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Old Feb 12, 2013, 09:30 PM
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appa609's Avatar
Ontario, Canada.
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Here's a prelim model. I based it half off of recommendation, half on intuition. It comes out to 56 in^2 (102.4 cm^2) lifting area, about 3.3g
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Last edited by appa609; Feb 12, 2013 at 09:39 PM.
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Old Feb 13, 2013, 01:41 AM
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Malaysia, Selangor, Kajang
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Interesting design. I would not have thought of having such a thick chord. Be careful what advice you give

I would add more dihedral though. With so little dihedral you'll find that it will fly well in a straight line but if you try to trim for circles it will spiral to the ground. 9 to 11 degrees is a good rule of thumb for free flight models. Alternatively you can go polyhedral as long as the average dihedral is around the 9 to 11 degree range.
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Old Feb 13, 2013, 03:37 AM
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The 'Wack, BC, Canada
Joined Oct 2002
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It is quite possible to go too far in achieving the greatest possible wing loading. And you've done it with your design as shown. The fuselage and tail won't be long enough or big enough to most effectively deal with the leverage of the large wing chord.

If the flying site has around a 10 meter ceiling that qualifies as a "low ceiling" as far as indoor hand or catapult launched gliders would go. I would do a Google or other engine search for "indoor hand launch glider". You'll soon see a variety of designs and find plans for such things. You'll note that they do not use delicate built up fuselage setups like you show. Such things are heavier than need be and will soon prove to be too delicate.
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Old Feb 13, 2013, 05:51 AM
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appa609's Avatar
Ontario, Canada.
Joined Aug 2008
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So, a Single stick for the fuse, reduce average chord to 3 inches, add two inches in front, and increase dihedral to 10 degrees. My searches also returned arched leading edges. Is this really beneficial, or would I lose too much wing area?
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Old Feb 13, 2013, 09:33 AM
Balsa Flies Better!
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Joined Oct 2000
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In terms of airfoils- you might want to check this thread-I suspect that what you've got is too thick.

http://www.rcgroups.com/forums/showthread.php?t=1792181

One of the issues you're going to run into is that your glider is smaller than a typical HLG- which necessitates a lower aspect ratio as you've surmised.

The required frontal area of the fuselage suggests that a pylon wing mount may be in order.

Also- agree with Bruce- you're going to need a lot more tail moment.

You may also want to look at the AJ Interceptor or other folding wing gliders for some inspiration- Guillows also produced some of these. The downside is the weight and reliability of the mechanism- but if you've got to minimize drag on the ascent- it's another approach.

Sam
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Old Feb 13, 2013, 07:08 PM
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Quote:
Originally Posted by Megowcoupe View Post
In terms of airfoils- you might want to check this thread-I suspect that what you've got is too thick.
His airfoil has slightly less than 3% thickness. Which is plenty thin enough for this scale. The ideal thickness for 10 to 12 inch span seems to be around 5% to 6% so actually his airfoil is a bit too thin.
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Old Feb 13, 2013, 07:20 PM
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Originally Posted by appa609 View Post
So, a Single stick for the fuse, reduce average chord to 3 inches, add two inches in front, and increase dihedral to 10 degrees. My searches also returned arched leading edges. Is this really beneficial, or would I lose too much wing area?
The reason for the long nose is to avoid adding too much weight to balance the plane at the right CG. The further out the weight, the less of it is needed. But don't make it too long because that would then affect spiral stability. Placing the wing 1/3 of the way down the stick is a good starting point.

Obviously, if you've been building RC and have been looking at real airplanes you'll notice that the 1/3 suggestion makes the nose look too long. But RC planes and real planes have something that free flight gliders don't: a heavy motor up front.

With regards to the arched leading edge it's mostly a style thing. When you build a competition glider you'll spend a lot of time flying it to get it to trim right. It's only fair to yourself to have something pretty to look at.

Aerodynamically the arched leading edge does two things. The first is that it adds surface area in front of the wing. This increases drag. The second is that it effectively act as a swept leading edge. Wing sweep increases dihedral. The rule of thumb is that every 6 degrees of sweep equals 1 degree of dihedral.

For models like the Swepette, the increase in surface area and dihedral is too small to matter much. Still, it does bother some people when their objective is speed. Which is why pylon racers usually have very straight wings with the taper on the trailing edge rather than the leading edge.
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Old Feb 13, 2013, 08:30 PM
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So... Say I have a glider ready, and I throw it hard at the ceiling. It streaks up, slows down... Then how does it get into a stable descent? Every scenario I can imagine involve either a loop around, losing much altitude, or stall. Is it a rudder turn?
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Old Feb 13, 2013, 08:43 PM
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Originally Posted by appa609 View Post
Is it a rudder turn?
Yes, it's a rudder turn. Or variations thereof.

Any recovery in the pitch direction will lose huge amounts of altitude. Just like you mentioned - either a loop or a stall. The trick is to trim it to recover in the yaw direction. But it's delicate. Too much and your glider will spiral on launch.

This is the first video I saw of a "proper" competition glider. Almost 10 years ago now when Youtube just started (before they were Google). It was what made me realize that the things I was building was crap and that I can potentially build much better planes.

Hand launch glider, 1994 (1 min 1 sec)
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Old Feb 13, 2013, 08:50 PM
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Here's another video of a more modern F1N glider (the previous video was an AMA class which has smaller wing span limitation):

F1N Ludbreg 2011 (1 min 12 sec)


Notice that the hard throw doesn't cause the plane to loop. This is another clue - the plane is trimmed with almost no up elevator! Keeping the nose up is done mostly by the correct placement of CG which can be as far back as 65% (instead of the usual 25%-33% for RC planes).

I should warn you though. Building planes like these take lots of trimming and your first will most likely frustrate you until you get it flying right. But just look at the videos for what the payoff will be: it's looks like magic!
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Old Feb 13, 2013, 09:02 PM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
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I don't know how you're measuring thickness off that- what I was looking at was basically a flat sheet with some camber in there. My guess- and it's only that- is that a bit less camber might work better, but hey- I'd probably put in a leading edge and thin the TE as well.

Sam
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Old Feb 13, 2013, 09:32 PM
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Quote:
Originally Posted by Megowcoupe View Post
I don't know how you're measuring thickness off that- what I was looking at was basically a flat sheet with some camber in there. My guess- and it's only that- is that a bit less camber might work better, but hey- I'd probably put in a leading edge and thin the TE as well.
Usually for bottom-uncovered wings airfoil thickness = camber height. Now that you mention the flat sheet I realize my calculations are a bit off:

1/16" sheet + 1/8" camber / 1.4" chord = 4.2% thickness - still thinner than optimum.

So actually a bit more camber might work better.
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Old Feb 13, 2013, 09:34 PM
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The 'Wack, BC, Canada
Joined Oct 2002
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Appa, read the write up below on how to trim an outdoor model. The difference for low ceiling indoor gliders is that you want the launch to recover and the model to turn to the same side as the launch instead of "S" turning to the opposite like with high ceiling or outdoor models. And I'll stress what is mentioned in the writeup that you must get the model to be VERY CLOSE to neutral stability. You want the model to be only just barely stable enough that it'll still recover from a slight stall in the glide. This marginal pitch stability is a major factor in achieving a good launch to glide transition.

Oh, and where I talk about the setup for the left hand circle in terms of stab tilt and dihedral joint skewing reverse them to aim for a right hand circle if you are a right'ie.

Also generally low ceiling launches are far more of an over the shoulder sort of throw like a spear rather than a side arm throw. Side arm is fine for the "S" launch outdoor style but it'll push the "launch right, glide right" style model into the floor quickern' you can blink. It needs to be more of an over the shoulder/slight pointed to the right sort of launch. You want the tail of the model to be pretty much touching your shoulder blade near your spine just before you push it up and out into a 70 or so degree launch.

And obviously you can ignore the references to wind and thermals for indoor flying....

Quote:
Building and Trimming Free Flight Hand and Catapult Launched Gliders.

The flight trimming for hand launch FF gliders starts on the building board.

If you're right handed you want the model to circle to the left in the glide. To achieve that with only adjustments to the fin means that there will be much too much turn in the launch. To get around this it's normal to use some stabilizer tilt to provide some left turn. This means you want the left side stab tip to be about 1/4 inch high compared to the center line. Reverse everything if you're left handed and will be going for right hand circles.

To aid in smoother recoveries from stalls you want a little bit of washin on the left hand wing tip. I used to warp some into the wood but now I "toe in" the outer dihedral break on the left side by 3/32 inch over a 4 inch chord. Tapered wings with smaller chords at the tip break would be angled appropriately less. I know this sounds odd since the washin at the tip is fighting the rudder. But trust me. Having the model set up so the turn fights the washin is what aids in avoiding those frustrating never ending series of stalls that never seem to damp out. Models set up with a touch of washout will settle back down a lot nicer and with a more rearward CG than "straight" models. If the wings are simple V dihedral then make the center line joint skew to the right at the leading edge by 1/16 inch over 4 inches of chord, or some scaled down amount for smaller chords, so you can glue the wing on straight across but still have a touch of washin on the left side and a touch of washout on the right.

Now to the flying field.

To avoid the looping over and hitting your self in the back of the head or having it gyrate around uselessly and stalling a few times before settling into a glide you need to trim the model to find the most rearward CG location that will still give you just a little bit of pitch stability. I know it seems odd but the best way to avoid these typical problems is to trim the model so that it is LESS strongly pitch stable. To fly their best and trim out the most easily it's pretty common that free flight models need to be set up with their balance point really far back and on the edge of being almost but not quite being unstable. Models set up this way handle the dynamics of the powered or climb portions of their flight with more control, a smoother overall climb pattern, gain more altitude and have a smoother transition to the glide..

To test this you're going to do two types of test gliding. The first is to adjust the stab warps by breathing on the balsa and curling it slightly to achieve a nice slow and floaty glide with no signs of any stalling. The second is a "pitch stability" test toss where you push it slightly faster and slightly down towards a point about 25 to 30 feet ahead. This faster test glide should show a mild but still noticable pitch up to a mild stall. Don't worry that it won't recover to a glide from such a test glide. You're just looking to be sure that it's still stable. Keep moving the CG back a bit at a time and retrim for a nice stable glide then do the stability test. When it won't raise the nose up to a mild stall before it hits the ground when aimed about 25 feet away you know you went too far. So move the balance back forward to the last test point and retrim the stab warping to get a nice slow and stable glide. You should also be looking for signs that the model is turning to the left during this test gliding. It won't turn much but you should see definite signs of it beginning to come around to the left. Add some left fin warp to where you're seeing that it's beginning to set up for such a turn.

Now it's time for the launching tests. Start with a fairly shallow angle and half power throws or catapult releases. For these you'll want more of an over the shoulder toss or a level wings release since it's not going to go far enough "under power" to really arc around. So more of a level wings and nose up at around 30 degrees is suitable. What you're looking for during this is that the rudder isn't so effective that it's over rolling the nose and making the model roll too far in the climb. Ideally you want the model to roll and turn in the climb about a third to quarter turn and be left hanging with the left wing low and almost no airspeed. The glider should then roll down into the glide and continue away. Play with the fin warping until you get this. In the longer glide that comes from this higher launch watch for signs of stalling or for the turn to get too tight and want to spiral in with a "death spiral". The glide turn should be roughly 30 to 50 feet in size. I like to aim for around 30 to 40 myself but remember that you're playing off the stab tilt and fin where the fin adjustment is used for the climb. And the climb takes priority in the fin adjustments.

If you're getting promising results from the low power testing then it's time to wind up more. You'll need to play with just where you and the model want to hold your arm by altering where the throw occurs between the "over the shoulder" and "sidearm" throwing. For a catapult model you're looking at full on side arm pull to stretch the rubber the most and you want to angle and bank the model to around 45 degrees up and roughly 45 degrees bank. Alter this as needed to where the model runs out of inertia up top with the correct attitude for the transition. Models with a strong rolling action in the climb can be thrown or released banked strongly to the "sidearm" position. Others with less rolling action in the climb will need something up more towards "over the shoulder". Some minor tweaking of the fin can add to or take away some of the rolling/turning in the climb. A little goes a LONG way so go easy.

When it's all working well you'll see the model arc up in an S curve to the right then roll over to the left. The model will end up somewhere overhead and over to the left havng performed somewhere around a 120 to 200 degree turn during the climb. Ideally it'll end up slightly nose high and strongly banked with left wing low. As the speed bleeds off it should transition smoothly into the glide with little or no stalling by rolling down into the low wing and leveling up and just go into a nice glide circle.

From there if you did your job right and found it some bouyant air the model will begin circling upwards. When you get a little better at picking the air you'll understand why so many folks are adding DT systems to chuck gliders.
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Old Feb 13, 2013, 09:38 PM
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Originally Posted by BMatthews View Post
Oh, and where I talk about the setup for the left hand circle in terms of stab tilt and dihedral joint skewing reverse them to aim for a right hand circle if you are a right'ie.

Also generally low ceiling launches are far more of an over the shoulder sort of throw like a spear rather than a side arm throw. Side arm is fine for the "S" launch outdoor style but it'll push the "launch right, glide right" style model into the floor quickern' you can blink. It needs to be more of an over the shoulder/slight pointed to the right sort of launch.
The rules for his contest requires catapult launch. So left or right circles is more a matter of taste since rubber doesn't have a handedness
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Old Feb 13, 2013, 10:17 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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Yer right. Still the same 70'ish degree nose up angle with perhaps 5 degrees to the right is how it would be held for the rubber band release. Left or right would be an option for the rubber but the model would still need to be built to suit the one direction or the other.
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