V2 and V2 min ???

[hamil]

Hi,
I'd like to know if someone could explain the meaning of V2 min used on DC-10 and MD-11.
What's the difference between V2 and V2 min?
Thanks

[Alex Whittingham]

Certification requirements specify only minimum values for V2. Manufacturers and operators often choose to use the minimum value of V2 because it will get the aircraft airborne in the shortest distance.

The disadvantage of using the minimum value of V2 is that, for a jet, it may lie on the wrong side of the drag curve and not offer the best climb angle. Some aircraft use 'increased V2' procedures where a V2 higher than the minimum is used giving better climb gradients but taking longer distances to get airborne. The benefit of improved climb gradient can either be used to improve obstacle clearance or to increase the climb limited TOM, sometimes called the WAT limited TOM.

[catchup]

Yes, that's correct. Airbus is doing it. They call it variable kvs-factor. Performancecalcs are therefore sometimes a little bit hazy.

regards

[Old Smokey]

V2 MIN = 1.2 X Vs or 1.1 X Vmca, whichever is the higher. Utilised as the lowest V2 allowed, and useful for short field operations, or Takeoff at less than maximum weights for the runway, with the 2nd segment OEI gradient suffering as a result of being at less than "Best Gradient Speed" (Vmd for jets).

Increased Speed V2 = A process of optimisation of 2nd segment OEI gradient generally up to Vmd (for jets) but no upper limit is specified. Field performance suffers as a result of the use of "Increased Speed V2", and therefore it's application is useful only on longer runways. Although no upper limit is specified by the regulations, it is usually approximately 1.4 X Vs at the highest.

A fully optimised Airport Analysis may choose a speed between V2 MIN and the optimum OEI best gradient speed, an equal trade-off between Field and Obstacle performance. Leave that one to computerised RTOWs, too hard for humans with sharp pencils.

Old Smokey

[john_tullamarine]

At the risk of being labelled a pedant ... a few qualifications to Old Smokey's post, above.

1. the current FAR 25 rule is a bit more involved - Old Smokey is referring to earlier two-motor rules (737 etc)
   
2. for earlier certifications, 3/4-motor rules permitted a stall-limited V2 of 1.15Vs. Note that the V2min term isn't used in the earlier reg, but the end result is much the same.
   
3. if one looks up the current rule for anything (as I did in (a)), please remember that it doesn't necessarily apply to Aircraft X. If you need to research rules applying to a particular aircraft, the first place to go is the TCDS library to find out the certification rule basis for the aircraft, and then to the superseded rule section of the FAA website to get a handle on what rules applied. However, be aware that all certifications involve a bit of horse-trading between the applicant and the regulator so there may well be a few further differences which are not easily unearthed .. however, by reviewing the relevant FARs, you will be close to the mark.
   
4. the climb performance curve (against speed) is a bit like an upturned teacup with V2min well down on the slow side of best climb at the top. As the speed for V2 is increased above V2min, initially there is a comparatively significant improvement in climb gradient which then rapidly tapers off as the speed approaches the maximum performance level. The penalty is a BIG bunch of extra runway distance required so the end result generally is a modest speed range above V2min scheduled in the AFM. The distance penalty sometimes is made more extreme by the need to use low flap settings to get any significant benefit.