Ultralight struts/cantilever/additional weight

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Riggerrob

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The basic problem is the logarithmic curve of loads on the spar: insignificant at the top, but steep at the root. Traditional strut-braced wings tend to be lighter, but external structure reduces top speed.
If you look at the World War 2 vintage Horsa glider, you will see mostly cantilever wings with a small diagonal strut far onboard.
Struts allow for a much simpler wing spar, maybe even a constant chord spar. The disadvantage is that plank spars are always heavier than tapered spars. In order to be strong enough at the root, they end up with too much strength/mass/weight near the tips.

There are a variety of ways to taper spars: lightening holes near the tips, varying chord, varying spar cap depth, nesting multiple spar caps, laminating extra layers to the front and rear, etc.
With enough internal taper, you no longer need external struts.
 

lr27

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Part 103 allows you to use some kind of gadget to keep the speed down, as long as the pilot can't turn it off while flying.
 

lr27

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BTW, there are some airfoils that are designed to perform well above a certain lift coefficient and have high drag below a certain lift coefficient. For instance, the Momfoil series. (See Mom, it CAN'T go too fast! I think this nomenclature may have been inspired by the Mom's hatch on submarines.) I'm not sure a Momfoil would be enough by itself, though. I suppose it depends on how much power is used,and other design details.
 

crusty old aviator

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In the 80’s & 90’s, ultralight manufacturers would go to a finer pitch prop (or adjust them to a finer pitch if they had one of Dale’s props) if the top end speed exceeded 100 kmh (62 mph), and that’s how the top speed is limited on the Ultracruiser. Wing airfoil selection should be focused mainly on the lowest possible stall speed and stall characteristics.
 

pictsidhe

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BTW, there are some airfoils that are designed to perform well above a certain lift coefficient and have high drag below a certain lift coefficient. For instance, the Momfoil series. (See Mom, it CAN'T go too fast! I think this nomenclature may have been inspired by the Mom's hatch on submarines.) I'm not sure a Momfoil would be enough by itself, though. I suppose it depends on how much power is used,and other design details.
I know you jest, but...
With a suitable lower surface, a momfoil is quite possible. Airfoils with a lot of camber have their lowest drag at highish Cl. Modding one of those to create higher drag at low Cl is the way to a momfoil.
 

lr27

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I wasn't jesting about the existence of a series of airfoils called Momfoil. You can look up coordinates and Xfoil plots. So if you came up with your own, the decent thing to do would be to pick a name like nannyfoil or grandmafoil.
 

MrHopkins

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Thanks everyone. To answer the questions from several posts:

Ultralight in this case refers to US, part 103.

I definitely considered Hummel's Ultracruiser. Hands down that is the most attractive ultralight I am aware of. The award winning one that looks like it was chromed is stunning. I decieded I will prefer looking down without obstructions vs alternitives.

My bona fides are I got a degree in Aeronautics. Knocked out a few ratings in a 172 and Decathlon for credit. Due to requirements to be a full time student an extra year, despite already completing the courses required, I got all the airframe and about half of the powerplant of A&P. I am very comfortable with fabrication of whatever material but definitely aluminum is my go to That was 10 years ago and I am way out of practice.

All that info above is me trying to sound qualified. Which is debatable.

I put off building too long for no good reason so Im making myself do the things I tell people i'll do.

I've built (assembled) a motor w/ a chain drive that spins 11 props on the trailing edge of a wing. 5 on left, 1 centerline, 5 right. The goal is lowering landing speeds/increasing AoA limits. Took a lot of adjusting but it seems effective so far.

Secondary goal is to be able to fly over the wife's horse as quietly as possible.

Third goal is attaching a recumbant bicycle for use in taxi, takeoff, post landing, and to get me the last mile to work. which is as close as I can be airborne wirhout being shot down or tackled.

I'm already at 218 pounds. Unbuilt yet is another 90 lbs of airframe + whatever im going to use as instruments + recharging system when not at home . So farther over the weight limit than I think I can reduce but im working on it .
 

maya.ayoub.32

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Hello!

I’m Maya, a highschooler in the San Francisco Bay Area, and a plane enthusiast. I’m helping design an ultralight with Flight Club, and had a couple of questions I was stumped on.

I’m currently working on our wing struts and attachment, and was wondering if the jury struts are necessary? While many similar ultralights use them, we noticed that the most recent Affordaplane gets away without them. If they aren’t there, they don’t create any drag or weigh anything so we’d ideally like to exclude them from our design. We were just thinking of making the design simpler, but don’t want to sabotage our plane. Hence, we were thinking of mitigating the absence of jury struts with larger diameter spars to resist bending. We’re still in the early conceptual phase of this so we still gotta run all the calculations but thought it would be good to get some input before we get too far down the wrong path. On a side note, we were looking at wicksaircraft.com and Aircraft Spruce for strut suppliers. Are there any other vendors we should look at?
Any advice is super appreciated and thank you in advance!
 

Dana

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Jury struts restrain the center of the strut to help prevent it from buckling under compressive (column) loading. Whether or not they're necessary depends on the specific design. As with everything, it's a compromise, the extra complexity of the jury struts vs. the extra weight of a larger strut. The smaller strut plus jury struts may have more or less drag than a larger strut without.
 

maya.ayoub.32

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Jury struts restrain the center of the strut to help prevent it from buckling under compressive (column) loading. Whether or not they're necessary depends on the specific design. As with everything, it's a compromise, the extra complexity of the jury struts vs. the extra weight of a larger strut. The smaller strut plus jury struts may have more or less drag than a larger strut without.
Thank you so much for the clarification! We'll look into how much force our current strut can take, and if we can substitute jury struts for stronger struts.
 

Ollie Krause

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the "average Joe" cannot design or build a safe ultralight.
I'm a firm believer that anyone can do it with enough intellectual curiosity, motivation, and an open mind.

 

lr27

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Actually, when using jury struts, you are substituting them for STIFFER struts, not stronger ones. The issue is buckling. Either style will have to withstand the same tensile or compressive loads.
 

lr27

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I'm a firm believer that anyone can do it with enough intellectual curiosity, motivation, and an open mind.

Believing it, unfortunately, doesn't make it so. People have different abilities and talents. Someone might not be able to design a safe aircraft but might have other, amazing talents beyond what any of us on this list can do.
 

BJC

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I'm a firm believer that anyone can do it with enough intellectual curiosity, motivation, and an open mind.
Believing it, unfortunately, doesn't make it so. People have different abilities and talents. Someone might not be able to design a safe aircraft but might have other, amazing talents beyond what any of us on this list can do.
Ollie clearly qualified his comment with “with enough intellectual curiosity, motivation, and an open mind.”

Having dealt with “engineers” who couldn’t design a lead weight, and non-engineers who were brilliant designers, I agree with Ollie. Intellectual curiosity, motivation, and an open mind go a long way in any field of endeavor.


BJC
 

lr27

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Of course they go a long way, but that doesn't mean there's 100 percent correlation with the required abilities. I'm not talking about whether someone has a degree or not. Someone may have as much talent for design as I have for music. And yes, I took lessons and tried to learn several times, but I took mercy on the ears of others. They didn't teach a lot about design in my engineering classes, but I designed an automated gadget using a bunch of actuators and other goodies that didn't need any revisions in three years* of service. I admit I had to tweak a few things before releasing it, but not very many.

*Or more, as far as I know
 

crusty old aviator

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The lightest configurations of wing struts I've seen employed 2024-T3 round with foam glued to the trailing edge, sanded to an airfoil shape and covered with Monocote. These struts do require small diameter juries with the same foam/Monocoat treatment. The only disadvantage of this occurs when some thoughtless clod grabs a strut to move the plane and crunches the foam...

Removing jury struts seems to be a latest fad: Lift strut question
 
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radfordc

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I have heard enough tales of grounded not-really-103s to want to stick to the rules that are current. It can be extremely difficult to make them legal if they are intially built to flout the rules. You can easily end up with an expensive pile of low value parts.
Finally! Someone who has some actual knowledge of slightly illegal ultralights that have been grounded by the FAA. I've been trying to find any documented cases for years and haven't found one yet. Please relate a few instances so that I can keep them for my files.
 

maya.ayoub.32

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The lightest configurations of wing struts I've seen employed 2024-T3 round with foam glued to the trailing edge, sanded to an airfoil shape and covered with Monocote. These struts do require small diameter juries with the same foam/Monocoat treatment. The only disadvantage of this occurs when some thoughtless clod grabs a strut to move the plane and crunches the foam...

Removing jury struts seems to be a latest fad: Lift strut question
What an interesting way to build a strut and thank you so much for linking that thread! We read through it, and our physics team did the calculations on column buckling, as the thread suggested. Based off the numerous arguments in favor of the Jury struts and the results of the calculations, we've decided that using Jury struts would be a safer way to go. Thank you again for all your help!
 

BJC

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Finally! Someone who has some actual knowledge of slightly illegal ultralights that have been grounded by the FAA. I've been trying to find any documented cases for years and haven't found one yet. Please relate a few instances so that I can keep them for my files.
The problem is that the slightly illegal ultralights were only slightly grounded, so documentation is somewhat iffy.

BJC
 
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