Prop Efficiency
 

Excuse the 'Jackanory' element here...

Whilst sweating my ar5e off in a hotel room, I stared at the ceiling fan above me and thought "if they put more blades on this thing it would push more air towards me and keep me cooler."

I came to the conclusion that adding prop blades would eventually make the whole setup less efficient due to drag and strain on the motor, and that therefore there was an 'optimum' amount of blades (and their AoA etc.)

Could someone clarify (or provide formulae for) the above?

Why is it that some helicopters use only 2 blades (jetranger[?}) and others need more? Likewise for propeller aircraft. What are the implications for adding more blades to a prop and at what point does it become ineffecient?

Ta :ok: Chris

You are right, up to a certain point. On 'most' propellers, maximum efficiency occurs with about 4/5 blades, because above this number, the blades start taking up so much area of the radius that there is not enough space for air to be accelerated through, thereby reducing the efficiency. How much of the propeller disk is 'taken up by blade' is referred to as the solidity of the propeller. Solidity can therefore be increased by increasing the chord of the blades, and/or increasing the number of blades.

Hope this helps!

Cheers, Jack.

Cool, but....

QUOTE

"On 'most' propellers, maximum efficiency occurs with about 4/5 blades"

UNQUOTE

If this is the case and 'maximum efficiency' is achieved with 4/5 blades, why don't all aircraft use this? As mentioned above, the Jetranger uses only 2.... :confused:

Cheers

Chris

I think part of the answer might be related to cost vs. amount of lift needed from the 2/4 helicopter blades. If you need a high lift helicopter, two blades ain't gonna cut it.

On turbo props though, I have heard that the six bladed props on the Dornier is alot quiter because of the extra blades. I think Jack is right about the point of deminishing returns on adding too many. High drag would only be a big problem if pitch was pushed up. Sorta like slamming on the brakes.

In fact the more blades a propeller has, the less efficient it becomes. Don't know of any full size examples, but most model aircraft speed records are held by aircraft using single bladed (counter-balanced) props.

bekolblockage has it right.

Single-blade props have been tried on light planes (~100 hp) and I don't know the outcome - but more blades are generally used only when there are diameter constraints, or (as a corollary) when tip mach number becomes too high.

Barit1 and bekolblockage,

Very interesting - I was always under the impression that a graph of 'number of blades/efficiency' would result in a rise, peaking at around four blades, then falling again. Could you explain (or point me in the direction of somewhere that will) very briefly the theory behind the high single blade efficiency?

Kind regards, Jack.

EXACTLY what I'm after Halfbaked_Boy

Am I correct in saying that the pitch of a single blade prop would be 'fatter' to counter the fact that there is only one of them?

Cheers

Chris

Not necessarily - because the best efficiency occurs when the angle of attack of an aerofoil is the one giving the best thrust/drag ratio - which is about 4 degrees for most (duly noting that angle of attack isn't the same as blade pitch due to the induced airflow reducing the A of A).

A single blade would need to be longer for best efficiency (like a long glider wing) - which would give ground clearance and tip speed problems, wider chord would be less efficient.... everything is a design compromise.

Multiple blades work well where the angle of attack can be kept close to the optimum thrust/drag ratio but too many blades are badly affected by the wake of the previous blade, reducing efficiency.

As far as I know, the less blades you have, the more time it needs to accelerate. With more blades, the prop radius will decrease (for the same efficiency), and you'll improve the prop time of reaction (especially with light synthetic blades, compared to metallic ones).
On the Bell helicopters, the two heavy blades are a good point for the autorotation, due to the rotor inertia (the blade tips are metalic). But in the other hand, when the rotor looses some RPM, it's more difficult to re-accelerate. Short radius-multi blade prop is like a low gear in a car, and a long radius, with few blade prop, is like a higher gear.
On some planes, they increased the number of blades, and also their shape. Maybe it's because with 4 bladed props, the blade tips are close to the speed of sound during cruise. By increasing the blade quantity, you'll reduce the prop radius, and the tip speed. From that, the plane can fly faster.

Its more efficient to move a large volume of air at alower velocity than a smaller volume at a higher velocity to produce the same thrust.
For un-shrouded rotors, the optimum tip relative Mach number is around 0.8.
So if you select the maximum diameter that can be packaged, gear to the RPM that will give you a good tip mach number, then the power available will dictate the approximate solidity that you require to absorb the power at that RPM. The choice then between chord length and number of blades will depend on the efficiency, weight, cost, noise, availability etc of the possible combinations.
No simple answer I'm afraid, and the demands of ceiling fans are rather different from those of aircraft powerplants.

Many compromises are needed in this area of propellor design.
To give but one example.
The DC-7 and the Lockheed 1649 Constellation.
Both had the same type engines (CurtisWright turbocompound) with the same BHP, yet the Connie had three blades whereas the DC-7 had four.

The reason was aircraft geometry, specifically wingspan.
The 1649 Connie had a brand new design long tapered wing (to hold more fuel, among other reasons) and as a result could use the three blade design, with slightly longer and wider blades, allowing wider engine spacing, which resulted in a slightly faster cruise speed.
The DC-7 on the other hand had a 4 blade prop because the wing was adapted from the DC-6B, and therefore three blades would not work well due to engine spacing considerations (wing bending moments, prop overlap etc).

Oftentimes, the devil is in the specific aircraft details, even though the engines might well be the same.

One of the big 4-engine (Consolidated-Vultee?) flying boats of WWII started out with 3-bladed props, but the inboards received so much erosion due to spray that they changed the inboards (only) to smaller-diameter four-blade props.

topic slew
 

and just to round off this topic to get a fuller picture:

Why do ducted fans (on jets) have so many blades and yet have an amazing efficiency? (At least I think they would better be efficient because there is too much power at stake.) Surely they are in each other's way.. I mean wake.

Could someone clear this up, please?

The compressor stages (including the fan) are generally trying to increase static pressure, not increase velocity. So you want a nice high solidity to 'hold the pressure' as it were. Plus its the gaps between the blades doing the work, not the blades themselves, and big gaps wouldn't work so well.

And--
the fan is shrouded, so tip losses are minimized.

(In fact, that's a good argument for a single-blade prop. Fewer blades, less tip loss... ) :8