bobkrech
Well-Known Member
- Joined
- Jan 20, 2009
- Messages
- 8,352
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- 55
- Hindsight is always 100%, but put yourself in the shoes of the 911 dispatcher and the emergency response folks on the receiving end of the 911 call. I'm sure is was very real and scary for them.
- Most aviation folks know helium will lift approximately 1 ounce per cubic foot at 32F MSL and the reported size of the balloon was 20' diameter and 5' high.
- A quick check from medical folks will reveal that the average 6 year old boy is 42" tall and weights 46 pounds.
- The 5 second back-of-the-envelop emergency response sanity check on whether this report was plausible or a prank is to approximate the balloon lifting power as 20*20*5*(3/4)/16 = 1500/16 = 94 pounds at sea level. This is greater than that the weight of a six year old so from an emergency response point based on a 911 call, it had to be taken seriously.
- As you are planning you method of response you can refine your calculations.
- Helium will actually lift 1.1 ounce per cubic foot at 32F MSL
- The lifting capacity of helium decrease with increasing altitude.
- The lifting power of helium decreases with increasing temperature.
- At an altitude of 5000' ASL and a temperature the lifting power had dropped to 0.85 oz. per cubic foot.
- Without knowing any more details, you perform a better 30 second approximation by correcting the lifting power for temperature and altitude. Without more information and assuming the balloon volume as a cylinder, the lifting power of the balloon is 1570 cuft. * 0.85 oz./cuft. * 1/16 lb/oz. = 83 pounds, a lower but still a credible number.
- This is enough to get the emergence response aircraft to respond.
- Even as the chase planes observed an ellipsoidal shape, the calculated volume is reduced to 1045 cuft but a quick calculation would show that the balloon could lift 55 pounds so from an emergency response point the report is still credible and the balloon is still capable of lifting a 6 year old boy so the search had to continue until the boy or the body was found.
https://en.wikipedia.org/wiki/Lifting_gas
https://www.aerospaceweb.org/design/scripts/atmosphere/
https://www.disabled-world.com/artman/publish/height-weight-teens.shtml
https://www.csgnetwork.com/volumeellipsoid.html