Units
April 12th, 2012I have a pretty sweet job. Currently when I’m not working on a tactical UAS for special forces and three-letter agency operators, I’m helping a buddy generate air loads for the Scaled Composites Roc (commonly known as Stratolaunch), the system Paul Allen signed on to fund which will put rockets in space by launching them from an airplane at airliner altitudes.
It’s projected to have a 385ft wingspan and 1.2 million pound maximum takeoff weight (not breaking any ground here, these are publicly available figures). When you compute loads for such a large airplane you end up with really large numbers. REALLY large numbers. You also end up with HUGE moments. Moment, also called torque, is a measure of force about an axis that causes rotation.
Imagine tightening the lug nuts on a car. You apply a force (say 75 lbs) on the end of a wrench (let’s say it’s a foot long) and you create a moment of 75 ft-lbs (75lbs x 1 ft). In English units ft-lbs are common although with airplanes we commonly measure things in inches (damn base 12 math) so instead of 75 ft-lbs, you’d get 900 in-lbs - same moment, different units.
Well when you want to calculate the root bending moment of the wing (ie. the amount of torque caused by the lift of the wing on the fuselage) and you have a 1.2 million pound airplane with a 385 ft wingspan, you end up with billions and billions of in-lbs of torque. While I know approximately what 75 lbs feels like and I know that 12 inches looks like, I have no idea what billions of in-lbs correspond to. Neither did the other guys at work, so they converted the loads into a different set of units…
mile-tons.
Imagine hanging a ton (2000lbs) at the distance of a mile (5280 feet). Imagine hanging a ton at 20 miles…
They better buy a lot of carbon fiber.






















