Why two-bladed props?

[David perry 1]

Posted by Steve Hargreaves on 28/09/2010 11:05:23:

 

The mass of air you shift equates to the thrust generated....Make the pitch coarser & you shift more (less dense) air & maintain the thrust.

 Stephen thats not quite true.

 

The reason we have VP props is because as the aeroplane gets faster the AoA reduces and we have to coarsen it off to get the alpha back to optimum.   There is an issue with torque and loading, hence we also add manifold pressure gauges to VP fitted engines.  Ultimately though, it is all to do with alpha.

 

D 

[Martin Harris - Moderator]

...and the thrust required to balance drag is reduced at altitude because of the less dense air - largely responsible for the difference between True Airspeed and Indicated Airspeed.

[David perry 1]

Got this from an electricity board website:

 

Why don't wind turbines have lots of blades?

People often wonder why there aren't more blades on wind turbines. The optimum number of blades for a wind turbine depends on the job the turbine has to do. Turbines for generating electricity need to operate at high speeds, but do not need much torque or turning force. These machines generally have three or two blades. On the other hand, wind pumps operate with plenty of torque but not much speed and therefore have many blades.

Why do some wind turbines have two whilst others have three blades?

The majority of modern wind turbines have three blades, as this design has been found to have a greater aesthetic appeal. The disadvantage is that each blade will add to the overall cost and weight and can be more difficult to install, particularly offshore.

Two bladed machines are cheaper and lighter, with higher running speeds, which reduces the cost of the gearbox, and they are easier to install. However two bladed machines can be noisier and are not as visually attractive, appearing 'jerky' when they turn. The engineering ideal would be to have only one blade, and some one bladed early prototypes were developed, but didn't stand the test of time.

[Myron Beaumont]

David & BEB

Very interesting . One thing comes to mind straight away .If the noise is an important factor then why are the same 3-bladers used onshore & offshore . Probably because its a question of economics I suppose versus efficiency .Make 'em all the same -in other words .

Regarding gearbox issues,surely it is just as easy to have "n" no of blades off a main annular ring gear as one or two or three.

Ex- gas turbine man Myron

[Erfolg]

I find the aesthetic clause (wind turbines) difficult to accept, as a sensible reason for choosing against 2 bladed designs.

 

When ever I pick up a learned journal (ImechE, Engineer, etc.) as opposed to vested interest and popularist publications, the economics are detailed as poor, for wind turbines. Assuming the descriptions to be essentially true, focusing on efficiency, of both design, build, operation, maintenance and decommissioning need to be focused upon. Not aesthetics. Efficient systems should not need long term subsidies.

 

I have wondered why old style windmills had 4 blades, instead of 2. I also assume the same argument of extracting power from a 2 blader, pushed to multi blades, I guess it was practicalities of the construction that settled on 4,. Rather than 6 blades or more. Although not related to models, given the free nature of the power source for windmills, why were they abandoned for pumping water (Norfolk etc) and grinding operations of grains. It seems at face value a no brainer, yet they went. 

 

I have seen a commercial one bladed electrical sytem for very large model gliders. It apparently works very well according to the scribe

[Myron Beaumont]

Erfolg

Don't forget that the industrial revolution came along with steam etc which could be controlled at will regardless of climatic conditions (thinking of the cotton mills in Lancashire ).Of course the only "naturally"powered energy conversion machines left from that day to this are water- flow powered (dams eg Mr Hoover/streams/rivers  etc.)

Think I'm going off thread

[Rob Sweeney]

Yeah we've gone off topic...all this talk of wind turbines

 

Rather interestingly I think we're (physically speaking) thinking backwards...about how propellors on engines work to move air. Of course wind turbines turn wind/"thrust" energy into rotational energy...so it's the other way round.

 

For the reasons already given about low torque and high speed, 3-bladed is probably the most efficient energy conversion of moving air.

A two-bladed prop would need to be far bigger (adding both weight and additional inertial resistance-meaning more energy to get it moving when in a "stopped" state).

Also, we know that these turbines create massive areas of low pressure disturbance (almost like a vacumn), the epicentre of which would be closest to the blades, meaning that more blades create more disturbance. You can look up how many bats these things kill on a regular basis...

Therefore, 4 blades, 5 blades, 6 blades etc would exponentially become less effective (converting the same amount of wind into less rotational energy) as they were added because of the additional air disturbance an extra blade adds (a point I think already made when we we're still talking about scale props!)

 

The bottomline to me would appear to be that 3-blades is (mathematically) a vast enough surface area (without being oversize as a 2-blade may be) without the compromise of efficiency created by having too many blades. Of course, I'm not stating that economic, constructional and other factors don't play a part as well!

[Biggles' Elder Brother - Moderator]

Posted by Myron Beaumont on 29/09/2010 09:46:28:

 

Regarding gearbox issues,surely it is just as easy to have "n" no of blades off a main annular ring gear as one or two or three.

Ex- gas turbine man Myron

 

 

Ah, but gas turbines have lots of very short blades. I think as I suggested - and as the excert David found also suggests - there are potential problems with impulse loading with two blades. So its not so much the weight of the blades or the balance as the way they capture the energy.

 

Erfolg - I don't think I buy the aesthetics argument either - good PR but but I don't think it stacks up economically or technically.

 

On the question of whether wind farms are cost effective at all. Well I don't know - its not my field - but like you when I read serious technical papers on the subject it doesn't look very convincing. I sometimes think for the politicians its a case of "we have to be seen to be doing something about renewable energy - this is 'something' so let's do it"! I'm pretty sure that if you removed the subsidises (both direct and indirect) that wind farms get then they would become a completely uneconomic undertking. The amount of power they generate - compared to the their scale and expense - is tiny.

 

BEB

[Tony Smith 7]

Posted by Biggles' Elder Brother on 29/09/2010 11:27:25:

 

On the question of whether wind farms are cost effective at all. Well I don't know - its not my field - but like you when I read serious technical papers on the subject it doesn't look very convincing.

 

The short answer is that they would not be cost effective without the subsidy.

[Erfolg]

As most of the contributors I do not know very much about propeller design, in reality.

 

I accept and have read some of the arguments put forward by Biggles and Rod previously.

 

I am sure that we can all accept that the larger the dia, the more air can in principle be moved (area relationship).

 

How many blades, is another issue. The masking effect in practical matters can be seen with high speed propellers (air and water are both considered to be fluid) on hydroplanes etc.  So I can feel that less is more.

 

Which then brings us to an apparent contradiction, steam and gas turbines. We can all see that the physical relationship is quite different, all the rotor blades are contained within a tube. Tip losses are generally controlled by a close fit, which is such an issue with an open propeller. It is the elimination (mostly) of tip losses which results in these blades normally being wider at the tip than the mid section or root. May be this is the difference? 

[David perry 1]

Well I think the Aesthetics arugment is probably the right one for the various reasons suggested so far: 

 

The economics of wind turbines are very poor indeed, as has been disclosed then hidden many times. HOWEVER, the economics for the operators and builders of the turbines (two different group) are vry GOOD indeed when combined with various incentives and grants available, political good will brought up and the hugely important "Euro targets" that wind turbines help us meet (miss).

 

The bottom line is, I reckon, that whilst for normal people the economics of WTs just don't stack up, for the politicos and CEOs of pseudo green companies, they stack up very nicely indeed thank you, kerching!  Thus, the only stopper is planning, and planners can be assuaged with public enquiries and these can in turn be assuaged  by being seen to listen to what normal people think.  So, make 'em pretty and you can build 'em. If you build 'em we all get rich despite them being somewhat impoverishing.

 

Of course, no one will wantonly pour money down the drain so, if four blades is too expensive and two blades are seen as ugly, then lets them have three blades.  A good British compromise. No racism intended.

 

 

As a simple demonstration of my cynicism in practise, I found this:

 

 The plans, for a £100 million, 30 turbine windfarm and forest development at Fauch Hill, 8km south-west of West Calder, were last night presented to the local community at a meeting at the Dalmahoy Hotel, near Edinburgh Airport.

The development is one of three planned for Scotland over the next five years by European Forest Resources, owned by French energy conglomerate the Louis Dreyfus Group. 

Another substantial project, near Inverness, is due to be submitted for planning in the first quarter of next year, while the firm is now considering sites for the third windfarm.

The West Lothian development, which will have a capacity of about 90MW and is located on the edge of the Pentlands Regional Park, will have to be considered by Scottish Ministers under rules governing planning applications for large windfarms that will generate more than 50MW of power.

Turbines on the site are to be about 125 metres high, with a capacity of about 3MW. The project will also include about 238 hectares of forest area, which the developers say will screen the turbines from nearby homes and will include paths for cycling and horseriding.

"This is a fairly substantial investment into Scottish renewables and forestry," said Debbie Chawner, a director of Fauch Hill Sustainable Energy - the subsidiary of European Forest Resources set up to run the West Lothian project. 

"We really want to make this a 'best in class' project and the plans are not firm yet. We will consult with the local community over their ideas."

In addition to existing woodland, the developers - who will also manage the windfarm once it is up and running - plan to create a large section of new forest following a plan to be developed in conjunction with the Forestry Commission Scotland. 

The project will also provide funding of more than £300,000 a year - £4,000 per megawatt generated by the windfarm every year - for community projects.

Co-director Laurence Caperton said the company was committed to using as many Scottish consultants and manufacturers as possible.

"There are currently no turbine manufacturers in Scotland, but if one sets up a manufacturing base by the time we come to build Fauch Hill, we will definitely take them into serious consideration," he said. 

 

 

 David

Edited By David perry 1 on 29/09/2010 12:56:44

[Biggles' Elder Brother - Moderator]

David I fear you cynicism well be well founded on this occassion.

 

The development you cite being a prime example. 30 turbines, 3MW each. Even at max output (very rarely achieved by windfarms in practice I believe) that's only 90MW - frankly a drop in the ocean. A main line current power station would have a typical capacity of 2000MW - just to put things in perspective - and would routinely be operating at 1000-1500MW. £100M for max 90MW - its crazy!

 

BEB

[Erfolg]

As I was saying about 2 blade propellers

 

Yet i totally share all your fears, particularly regarding "Stakeholder Management" we heard, we considered, yet disagree.  Even the selection of a "safe pair of hands" for any enquires has a major bearing on the out come.

 

It is time to get back to propellers

[Biggles' Elder Brother - Moderator]

Conditions are very different in a turbine. As you say Erfolg the close proximity of the casing dramatically reduces tip losses. Also the flows are often high sub-sonic - even trans-sonic and the at these speeds the compressibility of the gas can't be ignored anymore. Not to mention the fact that at the temperatures involved the gas properties are very different indeed. So in short I don't think turbine blad design has a lot to teach us where propellers are concerned - or at least the transfer of knowledge needs to be done with great care.

 

BEB

 

PS Fire alarm just gone off - have to go!

[Erfolg]

Yep

 

I vaguely remember doing calculations during my studies on heat engines. A  compressor system, which i remember as a closed loop system, many examples of steam turbines (the important thing was the feed water, condenser pump), and one example of an open system gas turbine. During the calcs I did not know what they were, other than Rankin, Carnot and sterling cycles etc.

 

What I guess is interesting on reflection, was that the performance of the compression components were not considered. Yet my employer, I was later to discover, had an early computer churning away on "Blade Design". That was to achieve those compression ratios. Up to that time the blade root was narrower, than the tip. The computer did all those boring repetitive calcs, to suggest, they had got it wrong, and a massive 1% efficiency improvement could be achieved.

 

Regarding model propellers, the situation that has become the norm, is the result of thousands of practical experimentation's by modellers, for example trying to get that Wakefield to climb that little bit higher, or perhaps a CL speed model, to go that bit faster etc. Although we do not know all the whys and answers, by empirical work, we have a good idea of a reliable starting point. Which for us a two blader..

[Biggles' Elder Brother - Moderator]

Spot on Erfolg

 

Yeah - Experiments rule OK - or at least that what I tell the students!

 

Not that they listen. Too much faith in the computer these days! I'm sure that if they predicted something with a computer model and the experiment gave different results their first thought would be "Reality's wrong"!

 

BEB

 

PS Now I sound like a grumpy old man! Ha ha, may be I am a grumpy old man (Thought I'd get that in before anyone said it )

[David perry 1]

On the money there BEB, reality is wrong.

 

Under certain emergency conditions my Airbus has a computer generated landing distance required of "x".   On a wet runway the distance will obviously be greater, in reality.  Unfortunately the compuiter has declared the maximum required disctance to be "x", and not a metre more, so, "x" has been multiplied by a safety factor"y"  to give "X".  Now the wet distance required has to be factored by "z" to give the same "X".  I phoned the company today to ask about this because it actually means, "X" that we have very few places left to land in the UK (about 3) if these damage conditions are met.  I was told, somewhat tongue in cheek it has to be said, "Computer says "x"! Thus everything is factored by a fudge factor to give the same end point even though we KNOW an aeroplane crashing down on a wet and slippery runway will slither for further than on a nice dry tyre-friendly runway. I asked why we cant multiply the DRY distance by the WET factor: I could almost hear the steam coming through.  Reality must not be allowed to get in the way it seems.  

 

I am now more than ever convinced that reality is simply a computer programme gone wrong and we must work hard to fix it.

 

David 

 

Important PS

 (To those of you scared of flying, fear not.  The situation in which this happens is most unlikely to occur and of course, in real life, my colleagues and I ARE professional airmen and we know exactly what to do with "the book" should this happen.  The books might be crap but the aeroplanes are capable little things). 

Edited By David perry 1 on 29/09/2010 17:11:11

[Martin Harris - Moderator]

What is the situation David, I'm intrigued..

 

Are we looking at total hydraulic failure, landing wheels up, mains OK but no nose gear, no brakes or something totally different?

[Mart61]

Ah!  If only UK airports would give us Mu factors eh?

 

For a little light reading, take a peek at the Flight Safety Foundation's Approach & landing Accident Reduction (ALAR) Toolkit web page:

 

http://flightsafety.org/current-safety-initiatives/approach-and-landing-accident-reduction-alar

 

Edited By Mart61 on 30/09/2010 08:13:07

[David perry 1]

Martin, Its a situation called Emergency Electrical Config.

 

Basically, all electrics have failed and we are on the emergency generator, the Ram Air Turbine (RAT).

 

The significant bits are: no main brakes (so on the reserve reservoir), no auto brake, no nosewheel steering, only two spoilers,  no thrust reversers.

 

The airborne situation is not really catastrophic as the RAT will keep you going on power for a long  while but the landing is the wage earner.

 

Why would I enter EEC? Well, if there was a fire or uncontrolled smoke on board I might have to elect it, generator failure might drop me into it.  On an electric jet its really not where you want to be.

 

As an aside, the RAT is a wind turbine with two blades! 

 

David 

[Biggles' Elder Brother - Moderator]

Posted by David perry 1 on 30/09/2010 15:00:23:

.

 

As an aside, the RAT is a wind turbine with two blades! 

 

David 

 

That's for two good reasons:

 

1. It benefits from very high, and fairly consistent, wind speed (at least while the likes of David are keeping the show on the road!)

 

2. It was designed by a real engineer who knew what they doing and not a 'lentil eating ex-hippy university drop-out' employed by a trendy "green energy" company! (sorry the cynic in me come out just occassionally )

 

BEB

Edited By Biggles' Elder Brother on 30/09/2010 15:48:36

[Steve Hargreaves - Moderator]

David I'm intrigued....why does the AoA change as the speed increases?

 

In my simple understanding the AoA is the angle at which the "flying surface" (wing, prop blade...etc) meets the oncoming airflow.....why would this change as speed increases?

 

If we take a simple analagy (& switch to a wing cos its easier to understand) with an aeroplane flying at 100mph in a straight line with an AoA of say 3 degrees....if we increase this speed to 200 mph why & how would the AoA change?

 

I'm not doubting what you say I'm just interested to know the explanation.....

[Tony Smith 7]

Steve, surprised not to see a highly technical answer, but here's my simplistic one.

 

The amount of lift varies with angle of attack and with airspeed, so if you increase airspeed and want to keep lift constant (for level flight say) then AoA needs to change to compensate.   If you kept constant AoA while doubling airspeed then lift would increase (fourfold I think?).   

 

Think of slowing down, in contrast.  Its commonplace that planes (even models) fly more nose high when they're slowed down.   In that case the AoA is increasing to maintain lift, and as for "how", that 's your increasing back pressure on the stick.

Edited By Tony Smith 7 on 01/10/2010 19:01:22

[Steve Hargreaves - Moderator]

Thanks Tony....I think I understand & agree with that bit but David wrote.....

 

"The reason we have VP props is because as the aeroplane gets faster the AoA reduces and we have to coarsen it off to get the alpha back to optimum"

 

So I can see that going faster for the same AoA produces more lift but David is saying that the AoA reduces with speed...given that AoA is a physical relationship between the oncoming airflow & the prop blade I'm wondering why this should be the case....?

[Martin Harris - Moderator]

I think you'll find that David was referring to the AoA of the blade when side on to the relative airflow - which effectively changes as the airspeed changes (imagine a conventional lift diagram with the wing being physically lifted as the airflow from forward motion hits it - the A0A is reduced dramatically). Hence the reason a coarse pitch prop makes a terrible racket when it's stalled i.e. the pitch angle exceeds the stalling angle as it rotates with the aircraft stationary. It's an annoying factor in the "official" DoE noise test where a model can be relatively quiet in the air but fail the test when being held.

Edited By Martin Harris on 01/10/2010 22:25:24