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Old Apr 07, 2011, 05:59 AM
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Sri Lanka
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You can increase the length of the (slack, unwound) motor up to 2 times the length of the 'prop hook to rear peg' distance
That is what I meant, attach the rubber to the prop hook and stretch it to attach it to the peg. This makes it run longer it seems the rubber thins out a bit.

You plane is very neatly and nicely built. 58 seconds is great - did you pull out the prop to stretch wind the motor? What are the motor specs - prop diameter, rubber dimenstions, prop to rear peg?

Quote:
You need to start thinking in terms of wingloading, not just weight.
I used to calculate wing loading etc but realized that it was simpler to simple calculate span loading and compare to a similar model, for example if I am building a 40 cm model, just see what the 40 cm (16 inch) models typically weigh. I find that for 40 cm, 20 g is very good. Larger models can support more weight than that.
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Old Apr 07, 2011, 06:34 AM
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Originally Posted by knlever View Post
... did you pull out the prop to stretch wind the motor? What are the motor specs - prop diameter, rubber dimenstions, prop to rear peg?
Thanks. Yes the nose block is removable. The prop is a 7" Peck. Rubber is 3/16 flat... a single loop of 17" (34" of rubber doubled up).

The fuselage length from hook to peg is 11.5". So the unstretched, unwound loop is 1.5 times longer than the fuselage. I'm going to make up an even longer loop - twice as long as the fuselage - to increase the flight times.


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Originally Posted by knlever View Post
That is what I meant, attach the rubber to the prop hook and stretch it to attach it to the peg. This makes it run longer it seems the rubber thins out a bit.
No... The rubber loop is not under any tension when unwound. It can be quite a lot longer than the 'hook to peg' distance. This pic shows another plane with the rubber attached to the rear hook, ready to wind.




Quote:
Originally Posted by knlever View Post
I used to calculate wing loading etc but realized that it was simpler to simple calculate span loading and compare to a similar model, for example if I am building a 40 cm model, just see what the 40 cm (16 inch) models typically weigh. I find that for 40 cm, 20 g is very good. Larger models can support more weight than that.
Span loading is important for some comparisons/calculations - induced drag for instance.

But it doesn't take into account the chord or aspect ratio of the wing. The wingloading tells you more about how hard the wing is working (Lift Coefficient) and is neccessary to work out stall speed.

In this context wingloading tells you more.

For instance:

- The Playboy was 20" span and 23 grams AUW. But it has a fairly high aspect ratio (long, skinny wings) it's 20" span but only 49 in2 wing area. So wingloading is 2.4 oz/ft2 or 0.47 grams/in2.

- The Redwing has 18" span but is already heavier - finished AUW should be around 30g.

So is it a worse model? Well the chord is quite a bit wider - the area is 57in2 so the wing loading at 30g will be 0.52 grams/in2 or 2.67 oz/ft2.

So it's only fractionally heavier in real terms... although by spanloading alone it looks much worse.

Another example is a biplane - same span: twice the area.


Jon

EDIT: Your rule of 20g for 40cm is not too far out - at an aspect ratio of 6 (fairly standard) it works out as close to the half a gram per inch square target. But this target is a bit heavy for smaller models (under 18" say)....
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Old Apr 12, 2011, 11:24 PM
Just shut up and build!
Sri Lanka
Joined Mar 2010
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Jon

Just realised your model in the above picture is sheet balsa! I was looking at the wing and I realized that it was rather flat and had some lines on it.

Bruce

I followed the plan for the paper wing, however I had a lot of difficulty keeping the spar straight and also avoiding ugly glue wrinkles on the paper. Any idea how to prevent this? Wing twisting was a problem as well, since I could not put a heavy book on the wing to straighten it without crushing the spar.

I finally built a 40 cm wing out of used A4 paper with a bamboo skewer spar and a cardboard centre piece. Tested it on the stick plane and it flies pretty nicely. I will be trying out painting techniques and also do some durability and impact resistance testing by flying the model around in the limited area I have.

I have also tested painting paper using oil paints and water colors straight from the tube without any water. The finish was dull and would rub off, so I spread a layer of white glue on the surface and it looks good for a finish. This opens up a lot of possibilities with paper. Only problem is weight but how much will weight affect flight times and is it worth it? I read that weight does not affect cruising speed or drag too much at cruising speed, so there is hope.

Thanks for all your help and encouragement - this opens up a huge lot of possibilities with paper.
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Old Apr 13, 2011, 03:24 AM
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Only problem is weight but how much will weight affect flight times and is it worth it?
You'll have to consider weight as your worst enemy...the only factor that may compensate for weight is speed...i.e. a higher energy input, which obviously is a bad thing when using rubber power...

Or, looking the other way...weight watching/reduction is your best friend when designing anything flying...and even more so, as the power supply is limited...

I admit that your attempts to design good performing paper models is an intriguing project, and I follow your efforts with great interest......although I beleive, acheiving results that even get close to the performance of balsa constructions would require a fair portion of 'out of the box' thinking, regarding choice of materials and how to make stable structures using a minimum of the same.....

A great challenge...keep us posted about your progress...
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Old Apr 13, 2011, 04:10 AM
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Gluehand is exactly right: the three most important things are weight, weight and weight...

Try painting it with thinned PVA? I would imagine oil paint is pretty heavy.


As a side point, I made a card mock up of a Radio model I'm doing, to check the proportions. I couldn't resist sticking some nose weight on it and seeing if it flew: I does! 8.5 inch span and 4 grams...

It's pretty darned awful(!) but I thought you might appreciate seeing it Knlever...


Jon
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Old Apr 13, 2011, 05:50 AM
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Sri Lanka
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That is such a nice model - even your mock ups are good! What is the card you are using? Box board? Bristol Board?

4 grammes? As in 4.0 grammes? Or you mean 4 ounces?
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Old Apr 13, 2011, 06:06 AM
Just shut up and build!
Sri Lanka
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I admit that your attempts to design good performing paper models is an intriguing project, and I follow your efforts with great interest..
Well I have in front of me a paper fuselage of a Cessna 180 weighing about 8 g, it is about 30 cm ( 12") long.. My other 1:24 Cessna was built mainly out of card so changing over to 80gsm paper will cut the weight quite a bit. That weighted 35 g, however the paper model should weigh about 25g which is quite comparable to a balsa model of 40 cm (16")

Don't forget that I have built a balsa model - the Guillows ME 109 - see my page here:

I am very excited by the idea of publishing plans for a working rubber powered plane out of waste paper. The cost will come to about US $ 2 - 3, for the prop and the rubber but this can be also made at home. This is the cost of a cheap toy, affordable for the worlds population. It is not the cost only, but the maintenance of the model, foam and balsa models break easily, I know I have used them. Also, paper takes paint very well. The finish that can be achieved is shown by the Chilton model with foam wing but paper fuselage here

My reward would be the creation of thousands of colorful scale paper models built and flown by children all over the world.
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Old Apr 13, 2011, 07:04 AM
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Originally Posted by knlever View Post
As in 4.0 grammes..
But a little more with the noseweight. It's just lifted off the preliminary drawings I'm doing at quarter scale, to check the outlines more than anything.

Card was just what I have lying around. Wings are single thickness - just about thick enough for this size - 8.5"/21.6cm. It's still very heavy for it's size - pistachios (8" span) need to be 2-3 grams to fly well...


Jon
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Old Apr 13, 2011, 06:24 PM
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Sri Lanka
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It's still very heavy for it's size
8" or 20 cm models need to be very light, lighter than I can build out of card. I am steering away from these for now. My plans are to build in the 16" to 20" range, that is 40 cm to 50 cm. At this size, due to the effect of air, heavier models are possible.
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Old Apr 14, 2011, 02:43 AM
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Probably a good idea.

For a 16" span model 20g would be a 'good' weight.
For 20" I would be aiming for about 33g.

The other consideration is how you plan to present the plans: something that can be printed an a standard paper size would help to make them accessible. 16" is the maximum you can get across a sheet of A3 for instance.

Also, you don't want anything too complicated in the construction, because it has to be 'repeatable' by someone with no experience.

It's a good project though, I like it...
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Old Apr 17, 2011, 08:14 PM
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Sri Lanka
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Quote:
For a 16" span model 20g would be a 'good' weight.
For 20" I would be aiming for about 33g.
Jon

Do you happen to have the weights of individual components of a 16" and 20" model? I am trying to compare wing and fuselage weights with my design - balsa versus paper and card.
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Old Apr 18, 2011, 02:58 AM
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As I recall from building the 20" Playboy 'bones' (balsa frame work) were 7 grams, prop and nose assembly probably about 3g, balsa wheels less than a gram... Then there's the wire undercarriage plus covering, wing retaining rubber bands and dowels etc...


Each component as shown is:

Wing - 3g
Tail - 1g
Fuselage - 8g
Nose/prop - 4g (Plastic props are HEAVY)

My scales are not super accurate - only to the nearest gram. They obviously under-read very slightly as the total is 19-20g.

Hope this helps,

Jon
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Old Apr 18, 2011, 03:09 AM
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The 18" span Redwing:

Wing - 6g
Fuselage (minus wheels) -9g
Tail surfaces - 2g


The one below is the 13" Tadpole. It's built very light for indoor flying.
Total weight is 5g, the fuse registers 3g and the wing and tail together 2g.

I need to get a more sensitive scale but that should give you a rough idea...

The figures you quoted - 20g at 16" and 33g at 20" - were what I thought would be a good weight for card construction - well, a fly-able weight anyway. I would imagine that if you go much over those wing loadings, the flight times will drop off a lot.
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Old Apr 19, 2011, 06:42 AM
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Ok here is my little contribution to the party!


I used a cornflakes box (truly awful cardboard) some sellotape, a bit of 80gsm printer paper and a 'proper' prop and nose button. The prop is 6 " and rubber is an old 17" loop of 3/16... I didn't bother lubricating it. It was still under powered and wouldn't climb under 500 turns (though I only tried it in the garden)

Span is 11" and chord 2.5" - this was the maximum I could get out of the box. Aspect ratio is very low (A = 4.4) to maximise wing area. The whole lot is 25g. That's a wingloading of nearly a gram per square inch!

It flies, but the wing is too flexible. The triangular fuselage is very strong - overbuilt in fact. You could use much lighter card for it. The tail surfaces construction works fine at this size. A fair amount of downthrust was needed, cut into the nose of the triangluar section.

Conclusions:
- Cereal box cardboard is rubbish! Very heavy for what it is.
- These models are going to need some serious rubber sizes (ie lots of power) to do any good.
- The fuselage/motor tube and 'nose block' work really well and could take some serious compression! (ie lots of rubber, lots of turns)
- There has to be a way of building a big, light, stiff wing if this is going to work...


The little Raingoose RC mock up flies really well considering. I'm thinking there may be a point at which small works just as well - where low weight and stiffness coincide again? Maybe around 10 inches...

Jon
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Old Apr 19, 2011, 09:25 AM
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So,

I've made a mod:
A plastic drinking straw taped to the LE... aerodynamically... well, less than subtle!

But it worked really well! The wing is comparably very stiff.

The model flew across the garden about 40 feet and started to climb! ... then it hit the fence... that was with about 800 winds. reckon it might do 20 seconds in a field perhaps...

I'm going to have to find some office rubber bands and see what happens with them...
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