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Old Jan 10, 2007, 12:48 PM
Joined Nov 2004
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Indoor duration competition

Hi,

We are going to have in Brazil the annual "vale Tudo", an duration competition but this year it will be held indoors

The rules ask for a battery no heavier than 7.5% of models total weight and 300mAh max. The wingspan is limited to 1.2 but bipes can be used, no ROG in needed.

I made some calculations and I may need 22g of thrust at 2.5m/s. Since it is a duration competition I need very a few Amps draw in the motor, I will try a LPS RXC C on a 10x4.7 expecting .48A at 3.6V. So flying a 2S 250 may take near an hour of flight. Is there a better motor? What can I use, is there a more efficient brushless for this?
Since the minimun weight is fixed (battery relation) a 22g motor is possible.

Thanks
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Last edited by Alexandre Cruz; Jan 10, 2007 at 05:58 PM.
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Old Jan 10, 2007, 05:41 PM
del
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north of Chicago
Joined Feb 2003
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That is an interesting concept. I presume the span is limited to
1.2 _Meters_? If so, an hour should be well within reach.

Is there an absolute minimum weight? Or is the weight simply
based on the 7% rule?

I have built indoor duration models with a wing span of around
1.2 meters that weighed around 30 grams, without the battery!
The rules we used were based on 50mAh NiCads, so the batteries
weighed as much as the rest of the model..

As you design your model, I would suggest that, while it is good to
think about the efficiency of the drive, it is more important to
think about wing loading. The lower the wing loading, the slower it flies,
and the less thrust you need.

--del
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Old Jan 10, 2007, 05:57 PM
Joined Nov 2004
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Yes, 1.2m
There is no absolute minimum weight, only the 7.5% rule.
Your plane weighting only 30g is amazing but since the weight is limited there is not much to do
Since we will be flying in huge gymnasium I think the bigger the plane the better, since you got reynold on your favor and motor efficiency...(batteries don't seem to vary too much)
By now my calculations showed a 2.7 aspect ratio would be the best, so 5.7sqft on a lighted e-tec 250 2S- (13.2g) total weight near 6.1oz what give us 1.1oz/sqft. The plane should fly near 2.5m/s.
My greatest problem is to know props efficience at this speed motor consuption, calculated 1.5W to 21g of thrust at this speed (assumed motor+ESC efficiency=0.7). Is that plausible? The LPS RXC pulls 34g at .48A and 3.6V
I assumed 20mAh to the receiver (GWS 4p) and 80mAh for two Hyperion 3.6g servos (but these are at 4.8V). Does that feels right?
A friend flyed for 45min on a IPS and this battery...
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Old Jan 10, 2007, 08:03 PM
del
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north of Chicago
Joined Feb 2003
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Quote:
Originally Posted by Alexandre Cruz
Yes, 1.2m
There is no absolute minimum weight, only the 7.5% rule.
Your plane weighting only 30g is amazing but since the weight is limited there is not much to do
Since we will be flying in huge gymnasium I think the bigger the plane the better, since you got reynold on your favor and motor efficiency...(batteries don't seem to vary too much)
By now my calculations showed a 2.7 aspect ratio would be the best, so 5.7sqft on a lighted e-tec 250 2S- (13.2g) total weight near 6.1oz what give us 1.1oz/sqft. The plane should fly near 2.5m/s.
My greatest problem is to know props efficience at this speed motor consuption, calculated 1.5W to 21g of thrust at this speed (assumed motor+ESC efficiency=0.7). Is that plausible? The LPS RXC pulls 34g at .48A and 3.6V
I assumed 20mAh to the receiver (GWS 4p) and 80mAh for two Hyperion 3.6g servos (but these are at 4.8V). Does that feels right?
A friend flyed for 45min on a IPS and this battery...
Alexandre,
The more I think about it, the more I think you are correct..

When I start doing some math, the 7.5% rule, or something like that,
becomes more interesting. But I have to ask, how is that calculated?

For example: If I build a model that weighs 50g, and the battery
weighs 50g, is that 100% or 50%? In other words, do you weigh
the model with the battery and then remove the battery looking
for a 7.5% reduction in weight? Or do you weigh the model without
the battery and then weigh the battery to see if it's 7.5% of the
airframe weight? At 7.5% that will not make a large difference, but it
will make some.

As for your aspect ratio.. I find that a bit extreme. Assuming you
will be flying above critical Reynolds Number, there is an improved
Lift/Drag ratio for higher aspect ratio. Which could lead to a
more efficient model.

Flight in this realm is little understood, at least by me.
But something I learned several years ago is that for span-limited
duration models, the design of the wing tips becomes important.
Years ago I built a tiny light weight smoke generator and put it
on the leading edge of a series of rubber powered model wings.
What I found was that tip plates made a signifcant difference.
They moved the wing tip vortex out beyond the tips of a
rectangular wing, and so, increased the effective wing span.

As for prop and motor efficiency, I probably can't help you much.
For me, a 7mm diameter motor is large... And those orange
GWS props weigh more than most of my models...

I guess, for me, it comes down to finding the highest energy density
battery you can get, then design the model to fit the 7.5% rule.

You have an iteresting problem. Almost makes me want to
move to Brazil... But, at my age, moving is a horrifying notion...
--del
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Old Jan 10, 2007, 08:19 PM
Joined Nov 2004
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"You have an iteresting problem. Almost makes me want to
move to Brazil... But, at my age, moving is a horrifying notion...
--del"

You should give it a try

This year we will be holding lots of chapionships (some of them with 10, spred in the calendar, days) like: 1.5m RE thermal duration (becoming veery popular); DLG; Free-Flight including CLgs and indoor duration (Roof-tramp http://www.e-voo.com/plantas/vl/imgs/rooftramp.zip ) and others, more information in the same forum....

If you have a 20g bat and a 50g model your bat weights 28.5% of the total. (20/(20+50)) so for a 13.2g bat the complete model must weight at least 13.2/.075= 176g

The great problem of enlarging the span is the reduction of area so more speed in needed and the prop becomes much less efficient. I assumed the prop will produce 0g of thrust at 7m/s (very low RPM).

I designed a wing for aprox oswald=1 but end-plates may be a good choice, I will have to fly with and without them to see the difference...(2 flights of one hour each will be really boring....but worthy).

As for the bat. I could not find anything better then Etec 250 specially because it will be discharging at 1C. And props may be balsa coustom made...
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Old Jan 26, 2007, 09:00 PM
Gustavo Exel
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Switzerland, GE, Geneva
Joined May 2003
1,763 Posts
Quote:
Originally Posted by Alexandre Cruz
There is no absolute minimum weight, only the 7.5% rule.
Actually the plane´s AUW must be in the 150-350g range.
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