> Prop diameter has nothing to do with aircraft top speed either....the C130
I don't agree (but am ready to be proved wrong)
The speed of an aircraft is limited by the propeller diameter as a factor of
hp and propeller RPM and number of blades.
There's two ways of looking at it:
1 You need to generate a column of air moving at a speed greater than the
aircraft speed. To accelerate a column of air in a given time requires a
certain amount of power. You only have a certain amount of power available.
Therefore the connected variables are power, speed and column of air
diameter (or propeller diameter). As the propeller diameter increases the
power required increases or the speed of the column of air must reduce.
2 The next way of looking at the problem is this. Take a ground
adjustable composite propeller of a given diameter. For the propeller to
generate thrust there must be a positive angle of attack on the aerofoil.
Consider a part of the blade some distance from the centre. This part of the
aerofoil is moving forward at aircraft speed and moving at right angles to
that at its rotational speed. Using a triangle of velocities it is easy to
work out the pitch necessary on the blade to keep it in positive angle of
attack for the aircraft speed. This also gives the twist in the blade for a
well designed propeller. For a given maximum rpm (max cruise power if you
like) there is only one pitch setting which will match max power. Practical
test - That is from a fine position keep coarsening the propeller until
maximum rpm can not be exceeded in level flight. If at that pitch the
aerofoil still has positive angle of attack then perhaps the efficiency of
the propeller can be improved by increasing the diameter. If however the
aerofoil no longer has a positive angle of attack then the pitch must be
increased for the aircraft speed to be increased. The pitch can not be
increased without loss of rpm and power without reducing the propeller
diameter. Therefore the maximum speed of an aircraft may be limited by the
diameter, number of blades and aerofoil size.
Hence - fast aircraft tend to have small fast turning propellers. Slow
aircraft tend to have large, wide, multi blade, slow turning propellers.
For the engine power C130s or maybe Constellations don't have big
propellers.
Jerry
LTS@avnet.co.uk
http://www.avnet.co.uk/touchdown
----- Original Message -----
From: Peter van Schoonhoven <pvans@pacifier.com>
Subject: Re: Engine selection
> Addendum,
>
> Prop diameter has nothing to do with aircraft top speed either....the C130
> or maybe the Constellation are examples of airplanes with propellers in
the
> 20 foot diameter range, and they go 300 mph easily. Or just look at the
> fighters of WWII, huge diameters going 400mph plus.
>
>
> >I am not sure why people think three blades are less efficient. Like
> >everything in aircraft design propeller selection is a compromise. The
most
> >important feature is probably that the propeller is well matched. The
blade
> >size needs to be matched to the engine hp. Large diameter props are more
> >noisy and having a large diameter prop may limit the tops speed of the
> >aircraft. In my experience a 64" three blade wide chord (130 mm) is a
good
> >choice on aircraft with the 912S at speeds up or near 160 kts. If you
think
> >your A/C can go faster it might be advantageous to have a smaller prop
but
> >at the loss of take off and climb performance.
> >
> >Jerry
> >
> > LTS@avnet.co.uk
> > http://www.avnet.co.uk/touchdown
> >----- Original Message -----
> From: Nigel Charles <72016.3721@compuserve.com>
> >Subject: Engine selection
> >
> >
> > > Message text written by INTERNET:clevelee@cswebmail.com
> > > >
> > > In addition to wondering why Europas use 3 blade less efficient (than
two
> > > blade wood) carbon fibre props, he offered the best advice yet (which
has
> > > been rattling in the back of my mind) on engine selection: The Europa
is
> > > an expensive aircraft (from his VW aircraft perspective) and
installing
> > > anything other than the factory recommended engine will severely
> > > deteriorate the resale value of the aircraft.
> > > <
> > >
> > > The loss of efficiency for the 3 bladed prop doesn't seem to be as
much as
> > > the theory would predict. The main reason for the 3 bladed version is
to
> > > improve ground clearance - very relevant for the mono wheel. The
necessary
> > > increase in diameter for a 2 bladed prop would increase the risk of
prop
> > > strike. It may be practical to look at 2 bladed props with the
tri-gear.
> > >
> > > Nigel Charles
> >
>
>
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