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I have always wondered what that string in the windshield of a glider was used for:

https://en.wikipedia.org/wiki/Yaw_string

Got me thinking. If you hung a weighted string from inside the cockpit (centered like an air freshener hung from the rear view of a car) - could that be used as a primitive bank angle indicator?

What am I not understanding about the physics that would cause the weighted ball to remain perpendicular relative to the aircraft wing - thus making such a device useless?

I'm not sure I fully understand how gyro's solve this problem either - is that missing link?

I have at best a gr. 10 understanding of physics - so please go easy :)

NOTE: No idea what to tag this question as

Alex.Barylski
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No, this will not work. There are a couple of problems. One is simply that the bob will oscillate, which is inconvenient. That problem can be mitigated by supplying some force of friction that damps out the oscillations.

The other one is fundamental and can't be worked around. One of the basic principles of physics is the equivalence principle, which states that a gravitational field is indistinguishable from an acceleration. Therefore a bob hanging in the cockpit will not actually tell you which way is down. What it will tell you is the difference between the gravitational field vector and the acceleration vector of the aircraft. For example, if a plane is making a turn, the bob will point down and outward, not just down. As a more dramatic example, if you do a loop at high speed, and you're upside-down at the top of the loop, the bob could point up.

This is why gyroscopes are used instead. Passenger jets use ring laser gyros. The gyros have to be initialized when the plane is known to be level and not accelerating.

  • I think I understand the oscillate issue - that problem could be addressed by making the string taut, correct? If you fixed the movement to 2 DoF - that is left and right bank, that still wouldn't work?

    I guess I thought it wouldn't work because of relativity (again my understanding of physics is elementary) that the "bob" would always be pulled down perpendicular to aircraft bank angle. Or that centripetal force would keep it there and prevent it from reflecting actual bank. I think I just confused myself more lol

     – Alex.Barylski Jan 04 '18 at 00:18
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    a properly-coordinated turn would show no deflection of the bobweight even though the plane is in a bank. this is why you need a gyro to detect attitude changes. – niels nielsen Jan 04 '18 at 01:52