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Ducted fan aircraft

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jthunt

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In a lot of RC aircraft an electric or gas ducted fan is used as a source of propulsion. Hypothetically could a small aircraft piston motor be used to create a life sized ducted fan style system for a small aircraft? Or would that engine not be capable of high enough rpm/torque levels to create enough thrust.
 

Topaz

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The short answer is that it can be done, but there will be some performance penalties. We've discussed this subject at length in several threads, so looking it up using the forum's search function should bring all that detailed information to light.
 

Dana

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The slightly less short answer is that model airplane engines develop insane power to weight ratios so you can do things that simply can't be scaled up to larger aircraft. Analogous scaling factors also apply to structures; models are proportionally much lighter than full size aircraft so less power is required.

-Dana

The difference between theory and practice is that, in theory, there is no difference between theory and practice, but in practice, there is.
 

jthunt

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The slightly less short answer is that model airplane engines develop insane power to weight ratios so you can do things that simply can't be scaled up to larger aircraft. Analogous scaling factors also apply to structures; models are proportionally much lighter than full size aircraft so less power is required.

-Dana

The difference between theory and practice is that, in theory, there is no difference between theory and practice, but in practice, there is.
That ps just blew my mind thanks for the input
 

jthunt

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The short answer is that it can be done, but there will be some performance penalties. We've discussed this subject at length in several threads, so looking it up using the forum's search function should bring all that detailed information to light.
Awesome thanks I'm new to this forum but I'll definitely take a look at some of those other threads - for me performance isn't really much of a concern. My main issue is whether or not it would be capable of sustained flight. I have a little experience piloting and I'm a decent fabricator but I wouldn't trust anything I built at any altitude higher than 15-20ft. I live on a bay and I'm basically looking at making a small craft that could fly just above the water...the slower the better
 

jthunt

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Yes that's almost exactly what I had in mind - is that flying with the assistance of the downward facing hovercraft fan or just the rear fan?
 

Lucrum

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Dana is right, based on my research a typical RC DF has a weight to power ratio of 1 - 5 Lbs per HP.
 

orion

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How about this > Video Gallery : Universal Hovercraft, The World Leader in Hovercraft Technology Looks like it could be a lot of fun, and it's definitely within your altitude range.
The Universal Hovercraft that's shown in the picture is an actual hovercraft, supplemented by a fabric low aspect ratio wing. The fabric is suspended between two tubes (one at the leading edge and one at the trailing edge) and essentially forms its own airfoil section. When it's not moving the fabric hangs limp. There are or two similar designs (one I'm thinking of is from New Zealand) that are configured as this one but they use actual foam inserts that provide an established and predictable section shape.

When operating as a hovercraft, there is a fan that blows downward, filling the skirt when the craft is on the surface (it's the big hole in front of the windshield).

The design came under quite a bit of criticism, especially from those that are familiar with wing in ground-effect flight, as there was no evidence of actual design or analysis work on this design. It was apparently developed through trial and error and as such, no-one can answer how safe the design actually is as there are no specifics as to the craft's capabilities or limitations. Despite that however, the craft is being marketed as a plans built, to the public.

A similar endeavor by another individual (almost identical configuration) displayed sufficient evidence of instability that that program was immediately halted and removed from public view.

The idea does look like a lot of fun but for most folks that have experience in this form of flight, there is a sizable element of risk and as such, I've mostly heard cautious warnings for those interested in heading in this direction.

One minor note - this really doesn't qualify as a ducted fan - it is more a shrouded prop, which quiets the craft's operation and if properly designed, the shroud does increase the prop efficiency and does provide for an increased static thrust rating.
 

Dana

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I have a little experience piloting and I'm a decent fabricator but I wouldn't trust anything I built at any altitude higher than 15-20ft. I live on a bay and I'm basically looking at making a small craft that could fly just above the water...the slower the better
You can get just as dead in a crash from 15-20' as you can from higher up...

-Dana

Politicians and diapers have one thing in common. They should both be changed regularly, and for the same reason.
 

Topaz

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I won't argue that but I think my odds aren't too bad flying low over water
You'd be surprised. At flight speeds, water can be mighty hard. You're much safer at altitude, where there's nothing to hit at all, and you have a chance to bail out under 'chute, if you're carrying one.
 

SVSUSteve

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I won't argue that but I think my odds aren't too bad flying low over water
Tell that to the 8% or so of folks who die in plane crashes whose cause of death was drowning. From a blunt trauma perspective, a hard ditching or uncontrolled low-altitude flop into the water is just as dangerous as an impact into solid ground. In some ways, it's more dangerous because the aircraft tends to "dig in" more and come to a much more abrupt stop than it would say sliding into a muddy cornfield.
 

el touristo

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The slightly less short answer is that model airplane engines develop insane power to weight ratios so you can do things that simply can't be scaled up to larger aircraft. Analogous scaling factors also apply to structures; models are proportionally much lighter than full size aircraft so less power is required.

-Dana

not trying to be a know-it-all or anything... RC aircraft are usually , not
always, overpowered. This has little to do with the airframe weight, in fact the models are generally NOT lighter, but heavier, and much stronger in scale terms. Weight scales with the cube of the size. In fact as you try to build planes very small it gets really hard to get them light enough, the wing loadings have to get lower and lower as they get smaller. The problem is that the density of air is not changing lol. They are often overpowered simply because they can be. Flight times range from about 3 mins to 20, with 5-10 being average. So you could put that much power in a full scale plane if you only wanted to fly a short time. They are overpowered because it can be more fun, and its safer because you can recover from more situations. Just wanted to point out that they do not require less power. I came to this thread since I'm curious about full scale electric ducted fans. I wonder if they would make sense if you were trying to make a very fast electric airplane, and what you were trying to do is created an 'electric jet' of sorts. Since props top out around 500 knots I wonder what sort of fan design would enable higher speeds under electric power. just something fun to think about, that I havn't yet found discussion on.
 
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