Ducted Fan propulsion

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dirk_D

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Sep 22, 2010
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93
Location
Victoria, Australia.
Does anyone think that 2 externally mounted ducts on a delta wing mounted just above the wing, at the rear and each side of the above blended fuselage seem like a good idea?
At high alpha landing would the ducts starve?
Or would the ducts draw more air over the wing and lower the stall speed?
Would the high alpha of landing create a wing tip stall that could saved by 'steering' with the ducts?
(Similar a dune buggies rear brakes when doing a wheelie.)

I have a design relying on this layout, if it's a turd please tell me.
 

delta

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May 26, 2011
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2,142
Location
Brookside Utah
Does anyone think that 2 externally mounted ducts on a delta wing mounted just above the wing, at the rear and each side of the above blended fuselage seem like a good idea?
It does to me

At high alpha landing would the ducts starve?
Anything's possible, but probably not.

Or would the ducts draw more air over the wing and lower the stall speed?
Anything's possible, but probably not.

Would the high alpha of landing create a wing tip stall that could saved by 'steering' with the ducts?
(Similar a dune buggies rear brakes when doing a wheelie.)
Anything's possible, but probably not.

I have a design relying on this layout, if it's a turd please tell me.
Probably not. I'm considering a similar package but it's one among many. First order of business would be to get the engine/motor/fan/duct worked out to your satisfaction, and size the delta accordingly. I've noticed a lack of enthusiasm for ducted fans in general because they're a lot of work to get set up, don't seem to work any better (or as good as) an unducted propeller, and you can imagine how much drag they'd have if the engine/motor quit.

RC experimentation should answer a lot of questions.
 

Swampyankee

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Dec 25, 2015
Messages
1,420
Location
Earth USA East Coast
Well, since some of you provided such thoughtful feedback on the question of fuel injection, I may as well put this one to you as well:

Why do ducted fan configurations seem to be entirely absent from LSA's and homebuilt's? Without having actually tried constructing such a propulsion system, I wouldn't imagine that cowling the fan with reasonable precision would be exceedingly difficult. Also, the challenges of weight and reliability of the cowling and drive mechanisms should be feasible too, given the indirect-drive configurations of many motorgliders.

The disadvantages that come to mind are potential fan balancing complications, and a drive mechanism that can handle the higher rpm's of ducted fans.

The benefits, aside from efficiency, would be smaller diameters and greater safety (given enclosure).

Just curious, really.
There are two or three aero things going on, but first a correction: they are not more efficient unless you have a stringent diameter constraint; if you do not, an open rotor and more diameter is better. (Before you ask, yes, there is a strict diameter constraint on turbofans). I worked on prop aero and did some time writing performance decks at P&WA, so I'm not completely ignorant of this.

Probably a big one is propulsive efficiency. Thrust is mass flow rate through the propulsor times by change in velocity across it. The power needed is mass flow rate times change in velocity squared. eta This is where the more-diameter-is-better thing came from (it applies to things with ducts, too: this is one reason why a modern high-bypass fan beats the crap out of a turbojet or early-generation low-bypass fan for fuel economy): the mass flow rate per unit area is pretty much independent of how much velocity added to the mass. Ten thousand newtons thrust -- about 2200 lbf -- can be produced by adding 500 m/s velocity to 200 kg/s, which requires 25,000,000 Joules/sec (500 squared times 100) or 25000 kW or by adding 5 m/s to 20,000 kg/s, which requires 250,000 Joules/sec or 250 kW.

As for safety?

Well, this is a major reason for the development of the fenestron by Aerospatiale, and a major reason for its adoption on the RAH-66 Comanche (apparently, people walk into to tail rotors far more often than into propellers), but a fenestron (or conventional tail rotor) and empennage can be designed to reduce tail rotor power needs in cruise to a small fraction of required power.
 
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pictsidhe

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Jul 15, 2014
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7,433
Location
North Carolina
Getting the design right is a lot harder than a prop. Get it right, you can approach prop performance. Get it wrong, performance is dreadful.
 
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