Elastic Launched Glider B

[bernard]

bernard,
Good job of documenting your current glider. It appears that the pylon that attaches the wing to the fuselage is not very substantial. This is an area that you want to be quite rigid so that the wing does not flex or change incidence angle too much during launch. Also, I'm not sure why you are building a shorter span wing than the allowed 32 cm? If I were you, I would build the a 32 cm wing and build to the required minimum weight of 4 grams. I am also noticing that your fuselage is quite thin. A thin fuselage can be stiff enough if it is reinforced with carbon, but I'm thinking that your fuselage may be too flexible. It would be a very good idea to build a glider from a proven design to minimize the potential issues (or, at least to construct a new, stiffer fuselage and more substantial pylon for your current glider). I have notes earlier in this thread describing how to modify the design we used last year for the current year rules. Much more detailed notes regarding construction and trim of this glider are written on the Hip Pocket Aeronautics website.

Thanks and good luck,

Bjt4888

Thanks for your feedback! I kind of followed the plans for the Time Machine, but didn't do everything exactly the same. What might happen with a fuselage that is too flexible? What might cause it to flex mid-flight? Would you happen to know why it doesn't transition well from high speed launch to slow glide? Thanks again!

[jander14indoor]

The Time Machine glider is actually a VERY sophisticated design with a lot of features that may not be evident to a new builder.
Example, your wing is sanded from single sheets of wood, the Time Machine is assembled from 5 pieces of carefully selected balsa with specific properties for that part of the wing.
Not stated in the design, but critical is the flexibility of the wing. If you don't match the original design (unlikely with your single piece of wood) your launch and transition behavior will be very different. Ideal settings may be nowhere near the same.
As already stated, your wing attachment is much less secure than the Time Machine design. Time Machine wing has a pylon supporting the front half of the wing chord and holding it rigid with respect to the fuselage stick. Your pylon is MUCH lest robust and as mentioned probably allows unintended changes to wing angle of attack during launch.
I don't see the carbon fiber on your fuselage that is called out on the Time Machine plan. Again, already mentioned, but this makes a big difference in fuselage stiffness. This is important during launch, a stiffer fuselage will better maintain the wing-stab relationship, allowing for more controlled/consistent launch and transition.
Weight difference, the Time Machine is a 2.0 gm glider. At 3 or rule minimum of 4 gm it has to fly much faster and again, ideal trim will be different.
And don't underestimate the importance of scaling this design to SO maximum. Giving up wing area is seldom a good thing for this kind of endurance event.

Hope that helps some.

Jeff Anderson
Livonia, MI

[SOCoach]

Is there a good ratio of weight wing vs. fuselage? We are trying to get the weight down on our planes . . . we are consistently in the 7 gram range currently and getting 7 to 11 seconds. I realize lowering weight to the 4 gram minimum should greatly improve times but not sure where to concentrate our efforts. Is the ratio 40% fuselage - 60% wing . . . . any suggestions?

[bjt4888]

bernard,
Fuselage flexing will result in erratic fast direction change. Transition issues, if not related to parts flexing, are often an issue of decalage. If nosing over, add 0.5 degree of wing incidence (if you have an incidence adjustable wing mount).

SO Coach,
See the posts at this link. Read all posts by Bill Gowen (OlBill). He is the current AMA national record holder in this event. Also, read my posts in this thread and in the previous year's thread. There is extensive detial on glider design, construction and trimming.

http://www.hippocketaeronautics.com/hpa ... ic=17413.0

Bjt4888

[bernard]

bernard,
Fuselage flexing will result in erratic fast direction change. Transition issues, if not related to parts flexing, are often an issue of decalage. If nosing over, add 0.5 degree of wing incidence (if you have an incidence adjustable wing mount).

Bjt4888

I adjusted my ballast so that the center of gravity is about 1/3 down from the front of the wing. It glides well when thrown slightly at a downward angle, and glides well when launched (with a rubber band) really fast at an upward angle. It doesn't seem to launch well when launched at "medium" speeds (when my rubber band isn't fully stretched.

[SOCoach]

My ELG students are 8th grade girls who are working pretty hard at this event, but they get tired of sanding pretty quickly. We are using 1/8 or 3/32 thick balsa for wings at the start and sand them down. Can we start with 1/16 thick balsa and make that work?

[bernard]

My ELG students are 8th grade girls who are working pretty hard at this event, but they get tired of sanding pretty quickly. We are using 1/8 or 3/32 thick balsa for wings at the start and sand them down. Can we start with 1/16 thick balsa and make that work?

When I did this event for Division C last year, I tried balsa sheets that were 1/32". 1/16" and 3/32" thick. For the thicker sheets, it definitely takes a lot longer to go through (I made a glider just a few days ago with a rather thick sheet since that was all I had left). I think a 1/16" thick sheet would work well, since your wings will likely be much thinner than that (the glider I made a few days ago had a trailing edge of about 2-3 sheets of paper thick). 1/16" is also thick enough so if you want a thicker leading edge it would work well.

If you have 1/8" or 3/32" balsa sheets left and don't want to let them go to waste, you could start with a lower grit sandpaper and once you're close to what you want use a very high grit sandpaper. I've made the mistake of using low grit sandpaper for too long and it has left gashes on my wings which have been hard to even out with high grit.

[SOCoach]

Thanks for the advice, I'll try that. I have good students with good intentions, but not enough patience.

[jander14indoor]

Probably just saying this another way, but sanding a glider wing should not be an exercise in patience. We're talking well less than an hour, should be much less.
Either the paper is too fine to start (or possibly dull) or the balsa is to hard (heavy, dense). Starting with 100 grit is probably appropriate, keep the force light. When close, switch to 150 grit, maybe finishing off with 220 grit.
You may also need to stop periodically to knock (vacuum) the dust off the paper so it doesn't clog and stop cutting. Some of the stearate coated or open coat papers might help there.

Make sure you use long sanding blocks to sand evenly. I use a piece of melamine covered shelving cut to 11 inches long by about 2 inches wide for the convex portions. For concave portions you will need some form of shaped block.

Jeff Anderson
Livonia, MI

[bjt4888]

SOCoach,
I am 100% in agreement with Jeff Anderson regarding sanding, sandpaper grits and sanding block shape and size. I'll add that my favorite sandpaper for grits 220 through 100 is 3M Sandblaster and my favorite extra fine sandpaper (320 through 600 grit), if you want to use it (not really necessary), is Gator Waterproof.

Also, as I have mentioned before, I have posted various narratives in Hip Pocket Aeronautics Science Olympiad discussion thread describing sanding of a 1/16" sheet wing down to target weight for a 4.0 gram flapper glider (this year, and also narrative for last year's HS event w/ a 3.0 gram glider of smaller span). A wing blank 1/16" thick of 6.0 lb/cu ft wood will weigh about 3.4 grams.

If you are intending to build to the 4.0 gram minimum weight, you will not want to use anything thicker than 1/16" as a starting point for a low-ceiling glider (up to 30-35 ft, or even a little more). Once a 1/16" thick wing blank that weighs 3.4 grams is properly sanded, it will weigh about 2.2 grams.

Brian T.