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## Aircraft Stability

• CameronD
• Topic Author

### CameronD created the topic: Aircraft Stability

Background: I'm one of those people who have trouble letting go of the controls in flight, especially in turbulence (possibly a result of ingrained poor trim technique, but that's another subject entirely).

Question: I've had someone say that, properly trimmed, a Piper Warrior is stable in pitch and yaw (in all weather conditions?). Is this true?

Thanks,
Cameron

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• Carello

### Carello replied the topic: Aircraft Stability

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• CameronD
• Topic Author

### CameronD replied the topic: Aircraft Stability

Very helpful link to explain stability in pitch.. but what about yaw?

I understand that dihedral is supposed to return the wings to level after an upset... (positive static stability) right? But after an upset will the aircraft hold some new heading (neutral dynamic stability?) or return to the old one (positive dynamic stability?)? or graveyard spiral (negative dynamic stability) and why?

I'm finding this all a little confusing..

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### bobtait replied the topic: Aircraft Stability

It's not just a matter of being stable or unstable. There are degrees of stability. All civil aircraft are designed to be strongly stable in the yawing plane (directional stability), moderately stable in the pitching plane (longitudinal stability) and weakly stable in the rolling plane (lateral stability).

This is a deliberate design characteristic because the other side of the coin is manoeuvrability. Whereas stability is the resistance to change, manoeuvrability is the willingness to change. We don't want the aircraft to resist any attempt to make it roll, but we would like to be happy to maintain direction without the need for constant rudder input.

I'm afraid you wont be able to 'hands off' for long before the aircraft will begin to bank one way or the other. That was the designer's intention.
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• CameronD
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### CameronD replied the topic: Aircraft Stability

Thanks, Bob! That about answers my question, but one little thing. You mention "you wont be able to 'hands off' for long before the aircraft will begin to bank one way or the other.":

Will the induced bank cause the aircraft to commence to yaw (turn)? That's the start of a "graveyard spiral", am I right?

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### bobtait replied the topic: Aircraft Stability

Yes, if the banking tendency is left unattended the bank will continue to increase and then the side-slip will induce a yaw towards the dropping wing. With no control input from the pilot, a spiral dive will result. The term 'graveyard spiral' or 'graveyard spin' is a typical American over statement.

Providing the pilot is conscious and has adequate visual reference to the horizon, a spiral dive should be no big deal. Simply reduce the power, level the wings and regain the level nose attitude. Then reapply the power. No drama and certainly no deed to mention graveyards!!!
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• Carello

### Carello replied the topic: Aircraft Stability

If the aircraft's directional stability is stronger then its roll stability, the aircraft will be spirally unstable - ie will enter into a spiral dive hands off.

On the other hand, if the aircraft's roll stability is stronger than its directional stability, the aircraft will Dutch Roll - a series of out-of-phase turns, when the aircraft rolls in one direction and yaws in the other.

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• CameronD
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### CameronD replied the topic: Aircraft Stability

Graveyard spiral Spiral dive..

That's awesome. Thank you both!

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