3 blade props

[the unmagnificent man]

Why is it that electric planes are quite often fitted with 3 bladed props but IC engined planes are only fitted with 2? I have seen them with 3 but not very often. I got asked at my club the other day and couldnt give a decent explanation.

[Tim Mackey]

Can't say I have seen a lot of electric with 3 bladers myself. Unless anyone knows anything different , I would say that it doesnt make one jot of difference what type of motor is turning a prop, as far as it  efficiency is concerned a 3 blader is generally always less efficient than a 2 blader. Something to do with more dirty air I believe, Eg each blade has to travel immediately behind the moved air of the blade in front of it, and this happens both faster and more often than with a 2 blader?

Again, from my limited knowledge of such matters, I think the only real reason a 3 ( or indeed any multiblader ) is used on a full size is because of insufficient ground clearance for a 2 blader - and some big meaty motors would have needed huge 2 bladers to absorb the power output correctly.

[Myron Beaumont]

Don't forget that your "dirty" air might well apply to a tiny prop but I don't think it has the same significance on full size aircraft .By the way I've wondered why the three blade configuration has been adopted for wind turbines .Any comments ? & WHILST i'M SLIGHTLY OFF THREAD AGAIN why THE SHAPE OF BLADE USED AS WELL  ?

Remember Windmills

[the unmagnificent man]

Cheers guys.  I would like to find a 3 blade prop to stick on the front of my messerschmitt. Any ideas as to a good place where to begin looking?

[Tim Mackey]

I think Steve Webbs has a good selection of props, and Sussex models too I believe.

[Martin McIntosh]

I used to use three bladers in competitions when we were strictly checked for noise at most events. The advantage is that the tips of the much smaller diameter prop run at a lower speed than a two blade and were therefore quieter, in fact 3dB down, i.e. half as noisy. The big disadvantage is that they are much less efficient. A 9 1/2" or less prop on a powerful 60 doing 14k+ produces a lot less thrust than an 11" two blade at the same revs. The generally much lower revving electric set up will not suffer the same fate because of the increase in prop efficiency at a lower speed. Explains why many models still tend to land like a bat out of hell when on idle. Anyone got a rule of thumb formula to convert two blade to three blade?

[Martin McIntosh]

Tried that. Goes all wrinkly though.

[Aslan]

Drop an inch in diameter or pitch by one.E.G. if using a 13x7 two blade you could use either a 12x7 3blade or 13x6 3blade.

[Myron Beaumont]

Martin

Didn't realise you were a contortionist !

[Martin McIntosh]

I try to be flexible in my approach to anything Myron.

Re the shape (props of course), I found that if the tips were rounded as per the wing tips of a model, the noise went back up by 3dBa. Square them up back to the original `Rev-up` shape and it went down again. Says something for the efficiency of CAP tips maybe.

[Shaun K]

Here's some fascinating reading that explains a whole lot of propellor theory, including facts and misconceptions around multi-blade props. Hope it helps.

 

http://www.bolly.com.au/1998%20Bolly%20Book%20v3.pdf

 

 The abridged version is here though, extracted from the above:

 "Contrary to popular belief, multiple-blade propellers do not operate in severely disturbed air from the previous blade (when in forward flight). The reason multi blade propellers often appear inefficient is the need to use considerable lower diameter propellers (in comparison to 2 blades), for the same horsepower available. Diameter for diameter, a well designed 4 blade prop will in some circumstances perform better than the equivalent 2 blade propeller.

It is rare (if not impossible), to find an efficient 3 or 4 blade propeller manufactured from nylon based materials. The reason for this is the most efficient shape (thin, narrow blades) for these propellers is difficult to produce in anything but a carbon or glass composite construction."

 So, maybe we can read from this that your beautiful scale model may fly perfectly with a nice big 3-blade prop, but you might just need a bit more grunt behind it...

[the unmagnificent man]

Cheers everybody for your contributions. I am now going to start a thread called where do you get spinners for 3 bladed props?

Seriously though I like the idea of a scale looking prop as long as it doesnt compromise the characteristics of the model.

[Myron Beaumont]

Nobody has explained to me why wind turbines have only three blades We've accertained from the above info that there is little interference done to the air on our tiny props so "in a word" why not re-invent big windmills ?

[the unmagnificent man]

Myron, We had this same conversation at work this morning. I also cannot understand why we dont start building windmills in the UK to produce electricity. After all they look nice and even generate public intrest. I imagine though the real reason is because someone would have to fork out a lot of money to build one and it would only generate enough power to power a certain number of houses due to the fact that they would be smaller to fit into the surrounding landscape and therefore would not produce enough profit for the person who built it. Shame really because when I lived in Holland people used to come from miles around to visit the windmolens of Kinderdijk. (7 in a row) This would lead to more money being brought into the area. ANYWAY how come we always end up off topic with you?  and why has Timbo only submitted 1 post on this thread ha ha. Im off to finish painting my plane. Al

[Myron Beaumont]

I Think he's gone into mass production of "sticky buns"

[winchweight]

I have a 3 bladed prop on my Tucano, and I power it with a 53 2 stroke. The model is designed for a 40 to 46 sized powerplant, so the 53 soaks up the 3 bladers extra power requirements.

 

As for spinners, try Just Engines as they sell all things ASP. My spinner is an ASP spinner, but I got it from Hong Kong via ebay.

[Tim Mackey]

Mass consumption maybe, but not production

"and why has Timbo only submitted 1 post on this thread ha ha"

2 actually, and with this one,that makes 3

Incidentally we are not short of those ugly great inefficient politically correct wind turbine generators here in the UK - 'tis a windmill really aint it ?

We are getting set here in North wales to have the largest offshore development of these ugly great things covering our lovely Irish sea / Liverpool bay area - right in the bay, smack in the back garden view of our house

[Martin McIntosh]

Shaun, thanks for that very interesting site. Not read all of it yet but must disagree with the tip shape bit so far from practical experience. Will peruse the whole thing later and it may enlighten me.

[the unmagnificent man]

MMM I like that prop on the Tucano, however after a short think I have decided against fitting one on my messerschmitt  as the flight yesterday proved that I need full power sometimes and I dont wish to lower that power with a 3 blader. But I now know that next time I fit engines I should go slightly above the reccomended size in order to fit 3 bladers. Thanks all.  Al

[Kelly]

Hi The unmagnificent man,

What is your Messerschmitt, have you any pics, here is mine ,yet to fly

 

Cheers

Kelvin

[Doug Ireland]

Re Prop tips... If I recall correctly, Hartzel (full size prop maker) came out with the "Q-Tip" some time ago. The blade tip was curled back and was apparently very good for noise reduction. Don't know how it could be made to work for our applications though.

[the unmagnificent man]

Hi Kelvin, I'll try again. I just wrote a post and it suddenly disappeared.

Sorry for not getting back sooner but I have just got back from France where I work, its a 700 mile drive and then I had to MOT my car when I arrived. I'm knackered. My Messerschmitt is still in France and I will try and prize the camera off my girlfriend for when I go back. Even then I will have to wait until I come home again to upload the pictures to my computer so it could take a fair few weeks. I think it'll be worth the wait though as I have recently re painted it with all the panel lines and added the weathered look to it. Even though I say it myself it looks OK. If I can somehow load the pics up before I will.  

OH, to answer your question, my messerschmitt is a Kyosho 109 e with an SC52 four stroke with on board glow and venom smart temp/voltmeter.   Al

[Kelly]

Hi The unmagnificent man,

Sounds good, look forward to seeing the pics.

I am going to video the maiden flight of my 109 if I can persuade my Son to come with me and  film it .

cheers

Kelvin 

[Walts]

Guy's, my ideas on the original question to this thread are that electric motors are so much more versatile than i.c. engines. With an electric motor you can still over load the motor with a three blade prop and achive poor results............thats if you don't melt something first! But there is a whole host of gear boxes that can be fitted to electric motors to allow the same motor to turn a much larger dia. coarser pitched prop more slowly. So you are now moving a larger amount of air at a lower speed, as opposed to moving a small amount of air at a higher speed, with the former being far more efficient. Hence why you do see, not only three bladed props but also more scale like sizes of props on leccy models. I'm still thumbling my way through this electric flight stuff, but I am toying with the idea of fitting an electric set up to the T.N. Typhoon that will be capable of swinging a scale size three bladed prop.

Walts.

[Martin McIntosh]

Dunno yet but a very low kV motor should be the answer. I have been trying a 670kV and the bigger the prop I fit the better the performance seems to be but of course the current drain goes up. I think a case of suck it and see at the moment until motor manfacturers/ retailers start to give more info on the recommended props/ battery capacity/discharge rate required/power output expected/rpm etc. instead of the utterly confusing diameter/length/No. of turns which do not mean a lot to me, and I presume many others.