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ducted fan to propeller conversion

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Old 10-15-2012, 12:23 PM
  #1
flybyjohn
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Default ducted fan to propeller conversion

I was wondering if you converted a ducted fan jet to tractor propeller driven airplane, what would be the best thing to do with the airducting that currently goes thought the fuse. Would it be better to close them off and have the air go around the plane or to keep them open and let the air go through the plane. Closing them up would create more frontal area drag, and but keeping them open may create more surface area drag. What would be the best way to handle this situation?
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Old 10-15-2012, 01:30 PM
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da Rock
 
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Default RE: ducted fan to propeller conversion

Where are you going to position the engine? It'll need cooling airflow. The original duct just might be an excellent way to get it.
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Old 10-15-2012, 01:34 PM
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Default RE: ducted fan to propeller conversion

Needless to say, you're going to need a firewall to mount the engine on, and the usual one would close off the duct, but in your case you'd simply design one robust enough to suffice even with holes in it.

Also, a lot of ducted fan engines are timed for quite high rpm. The one you have now just might not give maximum performance trying to swing a prop. If you have other engines they might work better.
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Old 10-15-2012, 03:04 PM
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flybyjohn
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Default RE: ducted fan to propeller conversion

It will not be used as a ducted fan engine. The plan is to cut off the front nose of the jet and mount the engine the conventional way. It will be a tractor propelled airplane, just like any other airplane with the engine on the front. The engine head would be out in front of the wing in the open as it is larger than the fuse at that point. I plan on using a 11x8 or a 11x10 prop on a supertigre 91 engine.
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Old 10-15-2012, 03:10 PM
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Default RE: ducted fan to propeller conversion

The ST.91 is not a high revving engine but more of a torque stump puller type engine and so IMO a 11x6 prop is a bit on the small side for it. I would suggest you start with no less than a 12" dia.

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Old 10-15-2012, 07:29 PM
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Default RE: ducted fan to propeller conversion

It is actually a Como 90 engine which was the closest to the s90k engine. I have had a 12x6 to a 14x6 on it before and if I remember right the 12x6 was above 13000 rpms. I think with the rpm numbers I have seen posted on this sight for that engine, I should see 12 to 13000 rpms with the 11x8. I know the engine will pull the plane. But that is not the question.

The question is should the large hole from the ducted fan design remain open going through the fuse or should they be closed for better aerodynamics? Will there be more drag from frontal area (closed off) or more surface area (kept open). I don't know too much about aerodynamics so I am putting it out there for the ones who roam this forum. What do you think I should do with the holes.
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Old 10-15-2012, 07:49 PM
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Default RE: ducted fan to propeller conversion

Close them off. If you want use a nice streamlined shape cut from a canopy or drink bottle to fair the opening. There will be much less drag than leaving a hole through the fuselage, goes for the outlet too.
Evan, WB #12.
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Old 10-15-2012, 09:03 PM
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Default RE: ducted fan to propeller conversion

It would depend on the design being modeled. A Mig 15 or Saber might do well to use the existing ducting as an airflow duct to carry away the cooling air needed by the motor or engine. A Northrop F-5 or F-4 Phantom would be best if the ducts were simply covered over and some slight rounding of the edges used.
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Old 10-16-2012, 03:26 AM
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Default RE: ducted fan to propeller conversion

Quote:
ORIGINAL: flybyjohn

It is actually a Como 90 engine which was the closest to the s90k engine. I have had a 12x6 to a 14x6 on it before and if I remember right the 12x6 was above 13000 rpms. I think with the rpm numbers I have seen posted on this sight for that engine, I should see 12 to 13000 rpms with the 11x8. I know the engine will pull the plane. But that is not the question.
It was understood that wasn't the question, sorry the advice sounded like an answer. Having screwed around with an engine designed for DF rpms compelled me to throw in the advice. It's kewl the engine you're using will handle the task. Not all of them will without giving us fits. Lots of people come through some threads, and I'm too often compelled to try and help everbody all at once. Sorry

Any time a screaming high rpm engine is used otherwise, it's always a good idea to try and run it as fast as you can. The carb opening often won't allow that engine to run very steady at low rpm. oops...... there I go again..... just can't help it.....



Quote:

The question is should the large hole from the ducted fan design remain open going through the fuse or should they be closed for better aerodynamics? Will there be more drag from frontal area (closed off) or more surface area (kept open). I don't know too much about aerodynamics so I am putting it out there for the ones who roam this forum. What do you think I should do with the holes.

When you said "the large hole" it suggests that your new firewall is going to block off the opening at the front and the hole through the fuselage. Now that we know the engine is hanging out in the clear, and there was one hole up front, by all means block it off. The rear hole won't matter too much.
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Old 10-16-2012, 03:41 AM
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Default RE: ducted fan to propeller conversion

BTW, if there are two holes up front and you leave all holes open and as much now unneeded interior junk is removed, there really shouldn't be a deadly penalty from leaving all that open and test flying to see what happens before and after closing the holes.

It'd be fun to find out if there is even a difference. You'd be the expert then.

You will be changing the model to a more 'torquey' system that should have more acceleration ability. And the difference in drag between blocked and open shouldn't be a killer at all. So flying open or closed first shouldn't be risky.

Stick the engine on the nose and fly the sucker either way first. Or you could simply masking tape the opening on the front and see what the takeoff roll is like. Then pull the tape and run her again.

It'd be enjoyable to take part in the testing. We'd all learn something, and everyone other than you would be risking nothing..... yeah, that's a joke. (you really shouldn't be risking anything)
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Old 10-16-2012, 04:41 AM
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Default RE: ducted fan to propeller conversion

Thanks for the replys. I could try both ways just by taping a piece of balsa wood over the openings and then taking them off. This would let me know either way. Here is a picture of what I am working with. The ducts/ holes are big enough to put my whole arm into, almost up to my elbow. There is one on each side. Right now it is quite clear but might need to put some stuff back there to balance the plane with the engine on the front. I was thinking about taking the engine mount out of the middle also to save weight.
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Old 10-16-2012, 04:44 AM
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Default RE: ducted fan to propeller conversion

da Rock, no harm done. I just thought you misunderstood my intentions. I didn't really give enough information to start with for the whole picture.
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Old 10-16-2012, 04:56 AM
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Default RE: ducted fan to propeller conversion

leave things the way they schould be dont close any ducts off> you do need lots air ive had 6 byron f16
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Old 10-16-2012, 08:07 AM
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Default RE: ducted fan to propeller conversion

My question is why?  The dumbest model I have seen is a blue Angels F-18 with a 2 bladed prop on the front.  And the dumbest airplane is the Thunderscreech.
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Old 10-16-2012, 08:10 AM
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Default RE: ducted fan to propeller conversion

You may have bigger problems than aerodynamics. You CG is going to be a pain when you change from DF to tractor propulsion. Think about moving servos, batteries as far aft as you can. some guys have resorted to breaking up packs to get thing into nooks and crannys. The last resort is adding lead, but sometimes ya just have to.
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Old 10-16-2012, 09:11 AM
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Default RE: ducted fan to propeller conversion


Quote:
ORIGINAL: oneaew@msn.com

leave things the way they schould be dont close any ducts off> you do need lots air ive had 6 byron f16
read the whole thread lol he isnt using a DF engine with a DF he is running a prop engine on the front of the plane.

Freakingfast said
"You may have bigger problems than aerodynamics. You CG is going to be a pain when you change from DF to tractor propulsion. Think about moving servos, batteries as far aft as you can. some guys have resorted to breaking up packs to get thing into nooks and crannys. The last resort is adding lead, but sometimes ya just have to. "

this is going to be a big pain for you as the heavy engine will be way out front of the plane. this will require you to add atleast the engine weight in weight in the rear of the plane to balance. again i say mount the engine in the rear of the plane and run a 11x8 pusher prop. you could mount it where the DF engine was and cut slots in the fus to clear the prop. this will give you the lightest and most scale looking plane. you would be able to start it thru the rear of the plane. this way you could leave the ducting alone. the tank could be mounted just in front of the engine also.

If youre intent on keeping the engine up front then you need to close off the ducts if going to leave all the old junk in the ducts. use a large heavy sheet of mylar and for a half cone shape to help streamline the air over the ducting. the clear mylar wouldnt be visiable from the ground inflight. if want to leave the ducting open then you need to clear out all the junk in the duct to let the air freely flow thru it.
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Old 10-16-2012, 09:56 AM
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Default RE: ducted fan to propeller conversion

The balance issue was a little of a concern but currently all the servos and onboard battery are mounted up in the nose of the plane, they all would go to the back as far as I could get them. I could also shorten the nose an inch or two and it would still look alright. Right now without engine I have to add a wieght to the front to keep the nose down. I will have to see what would be invoved if I did make it a pusher. See how much cutting I would have to do.

Oh and rgburrill, I have flown quite a few dumb looking things. I am not one who has to show up at the field with the coolest plane, I just like to show up with a lot of stuff to fly.
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Old 10-16-2012, 10:08 AM
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Default RE: ducted fan to propeller conversion

Do a mock up. Tape the engine, mount, prop and spinner in the approximate position on the nose. do the same on the rest of the plane with the servos, battery, LG, tank, etc.
It may save you a lot of time or change your mind. Planes that are designed as a prop jet have subtle changes like wing placement or shorter nose.
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Old 10-16-2012, 11:03 AM
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Default RE: ducted fan to propeller conversion

it is all energy stealing boundary layer... plug'em
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Old 10-16-2012, 01:28 PM
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Default RE: ducted fan to propeller conversion

If you decide to seal them, think about using solid foam insulation from a home improvement like Lowes or Home Depot. Then get ready to laugh at me for suggesting it. They sell it in 4'x8' sheets and it's 3/4" thick. Yeah, 4 foot x 8 foot..... I'm actually not kidding. It's excellent stuff for models if you're a fix and repairer. It's also great at making stuff with. anyway...

It's light weight is almost scary. It's thickness makes it dead simple to fit into holes or against bulkheads etc. without needing supporting structure. Every cowled plane I've done in the last 6 years had had engine baffles made out of the stuff. The sheet I bought 6 years ago looks to me like it's got maybe a year or two left, and the scrap stack might provide another year. It carves easy. So angling the top and bottom of the plug you'd need on each side of that jet would be a snap. It sands easy too, so if you usually sand things to get a perfect fit, it works with you there. Get a good fit and you can use canopy glue to seal it in. Canopy glue holds great, and this foam takes very little strength to hold even when blocking airflow. Canopy glue can also be removed with moderate effort so you got the best of all worlds.

You can find the stuff in blue and pink. Lowes here sells one color, Home Depot the other. I've used up a sheet of blue and am working on a pink one now. I've been painting it with acrylic colors you can find for $1 or $2 at craft stores.

The semi-gloss (acrylic painted) stuff in the P47 cowl was made from some of the blue stuff. It started out as a disc shoved in from the back. Took a minute to trace the shape of the cowl on, a minute to make figure out how much smaller that shape needed to be to fit forward into the hole, a minute to scroll saw it out. It took about 20 minutes cut out where the engine and muffler needed to be exposed. It's so cool to do that I sorta wish those jobs took longer.
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Old 10-16-2012, 07:08 PM
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Default RE: ducted fan to propeller conversion

The balance issue will be a PITA for sure. Even balanced you will have a jet with the mass located at the extremes of the fuse. A lot of weight in the nose and a lot of weight in the tail. This layout is a lot different from a jet balanced with the critical mass near the CG (as the ducted fan), as should be acording an optimal design.

You will have a plane very temperamental in takeoffs and landings.

Just my two cents from my own experience.

Javier


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Old 10-17-2012, 07:44 PM
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Default RE: ducted fan to propeller conversion

A model of that size with a suitable engine in the nose will require a BUNCH of lead in the tail to obtain the proper CG. When I posted my prior answer I did not realize that the design you were considering was something this extreme. I was thinking more along the lines of a Mig15 or Sabre. This design is a whole other level above the difficulties of those other designs. And as such I totally agree with what Javier posted just above this post.

Frankly if you want to stick a prop on a model of this sort I'd suggest you go for a pusher. It would require far less lead to balance out the engine at the tail on this model than the engine in the nose. Make the distance from the desired CG to the engine work for you rather than against you.

And since it's this sort of design and the openings are so massive I'd have to say that it would be best to leave them open to conduct air back to the portion of the prop shielded by the fuselage.
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