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Finding the centre of mass













Unstable suspended bottle

Using a natural air stream to provide your own basic wind tunnel.

 The bottle is suspended above its Cm by a nylon thread.


<<<direction of airstream <<<

unstable.jpg (64224 bytes)

watch a video

Finding the centre of mass Cm

unstable_front.gif (14292 octets)

Suspended point to left of center


center-mass.gif (14554 octets)

Suspended point above Cm


unstable_rear.gif (16651 octets)

Suspended point to right of Cm



Stabilised suspended rocket

center-mass.gif (14554 octets)                            


<<<direction of airstream <<<

stable1.jpg (62793 bytes)  

watch a vidéo



Rocket with centres of mass and aerodynamic drag centres marked by small white rings

stable2.jpg (50391 bytes)

watch a video

(wind coming towards you) 

Note: The rockets centre of mass needs to be above the areodynamic center.



The requirement for the center of mass to be placed  in front of the aerodynamic center of pressure along the direction of flight of aircraft. Had initially been identified by Sir George Cayley in 1804 and confirmed by the designs of Otto Lilenthal in 1891.

This also applies to the stability of a kite in flight. Which is why you need to have a long tail added to the kite to give it flight stability.

So I reckon that the Chinese inventor of the kite should be given some credit.770- 476BC.

Kites imported into Europe during the 19th century propably inspired and encouraged the 

development of flight.

P28  History of Aerodynamics . J D Anderson < Click


The sling shot air dart.


A very useful project for discovering flight dynamics and the importance of the center of mass relative to the aerodynamic center of pressure is the sling shot airdart.

Its very simple to make and can be used to get hands on experience and understand low velocity projectile flight dynamics. Whilst having alot of fun.

  • Take a straight stick or length of dowel diameter 10 >12mm  and 750 > 800mm in length.

  • Cut a cross slit in one end with a saw or a knife. Then insert the flight fins by sliding them into the cross slit. You can make the fins inpairs cut from the flat plastic lids recoverd from icream or other plastic boxes.

  •  Locking them in place by using a cable tie to clamp the cross cut behind the flight fins. Or simply by binding them shut with a length of cord.

  • Cut or file a notch in the rod just in the front of the flight fins  say 30mm infront. 

  • Then take a piece of string 20cms longer than the dart and tie a knot in one end.

  • To make the sling shot, what you know need to do is to place the knot next to the notch you have cut into the shaft of the dart. Then  wind the cord around the shaft locking it behind the knot.

  • Find a field or deserted stretch of beach to launch your dart.Its a lot more fun if you have some comnpetition.

  • Launch the darts with and against the wind.

 Wind the free end of the cord around your hand and keeping the cord under tension grip the front of the dart between finger and thumb.

And launch....

See what combination of fin design (area) and center of mass work best.(ballast)

Measure the flight distance covered by pacing out the distance.from the launching point to recover the dart.

Always keep spectators behind the point of launch.

Conclusion : You should be able to

Our standard rocket nose cone with clip top retainer for holding a variable nose mass.


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This site was created on the 15th April 2003


 ©John Gwynn and sons2003 

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