Same wing, different Vspeeds

[XPMorten]

Was looking through the manual, and I noticed
that e.g the 737-700 has LOWER Vspeeds than
the -800 for the same weight & conditions!

example 140.000lbs:

.........V1....Vr....V2
-700 132 135 142
-800 139 141 151

Since the acf are practically indentical except for the the length, I'm wondering why this is.
The only explenation I can think of is to avoid tailstrike - since the -800 is longer.
If anything, the -800 should have more lift due to a bigger fuselage.
Any ideas?
(same thing with the classic 737's)

Cheers,

M

[Jetstream Rider]

I reckon its a mass issue. More lift is required for the -800 as it has a higher mass so you need to go faster. Fuselage won't give that much lift.

[Mad (Flt) Scientist]

Well, those speeds may be determined by some or all of:
Vs
Vmca
Vmcg
Vmu
Vmbe

Assuming it's the same wing and tyres/brakes, I can't see that Vs or Vmbe would change (if anything, Vs might go down, assuming the same cg envelope, though I doubt they'd bother trying to take credit for it). One would expect the longer aircraft to have if anything a slightly better Vmcg/a (though these things can be odd) which only leaves Vmu as a candidate. If the longer aircraft are geometrically restricted over their shorter brethren, that would be logical.

edit: it's not mass, because those data are quoted at the same weight for both types. Unless Boeing are playing fancy games with minimum flight weight on the larger aircraft in order to reduce Vmc's - again, something I doubt

[XPMorten]

The pattern is the same on Vref. The -800 has slightly higher numbers. On another note, the
-900 (!) has even higher numbers on all speeds
which again fits this pattern...

The -800 also has a slightly larger CG envelope
3-36 % MAC vs 6-33 % MAC on the -700.
Probably since the horizontal stab is further away
giving a larger moment.

M

[Mad (Flt) Scientist]

Ah. If the fwd limit is different then although it's the same wing the stall speeds WHEN CORRECTED TO FWD CG will be different - higher on the -800 with a further forward limit. A delta of 3% on cg doesn't sound big enough, though, to justify the speed changes seen, though - 9knots on V2 would be something like 7 knots on stall speed - not credible for a 3% cg shift.

[Jetstream Rider]

Doh! Top marks for observation to JR!

RTFQ me thinks....

[slice]

-900 V speeds greater than -800 speeds which in turn are greater than -700 speeds (all at the same TO mass) ?

Is it possible that with the additional length of the 800 and 900 they are 'geometry limited' like the MD80/MD90 ie the length of the fuselage behind the Main UC prevents the AC being rotated to the optimum aerodynamic pitch angle at take off, thus increasing the TODR and in turn affecting the V speeds.

Are the different models (700/80/900) rotated to different pitch attitudes at take off ?

[esreverlluf]

OK - on a similiar topic, my airline operates B767s with both GE and RR powerplants.

We also see different V speeds for the same weights in what is not only the same wing, but also the same fuselage.

Anyone care to comment on why this is so?

[Old Smokey]

esreverlluf,

If the two B767 variants that you describe are indeed identical in every way, excepting the engines, the answer probably depends upon whether the Takeoff thrust delivered by the (differing) engines is also identical, or not.

If the Takeoff thrust is identical, you'll have to dig a little deeper for the answer. If they differ (which they probably do), then Vmcg and Vmca will be higher for the higher thrust engine, thus changing the V1/VR/V2 speeds which relate to Vmc.

Another factor to be considered is that the two different variants may have been "mission optimised" differently. The manufacturer can choose V2 starting from V2min with no defined upper limit. Choosing the lower (V2 min) optimisation will provide improved runway performance, thus a good choice for short / shortish runways, but paying a price in degraded OEI gradients impacting upon obstacle limits. If a higher V2 schedule is chosen, then runway performance will suffer (not a problem if you're operating out of long runways) but OEI gradients / obstacle performance will improve.

Regards,

Old Smokey

[esreverlluf]

Thanks Smokey - but both engine types claim 60,000lbs thrust - and using airline specific charts which I believe are optimised for the "Go" case.

It's always been a msytery to me, I probably should take it up with our performance engineers . . .

[Old Smokey]

esreverlluf,

If your engine ratings are indeed identical, methinks that you will find the answer in referring to your Performance Engineers.

I suspect that the answer will be that the later version of the aircraft (whichever that one is) is following an improved "improved climb schedule" if indeed runway lengths are not limiting for your operations.

Regards,

Old Smokey

[Sniff]

Are the engine mounted chines different in any way?