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Sometimes in wind tunnel tests, models are suspended upside-down: enter image description here

Image obtained from DNW - Interference corrections

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Image obtained from this Youtube video of testing at AEDC

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Image obtained from UWAL - Model Support Effects

And sometimes they are in the normal flying position:

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Image obtained from TRI models - wind tunnel testing

What is the specific reason for putting wind tunnel models upside-down?

I thought it had something to do with minimizing the influence of the support, but putting the plane upside down doesn't change much about the location of the support.

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    $\begingroup$ It looks like there's a Storm Trooper in that second picture! $\endgroup$ – FreeMan Sep 3 '15 at 13:07
  • $\begingroup$ @FreeMan it looks more like a screaming frog with really long arms :) $\endgroup$ – Ethan Sep 5 '15 at 15:19
  • $\begingroup$ Its because to test to see if the plane could fly upside down. It could happen so they need to test just in case. Test inspired by Will Smith In the movie FLIGHT. $\endgroup$ – Ethan Sep 5 '15 at 15:33
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In the times before strain gauges and electronic transducers, the forces on the wind tunnel model were measured with scales. To avoid interference as much as possible, the model was hung on strings, the scales being mounted above the measurement area of the tunnel. Since the forces in upright flight were of the highest interest, gravity plus lift would make sure that the strings were always taut. If the model were hung upright, the lift might grow bigger than the weight, and the model would move towards the scales.

Today, the orientation of the model can be either way, and only in exceptional cases there is a strong reason for placing it upside down. This might be when unusual attitudes together with the supporting sting (the long spike on which the model sits) would block one part of the wind tunnel cross section too much, or if movement restrictions of the sting force a certain orientation.

Another reason is flow visualization techniques: Sometimes it helps to put a coat of a mixture of petroleum and pigments on the surface and to watch where the pigments are blown by the flow (this is called an Anstrichbild). Obviously, it helps to place the surface of interest on top.

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  • $\begingroup$ But in the first picture from the DNW the model is fixed using a rigid support, so any movement is prohibited. Yet, still the model is fixed upside down. $\endgroup$ – ROIMaison Sep 3 '15 at 13:05
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    $\begingroup$ @ROIMaison, that could still be a power of habit. $\endgroup$ – Jan Hudec Sep 3 '15 at 13:07
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    $\begingroup$ @ROIMaison: Those aren't really rigid supports. If they were rigid they'd defeat the purpose of the wind tunnel. They're strain gauges which is a springy connection that measures forces (exactly the same as a digital kitchen/bathroom scale but more expensive). Depending on the design of the gauge/support, you may need to install the plane upside down either for the same reason as with older mechanical gauges or because one direction have better resolution or noise cancellation than the other. $\endgroup$ – slebetman Sep 3 '15 at 20:31
  • $\begingroup$ @slebetman: I understand that they're not actually rigid, but I used the word rigid to convey the contrast with wires. $\endgroup$ – ROIMaison Sep 4 '15 at 8:50
  • $\begingroup$ @ROIMaison: My point is the contrast may not be that big. Springs are springs be they thin coiled wire springs or straing gauges. It depends on how you design the spring. $\endgroup$ – slebetman Sep 4 '15 at 9:48
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The main reason for placing the wind tunnel models upside down was that it was easier to measure lift that way.

The first wind tunnels faced two main problems while measuring body forces:

  • How to avoid interference- This was overcome by either suspending the model on wires or placing the model on a metal pole called stinger that projected to the rear side of the model.
  • How to measure the forces accurately.

The forces on the initial wind tunnels were mostly measured using force balance. In case of the models suspended on wires, 'tare' weight can then be set and any additional balance weight required is the lift.

Wind tunnel Mass Balance

Though the wind tunnels became more sophisticated,the models were mounted upside down both as force of habit and as measuring equipments were designed for that.

Of course, the lift could be measured the other way too and some wind tunnels followed that. There is no fundamental difference between the two. Certain wind tunnel tests like separation studies require the models be mounted in a particular (usually right way up) orientation.

In today's wind tunnels using strain gages among other things, the models can be mounted in any required orientation.

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  • $\begingroup$ "Certain wind tunnel tests like separation studies require the models be mounted in a particular (usually right way up) orientation." I'm guessing this is because of the sensitivity of separation? Does this mean that gravity has a small but significant influence? $\endgroup$ – ROIMaison Sep 4 '15 at 8:59
  • $\begingroup$ The mounting of models for (flow) separation studies is determined by the visualization technique used. In the scales seen in aviation, gravity is not a significant factor, though it is in fields like geophysics. On the other hand, it is important to take into account the effect of gravity for payload separation studies. $\endgroup$ – aeroalias Sep 4 '15 at 18:53
  • $\begingroup$ This answer is a good one, but the final paragraph is misleading for two reasons : 1) many of today's wind tunnels were built decades ago, and 2) the direction/orientation can still be important because of the resolution or range of the tunnel sensors. $\endgroup$ – Gürkan Çetin Sep 5 '15 at 18:10
  • $\begingroup$ You are right, especially the first part. I should've qualified the statement. $\endgroup$ – aeroalias Sep 5 '15 at 18:14

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