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Eddy Small

Flying speeds.

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Jef Ott

I had often wondered why the tail droops in sink. I think Richard has nailed this.

When the airframe is moving in air that is moving downwards, the airframe simply adjusts its attitude until equilibrium is reached (without any adjustment of the sticks in our hands).
- ie the airflow over the airframe must generate sufficient lift to keep the weight of the airframe supported,
the airflow is essentially at the same angle over the airframe, but the slightly downward vector of the airflow dictates that the tail has to drop (from the ground based pilot's perspective) to keep the air flowing in the right direction from the section's perspective. 

And yes, the tail really does drop, in relation to the observer, when the model is flying in sink, of that I am certain.
Because the vector angle (from horizontal) will be increased for lower airspeed aircraft, the effect will be more visually apparent the lower the flying speed of the airframe - hence why it is not as pronounced in bigger airframes (I'm thinking full size here) with higher airspeeds. 

Thanks for solving the mystery for me.   

Jef  

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Richard Swindells

Jef. I think you misunderstood my post, I stated the opposite. 

There is a Drela post on it somewhere. 

It's like the downwind/upwind turn. We all know that there is no difference between them, but our perception is always telling us something different 

 

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Jef Ott
25 minutes ago, Richard Swindells said:

Jef. I think you misunderstood my post, I stated the opposite. 

There is a Drela post on it somewhere. 

It's like the downwind/upwind turn. We all know that there is no difference between them, but our perception is always telling us something different 

 

Richard,

I didn't realise you were stating the opposite. We will have to agree to disagree then. 

Thanks for solving the mystery for me, anyway. I know what I have seen and know that it wasn't caused by my stick inputs, the tail actually drops in sink, and I am convinced that this is because of the direction of flow of the air over the airframe not being horizontal, but moving closer to the earth. 

Thanks for putting me out of my misery though - I thought it was strange that we agreed on something ;) LOL 

Jef  

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