B737-800 Ref Speeds

[framer]

Hi,
Can anyone explain the relationship between Vspeeds on the 73-800.
Ie I have heard people saying that Vref Flap 40 is equal to V2 Flap 15 or something similar. I am not sure that that is correct.
Is there a definate relationship between all the speeds? If so, how did they design that? Did they choose the area/size of the flaps/slats in such a way that it gave the appropriate speed?
Cheers,
Framer

[captplaystation]

If I remember well, Vref 30 has been set "artificially high" (much bigger increment over Vref 40 than on a Classic) in order to increase the tail clearance on the 800, over what it would have been had the " correct" Vref 30 been utilised.

[flyingchanges]

Not true, Vref 30 is 1.3 Vso. Vref 40 is 1.25Vso.

[B737900er]

Framer

Vspeeds, Vref and flap schedules are based on weight of the aircraft.

Flap speeds in the NG are updated via the FMC unlike the classic where its a standard speed.
In the NG flap schedules are based on Vref 40 + additive.

Flaps up: Vref 40+70
Flap 1: Vref 40+50
Flap 5: Vref 40+30
Flap 15: Vref 40+20

The classic uses standard speeds.
Flaps up 210
Flaps 1 190
Flap 5 180
Flap 15 150.

[BOAC]

Framer - in general terms, the Boeing is built for simplicity. IF you maintain Vref+5 at any flap setting you will be ABOVE V2 for the g/a flap for that config (F15 or F1). The margin is generally positive (F15/30 and 40) except at very low weights where it can be negative, hence the 'teaching' of Vref+5 to be safe.

Certainly the 800 has about 4kts added to Vref F30 compared to the 700. I guess the 900 will be even greater but I do not have figures..

[Tee Emm]

The classic uses standard speeds.
Flaps up 210
Flaps 1 190
Flap 5 180
Flap 15 150.

Depends on the weight schedule. The above quoted speeds are for 53,070 kg and below. However, assuming that by now all 737 Classics have the required rudder mods installed, the Flap 5 block speed is 170 knots at 53,070 kg and below.

[ImbracableCrunk]

On the 737-9W, it looks like Vref40 varies from 126% to 128% above the 1-g stall speed. At F30, it's 132% to 134%.

One of my Boeing books states the Vref is 123% of the 1-g stall speed. It looks like F30 is padded by ~10% and F40 by ~5%.

[captplaystation]

flyingchanges,

so, about .05 Vs more than needed then (+/- 6kts? ) hence the bigger difference between Vref40 & 30 on the 800 than the Classic. And, if this isn't the case on the 700 (never flew it) possibly added, as I was told, to give greater tail clearance margin on the longer frame.
900 has 2 position tail-skid, so maybe they didn't bother?
2 position tail-skid an option on 737-800 Short Field, but my employer didn't tick that box , so don't know if it reduces Vref30 by a commensurate amount. . doubt it, as if it is a short field you are going for . . . well, I guess you will opt for F40 landing.

[ImbracableCrunk]

I don't think any of the 737NG have the 2-position tail skid automatically. None of the 8W, 8S, or 9W that I fly have the 2-position.

[captjns]

I don't think any of the 737NG have the 2-position tail skid automatically. None of the 8W, 8S, or 9W that I fly have the 2-position.

The B737-900 ERs have the two position tail skid.