fspro
Topic Author
Posts: 8
Joined: Tue Jan 10, 2017 9:47 pm

Is fewer fan blades better?

Sun Jun 10, 2018 6:27 pm

As we are probably aware, the number of fan blades in a jet engine has been decreasing as technology has progressed. The GE90-115B had 22 blades, the GE9X has 16 blades. The CFM56-3 had 36 blades, the CFM56-7 has 24 blades etc. This leads to me to believe that fewer, larger fan blades are preferable over more, thinner fan blades. I believe (correct me on this if I'm wrong) that weight is a limiting factor on the size and therefore number of blades. Thus, with better technology, we've been able to reduce the weight of the fan blades, allowing us to make the fan blades larger.

My question is: Why does the GE9X, a large engine that has 16 blades, have fewer fan blades than for example the LEAP-X or the PW1100G, which have 18 blades and are smaller? It seems to me that it would be easier to make a smaller engine with fewer blades than to make a larger engine with fewer blades?
 
goosebayguy
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Re: Is fewer fan blades better?

Tue Jun 12, 2018 6:57 pm

Fan blades are an interesting topic. I have a few from the Vulcan which bombed the Falklands. 1950's technology solid aluminium. Many large early engines had blades which featured a ledge about halfway along or two thirds. this would rest against the next blade and provide strength and support. However as you can imagine this restricted airflow slightly so the first move was to make the blades stronger and to remove this ledge which was about half an inch wide. To make the blades stronger their width increased, which then led to fewer blades being used. Fewer blades and weight loss leads to faster starts or acceleration. Each fan blade is made up of a sandwich of metal with a honeycomb centre and costs as much as a new car. Hope this helps a little.
 
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Florianopolis
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Re: Is fewer fan blades better?

Tue Jun 12, 2018 7:43 pm

But turboprops have been sprouting more propellers?

 
acjbbj
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Re: Is fewer fan blades better?

Wed Jun 13, 2018 12:11 am

The CFM56-3 had 38 blades, the -5B has 36.
Favourite plane: "L-1011-800 TriStar Next Generation" :mrgreen:
(3-Engine cargo jet approximately the size of a 77F, with three Trent XWB-97. Two engines on the wing, the third in the tail with an S-duct.)
 
strfyr51
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Re: Is fewer fan blades better?

Wed Jun 13, 2018 5:13 am

Florianopolis wrote:
But turboprops have been sprouting more propellers?



Those airplanes are nearly 30 Years apart. Technology has Certainly advanced between when the 3 bladed C130 was introduced and the 5 bladed
Herc was introduced One has the GM -Allison T56 engine and the other, The Rolls AE2100 (I think).
 
727glasair
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Re: Is fewer fan blades better?

Wed Jun 13, 2018 6:33 am

There are many factors to be considered when designing the most fuel efficient fan blade arrangements. One of them is called PROFILE DRAG. The fewer the blades, the less profile drag. That also applies to airplane propellers. Generally, the fewer the blades, the more efficient. But more blades promotes smoothness and less vibration.
 
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Balerit
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Re: Is fewer fan blades better?

Wed Jun 13, 2018 1:10 pm

I think it is just because the fan blades have gotten wider, therefor fewer blades fit into the same space. The frontal area is probably still the same. Just look at how the aspect ratio has changed.

Image
Licensed Aircraft Maintenance Engineer (retired).
 
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Florianopolis
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Re: Is fewer fan blades better?

Wed Jun 13, 2018 4:28 pm

Balerit wrote:
Just look at how the aspect ratio has changed.


Isn't a high-aspect airfoil better, though?
 
OldAeroGuy
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Re: Is fewer fan blades better?

Wed Jun 13, 2018 7:33 pm

Wing aspect ratio can't be generalized to engine fan blades.

The cowling interior wall acts an end plate/super winglet suppressing the tip vortex. The physical dimension of a fan blade does not reflect its operating induced drag characteristics.
Last edited by OldAeroGuy on Wed Jun 13, 2018 7:44 pm, edited 1 time in total.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
OldAeroGuy
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Re: Is fewer fan blades better?

Wed Jun 13, 2018 7:41 pm

strfyr51 wrote:
Florianopolis wrote:
But turboprops have been sprouting more propellers?



Those airplanes are nearly 30 Years apart. Technology has Certainly advanced between when the 3 bladed C130 was introduced and the 5 bladed
Herc was introduced One has the GM -Allison T56 engine and the other, The Rolls AE2100 (I think).


I believe the RR AE2100D3 has 6 blades not 5.

http://www.backroadswest.com/images/Edwards/C130-2.jpg
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
kalvado
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Re: Is fewer fan blades better?

Wed Jun 13, 2018 10:18 pm

Another example of sprouting prop blades is WWII Spitfire which started with 2-blade wooden prop and went through 13 prop designs (Thirteen!!!!) - and up to 6 blades, I believe. Or maybe more.
I believe that increase of blade number, with pitch being the same, requires more power from engine - or, twisting it around, more blades are required to use higher power engine without going into overspeed. Assuming same prop diameter of course..
I am not sure how that is applied to turbofans as those cover pretty much entire airstream facing area. Maybe more narower blades as opposed to fewer wider ones?
 
fspro
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Re: Is fewer fan blades better?

Thu Jun 14, 2018 4:21 pm

My question was in regards to jet engines, not turbo-propeller engines.
 
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Balerit
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Re: Is fewer fan blades better?

Thu Jun 14, 2018 7:00 pm

OldAeroGuy wrote:
Wing aspect ratio can't be generalized to engine fan blades.

The cowling interior wall acts an end plate/super winglet suppressing the tip vortex. The physical dimension of a fan blade does not reflect its operating induced drag characteristics.


Aspect ratio is the ratio between the width of an object to it's height or length . As you say the duct (fan cowl) prevents the air flowing around the tip which is in essence similar to a longer wingspan. If you look at the diameter of a propeller it is much longer than a ducted fan. When I look at the photo above, the older blades look like they could choke up or reach a point where the airflow battles to pass through the fan whereas the wide chord blades are more open allowing more airflow. The ideal would be to have variable fan blades like an adjustable propeller.
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Faro
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Re: Is fewer fan blades better?

Thu Jun 14, 2018 10:13 pm

727glasair wrote:
There are many factors to be considered when designing the most fuel efficient fan blade arrangements. One of them is called PROFILE DRAG. The fewer the blades, the less profile drag. That also applies to airplane propellers. Generally, the fewer the blades, the more efficient. But more blades promotes smoothness and less vibration.



But there is a limit to fewer blades.

The GE9X for example is actually (slightly) heavier than its predecessor the GE90-115B even though it is state-of-the-art and produces less thrust. Why? Because each of its 16 fan blades are necessarily heavier than the GE90-115B’s. So a heavier containment ring setup is required in case of fan blade-off incidents, leading to an overall heavier engine...


Faro
The chalice not my son
 
OldAeroGuy
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Re: Is fewer fan blades better?

Fri Jun 15, 2018 12:22 pm

You can't say that the GE9X weight increase is due only to the fan and containment ring. Its core has the highest compression ratio of any engine. Creating and containing the higher pressure can increase weight.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
LU9092
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Re: Is fewer fan blades better?

Fri Jun 15, 2018 4:36 pm

Suppose we're able to make fan blades of unobtanium and mass or strength aren't limiting. Would there be a point where fewer, larger blades is no longer more efficient? Would two very large blades be the ultimate in efficiency? Or even a sort of single-blade corkscrew? I imagine the latter would have tricky balance issues.
 
WIederling
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Re: Is fewer fan blades better?

Sun Jun 17, 2018 2:02 pm

Florianopolis wrote:
Balerit wrote:
Just look at how the aspect ratio has changed.


Isn't a high-aspect airfoil better, though?


open ended yes ( wing or propeller blade.)
different for an arrangement that sports some kind of endplate ( here the fan shroud.)
Murphy is an optimist
 
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DocLightning
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Re: Is fewer fan blades better?

Mon Jun 18, 2018 8:12 pm

OldAeroGuy wrote:
You can't say that the GE9X weight increase is due only to the fan and containment ring. Its core has the highest compression ratio of any engine. Creating and containing the higher pressure can increase weight.


I figured most of it came from the larger diameter, which means more nacelle.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
buzzard302
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Re: Is fewer fan blades better?

Thu Jun 21, 2018 1:11 am

When generally talking about fan blades, a greater quantity of blades will increase pressure capacity of the assembly (at a higher power consumption and lower overall flow rate). This would be comparing identical design blades in assemblies of varying blade quantity. Change the blade design between the two units being compared, and there is not a direct comparison based on quantity alone.

Fan design is comprised of a combination or parameters including diameter, tip clearance, desired flow volume, pressure, power consumption, noise, tip velocity, etc. etc. I suspect materials used are based on weight and reliability. Fan design is a science in itself and they have some serious engineering and modeling involved. It is highly complex.

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