jul 29 '16 at 14:18
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    $\begingroup$ @FreeMan, both the F-16 and the C150 have only a single engine so the point of discussing single-engine-out performance is becomes rather moot. However for multi engine non transport category aircraft the same story line can be used. $\endgroup$ – DeltaLima Jul 29 '16 at 14:18
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It's all about safety. The main reasons are:

1) Rotating too early will result in, best case scenario, a tailstrike, worst case would be leaving ground effect at too slow an airspeed and just stalling right away.

2) Before every flight, the pilot must determine if there is enough runway length to takeoff in. It goes without saying that there's no point doing this calculation if you're not going to stick to it. Even a bit of extra speed greatly increases the amount of runway used for takeoff, and chews through the stopping distance available if something goes wrong. It will also decrease the climbing angle, reducing obstacle clearance.

3) Tyres have a maximum speed, after which they may burst.

Aviators can't afford to be lackadaisical and rotate whenever they feel like. In fact, absolutely nothing in flying should be done without solid planning beforehand.

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It is not as exactly as you may think. The optimal speed may is even higher/lower due to unknown factors (human load weight distribution, exact winds etc...).

But the speed you calculate is, roughly, the optimal speed. There is really no meaning in choosing another one.

Imagine you are pressurizing your cars tires. There is mostly a value written on it which is optimal. Then you go for that value, don't you? Why would you just give away free optimization?

In aviation, this is even more critical, as you never ever just make things unnecessarily worse due to safety reasons. Accidents don't happen due to one bad thing, but due to four, five not quite optimal things (a controller, who is a little tired, a radio which is slightly to noisy, wind which is slightly stronger then expected, slightly more resistance on the runway due to heat... that's how accidents happen)

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  • $\begingroup$ Why downvotes?! $\endgroup$ – Mayou36 Jul 31 '16 at 13:33

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