Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 09/06/2015 12:50:02
-
Mike Greenland asked the Naked Scientists:
Why do wind farm generators generally have 3 blades yet aeroplane propellers usually have 4 blades? Is it not the same principle - though opposite...
What do you think?
-
At low windspeeds you want fewer blades, so that they are not turning in each others' wake. Hence small planes travelling at less than 150 knots tend to use the cheaper 2-bladed prop, and mid-size turboprop aircraft use 4 or 5 bladed propellors.
More blades = more effective swept area so a better coupling between the engine and the air at the design speed, but a rapid falloff in efficiency below the design speed. 3 blades seems to cover the requirements of windfarms for a broad speed range: if you reduce to 2 blades and a bigger span you can extract the same power at lower speeds, but the tips can become supersonic in a stiff breeze!
When the airspeed past the propellor/compressor/fan is really high, as in a jet engine, you can use lots of blades and get a really efficient fuel burn, but taxiing with a jet at idle speed burns money at an amazing rate!
-
I saw a mathematical analysis of the efficiency of wind turbines (some time ago).
You can extract more energy from the wind as you increase the number of blades, but the biggest gains come from increasing from 2 to 3 blades.
Increasing from 3 to 4 blades increases the efficiency slightly, but costs 33% more, which is not cost-effective for the small increase in efficiency. Traditional Dutch windmills use 4 blades, perhaps because you can use a long span of wood to balance the two opposing blades; it may be harder to find a piece of wood with a 120° degree split.
Windmills used on Australian outback farms to pump water commonly use around 20 blades.
Some examples here (http://www.zardec.net.au/keith/windmills.htm).
(I have not seen people intentionally use a single-bladed wind turbine; despite its low cost, the asymmetrical forces will quickly distort the drive shaft...)
-
Single bladed propellers do even exist- they're used on some powered gliders. They have a counterweight instead of one of the blades; they're used because they're easy to fold away, and they're slightly more efficient.
It wouldn't be particularly useful for a wind turbine, the counterweight would actually be heavier than a blade and you'd only get half the power. Weight is actually important because the mast needs to hold everything up.
-
small planes travelling at less than 150 knots tend to use the cheaper 2-bladed prop
I have seen some small fixed-pitch propellers formed out of a block of laminated wood. This provides considerable strength for a 2-blade propeller, without the complexities of a gearbox for a variable-pitch blade. (The variable pitch allows a greater range of speeds - like changing gears on a car.)
For commercial planes, where time is money, a variable pitch blade allows greater speed; a variable pitch blade can no longer be made as a single piece, so the hub and blade becomes much more complex. Perhaps the additional thrust of an extra blade (or two) becomes economic at this point?
Some aircraft have adopted counter-rotating blades (http://www.theguardian.com/uk-news/2014/nov/01/raf-russian-bomber-uk-airspace), as this is more efficient than a single propeller (the counter-rotating blades don't leave the air with so much of a residual swirl). However, I understand that the propellers passing each other at high speed can cause a lot of vibration and noise.