# Wrapping my head around U/L design practices

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#### Grimace

##### Well-Known Member
Coming from a background in certified aircraft, there's a lot about ultralights that scare me. I see some things that you wouldn't be caught dead doing on a normal-speed experimental. Sometimes I think, "well, they'll never see anything above 80mph in a dive, so it should be fine.." Other times I think, "That'll never hold up over time...."

I see pop rivets used extensively and that makes sense for the most part. But I see people (pop) riveting metal to composite structures and that gives me the willies. I see some metal tubing ribs use nothing more than a dimple in the tube to secure it to a plastic attach point on the aft spar. And then there's foam ribs. Yikes!

I don't mean to disparage what any builders are doing. You all know better than I do and you can't really argue with success. I guess what I'm trying to do is just come up with a method of construction that I would feel comfortable with.

One thing I thought about is a box spar made of foam wrapped in uni of various orientations. The spar thickness would provide torsional stiffness and you could mount plywood ribs to it very easily.

Cutting all the ribs at once and then gluing them on with some angle stock would be easy as pie... but then how do you sew on the fabric? Or is sewing the fabric really unnecessary, as some U/L manufacturers have suggested?

Any thoughts?

#### rtfm

##### Well-Known Member
Hi,
As far as spar construction is concerned, probably the lightest, easiest (and strongest) I have seen to date is that pioneered by Jim Marske on his sailplanes. A simple glassfibre shear web with graphite rods for spar caps. Laid up in a simple "U" channel, and then back-to-back for a "T" spar top and bottom.

Duncan

#### Norman

##### Well-Known Member
One thing I thought about is a box spar made of foam wrapped in uni of various orientations.
The RC gliders are using a system called a socked spar that's apparently pretty good.

#### cepheusglighter

##### Active Member
I too think Jim Marske has the right idea on spar design. I've personally built 2 spars using a method so similar to his there is little reason to delineat. Of course you do have to plan for some special treatment at connect positions but this is not all that difficult and a well documented method is readily available for the builder.

Mine unlike his is basicly a plywood 45 degree web with the carbon rods or bulk strands laid and encapsulated in a very minimal spruce box. When I say minimal I mean say a spruce U shaped box with the sides of 3/16" X 5/8" and a bottom of 1/32" birch ply. Fill these boxes with carbon...most at the root of a wing and step off to less at each rib outboard so by the tip you have as little as say a single .125" X .250" rod and you have a spar that can exceed 10 g. with very small overall deflection.

As for foam structures....these are fine if they have some additional stiffening structure applied. As an example, I have utilized 1/4" 4 lb/ft. cubed divinycell with a solid .8 mm finnish birch skin bonded with epoxy micro slurry on both faces and 1/4" sq. spruce stick in contact between both faces bonded in at any place where I am going to bond attach to other structure such as a web for instance. With corner blocks or like triangular gussets in the corners top and bottom you have a good light rib with qualities more than adequate to the task.

Look at my post in completions on my powered alnair sailplane and you will see spars of this design D-tube, and ribs of conventional built up spruce stick applied thusly to the web. I loaded this one static to 5.41 g with only 8 inches deflection. These wings weighed about 32 lbs. each covered and without the full span flaperons. Each held nearly 970 lbs. in the tests without any problems whatsoever.

Composites give us all great latitude in design but as always, design with materials has to be executed with care and testing.

Always happy to talk design and technologies....anytime Grimace.

GusMan

G. Malm

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#### Grimace

##### Well-Known Member
Hi,
As far as spar construction is concerned, probably the lightest, easiest (and strongest) I have seen to date is that pioneered by Jim Marske on his sailplanes. A simple glassfibre shear web with graphite rods for spar caps. Laid up in a simple "U" channel, and then back-to-back for a "T" spar top and bottom.

Duncan
I like Marske's configuration for a "normal" aircraft or sailplane because you have a lot of structure around it to provide for compression loads and resist twisting. For instance, Marske uses composite skins to take up the torsion loads. Covering in glass strikes me as being too heavy for ultralight construction, so you've gotta do something else for the torsion loads. As another example, Rutan uses solid foam to build a torsion box to resist the load. Both of these systems are pretty optimal for experimental aircraft, but largely seem excessive for ultralights.

By contrast, using a foam core in the spar, I would think you could use that to handle the torsion loads, much like they do with the hollow metal tubes. The foam/box structure would allow you to save a bit of weight and cost with the composite, while also incorporating your torsion resistance. So then the rest of your wing structure can be of more "traditional ultralight" construction.

As a case in point, I'm guessing one of those big aluminum tubes that Kolb and other manufacturers use probably comes in close to twenty bucks a foot? Just making a napkin calculation for an ultralight... You can build adequate sparcaps out of uni for about $5 a foot, 5" tall 3/8" thick divynicell is$1 a foot, and you can do your spar web out of BID for about \$1 a foot.

And when you're done, you have a flat-surfaced box structure to which you can bond your wing ribs and other doo-hickeys with minimal fuss... just a simple bond between two completely flat surcaces... It would seem to make construction simple...

I do think I could get used to foam ribs skinned with ply... but why not just make them out of ply? fewer pieces to cut and the cost for an ultralight of modest wingspan shouldn't be too great...

#### Dana

##### Super Moderator
Staff member
I fly ultralights, and there's some things about [some] ultralights that scare me, too!

Certainly you can get away with a lot due to the low speeds. In some cases the pilot has to be very careful... there were a number of crashes of Pterodactyls due to the pilot exceeding Vne. However, for most ultralights, the low speeds and light weights means low energy and low g-forces, so they can be built light... as they must, if you want to remain legal.

The dimple in the rib tubing... if you're thinking of the Quicksilvers, that doesn't even "secure" it to the aft spar; it's just a tab to keep the rib from cutting the fabric... they're more like sailboat battens, really, slid into pockets in the fabric. So rib stitching doesn't apply, since the rib is encapsulated in the fabric. On the more "conventional" ultralights, like my Kolb, which uses Stits, there's still no rib stitching... it's not necessary at the low speeds.

Certainly you could make some fancy lightweight composite structures, but it'd drive the cost up... if not in materials, then in labor.

In the end, look at the safety record... once the "bad old days" were over, the safety record of modern ultralights is comparable to certified aircraft. And just like certified aircraft, most accidents are pilot error.

-Dana

Me... a skeptic? I trust you have proof...

#### Grimace

##### Well-Known Member
I fly ultralights, and there's some things about [some] ultralights that scare me, too!

Certainly you can get away with a lot due to the low speeds. In some cases the pilot has to be very careful... there were a number of crashes of Pterodactyls due to the pilot exceeding Vne. However, for most ultralights, the low speeds and light weights means low energy and low g-forces, so they can be built light... as they must, if you want to remain legal.

The dimple in the rib tubing... if you're thinking of the Quicksilvers, that doesn't even "secure" it to the aft spar; it's just a tab to keep the rib from cutting the fabric...
I was actually referring to the Affordaplane which, I know, isn't much of a standard for ultralights... but I still think the process needs to be reconsidered... alumribs4

On the more "conventional" ultralights, like my Kolb, which uses Stits, there's still no rib stitching... it's not necessary at the low speeds.
Ok... I'm willing to "buy into" the unstitched fabric concept.. but only if I can find a discussion/reference/article/etc that talks about the necessary surface area for the fabric. I mean, surely there has to be some agreement, or at least a common practice with regard to how much of the surface has to be glued in order for it to hold...

Certainly you could make some fancy lightweight composite structures, but it'd drive the cost up... if not in materials, then in labor.
That's why I'm excited about my spar concept. It can be made using only a flat table, all the parts can be cut with a knife and a straight edge (no hot wiring necessary). It should take 5 "trips to the shop" to finish. Each trip should be well under 2 hours, including prep and cleanup, if you know what you are doing. All pieces are flat and straight. Depending on your resin system, it could be done in a weekend.... And best of all, it makes fitting all other parts really easy because you're left with a perfectly rectangular shape, ready to be bonded to...

#### Grimace

##### Well-Known Member
A few more questions. Sorry for the double post.

What kind of thicknesses do you typically see on plywood ribs for ultralights?

Are ultralight folks bonding the skin to aluminum tubes, sheet metal, wood, and composite ribs with equal success? It seems to me that a metal tube would either not offer sufficient surface area for bonding the fabric, or else would tend to cause the fabric to adhere in a saggy manner and get pulled apart with the tautening process.

#### mstull

##### R.I.P.
Grimace,

You have a lot of good questions, so you must be getting serious about building an U/L.

There's nothing wrong with your foam box spar. But the foam itself won't take the shear loads. So the box will need rounded corners, so before or after your uni carbon caps, you can spiral wrap both ways on a 45 to connect the spar caps. The shear loads are surprisingly strong when you're pulling Gs.

I like foam ribs. I've learned to scrim them on both sides with very light (less than 2 oz) glass. Then you can cut them out on a band saw. Add 1/32 plywood rib caps so the Stits chemicals don't dissolve the foam. You can use scrimmed 1/4" balsa the same way. It's much stronger, and not much heavier. Tip and root ribs have to be much stronger to take the tremendous load of the fabric shrinking. If you have 3/4" or wider rib caps, you won't need rib stitching, unless you use an under-camber.

Some generalizations... Try to make as many parts as possible serve 2 functions. Keep reminding yourself it's only an ultralight, or it will come out too heavy. Keep everything as simple, minimal, and light as possible. Weight tends to add up fast, so save weight with every fastener, cable, etc. Find a lot of things to do without, like cockpit enclosures, instruments, and creature comforts. Flutter is a fickle beast. Having fairly tight control cables helps.

Read AC103-7 to see how much power and wing area you'll need to pass the speed and stall speed limits. I've tried all different types of structures in my U/Ls. They all work. How things fasten is a pretty big deal. With composites, you can just attach ribs to spars with a mixture of epoxy micro and flox, making a generous fillet.

I've learned to use thin wall 2024 T3 for most everything, and reinforce the ends and any attachment points. Parts can be fastened with gussets or eye bolts among other things. Stits fabric glue sticks to aluminum well enough for an U/L without doing any pre-coating. Just make sure the surfaces are clean with virgin acetone.

You didn't mention engines... which is one of the biggest problems U/L designers face. Rotax twins are too heavy for most U/L designs, as are most 4-strokes. Most of the single cylinder engines vibrate tremendously and are less reliable. Engine vibration is a huge problem with U/L design. The frame is so light that the engine tends to vibrate it to failure.

I'm building my 7th U/L design now. Once you find out what works, it's very quick and easy. See my post "New Design".

#### Dana

##### Super Moderator
Staff member
The A-Plane link you gave looks like they'e using Quicksilver rib ends. The dimple holds it in place, but the bent rib is under compression, so there's no tendency to come out. Anyway, you don't see ultralights falling out of the sky due to rib or fabric failure (unless the owner failed to replace old sun damaged fabric, and even that's rare).

You can do some cool stuff with composites, as Mark has clearly shown, but I doubt this kind of construction will become mainstream in the kit market because it's somewhat complex... the average kit builder wants a simple bolt together structure. And they're more unknown... metal structures are easy to build, easy to verify that it's built right, and easy to analyze using known strength values... could be a liability issue for kit manufacturers.

-Dana

If vegetarians eat vegetables,..beware of humanitarians!

#### Grimace

##### Well-Known Member
Grimace,

You have a lot of good questions, so you must be getting serious about building an U/L.

Oh I don't know about that... I've been collecting data for my single seat experimental design now for years... and I'm pretty gung-ho on that. But ever since somebody posted a picture of that dePischoff plane, my gears have been turning...

The thing is, I must admit, some of my departures from the normal design practices sort of bridge the gap between ultralights and experiemntals (my target is an experimental, 170mph, under 300 lbs empty). So I'm definitely pushing the boundaries... and then somebody posted the picture of dePischoff's plane.. then I started thinking about how cool it was... and how an ultralight would give me the opportunity to test/demonstrate some of my oddball depatures from the norm...

So in a way, it sort of makes sense, start slow and small... find out if my processes are really as easy to construct as I think they will be... If they are.. and if they are as light as I expect, then I'll have something to fly around in as I work on the experimental...

Besides, it's cold here.. I need something fun to keep me warm inside...

#### mstull

##### R.I.P.
Grimace,

If dePischoff's plane excites you, you should build something similar. It helps to be excited. I love the freedom from regulation and inspection in Part 103. You can really let your creative juices flow. We'd be willing to discuss any of your "oddball departures from the norm". I've had more than my share.

#### Grimace

##### Well-Known Member
Alright, well, let's start batting an updated version of the Aviette around the ball park. I'll give you some things I've been thinking about...

The firewall could be a simple piece of thin sheet steel. With the engine that close to the cockpit, I think you do need a "real firewall"... but it shouldn't weigh more than a pound or so. Rivet the steel firewall to square aluminum tubes with a barrier between them to prevent corrosion. From this structure, it's 4 tubes straight back to the upper and lower spars. Total: 1 firewall, 4 gussets, 8 pieces of aluminum tubing, 4 attach points between the front structure and the wing.

Also 2 tubes from the lower spar to upper spar for rigidity... if necessary.

Wing: fiberglass box structure, bonded to ribs of an as-of-yet unspecified type. 2 to 4 foot center section with up to 8 foot outer sections.

Fuselage: Hrmm.. if the pilot can be completely seen from the side, then it's not an enclosed cockpit, right? So could I fair from the back of the pilot seat rearward to make it look like a normal airplane while leaving the sides completely open? Well, just a thought there. Aft fuselage could be a 2 or 4" thick sandwich core with roving on the top and bottom. Again, personally, I'm thinking composites are great so long as you don't have to make a mold or do a bunch of finishing work. A flat-sided tail boom could be laid up on a sheet of MDF or aluminum. You'd get a perfectly smooth part with virtually no finishing work. It's not as "transparent" as the aviette's structure, but for me it would be easy to construct. But how do you mount it? And you still have to accomodate the pilot's seat. Maybe a traditional aluminum box structure would be better since you could mount the seat to the top of the box and the 4 corners of the box could mate up to the vertical tubes between the upper and lower spars...

I really like the landing gear on it, but I think it's terribly impractical. Anybody care to disagree with me on that? I'd be happy if you convinced me that were practical...

Looking at AC103-7, the most accurate description would be "lower have of body enclosed"... or maybe "head exposed". But technically, I think it is sitting upright, not enclosed. The root word there is "closed", which the Aviette fuselage is not. So that give's me 5.5

Let's try 120 sqft of wing. Gives me 1.2

24sqft of control surfaces gives me 0.33

Eight flying wires gives me 0.4

I doubt the struts will be 4 feet high. Maybe I could make the top wing longer than the bottom wing and angle the struts to make 4 feet? Otherwise, I could just not get the "points" for it.

A pair of unfaired fixed main gear is 1.0

"Engines placed in front or rear of a fully streamlined fuselage or behind a semi-streamlined pilot cockpit are considered partially exposed..." Well, I don't have a fully streamlined fuselage on the Aviette, nor do I have a semi-streamlined cockpit. But it would have a streamlined fairing. So that gives me a factor of 1.5

Total: 9.93
(total +total x .2) = 11.916

I could use a half VW with just a smidgin bigger wing.

Now, with regard to the stall calculation, what airfoil do you ultralight folks use on biplanes? I'm guessing go with about 7.5% camber?

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#### mstull

##### R.I.P.
Grimace,

Most U/Ls don't have a firewall. But if you must, use thin aluminum sheet or 1/8" plywood. We don't worry about corrosion issues with dissimilar metals... so no barrier. Many/most U/Ls use stainless steel pop rivets and aluminum.

We try to avoid square tubing as much as possible, because it's heavier than other choices. Consider some thin aluminum angle, or thin wall round tubing.

3 piece wings (with a center section) are not uncommon. But you'll save weight if you can make it with no center section. End (tip and root) ribs have to be very heavy duty to take the fabric tension. So you'll save weight to minimize the number of them. Ideally (especially with a biplane), it's lightest to make the wings single piece for that reason.

Generally you'll need at least 120 square feet to wing area to pass the stall speed limit. Making a plane light enough to pass the weight limit would be easy... but making one with enough wing area to pass the stall speed limit, without coming out too heavy is challenging. If you really care about being truly legal... biplanes are inherently inefficient because of their low aspect ratio and the wings' proximity to each other. The formulas in AC103-7 were written with a monoplane in mind. A biplane will need about 20% more wing area to truly pass. See my post "U/L Biplane" to see the one I built, to maybe get some ideas. It's way back on page 4 of this forum. I used 4 flaperons (which I never used) to pass the stall speed limit.

Yes, open sided cockpits are common. Consider leaving the whole empennage open too, to save weight.

Making a strong but light landing gear is challenging. A welded steel frame that has nice triangles all the way to the wheels comes out light. I've always made mine out of triangulated aluminum struts/legs with a full width axle, since I'm not a welder.

It is REALLY hard to make the weight limit with a half VW, or any other 4-stroke. The whole rest of the plane would have to be very minimal, maybe even flimsy... which might make you grimace. Consider lighter alternatives.

The Gottingen 387 is the most common/proven U/L airfoil. It is extremely stall resistant, which is the main goal. That's what I've used on all my designs... usually slightly modified with a flat bottom... occasionally with added under-camber.

#### Dana

##### Super Moderator
Staff member
That design is really neat. Landing gear aside (which seems to be "clever" for the purpose of being clever), the only thing I don't like is the engine location... there's a reason that most open ultralights are pushers; namely the hellacious breeze you get when you're on the windy size of a prop! 40-50 mph cruise speed can get fatiguing (and chilly!) even when you're sitting in front of the propeller; personally I prefer to relegate tractor designs to aircraft with nothing more than my head exposed, and that behind a windshield.

But it's still a neat design, and I'd love to know how (or if!) it flew.

-Dana

Everyone who lives dies; yet not everyone who dies, has lived.

#### BBerson

##### Light Plane Philosopher
the only thing I don't like is the engine location... there's a reason that most open ultralights are pushers; namely the hellacious breeze you get when you're on the windy size of a prop! 40-50 mph cruise speed can get fatiguing (and chilly!) even when you're sitting in front of the propeller; personally I prefer to relegate tractor designs to aircraft with nothing more than my head exposed, and that behind a windshield.

But it's still a neat design, and I'd love to know how (or if!) it flew.

-Dana

Everyone who lives dies; yet not everyone who dies, has lived.
True, a tractor propwash is strong in a static condition. But what about cruise? The propwash should be about 10% more than the airspeed I would guess, depending on the design and diameter of the prop.
BB

#### Dana

##### Super Moderator
Staff member
Good point, you made me wonder... some real rough back of the envelope calculations based on the 30hp engine with 50" prop in my plane and arbitrarily assuming 50% propeller efficiency, I get a propwash speed increase of about 33% at full throttle, 25% at low cruise. quite substantial. For a faster plane the percentages would doubtless be lower, but we're talking ultralights here.

-Dana

Everyone who lives dies; yet not everyone who dies, has lived.

#### Grimace

##### Well-Known Member
Ok so the propwash is flying past you at 70mph or so.... about like my naked motorcycle... not pleasant at high speeds, but fun all the same. I could see it getting really tiring though...

Excellent point as well about angle aluminum rather than full tubes. I will have to keep telling myself "think light... think light..."

With the engine in front, I'd be pretty insistent on a firewall, especially since the stick wouldn't be more than a couple of feet aft of the motor...if that got hot, it wouldn't be a nice place to be... but given how small the plane is, even just a sixteenth of an inch of steel, around two square feet, wouldn't add much weight...

And I would think about corrosion. While I understand you need to keep it light, the concept of a disposable airplane does not appeal to me in the slightest. It's like the B-52's or 727's that are in service well past their intended life span... you might think you're designing an airplane to last 20 years, but you never know. And if it's not too tough to design in the little features that make a difference, then I think it's downright irresponsible not to do so. That opinion is tempered somewhat, however, by the large amount of inspectability you can have on an ultralight. Nevertheless, I'd want something that is at least intended to withstand the test of time...

Another thing I would do... if I wound up releasing plans... would be to make all of my calculations, assumptions, etc available in the manual. I'd think it would be irresponsible to not at least give people the opportunity to conveniently peruse that information if they were so inclined...

#### Dana

##### Super Moderator
Staff member
I don't think a firewall would do much good unless the engine was fully cowled. For that matter, I can't see it being necessary unless the engine is fully cowled, as an engine fire is extremely unlikely without an enclosed place for heat and/or flammable fumes to build up.

Now here's something almost in the same spirit, but a pusher:

This Hovey Whing Ding was by all accounts an extremely marginal plane that barely flew with a very light pilot... but the concept could be worth revisiting.

-Dana

The early bird catches the worm, but it's the second mouse that gets the cheese.

#### Midniteoyl

##### Well-Known Member
I would think clear lacquer or even simple packing tape (clear), or something just alittle thicker like good duck tape, would suffice as a barrier between metals. Might not last 20 years, but would take awhile before the process even started, giving you a few more years of service.

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