Recycled Flyer

Something not touched upon here in regards to ultimate performance is cowl drag and one of the main reasons that single blade props rule in efficiency.

To elaborate, every model aircraft has a bluff cowl either directly in front or behind of the prop, and its usually behind it.
So that cowling suffers the spiral vortex dragging over it caused by each blade, the less the number of blades the less the drag.

This is what was told to me by speed flyers when quizzed about the reason why they use single blade props.

Two reasons against multi bladed props, one is that it makes hand starting quite perilous for your finger ( just try hand flicking a 5 blade engine), that and the extra cost.

And I have always held that diameter equals thrust if you keep the same reynolds numbers on the blades, whereas pitch equals forward speed.

Just picture the before mentioned helicopter with its massive diameter low pitch blades, massive thrust at a low engine speed but a ducted fan model needs to grab so much more air with its high pitch and number of blades in order to approach anywhere near that amount of lift.

Different applications I suppose and neither one is wrong.

earlwb

Single blade props were popular with the Free Flight crowd as it has less drag after the engine stops and the plane goes into its glide mode. But nowadays they have reliable folding props, so there isn't much of a reason to use a single blade prop in free flight planes now.

For speed, it was the small high pitch prop at something like 30,000 RPMs or more that tended to cavitate at a stop or slow speeds, so it was hard to get the plane moving for takeoff. But a single blade didn't cavitate as bad so you could get up to take off speed better. You see something similar to that with RC boats, and how much trouble it is to get the boat moving as the prop just spins and throws water all over, but doesn't give any thrust. The airplane prop has the same problem too, but air being less dense that water and is invisible, you can't see it.
There was also the thought that a single blade prop has less drag to it at air speed as there are less things to cause drag, such as turbulence and vortices in and around the prop. Plus you can run a larger prop diameter which improves efficiency too, even if it is one blade. Of course at 36,000 plus RPMs it is difficult to say if the prop doesn't still have cavitation problems even at around 200 or more mph on a CL plane. The other issue you have is with mulit-blade props is that at high RPMs the trailing blade is running into the wake and turbulence from the leading prop. Thus a single blade winds up having an advantage in that respect. Now a 3 or 4 blade prop makes it worse as the closer spaced prop blades have the problem of the trailing blade encountering the wake and turbulence of the leading blade too.

So far has anyone set any control line speed records using a regular custom made two blade prop yet? All the speed record CL planes I saw were still using single blade props. I had a thought that maybe a gear reduction unit and a larger higher pitch prop on a engine turning some outrageous RPMs might go even faster than a direct drive engine and prop combination.

Years ago, Hobby Lobby once sold a two, three, four or five blade variable pitch prop unit. You could make it whatever type you want. It used a adjusting screw to set the pitch. I bought one, but never used it and now I can't find it. it would have been neat to try it out now. But it figures I lost it somewhere. besides I probably couldn't get spare blades anymore either. I think it was a German innovation at the time. But I forget who made it.

well I guess I could break down and make one though.

earlwb

What brand props and size three blade props are you using? The ones I see available to me have thinner blades that screw up the Reynolds numbers making them less effective.
The only three blade prop I found that had wider blades was some of the Graupner props. The Master Airscrew three blade props all have smaller thinner blades on them.

Recycled Flyer

Earl, if you want low pitched three blade props that are beautifully made try googling "EATHER PROPS and PIPES"

Brian Eather would probably know more about this subject than most of us, just give him a call.

earlwb

Cool, thanks for the info. I will contact him. he does have some lower pitch props that would be of interest.
ref http://flystunt.com/download/INT%20-...0Dec.%2008.pdf
His last PDF seems to be from 2008 so I don't know if he is still molding them or not until I contact him.

Paul_BB

Very interesting discussion and links. By the way, how do you balance a three blade prop ?

Hobbsy

My Feedback: (102)

Master airscrew 3 bladers are pre balanced, you balance 3 blades the same as any other just reduce the weight of the heavy blade or blades.

jrb1

VarioProp: http://www.ramoser.de/home_e/variopr...rioprop_e.html .

hsukaria

This is definitely not an area of expertise for me. But one thing to consider with RC planes is that a plane will have either a 2-stroke or a 4-stroke engine with its own unique torque-speed curves. So when we compare 2 vs. 3+ blade props, we must also account for the engine and the application. Are we going for top speed with a 2-stroke race plane, or higher static thrust for a 4-stroke 3D plane?

Edit:
I also would consider the difference of angular inertia of the 2 vs. 3+ blade props for throttle transient response on 3D applications. You have 3 vs. 2 blades' worth of inertia, BUT the smaller diameter makes a big difference since inertia is a function of the SQUARE of the diameter.

blw

My Feedback: (3)

Full scale helicopters have a good bit of angle of incidence built into the system, but you apply a very large amount of angle of attack for lift. Main and tail rotor blades can apply enough angle of attack to reach stall points. Rotary wing blades have several ways they can stall, and most are significant emotional events for the crew. Helicopter lift efficiency always begins with directional airspeed past @ 36 knots. Since helicopters were drug into the talk, you go to extra numbers of blades for structural reasons (shorten the tailboom), and mostly to determine the type of rotor system you will use as in teetering, fully articulated, and rigid. Most people know of the Bell OH-58A helicopter. It didn't have the power to perform out of ground effect hovers, so the tail boom was lengthened to support an increased rotor diameter.

Airplane/helicopter blades do not cavitate air like boat propellers. Cavitation is the production of steam bubbles produced in agitated water.

Hobbsy- we used to fly into Salina, Ks all the time from Ft. Riley. The only reason for the trip was to get fueled at Flowers Aviation. Think Hooters with tiny miniskirts and tanktops all over the ramp. Then think of the girls guiding helicopters to parking with all that available downwash just waiting for those skirts.[8D] http://www.floweraviation.com/

Radical Departure

My Feedback: (13)

Wow... thats pretty slick... has it flown? If so, meet expectations?

pe reivers

Earl,
Please check the definition of cavitation. That is something that cannot happen in air. It needs transition of liquid to gas and back.
Prop flow burbling, sonic waves and blade stalling is probably what you mean.

earlwb

You guys are correct, my mistake, a poor choice of words.

jaka

Hi!
To end this story about using 2 or 3 (or more blades) in our glow engined world with engines ranging from 0,8cc to 26cc two or fourstroke, there will always be a 2 blade prop outperforming a 3 or 4 blade! Simple as that!
Ever seen a .40 pylon racing plane with a 3 or 4 blade prop outperforming a 2 blade racer? I haven't! Ever seen a .60 size P-51 semi scale Mustang or Spitfire perform better with 3 or 4 blade prop versus one using a correctly choosen 2 blade prop...?

Hobbsy

My Feedback: (102)

Jan, as long as you are alive and posting here this will not end this discussion, not even close. And we will never agree that APCs are anything but ugly glue sticks and butcher knives.

earlwb

Well I can agree those APC props and the other scimitar shaped props like them scare me a lot.
So I tend to be pretty paranoid around them, but heck accidents still happen and those things can slice up someone really bad.
Unfortunately that scimitar blade shape reduces prop noise a lot and if I remember correctly it isn't bad for efficiency either.
So we may eventually all be using them due to noise rules at the flying fields.

earlwb

I know I thought so too. Unfortunately it wasn't me though, it was someone else.
It did have me wondering about how much servo torque was needed to change the pitch on the blades though.
But this design looks robust enough to actually work well.

The electric guys are using one that can reverse pitch so they can hover upside down or even maybe fly backwards too. usually they go with a mid engine setup for reversing the pitch though.

jaka

Hi!
Amen!

Radical Departure

My Feedback: (13)

That is quite a setup. Seen the variopitch electric stuff, think this was the first variable pitch I'd seen on a gas motor. Damn I knew I should have taken machining instead of computer stuff...

hsukaria

Ya-but, somebody will have to program the awesome variable pitch controller to vary the pitch based on torque load and airspeed, right?[8D]

Radical Departure

My Feedback: (13)

Touche!

pe reivers

To think in reverse:
Why have ALL large modern wind turbines three blades? Only the fast small diameter mills have two blades. If in these sizes a two blade would outperform a three blade, I bet you it would be two!
Fact is, that two blades don't cut the cake anymore on a large prop disk surface. The larger the diameter, the more blades you will need to move the air column in an efficient way with the least amount of eddy vortices inside the disk.

xcz5785

My Feedback: (21)

I work for n aircraft propeller company. Look at all the aerobatic planes. How many 2 bladers do you see. Even the high performance general aviation has 3 bladed props.[8D]

Recycled Flyer

Wind turbines operate in an entirely different environment where harmonics (and the detrimental pulsed effects on the quality of electricity it produces), tower strength and height considerations, cascade failure and its computer control, tip speed and the noise it produces (that awful whoop, whoop noise on a 3 blader is bad enough for rural folk to listen to), the ability to withstand hurricane winds and not fail, cyclic stress and vibration as the upper most blade will always produce more torque, the greater the number of blades the easier the start up will be due to a larger surface area and probably the most important of - aesthetics.

A lot of the blade number choice revolves around being an odd number rather than simply choosing the number 3.

The air flow 'through' a turbine is not the same as that of the airflow being pushed by a propellor - just see 'Betz Limit.'
http://peswiki.com/index.php/PowerPedia:Wind_turbine
And a quote -


"Most wind turbines have three blades. Very small turbines may use two blades for ease of construction and installation. Vibration intensity decreases with larger numbers of blades. Noise and wear are generally lower, and efficiency higher, with three instead of two blades.

Turbines with larger numbers of smaller blades operate at a lower Reynolds number and so are less efficient. Small turbines with 4 or more blades suffer further losses as each blade operates partly in the wake of the other blades. Also, the cost of the turbine usually increases with the number of blades."

To sum up I would not exactly equate wind turbine theory to small model aircraft propellers.

hsukaria

Recycled,
Like you said, small high-revving engines would be different than slow-turning wind turbines. Also, the model airplane engine is moving through the air, so the prop wash is probably not as prevalent, as has been mentioned in this forum also.

I think in the end, putting in the right numbers will determine the best choice. So far we have been talking qualitatively, not quantitatively.