Air Speed ( how fast are you going) - RCCanada - Canada Radio Controlled Hobby Forum
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Old 01-19-2004, 06:23 PM   #1
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I am: John Davidson
Join Date: Feb 2003
Location: Vulcan,Alberta
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Air Speed ( how fast are you going)

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I don't know if I am in the right area for this but Perry was inquiring in another forum about how fast his Diamond Dust would go. This is how I figure it out and adjust accordingly.
pitch x rpm div by 63,360 in.mile x 60 mi/hr.
This should give you the top end but as everybody knows propellers are not the most efficient. For most of my calcs. I use 80% but if it is a trim a/c and when racing I would use my airborn tach and speedo. For instance my Rossi (stock Q500) would ownly achieve 18.5k in a dive. So I proped for the max. rpm in the air which worked out to a 9 x 8 Revup. It is very slow on acceleration but after a lap it unloads. It is also no good for sport. On the otherside my FAI would come over 30k airborne so I would go up in pitch and down in dia. This is also why most FAI's use carbon props. If you ever here or see a (punky) wood prop flutter or explode you will know what I mean. I also know I was getting about 90% efficiency with this setup.
Warning when playing with high rpm it is essential to stay out of the arc of the prop incase it throws a blade. That also goes for any engine. Get somebody to hold it for you from behind and walk around and adjust the needle valve from the rear. Don't reach over the prop.
I know this will probably start some things but this is the way I do it. I also have an airborn tach & speedo.
John Davidson
Keep the shiny side up and the wheels down
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