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Center of Gravity With Lift

  • Cranenium
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Cranenium created the topic: Center of Gravity With Lift

Does the position of center of gravity affect the lift ? What i think but i may also be wrong. When Center of gravity is behind Center of pressure it will have a pitch up attitude and for straight and level that will be a increase AOA. So can we say that with Center of gravity at it most aft position less lift will be required than normal due to the increase in AOA.

Anyone?
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bobtait replied the topic: Center of Gravity With Lift

If the center of gravity is behind the center of pressure, lift and weight will act to produce a couple that results in a nose-up pitching tendency. This can be countered by applying down elevator to produce an upwards force at the tail to maintain a level attitude. However, this upwards force at the tail is acting in the same direction as the lift on the wings.

The local lifting force at the tail is therefore helping to support weight, so less lift is required at the wings. You would actually require a SMALLER angle of attack to maintain level flight with a center of gravity behind the center of pressure. This also explains why the stalling speed actually reduces when the center of gravity is in its most rearward position.

Of course in the real world it is more complicated that. If you define lift as all of the aerodynamic forces acting at right angles to the relative airflow (i.e. opposite to the direction of motion), you would need to consider the entire aeroplane. In many cases the fuselage and even the canopy can also contribute to lift, so it can be misleading to pretend that only the wings and tailplane contribute to lift. This should be kept in mind when we see vector diagrams representing the distribution of lift and weight.

Bob
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  • Cranenium
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Cranenium replied the topic: Center of Gravity With Lift

Thank you Bob
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  • John.Heddles
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John.Heddles replied the topic: Center of Gravity With Lift

Some observations.

Initial pilot PoF training is a bit unfortunate in that it tends to look at this area too simplistically. Generally, for a conventional aircraft, there will be a nose down pitching moment for the wing/fuselage balanced by tailplane loads. A tragic example of this (which was fortuitously captured on film) saw the loss of a Caribou during the Vietnam conflict. At that time, comms and procedures were a bit average - exacerbated by Service changes - and the aircraft had its tail section taken off by an artillery round while making an approach at a forward base on an ammunition resupply mission. The photo shows both the separated tail and the very clear result of a nose down wing/fuselage pitching moment .. www.c-7acaribou.com/album/photos/photo02.htm .

The effect of CG moving aft within the approved range is to reduce stick loads to maintain a given manoeuvre. This eventually gets to a point where the aircraft longitudinal stability is grossly affected to the point of becoming dangerous. Of interest, while a statically unstable aircraft can be flown for a short while by a knowledgeable pilot, most will lose the plot (along with the aircraft) with minimal delay. Dynamic instability is beyond the human pilot's capability.

As Bob observes, stall speed is CG dependent due to the variation in wing and tail lift with CG. This is why certification stalls generally are for forward high weight conditions - to give the highest value for performance related matters - and why the preferred cruise CG is towards the aft limit - to minimise tailplane download and associated wing lift .. ie lower drag situation.

Looking at the OP's final question, the net lift will be unchanged (that's tied up with the weight) but the wing lift is reduced as there is less tailplane downforce to balance out.

Engineering specialist in aircraft performance and weight control.
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