PHYSICS FOR THE COFFEE TABLE -QUIZ # 7

Put your thinking caps on again.

The Physics for the Coffee Table (PFTCT) Quiz number Seven will be broadcast on Wednesday 4th December at:

  • 7 am-  New York
  • 4 am – Los Angeles
  • noon – London
  • 1 pm – Paris
  • 8 pm – Singapore
  • 11 pm – Sydney

Good luck!

Pelican (Photo Sophia de Crespigny)

Pelican (Photo Sophia de Crespigny)

4 comments

  1. Hugh Ferrar · · Reply

    I was going to answer this question – approximately correctly, but I missed the check-in.

    However it left me wondering – given the effect of precession, when you want to pitch a helicopter forward to make it accelerate forward, does the cyclic pitch control increase the lift as the blades are moving perpendicular to the direction of travel across the back of the rotor disk, or 90° before that, when the blades are moving backwards? In which case as soon as you start moving forward the lift from those blades would decrease, making it harder to generate the differential lift needed to accelerate. No wonder helicopters took so long to develop!

    I would also have claimed that a boomerang is made from a single piece of wood, so could only have 1 or 2 airfoils! There are asymmetric hunting boomerangs that have 1 airfoil and 1 short heavy end.

    1. Hi Hugh,

      You are correct re the helicopter – especially regarding why Igor Sikorsky had so much trouble designing the first helicopters to be controllable.

      The helicopter cabin hangs (like a pendulum) from and is pulled around by the rotor “disk”. This disk acts as a massive gyroscope, exhibiting classic precessional behaviours.

      The rotor displaces in the direction that is 90 degrees after the position where the (net) force is applied. For example, to make a helicopter pitch forward, the disk must displace nose low, which means that the position of minimum lift is abeam the cockpit, 90 degrees of rotation prior to the front of the disk.

      The helicopter would roll right/left (for clockwise/anticlockwise rotation respectively) if the cyclic controls rotated the blades to set the minimum lift at the front.

      Thanks for info about “hunting boomerangs”. I can now state that boomerangs only need one airfoil!

      Best Regards, Rich

  2. tomalbertsson · · Reply
    1. Dear Tom,

      Thank you for your long and interesting email.

      I share your thoughts. I think that the semi-technical descriptions in “QF32″, infused with a few fun PFTCT questions and finally my full blooded “Big Jets” book will go towards addressing some of these issues.

      We both clearly share a passion for aviation – the most wonderful profession.

      Kind regards, Rich

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