# New Ultralight and LSA Trainer design PAIR 2

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#### Rienk

##### Well-Known Member
Well, I haven't given up on this project, but I wanted to get rid of the poll :ermm:

I keep wondering about the whole 103UL thing… it’s hard not to consider the options for making a truly inexpensive RTF aircraft, and realizing that the weight limit will be hard to meet – but going LSA is doable – even if the aircraft is not much heavier than 103. The problem again becomes flight training and expense… the beauty of 103 is limited training, no insurance requirements, etc.

I can’t help but think a fully rotomolded fuselage would be amazingly cheap. If we could get a fairly consistent 1/16” wall thickness (or less), the fuselage shell on something like the Solo would weigh about 60-80 lbs (depending on the plastic, and if it is glass reinforced). This doesn't seem to be a lot more than the weight of fabric and paint – if you take into account the extra structure required to hold an aerodynamic shape.
This would be in addition to the metal sub structure required.

If we picked a wing loading between UL and LSA – I’ll arbitrarily pick 7 – then a plane with a gross weight of 700 lbs would need a wing area of 100 sf. If that were rotomolded, the wing shell would weigh another 60-80 lbs – which is about double what composite would be. Thus, there would probably only be 60-80 lbs extra compared to the UL, which is really nothing in the grand scheme of inexpensive flight.

This is even less of a concern in the two seat version – which makes me wonder if it should be the only model made (no 103UL, but maybe both low wing and high wing LSA variants). If we assumed 1000 lb gross weight, wing loading of 7 (thus wing area of 140 sf), total surface area would probably be less than 600 sf, for a shell weight of 180-240 lbs – likely an extra 100 lbs over other building methods. If that building method can bring the cost of a “basic” LSA trainer below 35k, I think it will be a winner (assuming it looks good and flies well). If the metal sub-structure were beefed up, the same wing would work at full LSA gross weight (1320 lbs) and give adequate wing loading for LSA stall speed! Still to be determined: • finish – color and protection. • Acceptable fuselage shape, and required metal structure. • Rotomolding tooling costs. • Ability to have framework pre-installed into rotation mold (so parts are made around structural components) Those are my latest thoughts. Comments? #### orion ##### Well-Known Member You think rotomolding would be cheaper than vacuum forming Acrylic? (question, not criticism) #### Rienk ##### Well-Known Member You think rotomolding would be cheaper than vacuum forming Acrylic? (question, not criticism) I have been in touch with Tom Derrer at Eddyline Kayaks, and we will be talking at length next week. I am very open to the process of thermoforming - we do it a lot in our core business. But to answer your qestion; YES, I believe that rotomolding would be a lot cheaper. Granted, thermoforming molds are much less expensive, but the labor of trimming, aligning and joining the various parts will soon overtake that. The caveat is really the strength difference between the plastics available, and whether you can take advantage of skin strength in either, other than simply handling wind loads (I don't know the answer to this). But I do know how cheap rotomolded parts are - even large ones! I don't have time to go into details as to how I would envision the possibilities - right now - as my bride is waiting for me to take her out to dinner... but maybe I'll have time to post more this weekend. BTW, I have a post in the other thread (cheap aircraft) about low cost development, which is applicable here as well. #### Buchon ##### New Member Making a blank to make a mold for a rotomolded piece isn't cheap. I have a 7 ft piece that is being made and the blank/mold was around28K. Once the blank is made, additional molds are cheaper (12-14k).
The low density poly is heavy. To give you an example, that 7ft piece weighs about 24 pounds.
It would definitely have to be re-inforced because extended period in the sun will make the plastic too soft and bendable for flight. But it's really tough stuff. You can take a hammer to it without ill-effect. The plastic can be uv treated, so that it doesn't fade too much.
hope that helps...

#### Dauntless

##### Well-Known Member
I applaud your innovation for thinking about rotomolding, but like Buchon I was thinking about its lack of stiffness and low thermal properties.

For volume production runs closed-mold thermoset prepregs show good promise, but the tooling is so expensive I question whether that approach can work in light aviation.

For the low volume work that comes to my mind in the LSA context, I think vacuum infusion is going to be hard to beat. It's cost effective, as well as EPA and OHSA friendly.

I like your ideas about "best performance to cost" materials and the impact of good design work. Combine that with numeric tooling and you can get away with reasonable costs for labor. Whether you can make it work at Flabob I can't say...

I also want to beat the drum again for composites as the only way to achieve radical parts count reduction. In my own personal experience properly designed composite assemblies can see 90% parts count reduction compared to aluminum assemblies, which reduces the labor input and cost. Ever seen aluminum airplanes being assembled? If so, you can understand why they cost so much!

Sorry so rushed, gotta run.

#### Rienk

##### Well-Known Member
Making a blank to make a mold for a rotomolded piece isn't cheap.
I have a 7 ft piece that is being made and the blank/mold was around $28K. Once the blank is made, additional molds are cheaper (12-14k). The low density poly is heavy. To give you an example, that 7ft piece weighs about 24 pounds. It would definitely have to be re-inforced because extended period in the sun will make the plastic too soft and bendable for flight. But it's really tough stuff. You can take a hammer to it without ill-effect. The plastic can be uv treated, so that it doesn't fade too much. hope that helps... i realize it's not cheap to make molds of any kind, but I happen to have connections fro getting them done inexpensively. I would not use low density PP... probably glass reinforced Nylon. Most plastics come in around 1.0 density, and it's easy to figure out how much a shell would weigh based on thickness and surface are. The plastic shell is just that - a shell. The structural component would be a metal frame. We do a lot of injection molding... that is the method I would prefer for all the flight surfaces, but the tooling cost for that is REALLY serious! #### Rienk ##### Well-Known Member I applaud your innovation for thinking about rotomolding, but like Buchon I was thinking about its lack of stiffness and low thermal properties. I'm well aware of this, but it is a design compromise to reduce cost. However, as Orion has mentioned, we are considering thermoforming acrylic. The problem with that is the stretch of the material is often in the areas where you really want the thickest part to be. For volume production runs closed-mold thermoset prepregs show good promise, but the tooling is so expensive I question whether that approach can work in light aviation. For the low volume work that comes to my mind in the LSA context, I think vacuum infusion is going to be hard to beat. It's cost effective, as well as EPA and OHSA friendly. We're already doing this for the Solo and Duet (Welcome to Ayerscraft). We're looking for a way to get an even more basic and less expensive aircraft (around$35k for a two seat LSA, ready to fly).

I like your ideas about "best performance to cost" materials and the impact of good design work. Combine that with numeric tooling and you can get away with reasonable costs for labor. Whether you can make it work at Flabob I can't say...

I also want to beat the drum again for composites as the only way to achieve radical parts count reduction. In my own personal experience properly designed composite assemblies can see 90% parts count reduction compared to aluminum assemblies, which reduces the labor input and cost. Ever seen aluminum airplanes being assembled? If so, you can understand why they cost so much!
Again, this is what we're doing with the Solo and Duet. For these lower cost aircraft, we are assuming low cost labor (offshore) would be required to get the cost of manufacturing down where it needs to be.

#### Gnarly Gnu

##### Member
Hmm.... why does the Ayerscraft website give a virus alert? Blocked by Avast.

I wonder if anyone has used explosive forming for complex aluminium aircraft parts? I've done a little bit of this but not to such a precise degree of repeatability that an aircraft would require. However it can be a cost effective method for smaller quantities of shaped parts where a full press process isn't economically viable. You only need a one sided mould and not such heavy construction typically. Size is somewhat limited though, can't see large panels being done this way. Where applicable a squirt of expanding polyurethane foam on the inside later creates quite a solid & well damped part.

#### Rienk

##### Well-Known Member
Hmm.... why does the Ayerscraft website give a virus alert? Blocked by Avast.

I wonder if anyone has used explosive forming for complex aluminium aircraft parts? I've done a little bit of this but not to such a precise degree of repeatability that an aircraft would require. However it can be a cost effective method for smaller quantities of shaped parts where a full press process isn't economically viable. You only need a one sided mould and not such heavy construction typically. Size is somewhat limited though, can't see large panels being done this way. Where applicable a squirt of expanding polyurethane foam on the inside later creates quite a solid & well damped part.
That seems to be an issue with Avast - it apparently doesn't like the script or some such nonsense.

hydroforming of metal is interesting, but as you say - except with the high end equipment - not very usable for aviation (and definitely not homebuilt).
i would think that thermoforming plastic would be a much better way to go -possibly with backup layers of composite.
This method might work for the homebuilder - but IMO you might as well spend the little extra time and money and do a real mold.

#### Dana

##### Super Moderator
Staff member
i would think that thermoforming plastic would be a much better way to go -possibly with backup layers of composite.
All thermofomable plastics are way too heavy and too weak for use as aircraft structure. If you back it up with layers of composite (and the company I used to work for spent millions trying to get that to work and finally went bankrupt), all the plastic gives you is a heavy scratch resistant paint job.

Not to mention the enormous cost of large thermoform presses and tooling...

-Dana

A pilot is a confused soul who talks about women when he's flying, and about flying when he's with a woman.

#### Rienk

##### Well-Known Member
All thermofomable plastics are way too heavy and too weak for use as aircraft structure. If you back it up with layers of composite (and the company I used to work for spent millions trying to get that to work and finally went bankrupt), all the plastic gives you is a heavy scratch resistant paint job.

Not to mention the enormous cost of large thermoform presses and tooling...

-Dana

A pilot is a confused soul who talks about women when he's flying, and about flying when he's with a woman.
I would never count on plastic skins for structural members - just as a heavy shell.
The shell on a two-place aircraft with 600 sf of surface area would weigh about 200 lbs, in addition to the minimalistic metal frame required.

If you were to back out the weight of fabric, paint, extra structure to give shape, etc, it would be interesting to see what the weight penalty is.

Frankly, I am intrigued with rotomolding. You could do the entire fuselage as one piece, with all insets and jogs in place, as well as wing and root fillets, not to mention any aesthetic shape features.

if this could be done with a weight penalty of only 100 lbs, and yet be able to reduce the cost of the airplane by \$10-20k, it would be worth it to a lot of people

#### BBerson

##### Well-Known Member
HBA Supporter
I build and fly RC models that sometimes come with ABS molded plastic cowls. The vacuum formed ABS cowls (about .016"- .020" I think ) are not very durable so I have used the ABS cowl as a mold to make a thin fiberglass cowl.
Sooo... here is an idea...

What if the airplane kit company sold ABS molds instead of parts and let the homebuilder make his own parts with cheap ABS molds?
The molds could be made quickly in a vacuum former, then the homebuilder would provide the majority of the labor making the parts.

The ABS molds would come with some simple plywood supports for the builder to assemble on a table or floor, just good enough for one use.
BB

#### Rienk

##### Well-Known Member
I build and fly RC models that sometimes come with ABS molded plastic cowls. The vacuum formed ABS cowls (about .016"- .020" I think ) are not very durable so I have used the ABS cowl as a mold to make a thin fiberglass cowl.
Sooo... here is an idea...

What if the airplane kit company sold ABS molds instead of parts and let the homebuilder make his own parts with cheap ABS molds?
The molds could be made quickly in a vacuum former, then the homebuilder would provide the majority of the labor making the parts.

The ABS molds would come with some simple plywood supports for the builder to assemble on a table or floor, just good enough for one use.
BB
Wait! I have an idea... what if we thermoformed ABS forms that could be used as molds for the homebuilder, with some sort of interlocking support structure (kind of like the dividers in boxes for bottles and such)? This would work REALLY well - especially for a design that was mostly wood or metal, but needed just a few fiberglass parts...

WOW! I CAN COME UP WITH SOME REALLY GOOD IDEAS SOMETIMES... wish you guys would help out a bit too :grin:

Thanks BBensen!

This is a clever idea... but it begs the question - at what point does it pay to have someone make the actual part for you, instead of molding a disposable plastic mold? It might actually take just as much time to thermoform the part, trim, check that it aligns with the support structure, pack and ship. How much time is saved compared to making the actual part in a production mold? Probably very little.
The extra cost that a factory charges (including profit) would probably not be much more at all - and well worth getting a part that is likely much lighter than can be made at home.
I guess it comes down to the age old question - how much are you willing to pay for each pound NOT purchased!

#### BBerson

##### Well-Known Member
HBA Supporter
I don't know how long it takes a factory to wax and prepare a mold and laminate a part and let it cure and remove the part for the next one. Anybody know?
But a factory could make vacuum formed molds about one every 5 minutes, I would guess.

This is not a process for ready to fly airplanes. This is for kits.
A kit should supply prefabricated critical structure such as welded frame but leave something for the builder to comply with 51% of labor and reduce cost. This noncritical molding is where the builder can supply the labor. The wings would remain fabric covered. The ABS mold would only be for the fuselage shell to get a nice look at minimal cost.
BB

#### WonderousMountain

##### Well-Known Member
Or, we could bolt some strait panels together with a really big engine and wing; huge independently suspended tires, and name it after a strange wilderbeast:mad2:!

Wonderous Mountain,

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#### Rienk

##### Well-Known Member
We're discussing methods to achieve missions that haven't been set, with no predetermined engine, manufacturing method, set of tools or financial window other than cheap if a thousand are made.:tired:

Why don't you keep talking amongst yourselves, and I'll go to the lumber yard with 500 bucks and pick up the supplies I need.
This is a continuation of another thread where most of these things were discussed, if not actually laid out.

#### Rienk

##### Well-Known Member
My dear Smee - I've had an epiphany!

• I've heard (but not yet verified) that you can get a glider rating with less hours than an LSA (which is better than NO hours required for 103UL).
• A glider rating does not require a medical (like an LSA).
• A factory built LSA can be a glider (ready to fly S-LSA).
• Gilder pilots do not have as many restrictions as Sport pilots (though I don't yet know how they compare)
• A motorglider can tool around chasing thermals, or stay under power all its life, cruising cross country.
• There are only two types of LSA that can use retractable gear; an amphib - and a glider.

Therefore:
• Design an LSA as a motorglider, and you can include retractable gear.
• Offer trainng for either Sport pilot or glider ticket.
• Motorgliders do not necessarily need stellar climb rates.
• Design with an engine more suited for cruise power than climb power (within reason).
• With retractable gear, you can attain the maximum level speed with much less power/fuel required.
• A lower cost engine is thus viable.
• Chase thermals or travel distances with power - freedom of choice!

At max LSA weight, stall speed with a high AR wing could probably be met with a wing loading of 11-12 psf.
To be able to fit in a sta ndard T-hangar (even though the wings will be removable) the span should be no more than 38-39'.
An AR of 12 gives a span of 38', with a chord of 38", and a wing area of 120 sf (11 psf).

Questions:
• What qualifies as a "motorglider"?
• Are there certain criteria that need to be met?
• Does this idea seem plausible?

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#### Gnarly Gnu

##### Member
hydroforming of metal is interesting, but as you say - except with the high end equipment - not very usable for aviation (and definitely not homebuilt).
High end equipment? You mean low. The main advantage of explosive forming is, of course, that high end equipment and complex presses are NOT required.

Rotary forming is a great & economical way to make large plastic parts but the main limitation is strength as these are thermoset plastics that are typically cured at not such high tempertures. Good for big plastic water tanks and non-critical things like that.

PS the retractable 'motor glider' approach sounds great if you can figure a way to retract simply / cheaply.....

#### Rienk

##### Well-Known Member
High end equipment? You mean low. The main advantage of explosive forming is, of course, that high end equipment and complex presses are NOT required.

Rotary forming is a great & economical way to make large plastic parts but the main limitation is strength as these are thermoset plastics that are typically cured at not such high tempertures. Good for big plastic water tanks and non-critical things like that.
I wasn't referring to 'explosive' forming, which I understand to be of marginal reliability or consistancy; I was referring to "real" (high end) hydroforming, which of course is expensive.

As far as plastic goes; I think we all understand and can agree that the plastic has limited strength, and should not be counted on for such.
But rotomolding is excellent for creating shapes inexpensively - which is what I am proposing as a possible solution for aircraft design - with the understanding that there is a weight penalty.
Though I'm not yet willing to commit to it, the idea is worth considering and warrants further exploration and research.

#### Dana

##### Super Moderator
Staff member
I think in the end you'll find that the weight of a large non structural shell, whether rotomolded, thermoformed, or whatever, will be an unacceptable penalty within the weight limits of LSA (and definitely 103).

-Dana

But if we LEGALIZE it, we can't take your HOUSE!

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