Designing FF model for SO: A few Q's

[KnifeEdge51]

I've designing a model for the Science Olympiad contest. So far, I'm using the max demensions for the wing (50cm WS, 12cm Chord) with polydihedral at the last 8-10cm of each wingtip. However, I can't decided what size to make the T.E. I'm thinking 3/32, but how long a piece (width wise)? I'm also using 1/8 for the L.E., so I'd need to taper the ribs a bit. I'm thinking about 12 ribs, including the "endcaps." I haven't gotten to the horizontal stab or rudder yet, or the fuse. Oh, also how long a fuse should I use, and what area percentage of the wing should I use for the horizontal stab (should it be 35% of the wings area)? Fuse is going to be a piece of 1/8, as well as the mounting pylon (ala MicroSport, but changed a bit). Does this sound good so far, or do I need a few changes? Any input would be appreciated! Thanks!

Nick

[BMatthews]

Go lighter....

Look at Pennyplanes for inspiration. There is basically NO loads on these models as long as you don't move them too fast in your hands. I'm not up on the rules for the SO event but basically 1/16 sq for the leading and trailing edges is all you need. And 1/32 sheet sliced ribs with a 1/16 deep section will do nicely. 3 or 4 inch spacing is fine. Don't worry about a few dips or wrinkles but try to minimize them just for your own pride in workmanship.

With the 50 cm span you may need 3/32 deep x 1/16 wide spars (in indoor the leading and trailing edges are just called spars) to handle the twisting at the root but these should taper out to 1/16 sq, or less if allowed, at the tips. The sliced rib depth should mimic the spar depth at each station. Or use 1/32 x 1/16 V bracing off the wing posts if allowed.

With indoor models 90% of the model's performance is tied up in only two factors. The weight and the selection of "just" the right motor size for each site. The other 10% is all the rest. This assumes the plastic prop that I gather the SO models are required to use. Otherwise the 90% would be 3 issues with the prop being added.

It's almost impossible to stress the light weight too much. Each slender stick needs to be selected from very light contest grade wood but with an eye for particularly stringy stock. Break test strips to determine this. Breaks should NOT be square across. Rather they should be over a long failure line with lots of slivers that interlock. Such wood will offer the most resiliancy and strength for the weight and cross section. If you can't find 4 lb wood to do this then move to 5 lb. Then to 6 lb but no farther. It's out there but you just need to find it and then treat it like gold.

Yes, I have flown my share of indoor models.... :D Hope this helps.

PS: That's not a very long dihedral section. I'd go for a "square" tip outline and use about a 30 degree dihedral angle. Even that may not be enough. An 18 cm tip would be safer. But don't let me keep you from experimenting. For all the time and material these take why not make 2 or 3 wings and try them all.

[ED B]

Hi Nick, I'm an Event Supervisor for the Wright Stuff event here in NJ. Don't forget the MINIMUM WEIGHT requirements for the airframe! Or the MAXIMUM WEIGHT restrictions on the rubber motor! Also, shave/scrape/sand the prop. You can cut the weight of the prop in half by doing this. It's possible to build an airframe to the maximum dimensions and still be only half of the minimum weight. Instead of adding ballast to bring your plane up to weight, build a stronger structure. Use 6-8Lb wood instead of 4Lb contest grade. You will make weight and have a more durable ship! Good Luck and keep asking questions and experimenting. Last years winner in NJ flew for 1 minute 38 seconds.

Ed

[KnifeEdge51]

Ed,

Thanks for that valuable insight! I reduced the LE edge to 3/32 square, and the TE to 3/32x.8cm. So far I've got 14 ribs including the endcaps and the two center ribs I'll use to mount the wing to the pylon. All I have for rubber so far is Guillows 7" loops (the green stuff). I need to get a hold of some TAN II or something pretty soon here. It looks good on paper, I need to get something built to test, tough. :rolleyes:

BMatthew,

I did reduce the wood a bit, but like Ed said, I need to stay above 8 grams, with no more than 2 grams of rubber. My biggest concern right now is finding a good prop with a high angle so I can get good thrust, and keep the rubber from un-winding too fast. It also needs to be 20cm or smaller. I need to give that some though. Guillows props are out of the question. :D

Oh, anyone know where I can find a large source of the curved Xacto blades needed to shave props? I can't find themin any of my LHS's, and I don't have a car to drive around to the craft stores. Thanks!

Nick

[BMatthews]

You're fitting the construction to the rules and that's what it's all about. I've never seen the SO rules so I had no idea of what I was advising about other than you were obviously building the model stronger than it needed to be for just flight loads.

And for the record a little underweight is not a bad thing. If the prop weight is factored into the model weight then make the tail light and let the prop weight keep the nose moment short and the tail moment longer or even add your legal ballast to the nose to help keep the nose moment short. In this case use the prop weight to bring your model to legal minimum. This longer tail will allow you to run the balance point father aft and the farther aft it is the more efficiently the model will fly as the stab is supporting more of the weight. Kind of like a tandem wing. This is why free flight often uses a lifting stabilizer airfoil. 60 to 80 % or more from the leading edge is normal and good. I have an EZB that balances behind the trailing edge and it's stable.

Look up indoor sites for a copy of the Zero margin of stability or CMOS. This is a procedure for producing a chart that will let you set the model so it has zero conventional pitch stability. How does it cope you ask? Simple, the wing is high mounted on posts and the high center of the wing drag couple provides all the stability you need for indoor flying.

[ED B]

Nick,

You can use a single edge razor blade to scrape the prop. Only scrape on the convex/front side of the prop. Peck Polymers sells good props which are easy to modify. If you feel that more pitch is needed, use a heat gun to soften the prop hub ang give it a twist. They also sell Tan II. A single loop of 3/32 Tan II that weighs 2 grams is about right. Ceiling height is a big factor in how you set up your plane. Find out how high the ceiling is at the event site. You must use rubber lube and stretch wind the rubber. You can go to the AMA web site (www.modelaircraft.org) and use the club locator link to find a club in your area. Experienced modelers are usually glad to share their knowledge with newcomers.

Ed