Calibrating the Wright Flyer Balance
December 23, 1998
By Chris Lockwood
The balance is a device inside the model that measures how much the model is being pushed, pulled or twisted. We put very precise loads on the balance in the lab, before the test begins to make sure it tells us the correct force or torque Ð like putting a 100 pound weight on the bathroom scale and making sure it reads 100 lbs. The way that we typically put a load on a balance is to hang a weight at a very precise location and angle relative to the balance. The problem is that you can't get gravity to go upward. You can't hang a load in one location and hang another load in the opposite direction somewhere else on the balance. It's very hard to get combination loads into these devices. However, balances experience combination loads in a wind tunnel tests.
We have developed a new machine here that can impart loads on to the balances in six axises. That represents all the dimensions of forces and moments. The three dimensions in which forces can be applied, up/down, fore-and-aft and side-to-side. Then you have twisting moments or torques so you have three rotations and three directions. This machine will apply loads in all six degrees of freedom simultaneously.
I have worked on this Automatic Balance Calibration Machine for the past six years. It senses the position of the balance. It also has rods with electric actuators that extend and contract to push and pull, and twist the balance in any direction. The machine is just coming into use now we are currently testing it's accuracy compared to the dead weight loads that we have used up to now. The test instrument for the machine, is also the balance that will be used for the Wright Flyer test. In testing the balance we are after repeatability. We should get the same results with the dead weights that we get with the Automatic Balance Calibration Machine. We will repeat the many different loadings, multiple times to make sure we are consistent.
The balance is a 4 inch (referring to the diameter) Mk II A. Made by Able Corporation this balance will take 8000 pounds of lift, that would be two and a half or three cars! It is a fairly strong device. The Wright Flyer model is very unique in that it probably only weighs 1000 lbs. And to fly, it needs enough lift to over come it's weight. This balance can take 8,000 lbs. of lift so it is only using a small percentage of the capacity of the balance. But the wings are so wide, the wingspan is so huge compared to the balance that if there is a couple of hundred pounds difference from one wing tip to the other it will make the model want to roll. We are very close to the balance capacity in the strain gage that measures roll. To prevent damage to the model during the wind tunnel test the AIAA team has built a roll stop into the connection between the balance (and model) and structure in the tunnel that supports the model.