CG Position & Vy

[uwochris]

Hey guys,

I accidentally posted this question in a non-related forum, so I'll repost it here:

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!

 

[ackattacker]

Aft CG does benefit climb rate. Vy will also be slightly lower for an aft CG. On a very large aircraft (think 747) the change can be several knots.

Essentially, an aft CG is reducing the induced drag for a given airspeed because the wing and tail are both operating at a lower angle of attack. Climb rate is proportional to excess power, that is power over and above that required to overcome drag. So less drag = better climb.

Vy is also reduced because Vy will be close to (but slightly higher than) the speed where lift over drag is at a minimum. Having less induced drag shifts the L/D curve towards slower airspeeds.

 

[time builder]

Good explanation ackattacker, I agree for the most part, but bear in mind that the power available curve is also reduced at lower speeds. You must compare the reduced drag to the loss of power available at a lower speed. Again, back to the main point, Vy the point at which you have the most excess power (power available minus power required for level flight).

My question is why do airliners with trim tanks take off and land with a forward C/G and move it aft in cruise (where induced drag is less of a factor anyway)? Are we just being safe in the case of stall recovery?

 

[Makesheepnervus]

Aft CG does benefit climb rate. Vy will also be slightly lower for an aft CG. On a very large aircraft (think 747) the change can be several knots.

Essentially, an aft CG is reducing the induced drag for a given airspeed because the wing and tail are both operating at a lower angle of attack. Climb rate is proportional to excess power, that is power over and above that required to overcome drag. So less drag = better climb.

Vy is also reduced because Vy will be close to (but slightly higher than) the speed where lift over drag is at a minimum. Having less induced drag shifts the L/D curve towards slower airspeeds.

Exactly what I was about to post ackattacker!!!!

 

[ackattacker]

My question is why do airliners with trim tanks take off and land with a forward C/G and move it aft in cruise (where induced drag is less of a factor anyway)? Are we just being safe in the case of stall recovery?

I don't know, I've never flown an airplane which used trim tanks. I understand the Concorde did something like that, but that had a lot to do with trans-mach aerodynamics.

 

[time builder]

Trim tanks are also used in A300's and 747-400's. I don't know about the 747's, but the airbus will typically take off with all the fuel in the wings, transfer to the tail (trim tank) in flight, and then from the tail to the center before landing.
Of course, another factor could be that they burn from the inner wing tanks first, and the remaining fuel in outer tanks have a more aft CG, so putting fuel in the center would counter that.

 

[ackattacker]

Trim tanks are also used in A300's and 747-400's. I don't know about the 747's, but the airbus will typically take off with all the fuel in the wings, transfer to the tail (trim tank) in flight, and then from the tail to the center before landing.
Of course, another factor could be that they burn from the inner wing tanks first, and the remaining fuel in outer tanks have a more aft CG, so putting fuel in the center would counter that.

Hopefully someone who flys one of those aircraft will chime in. I did some basic googling on the A300 and apparantly the "trim tank" was added to the latter models (A300-600R and up) as a way to extend range. I can only speculate that perhaps they are allowed a more aft CG in cruise flight than is certified for takeoff and landing.

 

[atldc9]

The A330 does the same thing. After T/O, the system transfers fuel to the aft tank, automatically, I beleive.

 

[ackattacker]

OK, I found this in an Airbus document titled "Getting to grips with fuel economy"

[SIZE=-1]www.iata.org/NR/ContentConnector/CS2000/Siteinterface/sites/whatwedo/file/Airbus_Fuel_Economy_Material.pdf[/SIZE]

AIRBUS has created a trim tank transfer system that controls the center of gravity of the airplane. This system is installed on some A300 and A310 aircraft and all A330 and A340 aircraft. When an airplane with a trim tank is in cruise, the system optimizes the center of gravity position to save fuel by reducing the drag on the airplane. The system transfers fuel to the trim tank (aft transfer) or from the trim tank (forward transfer). This movement of fuel changes the center of gravity position. The crew can also manually select forward fuel transfer.

The Fuel Control and Management Computer (FCMC) calculates the center of
gravity of the airplane from various parameters including input values (Zero Fuel Weight or Gross Take-off Weight and the associated CG) and the fuel tank contents. It continuously calculates the CG in flight. From this calculation, the FCMC decides the quantity of fuel to be moved aft or forward in flight to maintain the CG between the target value and 0.5% forward of the target band.

Usually one initial aft fuel-transfer is carried out late in the climb to bring the CG within this band. During the flight there are several smaller forward movements as the fuel burn moves the CG more aft. Finally a forward transfer is made as the aircraft nears its destination to bring the CG within the landing CG range.

 

[time builder]

Ah, I see, so the landing CG limit is different than in cruise, like you said earlier.
Good site, by the way, especially the aerodynamic deterioration section. Who would have thought that misrigging a slat 15mm on an A300 would cost you 30 extra gallons on a cross country flight.