Everyone keeps looking at formulas for speed that a prop will give you and they will never work because they are not air screws. They are air foils that give a certain amount of thrust at a certain rpm. Thrust overcomes drag and unless you know how much drag you have you can't predict your speed. I don't know anyone that knows how much drag a KR has especially since each one is different. A B25 will go 300 mph or 9.58 ft per revolution of the prop. A smaller prop on a P51 gives it 400 Mph or 14.08 ft. per revolution. Go figure. ----- Original Message ----- From: <pe...@heroic.co.uk> To: <kr...@mylist.net> Sent: Wednesday, December 14, 2005 11:55 AM Subject: KR> Props
> Hi, the size of a prop is given as 2 numbers diameter/pitch. > > In terms of maximum speed, the pitch is the important number a one inch > pitch would give 2.84 mph, so a 49in pitch prop would give a max speed of > 139 mph at 3000 revs and 64in pitch gives 181 mph at 3000 revs. > > VWs are normally limited to around 3400revs( in UK by PFA edict ), so a > 49in prop would give me a max of 138mph. this is regardless of how big the > prop is - that's the max speed. At this speed the blades angle of attack > becomes zero, so it cannot provide any more thrust. The only way I can get > to go any faster is by increasing revs. > > So given that I want ground clearance, I am limited to say 60 inch diam. > > Now if you imagine a prop that is about the same width as a tape measure, > it will obviously need less energy to turn it than big fat wide blades, so > the designer has to come up with a prop that will have:- > > 1 have a theoretical max speed somewhat higher than the required max speed. > 2 shovels sufficient air to overcome the aircraft drag at the required max > speed. > 3 is not stalled at speed below the aircraft take off speed. > 4 does not produce so much drag that the engine cannot produce full power > at take off speed. > > I am puzzled why prop manufactures only give the diam/pitch figures for a > prop, when as explained above the prop width/cord is vitally important > when choosing a prop > > I suspect that it is not that Senns & Sturba have different ways of > specing a prop, rather one makes fatter props. > > Pete Diffey > St Albans, UK > > > > > > _______________________________________ > Search the KRnet Archives at http://www.maddyhome.com/krsrch/index.jsp > to UNsubscribe from KRnet, send a message to krnet-le...@mylist.net > please see other KRnet info at http://www.krnet.org/info.html
