Design

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Design

Postby Jim_R » September 8th, 2014, 6:57 pm

Discussion for design.
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Re: Design

Postby bernard » November 24th, 2014, 10:24 am

Just noticed that the Wright Stuff Kit from Freedom Flight Models is now available! (Yes, I've been waiting for it!) These kits can be very competitive (but are not necessary, just helpful).

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Re: Design

Postby 28builder » November 25th, 2014, 8:39 am

weight?? 8 grams min. for plane and prop and max 2 grams for rubber and o-rings...

didnt buy kit but trying to use old plan from previous yrs. is there an advantage to maxingbout the wingspan to the allowable rule over going with smaller wingspan. having trouble wuth weight when hoing max wingspan per rules at least without buying a kit

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Re: Design

Postby chinesesushi » November 25th, 2014, 9:08 am

weight?? 8 grams min. for plane and prop and max 2 grams for rubber and o-rings...

didnt buy kit but trying to use old plan from previous yrs. is there an advantage to maxingbout the wingspan to the allowable rule over going with smaller wingspan. having trouble wuth weight when hoing max wingspan per rules at least without buying a kit
Well maxing the wingspan in general leads to longer flights, which is part of the reason they added the rule this year, to limit the length of the flights of some competitors. However, the smaller wingspan gets you that bonus. In the end, testing is what will help you get to your conclusion.
Increasing the weight should be simple enough, add however much clay you need to the center of mass of the plane should work, I think.
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Re: Design

Postby 28builder » November 25th, 2014, 9:53 am

my wingspan currently is at 38 cm but my weight with prop (less motor and rings) is already at 8 gms. i will really have to work to get more wingspan without adding even more weight. i am amazed at how light these planes are and...fragile! the freedom flight 2015 kit has 50 cm wingspan and yet is still at 8 grams without motor.

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Re: Design

Postby A Person » November 25th, 2014, 11:05 am

my wingspan currently is at 38 cm but my weight with prop (less motor and rings) is already at 8 gms. i will really have to work to get more wingspan without adding even more weight. i am amazed at how light these planes are and...fragile! the freedom flight 2015 kit has 50 cm wingspan and yet is still at 8 grams without motor.
Freedom Flight makes really good products. It's really worth the price, but I wouldn't say that they are good planes for someone who hasn't worked with balsa before. I wouldn't say that you should worry about the weight too much. What you should probably do is build a full size plane, maybe 10g, but slowly as you build different models of it shave the weight down. I think it's easier to build a design that's over then move the weight down through lighter wood than to build a smaller design and have to redesign it every time to keep the weight at 8g. I'm not sure, never tested, but I think a full 50cm at 10g will out preform a 38cm at 8g.
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Re: Design

Postby jander14indoor » November 25th, 2014, 11:47 am

CAREFUL, you are confusing wing span (width of the wing from side to side of the plane) with wing chord (length of the wing front to back). The bonus is on the wing chord, not the span.

Wing area. YES, it is critical to maximize the wing area. Lift is proportional to wing area and velocity squared. Since you have to lift the minimum 10 gm (plane plus rubber) whatever the wing is Lift is actually a constant. SO, if you give up wing area, you HAVE to increase speed OR angle of attack. Both increase drag FAST. Square of speed fast. It just takes more power to generate the same lift, that power comes from the motor (and again as a practical matter the energy in the motor is fixed, more power expends it faster), so your flights WILL be shorter if you have less wing area. That assumes of course all else equal, especially weight, I'll come back to that.

So, for regionals, you really want a 50 by 8 cm wing (actually 49.9 by 7.9 or so wing, don't want to be tiered).

Oversimplified analysis of bonus vs chord. 50 by 8 to 50 by 7 wing is a loss in area of 12.5%. You'll have to fly 7% faster to make that up which is 14% more drag against a 10% bonus. Hmmm... might be over simplified... the stabs provide some lift so the percentage is a little smaller than 12.5%, you check it... feel free to check my math both theoretically AND with hard data...

That 38 cm wing gives up even more area. You are penalizing your self before you even start! Oh, and fragile. Guess what break's planes. Kinetic energy, which goes up with the square of velocity so that smaller wing has 14% MORE energy to break the plane when it hits something. IN FLIGHT, slower, lighter planes break LESS often than faster, heavier ones. And it's easier to trim a slow plane because it is just easier to see what it is doing before it crashes.

In addition, the bonus only comes into play at state and national levels. Why would you give up 10%, or more, at a regional for a bonus that you don't get?

Weight, these rules are designed so you should have NO problem hitting the minimum weight with a max size plane with a little care. Have you developed a bill of material with target weights for every part? You need to if you plan to hit 8 gm. At assembly level, prop < 2 gm, motor stick 2-3 gm with prop hanger and rear motor hook, wing 2 gm (covered) tail boom and rear stab 1 gm or so, balance ballast. To hit that does not require anything special. I use hobby store balsa in 8 lb/ft3 range for most parts except tail where I try to find 6 lb/ft3 wood and wing posts where I use 12-15 lb/ft3. Covering is grocery store bags, lightest I can find, its lighter than Japanese tissue, can be as light or lighter than condenser paper, and far more stable. You don't need specialty covering to hit 8 gm. Oh, and you HAVE to have a scale at hand to build. How else can you select the right strength/density balsa for a part?

Note, in the past, we've had rules with 50 cm wings, no chord limit, tissue covering, and students hit the minimum weight. You can do it if you plan.

Jeff Anderson
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Re: Design

Postby jander14indoor » November 25th, 2014, 11:56 am

While I think the Freedom Flight kits are very good designs with very good materials, I would never call them a good buy, unless you'd NEVER built a plane before. The cost of the kits will cover equally good quality materials bought seperately for 5-10 planes, if you know what you are doing and know sources, which we'll happily point you to.

The BIG advantage of kits is that they provide (at least the Freedom Flight ones do, some others too) ALL the materials you need for typically 2 planes that will (with care) meet the rules and fly very will. Not the best, but very well. Great for teams starting out without an available/knowledgeable mentor.

As to building to weight, have to disagree. Its just not that hard to build these things to 6 gm with care, building to 8 is easy. Set up a spreadsheet and develope a bill of materials. You'll quickly see where you need to manage weight as in my last note

Jeff Anderson
Livonia, MI

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Re: Design

Postby 28builder » November 25th, 2014, 1:36 pm

thanks! great advice! i truly appreciate

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Re: Design

Postby 28builder » November 25th, 2014, 2:35 pm

that probably explains why it flew fairly well with a fast small plastic prop but went to the ground when i used an ikara prop which was slower aha!!!


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