How is the centre of lift estimated for airplane models before they are being mounted in a wind tunnel?

They mount the airplane models or parts of an airplane like just a wing, or a rudder in wind tunnels to test their drag and lift, stability, etc.

How do they know where to hang it from or mount it on. How do they estimate the lift center of a new design of a jetliner which has no predecessor to use as a reference.

I am guessing it is a process of trial and error. But there must be an algorithm or method to arrive from an estimate to actual lift center.

you mount it in a convenient way, and then with all the load sensors reporting, you use a spring gauge to apply known forces by hand to the object in known directions and record the load sensor outputs for each. this will tell you how to add and subtract the individual sensor outputs to yield the correct signals (pure lift, drag, pitching moment, etc.).

The previous answer is correct. I would like to add to the answer that today's technology allows to simulate the airplane behaviour quite accurately.

Using CFD allows to simulate the behaviour of the airplane, however, the current CFD models are not perfect, they work quite well in the linear range and when flow is not attached, so you can actually estimate the values you are looking for quite accurately.

However, this technology, when applied to current designs lacks of several capabilities, the first one is to predict the drag with absolute accuracy (and not relative) and the other one is the detached region of the flow.

Finally design of airplanes is a join effort between CFD and wind tunnel when one is supporting the other. You have questioned about one of those joined areas.

• I know a bit about the ways they measure with sensors in rack of small pipe sensors. My question is not that my question is how they guess where is the aerodynamic center if lift to hang the model from that point. Otherwise all the readings will be wrong. Mar 25 '18 at 14:25
• Using CFD you can also estimate that value Mar 26 '18 at 8:26

You don't need specific mount point to determine a force and its center. If you measure the force at two points, you can measure it's magnitude and center along the axis between those points from that. Add more points for more axis.

So the model is mounted so as to minimize disruption of the airflow by the mount, with load sensors in various points of the mount. And then the force at various airspeeds (including zero to properly measure effect of gravity to subtract) is measured and the moments calculated.

• I am not sure. I know they need to know the exact center of lift point, and it's path, following change in angle of attack or how it relates to the changing center of gravity while the distribution of load changes! The behavior of an airfoil is most pronouncedly related to its connection to the fuselage and the relation between its center of lift and its distance from the supporting bracket or joint! Or a whole airplane if not mounted correctly will tend to pitch up/down or bank or basically have poor balance! Mar 25 '18 at 20:10
• @kamran, the mount to determine the centre of pressure is fixed. It can't move under aerodynamic loads (more than tiny amount needed to register the loads) and the loads are measured at various fixed angles. From that the centre of pressure can be determined and then it can be mounted so it can move if needed. Mar 26 '18 at 11:27