Propeller Blade Count.

[markstuk]

Hi,

I've been digging around for a while but can't really pin down why only the British went to five bladed props, whereas the US went from 3-4 and the Axis generally stuck with 3 blades from what I can tell. I understand that in some of the cases the move to 5 bladed props was to absorb the extra power and cope with ground clearance issues, but surely the same issues affected the late Me109's ?

ANy observations?

 

[swampyankee]

I suspect that the Germans continuing to use three blades had to do with their preference for synchronized guns. The British use of five blades may have to do with manufacturing— they may have been able to use an existing blade and just add one to get the blade area needed at higher powers. The US makers found it easier to design a new blade to get the area. I'd really have to dig into the design documents, which are probably in one of those university libraries that just closed due to covid 19

 

[wuzak]

The Spitfire XIV went to 5 blades because the engine was slightly tilted down at the front to aid pilot view and, thus, the diameter of the prop was smaller than for the Mk VIII Spitfire, which was, essentially, the same airframe with a 2 stage Merlin.

The undercarriage legs could not be extended any further either.

Not sure why the Tempest II went to a 5 blade. Maybe for similar reasons, possibly because the prop centre was lower on the Centaurus than the Sabre version.

 

[aircraftclocks]

The following is from the AAF News , April 1945, discussing technology.

As more horsepower has been built into engines, larger and larger propellers have been developed to convert this power into thrust. But there are limits to the diameter of a propeller that can be mounted on an aircraft. Instead of extending blade diameter, the width and number of blades can be increased to give the propeller more total blade area.
Today, our high-performance propellers have four blades. Some British props are five bladed. Dual-rotation props have six and eight blades. Such combinations increase total blade area but do not increase diameter of the propeller and can be used on planes with relatively short landing gears.
A further trend in propeller design has been to the stubby-end paddle blade, which provides additional blade area. This change has increased the power-converting ability of our propellers.
With development of the gas turbine engine for aircraft, basic design problems of the conventional propeller have not been greatly affected. To transmit the power from a gas turbine unit into the propeller, a reduction gear system is required, but the shapes and numbers of blades still are determined by the amount of power to be harnessed and the speeds and altitude of operation to be encountered. This turbine-propeller powerplant not only eliminates the internal combustion engine, but retains the advantages of short take-off runs.
If the gas turbine ejects its exhausts directly into the air, instead of transmitting its power to a propeller, the airplane becomes jet propelled. This type of installation eliminates propeller and reduction gears as well as the internal combustion engine. At very high speeds, where propeller efficiency falls off, jet engines become more efficient.

 

[keithwwalker]

Think of each propeller blade as a wing (which just rotates) The most efficient propellers are 2 bladed (actually in propeller theory a 1 blade prop is most efficient). The reason it is most efficient is similar to why monoplanes are more efficient than biplanes or triplanes - each prop blade has a detrimental effect on total blade efficiency on the other blade(s).

So you have competing issues in terms of detrimental effects:

• More blades have greater inefficiency (pre1937 or so, power wasn't the limiting factor)
• Less blades can not trasmit the power (why post 1940, very few military props were 2 blade)
• Landing gear can only be so long (think Corsair)
• Once blades get so big, the blade tip goes supersonic (think Thunderscreech ), this is why a helicopter will probably never go >Mach 1
• Finally the big one, what materials were used? (the Germans and Brits had to use wood to an extent, and therefore their blades were wonderfully designed with material limits in consideration, Germans went to large paddles, Brits used more blades)

Each company and country came up with solutions. Clearly, the leaders were USA, Britian, Germany.

 

[K5083]

So, how many blades do high powered turboprops use nowadays? Lots, that's how many. I always reckon 500hp per blade in WW2 tech. But if you can make the diameter bigger and the rpm less you can do with less. See the F4U. But that went four-blade too. The 109 was planned to go to four blades. 2000hp with a 109K with three small-diameter blades seems under-propped. I wonder whether German manufacturing could deliver a suitable prop earlier. Germany had four-bladers on He177 and Do217M/N, nothing else springs to mind.

 

[swampyankee]

So, how many blades do high powered turboprops use nowadays?

Up to eight for single-rotation propellers. We also need to suppress, smash, annihilate, and otherwise destroy the idea that there is some magic, general optimum for efficiency: there isn't. Thrust (actually and lift coefficient) determines the needed blade area, which determines the profile drag. Induced drag is inversely proportional to the number of blades, so that if a propeller needs x square meters of blade area, a propeller will be more efficient with that area spread over the maximum practical number of blades. That's set by cost (fewer blades will tend to be cheaper) and structure (constant speed propellers are usually operating with extensive stalled areas on the blades during takeoff; that dictates minimum second moment of the area -- thickness and chord -- at the root).

A problem that starts showing up with high-solidity propellers is that there may be areas of choked flow in near the blade roots. This can be dealt with by contouring the spinner carefully, but WW2-era propellers weren't highly enough loaded for this to be a problem.

 

[Sid327]

Question for the more knowledgeable...

Was the idea, or advantage of wooden blades such that they were lighter and so absorbed less power?
Also if the aircraft nosed over on landing or belly landed, they caused less damage to the engine when it suffered a sudden stop?

 

[swampyankee]

Question for the more knowledgeable...

Was the idea, or advantage of wooden blades such that they were lighter and so absorbed less power?
Also if the aircraft nosed over on landing or belly landed, they caused less damage to the engine when it suffered a sudden stop?

Early on, it was because they could be made reliably. The internal damping properties of wood probably helped. Wood is also less dense than aluminum. Aluminum propeller blades were solid.

 

[pbehn]

The 109 was planned to go to four blades. 2000hp with a 109K with three small-diameter blades seems under-propped. I wonder whether German manufacturing could deliver a suitable prop earlier. Germany had four-bladers on He177 and Do217M/N, nothing else springs to mind.

The 177 and 217 were both bombers, no guns firing through the prop.