This question is based on a conversation I had with a buddy.

If I’m understanding things correctly (based on what I’ve read on this thread), a two bladed prop is more efficient than a three, four, or five bladed prop due to the second, etc blade being within the ‘dirty’ air of the fist blade.

If the above is true, why do aircraft such as the C-130 use four bladed props? Do they produce more power, speed, etc, and if so, how? Also, if this is so, what determines ‘efficiency’?

(I was under the impression the main reason to switch to a prop with more blades is to raise ground clearance.)

Could my confusion be caused by mixing fixed pitch props with variable pitch props? Could it be because of the difference between models and actual aircraft? Does this difference in props become a factor?

Any help would be greatly appreciated.

My knowledge (or lack therof), is solely from this thread…extensive reading. I will be either signing up for a college physics and aerodynamic course, or obtaining the college books for those classes to lessen my ignorance, and further my understanding and knowledge.

Thanks again.

I, of course, don't know what was in the designer's mind when he choose a multi bladed prop when there was already more than enough ground clearance but I will take a guess. The efficiency of a propellor goes down as the tip speed approaches the speed of sound. This can be controlled by limiting the diameter for a given RPM or by reducing the RPM for the same diameter. However, reducing the RPM increases the need for blade area even more to be able to transfer the power to the propwash. When the power available is large enough the only way to transfer so much power to the propwash, without the efficiency and structural problems associated with near supersonic tip speeds, is to resort to multiple blades.

KitBuilder,
Having flown aircraft like the C-130 for many years and having a BS in aeroengineering i'll take a stab at answering your question.
It is correct that the fewer the blades the higher the efficiency. But the aircraft designer has many other constraints when building an airplane specially one with 4 engines like the C-130 or one with one very powerfull engine such as trhe Corsair.
On a multiengine airplane there is only so much room to hang engines. The engines need to be as close to the fuselage as possible for safety reasons incase an outboard engine quits. if they were to use one or two bladed props engines would be strung out all the way to the wing tips in order to keep the tips of the blades on adjacent engines from overlaping.
A similar thing happens on a fighter design with one big powerful engine. A two bladed prop on say a 2500 horsepower Corsair would have to be much longer to absorbe all those horses . As the tail comes up for takeoff the designer would be worried about touching the runway with the prop tips. If he just shortens the prop then he'd have to run the pitch higher to keep from overspeeding the engine. So what he really needs is another blade or maby two on the prop.
Some modern airliners are using 5,6, even 7 blades.

You really don't need much formal education to see why longer props are more efficient -Just look at different full scale aircraft types and see what is used .
If you want maximum thrust at minimum speed - a helicopter "prop" (Yes- the rotary wings)demonstrates.
If you want maximum thrust once high speed is attained - -look at racing plane props .
Some thinking about the types in between these two examples will put you in the right direction .
As for the best prop design and most efficient?
That is a moving target.

Dick Hansen couldn't have said it better.

Thanks all for your help, and for expaninding my understanding.

I might add- if finances permit - buy a few props - in the largest dia -lowest pitch recommended and the smallest dia , highest pitch recommended -
Note the differences in how the model flies.
If you can't tell any difference - Have a nuther beer -

First of all, let me say that I agree with you guys.

But what I am noticing on the airport is something very different.
I am seeing ATR's and DO328's with these incredible multiblade simitar looking ginsu props with 5 or more blades. (I'm not sure how many blades, but its a bunch).

This seems to be the trend lately and in the case of the DO328 I have seen ground speeds in excess of 400 knots. These things are almost like unducted fans.

Is it because these new generation turbines produce THAT MUCH power? And another thing, these puppies are super quiet!

As long as we are on the subject of different sizes and shapes of props, An interesting thought occurred to me a while back. Maybe some of you gurus can express your thoughts.

A lot of energy is lost by air slipping off the tip of a prop, hence the ducted fan. If they have found winglets to reduce this same effect on wingtips, what do you think of a prop that had winglets (or "proplets" as the case may be)?

Something like this...

Another reason, at least in the modern cases with very powerful turboprop engines, may be the mach number of the blade tips. I know it's important for noise and airliners need to be designed with that in mind as much as efficiency these days. Perhaps that's a key reason for the 5, 6 and 7 blade props. Just guessing....

Hey Minnflyer,

I have seen prop tips similar to that except that they sort of bent back, nothing in front. Guys paid big bucks for a "Panther" conversion . That was a speed kit for Navajos in the late 70's and early 80's.

These props are used to reduce noise.
They only do like 700 to 800 rpm. That's the noise reduction part. Now, in order to convert shaft power (these turbine engines generate like 5000 hp) into thrust with a prop spinning that slow, you need a lot of blades. Simple...at least the theory. In practice, a lot of engineering is needed to achieve succes in this matter.

To a good approximation, the power absorption capability of the prop of a fixed diameter and rpm is proportional to the total blade area. So doubling the number of blades has about the same effect as doubling the blade chords. But there are a number of secondary effects to consider...

Having lots of blades with narrow chord reduces the blade chord Reynolds numbers. This is bad on models, but nearly irrelevant on a big turboprop.

And if you already have the tooling for a blade and need more power capability, just making a new hub to take more blades is much more attractive than making new tooling for wider blades.

A significant concern is the inertial torque on each blade about the spanwise axis which tries to rotate it to zero pitch. This torque is roughly proportional to the cube of the blade chord, so using lots of narrow blades puts much less stress on the variable-pitch mechanism than using fewer wider blades. I've dealt with wide-blade props where the pitch-change torque was bigger than the propshaft torque at full power! It's good to avoid this.

A little bit of trivia:
I work with a guy who has crewed for the Rare Bear. It has a 13 foot 4 blade prop, a 3:1 gear reduction from a P-3 orion. The throttle is set to wide open and pitch is adjusted to limit rpm to 3200 rpm, above that and the tips of the prop go above mach which can be heard. The blades are 18" wide. This combination has produced speeds in excess of 500 mph.

Ground clearance and prop tip speed are two big considerations for full scale aircraft.

Ground clearance and prop speed are just as important on our models, mainly scale 'tail-draggers' ( which nose over on rough fields).

I was meaning to say that whilst working at the airport, I can confirm that some of the modern transport must have powerfull turbo-props judging by the noise of the tubines compared to other JETS.

Also, as already mentioned, the prop blades consisit of at least six blades, and they are very wide in section. Noise is a big issue these days - for full size.

I was wondering that multi-bladed props became popular after they were uses in aerobatic model competitions ie - pattern ships.
Is this due to having a braking effect when having the throttle right back when diving straight down??

A larger blade area with low pitch would be best??
Thats my understanding....