Helicopters B

retired1
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Re: Helicopters B

Postby retired1 » November 29th, 2012, 6:33 pm

You are on the right track. That should put you in the middle of the pack. Medalists will optimize as many factors as they have time. The River City Rocket is a 2011 chopper with 40 cm span. This year it is 30 cm. You get a substantially different amount of lift out of the shorter rotor, so pitch or rotor speed has to increase to get a decent flight. There were a lot more Freedom Flight choppers than the RCR, but it did not do proportionally as well. It was a late entry in the availability of the model and it is noteworthy that they did not make it available again.
I would use a bit more curvature in the rotor ribs. Those pictured are rather flat. How much higher do you plan to have the leading edge than the trailing edge (pitch)??

If you or your group can afford it, I would surely start with a Freedom flight chopper for a first one. The jigs and the instructions are worth it. OK, I am biased.

hogger
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Re: Helicopters B

Postby hogger » November 29th, 2012, 6:53 pm

Thanks for the comment. So you are saying that the Freedom Flight generally did a lot better, even though it looks to me like the very same design. I guess Freedom Flight advantage is the jig that gives you a more precise pitch. I am thinking that the difference in prop attachment system has very little effect on overall time and it comes down to the propellers pitch and blade width that matches the rubber and motor stick length.

Another thing, I wanted to get the Freedom Flight, but that is still last year kit as far as I can tell. Don't we want to wait to get the new kit for this year with the new 30cm propeller diameter and with no limit motor stick and rubber weight? Also, the new 3 grams weight vs 3.5 or 4 grams from last year.

retired1
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Re: Helicopters B

Postby retired1 » November 29th, 2012, 7:33 pm

FF is so busy building kits for choppers and gliders that he has not updated his site. The 2013 kit is the same price, builds 2 choppers and has the option of a 4 blade top rotor or a 2 blade rotor. The bottom remains a 2 blade. The 4 blade one comes in right at the 3.0 g min or a bit heavier, depending on how you build. If you want to be competitive, use the kevlar truss option. You can go to heavier rubber, either longer or wider and it will take the stress on max winds. From what I saw at the nationals last year, the shorter 4 blade rotor appeared to give a more stable flight than a 2 blade.
I am having the kids that I mentor start with a stock kit (4 blade top rotor). It comes with .125 and .140 rubber. Will have a couple of them try the .125, and the rest the .140. Later, we will cut out own rubber to try larger sizes.
In the past, with HS students, we normally built the fuselage a bit longer because we could make weight. At 3 g, I do not know.

_HenryHscioly_
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Re: Helicopters B

Postby _HenryHscioly_ » November 30th, 2012, 12:07 am

What should I call the, "chord"? or the distance between leadng edge and trailing edge tips? width?
I was wondering, why more blades instead of wider blades?
Is it because, even if make blades very wide, still not reach optimal lifting surface area..?
More blades would be less efficient, but if wider, also less efficient. Is that correct, to some degree..?

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Re: Helicopters B

Postby retired1 » November 30th, 2012, 7:43 am

The chord is called a rib. The width of the blade is an open question. Looking at 40 years of free flight, props have tended to be longer than wide by a factor of 2 or 3. Real choppers tend to have long blades because of the speed that they rotate, wide one would have problems. You can hear a "Huey" from a fair distance with a whup-whup sound caused by the following blade in disturbed air from the previous blade. As choppers needed to lift more, the blades got longer and then they added more blades. I tried one set of blades that was 4" vs the normal 3" at the tip and the results did not warrant the extra weight.
With 30 cm span, a wider blade might work better than it did with a 40cm span.
At nationals last year, I was impressed with the 4 blade rotors. They seemed to be considerably more stable. Wobble kills flight time.
I generally do not like the word efficiency as it depends how it is used- comparison of what to what.
With the shorter 30cm blade limit, it will change the dynamics a lot. You might be able to stay aloft longer with a 2 blade and light rubber, but it will have a very sharp critical point. A wider rotor blade or the 4 blade will both be able to generate more lift, but will require wider rubber and thus more weight which might cause it to land with more unused turns or wasted weight.
My gut feeling is that rotors that resemble free flight are an optimum, BUT they are a lot harder to build and require precise pitch settings. A big plus is that if it is properly designed, you can change the pitch and try more rubber combinations.
In short, there is not a lot of time left to build and experiment, so we are going to start with the basic 4 blade FF kit and make changes as we have time.

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Re: Helicopters B

Postby jander14indoor » November 30th, 2012, 8:10 am

retired1 is right (except for minor thing, chord is the correct term from the distance from the leading edge to the trailing edge of any 'wing' like surface, including a rotor blade, the rib is the mechanical thing which typically is along the chord) most things are it depends.

Wider chord blades have more area for more lift, but the aspect ratio gets low increasing drag and reducing lift. More blades also gives more area without the aspect ratio problem, but with the blade interference problem (also reduces lift). I suspect, but you'll need to confirm this, that up to a point (say 4 to 6), more blades is better than more area.

Also, better blade shape (eliptical) will be more efficient (better lift/drag ratio), but harder to build.

In all of these, be careful to focus on the factors that really matter until you have them firmly under control, that will get you within 90 to 95% of a competitive nationals time. The rest is to get that last 5-10%. Beginners should focus on the first couple of factors and ignore the rest, its a distraction. Advanced teams will have that right and can start working on the more esoteric stuff.

Critical factors in rough order are, WEIGHT, matching rubber to a given rotor, optimizing rotor design (number of blades/pitch), blade shape, motor stick length (assuming its stiff enough). The first two are by far the most important. Rotor design needs to be basically right (start with proven design), but is clearly the next place to work once you know how to hit weight and optimize rubber/rotor match. Blade shape (both outline and cross section), motor stick length, etc are way down in the weeds for things to mess with. Start at a reasonable point on these and you will do fine.

Jeff Anderson
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Re: Helicopters B

Postby hogger » November 30th, 2012, 9:43 am

I just want some clarifications to be sure I understand fully what it means to match the rubber to the prop. Since we no longer have limitation on the rubber weight or length and we no longer fly in circular pattern risking getting caught in one of the rafter of the gym, don't we want the longest rubber and thickness that would provide enough lift to the helicopter so it goes all the way up to the ceiling? The longer the rubber, the more windings you can put in it, right? The longest here should also mean the longest motor stick you can do within the weight limit?

If my thinking is somewhat correct, wouldn't you want to build the longest motor stick first and then find the rubber thickness that would get the helicopter all the way up and down as the windings run out as it lands on the ground.

Otherwise, what do you mean by matching rubber to the prop?

Thanks as usual for your input.

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Re: Helicopters B

Postby retired1 » November 30th, 2012, 10:30 am

Jander has said it well.
You seem hung up on just getting to the ceiling. Competitive choppers will get there in just a few seconds, depending on height. The trick is to stay there as long as possible. You have the most torque at the start and it steadily decreases from there. A high pitch prop matched to a thin band will land early because there will not be enough torque to keep it up. Lots of turns left on it.
Conversely a low pitch with a light band will expend its turns very rapidly and land early with few turns left.
A big band will weigh more so it will take more torque to keep it up. As Jander said, weight is bad! Somewhere, there is an optimum. Start with a known plan and then experiment. Tiny changes make an enormous amount of difference.

Semantics: Chord is the distance from the leading edge to the trailing edge and has a curve. A rib is the physical piece of balsa that you put in to become the chord.

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Re: Helicopters B

Postby jander14indoor » November 30th, 2012, 11:14 am

trying NOT to repeat what was just said...

This concept of matching rubber to rotor is a more advanced mathematical topic than generally dealt with in middle school, but lets see if we can give you a plan on how to go about doing it systematically.

First, as already noted, motor stick length isn't really critical (as long as its in a reasonable range, say 8-12 inches, and you hit the minimum weight and its stiff enough). So, build a nice, light, stiff one about 10 inches long and you will be fine for now.

OK, on the rubber. You have TWO control factors to play with to match the rotor for max time. Thickness and length. And they are not independant so you HAVE to test them together in an organized fashion (this is called a Designed Experiment), one at a time testing WILL NOT WORK.
- Starting assumption
-- Every test flight will be flown with a max wound motor, just short of breaking.
-- Every flight is in basically similar conditions. PREFERABLY no moving air, smooth ceiling, same height.
- Make a 3-d graph, rubber length on one horizontal axis, thickness on the other. Verticle axis gets the response, flight duration.
- Select a range of thicknesses and lengths you want to test. Divide those up into three. Minimum thickness, mid thickness & max thickness. Min lenght, mid length and max length.
- test every combination of those, that's 9 test points, at least twice each. Throw out and repeat test if flight gets caught in ceiling, or is otherwise bogus. But only if you really think its bogus.
- Graph all that data, preferably with a program that gives you nice output, maybe does some nice averaging.
- Interpret results.
-- IF you are lucky the average top time is the mid-point. Reduce the range you are testing and repeat.
-- More likely, one of the edge points is high point. Extend the range around that point and retest new points.
-- Repeat until your range collapses around the high point enough so there is little change in the max

Now, if you have someone who can interpret designed experiments for middle school students (its doable, I've done it as early as elementary school with my kids)you can make this more efficient, but only some. It will reduce some of the test points and make the data interpretation rigourous.

Hope that makes sense, never said it would be easy. Hey getting this test and data collection idea across is the whole point of the logs in the rules.

PS, after you've done this right once you'll be the expert and can 'guess where that optimum point is on the next rotor system more accurately and save a LOT of test time.

Jeff Anderson
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Re: Helicopters B

Postby hogger » November 30th, 2012, 11:37 am

How do you vary the length of the rubber in this experiment? Are these experiments supposed to be for the same motor stick length and the rubber just either has a lot of slack or very little slack compared to the motor stick length before winding?

retired, I agree I have unfounded hang up on the ceiling. That was the key in wright stuff but not so much in this event except maybe it is probably not very efficient to bounce up and down at the ceiling instead of no bouncing, but I don't know.


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