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Old Oct 17, 2004, 05:36 PM
Flap chap
snakeX2's Avatar
Most Southern England, Brighton UK
Joined May 2004
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Quick help 6.7:1 gearbox!

I put together a gearbox to temparary fly my model. It's ratio is 6.7:1 and if i am correct, this should produce 14.8 grams of thrust with a GWS 5x3 prop. I can only get 12.5< at most and the motor runs rather warm for a Didel 4.5 6mm. i tried everything to get it to run smoothly, it works but i just can't get that magic number. I need a quick solution (my didel gearbox will be the replacement). Thanks!!
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Old Oct 17, 2004, 06:16 PM
Team 30 Micro EDF
Mike Taylor's Avatar
Camarillo, California
Joined Apr 2002
4,516 Posts
Things that come to mind:
Some numbers you hear may be optimistic...
Unbalanced props produce loads that the motor has ot overcome, and you lose a LOT of thrust.
Too much flex anywhere in the assembly produces the same as above.
Gear mesh? How much drag are you imposing in the mesh?
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Old Oct 17, 2004, 06:47 PM
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Pete P.'s Avatar
South of Boston
Joined Mar 2004
846 Posts
Try some lubricants, and possibly melting the gears for a better mesh. Somebody mentioned that they used to do that in slot-car days, when I had mentioned I tried it with success. But be careful!
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Old Oct 17, 2004, 08:40 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
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How about a picture of your gearbox? What are you powering it with when doing your static test? Your volts may be lower than you think. What kind of bearings did you use? In addition to what Mike mentioned, there are a variety of reasons, like gear mesh if you didn't get everything aligned just right, etc.

Gordon
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Old Oct 18, 2004, 12:05 PM
Flap chap
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Most Southern England, Brighton UK
Joined May 2004
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Now when i think about it, it is somewhat unbalanced. I rebalanced the prop and i gained another 2 grams thrust. The shaft is very smooth and i can see no problem there. I notice when the prop is low speed it shakes violently. This must be an unbalance? I also think the gear is very slightly offset by about 1 tenth of a mm but it is noticable. Your advice suggest that i can lose thrust even by the slightest unbalance/offset. Thanks very much for the advice. I will post a pic for an idea of the problem.
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Old Oct 18, 2004, 12:31 PM
Flap chap
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Most Southern England, Brighton UK
Joined May 2004
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pics- The gears are a 40 tooth and a 6 tooth so thats a 6.67:1 ratio.
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Old Oct 18, 2004, 12:46 PM
Sticky Shepherd
Graham Stabler's Avatar
Oxford/England
Joined May 2001
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"this should produce 14.8 grams of thrust with a GWS 5x3 prop."

Have you tried a GWS 5X3 prop?

The thrust rig does not look overly stable, if the thrust line is not vertical that will also reduce thrust.

what are the bearings made from?

Gear quality may also be an issue.
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Old Oct 18, 2004, 01:05 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,432 Posts
In general CF under camber props generate a bit less thrust than their GWS counterparts. Graham is correct here. Also, it looks like your setup is pulling down and may be somewhat close to the scale. I've never used this type of thrust rig because of the potential of the disrupted airflow around the scale or work surface to introduce error. I'm not sure how much if any error there will be. Many people use this type of stand. I just don't have any experience with it. But, I suspect that the closer the prop is to the scale surface the less accurate the measured thrust will be. Yours looks sort of close. My test rig is a "L" shaped setup with the motor horizontal, pulling on the top of the L and pushing down on the bottom of the L on a scale. It also pivots on 2mm id ball bearings and moves pretty freely.

Gordon
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Old Oct 18, 2004, 01:16 PM
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Germany, NDS, WOB
Joined Nov 2000
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Static thrust is not a very useful measurement unless you want to hover your model. Sure, it gives an indication of what you can expect from the setup but it is not until you fly it that you get the real results. Matching prop and gear ratio to a certain airframe is more important. Better have a couple of different prop sizes and find out what works well in flight.

BTW, if you want to compare data make sure the setup is EXACTLY the same otherwise comparison is pointless.

Michael
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Old Oct 18, 2004, 01:21 PM
Registered User
Antwerp Deurne, Belgium
Joined Dec 1996
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my experience is indeed that carbon copies of GWS go up to 10% below the GWS performance.
The gear on your shaft looks rather thick, that may induce more friction too.
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Old Oct 18, 2004, 01:48 PM
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Dave Wulff's Avatar
USA, FL, Fort Lauderdale
Joined Feb 2002
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I am a bit with alexander on the gears. Are they a "matched pair", if the mods are just a bit different that might result in a lot of friction. At this size it dosn't take much.

Dave
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Old Oct 18, 2004, 01:58 PM
Team 30 Micro EDF
Mike Taylor's Avatar
Camarillo, California
Joined Apr 2002
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To add my observations from the photos:
It looks like your prop needs more work. The bends arount the blade outline should be ground or sanded away. You are not getting close to the real edges.
A CF prop doesn't have the airfoil shape of the prop used for the mold, so there is less thrust due to a different airfoil, and also your airfoils is now a flat plate with more camber than the average camber of the modeled prop.

(As an side on CF props, I wonder if using a 0.5 mm, sanded to shape piece of depron as an inner core, and tha back side impression of the prop as the mold, wouldn't produce a better copy prop? I made one that way, and it took a LOOONG time to lay up correctly, but it worked OK. Broke it instead of bending it, though)

Is there a thrust bearing between the frame and the shaft retainer? If not, the retainer is dragging on the frame producing friction.
What are you using for bearings - just the hole drilled in the ply wood? How much clearance is there (too little = friction, a bit more = slop)...
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Old Oct 18, 2004, 02:24 PM
Flap chap
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Most Southern England, Brighton UK
Joined May 2004
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Great advice!...Ok, isn't my prop a GWS 5x3?? It looks like it in the picture. I am not sure about the prop quality, so sanding to a better shape sounds like a good idea. I never realised a CF prop would be less efficient, so whats the point of using one??
I will be getting a Didel 6.6:1 gearbox for 6mm 4.5 ohm pager soon so i will compare mine to see what the problem is, could be a prop problem. I guess my thrust measurement is'nt totally reliable, but the thrust sure doesn't feel like 14 grams thrust, so 10 sounds about right. Also my 4.5 ohm pager was getting rather hot, which hasn't happened to me before. The gears are not matched pairs and are rather good but not brilliant.

If i am not correct, i should get between 14 grams to 14.8 grams with a GWS 5x3 on 6.5:1 ratio to 6.7:1 ratio? Overall, my gearbox isn't very good, it can be improved and it only relies on the ply as a bearing, what could i use instead? For a 15gram< model (light wing loading), is 10 grams of thrust going to keep it airbourne? That is my bare minimum thrust. Thanks for the expert advice!!
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Old Oct 18, 2004, 02:25 PM
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Jackson, Michigan, United States
Joined Nov 2002
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Maybe it's just me but i always prefer a bearing tube of some sorts. Seems to add more stability. Just 2 more cents

Greg
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Old Oct 18, 2004, 02:34 PM
Flap chap
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Most Southern England, Brighton UK
Joined May 2004
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Two prop pics. The prop isn't brilliant, (the odd underneath of the prop is just glass firbre tissue) but it is light ( 0.6 grams ). I just reliased i have a few heavier, possibly better CF props. A bearing tube may well be the choice in the next gearbox, thanks!
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