I have a need for speed but can't get there.......help!

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Skitdc
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I have a need for speed but can't get there.......help!

Post by Skitdc » Thu Feb 03, 2011 5:40 pm

My son and I are getting ready for our first space derby and we cannot get his ship to go the full 40 feet, we get about 30. We have the weight down to 20 grams fully assembled with fins and three rubber bands. We have hollowed out the middle nicely and made sure nothing is sticking out into the center. The ship is balanced front to back. We sanded down the prop slightly to make sure it is more balanced. The ship is pointed slightly down. I have added, though not permanently, three tail fins - the center in line with the hanging bracket and two side fins angled slightly upward. I have flown the ship with and without the fins with no difference. I have also tried dry and moistened rubber bands with no difference. At the start it goes slightly higher on the right side but does not circle over the line. Any ideas ????????????? I would greatly appreciate any input.



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Re: I have a need for speed but can't get there.......help!

Post by pwrd by tungsten » Fri Feb 04, 2011 12:45 am

Does the wire engage the prop firmly?


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Re: I have a need for speed but can't get there.......help!

Post by Skitdc » Fri Feb 04, 2011 6:01 am

Yes it does. I had originally bent the wire in a U shape without cutting off any excess. I then bent the wire in closer to the shaft and his ship did finish the full 40 feet a couple of times but glided slowly to the end. I will go in tonight and make sure the wire positioned correctly. Thanks for your post and help.

skitdc



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Re: I have a need for speed but can't get there.......help!

Post by dna1990 » Fri Feb 04, 2011 11:14 am

How many turns you putting on? With three bands, you can't go super high, but I would start testing at around 120 ish.

Does the prop seem to 'spin out' only in the first 5-10 feet? If so, could indicate something amiss about hanger or line. It is getting stuck or binding the hanger to the line (the large torque released can really twist the hanger).

Is the guide line super tight? No sag? What is the line, fishing mono or similar?

Does your prop look 'curved' right? I know, who knows the proper propeller curve, but any obvious deformities from the plastic making process? Excess plastic flashing around leading edge? It is balanced? When holding rocket in hand and you let prop go - do you feel a vibration? Do you feel wind blowing back on your hands?

Does the band holder at the back of your rocket sit down well in a wood gap, so it is not spinning or slipping. We saw some wind their rocket while holding that white cross bar thingy, and all was fine. But when launching, it would slip and much rubber band energy was lost and not going to prop.


We had plenty of what-looked-to-be-well-made rockets not traverse the whole track...never figured out why. SD is a persnikity event...no doubt. Good luck.



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Re: I have a need for speed but can't get there.......help!

Post by Skitdc » Fri Feb 04, 2011 1:03 pm

I have been warming up the bands by going 50, 100, 120 and then 150 turns. From 100 to 150 turns the ship doesn't seem to make much significant gain. I bought the space derby set from the scout store which has 50 lb mono and I have it as tight as I can get it. I also trying to tighten it from time to time as it stretches out. No obvious line deformities.

At first it does seem to spin out. I will check to hanger to see if there are any problems. The hangers seem to be different from the pictures on the directions. There is no longer two small tubes for the line. Now there are two pieces which need to be glued to the main hanger and then two metal pieces are bent to hang on the line.

I did sand down the prop to get it close to balanced...did not want to go too far. It does look correctly curved. I will check for vibrations and the amount of winds blowing back.

I did wonder about the cross piece in the back. Initially I had it sit about half way in the ship. I did deepen it to about 3/4 in and did not see too much difference.

Thank you for your time and information. Greatly appreciate it!!



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Re: I have a need for speed but can't get there.......help!

Post by Darin McGrew » Fri Feb 04, 2011 1:16 pm

How are you keeping tension on the line? The best way I've seen is to hang a weight from one end, so the line is always under tension. It doesn't take much slack to keep the rockets from gliding smoothly along the line. (This is the same principle that allows carnivals to rig their "ring the bell" games.)



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Re: I have a need for speed but can't get there.......help!

Post by Skitdc » Fri Feb 04, 2011 1:27 pm

we have an unfinished basement and have the ends tied to hooks. To tighten, I wrap the line around the wood stud keeping the tension as high as possible. Last night it seemed pretty tight. Will hang a line over two saw horses and weight them down with weight to compare the tightness.

Thank you for your time and post



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Re: I have a need for speed but can't get there.......help!

Post by dna1990 » Fri Feb 04, 2011 1:51 pm

when using sawhorses, or eventually at an event with free standing stands - be prepared for them to either tip over or scoot. Once you tension the lines, the tops will have a tendency to tip toward seach other of course, if you counter weight that, on a smooth gym or cafeteria floor - they can still scoot to each other. So a rubber type base, and counter weights is usually key to a good track setup.

If the cross price is 'seated' 1/2 way into the wood surface, should be no issue. If it were, you could witness it slipping while just holding a 150 turn prop.


Is this testing just free hand, or do you have a 'release bar'? Even though we had a pretty well-made one, the variety of rocket sizes/shapes - often meant the release bar (starting gate) was in the way for the first second or so...? Like I said, persnikity.

I don't have a reference to the weight. But in terms of mass, how much of the original block did you remove in shaping? I think you mentioned hollowing the inside some, good. But as you already know, get rid of all balsa weight you can.


If you have any influence over the event, try setting up the track with a downward slope. Maybe only a 3" drop over the 40' length. If you are having this much trouble given the care and interest you have shown, imagine the scout who just whittled theirs together last minute...



Try video. You can then maybe analyze when the prop (rubber bands) have played out in reference to the track. Is the rocket still 'powered' that last 10'?

How much rubber band sticks out the back when not wound? We saw variance in lengths of rubber bands. Some very long and sloppy, but could hold more winds. Others were alread 'stretched' when putting the cross piece in, and would pop at 100 winds or so.

Can you get more than 150 on there? Once past say 100, are the bands winding 'on top' of themselves?



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Re: I have a need for speed but can't get there.......help!

Post by John Shreffler » Sat Feb 05, 2011 10:50 pm

I had the same problem when I made my first spaceship. It was too heavy for one thing, which I worked on. The major problem, however, was that it was nose heavy. Constantly trying to fly to the floor, and putting all kinds of friction on the line. Once I carved out enough weight from the nose to make is slightly tail heavy, it really took off. Hope this helps.


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Re: I have a need for speed but can't get there.......help!

Post by Skitdc » Sat Feb 05, 2011 10:58 pm

Thanks for the tip Darin. I ended up running the line over the hook, tying the line to a weight which kept the line tighter than I was getting it. Everything worked better as there was less give in the line.

Dna1990, I have been testing free hand but have been trying to imitate a release bar as best I can. I do not know how much balsa weight I took off but there is a great deal less now. I will weight the wood prior to shaping next time. I don't think I can get much lighter though.

As for the bands, I bought a box of 24 because they were breaking so frequently. The bands in that box are pretty uniform and start just outside where the crossbar sits. The more you use the bands the more they stretch and it seems to happen quickly. We are supposed to use 3 bands and I have been getting the best results around 150. I think hollowing out the middle helps the bands unwind better too.

Thanks for your post John. We shaped our ship before reading up on things and we had the same problem. I drilled a hole in the hanger and strung the ship up to find the problem and have since balanced the ship. Did you balance your ship with the fins, prop and crossbar or the ship alone??

Thanks for your continued input and help. I greatly appreciate it!!!



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Re: I have a need for speed but can't get there.......help!

Post by sirslotalot1 » Wed Feb 09, 2011 12:22 pm

Skitdc,

I know this sounds dumb, but I did not see in your original post about this. Is the propeler on the correct way? If it is on the same way as the picture on the box, then it could be on backwards. The rounded part needs to face backwards. I know it seems weird, like the pointed or rounded part should go forward like you see on propellers on real airplanes.

At our Space Derby last year, many ships, including mine, had the propeller on backwards. Result was the prop turned fast, but very little thrust.

Just a thought.



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Re: I have a need for speed but can't get there.......help!

Post by ah8tk » Wed Feb 09, 2011 6:51 pm

Here is a diagram I created a while back to show others how to assemble the propeller assembly:

Image



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