VMCG vs VMCA - 747 400

[snowbirdbrad]

Hey...just wondering is someone can shed some light:

One of the questions I have come upon:

What is higher on the 747 400: VMCG or VMCA and Why? anyone know the approx or actual numbers?

From what I have looked up it is VMCG...which if equal to V1 would make sense to me.... becuase on the one's I have flown the V1 speed at max gross is higher than the VMCA speed...

Why? I could not find this in any of my books...but I figured it had to do with the fact that the speed would be higher on the ground to offset the yawing moment becuase of the additional friction force required with the wheels on the ground...

????

[Penske]

I was always puzzled by this question as well and never did find a definitive answer. This is my information on the subject:

Vmcg is higher because:
•Does not have the advantage of Vmca’s 5 degree bank towards the 2 operating engines
•Smaller moment arm on the ground (rudder to wheel area) than airborne (rudder to CofG)

I don't have specific data from the 747-400, but on the 747-100 at the example weight Vmcg 118KTS Vmca 102 KTS (HBJp.263, 264)

[BlueEagle]

I'll put this thread in the Tech Log forum, I feel sure you will get the answers there!

[clear_right]

Both deal with the turning moment created by the rudder offsetting the yawing moment created by asymmetric thrust.

Both speeds are in the least favourable set of circumstances:
eng fail of the critical engine
takeoff thrust on the remaining engines
takeoff flaps
gear up - Vmca
slippery runway - Vmcg

On the B727, the Vmcg is 60 kts, while there is no Vmca, as Vmca is lower than stall speed. Therefore you will stall the aircraft before you will lose control. This may also be the situation on the 747.

[slice]

Is it true that aircraft with all engines mounted at the rear could in practice be treated as centerline trust ie the engine thrust lines are so close to the longitudinal axis that any assymetric effect is negligible ?

[*Lancer*]

clear_right, it will depend on the weight of the aircraft... at heavy weights you will probably stall before you lose control. The 400 also has 2 Eng INOP Minimum control speeds - which are obviously more limiting. At 300T or so, the aircraft is just flyable from Vr with 2 engines going (on one side)... You'd want to be tracking straight out over water though! The crosswind will also be an influence on controlability, as its not part of the certification e.g. a failure at V1 with max crosswind will probably result in a runway excursion.

Lancer

[john_tullamarine]

(a) regular sort of topic .. please don't forget to run a search of the archives to find out what has been discussed before

(b) a couple of typical threads - one (http://www.pprune.org/forums/showthread.php?s=&threadid=98737&perpage=15&pagenumber=1), two (http://www.pprune.org/forums/showthread.php?s=&threadid=71279&perpage=15&highlight=Vmcg%20and%20Vmca&pagenumber=1).

(c) in general, no simple answer. For a specific aircraft Type and Model .. whatever the tests come up with ....

[Medwin]

VMCG- 118 kts
VMCA- 107 kts

This is 747 digits according to Handling the Big Jets book. Didn't explain why VMCA is lower than VMCG though.

[Mad (Flt) Scientist]

Is it true that aircraft with all engines mounted at the rear could in practice be treated as centerline trust ie the engine thrust lines are so close to the longitudinal axis that any assymetric effect is negligible ?

Absolutely not! If that were the case my company's jet engined productes - which are all rear engined twins - wouldn't need to worry about minimum control speeds, and I assure you that we certainly do!

One important reason is that acceptable minimum control speeds (for performance limiting reasons) are an important sizing criteria for the rudder, and sometimes other surfaces as well. Since the rudder does nothing useful in 99% of flight, one will make it as small as possible to meet the design requirements.

So, while an underwing twin with engines magically moved to the rear fuselage position would undoubtedly see an improved Vmc as a result, a rear engined twin of similar performance class *designed as such* would have a rather similar Vmc to the underwing-engined aircraft, as it would (simplistically) just have a smaller rudder too.