3 blades to 2 blades

[Night Crawler]

Hey all. My first question on this forum.
I have an Art-Tech Cessna 182 (brushless). It came with 2 sets of 3 blade props. To me, they seem pretty flimsey and i have already broken 1. I have fixed it, but not used it incase it decides to come appart. I was gonna get some more props, but i read on here that 2 blades are better than 3. Also, there would be less chance on me breaking a 2 blade prop (because there is less to break) and it would also be easier to balance.
The original prop has 8x6 stamped on it. So im presuming thats the size.
So, my question for you guys.
Should i replace the standard 8x6 3 blade with an 8x6 2 blade? Or would i need a different size alltogether?
Also, appart from carbon fibre props, whats the best to go for? Im not fussed about how it looks because its my first plane. I just want it to last and perform well.

[Danny Fenton]

I beleive you would be looking at a litlle bit larger diameter, you could try a 9 x 6 two blader. A two blader is more efficient than a three, and in fact a one blader is the most efficient. Apc electric are IMO the best props for these types of models.

 

Do you have the ground clearance to go up in diameter?

 

Cheers

Danny

[Alan Cantwell]

i read this as you have glued back a blade on your 3 blade prop, PLEASE, just bin it, even on a small model like this, you can lose an eye, or even worse, someone else can, i also get the feeling you are flying without insurance, can you imagine the damages if the prop threw this glued on blade? regarding the model, APC make specialist electric props, lighweight, and wide bladed, i am not an electric person, but have seen these used, as mentioned, go up 1" on diameter

[Night Crawler]

Thanks for your replies.
First off, i did mention that i wont be using the fixed prop, incase it came appart. I know a guy that runs a carbon fibre buisness, so if i had to stick with 3 blades, then i could used the fixed prop as a template for a mould.
As for insurance, i didnt realise that it was a must have. I thought it would be like home insurance, its there if you want it. But i'll be looking into it this weekend anyway.
As for the 2 or 3 blades.
As the plane stands at the mo, there isnt much room to go up a size with the current landing gear, but i do intend to replace them because those too are a bit flimsey.
You mention that 2 blades are more efficient than 3, but if thats the case, why go up a size? Or is it not that much efficient?
My basic understanding with props is this (just example figures).
Say a 1 blade prop gives 1kg of thrust at say 5000rpm, a 2 blade prop would give 2kg of thrust at 5000rpm and a 3 blade 3kg at 5000rpm and so on.
Im guessing that this isnt correct.
Also, what is the second number on the size? Could i change to an 8x7 2blade instead of using an 8x6 3blade?
Sorry, this is all still new to me.

[Alan Cantwell]

three blade props on a full size are generally run through a gearbox, on our models, we can get nowhere near full size without such a box, 2nd number on the size is the pitch, the amount the blade is twisted, go up on this, and the motor must have the torque to swing the increased pitch, for it will take more punch to get it through the air, props are a funny thing, just last week, we where trying out differant props, on a friends 88" span hawker hurricane, fitted with a 35cc petrol engine, we fitted one, it seemed way down on what revs we expected, model was tethered to a spring fishing scale, a note was made of the amount of pull, we fitted another, smaller, finer pitch, and the engine was really revving, we thought we had found the best prop--not so, the pull on the spring balance was down by 4 lbs!! so, revs are not everything, the amount of pull is the criteria,

 

as for isurance, it isnt illegal not to have any--just foolhardy, the biggest single claim for personel damage to a human, was from a chuck glider taking out someones eye,(bmfa info) on joning a club, not only will you be taught to fly correctly, and safley, you will also benefit from 1£10.000.00 insurance third party, and its about £25 for this remarkable bargain

[Craig Spence]

Hi night crawler,

You don't have to have BMFA insurance it's not the law. However I have the insurance, any flying club registered with the BMFA requires that all it's members have flying insurance, if they don't they can't become a member or have their membership revoked if they don't renew it (this is currently happening at my club).

Also a personal view from me, you really do need it. You are a learner but even the most experienced pilots have accidents out with their control and even major equipment failures.

I'm going way out there with this but I think yourl get the point, if you were flying then lost control and crashed into a moving car making crash causing others to crash and possibly being responsible for the loss of their lives, in a court you wouldn't have a leg to stand on it's like driving without insurance. You would probably go to jail.

If the same said make believe incident happened and you had insurance the BMFA would cover your liability up to a ridiculous amount (10 million I believe), also the judge/ jury wouldn't see you as irresponsible as you have token out insurance.

Even the little miss haps are covered ie.., amputated fingers, accidental minor damage of property etc...

Really, it's only cheap and you can't afford not to have it these days.

Back to your topic, sorry for the novel. I'm not sure about the props either, my view is if a three blade prop is less efficient than a two blade then why do they employ multi blade props on larger commercial planes.
Im thinking maybe turbo props as they call them, or the engines are simply more powerful.

I think this may just be in rc terms really, multi blade props in them selves are probably more efficient just not on rc models as they have not got the same grunt as real plane engines.

That's the only way I can look at it, again why would all larger planes have multi props if they are less efficient?.

Also I assume the was from multi blades makes the next blade less efficient.

Oh and could someone please explain how a one blade prop works lol!, the engine must wobble some.

Hope it works out for ya.

Edited By Craig Spence on 26/03/2011 10:29:41

[Alan Cantwell]

a one blade prop works with a bob weight taking the place of the other blade, quite a familiar site in rubber powered free flight, but not on our R/C engines/motors,

 

looks like we where typing at the same time, i won!!!

[Craig Spence]

Ha yeah I thought the same thing lol!.
Interesting about the one blade prop though, never knew that, thats why the forums good, learn something new every day.

[Night Crawler]

Thanks guys. I'll definatly look into geting some insurance. It makes sence really. If i join a club and got insurance through them, would it only cover me at the club, or is it mine to use anywhere?
Where i flew the other day, was a big open field. Well away from any roads and houses. When i saw a guy walking his dog, i didnt bother taking off until he was off the field.
Anywhoo...
Looks like its pretty much trial and error then with selecting props. I'll probably setup some sort of test rig to find out which prop is best. I dont fancy forking out for a watt meter though. Would a multi meter work set up to measure amps (as long as it can take it)?
Also, if i go with a 2 blade prop and keep the size the same, would it be ok to increase the pitch because theres 1blade less to cause friction...?
Oh, i havnt seen a 1 bladed prop on a model aircraft, but if you watch the first few mins of I Robot, you see that Will Smith has a 1 bladed celing fan. Its basically 1 normal blade and a small counter weight on the other side. Not sure one would be good on a model though because the pull from that 1 blade will always be on one side of the shaft and cause excessice wear (wear, ware? dunno, its to early for me) on one side even though its ballanced. If you get what i mean.

[Craig Spence]

It's your to use anywhere, ie.. Any club or area where ethe land owner gives you permission to fly and even then you have to consider how your flying will effect the public, livestock and property etc..

Another big tip don't fly near electrical stations, pylons, main roads etc..., some people may think this is common sense but you'd be surprised at how some peoples perceptions of imminent danger are realised.

But by the sounds of it you have the right idea, don't take off if there is a possibility ( if something goes wrong) you may have a mishap.

I did the same thing as you but with a multiplex mini mag ( excellent learner plane) but after spending so much money on repairs and new kit, plus I wanted to go ic I joined a club and really was the best thing I've done. There you will get all the advice you will need reference props and what not, I can't advise on electric as I'm more an ic man and have a chart for all my engines for props. Electrics a lot more complicated and just gives me a headache lol.

Hope this helps.

[Tim Mackey]

Craig says...

"Electrics a lot more complicated "

Not always - some excellent ready to go electric models are arriving at a store near you nowadays.

[Craig Spence]

Not to cause and electric/ ic drift tim.
I just find that sometimes there's a lot to consider in electric planes other than rtf or bnf.

I like electric but prefer the simplicity of ic and the sound.

However night crawler, ic is a lot dirtier and you have to clean your plane after every flight if you wish to maintain it well.
Plus engine maintenance and tuning to start with it can be a pain, starting out in electric is way better.

[Biggles' Elder Brother - Moderator]

Craig, you asked the question "if two blade props are more efficient why do full size commercial aircraft use 3 and 4 blades etc."

 

Well its a good question and it deserves an answer.

 

The simple answer is "power". Its a bit like a F1 car, there is no point in having a zillion horse power if you can't "put it down" on the road. So they have really big fat tyres to try to get all that power to do something useful - ie to drive the car forward very quickly.

 

Well the same applies to planes. If you want your plane to go faster, or lift a heavier pay-load, you need more engine power. But now you have the problem of converting that power into thrust. You could, in principle, do that by just running the prop faster and faster. Unfortunately there is a limitation - once the prop tip approaches the speed of sound they don't work very well any more - so you can't use that solution. The only other thing you can do is make the prop blades longer - so that they sweep more air backwards. This is exactly what they did at first. See WW1 aeroplanes. Right up to the prototype Hurricane and Spitfire. All with bigger and bigger two bladed props.

 

But soon, as engines get more and more powerful you hit a another problem. The prop diameter to handle the power starts to get silly - you simply can't have single seater planes with 24 foot diameter propellors! Not only is it bad from the practical ground clearance point of view, the tips of such large props again quickly run into compressability or "sound barrier" problems. So how can we reduce the prop diameter and yet still keep using all the power? Answer add another blade - for the same power transfer a 3 bladed prop is about 10-15% smaller in diameter. See the production Hurricane and earlier production Spitfire.

 

Then engines became even more powerful - and the three bleaded props got bigger and bigger. Eventually there was no choice - they had to go to 4 bladed props to handle the power whilst keeping the diameter reasonable. Towards the end of piston power, and even now on some turbo-props, they have five and six bladed props.

 

But, there isn't one of those designers that wouldn't much prefer to have a two bladed prop if they could - but it would be enormous!

 

BEB

[Night Crawler]

Im getting more confused now. Ok, 3 blades has more surface area than 2 blades, so they can push more air back. Why do people prefer 2 blades when clearly 3 or more blades is better? Ok, 2 blades might be more efficient (i cant really see how) but you clearly can get more thrust from 3 blades of the same size.
Why not make the blades wider instead of adding an extra blade.
Also, the Lynx helicopter has specially shaped rotor tips, because they spin so fast. Why not do the same with props?
Another thing i noticed on my 3 blade props, as you get further away from the root, the blades start to level off. Why?
1 more thing. Large 2 blade full size props spin so fast that the tips start to break the sound barrier. Surely our model props dont get anywhere near that speed, do they?
Sorry, i end up asking loads of random questions late at night.

[Alan Cantwell]

The answer to the above is simple, we would all like to swing that scale size 4 blader in our mustang, but we cannot, why? we are direct drive, now then, a lot of friends of mine are in the Dawn Patrol, these are a bunch of guys who fly the show circuit and fly 1/3rd scale models of WW1 machines, these machines, by rites, would want power plants of minimum 62cc, up to 140cc, these would swing props from 24x8, right up to 28x6, BUT, they use 38cc motors, and swing a scale size prop, of 32x12, this revolves at scale revs, of about 4500 rpm, how is it done? with a gearbox, this is why the smaller model cannot swing multi blade succesfully, straight shaft drive has its limitations,

 

now another thing, what frequency is your plane on? if its 35mhz, or 2.4, then ok, but if its 27mhz, a lot of clubs have banned its use, 40mhz is for surface vehicles only, best check before you find that club

[Craig Spence]

Hi BEB,

That's a good answer, always happy to learn new info.
So I am guessing that our small engines never reach a speed that truly justifies adding more blades, hence making the two bladed more efficient because they can increase the diameter without a problem. Adding more blades to our engines is then just for appearance and scale looks because in fact adding more blades will definately make it less efficient.

Oh and Alan that's a good explanation from our terms, I have seen those gear boxes on Mick reeves models, cost a penny or two as well.

Cheers guys.

[John Privett]

Posted by Night Crawler on 27/03/2011 02:33:12:

1 more thing. Large 2 blade full size props spin so fast that the tips start to break the sound barrier. Surely our model props dont get anywhere near that speed, do they?

 

Let's do some quick calculations then...

 

An 11" diameter prop has a circumference of 34.5 inches. Let's call that 3 feet. So the blade tip travels 3 feet per revolution.

 

At a modest speed of 10,000 rpm the tips are travelling 3 x 10,000 = 30,000 ft per minute. That's just over 340 mph or roughly half the speed of sound. Double the speed to 20,000 rpm, or drive a 22" prop at the same speed and you're very close to the speed of sound.

 

[John Privett]

As for efficiency, consider the problem this way. A single blade is always working in "clean" undisturbed air. Add a second blade and it work slightly less well as it's working in air that has already been disturbed to some (smallish) extent by the other blade. Now add another blade, and maybe a fourth or fifth, and the blades are now all affecting each other to an increasing extent.

 

So, in general, fewer blades is better. But practical considerations (as mentioned earlier) means that more blades are often required.

 

I'll leave you with this picture of the prop on the model that holds the F2A world speed record. A single blade - with counterbalance in the hub! When chasing world records everything has to be done to get the very best possible performance.

 

[John Privett]

Posted by Night Crawler on 27/03/2011 02:33:12:

Another thing i noticed on my 3 blade props, as you get further away from the root, the blades start to level off. Why?

 

Your 2-blade prop should be just the same.

 

Think of the prop "screwing" its way through the air. The part of the prop close to the hub doesn't travel very far round in its circle. The tips travel a long way round in their circle. But asthe whole prop moves forwards through the air the tips and the hub have to move the same distance forwards! So the blade is at a steeper angle at the hub than at the tip.

 

Alternatively, think of the angle between the steps on a spiral staircase. On the outside the steps are wide so it's a small angle between the steps. On the inside the steps are very narrow so the angle is much steeper. Same as a prop.

[Night Crawler]

Wow, thanks guys.
Never thought it would need so much more effort from the engine to spin another blade or 2. Its just cutting through air.
I also never thought about our motors spinning so fast either, bit it makes sence.
I also get the need for the angle to change as well. Kinda obvious really.
The one thing i still dont get 100% is the efficiency.
Ok, so a blade cuts through the air and leaves a disturbance behind it that the next blade goes through. Wouldnt this only happen while the prop is spinning in one place? Because as the prop is moving forward its always cutting through clean air?

[Craig Spence]

"Wouldnt this only happen while the prop is spinning in one place? Because as the prop is moving forward its always cutting through clean air?"

Fair comment, if your traveling forward the air will be clean as if to say and also going back to the screw effect it makes sense.

[fly boy3]

Hi night crawler, I know very little about electric models, Having said that,2 guys at my club were flying (not on the same day) small scale foamies with 3 bladed props. They both damaged one blade on landing. After changing to a 2 blade prop they flew again, and swore the models were flying even better than with a 3 bladed prop. Cheers

[Night Crawler]

Looks like im gonna have to do some prop testing then. Maybe even make some sort of forced air wind tunnel type thing to simulate the props movement through air.
I just need to figure out some way of measuring the thrust, twist and also how to measure the air going into the mini wind tunnel.
Would i be able to measure the watts the motor is using with a multimeter instead of using a proper watt meter?

[PatMc]

If you have a meter that will measure within the current range that the motor draws & a second meter to measure the voltage then it's a simple matter of multiplication. There will be some drop of in both readings as the battery runs down but if you start with a fresh charge it should be reasonably steady after a 10 - 20 seconds.

 

If you want a good explanation of how props perform this site is better than any I've seen. Just click "Propellors" & take it from there.

Edited By PatMc on 27/03/2011 20:02:01

[Biggles' Elder Brother - Moderator]

NC its not so simple as to say "The prop is going through clean air because its moving forward"! It may very well be moving forward but the air is far from "clean". Run your engine/motor with the model restrained on the ground. Very carefully put your hand in front of the prop - you can feel the air being drawn in. In other words a prop effects the air in front of it as well as behind. In reality all props "slip" they never perfectly screw through the air this means that even in flight the prop is having an effect on the air in front of it.

 

The more blades a prop has the closer they follow each other - the more they move in the disturbed air of the preceding blade - this applies just as much to a blade in flight as to one on the ground. In fact it possibly has an even greater effect in the flight because the relative axial speed of the air relative to the prop is much lower in flight - and so the air "hangs around" the prop for longer IYSWIM.

 

No need to build a wind tunnel - all this has been very well researched already. A suitable Google search will reveal 100's of technical reports and papers on this.

 

BEB