weight. thoughts?

28builder
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weight. thoughts?

Postby 28builder » January 22nd, 2015, 7:56 am

ok 1st im a novice parent trying to help my son and his teamates. mr anderson has been EXTREMELY helpful and who knows if our plane would even fly without reading his posts and others so big big thanks

question everywhere we read is keep it light light light! we now have a flying plane and trimmed fairly well but kids weighed it in at 8.52 grams. is it worth trying to sand it down somewhere to get to 8.00? the ither competetive planes in out region are coming in at 8 grams and some have to ballast up to get to 8 gms.

1) is it worth it to sand to cut weight because kids dont really want to go with new wing or fuse and..

2) if the weight is cut down .58 gms. that seems significant on an 8 gram plane. around 5% weight cut if my calcs are correct. would that give the kids 5% more flight time? i dont think those two things directly correlate like that perhaps its even more benefit but before we sand we thought we would throw it out for discussion. by the way so far they havent has problema with motor stick bend messing with launch. they are flying less than .089 rubber

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Re: weight. thoughts?

Postby bjt4888 » January 22nd, 2015, 9:51 am

28builder,
Great job helping students with this project! As you deduced, weight is pretty important to achieving maximum duration. A heavier airplane will usually require thicker cross section rubber motors which will take fewer turns resulting in lower duration. The heavier airplane will also need to fly faster to generate needed lift and this will make it more susceptible to damage. This being said, the students can still have a lot of fun and may be able to get two minute flights, or a little more maybe, in a reasonable ceiling height gym with a good rubber and propeller combination with this current model. If the students want to attempt building a lighter airplane, they could shoot for the following part weights: wing 1.3g, fuselage 2.0g, Ikara propeller (from Freedom Flight Models) 2.0g, stabilizer .4g and tailboom .5g. After adding remaining parts to the total, this will give them an airplane of about 6.8 grams. Being underweight like this is helpful as the students can test placement of the 1.2g of ballast (clay) for best performance.

If you can reply and include data like part weights, part sizes (ex. fuselage thickness, width, length; tailboom same, propeller brand/model or description and weight if selfmade, etc.) and descriptions, I may be able to suggest specific weight reduction ideas. The more data provided the easier it is to help via this forum. For example, if you have a relative light stabilizer now and a heavy propeller, you could substitute a better, lighter propeller (if the center of gravity location can handle this). Or, if your current model has an oversize motor stick (a typical good motor stick is 1/4"x3/8"x12"), it could be sanded if too thick or cut shorter from the back end and the tailboom reattached if longer than necessary.

Brian T.
AMA since 1972 (off and on)

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Re: weight. thoughts?

Postby 28builder » January 22nd, 2015, 10:53 am

well i feel bad because we didnt weigh the origanal pieces as we built. i do happen to know that the fuse including hook and prop assembly weighs 4.4 and the ikara prop we have weighs 2.05 and the wing weighs about 2.13. we are using a combo of the freedom flight model with a home made wing design the kids made similar with tip dihidral. they are getting around 1:50ish if all goes well and they say enough prayers. they are happy but still getting passed by teams in our region by quite some bit.

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Re: weight. thoughts?

Postby bjt4888 » January 22nd, 2015, 4:27 pm

28builder,
If I am understanding correctly, it sounds like your tailboom/rudder/stabilizer assembly is about 2.0 grams. As these parts are not that difficult to make, your students could consider cutting a new tailboom from some some 7lb. wood (should be about 25 grams for a 3"x36"x1/8" sheet). This sheet weight will make a .5g tailboom that is 11.5"x.25"tapered to .125" (even .6g would be ok). Then select some 10lb. 1/16" sheet to construct a new stabilizer (should weigh about .4g after covering with ultrafilm). This should remove about 1.0 grams from your student's airplane.

Lightening the tail like this will probably help, but you'll need to be able to adjust for proper center of gravity. What are your current tail moment and nose moment lengths and where is you center of gravity currently (usually expressed as % of wing chord back from the leading edge, or number of inches behind the wing trailing edge)?

1:50 is not too bad. How many winds are you getting into the 0.089 motor and how long is the motor you are using.

Brian T.

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Re: weight. thoughts?

Postby Toms_42 » January 22nd, 2015, 8:25 pm

well i feel bad because we didnt weigh the origanal pieces as we built. i do happen to know that the fuse including hook and prop assembly weighs 4.4 and the ikara prop we have weighs 2.05 and the wing weighs about 2.13. we are using a combo of the freedom flight model with a home made wing design the kids made similar with tip dihidral. they are getting around 1:50ish if all goes well and they say enough prayers. they are happy but still getting passed by teams in our region by quite some bit.
I take it you are using the smallest prop ikara makes? We made a couple planes with those props barely over 8g, and we still thought the props were too big. Trimming them down a little on the trailing edge should cut a little weight (to add to the weight lost by sanding) and the prop will take less force to spin. We are trying to get the plane to fly on 81 rubber, as it flies on our 87 no problem.

How many winds are you getting on the 89? with armor-all lubricant we can usually wind the 87 to 1800 winds and up before we're too worried about snapping the band, but after that it climbs too high. Is your plane climbing easily and falling back down too fast? or is it simply not gaining much altitude and remaining within 15 feet of the ground?
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Re: weight. thoughts?

Postby jander14indoor » January 23rd, 2015, 5:16 am

Congratulations on your progress so far. Just under two minutes with an overweight plane means your students have at least figured out how to fly them. But losing half a gram will really help bump up the time.

I wouldn't approach it by sanding, more likely to break things and have to rebuild anyway. If you are using jigs, it will be much easier to just build new parts.
As already said, if you are using the BIG Ikara props, try a smaller one with less blade area.
Alternatively, at 2.0 gm that wing is pretty heavy. I generally shoot for 1.25 to 1.5 gm for my wings. What stick sizes and densities are you using? What did you cover with? One nice thing about rebuilding the wing, you probably won't have to adjust the center of gravity much.
Also mentioned already, your tail group is overweight. I generally shoot for .80 to 1 gm there. To help lose weight, again, what stick sizes, densities and covering are you using.

The one place I probably wouldn't shave weight is the motor stick. Most stressed part of the plane reacting all the rubber tension and torque. 2 to 2.2 gm complete (no prop) is about right.

Jeff Anderson
Livonia, MI

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Re: weight. thoughts?

Postby 28builder » January 23rd, 2015, 6:47 am

thanks again. i knew the back/rear of the kids plane was slightly heavy. i think they will build a new wing. like i said they are using the fuse and tail boom from ikara kit but they went and built their own wing with slant dihidrals. they cut their own ribs and whe it flies well and has surprised me ;) i think they can be more careful on the ribs and make lighter weight. they found the mylar they were using too hard to work with so they went with veggie bags from local grocery. thanks for the input as i would have told them to sand the fuse motor stick and i see thats not the way to go.


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