NFFS Super Simple Div B Airplane - build video, etc.

[bjt4888]

All,

As is already posted in the "Flight B/C" thread, the National Free Flight Society (NFFS) has quite a lot of new video, picture and narrative resources for 2023-24 here: https://www.freeflight.org/science-olym ... resources/ These resources are organized so that the first few links in the "Build" and "Flying" and "Video Library" tabs are materials created new this year to address the most common questions we see from Flight Event participants (simple building, trimming, rubber winding descriptions, video and pictorial resources).

This airplane is flies two minutes in a gym takes 30 minutes to build, can be built from craft store balsa and costs $10-$15 (depending upon how many copies you build and having a local craft store; more copies spreads shipping costs for propellers and parts).

A super simple Div B airplane build video is the first video here on this new NFFS website: https://www.freeflight.org/science-olym ... ad-videos/

I am starting a new thread for this video as I am getting questions related to the video that I want to share.

Brian T

[bjt4888]

All,

Here is one of the questions that came to me as a PM, and my reply, regarding the Super Simple Div B Airplane build video:

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Hi Coach Brian,

I'm currently ordering materials to build your super simple Division B design with a group of 4 teams, and I'm very excited! A couple of questions/clarifications:

- My team is planning to build 4 planes exactly as built in the video, as well as a separate set of 4 "experimental wings" with the goal of using lighter wood to maximize the rules-allowed wingspan and potentially trying larger chords. Using some rough pythagorean theorem, I calculated a maximum allowable wingspan in division B of 46.5 cm (18.3 in). Does that sound about right? I was planning on building these "experimental wings" by ordering light balsa from Specialized Balsa for the wing spars and ribs (vs. your suggested medium density balsa) and extra light balsa for the stabilizer and fins (vs. your suggested light balsa).
- I was hoping to try 5'' or 6'' wing chords with some of these experimental ones, inspired by Gowen's Super Finny. Would I just uniformly scale the stabilizer and fin?
Thank you for all your help!

Hussain
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Hussain,

Sounds like and interesting strategy. Build the design and then modify.

The box we have constructed is 40.5 cm in width and 28.2 cm in height and is made of 1/8" thick corrugated box material. The diagonal measure of the inside dimension of this box is 18.75", so 18.3" planned projected span is a good target and is a span measure that I would have no problem with using with my students. I don't like to push the limits on this too close as this potentially can make judging stressful for the students and another 1/4" span will gain almost zero performance increase. I’m sure you have this covered, but remember that flat span of the tips and projected span are different once the wingtips are angled up for dihedral (more trig to calculate).

Also, a quick note; this Super Simple Airplane will fly pretty well for the time invested to build, but is not likely to be a Regional or State winner in the more competitive parts of the country. The purpose of this design and video is to demonstrate that pretty good performance can be achieved from simple, cheap and quickly built designs. For teams that want to win competitive States or Nationals, one of the top-level kits would be a better idea. Note that the airplane in the build video is so simple it doesn't even have a curved wing/stab airfoil, just staight sticks for wing/stab ribs. Modifying the Super Simple airplane is interesting science and can lead to improved performance, but, fair warning, getting SSM/CG/Nose Length/Trim etc. set as wing and stab area are changed can require lots of testing.

You will probably find that balsa that is less than 8 lb cu ft density might be too fragile for wing spars for this design. I would go with at least 10 lb cu ft for the design as is and for the longer span (18.3") at least 12-14 lb cu ft otherwise there might be too much breakage. Going with light balsa for wing and stab ribs is fine though (5-6 lb cu ft) and 6 -7 lb cu ft for fin parts and tailboom. Stab spars can be lighter 8 lb cu ft probably would be a minimum. You can definitely try lighter wood for spars, but my experience with middle schoolers is that, even with careful coaching, they occasional brush their airplanes against things (clothes, tables, boxes, etc.) and this could break light spars.

Be sure to test your glue stick adhesion to the HDPE grocery store bag material. I found the best glue stick was either Elmers Extreme or Elmers Craft Bond extra strength and even these just barely adhered HDPE to balsa. I had to rub back and forth on the sticks to get a little more glue applied.

A different glue that works perfectly on HDPE is Weldwood Non-flammable Contact Cement thinned 50% with water and applied per page 16-17 of this article by Chuck Markos: https://www.freeflight.org/wp-content/u ... Manual.pdf This article is also on the NFFS SO website Student Resources/Build Tab. You can also use gift wrap tissue for covering.

Tissue will bond with any glue stick, but will increase weight of the airplane in the video by about 0.6 grams (7.75 grams instead of 7.15 grams).

If you increase the wing chord and/or span, you will want to also increase the stabilizer size to keep its area about the same percent of the wing. The stab in the current design is 43% of wing area. A bigger stab (50% or more of wing area) is also a reasonable thing to test although bigger will mean a little heavier which shortens the tail moment arm (big stab is beneficial, shorter tail moment is not beneficial); testing determines best combo).

The fin, fuselage and tailboom can stay the same unless you want to extend the tailboom to use the full length of the box (current design is only 20.5" total length). For bigger wings and stabs, this is effectively a new design and you will want to calculate the Static Stability Margin to locate the CG correctly . This SSM/CG calculator works fine: https://rcplanes.online/cg_calc.htm. It's a CG calculator, but you can plug in different values till you get the SSM desired. The SSM on the current design is 17%. A quirk of super wide chord wings is that they can sometimes fly well with very low SSM. Our B Div airplane last year trimmed at 2% SSM.

A possible issue with large wings/stabs with the Super Simple design is that balsa spars are not very stiff unless relatively high density (heavier) and a wing that is not stiff enough will have issues if it is in rough air or is recovering from a ceiling hit. Sometimes, after a ceiling hit that causes the airplane to dive a little, instead of recovering, the airplane wing will "tuck" (twist the wrong way due to air pressures of fast flying) and this will lead to a fast dive all the way to the floor. Larger wings might be better with carbon rod spars. You can test this though. A larger wing/stab version of this Super Simple airplane, built with stiffer spars (14-16 lb cu ft) will probably be over 8 grams, but if you want to try, I'd say go for it. You can also use harder balsa spars in thinner dimensions as an option, like 1/16” square 16 lb cu ft spars.

If you use winglets, like the Finny, you will get less span in the box, maybe about 17.25" (if using 1.625" tall winglets). Winglets of this height cost us 5% projected span reduction last year (from absolute maximum) vs. tip dihedral or V dihedral wings.

Absolute maximum span this year with a box that is 1 mm from the limits in all dimensions is 19.12". I definitely don’t recommend building an absolute maximum dimension box or maximum dimension airplane. Make the fit in the box easy with ¼” to spare in all dimensions and make the box 0.5 cm undersized in all dimensions (maybe 0.3 cm undersized in the height dimension is ok too as height can be trimmed after box construction if needed).

I'd like to post this in the public forum to benefit other students. Let me know if there are elements of your question you would like me to remove before I do this.

Have fun!

Brian T

[Maxout]

A team at the Dodgen-Walton invitational was flying a variant of this airplane. It flew impressively well, about 100 seconds as I recall. They had an initial problem getting it to fly because the covering was peeling. Once that was fixed to floated away beautifully. Very nice design.

[bjt4888]

A team at the Dodgen-Walton invitational was flying a variant of this airplane. It flew impressively well, about 100 seconds as I recall. They had an initial problem getting it to fly because the covering was peeling. Once that was fixed to floated away beautifully. Very nice design.

Josh,

Glad teams are getting good flights from the airplane. Too bad about the peeling covering.

Definitely it is important to for students to use the Elmer’s craft bond extra strength glue stick that I noted in the video instructions. The glue stick I am using in the video is a very old UHU purple that is quite sticky. I tested the newer UHU sticks (and quite a few others) and they are not good enough.

The Weldwood contact cement that Chuck Marcos recommends in the article linked in the build video is even better and recent tests are showing that Elmer’s rubber cement works well (applied with a small disposable brush).

Brian T

[Maxout]

Yeah we had a conversation about glue. Elmer's school glue is absolute trash and most people are unaware of this. After all it's heavily marketed.

[jander14indoor]

Has anyone found a good, currently available, glue stick for this plane?

[bjt4888]

Jeff,

I have tried the following glue sticks: UHU white: UHU blue, Gorilla glue, Elmers Craft Bond Extra Strength and Elmers Extreme. Craft Bond Extra strength was the most promising, but after a couple of weeks, the demo stick I attached to grocery bag covering came off a little to easily.

I am trying Weldbond, a nontoxic "rubbery" drying white glue, and a standard hardware store item commonly available. This is a liquid glue with the consistency a little thicker than elmers white glue. It has reasonable working time and can be applied from a small puddle with a toothpick. I've used this glue as a substitute for epoxy to attach foam flaps to catapult launch gliders and it works well for this application even in very small amounts. I do think that students could use this glue.

However, this being said, I would be inclined to use tissue covering instead of the grocery bag covering for workshops as it is only adds 0.6 grams overall and can be applied with any generic school glue stick. I should have done the video with tissue. Use of tissue and recommendation to test glue stick bond overnight before using is in the video description/build instructions.

Thanks for looking into this too.

Brian T

[jander14indoor]

Thanks, that gives me ones NOT to try, and an alternative covering.
I did try Alene's Permanent glue stick and it seems OK, not great, after a couple of days.
Weldbond is reliable, just a little messier for clinics.

Jeff Anderson
Livonia, MI