I had the privilege last week to address a large audience at the Royal Aeronautical Society at the Engineering School at Cambridge University in the UK. (click here for more information).
We returned to Hugh’s office after the tour where we played with his bi-metallic clock pendulum, tuning cylinders, tuning forks and other remarkable (engineering) toys.
I thought of this week’s question whilst throwing Hugh’s boomerang around his busy office (and trying not to injure his students).
a. Why does a boomerang return to the thrower?
Lifting and pitching forces cause the boomerang to fly in an orbit path back to the thrower.
The boomerang is similar to a helicopter.
Imagine the paths of the rotating blades tracing-forming a rotating disk.
The rotating blades (wings) generate lift. The blades passing perpendicular to the flight path (front and rear) generate the same lift. The advancing blade generates more lift than the retreating blade.
Thus the net forces on the disk are:
- lift (to keep the boomerang airborne); and
- an additional “rolling” force (moment) that is positive/negative for the advancing/retreating blades respectively.
The “rolling” force causes the spinning boomerang “disk” to precess like a gyroscope or bicycle wheel. The disk displaces in the axis that is rotated 90 degrees forward from where the force is applied.
For the boomerang, the precession force provides the result similar to a canard or elevator on an aircraft, pitching the boomerang’s nose up.
The constant bank angle and pitching causes the boomerang to “orbit”.
b. What is the maximum and minimum number of airfoils (wings) on a boomerang?
Just the same as a helicopter:
- Maximum blade count – unlimited
- Minimum blade count – one. Modern hunting boomerangs have one blade with a counterweight on the other side.
It’s easy to work out how to throw a boomerang when you appreciate that the “rotor disk” needs a high rotational rate to generate lift, and then a high forward speed, to generate the precession forces that powers the pitching effects.
The first person to answer both questions correctly may chose their prize from the selection of Category 1 prizes.
The winner will be announced on Thursday 5th December at:
- 7 am – New York
- 4 am – Los Angeles
- noon – London
- 1 pm – Paris
- 8 pm – Singapore
- 11 pm – Sydney
Again, unfortunately, there were no people who correctly answered both questions.
However, I appreciate the effort you all took to tackle this question in the true spirit that I intended when I wrote “Physics For the Coffee Table”.
We are also approaching the Christmas break at the end of another challenging year.
So to kick start the festive season, I therefore deem all seven people who submitted answers to be “close enough” to get a prize. I’ll be in touch…
Thanks for joining in the fun, and Merry Christmas!
The QF32 Web Site Crew thanks Pan Macmillan and Airbus for providing these excellent prizes.