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Author Topic: Any word on Div. C Wright Stuff planes for 2018-2019 season?  (Read 3027 times)
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leop
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« Reply #100 on: November 05, 2018, 07:48:17 PM »

In reply to the last two posts, this year's Wright Stuff event is made somewhat more challenging by the lack of restrictions on propeller specifications and maximum motor weight.  Further adding to the mix is that the stabs are free in design, only restricted by the need to be of lesser area than the wing.  This means that a team will need to do a great deal of work to optimize the plane design, construction, and setup to maximize the flight times.  There is more than aerodynamics involved in the optimization.  For example, a heavier motor may require structural changes to provide for the necessary air frame stiffness.  Also, the theoretical optimal motor weight is two times the air frame weight.  However, the relative gain from increasing the motor weight above about half than of the air frame weight is not large.  This, combined with the above mentioned structural considerations, will contribute to the challenges this year.  Lastly, flying well (winding, trim, and setup) will be a big factor in flight times.  A non-optimal plane flown well will nearly always have longer flight times than an optimal plane flown poorly.

By the way, the aerodynamic drag (the drag force) increases as the square of the speed.  But, the lift (thrust force for a prop) also increases as the square of the speed.  One advantage of a larger diameter prop is that the aspect ratio of the blade airfoil can be larger compared to prop with a lesser diameter when both have the same blade area.

LeoP
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bjt4888
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« Reply #101 on: November 05, 2018, 10:24:51 PM »

Leo,

Thanks very much for the analysis of this year’s design; much appreciated.

Brian
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calgoddard
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« Reply #102 on: November 06, 2018, 07:43:32 PM »

Crtomir -

Get the weight of your models down to 8 grams and you will see a significant improvement in your flight times. You must be using very heavy balsa wood as most 2019 WS models come in under-weight.  Certainly a balsa wood prop will solve your over-weight problem.  Indoor Ikara props are much heavier.  You may end up having to add ballast.

Actually indoor Ikara props are quite durable, probably more so than a lightweight balsa wood prop.  However the latter can survive many head-on encounters with obstructions without damage.  I must comment that the plastic film blades of the Ikara props have an annoying tendency to detach from their injection molded plastic spars.  I never had any luck re-pitching indoor Ikara props with heat or cold-forming.  I have cut off the blades at the base of the spars and then inserted the spars into a small segment of Aluminum tubing that serves as a hub.  You can then set the pitch and glue the spars in place in the hub.
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Crtomir
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« Reply #103 on: November 07, 2018, 05:01:24 PM »

calgoddard:

We will definitely try lighter wood to bring the model weight down.  The wood we used was a little heavy.  The wing and stab leading and trailing edges were 3/32" x 3/32" sticks stripped from 3/32" sheet that was about a 10-12# density.  The ribs of the wing and stab were 1/16" x 3/32" cut from 1/16" sheet that was about closer to 15# density.  The fins on the wing and stab were cut from 1/32" sheet that was very light, probably around 5-8#.  The motorstick was 3/16" x 1/2" x 10" long and was 5# density.  The tail boom was roughly 3/32" x 1/4" x 12" with a density of 10-12#.  The Ikara prop weighed about 2.25g.  The rear motor hook was too heavy for sure, although I don't remember what it was.  The music wire was thicker than we needed.  The plastic film covering for the wing and stab probably added up to 0.6-0.7g.  So there is definitely room for improvement. 

We also found that the Ikara prop blades tend to split away from the spars.  Definitely a drawback.  Can't wait to try some balsa props, but we need to build a prop block first to give the correct pitch to diameter ratio. 
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Olbill
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« Reply #104 on: November 07, 2018, 05:28:24 PM »

You can put some thin CA along the Ikara spars to make them stay attached. I've personally never had them come apart.
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calgoddard
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« Reply #105 on: November 07, 2018, 05:47:07 PM »

Crtomir -

Only use 1/16 x 1/16 balsa strips for the LEs and TEs.  3/32 x 3/32 strips are way too big and heavy for an indoor duration stick model like a 2019 WS airplane.

In general, don't use balsa wood any heavier than 6 - 8# density for constructing WS airplanes.  You probably can't find 3 - 5# balsa wood.  It is too light and not necessary for a WS airplane. It is also too weak for the durability needed in a model handled by students. Save your super light balsa wood for building serious LPP, F1L, F1D, A6, etc. models down the road.

Read up about using deflection tests for selecting the stiffest balsa wood strips from a pile of strips of similar weight.

Ribs cut from 1/16 sheet balsa are fine for a WS airplane.  It is better to laminate two 1/32 sheets on a curved form, and slice the ribs from the cured laminated product with a balsa stripper. Ribs cut from a 1/16 sheet can break along the grain.

There is one exception regarding the density of the wood that you use to build a WS airplane.  Use the heaviest balsa wood you can locate for the wing posts. They should be 1/16 x 1/16.
 
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