Elastic Launched Glider C

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jander14indoor
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Re: Elastic Launched Glider C

Post by jander14indoor » March 4th, 2013, 7:52 am

First, try increasing the chord WITHOUT gaining weight. Sometimes possible by improving build technique or materials. Better balsa, lighter covering, etc. What is your current weight?

Any weight gain MUST be more than offset by the increase in wing area to be worth doing. Example, if you increase wing area 20% you want to increase mass by say 10% at most. Otherwise you don't fly slower, reduce drag, etc.

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Re: Elastic Launched Glider C

Post by Asteroidea » March 4th, 2013, 10:06 am

Thanks for the response! Our glider is pretty small it has a chord length of around 2 1/4 of an inch and is 2.7 grams but it's wingspan is fairly small at only 19.5 cm. Should I rebuild our glider using the same design but increase the wingspan/chord length while trying to keep the weight as low as possible? Thanks

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Re: Elastic Launched Glider C

Post by jander14indoor » March 4th, 2013, 11:28 am

YES!

OK, got caught with an assumption when I said weight first. The assumption was you are already maximizing the span allowed under the current rules.

In many SO flying events the wing area is limited by the rules, so all designs use the full allowed area and weight becomes the only concern.

In reality, its the wing loading, or mass per unit area that has to be minimized. So like I said, you MUST increase wing AREA faster than total aircraft MASS to improve time.

Increasing span almost never has a negative impact, so take it right up to just under the limit.

Increasing chord is more problematic. Mainly because drag increases with chord in a messy fashion. Up to a point it is worth it, beyond that not. Where that point is depends on multiple factors, theoretically calculatable, but as a practical matter cut and try is probably better at this stage. But chords above 7.5 cm are doable with a 30 cm span.

Oh, and a larger wing will affect the needed length of the tail and stabilizer area.

Careful wood selection and building practices should allow you to build a glider with max span and reasonable chord under three grams. I've heard of reports in the 2 gram range.

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Re: Elastic Launched Glider C

Post by _HenryHscioly_ » March 5th, 2013, 12:29 am

What is the difference between Glide Angle from Angle of Attack from Incidence..? glide angle and sink rate?
If I want "x" angle of attack, is that influenced by the Glide Angle? or does the glider actually line up with its flight path so that only Incidence matters... no..?lol ._.
I read that angle of attack greater than 15degree generally leads to stall, and that 4degree is possibly optimal for lowest sink rate.
However...my glider's airfoil seems to be parallel with the ground, and meeting incoming air at almost 15,20 degrees while descending..
or are the gliders actually pitched down so that the wing meets air at 4 degrees, and the glide angle is greater than 4 degrees?

So... Cd/Cl relates to glide ratio and Cd/CL^1.5 relates to sink rate... kinda confused

reduce weight and angle of attack can be reduced since less lift needed..and also lower sink rate because its lighters...and thus approach 4degree glide angle....?

uh, sorry for all the jumbled questions... >.<
:?

Adding more noseweight and more up elevator changes incidence, does it also change angle of attack? glide angle? its lighter so one effect is lower sink rate, it'd somehow affect CL and CD?

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Re: Elastic Launched Glider C

Post by fifty_missions » March 5th, 2013, 5:33 am

Optimum glide would be right at the cusp of a stall. The more wing area while being lightweight allows more opportunity to fly close to the stall without entering. Ratios in design that have that potential would be...

Wing area- 30 cm span with 7+ cm chord.
Tail area- 15% of wing area
Fuselage length- 34+ cm, note: do not trade weight with strength on the fuselage!! Try for strength and lightweight.
Glider weighs 3 grams or less.
Also note that gliders will gain weight quickly with too much glue and too many repairs.

Note that slow flying CAT 1 AMA gliders will, with many designs, actually fly nose high by a very small degree. These all have low wing loadings. Lots of wing area but very light construction.

Just be sure that the glider's wing and stabilizer set-up has 0-0 degrees of decalage where the wing and stabilizer are dead parallel. Adding any positive incidence in the wing will mean more clay than is needed in the nose.

I am always hearing a student tell me that they built a glider under 3 grams but no nose weight . With a glider too short coupled (ie short nose or short tailboom) the pitch sensitivities far exceed a good glide ratio. Start with a fuselage longer than the wingspan and always optimize the wing area (span x chord).

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Re: Elastic Launched Glider C

Post by Bizzle » March 5th, 2013, 3:20 pm

So I have a question....

I have been getting ok times of anywhere from 10-18 seconds, depending on the launch, but i find myself building lots of gliders (3 a week) because they are constanly breaking on my trimming flights. The same thing seems to be happening when i am launching it for the first 5-6 times i do not know which way to roll the glider (to the right or left) and when the roll is off it goes into a nose dive into the floor and usually ends up breaking off the nose of the plane.

Is there anyway to tell BEFORE launching for the first time which way I have to roll it (left or right)?

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Re: Elastic Launched Glider C

Post by wlsguy » March 5th, 2013, 3:32 pm

Bizzle wrote:So I have a question....

I have been getting ok times of anywhere from 10-18 seconds, depending on the launch, but i find myself building lots of gliders (3 a week) because they are constanly breaking on my trimming flights. The same thing seems to be happening when i am launching it for the first 5-6 times i do not know which way to roll the glider (to the right or left) and when the roll is off it goes into a nose dive into the floor and usually ends up breaking off the nose of the plane.

Is there anyway to tell BEFORE launching for the first time which way I have to roll it (left or right)?
Try launching flat and level at 25-40% power. The glider will tell you a few things.
1) it will turn right or left (or go straight). This tells you which way it wants to go.
2) it will dive, rapidly climb, or climb slightly and go into a nice level glide. Your goal is the last one. if you have one of the other two cases you may need to adjust incidence.

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Re: Elastic Launched Glider C

Post by fifty_missions » March 5th, 2013, 10:09 pm

Inspect the glider for any asymmetry issues. Check the leading and trailing edges of the wings, stab and fin. A trailing edge that is lower than the opposite location on the glider, the lowest trailing edge induces lift. On a wing, that lower trailing edge will cause a roll or banking condition. Before modifying the asymmetric issues, give the glider a test flight. But just a handlaunch is enough to see the effect. The trick is to launch slightly nose down otherwise the glider may stall with the sudden thrust on launch. If there is any tendency to dive, give some up elevator. If there is any roll, reduce the down aileron effect on the lowest trailing edge. If the glider enters a glide smoothly then it is ready for an elastic launch. Start just above level and gradually increase the angle of attack (angle of launch) through a progression of launches. At some point you should find the "sweet spot" for launch and smooth transition into the glide.

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Re: Elastic Launched Glider C

Post by 314chocolate » March 6th, 2013, 9:38 am

First, try increasing the chord WITHOUT gaining weight. Sometimes possible by improving build technique or materials. Better balsa, lighter covering, etc. What is your current weight?
My current weight with ballast is ~3.5 grams. What would you recommend for balsa?

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Re: Elastic Launched Glider C

Post by Asteroidea » March 6th, 2013, 12:28 pm

I have a quick question regarding launch tabs, are they necessary and what are the effects of using them? I've always assumed that they were more for HLG rather than CLGs, but it seems that based on the plans I've been looking even CLGs have some sort of a launch tab. Thanks!

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