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Sep 15, 2019, 01:24 PM
Everything's A Composite
Knoll53's Avatar

more 3m wing bending


As a point of reference, I did a little testing on what I consider a lightly designed wing. It is a Flair kit, Ka8 with an upgraded spar. At least 50K carbon tow was added to the wood spar caps plus upgraded shear. Also the joiner is 3/8" "Stressproof" steel which is a very high grade. Span between supports in this video is about 3m. 13.2 lbs. was added at the center. Certainly noticable flex. I would say somewhat scale-like at this loading which is about 3Gs. Note to self: I would hate to see this wing at 10Gs. Happily, no cracks or pops during this testing.
Last edited by Knoll53; Sep 15, 2019 at 01:39 PM.
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Sep 15, 2019, 03:36 PM
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Kenneth Paine's Avatar
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Are the wings resting one their MAC?
Sep 15, 2019, 04:08 PM
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Aerofoils Revisited


In post #4 I stated that I had chosen the Selig 7035 to 7037 family of aerofoils and the reasons why. Then rrcdoug reminded me of the Drela AG 24 to 27 family of aerofoils. I compared them side by side to the Selig ones at the correct Re for each panel break and the Drela aerofoils have less drag up to Cl 0.8. both at low Res (thermalling) and high Res.

This comes as no surprise for the Selig airfoils were designed in the late 80s and early 90s when no one was contemplating the extra low Res that Dr Drela has designed his aerofoils for and structural limitations of the day made aerofoils thinner than 9% a challenge. The Drela aerofoils chosen go down to 6.1% thickness vs. the 9% thickness of the Selig ones.

My revised airfoil schedule is below. With this decision made I can get on with making a meal out of my templates.
Sep 15, 2019, 09:58 PM
Everything's A Composite
Knoll53's Avatar
Quote:
Originally Posted by Kenneth Paine
Are the wings resting one their MAC?
The supports are way out near the tips, about a 3m span, so significantly out further than the MAC. The deflection shown is exagerated as compared to actual flight. I like to test as much of the spar as possible. It is much safer and accurate to support the wing at the MAC................but what fun is there in that. I know that there is cf tow all the way out to the tips so I was unconcerned about such testing.

For very accurate deflection test you can flip the wing upside down and add sand bags over the entire wing.
Sep 17, 2019, 11:26 AM
Everything's A Composite
Knoll53's Avatar

2m vs 3m


Here's a comparison between bending moments of this wing at 2m vs 3m wing span.
A thousand pardons for working inch/pounds.

From your previous projects, it's a given that you have a proven wing design at 2m . The question I have is, can that design withstand the higher bending moment of a 3m version. Of course the 3m version has a deeper spar to resist the higher bending moment.

The assumptions are.....................
10G loading for half span at 3m wing is 10Kg = 22 lbs.
10G loading for half span at 2m wing is 5Kg = 11 lbs. ( the smaller plane weighs less)
Max. moment occurs at the sub rib that is at the end of a 4" long joiner sleeve.
The joiner fully supports the wing along it's length, so wing bending starts at the end of the joiner.

I knew that the moment would be much higher at the 3m version, but was surprised at the result of the quick calc.
Sep 17, 2019, 12:49 PM
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Kenneth Paine's Avatar
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Kent,

Thanks for taking the time. Let's forget about the 2m model because the Rotors in the pictures are 60" span and have not flown yet. The original plan calls for 1/64" ply skins which I replaced with glass.

With reference to this 3m model, my calculations give 500 lbs/in (41 lbs/ft) at the end of a 4" joiner tube so very close to yours . To check my layup choice I have used Joe Wurts's Layups.xls spreadsheet which was designed to calculate the layup schedule for winch launched multi-tapered models using stressed skin only on blue foam. The author reports that it is empirically accurate enough, i.e. no wing failures at full winch for 8 to 9 years. I modified the spreadsheet in that I replaced the winch tension with the projected fuselage load at 10G.

The attached chart shows the relationship between the bending moment along the semi-span at 10G (red line) and the bending moment my chosen layup can cope with (green line). The spreadsheet assumes a constant aerofoil thickness. I had to go up to 11% thickness to get the green line to go above the red line at the root. If I had chosen carbon (that would be the blue line at zero across the bottom of the chart) I would have got away with a thinner section at the root.


The point is that at 10G my layup should just cope with the bending loads at the root and comfortably cope 4" in at the end of the joiner (wider gap between red and green lines). Note that the contribution of the woven glass is ignored ( so is the contribution of the filler, primer and paint!)

Will the wings flex? Probably, and certainly more than with comparable carbon skins but will they fail? Only if I am extremely heavy handed.
Sep 17, 2019, 05:03 PM
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Knoll53's Avatar
That looks promising, especially since you do not plan on hooking up to a winch.
Some day I will have to try glass UNI.
Sep 29, 2019, 04:33 PM
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Sections


Below is what the aerofoils should look like as per the previous table in post #33. The dimensions are in millimetres as and show the maximum thickness and the thickness at the hinge line. Check out the tip. Eeek!
Sep 29, 2019, 04:37 PM
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Cores


I finally got round to core cutting. I got through seven cores without an hitch and as the wire was sinking into my final blank, a wing root blank, I was thinking to myself: “this will be the first project where I don’t fluff any of my blanks”. Then I noticed the root chord wire emerging long before the tip chord wire. Aaargh! I had changed blanks but not the taper ratios on the Feathercut. Did you hear my rage earlier on?

I hastily prepared a new blank and fluffed that one too! Third time lucky; results below. I am glad I made the effort with the kerf calculations; my trailing edges have a little meat on them to taper down with the sanding bar. Better that than short trailing edges.

Notice the wing tip in the middle top of the third photo. It looks thick and misshaped but one inch inboard (inside the shucks - not visible in the photo) it looks much better. The sanding bar will bring it in line.

This time I tried cutting the cores upside down. It is much easier to push the wire down and round the LE at the beginning of the cut than to pull the wire up and over as you do with conventional cutting. I got a much better result than in my previous project. However, I have now come to the conclusion that trying to cut LE radius with a hot wire is futile. In future I will just have little ramps into the LE and round the LE radius with a sanding bar.
Last edited by Kenneth Paine; Sep 30, 2019 at 06:41 AM.
Sep 30, 2019, 09:37 PM
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Trent Collins's Avatar
Nice looking cores. interested to see how this comes together and if you get the nice scale flex in flight.
Oct 01, 2019, 01:45 AM
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Kenneth Paine's Avatar
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Quote:
Originally Posted by Trent Collins
Nice looking cores. interested to see how this comes together and if you get the nice scale flex in flight.
Thank you Trent.

I don't know what level full sized gliders are stressed to; 5-6G? I am going for 10G to prevent my wings clapping when horsing around the slope or flying at spec height while thermal soaring. I don't want the FlyFly experience so I will be very worried if I do get scale flex during normal flying.

K
Oct 01, 2019, 09:58 AM
Everything's A Composite
Knoll53's Avatar
Full size gliders always have a safety factor built into the rated G loading. Models need not adhere to this and can be designed to failure. That is, if the wing does not explode on the winch launch, it does not matter how much it flexes. If you actually want to control the flex to a certain limit, it becomes a much more interesting engineering calculation. Although I have some engineering background, I prefer to simply build and test.

This giant wing had everyones attention. It had some flex under near 1G loading. I asked the pilot (before he flew) if he planned on doing some aerobatics. His answer was quick. No. I'm not sure what his thinking was. He said the joiner was solid fiberglass. 1 1/8" diameter IIRC. To my mind fiberglass guarantees flex. Apparently the designer choose fiberglass for that reason.

Oct 02, 2019, 03:04 AM
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Kent,

I like to conclude my threads with a video of of the model in flight. When I eventually film the Discus I will deliberately put it in some "high flex" situations so that we can all satisfy our curiosity. It would be good to have a G-meter in the model to correlate with the footage but I I am still flying with a telemetry-free Multiplex MC4000.

K
Oct 02, 2019, 09:48 AM
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Knoll53's Avatar
Correction: You will put it into high load conditions. Whether or not it flexes is yet to be determined...........
I am preparing myself to be amazed.

Stellar build thread so far.

Video !
Oct 02, 2019, 10:08 AM
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Kenneth Paine's Avatar
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Quote:
Originally Posted by Knoll53
Correction: You will put it into high load conditions. Whether or not it flexes is yet to be determined...........
I stand corrected

Quote:
Originally Posted by Knoll53
Stellar build thread so far.
Thanks. Starting the thread invoked Murphy's Law as I expected and contract work came flooding in, so my pace has slowed down. Still, without the thread motivating me I would be even slower.

Nobody wants a video more than me. I still haven't maidened my Rotor which I finished in early August.

K


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