Rotors

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Re: Rotors

Post by wlsguy » October 13th, 2010, 6:16 pm

franklinknights wrote:we plan on using heli rotors made out of taco bell cups. w ewill use same design from plastic cups of last year
You have already been told by Jander, plastic cups are likely not allowed. I have a different opinion however.

The definition of "Rigid" is "Firm; stiff; unyielding; not pliant; not flexible. [1913 Webster]"
This leaves plastic cups (which are pliable) in the grey zone. Rotors made from these are not firm, not stiff, and are still flexible.
The rules also allow "plastic film" but did not specify a thickness. Since many plastic cups are vacuum formed, they most likely started as thick sheets of "plastic film".
It could therefore be argued plastic cups are legal.

I would advise you to submit a rules clarification. This is another area that needs closed up and not left to the individual event supervisors.
While I think they will likely agree with Jander, it's always better to have it in writing.

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Re: Rotors

Post by jander14indoor » October 15th, 2010, 3:45 pm

I struggled with whether to respond back. Finally decided to, obviously.

First, a general disclaimer. This is NOT the NSO clarifications web-site, so nothing I say here can be anything other than my opinion. If that.

With that out of the way, let me address this as an occasional event supervisor, again. If I used the term 'rigid' literally as in your definition, there is NO material in existence that meets it. Even something as hard as a diamond is flexible or pliant if you want to exert enough force. So, nothing would be ruled out and I'd have to wonder why they added a non-operational word? Assuming they meant it to add to the meaning of the rule, I'm left how to interpret it on my own (unless an official clarification is released AND I see it before the competition). I'm going to have to figure out how stiff is too stiff and considered rigid, and how much is un-stiff enough to be not rigid. I suspect most will come to a conclusion something like this: If it is self supporting, it is rigid. If it flops due to simple gravity, it is not rigid.

Now, let me speak as a mentor coach (another role I play more often). Several things come to mind.
Do you want to take the chance at a competition that the ES will disqualify your device over such an interpretation? Maybe if it gives a big performance advantage, but you (the student) had better be prepared with a good rationale about why the device meets the rules. And expect to lose most (not all) of the time anyway.
So, lets look at what performance advantage plastic props might bring. First, this is a flying event. Weight is KING. You MUST build your copter to the minimum 4.0 gm (from memory) of the rules. Just as a reference, the 12 inch Ikara props used in WS (illegal in this event) weigh more than two grams each. These things were about as light as you could get for a plastic prop, you are going to be challenged to beat that weight with home made props. If you use two for a counter rotating system (and the dual rotor copters generally beat the single rotor ones last year), you just used your WHOLE weight budget on the props, nothing left for the motor stick, prop hanger, tail hook.
So, what's the advantage of a plastic prop then??? Easy to build I guess, but not very competitive. And frankly, balsa props are darn near as easy to build and can be built for far UNDER 1.0 gm for a two bladed prop of 40 cm diameter.

Summary, you might get away with plastic bladed props cut from a cup, but why risk it for no advantage??

Jeff Anderson
Livonia, MI

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Re: Rotors

Post by calgoddard » October 16th, 2010, 8:01 am

I wanted to give some context for people to consider when they are debating what is, or is not, legal under SciOly event rules.

SciOly event rules are drafted by a knowledgeble person in the field and presumably approved by a committe before being tested for two years under the trial event process. Feedback from event captains, coaches and competitors often results in rule changes. If all goes well, the event becomes official.

It is simply not possible to draft SciOly event rules that expressly and umambiguously cover all possible contingencies. This is why the nationals web site provides for FAQs and rule clarifications.

SciOly event rules can be analogized to a contract or a statute. When a question arises about coverage or whether something is legal, you first look to the intrinsic evidence, i.e. the wording of the rules themselves. Usually the ordinary plain meaning of the rules should govern. In some cases the meaning of a term can be further defined and restricted in the rules themselves. See the definition of "rotors" in this years HELICOPTERS rules as an example.

Ordinarily, if a particular design or material is not expressly prohibited, and is not unsafe, it will be allowed, unless it violates the spirit of the rules. An example of the latter was the bottle rocket at the nationals competition in 2005 that had a trash bag filled with helium balloons secured to the top of the rocket.

In judging whether a particular design is legal, those determining the outcome should consider the intent of the drafter or the committee and try to come up with a ruling that follows that intent.

Sometimes a design is on the margins as to whether it is legal or might result in a DQ. Typically whether to DQ a device is up to the EC at a particluar competition. I agree with Jeff. Unless the potentially prohibited design offers a clear advantage don't waste your time with it and don't risk a DQ. If, on the other hand, the design seems to provide a clear advantage consider whether you should submit an FAQ or a rule clarification to nationals. Bear in mind, however, that your posted question may give away the design to everyone else and the adavantage may be lost if the answer by nationals approves the new design. This is because both the questions and answers are published on the nationals web site.

The formulation and application of SciOly event rules is analgous to the process by which a legislature drafts and enacts statutes and the way they are enforced. The application of statutes to borderline fact situations often ends up in court and a judge decides whether a particular activity is, or is not, permitted under a statute.

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Re: Rotors

Post by wlsguy » October 16th, 2010, 10:57 am

jander14indoor wrote:Summary, you might get away with plastic bladed props cut from a cup, but why risk it for no advantage??

Jeff Anderson
Livonia, MI
Jeff, let me start by saying I agree with you. I think using plastic material for Props is not the best way to make them. Too heavy.

But I also understand that the Science Olympiad rules need to be flexible enough to allow everyone to learn something. Not every school has an experienced mentor to help them out. Some teams have a single coach who is often overworked. The flight principles are the same regardless of the material for the blades. The knowledge gained is the same. Is it easier to use plastic cups? maybe. Is allowing plastic cups an interpretation of the rules to gain an advantage? probably not.

So, if the learning is the same and it is not an advantage, it really doesn't matter.
It's better to have the students make a prop out of taco bell cups and learn something rather than not participate in the event because they think it is too hard and learn nothing.

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Re: Rotors

Post by blue cobra » October 17th, 2010, 10:03 am

What's the best way to mount the free rotor using a nylon thrust bearing such as those at this [hardurl]https://www.a2zcorp.us/store/ProductDet ... uctID=1524[/hardurl]. I was considering a tissue tube, but I don't feel comfortable entrusting such a high stress and important area to tissue tubes. As well, I need at least 1/16" additional clearance. How does a 3/32 to 1/8 spacer glued onto the MS, and then glue and thread sound for securing it?

With the Ray Harlan bearings, is the Penny Plane model really significantly better than the SO one? You can save 0.06g, but you have to go from an already fairly small 0.020" prop shaft to at most 0.018" and you lose almost half your clearance.
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Re: Rotors

Post by jander14indoor » October 18th, 2010, 3:52 am

For those Peck bearings, you can:
- Tie the bearing directly to the motor stick (I'd add a sort spacer from clearance of the spinning prop hook). I don't like this as it makes it hard to change the free rotor.
- Make some sort of hanger with a hole to fit allowing the bearing to be slid in and out. I've used .030 aluminum successfully. Not too heavy. Cut out a piece with a circle of 1/4 inch or so (to allow a rim around the hole for the bearing, and a 1/2 inch by 1/8 tail off one edge. Drill the hole for the bearing in the circle just large enough to fit, I seem to remember that 1/8 or so fits the smallest bearings. Then bend that long tab 90 degrees to the circle. Glue it to a spacer on the motor stick, reinforce with a few wraps of thread, glued down.

Harlan bearings. The penny plane bearing is much better for one BIG reason. It uses a split tail so you can change rotors. Its probably strong enough for this, depends on rubber size. The SO bearing does not (unless he's changed it) allow prop changes as the tail only has a hole.

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Re: Rotors

Post by smartkid222 » October 18th, 2010, 2:49 pm

blue cobra wrote:With the Ray Harlan bearings, is the Penny Plane model really significantly better than the SO one? You can save 0.06g, but you have to go from an already fairly small 0.020" prop shaft to at most 0.018" and you lose almost half your clearance.
I don't understand the picture or the rest of that paragraph but for the PP Harlan Bearing the MAX prop shaft you can use is 0.017". 0.018 is the size of the hole and I can tell you that a wire of that size will definetly not fit. 0.017" barely makes it and may cause problems with friction (the thrust bearing around the wire turned black probably from the heat created from friction-- as a side note I never really analyzed that to see if that had any effect on time; what do you think?)
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Re: Rotors

Post by lllazar » October 19th, 2010, 9:56 pm

How can we cut specific size props out of balsa - wouldn't we have to bend it. I'm very confused on this topic....
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Re: Rotors

Post by jander14indoor » October 20th, 2010, 9:02 am

lllazar wrote:How can we cut specific size props out of balsa - wouldn't we have to bend it. I'm very confused on this topic....
I know of three basic approaches to making props/rotors out of balsa. Not sure which are better/worse from a time or effectiveness point of view, for that we'll need a couple years of data from competitions! OK, the basic approaches:

1. Build a balsa framework in a helical shape and cover. Not sure if there's an explanation on the web, so I'll try a quick one here until I can get all the pictures of the copter I'm building now finished and load them up.
a. You need two 38.9 cm spars, a bunch (8 or so) of ribs and covering. A building board and parts to make a jig.
b. Pin the first spar to your building board, mark its center.
c. Take the second spar, locate it say 1.5 inches above the first (you ARE going to need a jig for this) and rotate (ccw for an upper rotor, cw for a lower) it about the center so the tips are spread say 4 inches or so looking down from the top.
d. Mark both spars every 5 cm in from the end.
e. Cut ribs to fit well between the spars starting at the tips and moving in at each 5 cm radial position. Glue in position. You may actually want to start at the first 5 cm position from the center and work out!
f. Remove rotor frame from board and jig. Cover from first rib at 5 cm radius out to tip.
g. If you made a lower rotor, glue it nice and square to motor stick about ¾ inchs down from upper bearing.
h. If you made an upper rotor, add another spar at the center and your prop wire to hook the motor too. Assemble to prop hanger.

2. Take thin balsa, soak it in water and mold it to helical shape on a form. Attach blades made this way onto a center spar.
a. See: http://www.indoorduration.com/INAVHobbyShopper.htm This is a multi-page article see especially the last section. Note, this article is about building a CRAZY light balsa plane including a prop only a little smaller than needed for SO. Use it as inspiration and procedure. Details of course need to be adapted to SO rules. Thicker wood, larger, etc. For SO I'd think in terms of 3/32 to 1/8 inch spars and 1/32 to 1/16 inch thick blades. But the process is right and explains it FAR better than I can.
b. In addition to using a form like in this article, you can also form the props on a curved surface and get a close approximation of a helical prop. Haven't found an online explanation, but form the blade about 15 degrees off axis on a 6 inch cylinder should be about right.

3. Carve from a raw block of balsa.
a. Here's an article that shows how to lay out a block for helical pitch props: http://www.indoorduration.com/ftp/GrantPropINAV94.pdf
b. Kind of assumes you know a lot already, hmm this is pretty detailed, see also: http://www.gryffinaero.com/models/ffpag ... carve.html or http://www.keveney.com/prop1.html
c. Another approach which takes more time prepping, but less carving: http://www.hbrf.org.nz/joomla/index.php ... &Itemid=59

A simplified approach to 2. would be to simply attach flat blades to the spar at appropriate angles. Works, if not very efficient. Probably fasted way to get a copter built.

Another approach to 2. would be to carve the blade, see: http://www.gregorie.org/freeflight/shee ... index.html and then attach to spar.

Yet another approach to 2 would be to build a balsa frame on that surface you molded a solid prop blade on and cover. See: http://www.indoorduration.com/F1DPropConstruction.htm FOR PROCEDURE ONLY. Again, this is about building crazy light stuff, but the process is right. Scale up for SO. Again, probably 1/16 inch square outline and ribs.


Approach 1. is the conventional approach for indoor copters. I suspect approach 2. with the built up blades will give a little better performance because it should be possible to reduce drag. Approach 3. will be a challenge to make rotors light enough.

Hope that helps, feel free to ask followup questions.

Jeff Anderson
Livonia, MI

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Re: Rotors

Post by franklinknights » October 28th, 2010, 11:33 am

how do i make the robot assembkly? im thinking about using paper cups for rotor shape and got design to cut them but idk how to make rest of assembly
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