Question about B737 flap40 landing

[bradlee737]

An instructor said that when 737 land with flap40, touchdown speed is just vref, there will be winglets stall occur, which cause horizontal control unsteadily.Is that right? I cannot find any depiction through the doc.
Any one have some idea? thanks.:rolleyes:

[TopBunk]

I haven't flown the 737NG, but on the older series the approach min speed is Vref+5, why would it be otherwise?

[bradlee737]

yes, appoach min speed is vref+5, vref is for touchdown(wind calm).:)

[BOAC]

Actually I believe 'touchdown speed' is nominally Vref-5 although it is rare to see that flown correctly. Vref is the 'target speed' at 50' I believe?

bradlee - I am fairly new to winglets on the NG but I know nothing of this claim and I don't think that the winglets contribution to directional stability is significant. Perhaps CaptainSandL can help here? There is nothing I have seen from Boeing about it.

[Tee Emm]

Vref is the 'target speed' at 50' I believe?

I think that is for certification reasons only. In other words the test pilot aims for that as part of the certification process.

For day to day operations the +5 is bled off approaching touch-down (defining "approaching" is a matter of many personal opinions). Touch down is acceptable at minus five from Vref. In practice it is extremely rare to see this as many pilots flare at anything from 5 to 15 knots faster than Vref due lazy flying techniques in calm wind and long runways.

[BOAC]

Correct, as I said.... but the point being that the NG does not fall out of the sky at Vref-5 and presumably does not suffer from this legendary 'winglet stall'? If it does, then Boeing do not know about it.:)

[ScottyDawg]

Boeing state that approach speed of Vref 30+5 or Vref 40+5 is flown in order that full manoever protection is assured at Flap 15 (go-around) configuration.

Stall speeds are therefore considerably less than Vref 40-5 or Vref 30-5 (although I can't lay my hands on the factor above Vs for Flap 30 or 40 configuration at the appropriate flap Vref speed).

Touchdown technique is to effect touchdown at a speed between Vref and Vref-5 as previously mentioned (power being slowly retarded during the roundout/flare) - note that on a -800(NG) tailstrike will occur at around +13 deg pitch (oleos extended) or +11 deg pitch (oleos compressed).


Cheers

ScottyD:O

[Flight Detent]

I agree with "Bradlee737", Vref40 is the speed to aim for at 50', touching down just a 'smigin' under.

Anyway, the normal B737NG landing flap is 30.

Cheers, FD :uhoh:

[BOAC]

Folks - we are slowly drifting further away from the original question! Has anyone else heard of this 'winglet stall' - whatever that might be?

[Lucky Angel]

I fly the NG with winglets and to be honest I DID notice a horizontal instability when near VREF. I don't know why but i had a scary ldg once in germany when i had a big wing drop very close to the ground with only a 10kt crosswind and no gust at all. Maybe your instructor knows something we don't.

[Mad (Flt) Scientist]

....but the point being that the NG does not fall out of the sky at Vref-5 and presumably does not suffer from this legendary 'winglet stall'....

You won't necessarily "fall out of the sky" because a relatively small part of the wing has stalled.

This is NOT B737-specific, but in a general sense, it's entirely possible that at the higher AOAs associated with the final stage of the approach/flare, the winglet is sufficiently off-design that there is a flow breakdown, or stall; if the winglet has been "tuned" for maximum efficiency in cruise, then the conditions at the tip may be very different. I certainly have heard of similar behaviour on other types (winglet stall, that is, not the control issues mentioned).

Personally, I would be surprised if there was a significant handling effect, as
(1) winglet retrofits have generally been accomplished without having to introduce handling limitations, and often on the basis of no change to handling (not perf, otherwise why bother!)
(2) if there were an effect, there would be numerous issues relating to asymmetric versus symmetric stalls, and I can't again recall of limitations which might be consistent with such a concern (xwind limit changes, etc.)

Again, personally, I'd be more inclined to attribute changes in control characteristics in the final stages of the approach to increasing ground effects, but that's just an opinion.