Design

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someusername
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Re: Design

Post by someusername » December 18th, 2014, 10:02 pm

Would anyone be willing to direct be to a good article on aerodynamics? I just feel like every time I read through these forums, the further behind I am because I am not understanding some of the "finer" details of powered free flying planes.

any sources would be greatly appreciated!
simplicity is key...sometimes

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Re: Design

Post by bjt4888 » December 19th, 2014, 5:15 am

someusername,

This NASA Glenn Resource Center website is a beginners guide to aerodynamics is an excellent resource. It even has a copy of "Foilsim" that you can play with. See the index and read the "Science Fundementals", "Aircraft Forces" and "Aerodynamics" topics in order.

Who knows, maybe if you study these materials diligently over the course of many years, you will be the one to win the $1,000,000 prize for the general solution to the Navier-Stokes equations. The study of fluid pressure as it flows around an object, like a wing, is one of the great problems in physics.

http://www.grc.nasa.gov/WWW/k-12/airplane/short.html

Good luck,

Bjt4888

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Re: Design

Post by torqueburner » December 19th, 2014, 10:23 am

bjt4888 wrote:artysophia,

. . .Here's a link to a video of an international level competitor winding his rubber motor off of the airplane. His rubber motor is attached to a torque meter. He is taking quite a bit of time to wind as in AMA competitions there is not the 8 minute Science Olympiad flight window. You will want to wind and load the motor to your airplane quicker than this.

https://www.youtube.com/watch?v=xyXeTJno0sI

BJT4888
Actually, the rules allow you to take your time winding. Rule 4i: . . .teams must be given an 8-minute Flight Period, starting when their first flight (trim or official) begins." So you could take your time winding for the first official flight, and even if you misjudged, and broke a motor, you'd have time to wind another, according to this rule. Then, after you launch the first flight, you'd have 6 or 7 minutes to wind for the second launch - there is no rule that you can't wind while the first flight is in the air.

Having said this, I will also observe that I have seen event supervisors, in Wright Stuff and Helicopter, who didn't know the rules, and who started the 8 minute window as soon as they said "go", so it wouldn't hurt to have a copy of the rules with you if you plan to wind slowly, and aggressively, as you saw in the video.

Speaking of torque meters, which you see on the left side of the video, they are an essential part of mastering the art of winding for optimum flights. If you don't have one, you may want to take a look on ebay, as I see there are currently some for sale. Just search for "rubber torque meter.

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Re: Design

Post by artysophia » December 19th, 2014, 3:12 pm

So 61 cm is probably the shortest design people go for? Also, the distance between my frontal and rear motor hooks is about 12 inches, but the length of my rubber is a couple of inches longer than that...is that normal?

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Re: Design

Post by bjt4888 » December 19th, 2014, 6:17 pm

Artysophia,

Yes, the motor will always be longer than the hook to hook distance. My students are using motors that range between 14.75" and 18" depending upon the propeller pitch and/or blade area and our hook to hook distance is about 13".

Bjt4888

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Re: Design

Post by jander14indoor » December 19th, 2014, 8:27 pm

artysophia wrote:<SNIP> Also, is a dihedral or wing tips more effective? <SNIP>
Someone else already answered your length question. Longer is better, tends to be more stable from bumps & drafts among other things. NO relation between stick length and motor length. Big problem with too long motor sticks is they bend too much when motor wound, that is bad because it changes the trim of your plane.

As to dihedral vs wing tips, they are actually the same thing. Tips are just and extreme of a two break dihedral. There is some theory that states the 'best' dihedral is to have your wings follow the bottom half of an oval. A pain to build and the difference is VERY small. Frankly, tips vs angled wings tends to go into and out of style with little more evidence (that I've seen) other than what the most successful flyer you last saw was flying. My suggestion, pick the easiest for you to build quickly and consistently and get flying.

You'll get more out of learning basic flight trim and matching rubber to prop than you will messing with which dihedral, or too much on plane length.
someusername wrote:Would anyone be willing to direct be to a good article on aerodynamics?<SNIP>
You've already been pointed to a good site for general aerodynamic theory. But a caution. Any plane is a compromise aimed at what the end use is. Most of those sites are written by folks studying full size man carrying planes. Very different requirements for stability, control and safety than an indoor free flight model. For more on that, you may want to find copies of Hey Kid! ... Ya Wanna Build a Model Airplane? by Bill Warner, a series of three books, or Rubber Powered Model Airplanes by Don Ross, or Building & Flying Indoor Model Airplanes by Ron Williams. You'll also find articles on the web that are more focused on indoor free flight.

Winding. Slow allows you to pack more turns into a given motor, and turns are fuel. Learn to wind slow. As already pointed out, the first wind is NOT part of your 8 minutes.

Jeff Anderson
Livonia, MI

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Re: Design

Post by nxtscholar » December 27th, 2014, 4:04 pm

I know that traditionally, spray glue is used to attach HDPE covering to the wing frame and stab. But I read that a permanent glue stick also works as long as your careful with it. Is that true?

I'm just wondering since I tend to be prone to being trigger happy with the spray.

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Re: Design

Post by A Person » December 27th, 2014, 7:42 pm

nxtscholar wrote:I know that traditionally, spray glue is used to attach HDPE covering to the wing frame and stab. But I read that a permanent glue stick also works as long as your careful with it. Is that true?

I'm just wondering since I tend to be prone to being trigger happy with the spray.
I personally use the spray(I try really hard not to use too much), but a normal glue stick works too. Just make sure to be careful, as you can easily break your wings.
"Any sufficiently advanced technology is indistinguishable from magic." - Arthur C. Clarke

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Re: Design

Post by jander14indoor » December 27th, 2014, 10:36 pm

You can also wet your brush with alcohol, dissolve some glue stick by brushing the wetted brush on the stick and then paint the glue stick on the wing. A little more effort, but less risk of breaking delicate parts. Also avoids globs of glue.

PS, I'm not sure how much HDPE is used for covering, the really light stuff is mylar. Grocery bags can be LDPE or HDPE, but I don't care much myself as long as its LIGHT.

Jeff Anderson
Livonia, MI

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Re: Design

Post by nxtscholar » December 28th, 2014, 6:08 am

Thanks for the tip.

I use HDPE simply because it's more accessible for me :P (I mean, as long as my plane is still about 8 grams, it's fine right? The other reason why I used HDPE was to meet the 8 gram minimum).

I was wondering also if there was anything I could do to test the performance (or get some early idea) of these indoor planes without actually using the rubber. What I mean is that I'm very limited in my use of my school's gym/cafeteria.

For ELG, one way to test to see if your glider was working without launching was to lightly throw the glider like a paper airplane. If the glider was built right, it should start to turn after a few seconds. If it didn't, then possibly some modification would be needed.

Does the same method work for Wright Stuff? I know that without a trim flight, you can't really get that much out of it. But what I'm asking is if something can be done to get some information on predicting flight patterns without a trim flight.

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