closed or open F/18 ducts?

[stevem1928]

I am sratch building an F/18 using 2 J250's and 3x3 props. it is not scale. I used a 3 view drawings. I am trying to keep the drag down, but keep some detail. Should I close the front of the intake ducts to reduce drag? They are not really functional. However, they do extend all the way back to the props which may allow better airflow to the props. I made the ducts smaller than scale to reduce drag. The nose is also a bit shorter. I did not have a piece of EPE (swining pool noodle foam) long enough. Here are a few pictures of the pieces not yet completely attached. I will use 2s2p Etech 1200's. The final weight AUW should be 15 oz. I have used this same power system on a Projeti like plane. I is fast with this set up.

[Mike4122KSA]

IMO, block them off. Less drag.

[DaveGherardini]

How about elaborating on that a little Mike4122ksa. Im curious. More wet area? Im not sure that the inside wall of the duct would be considered wet area.... Im not trying to dout you just merely needing some evidence. I would think diff situation might have diff isuess to go one way or the other.....its is interesting that it would reduce drag...

Dave G

[stevem1928]

Thanks. I think I will bevel the plug to reduce the drag even more. It won't look the best, but will keep the plane flying better.

[DaveGherardini]

BTW..Nice jet stevem1928. Im a blucore nut but i love about anything i can use to build a plane.

Mike4122ksa another reason im asking is i have a simular situation and im debating opening up this area. mainly for cooling on a ele jet pusher. My battery is in this area. If it would add drag if i open this area up then where would be the best place to put some vents with out increasing drag. On the bottom sides or would that add drag to. sorry for asking so many questions but i need some good advice.

Dave G

heres a pic of the ship. You can see the black ramp...or area that im thinking about cutting a hole in. would it be better to cut vent in the bottom...

[rcjetpilot]

IMHO, if the inside area of the duct is clean with no obstructions, I'd leave them open for good cooling.

If you seal up tightly around the motor area, I would close them.

Looks great... Gotta luv that FanFold:)
Bob

[BMatthews]

If the ducts are tight then the turbulence in there might make them act like they are closed but with a fair amount of turbulence coming off the lip. I won't matter until you start doing some high speed dives and then you may find it whistles or seems to make the model slow down quickly.

For now I'd leave them open and once it's flying try taping on a couple of clear bubble formed canopy like sections to fair them in. Then if the high speed goes up and/or the model holds it's speed energy better then you know the ducts were dragging it down and you can leave the clear fairings on the model so it looks decent in the air but has the streamlining for speed and performance. Similarly you can then remove the fairings and add simple blocking plugs and try it again. At that point you can choose the one that works the best for performance and looks.

[stevem1928]

Bmatthews,

That sounds like a real good idea. I will try it open to begin with, then close them with tape. I will keep the design which will allow for them to be open or closed.

[Mike4122KSA]

Sorry, haven't checked this thread in a couple of days.

My reasoning for saying 'block them off, less drag' is because the air flowing through the ducts would be subject to skin or surface drag. The entire interior surface of the ducts would add to the exterior surface of the plane in the skin drag total. So, blocking the ducts off would eliminate the extra surface area, and I'm pretty confident that this would be superior, drag wise. Not sure that reads very well...

But hey, try it both ways, and post the results.

If you want additional cooling for interior bits, you might want to add "NACA" ducts. They have very low drag since they don't stick out into the airflow.

Mike in Arkansas

[stevem1928]

I should have it finished in a week. But, it will be a while before it it warms up, and the snow cover is gone.

I will have to do some research on the NACA ducts. I don't have a clue as to what they are. I have been trying to find info on EDF specifics, but have not found much. Is there any good reading on the characteristics (effects) of a nacelle and ducting of an EDF. I see the GWS info on the EDF lists increased thrust when using a duct & intake ring.

[Mike4122KSA]

I googled "naca duct picture" and this popped up, pretty good stuff for a quickie search

http://www.wisil.recumbents.com/wisil/nacaduct/naca-duct.htm

HTH
Mike in Arkansas

[stevem1928]

FLIGHT REPORT

all fan fold foam construction
carbon tube down the fuse, and across the wing span
folded wing airfoil w/ 1/4" former, flat bottom
26" ws
18.5 oz.
twin Johnson 250's
3x3 GWS props
Etech 1200 2s2p
2 - 18 g generic servos
15 amp ESC
full flying tailerons
GWS 6ch RX


It flew!

I took the plane out today for a test. The winds were a little high for this plane. 12 mph as reported by our local airport. The plane came out a little heavier than expected. 18.5 oz. That is 3 oz. above the target weight. I also had to clip the 3x3 props. They were just about touching each other. Now they are 2.9x3. Static thrust while holding the plane in my hand seemed to be very weak. But, the plane took right off with a light toss into the wind. It seemed to skid in the corners some. I will put ailerons on the next one. The wind was blowing hard, but the plane kept right on course. I lost orientation a bit due to the yellow line running down the fuse on top and bottom. I will take the yellow line off of the bottom. It will loose some of its scale looks. I had the ducts open for this test. The plane flew much slower than expected, guessing ~ 35mph. I was thinking the stall speed was going to be about that. It did not tip stall or stall in any way. If anyone has any input or ways to improve this plane, please let me know. I may put a Himax 2025 4200, or a Mega 16/15/4. Any thoughts?

[stevem1928]

Voltage drop?

I have a similar plane which I call a Twinjeti. It uses the same power system. It is 14.5 oz, but flies twice as fast as the F 18. I am wondering if I have a voltage drop due to the lenght of wire from the ESC to the motors. I had to position the ESC toward the front. The 16 gauge wires are almost 2' long. The motors are connected in parallel with short wires, and then a single pair of wires runs the lenght of the fuse. Does this set up cause a voltage drop? I amu using Deans Ultras for the motor, ESC and battery connecitons. Here is a picture of the Twinjeti.