Ducted propeller, not fan, jet.

[eflightray]

I suppose this thread is the other extreme to to the 'Mach sound barrier' one.

Since building quite a large prop driven model in depron, I have been contemplating a fairly large jet, something like a Sabre F-86/Mig 15 in depron, a hole with wings attached.

But I don't want to go ducted fan, (I'm a cheapskate and the thought of a big diameter ducted fan gives my wallet palpitations), so a biggish ducted propeller is being contemplated.

My logic says, if one on the front would work, one inside may stand a good chance.

I'm not looking for speed, or shear power.

So.......how about a 9" diameter prop, (or bigger, ....maniacal laugh... :D )..... in a tube work, regarding the 'right way' of going about it.

The sort of questions are -

Where to put the prop, towards the front, towards the rear, or wouldn't it matter much ?

Would flow straighteners help ? (Balance that 'help' against complexity).

Now don't laugh at this one.........Oh all right, :p, but at least read it first.

Two motors/props ?, a bigger diameter at the front and a smaller higher speed at the rear, (two stage ?).
No advantage ? (mostly owing to the extra weight I would think).

The two stage one's only a wild idea, but a fairly large depron jet isn't :), you could say I have a twinkling of an idea in my tail-pipe ;)

[JetPlaneFlyer]

The problem of putting a normal prop in a duct is that the duct would have to be huge, so if building a scale jet the overall model would be enormous. The power provided by a normal prop would be inadequate for such a huge model. Take a look at any prop powered jet model and consider how big the model would have to be to contain the prop within the fuselage.

Two stage ducted fans have been tried before and found not to work very well, if at all.

Sorry to be a killjoy :(

Steve

[eflightray]

Oh ye of little faith :D

Bear with me, I have a fiendish plan.

A 62" span F-86 has a 10.5" wide body.

Now come on, 62" isn't that big.

My depron Dauntless is 67" span, and like I said, I'm not looking for speed or power. I could build it, put a prop on the front, (luckily I'm not that mad :p ), and it would fly. Ok, so the prop gets swallowed.......

I kinda guessed the two stage was a no-no.

But.

"I have a dream". ;)

[HerkS]

I've seen several successful arrangements with ducted props.

Cut the prop down somewhat to a wider part of the blades. Three blade and four blade props work better than a two blade. By cutting the prop down a bit and then putting it in a duct, you will probably get as much thrust as an uncut prop without the duct.

You do need a good fit with minimal clearance between the prop tips and the duct -- but you don't have to obsess about it.

More blades also add thrust without adding diameter. It may not have to be as big in diameter as you are thinking.

I think it's doable. Build your ducted fan first and do some testing - then size the model to your results. You must tell us how it works and you must send pictures ;)

[kcaldwel]

You can even make it scale:

http://www.airbornegrafix.com/HistoricAircraft/ThingsWings/StipaCaproni.htm

http://www.youtube.com/watch?v=xYqr2h_xQRk

http://www.airliners.net/photo/Untitled/Caproni-Stipa/0320209&tbl=photo_info&photo_nr=1&sok=WHERE__(aircraft_generic_%3D_%27Caproni_Stipa% 27)_&sort=_order_by_photo_id_DESC_&prev_id=&next_id=0320208

Kevin

[eflightray]

Thanks HerkS

I know it's doable.

Heck!, I bet the Chinese model manufacturers are tooling up this minute. ;)

Must get round to ordering some more depron, and some blades off a wind turbine, (about a 100 megawatt should do).

Excuse my 'sense of humor', at my age it helps with my scratch building. If I was to look at things seriously I wouldn't have a scratch built 100" B17, a 93" B25, and a 67" Dauntless.

Next ...............a 60 something inch Sabre, (prettier than the Mig). :)

Oh no!, just realised 60"+ isn't all that big really ;)

[JetPlaneFlyer]

A quick bit of scaling from 3-views shows that the ratio of fuselage inlet diameter to wingspan is in the order of 1:13 for the Mig 15, probably a larger ratio for the Sabre. So if the fuselage duct was 10" then to be scale the wing span would be 130". Ok you could get away with a little cheating without spoiling the look too much but if you did a 60" span version with a 10" dia ducted fuselage it would look, quite frankly, ridiculous... Have you sketched it out?.. It would make the Stipa flying barrel look sleek!

As for using a cut down multi blade prop... Is that not just a difficult way to make a (not very efficient) ducted fan impellor?.. Why not just have done with it and buy a ducted fan impellor?
Compared to the cost of high performance batteries and motors that would be needed whatever prop/impellor you use then the ducted fan impellor is a minor expense, but it's a component that will ruin the performance of the model if it's done wrong.

Steve

[JetPlaneFlyer]

To illustrate what i mean, here is a Sabre drawn to a scale to give 10" dia fuselage duct and 60" span.. in fact i've cheated a bit to make it look less stupid:
http://i3.photobucket.com/albums/y65/Rapier_HP/Sabrebarrel.jpg

Would anyone recognise it as a Sabre?

Steve

[eflightray]

To illustrate what i mean, here is a Sabre drawn to a scale to give 10" dia fuselage duct and 60" span.. in fact i've cheated a bit to make it look less stupid:
................

Would anyone recognise it as a Sabre?

Steve

Are you trying to put me of Steve :)

Plenty of scale(ish) model jets also have 'cheater holes' to augment their small intake(s).

What I'm saying is, at 62" span I can get a 9" diameter prop inside with room around it, the outlet would obviously be small diameter and with a little cheating could be a little bigger than scale.
By the way, I don't do 'scale' anyway, I do models that are recognizable and 'scalish' in profile.

"I still have a dream".



Kevin, I said a 'dream', not a nightmare, (the flying barrel) :D

Boy, that really is a hole with wings on. I could visualize the pilots legs dangling down in the 'barrel', ............Hey!, sounds like a great idea for a pedal powered 'jet'.........nooooo.., I'll stick with the Sabre. ;)

Now, has anyone anything helpful to say ????????

[JetPlaneFlyer]

Now, has anyone anything helpful to say ????????
Sorry if I'm appearing a bit negative on this one.. The illustration is not skewed to support my argument, its simply a scale outline of a (about 60 -65" span) F86 'fattened' to take a 10" prop (in fact it's drawn at 9.5")... that’s' what it would look like, no getting around it.

Here's a helpful comment.. Sketch out the fuselage with the 9"/10" prop inside, allowing adequate room to wrap a bit of structure around the prop and with an outlet of adequate area (the EDF boys say not less than 80% of prop swept area).. and see if it looks viable to you. The Sipa Flying Barrel is a pretty good example of the proportions you can expect if you put a duct around a conventional prop.

But go ahead prove me wrong :D

Steve

PS.. if you want to use a conventional prop why not just mount it in the tail as a pusher or even conventionally on the nose?... Once in flight you would not notice the prop and in my opinion the prop would be a much more acceptable departure from scale than a ridiculously out of proportion fuselage, plus cheater holes etc.

[eflightray]

Ok Steve your right, I'm wrong.

On re-checking my 3-view, the fuselage diameter to wing span is approx a 7:1 ratio.

I've now dropped the whole idea.

[BMatthews]

And this is why all the early models of F86's were of the D model with the radome bubble so that a regular prob could be added to the "radome" spinner.

In flight props are not that obvious and even a prop shaft sticking out the front would not be that noticable. Mount the motor up near the upper "lip" and in flight it would be all but unnoticable. Mind you the radio and battery pack would need to be down near the tail to balance the combination.

Um.... mid mount the motor and run a drive shaft up to the nose to run the prop?

[eflightray]

Shhhhh.......I had my fingers crossed ;)

[BMatthews]

Yeah, I know. I think we've all been there at one point. I know I was sketching up a cartoon Mig15 that would take a 4 inch prop in the "tube" and would have around a 36 inch wing. It definetly qualified for the cartoon label but it actually didn't look all that bad. Cute in fact. It was a little pen and paper sketch that went astray years ago but the idea is still sound. With todays higher Kv value motors it may be worth looking at again. The heli motors up around the 2800 to 3200 Kv values should be OK to turn a little 4 x 3 to 3.5 x 3 inch prop I'd think.

Hmmmmmm..........

OH GREAT! ! ! ! ANOTHER PROJECT! CURSE YOU EFLIGHTGUY ! ! ! !

:D

[Jurgen Heilig]

...
But I don't want to go ducted fan, (I'm a cheapskate and the thought of a big diameter ducted fan gives my wallet palpitations), so a biggish ducted propeller is being contemplated.
...

Seen this one?:

http://www.metacafe.com/watch/2572020/indoor_jet_with_thrust_vector_control/

:) Jürgen

[Brandano]

interesting plane Jurgen. No conventional rudder even though it could have been linked in. This reminds me that there was the proposal to add thrust vectoring to the EFA as a mid life upgrade, though the advantages brought by it don't really outweigh the costs and increased risks. Essentially, the EFA is already extremely agile. Vectoring thrust would just give you a mean to stop in midair during combat to make a better target.
Eflightray, without having to go the Stipa-Caproni route, perhaps you could make one of these:
http://www.flugzeuginfo.net/acimages/fantrainer_mv.jpg

[eflightray]

The Fantrainer is always a possibility Brandano.

I have always fancied building an Edgley Optica as a 'scale' model for some AP/FPV work.

http://upload.wikimedia.org/wikipedia/en/8/83/G-BGMW.jpg

But I still intend a biggish Sabre/Mig,......... one day. :)

[Jurgen Heilig]

...
I have always fancied building an Edgley Optica as a 'scale' model for some AP/FPV work. ...

More interesting information on this "strange looking bird":

http://en.wikipedia.org/wiki/Edgley_Optica

:) Jürgen

[Brandano]

The trouble with the optica is that it's hard to make it "boxable", though I agree that it's an intriguing design.