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- Division: C
- State: IL
- Location: UIUC (Formerly BGHS)
2012 Helicopters State Champion
2013 Robot Arm State Champion
You need a hard teflon or polyethelene tube about 1/8 to 3/16 inch OD by 1/32 or so wall thickness. You can buy appropriate tubing at the hobby store, they call it push rod housing, http://shop.dubro.com/p/30-pushrod-housing about a 30 inch piece costs a couple of dollars, and I just bought my second one after 10 years of coaching the flying events. Slice off about 1/32 off the end of the tube and there's your o-ring. Alternatively you can use the ink tube from an old ball point after the ink is gone. The suction tube on most spray bottles works fine too. Oh, be careful cutting off the slice, I ended up making a little safety jig after slicing my finger to the tune of about 7 stitches in front of a class of high school students coaching Wright Stuff one year. OUCH.
Don't worry about the hard edges, the plastic is soft enough that I haven't seen a rubber motor cut yet.
State: Shock Value - 8th place
Nationals: Helicopter Egg Drop - 6th place
Rotor Egg Drop-3rd place
Rotor Egg Drop-5th place
Rotor Egg Drop-46th place
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http://scioly.org/phpBB3/gallery/image_ ... age_id=606
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Please respond with recommendations. This is my second year on the team, and first year on this event.
Is this what you called cross-shaped?
These work well.
Read back up this thread and last years helicopter thread for more discussion on blade shape.Maxwellc007 wrote:I have an urgent question. What kind of rotor works well? I've tried the basic set kinds that are cross shaped, but there are also some more "exotic" ones. I couldn't find better pictures for what I'm talking about. Here's some links to show you what I'm talking about:<SNIP>
The pictures you show with flat blades that do not change angle with radius are LESS efficient/effective than the cross shaped style. While rotors are 'just' wings that fly in a tight circle, that circle does affect their shape. Near the center they are moving very slow and to have the proper angle of attack to the relative wind they need a higher angle than the tips that are moving relatively fast. The cross shape blades do all that automatically, but are not the only way to get there.
The disadvantage of the cross shaped blades is mainly at the tips. Those hard corners are draggy reducing efficiency some. To help this, you can look at elliptical blade shapes with rounded tips that follow that same twist the cross rotors have. Again, more discussion of this back up this string and in last years helicopter discussion.
Comment, unless your helicopter is flying in the 2-3 minute range, you are worrying about the wrong thing looking at blade shape. X shaped rotors will get you that far. Spend more time on matching rubber to rotor, and possibly more time finding the ideal pitch and blade width.
I was very impressed with the 4 blade rotors that I saw at nationals last year. With the rotor length reduction this year, I think that the 4 blade is the way to go for the top rotor. 2 blade is fine for the bottom. The fin on picture 3 is a waste of effort. A 2 blade rotor that is free to rotate as the top one does is far more efficient and possibly less weight.
Janders comments on shape are certainly true, but are well above most competitors experience level. A mylar covered square blade is quite adequate for this event IF it is well constructed, blades absolutely identical in shape and weight. Once you have a jig, they can be fairly fast to build and make modifications to find the optimum which involves lots of flying.
There are so many tradeoffs in design that it would take pages to discuss. The only real constant is rotor length.
Generally pre-stretched, however if you are using an appropriate amount of lube and letting the rubber band rest after winding, it will normally retract back to close to its initial dimensionshogger wrote:So when we are talking about the length of the rubber, are we talking about pre-stretched or post-stretched? The rubber seems to stretch up to 20+% more after the first winding. I would think that you would record the initial length and go from there so that you can cut another somewhat identical piece of rubber.
Hawaii State Director
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