acjbbj
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Number of blades on an engine

Fri Jun 01, 2018 11:41 pm

I've always wondered, how do GE, Pratt, and RR et cetera determine how many blades to put on their engine fans? And also, why did they decide on such non-standard numbers, and also why is the number of blades mostly even? (I say non-standard because 2, 3, and 5 are most "pleasant" to work with.)

Trent 800 = 26 blades
PW4168 = 34 blades
CF6 = 38 blades
JT9D's on the 747 = 46 blades (one of the best-sounding engines IMO)

And several Russian/Soviet engines (D-18T, D-436, PS-90) have 33 blades! First time I've ever seen an odd number of blades on a turbofan.
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.)
 
ChrisKen
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Re: Number of blades on an engine

Sat Jun 02, 2018 10:36 am

Balance
 
Dalmd88
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Re: Number of blades on an engine

Sat Jun 02, 2018 1:13 pm

Yes all about balance. Say a blade is damaged by fod. The manual has limits on damage blends. If the repair blends exceed the limits the blade is replaced on the engine. You also replace the the blade that is 180 degrees opposite of the damaged blade. They are a matched set by weight. The two new ones are also a matched set so the overall balance of the fan should be pretty close and after a balance run the spinner weights should only need minor adjustments to give a nice smooth run.
 
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lightsaber
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Re: Number of blades on an engine

Wed Jun 06, 2018 5:30 am

Not just balance, but each row of blades must have a different part count to the next row or a bad resonance will happen. Best if a widely different prime number.

If you notice the blades are a multiple off 7, 11, 13, and 17. The compressor blades will add a multiple of those or 19 or 23 or 29.

Lightsaber
You only have the first amendment with the 2nd. If you're not going to offend someone with what you say, you don't have the 1st.
 
LH707330
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Re: Number of blades on an engine

Wed Jun 06, 2018 2:06 pm

lightsaber wrote:
Not just balance, but each row of blades must have a different part count to the next row or a bad resonance will happen. Best if a widely different prime number.

If you notice the blades are a multiple off 7, 11, 13, and 17. The compressor blades will add a multiple of those or 19 or 23 or 29.

Lightsaber

That makes sense. Regarding the fan, do the rotor and stator typically differ by similar counts as well?
 
acjbbj
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Re: Number of blades on an engine

Thu Jun 07, 2018 2:13 am

So, prime numbers, essentially...

But still, makes me wonder why anyone would use an odd number of blades on a fan... although the PS-90's have a very nice sound.
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.)
 
JayinKitsap
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Re: Number of blades on an engine

Thu Jun 07, 2018 7:08 am

I would have expected even numbers on the fan so the matched blade pairs can be replaced and be close to in balance. Lightsaber's introduced the concept of resonance, so important but isn't this mainly in the multi staged areas of the engine, the compressor and turbine areas. It would be for both rotating and fixed blades.
 
Tristarsteve
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Re: Number of blades on an engine

Thu Jun 07, 2018 8:59 am

I was pretty sure that the RB211-22B had 33 fan blades. But having read the thread, I must do some research but can't access my notes.
RB211-22B, 524B/C, and 535C had fan blades with clappers. The later RB211-535E and 524G/H had no clappers and much larger fan blades, and less of them.
But I am sure that 22B and 524B had 33 fan blades. Just need to find some proof.
 
kurtverbose
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Re: Number of blades on an engine

Thu Jun 07, 2018 9:37 am

Tristarsteve wrote:
Just need to find some proof.


Wikipedia pic of the 22b has 33 blades.

https://en.wikipedia.org/wiki/File:Rolls-Royce_RB211-22B_(1).jpg
 
masi1157
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Re: Number of blades on an engine

Thu Jun 07, 2018 10:01 am

JayinKitsap wrote:
Lightsaber's introduced the concept of resonance....

I wouldn't call it resonance, the term here is "interaction tones" and a question of which of the excited modes can propagate or not. But I would have to dig into my literature to explain a bit more about it.


Gruß, masi1157
499 different segments on 98 airlines to 208 airports in 55 countries
 
OldAeroGuy
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Re: Number of blades on an engine

Thu Jun 07, 2018 4:02 pm

Isn't also a trade between blade structural capability and weight?

For a given thrust level, fewer blades -> each blade carries more load so each blade must be stronger i.e. more structural capability

But fewer blades -> less weight, both in the blades and the hub.

Though as an AeroGuy, I'll yield to Engine experts.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
acjbbj
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Re: Number of blades on an engine

Fri Jun 08, 2018 3:24 am

Yes, the RB211-22 and many Soviet (former) engines (PS-90, D-436) etc. use 33 blades.

Also, yes. That is why today's new generation engines use fewer blades, is that they don't need as many blades since fewer, thicker, curved blades offer higher efficiency than a larger number of straight, thinner blades with clappers.

Also, those in music will note that the ratio between the pitches of the high-frequency whine (fan blades) to the low-frequency "roar" (fan itself spinning) approximates several nice musical intervals. Trent 800 = Trent 700 = 26 blades (13/8 = approx. 8.5 half steps), Trent XWB = 22 blades (11/8 = approx. 4.5 half steps). CFM56-5B = 36 blades (9/8 = approx. 2 half steps), CFM56-3 = CF6 = PW4062 = 38 blades (19/16 = approx. 3 half steps), JT9D-7 on the 747 = 46 blades (23/16 = approx. 6 half steps), PW4168 on the A330 = 34 blades (17/16 = approx. 1 half step).
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.)
 
masi1157
Posts: 272
Joined: Sun Feb 06, 2011 11:56 am

Re: Number of blades on an engine

Fri Jun 08, 2018 5:42 am

acjbbj wrote:
Also, those in music will note that the ratio between the pitches of the high-frequency whine (fan blades) to the low-frequency "roar" (fan itself spinning) approximates several nice musical intervals.

Well, if that is the case it is probably just coincidence. And I don't quite understand your calculations. For all the engines the ratio between the blade pass frequency and the speed of rotation of the N1 shaft equals (of course) the number of blades. In all of your examples they are more than 4 (factor 16) or even 5 (factor 32) octaves apart. The number of half tones (is that what you call half steps?) between them can be calculated with 12*log2(blade count), their number lies between 53.5 (blade count 22) and 66.3 (blade count 46).


Gruß, masi1157
499 different segments on 98 airlines to 208 airports in 55 countries

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