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flylku
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Fan Diameter And Drag

Sat May 19, 2012 9:07 pm

There was a recent forum thread in Civil Aviation regarding the decision to go with the 69.4 inch fan on the next, next, next generation 737 (AKA the 737MAX). In it there was a statement that increasing fan diameter increases drag. I would like a little clarification. As the fan is part of the propulsion system it sounds like we are not talking about drag for the aircraft itself but rather drag within the the engine itself. In other words it would take a more powerful high pressure section to drive a larger fan?

In addition, what are the tradeoffs (in general) with larger versus smaller fan diameters?
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tdscanuck
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RE: Fan Diameter And Drag

Sun May 20, 2012 1:27 am

Quoting flylku (Thread starter):
As the fan is part of the propulsion system it sounds like we are not talking about drag for the aircraft itself but rather drag within the the engine itself. In other words it would take a more powerful high pressure section to drive a larger fan?

In the context of the thread you references, they were talking about drag for the aircraft itself. The engine has to push it's own nacelle through the air (along with the rest of the aircraft). Bigger fan means bigger nacelle means more nacelle drag.

It is also, and independantly, true that a higher thrust fan needs a more powerful core. If it's a bigger fan that's not generating more thrust (i.e. lower disc loading) then it may or may not need a more powerful core, although the larger fan will tend to have larger skin friction drag but lower aerodynamic drag so the balance isn't exactly clear to me.

Quoting flylku (Thread starter):
In addition, what are the tradeoffs (in general) with larger versus smaller fan diameters?

Big fan: higher bypass ratio (for equal core), lower disc loading (for equal thrust), heavier, more expensive, more fuel efficient, quieter (for equal tip Mach number), larger/slower/heavier LP turbine, lower maximum Mach

Small fan: lower bypass ratio, higher disc loading, lighter, cheaper, less fuel efficient, louder, smaller/faster/lighter LP turbine, higher maximum Mach.

Tom.
 
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flylku
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RE: Fan Diameter And Drag

Fri May 25, 2012 6:51 pm

Quoting tdscanuck (Reply 1):
Bigger fan means bigger nacelle means more nacelle drag.

That is an aspect I had not thought of: parasitic drag of the nacelle. Thanks.
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KELPkid
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RE: Fan Diameter And Drag

Fri May 25, 2012 7:41 pm

Something which may also be worthy of note here:

In GA (multi-engine) realms, it is fairly common knowledge that the drag of a windmilling prop is roughly equivalent to a flat plate of the propeller diameter, so it behooves you (as a twin pilot) to feather the prop once the dead engine is identified and verified  

I would assume that in an engine out scenario, a larger dead engine on a jet is going to cause (larger) drag problems which must be overcome. Anyone know how much drag a windmilling jet engine produces? I'd assume it's significant, because on the 747 "5th engine" ferries, the core and fan blades are locked, and a protective cover is placed over the 5th engine for the ferry..

More drag might even do something like raise Vmc above an acceptable (for certification) threshhold.
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Starlionblue
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RE: Fan Diameter And Drag

Sat May 26, 2012 12:17 am

Quoting KELPkid (Reply 3):
I would assume that in an engine out scenario, a larger dead engine on a jet is going to cause (larger) drag problems which must be overcome. Anyone know how much drag a windmilling jet engine produces? I'd assume it's significant, because on the 747 "5th engine" ferries, the core and fan blades are locked, and a protective cover is placed over the 5th engine for the ferry..

According to "Handling the Big Jets" by Davies, windmilling jets produce less drag than windmilling props. But I can't remember how feathering changes things. Then again the last edition was published in 1971 so this was before today's gigantofans.

I think the cover on the 747 engine ferry is to stop the engine from windmilling and thus being damaged by the lack of lubrication. And in any case the cover would decrease drag.
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tdscanuck
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RE: Fan Diameter And Drag

Sat May 26, 2012 12:25 am

Quoting KELPkid (Reply 3):
Anyone know how much drag a windmilling jet engine produces?

I think it's considerably better than a piston prop. A windmilling fan doesn't get anywhere close to full speed and, more importantly, it takes very little torque to turn the fan. A windmilling prop is turning the engine over (i.e. overcoming compression in the cylinders), which takes a lot more torque. Hand-propping is challenging on even a moderately sized piston engine, "hand spinning" even a very large jet is easy. A free-turbine turboprop should be similar to a jet.

Tom.
 
timz
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RE: Fan Diameter And Drag

Sat May 26, 2012 7:34 pm

I think Gunston once said Rolls-Royce made a mistake in the 1950s, overestimating the disadvantages of high bypass. Anybody know what they miscalculated?

(Yes, the Conway had to fit into bomber wings, but as I recall he suggested that wasn't the only reason they gave the Conway that low bypass ratio.)
 
tdscanuck
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RE: Fan Diameter And Drag

Sat May 26, 2012 7:45 pm

Quoting timz (Reply 6):

I think Gunston once said Rolls-Royce made a mistake in the 1950s, overestimating the disadvantages of high bypass. Anybody know what they miscalculated?

As I recall, they made an error in calculating the drag of the total installation that inaccurately penalized large fans. In other words, they thought a high bypass engine would be draggier than it actually is, which biased all their trade studies towards a smaller fan.

Tom.

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