Another GQ thread links a photo of a Beechcraft Super King Air. The first thing that caught my eye was that it has four-bladed propellers.
When engine power increases, you need some way of making use of it. Increasing the length of the blades or increasing the RPM of the propeller can put the tips into supersonic range, which is inefficient. (And of course longer blades brings them closer to the ground, which might be problematic. This was solved in the F4U Corsair by ‘cranking’ the wing.) Increasing the pitch would, I think, be inefficient during some phases of flight. (Though variable-pitch blades can compensate.) Increasing the chord seemed to be very effective on such planes as the P-47, whose performance improved greatly after replacing the skinny blades with ‘paddle’ blades. And of course you can make use of increased engine power by adding more blades.
I’ve seen Griffon-powered Spitfires with five blades, Russian transport aircraft with eight contra-rotating blades, B-36s with six contra-rotating blades, and so forth. Most civil aircraft have two blades or three blades. This makes sense, because most civil aircraft don’t have monster engines. Even most turbines seem to turn three-bladed props. (Say ‘three-bladed props’ five times fast! )
Where’s the ‘tipping point’? When does someone decide to use three blades instead of two? For example, many Cessna 182 Skylanes have two-bladed props, and many have three-bladed props, even though they all share an O-470 or O-540 engine producing 230-235 hp. Three-bladed props certainly look better/more spiffy/more powerful, but I’d want more performance for that kind of money instead of just appearances. Given an engine of a certain horsepower, what is the advantage of going from a two-bladed prop to a three-bladed one?
ISTM that propellers with an odd number of blades would be quieter than ones with an even number of blades due to structural interference. True, or false?
How many blades are too many? At some point won’t the ‘wakes’ interfere with the efficiency of the following blade? How do ‘a large number of’ propeller blades compare to turbine compressor blades or turbofans?
There are probably other questions, but these are the ones that came immediately to mind when I saw the photo.
Isn’t acceleration an issue when going to more blades, as well as a balance between response, engine RPM and fuel economy?
Don’t some planes need 2-bladed props to generate enough acceleration, whereas a 3-bladed prop hurts acceleration/response but knocks down cruising RPM? So, if a plane would lose too much response with a 3-blade setup, it uses a 2-blade setup for response and acceleration?
Do 2-bladed props give more top end (less resistance) versus 3 and 4 bladed props?
Probably not related, but what the heck. When I was shopping for a ceiling fan a few years back, I read that a three bladed fan was the most efficient at moving air. That probably wouldn’t apply to an aircraft as the aircraft would be moving through the air whereas a fan is basically static. It would make sense that the faster the AC moved through the air, the more beneficial multiple blades would be.
I recall reading a detailed breakdown of all the factors some years ago, but I don’t recall much of the content. One factor was small experimental planes that used higher rpm engines and needed shorter props to avoid the weight of gearing down the engine. The noise increases with more blades and modern materials like carbon-fiber reduce that effect making more blades more practical. I’m not sure about this, but maybe there was something about reducing total drag through multiple thinner blades. Then there were a bunch of reasons for many blades on modern turbo-props, and the advantages of the scimitar shape on those.
You don’t want the ends of the blades go supersonic. So there is a maximum diameter for a given rotational rate. By the same token, you do want the blades to go as fast as they can, as efficiency increases, so you want the blades as long as is mechanically reasonable (which includes such mundane things as not touching the ground), without the ends going supersonic.
I don’t know if it’s related, but it’s my understanding that for windmills, the larger the radius, the smaller the optimum number of blades. So small farm windmills have dozens of blades, the old Dutch windmills have four, and modern electricity-generating windmills have three or two, because they’re larger.
Application is important. A plane built for racing has 2-blades… right? It’s about speed. A 2-blade setup won’t offer the best acceleration or response, but she’s more likely to hit a higher top end. She would have to offer ‘sufficient’ accel or response – of course – just not the best.
Correct me if I am wrong, but plane who has a life spent on tighter runways or in need of more acceleration and response needs more bite, such as that offered by three blades. The extra blades also knock down cruising RPM, but overall top speed takes a hit. There might be a risk of lugging the engine with too many blades, too, which is not good on the internals. More blades might be wise when the plane has a wide array of loads.
Certainly, it’s a compromise. There is NO overall optimum/best blade number, but there is one based on the intended use, the plane and the desired outcome. <<<< Is this what we are getting at here?
Of course, all this has to be setup with gearing, too.
Most piston-engine airplanes are direct-drive. Experimentals using converted car engines, or planes using Rotax engines have reduction gears, but they are the exceptions. Cessna experimented with a geared engine in the late-'50s/early-'60s on their 175 Skylark. Possibly because of a slightly higher RPM that pilots found strange, so they ran the engine at a sub-optimal RPM, or possibly for another reason, the Continental engines used in that application were found to be less reliable than the non-geared ones. Overhauls were also more expensive. So they went back to direct-drive engines.
I don’t know squat about aeroplane details such as direct drive (didn’t know they were mostly direct drive!), but I’ve been solving performance boat prop issues for years!
Figured I’d see how far I could get before getting outed!
My understanding is that it isn’t the diameter, but the need for torque that keeps those farm propellors large in number. You’re not talking about a system that needs long sustained operation – those farm propellors are used to intermittently pump water up, and the hard part is getting the thing to turn at all (especially in the face of indifferent ukeep – who wants to climb al the way up and oil the damned thing?) You need a lot of vanes so that there’s a lot of torque to get the wheel turning for a little while. Keeping it going is much less important, and worrying about the tips of the blades going at a speed where their drag will interfere with operation is virtually impossible.
Maybe available torque is a factor? For instance, it might be that you get maximum cruise efficiency for an engine at, say, 2500 RPM, but that a two-bladed prop turning at that speed leaves some engine power unused because it can’t move enough air. If you can’t make the blades longer to increase the effective area, you need more of 'em.
Of course there’s also the issue of blade pitch, but maybe it’s more efficient to add more blades and cruise with minimal angle of attack than to increase pitch and take bigger bites with fewer blades.
One is ground clearance. For a given power/weight ratio and required performance, and a given landing gear height, it may be necessary.
False. Noise is a function of shock waves at the tips, and general wake turbulence behind the prop. More blades means a higher fundamental frequency, though, and the human ear/brain will *perceive *that as quieter.
Yes, but mainly at the hub where the disk coverage is higher and the flow is more turbulent anyway. Turbulence strength dissipates exponentially with distance, and you don’t have to go very far out the blade for it not to matter much - the wake of the outer part of the blade is already downwind of the following blade before it gets there.
Probably can’t hear yourself think within 10 miles of that thing. The aft blades operate in the wake of the forward ones and will largely just chop up the air more.
That arrangement can apply more power into the air for a given disk diameter, keeping the engines closer together and the wing lighter for a given payload. That’s pretty much it, though. The differing number of blades in the 2 disks does mean you don’t generate a pure-tone blade-passing frequency, though, but that’s probably more about fatigue loading on the blades.
Imagine a propeller is sending a thin stream of pressurized air backward. As time passes, the air begins to depressurize. Perhaps another blade following closely behind the first can get to that air before it depressurizes, and gain additional (thrust? efficiency?) than it would have if the air had more space/time to dissipate.
This would suggest the more propellers the better, but even if it’s true there must be tradeoffs or every propeller would have a dozen or so blades.
I think just about all the fastest planes in WWII used 4-bladed props, though I don’t know how the 5=bladed Spit mentioned in the OP stacked up. If it was a prop for a particular engine, that suggests to me the number of blades might be closely related to the expected engine RPM. Did the Griffon engine operate at a considerably different RPM range than the Merlin?
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