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Old Aug 02, 2002, 08:59 PM
What goes up must come down..
Canada, QC, Saint-Laurent
Joined Jan 2002
1,724 Posts
expert/experienced micromodeler req'd!

Dear xperts, and/or experienced micro-modeler this is for you, i need help.
I just start this project of a Cessena Bird Dog semi-scale and need expertise to finish it, I draw a profile "a la "rodkill" type....right now I would have the wood (contest) to build to the size I initially draw ( ~ 120 sq.in.). I intend to fly this one rudder only "pulse propo", I just finish a set that runs on three button cells and I made my own actuator using 37 awg copper wire and neo magnets ....all that is control by old Tx and an analog Rx swamp from a toy! but it work and it's proportinnal control and have the req. range.
A this point I need you, ... here are my questions for you (dear xpert la la la la!) for initial setting of incidence of wing/ stab/ motor downtrust / righthrust? my airborne pack batteries / rx / actuator (~60 ohms) is may be to weak? I don't I never saw one before! but it can swing the rudder that is fairly large in area! I will probaly have to aerodynamicly balance it and reduce the area rudder/fin ratio.
the wing span is about 28" total the weigh are: landing gear and wheels* approx weight 1.5 gr. per wheel total with M.W. 5gr.*so the model w/contest of balsa my model should weight 11 gr.plus 5 gr. for the wheels (TBD*)+ 15 gr.for the r/x set = Total of 31gr. AIRFRAME / LG (no glue and mess! yet) this WHITHOUT THE POWER PACK IE. MOTOR/BATTERY COMBO. Here is what I have on hands to power / finish this model asap:
Motors:
a firecracker motor DD with 3 1/4" props (Takeoff 2000, 13 gr.)
a smaller one 6gr. DD /U80 prop (Takeoff 2000, 8 gr.)
Twin Turbo (N-20) motor with a 4.2:1 gear / U80 prop( ? gr)
2- Cox ." black" motor ( twice the size of th N-20 dd with original 4.5" x 3 prop) Looks (feel) lika a lot of static trust compare to the other DD with only 2 x 50 Ni-Cds!..even more then N20/gear/U80 this one would really would fit the 120 sq.in. of span and the DD would be be more then happy at 4.8 volts instead of the original 2x50 nicds....DD no gears! Is this an illusion?
Props (weight unknown)
qty - type
1 -Gunther plastic 5-1/4 " -.079" hole
1- grey Peck Poly 6" .- .079" hole
1- green Peck Poly 7" - .079" hole
6- 3'1/4" takeoff 2000 - .062" hole
1- U80 3-1/4" 1mm hole
4- cox electric red prop (special 4 3/8"x? - .079")
Batteries (weight unknown)
4 - 50 Mah nicd's
5- 70 NiMh's (1/3 AAA) ?
I need to know from you modelers are suggestions about what I should use for power/combo as well as the setting of incidence of wing /stab/motor downthrust right thrust for a start base on these spec's? may be I am asking to much!?
Thanks in advance for these whom reply.
Roger
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Old Aug 03, 2002, 05:33 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,432 Posts
Roger,
If the 70mAh NiMH cells are the ones Westech and Dave Lewis sell, I have never heard of anyone with any success with these. If they are they are button cells, I don't know much about them, but doubt they will work. I'd go with your 50mAh NiCd's, if that's what you have. The Takeoff prop provides an amazing amount of thrust (It's 4-3/8). Give it a shot on the Firecracker. In the Nov '99 RC Microflight, John Worth says it weighs 15g, and puts out excellent thrust on 4 50mAh NiCd's. This is probably a good bet for what you have on hand. Id' suggest starting with about 2 degrees down thrust and 2 degrees right thrust.

If you can rubber band your wing on that would be helpful for setting the correct incidence since you have no elevator control. You can also make the elevator control surface adjustable by using two pieces of soft wire glued to it, so you can bend the wire slightly when adjusting the elevator surface (this idea is from Ralph Bradley).

You may be able to fly this plane with a N20 geared 4.2:1 or 5:1 with a GWS 6.x5.0 or the Gunther prop. But, it will probably take 5 cells. 6:1 gearing would be better.

I'd suggest starting with the direct drive Firecracker on 50x4 NiCd's. If that doesn't work, you can probably save the model by using a N20 geared 6:1 with a GWS 6x5.0 or Gunther prop and a 140x2 LiPoly pack. This pack will solve any/most weight problems you might have, and give you the power of 7 120mAh NiMH cells. I have two planes I'm flying right now with this setup. See the theads on the Punkin Bipe and the 1/2-size And Now on LiPoly's. The only thing you would need is 2 LiPoly's and a charger, about $45 total.

Do you have pictures you can post? That would help us see your setup and make suggestions. I think you can be successful, but thrust could be an issue. With your airframe, a Firecracker, and 5-cell 120mAh NiMH pack (I think it will be very sluggish on 4 cells) your AUW will be about 65g. This will give you about a 2.8 ounce/sq.ft. wing loading, which is fine. I just think that with 4 cells your performance will be marginal. That's what I found with a Flying Aces Stick on a geared N20 with 4 cells. 5 cells made it come alive.

I have no experience with the Firecracker motor. Sorry.

Hope this helps. Ask more questions as you need.

Gordon
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Last edited by Gordon Johnson; Aug 03, 2002 at 05:39 PM.
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Old Aug 03, 2002, 07:47 PM
What goes up must come down..
Canada, QC, Saint-Laurent
Joined Jan 2002
1,724 Posts
Update....

Yhanks Gordon,...for the reply I was wondering if I was at the right forom.....so then what is the perfect wing/loading for a trainer remeber I will mostly start flying outside I hava a small park next to my house.
I sorry I don't have a digital camera so I can't post pic's, but reading at what you suggest I have to make an update of the NiCd's there not 50 ma but really the Sanyo N-110 AA..the even put the charge rate and amount of time for charging them, and have 4 off these.
Someone offer me a Tyco tx/rx and it weight 9gr, if I use this one then I would able to have rudder as a bang-bang and motor control thus reduce weigh (3gr. by using a single set of battries and a watch to stop the motor before the batteries are dead? does it worth it? I loose my propo! and have to redo my actuator!
Roger
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Old Aug 03, 2002, 08:11 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,432 Posts
What do your batteries weigh?
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Old Aug 03, 2002, 10:37 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,432 Posts
Roger,
110mAh NiCd's weigh ~7.6g each, so your 4 cell pack would weigh 30g or so. Is your intention to fly this plane indoors? Or is it supposed to be an outdoor plane for calm conditions? It makes a difference as the outdoor plane can get away with a higher wing loading and faster flying speed since it has more space. One thing two consider is that a geared N20 (e.g., Kenway KR-1, or your own self made gearbox) will weigh somewhere around 6.6 to 7.0g, which is lighter than a direct drive Firecracker motor.

Also, I don't know anything about it, but there is a profile Bird Dog on Bob Selman's site, with plans. You may be able to get some ideas and comparison statistics there.

Gordon
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Old Aug 04, 2002, 01:45 AM
What goes up must come down..
Canada, QC, Saint-Laurent
Joined Jan 2002
1,724 Posts
the bird dog.....

Gordon,
yes at the beginning and since it's summer, I plan to fly outdoor at evening or morning first in calm weather then if everything goes right I'll scale down to 100 may be 80 sq.in of wing area, the most important thing at the moment and you did almost provide me with is the use of detachable wing to adjust the angle of attack if req'd 2 degree down and 2 degree right thrust.
( major design review and weight added!) BTW you didn't suggest any incidence angle to stat with?)
I'll send you a PM
Roger
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