A Question About Aircraft Design

[Subdeacon Joe]

How do the engineers determine the number of blades on a propeller?   

[Wallaby Jack, SASS #44062]

 ...... probably by counting them on their fingers ........... 

[Alpo]

I was pondering that the other day, looking at that video on Spitfires (four blade) which also contain pictures of gooney birds (three blade).  And in an online story I was reading they have the Pilatus, which is a single engine turbo prop, five blade.

 

[Marshal Mo Hare, SASS #45984]

I bet the subject could fill a bookshelf.

 

 

 

[sassnetguy50]

14 minutes ago, Marshal Mo Hare, SASS #45984 said:

I bet the subject could fill a bookshelf.

 

 

 

I bet it already has. 

https://vspu.larc.nasa.gov/training-content/chapter-1-vspfundamentals/propellers/propeller-design-diameter-blades-and-pitch/

[Alpo]

And of course we have the Yellow Peril. Stearman 75, US Navy 1930s/40s training aircraft. Two blades.

 

[DocWard]

And then of course...

 

 

[Equanimous Phil]

As I remember from back when I did model aeroplanes, the more blades the less efficient is the propeller. You just chose more blades for less diameter to get ground clearance if you don't want to have a super hight alighting gear.

[Duffield, SASS #23454]

Umber of blades is determined by power of the engine and the clearance available for the blades tip Path. The propellers efficiency decreases as the number of blades increases, but ground clearance (that's why the F4-U has bent wings ) or fuselage clearance  can limit blade length so more blades are added to use the available power.

[Subdeacon Joe]

1 hour ago, Duffield, SASS #23454 said:

The propellers efficiency decreases as the number of blades increases, but ground clearance (that's why the F4-U has bent wings )

 

 

And why the P-47 had gear that extended as they deployed.

[Forty Rod SASS 3935]

1 hour ago, Duffield, SASS #23454 said:

Umber of blades is determined by power of the engine and the clearance available for the blades tip Path. The propellers efficiency decreases as the number of blades increases, but ground clearance (that's why the F4-U has bent wings ) or fuselage clearance  can limit blade length so more blades are added to use the available power.

I believe I'v seen F4-Us with three blades and later versions with four.  I seem to remember something about the four bladed ones having more powerful engines.

[Cold Lake Kid, SASS # 51474]

Showing my age a bit, but I recall the Spitfire Mark 1s were a two blade version, while succeeding Marks had three and eventually four blade props as Rolls Royce developed better engines.

Loved that Spit 9

 

[Abilene Slim SASS 81783]

2 hours ago, DocWard said:

And then of course...

 

 

The cool thing about counter rotating props is they eliminate a lot of the torque acting on the airframe. Many pilots were killed in training because they mishandled it. 

[Abilene Slim SASS 81783]

28 minutes ago, Cold Lake Kid, SASS # 51474 said:

Showing my age a bit, but I recall the Spitfire Mark 1s were a two blade version, while succeeding Marks had three and eventually four blade props as Rolls Royce developed better engines.

Loved that Spit 9

 

Yup. The earliest were fixed pitch and made of wood. Variable pitch were added shortly. 
 

 

[Sedalia Dave]

The theory that the # of blades reduces efficiency is  true however the drop in efficiency can be more than offset by the ability to make use of a more powerful engine.

 

Engineers testing eight-blade prop for C-130

 

https://aviation.stackexchange.com/questions/23009/what-are-the-advantages-of-more-than-4-propeller-blades

 

[Savvy Jack]

I always thought it was to quiet down the noise. Ever hear a two bladed Cessna 180 depart?

[Colorado Coffinmaker]

Actually, it's slightly more complex than "efficiency."  Yes, a two blade propeller is more efficient as it's traveling thru undisturbed air.  However, the real question determining the number of blades, is "thrust."  Increasing the number of blades increases the amount of available thrust.  That thrust is what propels the aircraft forward.  And there is an entire book about Propeller length (arc) vs cord vs pitch vs angle of attack.  And, of course, it takes more power to effectively turn bigger propellers with more blades.  As an example, we have the B-29.  Enormous horsepower, yet to turn it's enormous propellers effectively, the engine/propeller interface required a gearbox to increase torque.

 

Contra rotating propellers are a whole different ball game.

 

Remembering, the desired result is Thrust. 

[Charlie Harley, #14153]

What Colorado Coffinmaker said. Thrust is king. Efficiency is well down the hierarchy.

 

Now ponder helicopter rotor heads. Why does the ubiquitous Huey have two rotors, the OH-6 Loach have four, but the Hughes 500, it’s civilian counterpart, have five blades?

[Wallaby Jack, SASS #44062]

4 hours ago, Charlie Harley, #14153 said:

What Colorado Coffinmaker said. Thrust is king. Efficiency is well down the hierarchy.

 

Now ponder helicopter rotor heads. Why does the ubiquitous Huey have two rotors, the OH-6 Loach have four, but the Hughes 500, it’s civilian counterpart, have five blades?

 

 

  ....... maybe the Hughes is just greedy ....  

[Imis Twohofon,SASS # 46646]

You had me at Thrust

 

Imis

[Colorado Coffinmaker]

It must be considered, Like the Ubiquitous Bumble Bee, Helicopters are unable to actually fly.  They just beat the atmosphere into submission.  And when the "loud" part get real quiet, they become metallic "rocks."  

[Subdeacon Joe]

27 minutes ago, Colorado Coffinmaker said:

It must be considered, Like the Ubiquitous Bumble Bee, Helicopters are unable to actually fly.  They just beat the atmosphere into submission.  And when the "loud" part get real quiet, they become metallic "rocks."  

 

 

Not so!  Those loose assemblages of oil leaks are so ugly the Earth repels them!

[Pat Riot]

I had two buddies that were aerospace engineers with a hobby of building homemade custom airplanes. Those two gave me lesson on propeller dynamics one day at lunch that gave me a headache. During my lesson (lunch) there was a table full of guys next to us that never said a word to each other. They just ate lunch. As we were finishing up I started thinking these two were BS’ing me when the guys at the next table complimented my two buddies on their grasp of propeller design and dynamics. Then a new discussion began. I left. Those 2 didn’t show back up to work that Friday afternoon. 
On Monday I asked what happened. These two spent the afternoon drinking and talking with that table full of Northrop Grumman guys. 
All of them were into airframe design and their hobbies revolved around propeller driven airplanes. 
My one buddy had a room full of books on airplane design. One wall was all books on propellers and their design. 
 

[Colorado Coffinmaker]

OOPS!!  I neglected to include, with all of the above, the biggest driving consideration in propeller design, is the GROSS design weight of the aircraft.  Propellers are designed to be able to move an aircraft at gross weight at the design'd airspeed and to be able to CLIMB.

 

In WWII Europe, if a Luftwaffe pilot wanted to "extend" and run from a fight, he just climbed away from allied fighters.  Until . . . . that big herky prop on the front of the P47 was redesigned.  Our pilots stated, with the "new" propeller no FW 190 was able to climb away again (dead meat) as the descriptive.

[Chickasaw Bill SASS #70001]

i guess , if , you were the have enough engine , you could get a Gray Hound bus to fly 

 

  CB 

[Savvy Jack]

1 hour ago, Chickasaw Bill SASS #70001 said:

i guess , if , you were the have enough engine , you could get a Gray Hound bus to fly 

 

  CB 

I do believe they call it the space shuttle!

[Trailrider #896]

And if you use a pair of powerful enough jet engines, you can get a metal brick to fly.  They called it the F-4! 

[Three Foot Johnson]

Aviators have also experimented with single blade props almost since day one.

 

[Savvy Jack]