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Author Topic: Flat buttom vs under cambre wings design  (Read 788 times)
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village boy
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« on: October 10, 2018, 03:35:03 PM »

Hello there, iam new to building a big plane, I been building pistachio scale from scratch, and with my office rubber band knotted I get 6 sec. Of flight
  Iam now building a 20 inch span and 4 inch chord. My question is should I make under cambre wing or flat bottomed wing should be good for me considering my type of rubber band? Iam targeting the weight of 40 grams or less. Thanks ..

VB

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strat-o
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« Reply #1 on: October 10, 2018, 03:58:26 PM »

I think either type of airfoil can be made to fly very well at that size.  Some designers prefer the undercambered airfoil for models of this size as they feel it gives a bit of extra performance, for example, John O'Donnell and his Teacher's Pet design (30" wing), however, it can be a bit of an extra challenge to build an undercambered wing using balsawood and tissue because it's a little more difficult to cover the underside of the wing.

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village boy
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« Reply #2 on: October 10, 2018, 04:11:12 PM »

Thank you malin, I was wondering which one would perform more better. I am planning 6 inch prop

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applehoney
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« Reply #3 on: October 10, 2018, 10:57:05 PM »

Try to keep the weight down - 40g is heavy for a 20" model
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village boy
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« Reply #4 on: October 11, 2018, 03:02:45 AM »

What is your suggestion please?

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village boy
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« Reply #5 on: October 11, 2018, 03:05:43 AM »

@Applehoney whad do you suggest the weight to be at maximum? And is the prop size OK? I want a fairly slow plane. Thanks

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applehoney
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« Reply #6 on: October 11, 2018, 07:04:56 PM »

Hard to  say, much depends upon the type of model, structure, whether it has  undercarriage, etc.  For 20" span a 7" prop would likely be okay.

I rarely build such small models.   The one in photo is 24" span, strongly built, 8" prop - weighs  22g without rubber
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Re: Flat buttom vs under cambre wings design
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TimWescott
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« Reply #7 on: October 11, 2018, 08:05:09 PM »

My two attempts at Bostonians (16" span) weighed in at around 14 grams.

But I've been building for years and AppleHoney is a F#$%ing Expert, so if your first attempt comes out at 40 grams don't despair -- just make it fly as best you can, and try to do better on the next one.  Well over half the battle in building good light planes is getting good light balsa -- that may be difficult in your part of the world.
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village boy
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« Reply #8 on: April 27, 2019, 09:04:26 AM »

iam not using balsa but a kind of grass that we have here.Though i have to use a motor stick because the grass alone can not stand the twisting moment
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lincoln
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« Reply #9 on: April 27, 2019, 12:15:26 PM »

It seems possible that a small amount of "undercamber" might be helpful, but not if you have to use more material to keep it stiff and strong enough. You might try a Neelmeyer airfoil, which you can find easily on the internet.
It's a very different kind of construction, but if you have extruded polystyrene foam around, you may be able to make lighter models with that. Bead foam also works, but doesn't look as good. Around here, you can find either kind used as insulation or packaging.
I once tested a kind of hollow reed and found that it was about as stiff as the same weight of good balsa, but I don't know what your local plants are like.
Do you lubricate the rubber? It lasts longer that way, and you can wind it more turns. Silicone grease or liquid soap with glycerine. Many other things can be used if they are slippery and don't attack the rubber.
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stovebolt
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« Reply #10 on: April 30, 2019, 01:18:24 AM »

 Village boy, I too am new at this. My first attempt was 44 grams for an 18 inch wing, it wouldn't fly.
 Heavy planes have to fly fast to stay up, which takes a lot of motor, which adds weight. The folks here set me up with a wing loading chart. http://www.ef-uk.net/data/wcl.htm try to find the square area of the wing you build, best you can, and the total weight.
 Either loose weight or build a bigger wing.
Flat bottom wings work just fine.
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