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rtfm

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Hi mcrae0104,
Well, I'm stunned. Not only by your grasp of the issues involved, or by your working knowledge of the major texts dealing with the issues, but also by your perseverence (NINE PAGES) and your ability to follow a logical train of reasoning. I'm tempted to print off those nine pages and fix them to the wall in my shed.

Makes my nine LINE spreadsheet calculation look plain silly.

Thank you VERY much for your hard work.

Duncan
 

rtfm

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Well, I finally grabbed the bull by the horns, and put epoxy where my mouth is (says he, wildly mixing his metaphors). Well, you know what I mean.
1620700988980.png
Basically, I bonded the leading edge reinforcement and two spars to the ribs. They form a convenient triangle, so I was able to ensure that the ribs weren't twisted. I used T88, mainly because the West System epoxy is like water, and I wanted the treacle-like consistency of the T88 to ensure it stayed where I put it.

Each spar cap consists of two pieces of 19mm x 19mm Hoop Pine - I bonded the bottom ones only. Next up will be to bond the shear webs to the bottom set of spar caps, and slip the top spar cap in the remaining gap. I'm not expecting that step to be so finicky as this one. Things should be solidly held in place by tomorrow.

I'm taking this terribly slowly, because I need to work out how best to do this.

Notice the spar web sections in place, but not in contact with the epoxy. They ensure that the spacing of the ribs is correct Quite convenient, really.

I have to say, my upright jig worked really well.

Duncan
 

mcrae0104

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I managed to get a spreadsheet to very nearly match the values I calculated yesterday* and then adjusted for the 40% load on the rear wing (and not applying all the lift to one wing panel--doh!). The stresses are lower than before--we know this because the max. shear and bending values are lower on the diagram below. The rear wing looks plenty strong. If you end up having weight problems, the spars could potentially go on a little diet (perhaps a few pounds?).

I've also persuaded myself that the interrupted shear web can be acceptable. If we round up the max. shear to 400 lb, then each of the "stubs" is carrying 100 lb shear over a cantilever of 1". Both the shear and bending stresses resulting from that cantilevered point load are pretty low. The moment carried by the wing (say 4,500 in-lb at station 44.49", the slot with the highest moment that's nearest to the strut) gets resolved as a couple in the upper and lower cap "stubs" with an axial force of about 900 lb. I have to admit that I've never done combined bending and axial using Bruhn's methods, but these stresses are off the low end of the FPL charts. Also your stress at the glue line between the caps and webs is quite low.

I would feel better, though, about adding some small vertical wooden stiffeners on either side of the webs where they touch the foam; relying on the bond to the foam on the thin edge of the plywood might not be best practice as a stiffening device, but the proof will be in the test, not Some Guy on the Internet With an Opinion (like me).

*I used a rather crude method to integrate lift to generate the shear curve, and then once again to integrate shear to generate the moment curve. The inaccuracies get magnified at each step, but I'm pretty confident the end product is within 5% or so--WAY less than the margin of safety you have here. You can tell there's a problem in my method since the moment curve doesn't quite return to zero at the left end. (The shear curve does not return to zero at the left end because of the small reaction at the fuselage attachment. Sometime I need to tinker with a way to do real integration in Excel and this would be more precise.

1620721549552.png

1620721590539.png

1620721656727.png

One way you might be able to save some weight is by attaching the strut further out. Of course, this makes the strut floppier in compression and adds to axial stress in the spar--there are always trade-offs. But notice how the diagrams change (lower max. shear and moment) if it's moved out to 1.2m (here again you see the non-zero problem at the left end of the moment diagram, but this is just a conceptual comparison):

1620723166466.png

Also, thanks for your kind words, but in truth I'm just a hack with a stack of books and a calculator. :) My opinion here is worth what you paid for it--trust your tests and not me.

I should be able to run the front spar relatively easily now with the spreadsheet. What does the support look like? One center point and a strut on each side, or just two struts? Similar rib spacing? I assume the front is a one-piece spar.
 

rtfm

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Hi,
I love the pretty graphs, although I don't really understand what they mean. It is comforting to know that the rear wing seems strong enough, even though a little beefy. At least it gives me some confidence that the load test will be OK.

This is a Flea, so the struts converge on a single point in line with the single point of the masts (just ahead of the 25% chord) allowing the wing to pivot. The inner wing panel on the front wing has a span of 1.25m and is joined to the outer 1.25m panel on hinges (like all Fleas do). However, the two inner panels are joined into a single 2.5m wing section. Similar rib spacing. So I can't move the struts further out...

Although in Fleas, the rear wing is fixed and only the front wing pivots, I am implementing a ground-adjustable rear wing pivoting mechanism allowing pilots to set their own rear wing incidence. Once the plane has flown and I have some considerable time on it, I'd like to experiment with a fully pivoting rear wing also, which is another reason I'm implementing the same pivoting mechanism on the rear wing.

Thanks, and regards
Duncan
 

Victor Bravo

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I believe you need to have someone involved in the project who does understand those pretty graphs.

Whether that person is involved at the very beginning (to professionally design the structure so it has a 99.999% chance of being safe), or that person is involved after you build the test wing (to know how many sandbags and where to put them, to prove out an amateur design)... is completely up to you and your wallet and your "hobby" aspect of designing an aircraft.

But the minute you are actually putting your arse (or mine) in to the aircraft, someone who understands that stuff (at a higher level than you or me) has to be involved.
 

Sockmonkey

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You could rig up a remote control and ballast it for the initial test flight. Get a buddy with a truck to take it out to the coast and do it over the water.
 

Victor Bravo

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You could rig up a remote control and ballast it for the initial test flight. Get a buddy with a truck to take it out to the coast and do it over the water.
Yes but that is still not nearly a substitute for formal engineering being applied to this design. Unless he is using highly advanced data streaming, a thousand stress/strain sensors, and a fully computerized system of applying and recording flight test loads to his aircraft.... there is simply no excuse for not having an engineer involved to prove out the design.
 

mcrae0104

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Hi,
I love the pretty graphs, although I don't really understand what they mean. It is comforting to know that the rear wing seems strong enough, even though a little beefy. At least it gives me some confidence that the load test will be OK.

This is a Flea, so the struts converge on a single point in line with the single point of the masts (just ahead of the 25% chord) allowing the wing to pivot. The inner wing panel on the front wing has a span of 1.25m and is joined to the outer 1.25m panel on hinges (like all Fleas do). However, the two inner panels are joined into a single 2.5m wing section. Similar rib spacing. So I can't move the struts further out...

Although in Fleas, the rear wing is fixed and only the front wing pivots, I am implementing a ground-adjustable rear wing pivoting mechanism allowing pilots to set their own rear wing incidence. Once the plane has flown and I have some considerable time on it, I'd like to experiment with a fully pivoting rear wing also, which is another reason I'm implementing the same pivoting mechanism on the rear wing.

Thanks, and regards
Duncan
I have to admit I'm not all that familiar with Fleas. I looked at some images, but I see differing strut configurations. I just want to make sure I understand what you're saying and that we're using the same terminology. Is this right? Also, I believe you said it has 6m span earlier so maybe I'm a little confused.

1620759040869.png
 

WonderousMountain

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I would feel better, though, about adding some small vertical wooden stiffeners on either side of the webs where they touch the foam; relying on the bond to the foam on the thin edge of the plywood might not be best practice as a stiffening device, but the proof will be in the test, not Some Guy on the Internet With an Opinion (like me).
Me too, I would feel more comfortable with sheer web stabilizers.
However, my spacing would be 4" , 2" off the vertical centerline on
each plywood panel. This would provide better support than spaced
37-40mm apart, allowing the plate to catch up with carrying some
of the load before introducing the tiny columns.

Sincere regards,
CK LuPii
 

Sockmonkey

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Yes but that is still not nearly a substitute for formal engineering being applied to this design. Unless he is using highly advanced data streaming, a thousand stress/strain sensors, and a fully computerized system of applying and recording flight test loads to his aircraft.... there is simply no excuse for not having an engineer involved to prove out the design.
I certainly wasn't suggesting that he skip the engineering. Only that the first flight be RC in case something was missed.
 

mcrae0104

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On the subject of pretty graphs...

We draw the shear & moment diagrams to identify the maximum shear force and maximum bending moment experienced by a beam. Without these maximum values, we don't know how big to make the beam. It would be a good idea to be familiar with "reading" these diagrams, even if you're not getting into calculating them yourself, because they help you "read" and understand the forces on the beam and they point out the critical locations at a glance.
  1. Loading diagram. How much lift is being applied at various points along the span? Where are the struts and attach fittings, and how much force are they applying to the spar?
  2. Shear diagram. With this, we can see how much shear force the spar experiences at any given point. (The shear force is not constant along the length of the spar.) We can also see at a glance the minimum and maximum shear forces, and their location along the spar.
  3. Moment diagram. Similar to a shear diagram, this shows the bending moment at any given point.


Here is how to read the diagrams:

Loading diagram:

1620764304541.png

Shear diagram:

1620762958186.png

Moment diagram:

1620763914677.png
 
Last edited:

challenger_II

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Fisher County, Tx. USA
You may want to try lemon-scented ammonia. You can use a hand sprayer bottle, from the Dollar Store, spray down the surface, and then form. When the ammonia evaporates, it will help prevent mildew, where-as water will encourage mildew. Also, it doesn't require boiling. :)
Method works on ply, and capstrips.


I think I've nailed it. I built a D-tube bending jig yesterday, and bent my first D-tube this morning. Worked like a charm! LOTS of boiling water painted on (mainly on the outside surface) with a broad paintbrush. Then gently pulled the ply into the former, while painting on hot water. In very short order, the skin was ready to be pulled all the way in using a SS pipe as you can see. Now I just have to wait for everything to dry.
View attachment 109654
 

rtfm

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Hi,
Perhaps I should have spent more time studying the graphs before writing, but I was excited by your efforts. Further examination, and with the help of your diagrams, it's all pretty clear now. Thank you.

Me too, I would feel more comfortable with sheer web stabilizers.
However, my spacing would be 4" , 2" off the vertical centerline on
each plywood panel. This would provide better support than spaced
37-40mm apart, allowing the plate to catch up with carrying some
of the load before introducing the tiny columns.

Sincere regards,
CK LuPii
Hi LiPii
Sounds like a good idea. Thank you.

Mcrae0104,
Your diagram of a Flea (and of the Fleabike) is exactly right. The inner two 1.25m panels are built separately (i.e. separate spars) but joined together, forming a single unit.

Duncan
 

rtfm

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You may want to try lemon-scented ammonia. You can use a hand sprayer bottle, from the Dollar Store, spray down the surface, and then form. When the ammonia evaporates, it will help prevent mildew, where-as water will encourage mildew. Also, it doesn't require boiling. :)
Method works on ply, and capstrips.
That's a great idea. I'll give it a go on the next D-tube. Thanks for the tip.
 

rtfm

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Well, I'm not a happy camper today. I left my leading edge and spars to bond overnight, and something went wrong. Either the cat jumped on the ribs or something else, but this morning, the entire thing was skewed to one side.
1620856129771.png

It's at times like this I just want to give up.

So after a prolonged period of cursing and throwing up of hands in the workshop, I cut a new set of foam ribs (no holes this time) as well as new plywood ribs. Take 2 today.
 
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