What criteria determine whether to use 3,4, or 5 blade propellers?

[TheMadPenguin]

What criteria determine whether to use 3, 4, or 5 blade propellers?

 

[pbehn]

What criteria determine whether to use 3, 4, or 5 blade propellers?

The power to be transmitted, maximum diameter usable (usually governed by ground clearance) and synchronised guns are factors. The Spitfire went through the full set from 2 blade up to six if you include the contra rotating props. As far as efficiency goes the fewer the better and the same for weight.

 

[TheMadPenguin]

The power to be transmitted, maximum diameter usable (usually governed by ground clearance) and synchronised guns are factors. The Spitfire went through the full set from 2 blade up to six if you include the contra rotating props. As far as efficiency goes the fewer the better and the same for weight.

I guess that, what I can't guess is why/whether 3 blades on a B-17 engine serve as well as 4 would. *pulls out the Magic Propeller Adapter Thingie by Acme*
B-17 & B-24: 139" 3-blade
P-47: 146 " 4-blade
*pulls out the "Magic Propeller Adapter Thingie" by Acme* What is the effect of putting the P-47 paddle-blade prop on a B-17 or B-24?

 

[pbehn]

I guess that, what I can't guess is why/whether 3 blades on a B-17 engine serve as well as 4 would. *pulls out the Magic Propeller Adapter Thingie by Acme*
B-17 & B-24: 139" 3-blade
P-47: 146 " 4-blade
*pulls out the "Magic Propeller Adapter Thingie" by Acme* What is the effect of putting the P-47 paddle-blade prop on a B-17 or B-24?

As per my previous post, what was the power output on your B-17 engine compared to your P-47? The P-38 never needed 4 bladed props as far as I know but it did use big three bladed ones.

 

[Deleted member 68059]

What criteria determine whether to use 3, 4, or 5 blade propellers?

what Pbehn said but I would add that to this is included available reduction gear ratio and hence consideration with respect to allowable diameter for keeping the tip speed below (about) mach 0.9 (you can go a little higher but efficiency and vibrations become worse).

There are definetly fairly intensive German studies on pure efficiency which show that 3 blades is better than >3 (agreeing with pbehn).

 

[swampyankee]

what Pbehn said but I would add that to this is included available reduction gear ratio and hence consideration with respect to allowable diameter for keeping the tip speed below (about) mach 0.9 (you can go a little higher but efficiency and vibrations become worse).

There are definetly fairly intensive German studies on pure efficiency which show that 3 blades is better than >3 (agreeing with pbehn).

The power to be transmitted, maximum diameter usable (usually governed by ground clearance) and synchronised guns are factors. The Spitfire went through the full set from 2 blade up to six if you include the contra rotating props. As far as efficiency goes the fewer the better and the same for weight.

Induced losses decrease as the number of blades increase. If the total blade area is fixed and structural considerations don't override aerodynamic ones, a prop with more blades will convert more power into thrust at a given speed. Increasing blade count increases costs, decreases blade damage tolerance, reduces loads on the pitch change mechanism, reduces weight of individual blades, and brings aboard the possibility of choking in the space between the blade roots.

 

[pbehn]

Induced losses decrease as the number of blades increase. If the total blade area is fixed and structural considerations don't override aerodynamic ones, a prop with more blades will convert more power into thrust at a given speed. Increasing blade count increases costs, decreases blade damage tolerance, reduces loads on the pitch change mechanism, reduces weight of individual blades, and brings aboard the possibility of choking in the space between the blade roots.

Is it a case of in different situations the significance of things changes? WW2 aircraft were all pretty much in the same ball park as far as power output goes and pretty much the same solutions. But if you take a sublime to ridiculous of extremes, model racers use a single blade with a counter weight while planes like the A-400 have 8.

 

[Reluctant Poster]

And yet the Q400s I often fly in have 6 blade props despite all the disadvantages

 

[Reluctant Poster]

And yet the Q400s I often fly in have 6 blade props despite all the disadvantages

 

[pbehn]

And yet the Q400s I often fly in have 6 blade props despite all the disadvantages

The advantages obviously out weigh the disadvantages, there isnt a universal "best".

 

[pbehn]

There are definetly fairly intensive German studies on pure efficiency which show that 3 blades is better than >3 (agreeing with pbehn).

Was that tied in to discussions of synchronised guns?

 

[swampyankee]

And yet the Q400s I often fly in have 6 blade props despite all the disadvantages

Because of diameter and acoustic constraints, aircraft like the Q400 need a lot of blade area. Putting that area on three blades would result in higher induced losses and much higher loads on the pitch change mechanism. Propeller design hasn't stood still for the past 75 years.

 

[MiTasol]

And yet the Q400s I often fly in have 6 blade props despite all the disadvantages

because the much larger three blade prop needed to absorb that many horsepower would bash holes in the fuselage and gouge grooves in the tarmac.

Everything is a compromise and if you want the longer blades then the engines must be moved outboard and undercarraige lengthened.

Moving the engines outboard means much more swing when you lose an engine which requires a bigger fin and rudder to keep the aircraft in a straight line during takeoff or go-around (that being the worst case scenario). Lots of added weight and drag that is avoided by adding blades and sacrificing efficiency.

 

[swampyankee]

because the much larger three blade prop needed to absorb that many horsepower would bash holes in the fuselage and gouge grooves in the tarmac.

Everything is a compromise and if you want the longer blades then the engines must be moved outboard and undercarraige lengthened.

Moving the engines outboard means much more swing when you lose an engine which requires a bigger fin and rudder to keep the aircraft in a straight line during takeoff or go-around (that being the worst case scenario). Lots of added weight and drag that is avoided by adding blades and sacrificing efficiency.

No diameter increase needed; just twice the blade chord.

 

[MiTasol]

No diameter increase needed; just twice the blade chord.

Killjoy

Seriously though twice the chord brings about its own greater inefficiencies which is why they use the additional blades.

The operative sentence is Everything is a compromise.

 

[swampyankee]

Killjoy

Seriously though twice the chord brings about its own greater inefficiencies which is why they use the additional blades.

The operative sentence is Everything is a compromise.

Unless structural issues come into play, spreading blade area X over more blades improves efficiency. From my time at Hamilton Standard (I did prop aero), blade chord was driven by vibration loads at takeoff.

 

[Reluctant Poster]

because the much larger three blade prop needed to absorb that many horsepower would bash holes in the fuselage and gouge grooves in the tarmac.

Everything is a compromise and if you want the longer blades then the engines must be moved outboard and undercarraige lengthened.

Moving the engines outboard means much more swing when you lose an engine which requires a bigger fin and rudder to keep the aircraft in a straight line during takeoff or go-around (that being the worst case scenario). Lots of added weight and drag that is avoided by adding blades and sacrificing efficiency.

ATR 72s and Dash 8s can be seen with both 4 blade and 6 blade props. The point is that prop efficiency doesnt seem to be ranked high on the list of attributes.

 

[TheMadPenguin]

Perhaps the total thrust delivered matters most. So the question is "How much thrust will this (e.g. B-17) engine give us with 3 blades vs a (e.g.) P-47 4-paddle-blade propeller?"
Avro Lancasters used 3-blade, running Merlin engines, while the P-51 and Spitfire used 4 & 5 blade props.

Gotta compare prop diameters, which is not always given in net-available search-findable sources.

 

[swampyankee]

ATR 72s and Dash 8s can be seen with both 4 blade and 6 blade props. The point is that prop efficiency doesnt seem to be ranked high on the list of attributes.

It actually is. The number of blades was chosen to provide best efficiency within installation and regulatory constraints. Also, once again, induced losses decrease as number of blades increase, and profile drag is determined primarily by blade area. Root thickness is driven by aeroelastic loads at takeoff; much of the blade is stalled and vibratory loads are very high. The design process at Hamilton Standard would include that in the selection of number of blades.

There is no magically universal "best" number of blades. There is no universal number for best efficiency. If you diagree with that statement, well there are a lot of reports from NASA, NACA, ONERA, DLR, ARC, and journal articles from professional organizations, such as AIAA to refute.

 

[Deleted member 68059]

Was that tied in to discussions of synchronised guns?

Most aero German reports I have are EXTREMELY obtuse, and tend to never give useful stuff like "why have I been asked to do this",
I suppose they never expected historians to get upset about it later. hah