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Swapable Rubber pod

jhitesma

Some guy in the desert
Mentor
#1
Before I finally got into RC planes and found FT I had always built planes - just never had the money to go RC. So I built a lot of hand toss and rubber powered free flight kits as a kid. Everything from the little balsa gliders and rubber planes to stick and tissue planes that could have been RC if I'd had the money to do it but I was just a kid so I built them for free flight.

Never got more than one or two flights out of the big ones because I got sick of fixing them before I'd get them well trimmed...but I had a blast with the cheap balsa rubber planes.

My daughter has had a huge interest in planes and I surely don't mind encouraging it (heck it's one of the reasons I finally got into RC planes!) She loves hand launched gliders, and carrying my planes back after crashes. So I tried to find her some balsa gliders as a next step up. Can't find them in town and don't want to pay shipping to order them. I did find HK had some cheap foam and balsa planes and tossed one into an order - it's actually kind of nice for the price...but I still prefer the old balsa ones.


Anyway, yesterday I saw in the ask chad thread a mention of the glider conversion they did on the flyer airframe in this episode (http://www.youtube.com/watch?v=7Q9AP7aK708). And the idea of adding a bit of propulsion with rubber came right to my mind.


I'd already tried making some balsa gliders from scratch but didn't have any appropriate props, and couldn't rig up a shaft that ran true enough to be useful. But I had some time yesterday and thought I'd give it a try and testing some ideas.

First I built up a swapable pod with a reinforced rear section to support the pull of the rubber. I just used an old hotel key card that I cut up, I left some legs on it that would go into the mounting holes since I know the front of those tabs get smashed up a bit in crashes and I expect this pod to have quite a few hits :)
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Next I glued a block of 4 or 5 pieces of foam board inside the firewall and then glued a 3mm prop saver onto the front of that:
20130629_161212.jpg

Once the glue had cooled I carefully used a T pin to try and make a straight hole through the center:
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And then opened the hole up some more using some coat hanger wire:
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I only have 8x4 props on hand...I think this may work better with a larger lower pitch prop...but am interested in what those with more experience would say. I cut a ramp into the front of the prop hub so the shaft I made could push but would let it freewheel in the other direction. I actually goofed and cut it the wrong way at first and then had to cut another and go back and remove the original. :
20130629_163118.jpg

I also used a straw off a capri sun to support the shaft. I just pushed it in through the foam and cut it off when it hit the back of the prop saver. I then cut a second piece to go in front of the prop saver to push the prop out a bit. The shaft is just a straight piece of wire which I bent an eyehook into the end of and then put through the straw. Once it was through I put the other bit of straw on followed by the prop and then bent it over and trimmed it:
20130629_163040.jpg

It worked...but very poorly. With just one "File band" (the longest rubber I had on hand) it moves some air and will run for 15 seconds. But I don't think it would actually help a plane as heavy as a FT flyer. And my shaft was not very straight causing a LOT of vibration. I was able to massage the shaft so it ran fairly smooth, and with 4 rubber bands it pulls almost as hard as my 24g motor! But with 4 bands I wasn't even able to get 100 twists in and it runs for less than 10 seconds. After a bit of experimentation 2 bands seems like the best compromise that makes noticable thrust but also allows for at least 150 twists and about a 15 second runtime.

(This was the initial test with one band.)

However when I went to attach it to a Flyer airframe I ran into some issues. First the servos are in the way of the bands. Not a big deal since I planned on making a servoless fixed version for this. Second, the gift card reinforcements hold the pull of the rubber band no problem...but they keep the pod from fitting well on a flyer. It goes on with some pushing...but it's far from optimal. And finally...I can't get the bands over the rear peg...they're just a bit too short and there's no way I can come up with to pull them back and make them go around the rear peg.

Watching the original FT video that got me thinking I noticed that their gilder pods were extended - I already know that this pod is going to need quite a bit of weight up front to get the CG right so I'm thinking maybe making an extended pod like this and using longer rubber bands to make sure there's enough slack in them I can get the peg over them.

My daughter is asleep again so I'll stop writing and start cutting and gluing and see how it goes. Would be a fun surprise for her to wake up to a plane...except it's too hot and windy to fly today :(

If I manage to get this to work I'll post an article. But right now it's way too much of a work in progress. Wanted to post something though to see if anyone with more experience would have any thoughts on doing the shaft better and what kind of prop would work best for this..
 

pgerts

Old age member
Mentor
#2
Rubber is a fantastic Power source. I was looking at a competition yesterday. They use 30 grams of rubber band it is slightly more than 1 oz. Take a look when they start to wind the motor about 1 minute into the vid. Check the launch at 3 min40 sec. The plane goes vertical for about 50 meters/yards and then still with the motor force another 50 m/y up. Then it is mainly thermal to get the long flight times. The weather was terrible with temperatures slightly above 10C/50F winds in average 8 m/s 18 miles/h and heavy rain most of the day. This was at 9 pm and the last round called flyoff with a time limit of 7 minutes flight.
This is a little to much rubber for small kids but 10 grams / 0,35 oz is really good performance for small girls.
 
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jhitesma

Some guy in the desert
Mentor
#3
It was actually one of your posts about that competition that got me seriously thinking about rubber again :)

Unfortunately yesterdays nap wasn't as long as Saturdays and I wasn't able to make any further progress. Hopefully the evenings this week will be more productive. She loves watching the propeller spin on "her" pod already so I'm going to have to follow this through.

I'm thinking if I stretch the pod about 4" in the front it will make it possible to use 4 bands (2 parallel groups of 2 in series) which should solve almost all of the problems. Moving the reinforcements to the outside of the pod isn't as clean...but would solve the other fit issue...just wish I had time to put all these ideas into foam already!
 

jhitesma

Some guy in the desert
Mentor
#5
Yeah, I had planned on designing the front so the shaft could be pulled out and you could wind the rubber that way. But I didn't have much time and wanted to get something together that would let me test some ideas.

I'm definitely planning on a way to stretch the rubber on the final version. But right now I'm more interested in just figuring out a way to get the pod to make up with rubber installed. I'm kind of thinking I could just add a second skewer further up so there's one for mounting and one for the rubber...but that seems just wasteful and like it would mean less room for rubber which means shorter flights...so I'm still looking at my pile of bits and trying to work out some ideas ;)

Would not at all be disappointed if someone else figured out a better way before me :D
 

jhitesma

Some guy in the desert
Mentor
#6
For those who are wondering I haven't abandoned this project. Just been really busy the past month with my daughter having swim classes and teaching myself Android development.

And with our average outside temps being in the 107f range lately I haven't exactly been getting to do much flying. Finally gave in today and put a storage charge on all my lipos until the weather breaks in a month or so. I did get out two weeks ago for a bit of afternoon flying...but well, in the heat its sometimes hard to make wise decisions and after "landing" my BW on my wifes head (and giving her a nasty lump but thankfully no prop gashes) I decided it just wasn't wise for me to keep flying in the heat.

Of course...building happens inside. But when it's 107 out even with the thermostat set to 77 inside I'm not exactly jumping at the chance to fire up the glue gun ;)

Usually mid september temps start getting tolerable again and I'll be looking to give this another try.

Then again the wife and daughter are taking a trip back east for a week without me soon so with that extra time on my hands I may find the glue gun calling me....
 

jhitesma

Some guy in the desert
Mentor
#8
Yeah, I was surprised the first time I wound it up and it tried to launch out of my hand.

I just had my week with no distractions (they get back in a few hours) and unfortunately didn't get to this project. I did get a second spitfire and a versa wing built - but didn't get to this :(