Fan & Propeller shapes: Sickle and Curved

[kach22i]

Air or Water:
I'm looking for information on fan and propeller shapes (ducted and non-ducted) which might lead to more quiet if not more efficient shapes.

Noise is lost energy just as heat is lost energy in a mechanical system, right?

I have a thread which contains many bits of information for the context of this request. It also has many pictures which if you cannot see tell me and I'll repost all the links here.

I'm no engineer, pretty pictures tell me a lot. However if you have recommended textbooks, graphs, charts or other information you feel you can share, please add it to this thread.

Reference Thread:
http://www.hoverclubofamerica.org/forum/index.php?showtopic=1244&st=0

Sample Picture and second link:
http://www.hoverclubofamerica.org/forum/index.php?showtopic=1232&st=0

Cheers, George/kach22i

 

[DaveC426913]

And cavitation. Big problem in propellors. That's why they're curved.

 

[FredGarvin]

Do realize that fan shape is an extremely active area of research. We are always looking for ways to tweak our fan blade designs to achieve exactly what you are asking about. In regards to noise, the tip geometry is pretty much king. That's where you are sure to go supersonic and that is where the noise is generated.

For example, the forward swept tip is now the standard.
http://www.turbokart.com/images/ge90_huge.jpg

You can also see the tips here on Honda's blatant copy of our engine:
http://world.honda.com/HondaJet/Background/TurbofanEngine/images/top.jpg

I'll see if I can dig up any more sources.

 

[berkeman]

For example, the forward swept tip is now the standard.
http://www.turbokart.com/images/ge90_huge.jpg

I'll see if I can dig up any more sources.

Beautiful picture, Fred. But I want to hear the story behind this picture in the same directory...

http://www.turbokart.com/images/DSC01746.JPG


.

 

[kach22i]

In regards to noise, the tip geometry is pretty much king. That's where you are sure to go supersonic and that is where the noise is generated.

Turbofan Video-1:
http://s184.photobucket.com/albums/x295/kach22i/?action=view&current=Imgp1438.flv

Turbofan Video-2:
http://s184.photobucket.com/albums/x295/kach22i/?action=view&current=Imgp1448.flv

These details look very odd to me, taken at a local airshow last week at Willow Run (FedEx jet). Do these steps and bevels or cants help reduce noise?

http://s184.photobucket.com/albums/x295/kach22i/

The Horten brothers and their WWII flying wings had some odd pusher propellers, I'll try to find my books and post those pictures too.

 

[FredGarvin]

I didn't get a chance to get down to Willow Run this time. I was bummed out. Did the MING have one of their CH-47s there?

Anyways, there are certain things that fan design need to accomplish:
- High thrust
- Low weight
- Low Noise
- Structural integrity

It's difficult to say exactly what each fan designer had in mind with each specific piece of geometry. As I mentioned before. the tip area, especially on a large diameter fan, is going to have a lot of supersonic shock issues. That means noise. So pretty much any design is an effort to increase the efficiency and decrease the noise at the tips.

 

[kach22i]

I didn't get a chance to get down to Willow Run this time. I was bummed out. Did the MING have one of their CH-47s there?

Yes, the double rotor did a mock rescue of a downed flyer.

My 34 picture album of the Willow Run airshow, mostly B-25 nose art.

http://s184.photobucket.com/albums/x295/kach22i/B-25 Mitchell at Willow Run 2007/

Back to the topic; does anyone know if NASA has any current or very recent projects dealing with curved blades? I know they did a super quiet prop. It was hollow and had air pumped through it. The escaping air controlled the vortex's which make sound. The air did not escape out the tips, rather it vented on the backside of the prop surface via the hub.

 

[kach22i]

From page 12 of the Random Picture thread in the O.T. section of HCA.

http://www2.nlr.nl/public/facilities/AVET-Info/Content/UK/PropBlades.html
http://www2.nlr.nl/public/facilities/AVET-Info/Content/Pics/Prop_apian_isolated.jpg
http://www2.nlr.nl/public/facilities/AVET-Info/Content/Pics/Prop_apian_kulites.jpg

Daniel T. Valentine, Ph.D.
http://people.clarkson.edu/~space/VALPG1.html
http://people.clarkson.edu/~space/VALPG1_files/image003.jpg
Photo from the Naval Institute Proceedings web site;
this propeller was the first highly-skewed propeller
installed on a U. S. Merchant ship. The Project was
supported by MARAD. It was designed by Valentine
(when working at DTRDC under Dr. Wm. B. Morgan).

From page 13.....of HCA thread.
http://www.hoverclubofamerica.org/forum/index.php?showtopic=779&st=180


http://frontier.cincinnati.com/blogs/art/2..._01_default.asp

GE Aircraft Engine's Jet Engine Fan Blade (2001) that the museum acquired just before reopening in fall 2004. That's the composite fiber resin, polyurethane and titanium blade at left.

http://frontier.cincinnati.com/blogs/art/uploaded_images/momageblade2-785195.JPG



http://www.geae.com/aboutgeae/presscenter/ge90/ge90_20041116.html
http://www.geae.com/aboutgeae/presscenter/ge90/GE90_Blade.jpg

It's Great Design Too: World's Biggest Jet Engine Fan Blade at The Museum of Modern Art
November 16, 2004 -- EVENDALE, Ohio - Today's three-dimensional aerodynamic designs create not only extremely efficient jet engines--but also, beautiful works of design.

The Museum of Modern Art in New York City has acquired the world's largest jet engine fan blade, produced by General Electric Company (GE), into its Architecture and Design collection. The GE fan blade will be on view when the museum reopens on November 20.

Nearly four feet long and with a unique curved design, this fan blade is on the GE90-115B, the world's largest and most powerful jet engine. Unprecedented in size, the GE90 front fan blade is also the only composite fan blade in commercial aviation. The composite material results in the blade's distinctive black color.

The GE90 engine powers Boeing 777 aircraft around the globe.

The GE90 blade blends form and function. Using three-dimensional aerodynamic computer design tools, GE engineers modeled these huge blades to pull massive amounts of air into the engine while operating at low noise levels. The artistic, curved design serves a key function--to enable the fan blades to withstand supersonic airwaves generated during flight.

The blades' composite material gives the GE90 engine a rare combination of unprecedented power and low noise. Here's how: The front fan turns more slowly than the front fan of jet engines of similar thrust, and thus it is quieter. But the blades have to be huge to pull enough air into the engine to produce the super-high thrust. Traditional titanium blades would have resulted in far too much engine weight.

The GE90 blade's carbon fiber polymeric material and a titanium leading edge for extra protection were a lightweight and durable solution.

Each fan blade weighs between 30 and 50 pounds. Every GE90-115B engine contains 22 of these fan blades, which add approximately 2,000 pounds to the engine's thrust capability, while providing better fuel burn.

GE Transportation - Aircraft Engines, a part of General Electric Company (NYSE: GE), is one of the world's leading manufacturers of jet engines for civil and military aircraft. GE Transportation is headquartered in Cincinnati, Ohio.

http://www.hoverclubofamerica.org/forum/index.php?showtopic=779&st=180

NASA
Novel Engineering and Fabrication Techniques Tested in Low-Noise-Research Fan Blades
http://www.grc.nasa.gov/WWW/RT2001/7000/7725cunningham.html
http://www.grc.nasa.gov/WWW/RT2001/images/7725cunningham-f1.jpg
Trailing Edge Blowing blade with top skin removed. Air enters at the retainer (bottom right) and exits through turning vanes at the trailing edge

A major source of fan noise in commercial turbofan engines is the interaction of the wake from the fan blades with the stationary vanes (stators) directly behind them. The Trailing Edge Blowing (TEB) project team at the NASA Glenn Research Center designed and fabricated new fan blades to study the effects of fan trailing edge blowing as a potential noise-reduction concept. The intent is to fill the rotor wake by supplying air to the rotor blade trailing edge at the proper conditions to minimize the wake deficit, and thus generate less noise. The TEB hardware is designed for the Active Noise Control Fan (ANCF) test rig in Glenn's Aeroacoustic Propulsion Laboratory.

http://www.grc.nasa.gov/WWW/RT2001/images/7725cunningham-f2.jpg
http://www.grc.nasa.gov/WWW/RT2001/images/7725cunningham-f3.jpg

 

[berkeman]

Wow! That NASA fan blade design is wild.

 

[kach22i]

Wow! That NASA fan blade design is wild.

It reminds me of a Aerial Tennis Birdie (badminton).

http://www.everbe.com/Products/Sports/cwdata/badminton%20rackets.html
http://www.everbe.com/Products/Sports/badminton%20birdy.jpg

 

[kach22i]

Why do helicopter rotor blades have weights in them?

Do other types of fans or propellers have end tip or leading edge weights?

Why?

Helicopter blades have weights in them?
http://www.cavalrypilot.com/fm1-514/Ch3.htm
http://www.cavalrypilot.com/fm1-514/IMG00039.GIF

 

[FredGarvin]

Why do helicopter rotor blades have weights in them?

For track and balance purposes. The blades need to be rotating in the same plane or as close to the same plane as possible. If you don't you get control and vibration issues. Since there is no real fine tune adjustment per blade on a rotor head, blade weights adjust the tracking.

Do other types of fans or propellers have end tip or leading edge weights?

I can't be for sure, but I can not say that I have ever seen them anywhere else.

 

[kach22i]

I thought this paper may be of interest to some.

Senior Thesis Project
University of Virginia Department of Mechanical and Aerospace Engineering
Raymond Scott Ciszek
March 25, 2002
http://members.aol.com/sciszek/propfan.htm
http://members.aol.com/sciszek/Image5.jpg

 

[AlephZero]

These details look very odd to me, taken at a local airshow last week at Willow Run (FedEx jet). Do these steps and bevels or cants help reduce noise?

If you mean the snubbers (the cantilevers sticking out of each blade at about 2/3 of the blade height) their function is to control vibration, not noise reduction.

When the engine is running, the blades twist and the snubbers press against each other to act like a solid ring. That has a big effect on the lowest vibration frequency of the blades, and eliminates some aerodynamic stability problems (a.k.a. flutter). The disadvantage is they partly block the airflow through the fan.

Modern fan designs have longer blade chords and "hollow" blades (actually, lightweight composite structures inside the blade) to control these vibration issues without using snubbers between the blades.

 

[kach22i]

snubbers

Thank you, I was not familiar with that term.

EDIT:
AlephZero are you calling the vane riblets "snubbers"?

Will the turbofan's blades will be pulled foreward and well into the larger part of the duct opening under full speed? You see the level changes there (right?), one is sloping a little.

This cannot be good for tip blade vortex generation, right?

http://s184.photobucket.com/albums/x295/kach22i/?action=view&current=FAN-1.jpg

http://s184.photobucket.com/albums/x295/kach22i/?action=view&current=FAN-2.jpg

 

[normofthenort]

Here's a related question: Both (air) fan blades and ships' propellor blades often have one side that's curved (convex) and another side that's either straight or concave. But the orientation is OPPOSITE on fan blades and propellors! I.e., props seem to lead with their curved (convex) edge, as the leading edge, while fan blades seem to lead with the other side and use the curved (convex) edge as a trailing edge!

How can the optimum blade shape for these two common fluids be OPPOSITE from each other? Seems weird to me.

BTW, I'm old enough to remember when fan blades looked like prop blades. I don't think the shape of props has changes hugely in those decades, but the shape of fan blades has. Now some of them seem to have concave leading edges with swept-forward "points" at the outside! If that were a universally efficient way of slicing through the air at an angle to generate lift, I'd expect modern airplanes to have wings with concave LEs with swept-forward wing tips. But I haven't seen anything like that -- at least not yet!

What gives?

 

[DaveC426913]

Here's a related question: Both (air) fan blades and ships' propellor blades often have one side that's curved (convex) and another side that's either straight or concave. But the orientation is OPPOSITE on fan blades and propellors! I.e., props seem to lead with their curved (convex) edge, as the leading edge, while fan blades seem to lead with the other side and use the curved (convex) edge as a trailing edge!

How can the optimum blade shape for these two common fluids be OPPOSITE from each other? Seems weird to me.

BTW, I'm old enough to remember when fan blades looked like prop blades. I don't think the shape of props has changes hugely in those decades, but the shape of fan blades has. Now some of them seem to have concave leading edges with swept-forward "points" at the outside! If that were a universally efficient way of slicing through the air at an angle to generate lift, I'd expect modern airplanes to have wings with concave LEs with swept-forward wing tips. But I haven't seen anything like that -- at least not yet!

What gives?

Can you dig up an example of a propellor with a concave leading edge? I've never seen such a thing.

 

[normofthenort]

No, Dave, I've never seen a propellor blade with a concave leading edge, they're all convex. But I've seen lots of FAN BLADES with concave leading edges, and some with straight ones, too. And their TRAILING edges are all convex, like the LEADING edges of the propellors. That's my point, the fact that they're totally opposite to each other.

Got an answer to the puzzle?

 

[DaveC426913]

No, Dave, I've never seen a propellor blade with a concave leading edge, they're all convex. But I've seen lots of FAN BLADES with concave leading edges, and some with straight ones, too. And their TRAILING edges are all convex, like the LEADING edges of the propellors. That's my point, the fact that they're totally opposite to each other.

Got an answer to the puzzle?

Can you dig up an example of a fan blade with a concave leading edge? I've never seen such a thing.

 

[normofthenort]

I've got one at home, but I'm not there now. Here, they've got ceiling fans that look exactly like ship propellors, but they go in only one direction, which would be reverse for a ship -- straight edge first, convex edge trailing. That's really what prompted the question. If you've got a relatively new table fan or pedestal fan around, check the blade shape, and I think you'll find the leading edges are either straight or concave.

OK, I just did a quick check of "table fan" on eBay: [I can't post URLs here yet, so I'm leaving out the cgi and ebay and com parts at the beginning]
Lasko-16-Performance-Oscillating-Table-Fan-Model-2506_W0QQitemZ160227774440QQihZ006QQcategoryZ20612QQssPageNameZWDVWQQrdZ1QQcmdZViewItem shows the kind of a fan blade with a concave leading edge that I'm talking about. Note that the fan blows air FORWARDS, so the back = farther-away edge of the blades is the leading edge -- that's the funny concave edge with the pointy bit near the circumference. If it turned BACKWARDS, it would look like a reasonable boat prop, with a "swept-back" trailing edge. Also, comparing it to the shape of an airplane wing yields paradoxical (= backwards) results, no?

For comparison, here's the proof that fan blades USED to look exactly the other way around: VINTAGE-ANTIQUE-ELECTRIC-TABLE-FAN-ROTOR-ELECTRIC-CO_W0QQitemZ330227727933QQihZ014QQcategoryZ4037QQssPageNameZWDVWQQrdZ1QQcmdZViewItem . That's an old one, and the convex edge leads, and the concave edge with the point trails. That looks like conventional "streamlining" to me, and like a ship's prop -- but somebody's apparently discovered recently that the opposite shape works better for fans.

(Judging by the pitch, both fans have to rotate clockwise, the way we're facing them. But the blade shapes are exactly opposite to each other!)
eBay also has lots of fans with straight blade edges. Again, the "vintage" ones have the straight edge trailing, like a prop, and the new ones have the straight edge leading, backwards from a prop.

What gives?

 

[DaveC426913]

OK, I just did a quick check of "table fan" on eBay: [I can't post URLs here yet, so I'm leaving out the cgi and ebay and com parts at the beginning]
Lasko-16-Performance-Oscillating-Table-Fan-Model-2506_W0QQitemZ160227774440QQihZ006QQcategoryZ20612QQssPageNameZWDVWQQrdZ1QQcmdZViewItem shows the kind of a fan blade with a concave leading edge that I'm talking about. Note that the fan blows air FORWARDS, so the back = farther-away edge of the blades is the leading edge -- that's the funny concave edge with the pointy bit near the circumference.

Hard to tell which edge is leading edge. http://www.jrelec.com/Files/Mall/bigimg/HF22W.jpg" - where the leading edge is indisputable.

I see your point.

 

[normofthenort]

Yes, that photo is a great example. The profile of those fan blades is JUST EXACTLY like some propellors I've seen, except that the rotational direction is exactly BACKWARDS! On the props, the curved (convex) shape is the leading edge, and the outside trailing-edge tip is "swept back", creating a concave trailing edge. But these fan blades LEAD with the concave edge, and TRAIL with the convex one.

So, now that you understand the puzzle, what's the answer??

In a way, there are a few puzzles:

1) How can an optimal blade shape for air be almost exactly the opposite of an optimal blade shape for water? (Or, "Why is. . .?)

2) Since OLD fan blades used to look just like prop blades, when did fan-blade shape get flipped around, and what was the discovery that led to the reversal, who made it, etc.?

3) If modern fan blade shape is really efficient, is it possible that propellor-blades should also be reversed? (Many powered boats I've driven have pathetic performance in reverse, though there are often other reasons for that -- exhaust coming out the hub of the propellor, bad boat geometry for the prop "throwing" water forwards, etc.)

 

[normofthenort]

Have I really stumped the assembly here?

 

[normofthenort]

And while we're at it, I've got another aerodynamic puzzle, also about fans:

A couple of years ago, I was introduced (by a Canadian science show on TV) to a fascinating and counter-intuitive fact: If you're trying to use your breath to inflate (say) a large plastic garbage bag, it is MUCH quicker and easier NOT to seal the bag against your face! If you instead hold the bag opening a foot or so in front of your mouth and direct a blast of breath in that direction, the bag fills with maybe 1/2 to 1/3 as much breath!

So the question that flows from that is this: Why are exhaust fans always "sealed pretty tight" to a window or wall opening? Wouldn't they exhaust 2-3x as much air if they were removed from it by 1-2'? Even if they're mounted adjacent to the outside wall, wouldn't a "leaky" mounting help??

Norm
Toronto, Canada

 

[DaveC426913]

And while we're at it, I've got another aerodynamic puzzle, also about fans:

A couple of years ago, I was introduced (by a Canadian science show on TV) to a fascinating and counter-intuitive fact: If you're trying to use your breath to inflate (say) a large plastic garbage bag, it is MUCH quicker and easier NOT to seal the bag against your face! If you instead hold the bag opening a foot or so in front of your mouth and direct a blast of breath in that direction, the bag fills with maybe 1/2 to 1/3 as much breath!

So the question that flows from that is this: Why are exhaust fans always "sealed pretty tight" to a window or wall opening? Wouldn't they exhaust 2-3x as much air if they were removed from it by 1-2'? Even if they're mounted adjacent to the outside wall, wouldn't a "leaky" mounting help??

Norm
Toronto, Canada

I think that blowing air into a bag to fill it is fundamentally different than exhausting air from a room to outside in regards to the number of uncontrollable factors involved.

Hey! You're from T.O. too! I'm in S. Etobicoke.

 

[DaveC426913]

I didn't think this answer was really relevant earlier, but the more I consider it the more likely it is the answer you're looking for:

The reason ship and submarine blades are designed the way they are is to avoid cavitation, which is extremely destructive. This is fact.

(Interestingly, humankind was not the first to suffer from this problem or to find the same solution.

"For powerful swimming animals like dolphins and tuna, cavitation may be detrimental because it limits their maximum swimming speed.[2] Even if they have the power to swim faster, dolphins may have to restrict their speed because collapsing cavitation bubbles on their tail are too painful. Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the painful bubbles because they have bony fins without nerve endings. Nevertheless they cannot swim faster because the cavitation bubbles create an air film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage."
http://en.wikipedia.org/wiki/Cavitation#Marine_Life)

So, I surmise the effect you are seeing is this:

1] Early fan blades had flat leading and trailing edges
2] Improvements on the fan blades' curvature, resulting in a convex trailing edge made them quieter and more efficient
3] Prop blades, being a relatively new invention, and low-powered, followed a similar design.
4] As engines got more powerful - enough to cause cavitation - prop blades had to change to avoid damage, resulting in a concave trailing edge.

I further surmise the following principles:
1] Blades are most quiet and/or efficient with a concave leading edge
2] As the medium in which the blades operate is increased in viscosity, cavitation becomes a bigger problem, and the concave trailing edge becomes the preferred design.

 

[vissarion.eu]

No, Dave, I've never seen a propellor blade with a concave leading edge, they're all convex. But I've seen lots of FAN BLADES with concave leading edges, and some with straight ones, too. And their TRAILING edges are all convex, like the LEADING edges of the propellors. That's my point, the fact that they're totally opposite to each other.

Got an answer to the puzzle?

Fan (computer fans) sweeped (maybe not sweeped, don't know how say, may "FAN BLADES with concave leading edges") to get more thrust at smallest size, i think.

 

[DaveC426913]

Fan (computer fans) sweeped (maybe not sweeped, don't know how say, may "FAN BLADES with concave leading edges") to get more thrust at smallest size, i think.

How would this work?

It sounds to me like we need to define some properties that need to be nominalized:
desired outcomes:
- thrust
- quiet
- cavitation
variables:
- rotational speed
- blade shape

 

[zfolwick]

god that boeing fan blade is a work of art...

I wonder what one of those bad boys would cost...