Warbird Information Exchange

I mean, really!

Does some airplane designer sit at his desk, fiddling with his etchings, and decide that two blades isn't enough? Three blades looks better, yeah. So I'll put em on this spitfire because the two blades was dumb looking. And then an anvil hits him on the head nine months later...and his assitant, who was always in favor of the four blade asthetic, goes for that nicely even number of blades. Then he manages to get some nightscap in his tea by mistake, and his replacement goes for six?

I was looking through a spitfire book tonite and noticed that it went from two all the way up to six blades before the war ended. How does the engineering desicion behind number of blades work?

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"I knew the jig was up when I saw the P-51D-20-NA Mustang blue-nosed bastards from Bodney, and by the way the blue was more of a royal blue than an indigo and the inner landing gear interiors were NOT green, over Berlin."

It's a function of available torque, the rpm that produces that torque level, take-off attitude clearance, airframe redline and design service ceiling, blade tip speed and acoustic footprint.

Despite all the advances in numerical analysis, CFD and historical data compliation, selecting a prop for a particular installation and design purpose is still close to a black art. The UDF program produced a very efficient prop for airliners, but it had an abominable acoustic footprint that the program could not overcome.

Even in this day and age, there is quite a bit of research that is ongoing in boosting prop efficiency.

Gee, you cracked the big secret first go! You only missed that they decided that they'd been going the wrong way all these years, and reversed the prop direction halfway through the development!

Spitfires (Hurricanes et al)...
The two blade wooden prop was an interim compromise until Britain had production of the new constant speed or variable pitch props. Like a bike with one gear, the wood two blade 'Watts' type prop was a compromise between take-off, cruise and full speed power conversion to thrust. It's easier when making a all-wood prop (i.e. without a metal hub) to make a two or four (two- twos bolted together) blade unit.

The three-bladers were the ideal for that level of performance, and required plumbing into the engine's oil system to change the pitch. These came in wooden (Jablo) blades and metal - each socketed into metal blade-root units that allowed variable pitch angles to be set by the hub. A four blade prop was a natural progression (simplistically) to take advantage of increased engine power over the three.

The five blade prop was fitted to the still more powerful Griffon engine (which turned the other way to the Merlin, incidentally) and five blades was a good way of absorbing and converting that power within a limited ground clearance. The six blade props were two-three blade cotraprops, to remove the significant torque effect of the five blade effort. (The oddity was the wide-chord four-blade props used on Griffon Spits and Seafires, which was a better compromise for the low level environment they were intended to excell in - see Spitfire Mk.XII and Seafire 17.)

A quick and dirty answer, I'm sure major inaccuracies will be corrected.

Regards,

_________________
James K

"Switch on the underwater landing lights"
Emilio Largo, Thunderball.

www.VintageAeroWriter.com

Herm herm herm...

Now I am wondering why my remote control plane uses a twin blade prop but I have also seen three bladers and four-same engine same aircraft...

Is more blades more power? Would I need more engine for more blades?
I would imagine at a small scale it wouldn't much matter. But at man sized you're dealing with new issues, eh?

_________________
"I knew the jig was up when I saw the P-51D-20-NA Mustang blue-nosed bastards from Bodney, and by the way the blue was more of a royal blue than an indigo and the inner landing gear interiors were NOT green, over Berlin."

To stretch a metaphor, probably for the same reason racing bikes can have three, five, eighteen, twenty four (or whatever) gears. There are performance advantages and tradeoffs, and the person in charge may also be following fashion or style as well.

Cheers,

_________________
James K

"Switch on the underwater landing lights"
Emilio Largo, Thunderball.

www.VintageAeroWriter.com

The more I read about the design of these suckers, and all the differentials that went into building them for low/mid/ high altitude, the more I realize what an amazing time in engineering it must have been. The 30's and 40's were really groundbreaking weren't they? Now I'm up to the Mk.V (with all it's variations) and WOW! I'm beginning to love the British birds more than my beloved Buffalo, Duck, and PBY...But I guess I'm not losinganything, just adding to the list of fave's!

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"I knew the jig was up when I saw the P-51D-20-NA Mustang blue-nosed bastards from Bodney, and by the way the blue was more of a royal blue than an indigo and the inner landing gear interiors were NOT green, over Berlin."

To keep it simple: 1 blade is the most efficient as it is always turning in "clean" air. They made 1 blade props in the 20's and 30's. It had a counterweight on the oppisite side. It was limited to low horsepower (like 50hp) As you increase in power, the blade gets longer and the pitch changes. When you get to the point where the blade will hit the ground or the tip will go supersonic, you have to add a blade. You lose some efficiency as the blades will now be in the wash of the others, but overall you gain power. As you gain blades you also lose noise. The more blades the quieter it gets. The Corsair is an example of change in horsepower. The early ones had 3 blades which barely cleared the ground, hince the gull wing design, to get the engine and prop higher. When the hosepower increased in later engines, they went to 4 blade props.

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Blue Skies,
Doug

www.cavanaughflightmuseum.com

R/C airplanes and real airplanes are actually 2 different worlds. In R/C a 3 or 4 bladed prop is only used when ground clearance is needed or scale-look is desired. I had the great experience of getting to talk to the owner of APC props several years ago (like 2000-2001) at a fly-in at the Jeff'co Aeroclub in Denver. I'd brought up the question to him about prop configuration on R/C airplanes and why so many used a 2-bladed prop on a plane that in the real world required a 3 or 4-bladed prop. He said that it was because most of the engines used on R/C aircraft can't run at full speed with a 3 or 4-bladed prop because they don't produce enough torque to overcome the weight of the larger prop, thus why most pilots run 2-blade props. The drop-off in power of R/C engines not running at full power is massive at the top end of their operating range, so even a 500 RPM drop can greatly affect the plane's handling (remembering that most of these planes are turning 15,000+ RPM at full throttle). So, what ends up happening is that to fly a 3-blade or 4-blade prop on one of these R/C aircraft you have to drop almost an inch off their diameter for the same size engine over a 2-blade prop for each blade you add. This gives you the same weight (and thus momentum) as the 2-blade prop and (sadly) also gives the same amount of thrust. The only advantage is (again) if you have low ground clearance or if you're just concerned about in-flight looks. With the advent of higher torque 4-stroke engines and the expansion of the IMAA-legal large-scale aircraft, we're starting to see more multi-bladed props as they are now able to operate efficiently, look good, and still clear the cowling. He showed me some pictures of a 1/8-scale Corsair with APC props on it. First had the 2-blade prop (which only cleared the cowling by a couple of inches). The second had the 3-blade prop for that engine and it only cleared the cowling by fractions of an inch, and the 4-blade prop for that engine was fully within the cowling, technically making it a ducted fan R/C.

The other issue you run into is thrust-to-weight and wing loading. It's nearly impossible to create the correct thrust-to-weight ration while keeping the correct wing loading with R/C airplanes because of the lack of true power in the small engines used on R/C airplanes. They cannot produce enough power to lift the weight required for the aircraft to have accurate wing loadings. This is also the reason you didn't see many R/C helicopters with scale blades until the advent of the turboshaft R/C engine.

Can someone explain for this non engineer how oil gets from a fixed engine to a rotating, variable pitch prop hub? Thanks in advance!

Oil to the prop is fairly easy. The crankshaft has the section the prop attaches to bored out to a point ahead of the the #1 rod. Within the section of the shaft between the front seal and that rod, there will be at least one relatively large oil feed hole. The govenor controls the oil pressure fed to a chamber that isolates this hole from the rest of the engine internals. The regulation of the pressure in this chamber causes the pressure to change within the prop, driving a piston within the prop that controls the blade pitch change mechanism. Geared engines feed the pressure basically the same, but within the gearbox area instead. Koppers et al. developed a variable pitch prop (not constant speed) that uses a bolt on housing with a modified crank for engines that were originally designed for fixed pitch props.
Featherable props use high pressure pumps and accumulators to provide enough pressure to drive the prop into or out of feather by changing the position of the pilot valve within the pitch change mechanism. The Hyromatics on the the Lockheed 10 that I used to maintain, used over 500 psig for the feathering system.

All of the manufacturers have their own variations of the process, but the basic method is all the same.

Don't forget harmonics. Some engine/prop combos never were happy at all speeds. The "bad" harmonic ranges can be shifted or narrowed with changes to propeller diameter, blades, aspect ratio, airfoil, pitch, twist, and material. It WAS likely less expensive to experiment with the prop rather than engine to resolve certain vibrational issue. It is a juggling act that on larger aircraft, have a specific role in mind for the majority of aircraft's operational life. The designers may have given up on getting maximum durability and efficiency at say 20% to 45% power and instead selected the best combination for 50% to 90%. Limiting operations in the other ranges to minutes. Very unlike what people expect from a car....operate it at any speed, load, geography. Can you imagine telling Joe consumer that he is not to maintain 1800 -2000 rpm in his new wondermobile for any longer than 3 minutes?
Aircraft are not alone in this respect....ships have same issues. Research the screws on BB55, USS North Carolina. They fought vibration the entire career of that ship. A couple of different screws were fitted meeting with partial success. If a 14' Hydromatic seems costly, imagine a 35' megaton bronze ship screw! Better get that right the first time (and they didn't)!

oooooh! This is a good threat that's is gonna get better! What is hapening when they feather a prop? Is that adjusting the angle so it slices the wind instead of catching it?

_________________
"I knew the jig was up when I saw the P-51D-20-NA Mustang blue-nosed bastards from Bodney, and by the way the blue was more of a royal blue than an indigo and the inner landing gear interiors were NOT green, over Berlin."

Feathering aligns a predetermined chord (the blade is generally twisted so the angle of attack changes as you move along the radius of the blade)of the blade airfoil with the windstream. This primarily reduces drag to a minumum (as it is going to get) but also serves to hopefully prevent windmilling an already ailing soft engine. The idea is to secure the engine to prevent further damage to the powerplant as well as airframe from undesireable vibration from a windmilling prop driving an un-powered engine. It can be a bad thing to drive some of the larger recips with the prop as it loads the bearings in all the wrong directions. I believe B36's even had brakes to prevent windmilling.

To amplify Wheel's comments, allowing a prop to windmill on a geared engine causes all sorts of problems with the gears. They are designed to be loaded in only one direction and reverse loading will lead to premature wearing out of the gears or breaking teeth off. In addition, when the engine is windmilling, it does not produce the minimum idle oil pressure, so the entire oil circuit does not get adequet oil flow.

Additionally, from the aerodynamic view, a windmilling prop presents an effective flat plate drag increase by the size of the area of the prop arc. For example, a 10' diameter prop, when windmilling would appear to the aircraft as a flat plate of approximately 80 sqft. When you consider that most spam cans in flight present a flate plate area of only 10-20 sqft., you can imagine what it does to the flight characteristics...can you say tossing out the anchor???

Hope all this helps.

Regarding the Spit's six blade prop, that would be the 2 contra-rotating 3 blade props, & IIRC what I've read, contra-props don't produce any more thrust than a comparable single prop, the contra-props just eliminate the torque. The a/c may have a little better performance since it's not having to counter the torque with the contra-props, but that's more aerodynamics than prop thrust. At least I think that's how I've read it.