TexMechsRobot
Posted a thousand or more times
Looking forward to it!
nnDLG
- Thread starter nerdnic
- Start date
Nic, quick question.
Why did you mount the H-Stab behind the rudder assembly as opposed to on a pylon in front of it? Are you getting a considerable strength gain?
Why did you mount the H-Stab behind the rudder assembly as opposed to on a pylon in front of it? Are you getting a considerable strength gain?
Without going back and reading the previous page(s), why does it matter to be the other way around?
A pylon mounted tail is just the only way I've seen it done on a DLG. Just curious if you were making the change for anything besides ease of build
I'm trying to be a trend setter over here
Joking, I talk about it here: http://forum.flitetest.com/showthread.php?16730-nnDLG&p=302118&viewfull=1#post302118
Basically I considered why it is the way it is on all other DLGs first and then made the change for build simplicity.
Joking, I talk about it here: http://forum.flitetest.com/showthread.php?16730-nnDLG&p=302118&viewfull=1#post302118
Basically I considered why it is the way it is on all other DLGs first and then made the change for build simplicity.
FAI-F1D
Free Flight Indoorist
Without going back and reading the previous page(s), why does it matter to be the other way around?
You want all the yaw damping you can get on one of these. DLG's are a royal pain in the neck in that regard. You also want the ratio of yaw inertia to yaw damping to be as low as you can possibly get it without having large, vulnerable, heavy, daggy vert stab. This is a major issue in terms of weight and balance because any weight on the nose increases yaw inertia, so having anything behind the V stab is a double whammy on hurting yaw damping performance.
All that said, there is a rear stab Japanese design out there that holds a world record for indoor hand launch glider.
Very cool thanks for that info. Here was my thought process from a previous post:
I didn't know about the Japanese design so that's reassuring to know it still works well (the best?)
The nnDLG is shorter coupled than a standard DLG already (mainly for CG reasons) so this might prove to be an issue. I had more than average yaw wiggle on launch in previous versions and I attributed it more to the rigidity of the main boom. Since then I have increased the size of the vstab and came up with an idea in a previous post to test boom rigidity.
To help narrow down where the yaw wiggle is coming from I'm going to run some line from each wing tip back to the tail boom right in front of the vstab. This should prevent the tail from moving much at all and thus increase the main boom rigidity by a large margin. I'll then test without the line and compare the high speed video.
If the wiggle is reduced with the line then I'll know the shorter coupled design isn't really as big a deal as feared. If the yaw is not reduced then I'll have to increase the boom length and/or move the stab positions back.
http://forum.flitetest.com/showthread.php?16730-nnDLG&p=253827&viewfull=1#post253827
nerdnic said:
I didn't know about the Japanese design so that's reassuring to know it still works well (the best?)
The nnDLG is shorter coupled than a standard DLG already (mainly for CG reasons) so this might prove to be an issue. I had more than average yaw wiggle on launch in previous versions and I attributed it more to the rigidity of the main boom. Since then I have increased the size of the vstab and came up with an idea in a previous post to test boom rigidity.
To help narrow down where the yaw wiggle is coming from I'm going to run some line from each wing tip back to the tail boom right in front of the vstab. This should prevent the tail from moving much at all and thus increase the main boom rigidity by a large margin. I'll then test without the line and compare the high speed video.
If the wiggle is reduced with the line then I'll know the shorter coupled design isn't really as big a deal as feared. If the yaw is not reduced then I'll have to increase the boom length and/or move the stab positions back.
http://forum.flitetest.com/showthread.php?16730-nnDLG&p=253827&viewfull=1#post253827
nerdnic said:I've been thinking of using fishing line to creatively solve a different issue.
One of the realities with DLGs is that you run into loss of energy on launch due to flex in the tail boom. This flex is hard on the DLG, robs you of overall height, and can be seen very easily with your eyes. Here is my understanding of how this plays out, but please someone jump in if you know more than me.
The angular momentum generated as you spin causes the tail boom to start to flex away from the pivot point (you), while the nose hooks in and leads the way in the rotation. This also means the tail, as being farther way from you than the nose, is moving faster in relation to the rest of the DLG.
Let's say you're right handed and spinning counter clockwise (left). Your DLG tail boom will be bent to the right as you build up energy. Once you let go, the DLG continues to rotate (yaw) in the direction of your rotation (counter clockwise/left) while at the same time moving in a forward trajectory from your release point. For a moment your DLG will be skidding sideways to the left ever so slightly. As the hstab grabs onto the air it will force a correction toward a 0 yaw path, however, remember the boom flex I brought up? The boom is still bent to the right and as the hstab grabs the air the energy in this bend is released and flings the boom to the left, past 0. Your DLG is now skidding sideways to the right. This process happens 3-4 times before your DLG is on a 0 yaw path.
With your eyes this looks like a wiggle back and forth on launch. Look at this early demo video I shot for a visual example of this happening:
https://www.instagram.com/p/2uLfVXi-Lt
Here's the problem
This wiggle is killing your DLG. It's putting huge amount of extra stress on the boom/wing joint as well as the hstab. It is also preventing you from getting the most height out of your launch. But to be clear, you can't remove all the wiggle. Wiggle exists even on the most expensive of DLGs. So the issue here is the amount of wiggle produced from the nnDLG is its current form.
So how do you fix this? The easy answer is use a better, stiffer boom. But this could mean more weight or potentiality more expensive material. When you're talking about CF or alum arrows, know that they are not all equal. There are a lot of specs on an arrow, a lot a lot. I spent a few hours researching them and I had no idea how complicated archery was. Check out this awesome video by Destin from Smarter Everyday for a quick run down on the interesting points.
Okay back on topic. The most important specs that we should care about is the arrows spine rating and overall weight. The spine, in the most basic terms, is an arrows stiffness. The stiffer the arrow, the less it will flex. I had no idea about this when I bought my first batch of arrows, btw, so this has been a cool learning experience so far. The lower the spine rating number, the better in our case. A spine rating between 250-300 would be ideal, but these arrows cost a bit more than the $3 Walmart special I'm running now. The arrows I bought have no rating advertised so I might test them myself, but I'm guessing they are closer to 500.
CF DLGs have custom booms that taper and are thicker diameter than a CF arrow. This allows the designer to tweak the correct amount of flex and limit the loss of energy as much energy as possible on launch. Going this route isn't an option for this project, though so it's unfortunately a no go.
Okay, so the issue with a flexing boom is energy being expended on stuff other than pushing your DLG higher. And the fix might be a really nice CF arrow ($$) or a real DLG boom, but that's out of the question. So what's next?
My idea
I want to use fishing line to prevent excessive tail flex/wiggle on launch. The thought is you anchor a piece to each wingtip and run it back to the tail boom just in front of the hstab. This will add stress to the wing that wouldn't have been there before, but I'm banking on it being less destructive than the excessive wiggle that already exists. It will limit the tail from moving too far in either direction and help to eliminate flex. And the best part? It will add almost no extra weight or cost (win!) and not impact flight/launch.
Thoughts?
rcspaceflight
creator of virtual planes
Yesterday I was at a local craft store and saw 30" bamboo skewers meant for roasting marshmallows. I don't remember how many was in the pack, but they were much thicker and it was only $1 for the pack.
I didn't buy them because even though they would make for a cheap DLG (hopefully) I don't think it's actually a common material. Amazon sells them, but for $20. Which isn't cheap. Seems pointless to make a cheap DLG that no one else can make for as cheap. Maybe "pointless" is the wrong word.
But I really need to start working on my design. I have a few great ideas flowing in my head that I need to test.
And watching that archer's paradox video really gets me thinking on how stiff to make the fuselage boom.
Oh, and the comment about using fishing line to stiffen up the tail. Great idea, but fishing line stretches. Use MIG welding wire. I know steel is heavy, but a little steel can go a long way. Steel is your friend.
I didn't buy them because even though they would make for a cheap DLG (hopefully) I don't think it's actually a common material. Amazon sells them, but for $20. Which isn't cheap. Seems pointless to make a cheap DLG that no one else can make for as cheap. Maybe "pointless" is the wrong word.
But I really need to start working on my design. I have a few great ideas flowing in my head that I need to test.
And watching that archer's paradox video really gets me thinking on how stiff to make the fuselage boom.
Oh, and the comment about using fishing line to stiffen up the tail. Great idea, but fishing line stretches. Use MIG welding wire. I know steel is heavy, but a little steel can go a long way. Steel is your friend.
rcspaceflight
creator of virtual planes
I kept thinking about the boom rigidity problem and you know Nic, you don't have to use a carbon fiber arrow shaft. You can use an aluminum one. I'm not sure on the weight differences or the rigidity differences, but it might be worth a try.
I've literally built a 30" wingspan "DLG" out of two aluminum arrow shafts put together like a cross and I couldn't break the thing if I tried. It finally broke when I put a motor on it and slammed it into a building. Even then it broke because I had drilled a hole in the two arrows so I could mount a peg thru them to tie them together.
I've literally built a 30" wingspan "DLG" out of two aluminum arrow shafts put together like a cross and I couldn't break the thing if I tried. It finally broke when I put a motor on it and slammed it into a building. Even then it broke because I had drilled a hole in the two arrows so I could mount a peg thru them to tie them together.
Christopher14
Driftin' with the wind...
I did a little research and found that CF is about 40% lighter than aluminium (per unit volume) and 10 times stronger than aluminium (once again per unit volume). Due to the weight difference, personally I'd go for CF, but hey, this is supposed to be an inexpensive glider. You can find aluminuim at Home Depot for fairly cheep. (Depending on how large of a selection of metal stock they have.) Arrow shafts can be found at places like North 40 or Walmart for ~$3 apiece. Can find them on line as well.
The-One-Who-Never-Crashes
TOWN-C Aviation
That was my first thought too. Why would one use aluminum if CF is stronger and lighter? The trouble with both is that they aren't very crash-resistant, and when they break they can't be fixed as easily as wood. Wood tends to be even heavier though, and if you use balsa, it'll bend like crazy... I think that a carbon arrow shaft would be the best option.
rcspaceflight
creator of virtual planes
The issue at hand isn't strength. It's rigidity. Flexibility. CF is lighter and stronger, but I'm unsure which one is stiffer.
I never tried to bend my aluminum arrow shafts that I used to have. I imagine that they're stiffer, which is the issue I'm trying to resolve, but sometimes my imagination is horribly wrong.
edit: I found one of my aluminum arrow shafts and it is flexible. Not sure if it's more, less, or the same as a CF arrow shaft though.
I never tried to bend my aluminum arrow shafts that I used to have. I imagine that they're stiffer, which is the issue I'm trying to resolve, but sometimes my imagination is horribly wrong.
edit: I found one of my aluminum arrow shafts and it is flexible. Not sure if it's more, less, or the same as a CF arrow shaft though.
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The-One-Who-Never-Crashes
TOWN-C Aviation
If you used a woven carbon fiber tube where the fibers are laid down at 45 degree angles to each other, you would get the best torsion strength. An example: http://rcexplorer.se/product/10x10mm-woven-carbon-fiber-arm/
Sky Pirate
New member
Nic, have you gotten a chance to test-fly it? Is V5 looking like it's going to be the winner? I keep seeing your Instagram posts and am getting excited!
