Propellers: less blades vs. more blades

[ludovico]

Hello!
I was wondering why some propeller airplanes have 2, 3, 4 or more prop blades. I've seen, for example, C210's with 3 (http://www.airliners.net/photo/Cessna-P210N-Pressurized/1347946/L/) and with 4 (http://www.airliners.net/photo/Motorfluggruppe-Olten/Cessna-P210N-Pressurized/1072397/L/) blades, this last one looks like a modification the owner did, and I don't know if it has something to do, but I have seen this mods more often in european planes. On the King Air 90 as well, seen them with 3, (http://www.airliners.net/photo/Beech-C90A-King/1543745/L/) 4 (http://www.airliners.net/photo/Beech-C90B-King/1543731/L/), or even 5 (http://www.airliners.net/photo/Beech-C90-King/1349665/L/) blades. Why is this?
My thoughts: :ugh:
More blades mean more drag, but the surface to move air is greater, so you'll need less speed to move the same amount of air as with a prob with less blades, so... is it the same thing in the end? also, more blades are more expensive, so what's the advantage on this?
It would be much appreciated if someone could technically explain this.
Thanks a lot! :}
ludo

[Wizofoz]

You are more or less right in you assumptions- More blades of the same area at the same pitch produce More thrust but also require more power. It's probably analogous to the gear ratio of a car- more blades equals higher gear and is more efficient at higher speeds (Hence may modern turbo-props have multiple bales), fewer blades equals faster acceleration (Needed for small aircraft using Short strips)

The reason, however, that you see more 3 or 4 blade retrofits in Europe is not because of this, however. More blades allows the blade Diameter to be reduce, reducing tip-speed for a given RPM. Much of the noise associated with prop aircraft is due to the prop tips operating at a high percentage of Mach 1.

More blades=smaller diameter= less tip speed= less noise. Necessary in noise conscious European cities.

[411A]

More blades=smaller diameter= less tip speed= less noise

Another benefit...more blades allows the propellor to absorb increased engine power (on those aircraft so modified) while allowing better ground clearance, due to the propellor diameter being contained.

[ClimbSequence]

More blades helps to improve climb performance but also account for a reduced cruise speed due to the parasite drag it creates

[bfisk]

You can google, or otherwise reserach the term propeller solidity :ok:

[IrishJetdriver]

More blades, less diameter then put a casing round them all and you have a multi bladed shrouded propeller, i.e a modern high bypass jet.

[barit1]

Just to complicate things, I've seen photos of a single-blade prop (with a counterweight, of course). Presumably less interference drag (think monoplane vs biplane...), but there must be quite a vibratory condition resulting from unbalanced aero load. :uhoh:

[ludovico]

bfisk: thanks for the hint, really interesting stuff.

Once more, questions solved!
Thanks for all your responses.

[rigpiggy]

Flow interaction reduces prop efficiency, more blades less efficient. However, more blades will allow more power absorption within a smaller blade diameter reducing noise. With newer airfoil designs the efficiency loss can be compensated.

[JammedStab]

Get ready for the seven-bladed prop from MT-Propeller of Germany. Already being tested on a Piper Cheyenne.

Apparently, more blades allows lower rpm gearboxes reducing blade tip drag. The number of blades, diameter, twist, chord, and thickness distribution all determine propeller efficiency which is driven by the lift distribution along the blade radius and the airfoils. Most modern designs have been designed with reduced lift at the tip reducing the vortex.

An interesting statement in the article I am reading about the 7-bladed prop is a quote in the article by a spokesman...."Everybody told us it's not possible"......."They said it was dangerous;that fuel control units will go into resonance and King Air's will crash ".

Anybody have more details about FCU resonance and number of prop blades?

[Pugilistic Animus]

Oh wow yet another blast from the past!

[zac21]

https://encrypted.google.com/imgres?imgurl=https%3A%2F%2Fcdn.avbuyer.com%2Fuploads%2Fcms_ article%2F104701_104800%2Fmt-propeller-tests-the-world-s-first-7-bladed-propeller-104799_724_969X727.jpg&imgrefurl=https%3A%2F%2Fwww.avbuyer.com%2Farticles%2Fga-buyer-europe%2Fmt-propeller-tests-the-world-s-first-7-bladed-propeller-104799&docid=9tRAdZGIRkhgYM&tbnid=cveaTQIlOlHRzM%3A&vet=10ahUKEwi_yJXi07jaAhWKE7wKHXU2BwQQMwg2KAEwAQ..i&w=969&h=556&bih=676&biw=1247&q=piper%20cheyenne%20with%20MT%20propeller&ved=0ahUKEwi_yJXi07jaAhWKE7wKHXU2BwQQMwg2KAEwAQ&iact=mrc&uact=8

[jolihokistix]

or "more blades vs fewer blades".

[Basil]

Anyone tried prop tip winglets?

[scifi]

If you go from two blades to three, then for the same rpm and power, the diameter will be somewhere around 10% less. For some single engine aircraft this gives a bit more prop to ground clearance, same cruise speed, but less thrust for the climb.
There are several design programs for model aircraft props, where you can make alterations to get the performance you require. Maybe this also works with full size props..... ( edit.. Yes it does.. )
.
http://www.godolloairport.hu/calc/strc_eng/index.htm
.

[scifi]

Just some more observations on model airplane prop design that should carry over to the full size aircraft...


The diameter will determine the static thrust, and the Pitch will have to be selected to match the cruise speed. Mathematically, because the diameter squared determines the swept area, any delta increase of diameter will need twice that amount of reduction of pitch. So starting with an 11x7 for the usual model engine, If you want more speed go for a 10x9, or for a better climb ( vertical.?) try a 12x5.


There are snags to going oversquare though ( 9x11 for this example) in that at a standstill or slow taxi speeds, the propeller will be stalled. Some speed freaks, put up with this slow take-off, as it gives them speeds of over 200mph in a dive. Conversely some Fun-Fly pilots use 13x3 props to give prop-hanging performance, with colossal amounts of static thrust. Increase the diameter ad-infinitum and you will end up with a helicopter !

[nomorecatering]

https://www.youtube.com/watch?v=uruXyPEjakU

[nomorecatering]

https://www.youtube.com/watch?v=1Z8D2MG1-8Q

[anxiao]

Another question I have about two bladed versus three. The question may be very simplistic but I have been out of prop aircraft for a long while and have delightfully gone back to them.

Does the P factor change when you switch from a two bladed propellor to a three bladed one? I have an aircraft which was designed for a two bladed prop (C177RG) and I bought it with a three bladed. Would you expect the three bladed prop to require more right pedal (or right trim) at high power settings than the two bladed one? I need an inordinate amount of right rudder trim in climb and cruise, and was wondering if this could be one of several reasons.

Thanks in advance

[jmmoric]

I guess that a big powerfull engine, with only two blades would get the speed of the tips so far out (to absorb the power), that compressability problems will occur... hence the bigger the engine, the more blades to absorb the power... without the tips spinning too fast.

I think?

[tdracer]

Ideally, you want a two blade prop - just as a long, thin wing is more efficient, a long propeller blade is more efficient. But at some point practicality comes into play. Long blades require greater ground clearance (the 'gull wing' on the F4U Corsair was to get ground clearance without needing huge landing gear with it's big prop), long props tips go supersonic sooner (noise and loss of efficiency). Adding blades means you can deliver more power to the airstream, and go faster without supersonic tip speeds, but the shorter blades are less aero efficient and you lose efficiency due to the interaction of the additional blades as they pass through the leading blade vortices.

Modern, 'swept' blade props have lessened the penalty of additional blades, but you can't eliminate physics.

I've found it personally interesting that wind turbines have pretty much settled on a three blade configuration. With wind, I figured more blades the better since you're not as concerned with efficiency as much as over all power absorption - but apparently the additional cost and weight of more blades offsets any power increase from adding more prop blades.

[BluSdUp]

Td, I think You summed up Aviation Design in a short and sweet sentence!
May I use it in the future?
Not that that has ever stopped the USSR. I think they have the record on power and propeller complexity. The Tupolev Bear comes to mind. An engineering delight, I am sure.

Long time since I did props, but the sound of a C185 with 300hp and standard two blade as it takes of , makes a wonderful sound as it passes abeam.
Tips chasing M1.
I also had the pleasure of flying nr 29 Beech 200 with pod and standard 3 blade. The fastest in the fleet and a delight to land. An old rented Ken Borek machine.
The 4 bladed was less forgiving in the flare if you thought you had a 3 blade, the drag was about twice.
The 6 bladed Dornier 328 was rather quiet in the cabin for a high wing, one of its selling points.
Anyway
Back to my 30% Jet with 24 blades, fixed pitch. ( CFM 56)
Regards
Cpt B

[fdr]

Just some more observations on model airplane prop design that should carry over to the full size aircraft...

!

Only to the extent that the blades are subsonic or not. Most model props have very high tip speeds, much greater than 12":1' scale. When the blades are sonic at the tips, the consequences are that positive camber results in less thrust than a negative camber would, from the effects of the shockwave formation. While the sonic tip speed results in oblique shock formation, the normal shock appears to be much more prevalent when tips are sonic. That oddity, of reversing camber at the outer radius of the blade has been on scale models by a researcher in NZL, (google it), and the only full scale evaluation was flown probably by me using rubber tabs on the TE of the blades to increase camber inboard so as to essentially reduce effective camber (well, really AOA) outboard. That was surprisingly effective from low speed up to the high speed WW2 high performance fighter we flew, but the blades physical profile remained as manufactured. In the latter cases, the tip velocities were subsonic, but there are normal shock formations on the blade in operation. tabbing inboard of the tip increased inboard loading of the blade, reducing the tip loading as AOA for a given thrust was lower, reducing the intensity of the normal shock outboard. The location of the tabs was chosen to give increased velocity profile inboard but trying to avoid excessive shock formation. The same tabbing was flown on helicopter rotor blades, only on a 2 blade system, and with a serious amount of safety precautions. Outcome was a very large change in the acoustic signature as was shown on propellers, and a very large change in required torque, and a much reduced blade stall rotor RPM. The limited flight test series includes some failure modes and the test helicopter, an R22B was in more ways than I care to recall less than an ideal test vehicle for extended testing. One set of comparisons comes to mind though; at test weight and conditions, normalised for w/delta, the rotor stalled out on normal blade at 82%Nr. At that time, directional control was in the process of being lost, full yaw pedal was attained.... (it is a test point with some risk attached, and is not done above a 1" hover height, stall a rotor at 10' and you are going to wake up at A&E, stall it at 50' and you are going to wake up in the after life. The risk of rolling up Franks infernal aerial contraption into an aluminium wad is non zero). The next test was with a series of conformal VGs on the outboard blade LE, and the torque required to hover reduced by 9%, and the blade speed for stall reduced to 77%Nr, 5% absolute, about 6% relative, and at the stall, residual left pedal authority remained, but only just, which is impressive). Adding a 1 meter tab mid span to the rotor with the VG on the outer blade for mass balance considerations, resulted in a reduction in power required by 17% from the base clean blade, and the rotor stall RPM dropped to 68% Nr. Approaching the stall RPM, 1/2 left pedal remained, and turns to the left could be accomplished without any difficulty, although anyone flying under speed rotors is nuts, and will have noted that the cyclic control becomes pretty wooly, while the aero affects are being amended the inertial effects become rather different to a normal rotor RPM case, so control is fun. limited transitional flight was conducted, and autorotation evaluated, with no adverse effects, the most notable difference being that transitioning through ETL was very soft, vibration was well down, and the rotor noise was considerably reduced. The concern on extended testing was that doing a failure mode in forward flight could result in tail rotor blade impact with a small piece of low density foam, and the RHC tail rotor is a swiss watch level of robustness... akin to tissue paper, and not overly robust. The failure tests in hover showed that there was low likelyhood of a shed piece of foam touching a blade, but it was not able to be ruled out, and an impact would lead almost certainly to a TR blade root failure, loss of 90 gear, and probable separation of the tail cone at the 5 point attachment, just the blade loss alone would result in a high probability of a mast bump, and being placed on aforementioned slab in 2 major pieces, head on one slab, remains of carcass on the other. The testing was discontinued for choppers awaiting access to a fensetron tailed beast or a UH60 which does a whole lot betterer with losing bits.

Back to subject, the model blades operate in a sonic tip velocity frequently, and more or less only the Thunder Screech did that on full scale. Almost all blades have some normal shock formation in flight, but oblique shocks are avoided where possible by limiting tip velocity. Sweep reduced the normal shock intensity...

[megan]

Wonder how it performs?

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/1049x786/zz6_a1501c9f0b9ee3d736fd8a6b74a3fad7d5f0d388.jpg

[Cheyennethree]

I thought more bladed less noisy and less vibration

[turbidus]

From MT....

Looks like reduced noise is a significant benefit:

World's first 7-bladed Quiet Fan Jet Aircraft Propeller MTV-47 is now supplemental type certified by EASA STC 10014953 R3 on the Pilatus PC-12 (http://www.mt-propellerusa.com/en/mtusa/stcs/pilatus_3.htm)powered by a Pratt & Whitney PT6A-() propeller turbine.
The installation of the 7-blade MTV-47 on the Pilatus PC-12 reduces Ground roll distance by approx. 10% and Take-off distance over 50 ft obstacle by approx. 15%. It reduces also the inside and outside noise significant and complies with the strict German Noise regulations.

5 balded prop:

Applicable Aircraft: Quest Aircraft Design LLC Kodiak 100 Series
Engines: Pratt & Whitney PT6A-34 Engine


Best vibration damping characteristics for almost vibration free propeller operations!
Bonded on nickel alloy leading edge for superior erosion protection of the blades!
Take-Off improvement by 20%!
No propeller speed restrictions on ground while operating in low idle!
Unlimited blade life and FOD repairable blades!
More ground clearance = less FODs!
Lower ITTs during start-up for less engine wear!
Unbeatable esthetic ramp appeal!
Significant cabin noise [up to 5 dB(A)]and vibration reduction!
Comply with the strict German noise regulations 2010 - „Landeplatz Lärmschutz Verordnung“ for unrestricted airport operation in Germany and other European Countries!
Max. RPM reduction to 2000 RPM with increased torque for 750 shp!

[Winemaker]

So that's a constant speed prop, correct? Do the seven blades increase complexity/weight of the pitch mechanism?