You are correct, props are airfoils and the airfoil one uses can have a
large effect on the performance. A supercritical airfoil can increase
the drag rise Mach number as much as 15 percent which means a larger
diameter prop can be used at the same RPM if the outside20 percent used a
supercritical airfoil.
Jim
On Wed, 14 Dec 2005 13:23:27 -0600 "JIM RALEIGH" <drale...@ktis.net>
writes:
> 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
>
>
> _______________________________________
> Search the KRnet Archives at
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>
>
