Certification of V2min

[Alex Whittingham]

From JAR 25-107(b)

V2MIN, in terms of calibrated airspeed, may not be less than –

(1) 1.13 VSR for –

(i) Two-engined and three-engined turbo-propeller powered aeroplanes; and
(ii) Turbojet powered aeroplanes without provisions for obtaining a significant reduction in the one-engine-inoperative power-on stall speed;

(2) 1.08 VSR for –

(i) Turbo-propeller powered aeroplanes with more than three engines; and
(ii) Turbojet powered aeroplanes with provisions for obtaining a significant reduction in the one-engine-inoperative power-on stall speed:

Can anyone involved in certification tell me what sort of provisions they might be referring to in paras (1)(ii) and (2)(ii)?

[Old Smokey]

Alex,

The 'clue' lies in 2 (i), i.e. "Turbo-propeller powered aeroplanes with more than three engines".

Vsr for the wing is the 'pure' lift derived from the wing alone, without any degree of lift augmentation from other sources. Propeller aircraft provide lift augmentation by way of increased airflow over the wing, the jet achieves this by way of providing a vertical vector of thrust, thus augmenting lift. In both cases, the actual Vs is reduced below Vsr, thus allowing lower V2's expressed as a function of Vsr.

Consider the 2 engined propeller aeroplane with one engine failed, the wing on the 'failed engine' side has no lift augmentation, and thus the actual Vs for that wing becomes Vsr. 1.13 Vsr thus becomes the minimum V2 to allow for suitable stall margin, in this case tolerance of 1.3 G. Now consider a 4 engined propeller aircraft (more than three engines as alluded to in 2(i)), with an engine failed, that wing is still receiving significant lift augmentation, reducing the actual Vs, and allowing a lower V2, as a function of Vsr. For the propeller aeroplane therefore, the deciding factor in allowing a lower multiple of Vsr as minimum V2 becomes whether or not BOTH wings are still receiving lift augmantation following engine failure of one engine.

Increased airflow over the wings as a factor in reducing stalling speed for a jet aeroplane is not a factor, as, except for a small amount of air which is entrained, no significant increase in airflow over the wing occurs as a result of engine operation. The vertical component of the thrust vector becomes the only lift augmentation available for the jet aircraft. The pitch attitude flown following engine failure for flight at V2 becomes significant, this attitude more specifically applies to the engine. Some aircraft have a quite low pitch attitude following engine failure, and the vertical thrust vector is insignificant (particularly aircraft without slats). If the pitch attitude is somewhat higher, then the vertical vector becomes a significant contribution towards lift augmentation. Consider a 300T aircraft with 2 RR Trent engines of 92,000 Lb thrust each (B777-300), at 10° pitch attitude, the vertical vector becomes 15975 Lb (7.246 T), reducing the actual Vs by a mere 1.2%. This will effectively reduce the actual Vs, although Vsr would remain unchanged. The question arises in 2(ii) of the regulations of whether or not this implies a significant reduction in the one-engine-inoperative power-on stall speed. In the case of the B777-300 it obviously does not, as Boeing specifies 1.13 Vs as the minimum V2 for this aircraft. The threshold of significance occurs when (in approximate terms) the same 1.3G stall protection is available at 1.08 Vsr. For the example quoted, the aircraft would require a significantly higher pitch attitude than 10° to achieve this. Some aircraft will, and some won't.

Regards,

Old Smokey

[Alex Whittingham]

Thanks Old Smokey. I hadn't considered the thrust vector at all, I was thinking of blown lift devices. Which aircraft use this?

[enicalyth]

G'day O_S

wasn’t the worry about Vs that if derived from the performance stalling manoeuvre then the load factor i.e. ratio of lift to weight could be significantly less than unity. In which case the multiplying factor would give illusory rather than desired margins. So the Vsr technique could be said to give “proper” V2 speeds rather than “lower”. [1-g stall speed as the basis for compliance… refers]

Otherwise very nice reply I have no trouble with. How’s it going?

[FE Hoppy]

As a comparison the Landing Climb speed is limited to

Not less than-
(i) 1.08Vsr for aeroplanes with four engines on which the application of power results in a significant reduction in stall speed; or
(ii) 1.13Vsr for all other aeroplanes.

CS25.119

Certification is I believe EASA these days no longer JAA

[Mad (Flt) Scientist]

wasn’t the worry about Vs that if derived from the performance stalling manoeuvre then the load factor i.e. ratio of lift to weight could be significantly less than unity. In which case the multiplying factor would give illusory rather than desired margins. So the Vsr technique could be said to give “proper” V2 speeds rather than “lower”. [1-g stall speed as the basis for compliance… refers]

The desire to use 1'g' stall speeds instead of minimum stall speeds arises from a desire for repeatable speeds, not a desire to make V2 (or any other stall speed related operational speed 'correct')

In fact, the intent of the reduced reference speeds is to achieve a situation of 'equivalent safety'.

Broadly, V2min=1.2Vs has been shown, empirically, to provide a sound basis for a minimum takeoff safety speed.

Vsr (1g stall speed) is going to come out higher than Vs (since the load factor at Vs is <1.0). In order to avoid penalising more modern designs for taking a more scientific approach to the determination of stall speed, the V2min requirement for a 'Vsr' aircraft changes to 1.13Vsr, precisely so that the same V2min will be arrived at whether one uses the Vs or Vsr methodology. In other words, a 'Vsr' aircraft has precisely the same margin between V2min and the 'real stall' as a 'Vsmin' aircraft.

(Which implies that for a typical aircraft, one would expect that Vsr=1.13/1.2=0.94Vs)