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Mar 18, 2009, 01:55 PM
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Ironbowser's Avatar
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Help needed on scratchbuilt catapult glider


hi, i am trying to make a slingshot glider using a PZ brick. ive gotten most of it done, but I just need some tips from more experienced R/C gliders personnel.

any help would be appreciated

current AUW is 12 g

51 cm span

all balsa

shooting for under 22 grams
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Mar 18, 2009, 02:45 PM
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One thing I see is the grain of the vertical stab. It will break easily the direction you have it. It should be vertical.

What are you planning for the wing?

I sometimes build HLG and CLG models. They're fun. Because of the high-speed launch, they have to be perfectly straight.
Mar 18, 2009, 02:48 PM
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Just re-read your post. You say ALL balsa?

A balsa boom will break, maybe on the first flight. Basswood is best, spruce will do. Of course, a carbon fiber tube works well.
Mar 18, 2009, 03:02 PM
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the v-stab is actually strong. it is mounted on a thin basswood frame, as is the h-stab.
Mar 18, 2009, 09:23 PM
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Quote:
Originally Posted by Ironbowser
the v-stab is actually strong. it is mounted on a thin basswood frame, as is the h-stab.
It may be 'strong' but structurally it makes no sense. Wood has it's most strength in the direction of the grain, parallel if you will.
You could save alot of weight by eleminating the balsa sheeting in the tail and using tissue/mylar etc instead.

I would make the wings with a series of ribs and two cap strips with LE/TE sticks nothing fancy. Try and see if you can find some small carbon strips to use for the build.

If you don't have carbon for the fuse, use a hardwoon as suggested. For small models like this I have created strong sticks by laminating hardwood strips on either side of a balsa stick and wrapping with CF tow.
Last edited by tommyeflight89; Mar 18, 2009 at 09:30 PM.
Mar 18, 2009, 11:33 PM
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As a matter of fact, I did this very thing. Here is my build thread:

https://www.rcgroups.com/forums/show...light=catapult

I would suggest that you find a CLG design you like and use that as a basis. Also, if you want any decent chance of success buy contest balsa. 4 lbs/ft^3 if possible.
Mar 19, 2009, 08:55 AM
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updates!


shortened, CF fuse, small tail fins, and 1 gram lighter.
Mar 19, 2009, 07:40 PM
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What are the dimensions on the DF tube? ID and OD?
Mar 19, 2009, 07:42 PM
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Id 1/16 Od 1/8
Mar 19, 2009, 10:41 PM
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Good luck with the project, I think there's a lot of potential for micro gliders, catapult launch is a good way to go at it.

Theres lots of forces applied to the wings and tail during a fast launch like from a catapult, you may find you need the wing to be reinforced at all the joints. If it seems squirrely in flight, a longer CF tube might help smooth it out a bit too.

What will you use for the catapult itself?

Nick
Mar 20, 2009, 12:14 AM
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You can never have too little camber but you can easily have too much.

There are several things modelers have learned over the years about violent or high-stress launches like hand launched gliders experience and presumably would be similar for a catapult glider:

1. The fuselage was best made from very hard balsawood (12+ lbs per cubic foot), not spruce, pine nor basswood. This is because balsa tends NOT to flex nor vibrate on launch where the others will. Flex and vibration are bad because they change the trim of the tail surfaces in unpredictable ways. I haven't worked with CF enough to say whether it would be good for a fuselage.

2. Undercamber is probably not desirable. Why? A glider that is catapult launched will experience a wide range of speeds: Very fast on launch (like 50-60 mph) and as slow a glide as possible (like 10-12 mph). If undercamber is used, I think it will cause a lot of drag during the fast launch phase and limit your final altitude. Therefore such a glider would probably best have a wing that's no thicker than, say, 1/4" and should have a flat bottom. This will help with controllability too because if you have a thick and undercambered wing you will have to deal with different trim settings such as "down elevator on launch and up elevator in glide"

Marlin
Mar 20, 2009, 12:31 AM
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One thing I experienced with RC CLGs is that it is difficult to transition from launch to RC control. I used Tx straps, obviously, but the launch is only like a second or two, and I have to fumble for the Tx to flip a toggle switch to thermal model. I did a lawn dart one time, because I wasn't quick enough.

If you have a helper, it would help a great deal.
Mar 20, 2009, 11:09 AM
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I couldnt agree more with that, Mr. Mulligan. I just law-darted mine yesterday due to the exact reason you mentioned. Snapped the pod off the fuse, threw the RX and lipo a few feet...spectacular crash, but could've been avoided if I had both hands on the TX. Maybe a foot release of some sort like the big gliders use on their winches.


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