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kukailimoku
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« on: June 12, 2017, 02:33:40 PM »

The goal is to make something tiny and light for dune soaring on gentle tradewind days. I have a 2-meter ship for the big wind on the big hills but nothing for the beach. I've taken a set of Stylus plans down from 40" to 16". I'll need to make some tweaks to the fuselage to make room for radio gear but should be able to reproduce the wing exactly. A couple of questions for the smaller-is-good crowd:

1- Increasing the height of the fuse would make the wing about 3/8" higher above the tail than it is now. Do I care?
2- Any pitfalls lurking that I can't think of 'cause I never made anything this small that was actually meant to fly?

Thanks all.

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OZPAF
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« Reply #1 on: June 12, 2017, 08:07:31 PM »

If you haven't built this yet I would suggest a couple of ideas.

First I would consider increasing the tail area over that of the larger model - say by 10% or so.

Second I would use a much thinner airfoil say 8% at the root and down to 4-5% at the wing tip.

Otherwise it should be ok but use light gear.

as an alternative you could consider either building mark Drela's Apogee full size or reduced. At 30- 36" WS it would be a very good light condition flyer and has much better airfoils

http://www.charlesriverrc.org/articles/apogeehlg/markdrela_apogeehlg.htm#Introduction

Hope this gives you some ideas.


John
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kukailimoku
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« Reply #2 on: June 12, 2017, 09:11:13 PM »

I've increased the elevator a bit along those lines, probably a little less than 10% but still significant. It was hard to imagine it having any usefulness beyond just staying level without a tweak.

As to the airfoil, the eppler 214 seems reasonably thin without sacrificing some of the lifting needs of scratching on the dunes. I expect to do a lot of very slow work on those super-light days and I've had good results with fatter airfoils on other aircraft without losing a reasonable ability to penetrated. Was there some specific limitation you had in mind with that thought?

Re: the Apogee, nice bird! One of the reasons I'm going with the Stylus is its similarity to the Airtronics Oly 650, flat center section and dihedral at the tips, none at the center joint. That's a much more challenging scale-down project and this first tiny build is going to be a learning experience to eventually build the tiny Oly. It would be the sister ship to my current 2-meter version that's fully sheeted top-and-bottom and heavy as nails for days like this (it actually slows down pretty well but can also deal with 30+ mph full of ballast with nary a twist to the wing):

https://youtu.be/RWZM2IYlSrw
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Yak 52
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« Reply #3 on: June 13, 2017, 04:36:41 AM »

As to the airfoil, the eppler 214 seems reasonably thin without sacrificing some of the lifting needs of scratching on the dunes. I expect to do a lot of very slow work on those super-light days and I've had good results with fatter airfoils on other aircraft without losing a reasonable ability to penetrated. Was there some specific limitation you had in mind with that thought?

John is absolutely right on the thickness - even at 40" the Eppler 214 is way too thick for a modern glider. The AG03 of the Apogee would do much better at 16" span you'd probably want thinner than that still.

Much depends on the actual conditions you will be sloping in ie wind speed and how much lift you get for it. I would suggest that for scratching on the dunes will be a real test of a models agility.

Cool little project though Smiley

I would suggest having a look at some of the micro DLG designs that have been done. Something like that with the ability to add a bit of ballast would work well. The link below is my 24" DLG based on the Ember brick: http://www.peterboroughmfc.org/membersmodels2015/06-Jon-Glider.htm
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OZPAF
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« Reply #4 on: June 13, 2017, 05:07:15 AM »

K I have had a few more thoughts about your scaled down glider and I'm inclined to feel that 16" WS will be too small to be really usable.

I think 24-30" would be much better and Yak 52 has made a couple of very interesting small  gliders similar to that on the link.

Ah E214! It has too much thickness as YAK mentioned and I considered it many years(don't ask) ago for a thermal glider and rejected it as I felt it had too much camber and would need too much weight to perform. ( this was based on my assessment and the results of another fellow's model - which suffered bad flow separation as indicated by the loud rumble it made when flying past, due to a wing loading which was way too low!

It depends what you want to do with your small model but if you want a small model for light slope lift and a fair amount of penetration at that low wing loading - then Drela's airfoils as per the Apogee are a good place to start.

However from a practical point of view I would definitely consider building a model similar to that YAK has on his link - at the same size or only slightly smaller -rugged and cheap.

John


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Hepcat
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« Reply #5 on: June 13, 2017, 06:56:47 AM »

Yak and Ozpaf are experts in this field and I endorse everything they say, however as just an interested bystander I wonder about your first statement that you want something small and light to fly on the dunes in gentle winds.  What matters in aeroplanes is not size or weight in themselves but only when combined together to give wiing loading.  So on that basis you need a model that is large and light.

Another thing to remember when making model aeroplanes is The Reynolds Number because if the RN is high the airflow is smoother and gives less drag. Big aeroplanes flying at high speed have a high RN and obviously model aeroplanes which are small and fly at low speed do not. Fortunately however over many years careful theoretical and practical testing has shewn that spcially shaped and usually thin aerofoils can be made to work reasonably well at the RNs encountered by models.
Sorry, I should have stopped after the first eight words.
John
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kukailimoku
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« Reply #6 on: June 13, 2017, 07:53:19 PM »

I've done large and light (and large and heavy for that matter!) but this is a unique set of circumstances. The soarable areas are small in both height and width with not enough room to even attempt a turn with a 2-meter. Many of the steeper sections have obstacles close enough to the edge that there's no place to tuck part of a large wing over the back to stay in the lift when scratching so tiny seems to be the logical move. I've got all of the micro gear needed and am going as light as I can. I'm not terribly concerned about penetration as I'm not planning on flying in anything but light and smooth.

Is 16" too small? Beats me but it's 1) going to be really fun to try to make it work and 2) I've got a whole bunch of extra balsa lying around to experiment with a whole lot of different airfoils and wing configurations. And seriously, wouldn't it be cool to get the little bugger cruising the beach? I know from personal experience that a thick line of trees can generate an amazing amount of lift and it would be a great challenge to be able to bump up into the "ridges" of ironwood trees that grow in many spots.

Stay tuned, I promise evidence of either the success or carnage.

Edit: an additional question for those in the know, any opinions on the Oly 650 airfoil in a mini version? I think it was e205, pretty much a flat bottom.
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Yak 52
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« Reply #7 on: June 14, 2017, 05:15:39 AM »

Small is not necessarily more manoeuvrable. The turn radius of a model at a given bank angle is directly related to it's speed - ie it's wing loading and airfoil lift coefficient.

So a large light model can have a tighter turning ability than a smaller heavily loaded one.

So for example a model with:

Span              16"
Wing area       36.6in2
Aspect ratio     7
Weight            40g

Will have a turn radius of 13 metres at 30 degrees bank angle.

Compare that with my Skidoo (http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=21923.0)

Span              59"
Wing area       330in2
Aspect ratio     10.5
Weight            200g

At 30 degrees bank angle the Skidoo turn radius is 5.6m Smiley

So Hepcat's comments on large and light are accurate.

I'm not saying you need to go to 1.5m - just a size that you can keep the wing loading right down, maybe around 30" with micro gear. In fact the full size version of this model may turn tighter if built relatively lighter than the 40% version.

I woulds suggest that ailerons are the way to go also. Perhaps aileron/elevator if you have only 2 channels. Rudder only models are generally less manoeuvrable or at least less precise.

The other way to improve turn radius is to improve the airfoil - a higher lift section allows a tighter turn. Eppler E205 is still way too thick I'm afraid. At 16" you need to be looking at 5-6% thick max. BE5017FB would be a good starting point (it has a flat bottom) or Mark Drela's AG19 and Gerald Taylor's Polyhot if you can build accurately with undercamber. I would consider thin sheet wings as found on a catapult glider or hand launch but with ailerons.

Definitely an interesting project and do get experimenting Smiley I have had it in mind to do a micro sloper like this myself (I only have micro slopes  Roll Eyes) so you are getting my creative juices flowing Smiley
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kukailimoku
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« Reply #8 on: June 14, 2017, 01:20:34 PM »

Interesting math but I'd sneak in the headwind factor as well. Assuming the min sink speed of the little beasty is considerable lower than the large one then the turning radius doesn't really figure in as the ground speed is so low during the maneuver.

Stay tuned...
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Yak 52
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« Reply #9 on: June 14, 2017, 07:09:59 PM »

Actually in the above case the 16" would have a speed of 8m/s and the Skidoo about 5m/s. Sink rate would be rather worse on the small one too.

But you are right in a way - the wingloading is more to do with what conditions you will encounter ie the wind speed and how much lift there is on a particular slope. But in scratching in light lift and light wind you need good foils and a light wingloading (which is often easier to achieve in a bigger model.)

As I said I would be more inclined to focus on manoeuvrability. Look forward to hear how this progresses. You've inspired me to draw up plans for a 20" micro DLG type sloper anyway Smiley
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kukailimoku
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« Reply #10 on: June 14, 2017, 07:18:01 PM »

Maneuverability is definitely high on the list as it is with all of the sailplanes I've built over the years. The range of motion and surface area of control surfaces are something I always increase. More is good (up to a point of course). My current Oly 650 rudder and elevator have a range WAY beyond what's on the plan. Full throw during the pull-up after a long dive is...violent.
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Yak 52
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« Reply #11 on: June 16, 2017, 02:07:09 AM »

Well I had a go Smiley designed and built this yesterday...

Wing span is 15.5", weight 16.5g (maybe my 40g estimate was a bit unfair!) Control is R/E with two cheap linear servos. The rx is a banggood 4ch with the pins removed (servos and plug directly soldered.) I've had these flying over 400m with no range issues  Grin

Flies across the garden nicely but one of the servos is sticky so it may not be up to much. I think a rotary servo for ailerons instead may make a better sloper as I said. And considering you could increase the wingloading (for more wind) it would be possible to go quite a bit smaller!

Jon
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kukailimoku
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« Reply #12 on: June 16, 2017, 01:47:04 PM »

Nice!

I'm hoping to finish the Stylus this weekend. Pics to follow, not sure about flying as the tradewinds might be a bit brisk.
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OZPAF
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« Reply #13 on: June 16, 2017, 07:36:33 PM »

That's neat and quick Jon. 16.5g that' good going.

John
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kukailimoku
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« Reply #14 on: June 19, 2017, 01:38:40 PM »

After two weekends of tinkering I've abandoned the Stylus. I just don't have the right toys and wood inventory to make it work and buying a bunch of balsa would make it way too expensive.

Back to plan A, an Oly 650 at 46% of the original plans that works out to a 36" span. An Oly 299?

It's a wider than I'd really like for the type of flying that I'm looking at but I'll go ultra-light and see how it works out!
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kukailimoku
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« Reply #15 on: July 03, 2017, 01:49:13 PM »

Finally got a little workshop time this weekend and things are moving along quite nicely. Tail surfaces and the wing center section are complete and the tips are well on their way. No spruce yet (the fuse will be getting a little), I raided the kitchen instead for the wing spars. Shaved chopsticks on top, bamboo skewers below. Weighs dang near nothin' and seems pretty sound.

A HUGE mahalo to this forum for the ribs. I've been the worst scratch builder of ribs since dinosaurs ruled the earth but the techniques discussed in here (and the purchase of a couple small files) made the world a better place. Rectangle on the top for the chopsticks and rounded on the bottom for the skewers, it actually fit like it was supposed to!
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kukailimoku
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« Reply #16 on: July 07, 2017, 01:58:10 PM »

Quick update and a question for the experts. The wing is complete, tips ready for covering and attaching. Many a chopstick and bamboo skewer gave of themselves to keep it crazy light without losing strength. I've got the thin balsa fuse sides cut but am looking at options for firming them up with ply in the forward and middle sections (and expect to take several iterations to get it right). Is there a preference for balancing strength and weight in the removal of ply, circles or squares (per the very crude drawing)? This is the first bird that I've tried to make as light as possible and am curious about which would be the best. Thanks!
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OZPAF
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« Reply #17 on: July 09, 2017, 04:49:01 AM »

Well my advice would be to build the fuse sides out of strip instead of sheet if you want to keep it light. Even strips of 3/32 may be lighter for your application of a light slope soarer.

however if you use sheet sides (1/32?) use circular holes in preference and some vertical 1/8"x 1/32' vertical strips in between the holes.

If you are trying to save that much weight I wouldn't use ply in the centre - front section - just use the full sheet sides and back them up with a lamination cross grain balsa from the TE of the wing to the nose.

John
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kukailimoku
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« Reply #18 on: July 10, 2017, 01:42:29 PM »

Thanks for the input, not a lot of progress over the weekend but the wing is ready to roll. I laid it down next to its big brother...
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kukailimoku
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« Reply #19 on: July 17, 2017, 01:33:44 PM »

Was able to spend some time in the shop, I'm down to the always entertaining hook-up-the-controls and trimming stage for next weekend. It's starting look very sailplane-like. I built a new fuse after carving out sections of the side ply (not as artistic as the regulars in here as I did it with spade bits, a coping saw and a file but it worked). The only double was balsa for the servos, the bottom is thin balsa and the top behind the wing mounts is just stringers from side to side so it turned out pretty light. Hoping for trim flights and a bit of soaring next week, weather permitting!

For sizing reference, there's one pic next to a ballpoint pen and another with its 2M big brother.
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OZPAF
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« Reply #20 on: July 17, 2017, 07:52:22 PM »

It looks neat - good luck with its maiden flights.

John
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lincoln
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« Reply #21 on: July 21, 2017, 02:32:48 AM »

Maneuverability is definitely high on the list as it is with all of the sailplanes I've built over the years. The range of motion and surface area of control surfaces are something I always increase. More is good (up to a point of course). My current Oly 650 rudder and elevator have a range WAY beyond what's on the plan. Full throw during the pull-up after a long dive is...violent.
As the span goes down, so does the response time. I once tried an Apogee with ailerons. For a minute or so, it was all I could do to keep it right side up!

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kukailimoku
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« Reply #22 on: July 21, 2017, 01:22:33 PM »

Interesting thought and I hope 1) it truly is a little squirrelly to keep the fun factor up and 2) that it's not too crazy and becomes a casualty. Sand is soft, that should help!
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kukailimoku
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« Reply #23 on: July 24, 2017, 02:00:39 PM »

Now that I'm getting to where the rubber meets the road and the controls are going in I'm discovering that there's a real challenge with pushrod angles and the like. Some things just don't scale down so nicely! And my go-to clear tape hinge technique is useless on the rudder because it's so small. Not enough surface area to keep things solid. Off to buy some hinges.

Live and learn, at least I'm "enjoying" the education (and my workshop is detached from the house so my wife can't hear my colorful commentary.
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kukailimoku
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« Reply #24 on: July 26, 2017, 02:00:24 PM »

I'd love opinions on this as it seems like it would 1) work pretty well and 2) simplify things a lot. The tail configuration of the Oly has always been a pain in the okole as the clearances for the control surfaces and horns has always been a challenge. Scaling down makes it worse by a huge amount so I'm looking to make a change. It seems that sliding the vertical stabilizer and rudder (with a little extra control surface area) forward would make the whole thing simpler and save a little nose weight. Sliding the horizontal stabilizer forward seems like it would save even more. I've attached examples of both, any thoughts?
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