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Factors affecting longitudinal stability

  • Aviatordan
  • Topic Author

Aviatordan created the topic: Factors affecting longitudinal stability

G'day Bob,

I've got a query relating to the explanation given on page 8.20 for question 5 of review questions set 2.

The question relates to the effect of loading an aircraft with its CG forward of the forward limit.

The explanation states that the reduction in elevator effectivenesss is due to the increased keel surface behind the CG and the decreased keel surface ahead of the CG creating a condition of super stability which negates the effect of the elevator acting on a longer arm. However, I can't seem to understand what the keel surface has to do with longitudinal stability; doesn't the area in front of and behind the CG only affect lateral and directional stability?

On page 11.3 of the aerody TB it is said that the arm determines the strength of the stabilising moment contributed by the tailplane (disregarding tailplane volume) which is the most important single item ensuring stability.

My understanding of the effect of loading an aircraft with the CG too far forward would be that the aircraft would indeed become super stable in pitch. However it would be due to the fact that the arm has increased so much that the ratio of stabilsing forces to destabilising forces would increase beyond design limits.
This in turn would decrease elevator effectiveness due to the fact that whenever the pilot made an input the aeroplane would hyperbolically 'snap' back to its original position, which again would negate the fact that the elevator is acting on a longer arm.

Hope this all makes sense!


Dan =]

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  • Richard

Richard replied the topic: Re: Factors affecting longitudinal stability

Hi Dan,

With a CofG too far forward, you will have a lot of keel surface behind the CofG compared to ahead of it. The elevator will actually produce more pitching moment with a forward CofG but it will be fighting against a much greater restoring moment created by the weathercocking tendency of the tail when exposed to the relative wind.

For example, consider a pilot who pulls back on the yoke in an aircraft with a forward CofG problem. The elevator creates a powerful pitching moment so the tail starts to deflect down into the relative airflow. However, the aircraft has inertia so the entire aircraft will momentarily continue motion in the original direction but now with the tail deflected downwards, deflected out into the relative airflow.

The relative wind will create forces acting on the tail which try to weathercock the fuselage back into line much like a canoe which is tied to a jetty will swing around to align itself with the current in the river. This is the restoring moment.

As the CofG moves forward, the restoring moment created by the relative airflow on the tail will increase and, even though the elevator acts on a longer arm and therefore creates a stronger pitching moment, the elevator's increased force will be progressively overwhelmed by the increasing restoring moment. Elevator control will feel heavier and changes in pitch will become harder.

That's why we say the elevator becomes less effective at changing pitch as the CofG moves too far forward.



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bobtait replied the topic: Re: Factors affecting longitudinal stability

Also Dan, I get the impression from your post that you are considering only the sides of the fuselage as the keel surface. The area of the 'belly' of the aeroplane is usually included in the definition of keel surface. That is an important factor affecting longitudinal stability.

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  • Aviatordan
  • Topic Author

Aviatordan replied the topic: Re: Factors affecting longitudinal stability

Hey guys!

Thanks for the reply, Rich! And also thank you, Bob. I was under the impression that keel surfaces simply meant the sides of the aircraft so that cleared it up!

I ended up getting 94% in the actual exam so I am happy with that! Now on to the flight test!! I'm going to submit an exam review but in another topic. =]

Cheers guys!


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