CG & Max Rate of Climb
Thread Starter
Join Date: Jul 2005
Location: heaven
Posts: 60
Likes: 0
Received 0 Likes
on
0 Posts
CG & Max Rate of Climb
Hey guys,
I was just curious as to whether or not the position of the CG will have any effect on Vy (i.e. the speed in terms of IAS), and, will it have any effect on your ability to achieve a maximum climb rate? (i.e. if the CG is more aft, will it be able to obtain a higher rate of climb at Vy compared to a forward CG?)
I have argued that the position of the CG should not have any effect on your ability to achieve a max climb rate; however, some people have told me an aft CG is favourable. I guess in case of stalls, an aft CG reduces the stall speed... are there any similar benefits for climb rates?
Thanks in advance!
I was just curious as to whether or not the position of the CG will have any effect on Vy (i.e. the speed in terms of IAS), and, will it have any effect on your ability to achieve a maximum climb rate? (i.e. if the CG is more aft, will it be able to obtain a higher rate of climb at Vy compared to a forward CG?)
I have argued that the position of the CG should not have any effect on your ability to achieve a max climb rate; however, some people have told me an aft CG is favourable. I guess in case of stalls, an aft CG reduces the stall speed... are there any similar benefits for climb rates?
Thanks in advance!
Wunderbra
Join Date: Aug 2006
Location: Bedford, UK
Age: 44
Posts: 313
Likes: 0
Received 0 Likes
on
0 Posts
An aft C of G is highly preferable for the cruise as it minimises the moment arm between the centre of pressure and the C of G, thus the force required from the tail is reduced. This allows the a/c to fly at a lower AOA as less lift is required and so the induced drag is reduced.
This should also apply in the climb as this tends to be at a lower speed, where the major component of the drag curve is due to induced drag. An aft C of G will again allow a lower AOA for the same lift and thus the max residual lift will be higher.
Obviously you also have the issue of stability being adversely affected by an aft C of G. Some A/C (most of the Airbus birds as far as I know) run an aft C of G which is very close to the centre of pressure. They require several computer systems controlling pitch to make the aircraft controllable!
This should also apply in the climb as this tends to be at a lower speed, where the major component of the drag curve is due to induced drag. An aft C of G will again allow a lower AOA for the same lift and thus the max residual lift will be higher.
Obviously you also have the issue of stability being adversely affected by an aft C of G. Some A/C (most of the Airbus birds as far as I know) run an aft C of G which is very close to the centre of pressure. They require several computer systems controlling pitch to make the aircraft controllable!
Guest
Posts: n/a
Hi Matt,
I thought all commercial transport aircraft had to have a system of manual reversion. For the Airbus I understood this to be non FBW rudder and trim. If the CofG is as far back as you say, makes this CofA requirement appear as just a box ticked .
They require several computer systems controlling pitch to make the aircraft controllable!
Wunderbra
Join Date: Aug 2006
Location: Bedford, UK
Age: 44
Posts: 313
Likes: 0
Received 0 Likes
on
0 Posts
I'm not saying that the C of G is so far back as to be completely uncontrollable if necessary, but that the workload to maintain control would be excessive in normal flight.
Join Date: Mar 2006
Location: Vance, Belgium
Age: 62
Posts: 271
Likes: 0
Received 5 Likes
on
3 Posts
The position of the CofG (provided it'swithin limits) does not influence much the AOA of the wings. The wing AOA will be what is required to produce the lift at the given speed.
However, the AOA of the elevator is directly influenced by the position of the CofG since the elevator has to exactly negate the moment arm produced by the pair weight@CofG / lift@CofP.
For the cruise, the optimum elevator AOA is the one for which the drag is minimal. For a symetrical elevator, this is AOA 0° producing no lift and thus matching a CofG exactly at the CofP.
For the best climb rate, the optimum elevator AOA is the one which produce as much lift as possible for a limited drag. This lift adds to the wing lift and thus increases the climb rate. This elevator AOA produces an upwards moment arm that matches a slightly aft CofG.
However, the AOA of the elevator is directly influenced by the position of the CofG since the elevator has to exactly negate the moment arm produced by the pair weight@CofG / lift@CofP.
For the cruise, the optimum elevator AOA is the one for which the drag is minimal. For a symetrical elevator, this is AOA 0° producing no lift and thus matching a CofG exactly at the CofP.
For the best climb rate, the optimum elevator AOA is the one which produce as much lift as possible for a limited drag. This lift adds to the wing lift and thus increases the climb rate. This elevator AOA produces an upwards moment arm that matches a slightly aft CofG.
Last edited by Luc Lion; 2nd Sep 2006 at 16:16.
Join Date: Oct 2004
Location: Los Angeles
Age: 61
Posts: 223
Likes: 0
Received 0 Likes
on
0 Posts
Interesting question. And I would agree with the previous posts.
Since the rate of climb in a steady state established climb is directly related to the excess thrust being produced over that required for the same airspeed in level flight it would follow that by reducing the induced drag, (by moving cg as far aft as possible and thereby reducing the total lift required and hence the induced drag) you would have less total drag and therefore greater excess thrust available, which would translate directly into a greater rate of climb.
As a side note I can recall a vector diagram that demonstrated that the total lift being produced by the wings in a climb is less than in a power off glide.
Since the rate of climb in a steady state established climb is directly related to the excess thrust being produced over that required for the same airspeed in level flight it would follow that by reducing the induced drag, (by moving cg as far aft as possible and thereby reducing the total lift required and hence the induced drag) you would have less total drag and therefore greater excess thrust available, which would translate directly into a greater rate of climb.
As a side note I can recall a vector diagram that demonstrated that the total lift being produced by the wings in a climb is less than in a power off glide.
Join Date: Jan 2005
Location: north
Posts: 319
Likes: 0
Received 0 Likes
on
0 Posts
Join Date: Mar 2006
Location: Vance, Belgium
Age: 62
Posts: 271
Likes: 0
Received 5 Likes
on
3 Posts
Sorry about that. I really feel ashamed to have stated that additional lift could help the climb. Not enough thinking before writing.
Yet, the positive influence of an aft CofG has to do with the transfer of lift from the wing to the elevator. At relatively high AOA (low speed), having part of the lift provided by the elevator (because of having an aft CofG) reduces the AOA of the wing and reduces the drag at the expenseof increasingthe AOA of the elevator and increasing the elevator drag. That can only be beneficial until the elevator D/L ratio becomes equal to the wing D/L ratio. The point where the D/L ratios of wing and elevator are equal may be outside of the CofG limits depending on the plane type. This can only have an importance at low speed, because it's the induced part of the drag that is influenced by the AOA.
