Eagler's Nest
Airframes => Single Seaters => XL => Topic started by: Dave Stroud on March 18, 2017, 03:56:37 PM
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More questions again...please. The plans and the narrative indicate that the wing angle of incidence is set by having the front spar one inch higher than the rear spar and that is fixed by the wing mounts being built into the fuselage, thus attaining the required 2 degrees. This whole scenario seems to be based on the idea that the bottom of the front spar should be one inch higher than the bottom of the rear spar. So...in order for that to be accurately done, could we assume that the front and rear spar wing attach fittings should have their fuselage mount holes the same distance from the bottom of each spar ? I would think so, but my plans on page 35 seem to indicate otherwise.
My main spar is about 6 7/8" tall. My rear spar is about 5" tall. If you look at page 35 in the plans it says the wing attach fitting hole at the fuselage end should be 5" down from the top on the front spar and that would leave the hole about 1 7/8" up from the bottom. On the same page, if you look at the 3 3/4" drop from the top on the 5" tall rear spar, you are left with only 1 1/4" up from the bottom. That seems to be about 5/8" vertical difference between the two spar fitting holes where they meet the fuselage.
Any ideas appreciated....thanks.
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I haven't built my wings yet...but I would space them the same from the bottom and disregard the top measurements...that was my plan anyway.
I'm sure somebody else will chime in with some better info.
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Don't worry about the spars... The bottom of the wing is flat. Go off that. Cut out a plywood (luan) airfoil from a copy of the rib drawing, and mock up the spar fittings to this. With a level on the backbone longeron, get the 2 degrees at the root either by trig or cell phone app and build the front and rear wing mounts accordingly. The bottom of the root is 2 degrees the tip is rigged level. This sets your washout...
https://play.google.com/store/apps/details?id=com.plaincode.clinometer&hl=en (https://play.google.com/store/apps/details?id=com.plaincode.clinometer&hl=en)
(https://lh6.ggpht.com/5GkDPgekECw1OSCqr5qztGNxxXbaKqf8wnbkI9Mwg5wchKYq6cWnLKUGLGrnTXfp=h900)
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Thanks, Dan...but we're not talking about washout here. We're talking about the vertical measurement of the wing root spar fitting's bolt hole in relationship to the bottom of the wing spar, front spar vs. rear spar. I think they should be equal....front and rear spar, yet the plans on page 35 say different.
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Thanks, Dan...but we're not talking about washout here. We're talking about the vertical measurement of the wing root spar fitting's bolt hole in relationship to the bottom of the wing spar, front spar vs. rear spar. I think they should be equal....front and rear spar, yet the plans on page 35 say different.
What I am saying is the bottom of the wing is flat... Take advantage of that fact to figure out how to proceed.
You will be able to take advantage of it when you are ready to rig the lift struts as well...
Cheers.
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I see what Dan is saying Dave. If you like I can measure my fuselage. Mine should be the same angle as I didn't change wing position at all. I'll do it later after the sun comes up. If the fuse has the required 2° the wing fittings should be spaced the same.
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No need to measure, thanks Dave. I get what Dan is saying. I guess I'm just checking more or less continuously as there's been a few things to wonder about. It throws me off when the numbers on the plans are dead wrong and left up for the builder to interpolate.
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Well, you got me curious...so I measured it anyways. My top tube is 1.8-1.9°
Thats with a smart phone app so no idea as to it's accuracy.
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I cannot seem to cut and paste an URL lately so to see the site I use for distance / angle calculator you'd have to google search angular size calculator . It's easy and versatile. It shows that 2 degrees over a distance of 29" is 1.0124". Conversely, if Arctic Dave seems to have only about 1.85 degrees, that would show a vertical difference of .93645". Another handy example is that the dihedral is supposed to be 3 degrees and the calculator shows that three degrees over 154" would be 8.0653 inches. Nice and easy.
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I cannot seem to cut and paste an URL lately so to see the site I use for distance / angle calculator you'd have to google search angular size calculator .
This it..?
http://www.1728.org/angsize.htm (http://www.1728.org/angsize.htm)
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Yep...nice one, Dan.
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Since the subject has been broached...why is 2° the standard?
I was just given a set of Air Camper blueprints that were purchased in the early 60's. The Air camper has 2° as target AOI as well. A PA-18 on the other hand is spec'd at around 4.5-5° bone stock. A Cub setup for STOL competition can have 7° or more. I've been pondering the benefit of increasing the root angle a bit for a couple of months, and am seriously considering dropping the front wing fitting an inch to double that to 4°. I think slower flight will be possible... at a cost of cruise speed
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Ok, I think I get what your original question is or was. Follow along there could be an error or two in this statement, and I would love to be corrected.
I will agree that a 1" drop over 29 inches (length of top tube) is for practical purposes 2 degrees
I also see that the front spar bottom and the rib bottom is the same.
The rear spar sits over the bottom cap strip which is 1/4 inch.
the bottom of the rear spar sits 3/8 inch higher than the front spar bottom by the differing mount locations.
lower the bottom rib cap strip 1/4 inch and we are within 1/8 of an inch
As in 1/8 inch too high in the rear.
If it is going to keep me awake at night I could fix it while drilling the rear spar mount holes, set the top tube to be a little heavy on the fall,
or work it out on the wing mounts as I fit and weld to the fuselage.
Or a little in all three places. Sort of sneak up on it.
There is also some slop in the rib to rear spar fit up.
I imagine many have been built without any concern and are all over the place. No two planes exactly alike yet they all fly well.
Thanks for pointing it out.
Tom XL-7
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Since the subject has been broached...why is 2° the standard?
I was just given a set of Air Camper blueprints that were purchased in the early 60's. The Air camper has 2° as target AOI as well. A PA-18 on the other hand is spec'd at around 4.5-5° bone stock. A Cub setup for STOL competition can have 7° or more. I've been pondering the benefit of increasing the root angle a bit for a couple of months, and am seriously considering dropping the front wing fitting an inch to double that to 4°. I think slower flight will be possible... at a cost of cruise speed
The air camper and pipers are different airfoils and each may have its own preference. STOL competition is a specialty situation after one goal at much expense to other missions. ultralights are usually very draggy and I would not want to build in any extra.
Make the stabilizer mount posts a little long and experiment where it can be adjusted.
(I have no idea why the font just changed)
Tom XL-7
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Idk why it changes like that. Does it to me too. It sorta makes it look like I'm yelling...weird.
I agree with with you Tom. It depends on mission. A guy trying to shoehorn himself into short strips could find more angle helpful. I'm sure 2° was chosen as a best "all around" angle.
The stabilizer suggestion is a good one...is there even room to get it down to -4°? Without going under the top longeron?
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I cannot seem to cut and paste an URL lately so to see the site I use for distance / angle calculator you'd have to google search angular size calculator .
This it..?
http://www.1728.org/angsize.htm (http://www.1728.org/angsize.htm)
Thanks for finding that Dan. Saved it to my bookmarks.
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I was fiddling around ( good at that ) with some sketches and drew a few adaptors that would raise the front fuselage wing mount an inch. Totally reversible if desired.
Tom XL-7
I have some changes I want to do to my build but have had a lot of good advice not to. So example: I want to use a torque tube to operate ailerons.
But I will weld the pulley ready plates on the fuse and have the ability to change to the tested cable system.
If some of my ideas are not so bright I want to be able to backtrack as painlesslly as possible. I have seen 3 legal eagles, A double , and Sam's XL.
Found them all pleasing. Would be proud to match any of them.
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David, look on page 8 under the heading struts and you will see that the dihedral is 3 inches not 3 degrees. Also the cord line is not the bottom of the wing, so it is about 3 degrees up from the bottom if I remember correctly added to the 2 degrees built into the fuselage and it is about 5 degrees total. Also on page 34 the rear spar is called out at 5 1/4 inches not 5. My wings are drilled to the specs called out on the plans on page 35. I have no dought it will fly fine as many others have already. I hope I have helped.
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Thanks for those clarifications, Poorman.
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Your more than welcome.
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Didn't even think of the fact the wing bottom is not the chord line. If it's at or close to 5° already, I'm not messing with it any further. Thanks Poorman!
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While we're at it, why do you think the plans call out the wing spar to fuselage attach fitting to have it's mount hole 2" out from the end of the spar ? The fuselage mount end of things only needs 1/2" to clear. If the 2" fitting dimension was brought down to say, 1"or 1.5", there'd be less stress on the end of the spar and still room to attach things. Just wondering if that extra space is needed for something.
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DS: Was wondering the same thing.
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just speculation but the 2" for the spar mount hole may just be a carry over from the legal eagle. Both wings mounted to a single hole. actually two, one front and one rear. Yes, there would be an overlap and one wing would be a mount thickness in front of the other.
For those readers with the question of which one to build XL or LE keep in mind that the xl not only picked up some wing area but the drawings contain some popular builder mods and running improvements. Think about mounting both wings at once.
Tom XL-7
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While we're at it, why do you think the plans call out the wing spar to fuselage attach fitting to have it's mount hole 2" out from the end of the spar ? The fuselage mount end of things only needs 1/2" to clear. If the 2" fitting dimension was brought down to say, 1"or 1.5", there'd be less stress on the end of the spar and still room to attach things. Just wondering if that extra space is needed for something.
It's to clear the cabanes. See the last pic.
http://www.mykitlog.com/users/display_log.php?user=ontherun&project=2554&category=0&log=233329&row=75
Darren :)
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And there's the good answer. Thanks, Darren. Not an issue with my squared up cabin so I figure I can shorten my fittings some.
I start the fuselage next week and a couple of days ago I gaffed a real nice building table for only $120 CAD. 10 feet by 30". A heavily built, wood topped retired bakery table with SS frame, shelf rack below and on casters. When the fuselage is done, I'll dump one of my older benches in the garage and install this against the wall with the casters off.
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While we're at it, why do you think the plans call out the wing spar to fuselage attach fitting to have it's mount hole 2" out from the end of the spar ? The fuselage mount end of things only needs 1/2" to clear. If the 2" fitting dimension was brought down to say, 1"or 1.5", there'd be less stress on the end of the spar and still room to attach things. Just wondering if that extra space is needed for something.
When straps are used in the XL fashion from the ends of the wing spar, There is not any undo stress being applied to the spars.
When your flying, the "stress" if you want to call it that, is trying to push the center together horizontally. Or compressing the center..
A large (although) in this case "small" reason for space it to give ample room for your fingers installing the wing attach bolts.
Certainly are those that like things tight, and consider areas best if they are cause of a battle, but I don't think you will find this area under "stress" that would change by taking out 1/4" to 3/4"
I relate this somewhat like the kingpin on a semi tractor- One small pin, pulling for millions of miles a load of 40-45,000 pounds... If you reduce the pins/straps/and holes for the XL to match the % reduction of the 45K load, there wouldn't hardly be anything left.
Looks to be an area where "if it is not broke, why fix it"
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Thanks for that, Scott. What are the most stressed areas on a LE wing spar in flight, please ?
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Thanks for that, Scott. What are the most stressed areas on a LE wing spar in flight, please ?
I am pondering this question Dave. In the mean time I will offer this.
- if there was a problem, we would have heard about it by now
- to really get down to the nitty gritty, one would have benefited to be at the stress test of the wing. I was not there, and even if I was, how the test was done, can effect where this "most stressed" area shows its face. But have never heard that it was at the compression zone of the root. In fact have never heard that of just about any wing.
- While perhaps it would be nice to know this info on just about any wing, when good enough is good enough, I am not sure what most can/or would do about it.. Sometimes, "more better" doesn't really make a difference than good enough.
Just the thoughts for the day!
There are all kinds of books, some really large ones that deal with this question, but that is more of a design aspect than anything else.
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Most importantly there is no professional engineers registration number with this post.
That means this is an opinion. Perhaps an educated one but no more than an opinion.
Therein lies the problem of changing things from a proven design. This wing and slight variations of it has been flying for a very long time. possibly 30 - 40 years.
In a simple world and we don't live in one, there are two types of wing structures. Not talking about materials or methods, just structure.
The cantilever wing is the modern one. heaviest loads are at the root, which is pretty easy to understand because there is no other attachment. Landing gear loads could exceed flight loads. They are outrageous, the gear wants to tear right off backward.
Then there is the braced wing we have. That can be old-time, like a kingpost and cables running everywhere, or strut braced like ours.
These have more drag but can be much lighter.
The vast majority of the lift is carried by the brace, so your highest stress will be in and around that strut mount point.
An attachment may help visualize. Notice in the third sketch the spar loads go from positive to negative at the attach point.
That's where the action is.
Tom XL-7
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Might as well throw this attachment on the pile...
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Gee Dan, I don't know. That looks pretty hard. Wouldn't it be easier to use the drawings? You know.
the ones we paid for.
Or might you be suggesting my attachment is of similar practical use?
There are some interesting things to see and the math has been worked out.
For instance, the load at the strut is 540kG while the load at the fuselage is only 41kG (R1, R2)
That's only 7%. not only that but the fuselage wing mounts are not being "lifted" up but are being pushed down into the cabanes.
That is not how most people would assume.
hey you English majors is this a metaphor:
consider the wing as a teeter-totter. It pivots at the strut attach. The outer section of the wings lift pushes the inner section down.
Tom XL7
We haven't even talked about the compression loads.
But I feel we have had enough for now
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Geeze....I find this stuff very interesting and do appreciate your taking the time to do it in almost layman's terms. Do the numbers 540 kg and 41 kg represent any legal Eagle design or are those numbers just an example for illustration purposes ? What happens at the bottom of the lift strut ? My LEXL plans say gross weight of 575 lbs or 261 kg. Any chance you could translate that gross figure to each end of the lift strut and wing root to fuselage point in say a 3 or 5 g scenatio?
I seem to remember my Christavia being rated for 5 g positive and 3.5 g negative with a safety factor of 1.5 at gross weight. I don't think I've seen numbers like that in print regarding the Legal Eagle designs but I may have missed it. Are they out there?
Thanks, Tom
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Or might you be suggesting my attachment is of similar practical use?
Absolutely not. I just sent that to show what a can of worms it is to account for all the considerations necessary to become a "designer".
The XL wing was load tested with 1000 lbs. The load was evenly distributed instead of a load schedule calculated to represent the lift distribution.
The deflection was 5 inches and no damage.
The 560 lb gross weight mini max (https://www.teammini-max.com/aircraft/1100r-mini-max/) is rated 4.4 positive and 1.8 negative. Which is squarely in the utility rating.
My guess would be the XL wing is at least this good or better.
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re Reply's #27 & 29: Thanks, Scott
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Thanks for that info, Dan. I've got a copy of the book Stress Without Tears and have dug into it a couple of times but the deeper I get, the tears start to flow from me getting stressed. :-) I'm new at the Legal Eagle thing and just trying to become more informed.
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Thanks for that info, Dan. I've got a copy of the book Stress Without Tears and have dug into it a couple of times but the deeper I get, the tears start to flow from me getting stressed. :-) I'm new at the Legal Eagle thing and just trying to become more informed.
I don't have that particular book, but I have some others I get the same sensation as you from...:-\
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Dave that was just an illustration that I thought could answer the question of what part of the spar is most stressed. Remember that question?
My interest in this is mostly curiosity. I want to know what's going on with my aircraft just as I am interested in the details of automobiles.
You might know what that noise is if you have a clue about what's under there and what it does.
Since we are building our aircraft there won't be any mysteries about the what and how it is put together. The why is what I am trying to learn.
I believe the illustration is a work up of a wing for the High-Max. It is very similar. Much of it is the same. Most actually.
Tom XL-7