RKAJFSAZ

AIR FRICTION SPEEDOMETER

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It may have aeronautical provenance. Airspeed is essential information for flight, in general, FAR more important than ground speed.  Relative headwind plays an important part, sometimes beneficial as when a tailwind reduces it,  in the operation of our ancient crates with such high drag coefficients. Even more critical in the case of air cooled cars. Drivers of our beloved old junk should be particularly cautious when driving into headwinds. Operating at high intake manifold pressure (high torque output), under such conditions may lead to engine destruction. Slow way down driving into strong headwinds, and keep a close eye on temperature and oil pressure. Even on modern cars, maintaining EXCESSIVE  speed into EXTREME headwinds can be detrimental to engine health. Hmmmm.........    You know ? I think I want one of these air friction speedometers. Anybody have one for sale ?   -  Carl 

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Well I was wrong about that then. It is not a Pitot Tube. It relies on the friction of air between two metal surfaces in close proximity. Here is a bit of guff about it.

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Very interesting, Professor Spinney', my friend. However, you have burst my bubble. All  : please cancel my order for an air friction speedometer.  ?,   -  Carl 

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If I'm correct, air friction speedometer was a Tesla invention. The inner drum is directly connected to the cable/gear assemblage and the outer drum is on a light spring The outer drum has the numbers around its circumference. The air between the two drums makes the connection between them. Calibration is made by either drum height or the spring.

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If the delicate aluminum drum (with the speed printed on it) gets distorted in any way by improper 

handling, the thing won't work (I have three like this).  Im surprised there's not an expert who repairs

these,  thought they were used in a lot of higher end cars, not just Franklins.  I think 1928 was the last

year they were used.

 

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4 hours ago, Friartuck said:

If I'm correct, air friction speedometer was a Tesla invention. The inner drum is directly connected to the cable/gear assemblage and the outer drum is on a light spring The outer drum has the numbers around its circumference. The air between the two drums makes the connection between them. Calibration is made by either drum height or the spring.

 

Interesting that Tesla, renowned for his work with electricity and magnetism, would invent a air coupled mechanism for a speedometer.

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Wouldn't the accuracy of such a device be dependent somewhat on barometric pressure i.e. air density?

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No. Read the 1916 Scientific American article which Dr. Spinneyhill links in response #5 above. Very clever, sensitive device, with remarkable accuracy, even down to less than one mph ! The thing does not lose function at high altitude either. It was tested up to 10,000'.   -   Carl 

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@RKAJFSAZ can you ring any instrument repairers and ask if they can do them and if not, do they know who does? Telephone calls are pretty cheap in the USA compared to many other places, e.g. here!

 

If the cups are bent, a very good panel beater or jeweler should be able to put them back into shape.

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