Can a C of G movement change your vr speed?

[jetman12]

I was asked a question and wasn't sure of the answer:
Can a C of G movement change your vr speed?
Just say your field length limited and wanted to get off the ground quicker?

[JLMF]

Just put flaps down and non takeoff completely stoped, you´ll be fine with a shorter runway,

Jose Luis.

[Obs cop]

Agree with Alistair here,

As I understand from my P of F which was some 2 years ago now, I get it as the following.

The forward c of g limit is limited due to the stick forces required, although the stability will be better. Rear c of g is limited by stability, ie. too far back gives significantly reduced stability, but with the advantage of light stick forces.

If you imagine moving a 1 ton load from the rear hold to the front hold, then the all up weight of the aircraft remains the same, but common sense dictates it will take more force to raise the nose. That increase in the force required to rotate the aircraft, then means that a larger downwards force is needed by the tailplane.

This increased downwards force from the tailplane increases the aircraft's effective weight (ie. it's actual wieght plus the downwards force from the tailplane). Given the increase in effective weight, you therefore need more lift which means an increased takeoff speed.

If I recall correctly, that is, :confused:

regards,
Obs

[fly_antonov]

The safe CG envelope (X% MAC to Y%MAC) as determined by the manufacturer must be such as to allow the airplane to rotate smoothly at VR rather than the VR needing change according to the CG position.

Hence, the CG envelope is taken (by the manufacturer) small enough to accomodate a whole range of VR' s. If the CG envelope were to be taken larger, then yes, you would have a problem and you may need to take CG position into account for Vr.

Before the flight you need to determine whether or not your CG is within the limits acceptable for taxiing, take-off roll, rotation, climb and flight. This includes all VR/Vmcg/Vmca speeds and combinations thereof. That' s what the loadsheet is for.

As a reference, if I'm not wrong CG data is not used on the FMS take-off reference page.
It is used in the perf init page for computation of cruise CG in order to calculate fuel efficiency. I don' t hink that cruise CG is used as reference for take-off performance data.

I can imagine that on certain smaller aircraft your CG could modify your VR because the CG spectrum is made to accommodate a larger range of CG positions. I have yet to see such an aircraft' s POH as most of the time the manufacturer will prefer to take safe limits with the CG rather than play around with airspeeds.

[Vone Rotate]

This is what stab trim is there for and why it should be 1- set correctly with reference to the trim sheet and 2 - within limits!

[wheelbrace]

An interesting point in your last, Fly Antonov, but the Arrow (for example) has a range of 5 KIAS for Vr from minimum to MAUW. There is a tendency to wheelbarrow if you decide that an unnecessary 5 kts. should be added to Vr.
:(
I have been there...

However that is mass calculated, not CG. Flame me - see if I care!

[mad_jock]

I don't think makes any difference at all.

Vr is a function of your stall speed but limited by a few other speeds if your in a twin. 1.3 Vs springs to mind unless limted by Vmca

CofG is a balance issue that you have to be within.

I have a spread of Vr of 15knts through the weight range on my turboprop.
CofG is used to set the trim for departure and thats it for takeoff planning. At the forward limit and at the rear limit Vr remains the same.

[mad_jock]

See below for the selection of Vr


Vr must be equal to or greater than V1.
· Vr must be equal to or greater than VMCG.
· Vr must be at least 5% greater than VMCA. This buffer is to assure that if an engine failed as the aircraft
was rotating to takeoff nose attitude, the rudder and ailerons alone is sufficient to enable the flight crew to
hold the aircraft straight.
· Vr must be compatible with reaching V2 as the aircraft reaches 35 ft above the takeoff surface with 1
engine inoperative. Vr infact guarantees that the aircraft reaches 35 ft/V2 in the minimum takeoff distance,
and no later than the end of the TODA.
Correct Vr will ensure that even if the aircraft is rotated at the maximum rate the aircraft will actually lift
off the ground (VLOF) without the tail striking the runway. Correct Vr will ensure VLOF is at least 1.1
VMU all engines and 1.05 VMU 1 engine inoperative
· Some very long aircraft have a Vr which is artificially high because they are prone to tailstrike during
rotation. This is often referred to as “geometry limited Vr”. It is a higher speed than would otherwise be
required to meet the other criteria listed above.
· Rotating at correct Vr will allow optimum acceleration to V2.


taken from this document http://www.aviationshop.com.au/avfacts/per/inst/inst2.PDF

And it contains all the speeds and what effects them that you could ever need.