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Feedback on DubCub ultralight design?

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Lendo

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Megan, Victor Bravo is right the written word in forums can be extremely misinterpreted.
I can see you have done your homework, but what is your background (briefly) or Qualifications, I would guess Engineer or maybe Aeronautical Engineer.
I'm sure members don't wish to preach to the enlightened.
George
 

Riggerrob

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Dear Meagan,

You might weigh less than 170 pounds, but the last time I weighed 170 was the day I graduated army jump school!
Hah!
Hah!
Old army joke!
These days - with a lot of hiking - I can keep my weight down to 200 pounds. At 200 pounds, I am too heavy for a half VW powered airplane. Shucks!
My current naked weight limits me to small airplanes powered by full-sized VW engines.

The FAA may count 170 pounds as an AVERAGE pilot weight, but many of us are closer to the 254 pound standard for parachute certification.

If you want to sell significant numbers of plans or kits, you may have to up-size for design for more realistic weights.
Not that many of us are proud of the way our naked weights increased over the years, but we have to be honest.
A common failing of plans-built airplanes is that the original designer (e.g. Ralph Mong or Ken Rand) built a tiny airframe around his diminutive stature, but few full-sized pilots could squeeze into the cockpit.
 

Riggerrob

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Dear Meagan,
You have some clever ideas in your ultralight proposal.
Since a full-foam core might be too heavy, may I suggest a partial core wing structure?
I am picturing a partially hollow foam D-spar (curved leading edge glued to flat main spar web). Vacuum-bag to minimize epoxy weight. Making the main spar up to 18 percent deep will help lower spar weight, maximize lift, but contribute little extra drag (see Heinz design rational on the Zenith series of kitplanes). Aft of the main spar, use a bit of foam to stabilize ribs. Maybe even wrap glass/carbon fabric 3/4 of the way around ribs.
A large cross-section, triangular/quadrilateral spar across the trailing edge will help carry torsion loads from your Junkers flaps. Building a sharp trailing edge is difficult and irrelevant. As long as the trailing edge is square/rectangular, it will be strong enough and simple enough to build.

As for wing tips, the primary goal of fancy wing tips is pushing wing tip vortex as far outboard as possible to maximize effective span (engineers, is that the correct term?). So integrating a swept wing tip into your D-spar will minimize build time and weight.
For an overly-sophisticated wing tip, look at the fancy Schumann planform now fashionable on competition sailplanes. Your STOL envelope will do fine with mostly constant-chord wings with swept tips.

While you are self-isolating - with a text book in your lap - you might want to read Harry Riblett's book on airfoils (available from EAA headquarters in Wisconsin). Riblett developed airfoils for Rutan, etc. His most notable accomplishment was substantially increasing the cruise speed of the Bearhawk series of kitplanes, while maintaining Cub-like landing characteristics.
 

Megan May

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Messages
66
Megan, Victor Bravo is right the written word in forums can be extremely misinterpreted.
I can see you have done your homework, but what is your background (briefly) or Qualifications, I would guess Engineer or maybe Aeronautical Engineer.
I'm sure members don't wish to preach to the enlightened.
George
I pecked this into my iPad for member Victor Bravo the other day: Feedback on DubCub ultralight design?
 

Megan May

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Joined
Mar 10, 2020
Messages
66
Dear Meagan,

You might weigh less than 170 pounds, but the last time I weighed 170 was the day I graduated army jump school!
Hah!
Hah!
Old army joke!
These days - with a lot of hiking - I can keep my weight down to 200 pounds. At 200 pounds, I am too heavy for a half VW powered airplane. Shucks!
My current naked weight limits me to small airplanes powered by full-sized VW engines.

The FAA may count 170 pounds as an AVERAGE pilot weight, but many of us are closer to the 254 pound standard for parachute certification.

If you want to sell significant numbers of plans or kits, you may have to up-size for design for more realistic weights.
Not that many of us are proud of the way our naked weights increased over the years, but we have to be honest.
A common failing of plans-built airplanes is that the original designer (e.g. Ralph Mong or Ken Rand) built a tiny airframe around his diminutive stature, but few full-sized pilots could squeeze into the cockpit.
Very good points. I will definitely scale things to fit a more representative size. ☺
 

choppergirl

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Wouldn't two props working in clean air (for example. one each on each wing), be far more efficient than one prop in clean air, and one trying to work in turbulent air behind it? Not to mention, in this configuration, wouldn't the rear prop have to spin faster or be pitched higher to create thrust, because it's working in faster moving air generated by the front prop and not airspeed air?
 

Megan May

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Messages
66
Dear Meagan,
You have some clever ideas in your ultralight proposal.
Since a full-foam core might be too heavy, may I suggest a partial core wing structure?
I am picturing a partially hollow foam D-spar (curved leading edge glued to flat main spar web). Vacuum-bag to minimize epoxy weight. Making the main spar up to 18 percent deep will help lower spar weight, maximize lift, but contribute little extra drag (see Heinz design rational on the Zenith series of kitplanes). Aft of the main spar, use a bit of foam to stabilize ribs. Maybe even wrap glass/carbon fabric 3/4 of the way around ribs.
A large cross-section, triangular/quadrilateral spar across the trailing edge will help carry torsion loads from your Junkers flaps. Building a sharp trailing edge is difficult and irrelevant. As long as the trailing edge is square/rectangular, it will be strong enough and simple enough to build.

As for wing tips, the primary goal of fancy wing tips is pushing wing tip vortex as far outboard as possible to maximize effective span (engineers, is that the correct term?). So integrating a swept wing tip into your D-spar will minimize build time and weight.
For an overly-sophisticated wing tip, look at the fancy Schumann planform now fashionable on competition sailplanes. Your STOL envelope will do fine with mostly constant-chord wings with swept tips.

While you are self-isolating - with a text book in your lap - you might want to read Harry Riblett's book on airfoils (available from EAA headquarters in Wisconsin). Riblett developed airfoils for Rutan, etc. His most notable accomplishment was substantially increasing the cruise speed of the Bearhawk series of kitplanes, while maintaining Cub-like landing characteristics.
I really like your wing and spar ideas. I am going to investigate a 18% deep wing vs the 12% I've got now. Maybe just "sqoosh up"--an engineering term 😅 --my Clark Y to 18% thick. I've heard and seen that strict adherence to airfoil science isn't so critical when designing ultralights. Basically just something round in front and pointy in the back. 😅 I do like the idea of perhaps a rotated trapezoid or just a sim0le square for the trailing edge.
 
Last edited:

Megan May

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Wouldn't two props working in clean air (for example. one each on each wing), be far more efficient than one prop in clean air, and one trying to work in turbulent air behind it? Not to mention, in this configuration, wouldn't the rear prop have to spin faster or be pitched higher to create thrust, because it's working in faster moving air generated by the front prop and not airspeed air?
I believe you are correct that this inline configuration I'm investigating is indeed less efficient than a more standard twin. But I want to avoid any yawing complications should one engine quit in flight, which is a real possibility when running what's essentially a high-strung go kart engine. My inspiration for this push-pull arrangement has been the Cessna 337 and the DoubleEnder.
 

Megan May

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Oh, an under-achiever. We'll try to take it easy on you :)

Just browsing through the recent discussion threads on this forum, I see that our dear friend from Switzerland has just brought up the Fly-Nano, which was a very promising little ultralight seaplane... sort of a flying Jet-Ski. Either nothing came of it, or there was some problem with the (unconventional joined wing) design that made it less safe to operate.

So if all that mfg. and plastics and engineering and drafting experience in your skill set is looking for a little ultralight flying machine to develop, something that accomplishes the Fly-Nano mission (not that specific design) might be a unique little niche to start with. It would set itself apart from the many small STOL ultralights that are around.

As a new idea, have a look at the Pereira X-28 / Osprey 1, which was a little wooden flying boat. Think of something along those lines, but in composite or plastic materials, that is small and light and could be marketed like the Fly-Nano???

Or create a small tandem wing personal sized ground effect aircraft along the lines of the larger Russian Ekranoplan and/or Lippisch ground effect aircraft.
Victor Bravo, you're a bad influence. 😅 I've had that FlyNano stuck in my head all day. But I think it's given me an idea for getting to production a bit faster: You might be familiar with the Goat open-source glider by Mike Sandling, I think it is? I was thinking of sort of a mashup of my current airframe design, the Goat, the Airbike, and the FlyNano. It wouldn't be amphibious. At least not this iteration. But something that you more "ride on" than "ride in." Maybe ditch the slats at the front--at least for now. Get a proof of concept flying. Then incrementally improve it/complicate it over time. In much the same way the Japanese typically take with products: more evolution than revolution, you know? And the first "evolution" would be essentially the very simplest airworthy twin to ever take to the skies.

Thoughts?
 

Victor Bravo

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The Fly Nano is an outlier within that group. A mash-up of the GOAT, Airbike, and your sketch is possible, but mixing Fly Nano DNA into that equation is like a restaurant serving tuna fish topped with ice cream :) ALSO... beware the darkest, most evil force in the aircraft design universe: Mission Creep.

I truly don't want to constrain or deflate any of your creativity... honest. But myself (and many others here who have been around airplanes for a while) also want to prevent you from rapid bunout. Many many people with high intelligence and cleverness went around in circles chasing after competing design priorities and competing configurations, and lost their energy. Belive it or not, after seeing only the first handful of your posts, a lot of us here really don't want to risk losing you to burnout.

OR, to quote another friend of mine who occasionally posts on this forum... "Perfection is the enemy of Completion".

So being more or less the largest mouth in these parts, I'll be the first to give you advice that others will soon back up: Pick a specific mission that the airplane needs to do, or a specific problem your airplane will solve better than others. Then focus your rceativity on accomplishing that in the most direct manner. A thousand different airplanes would be "really cool". But nobody would buy it, or it would be too complicated to build, or it would cost too much, or the tooling to make the kit parts would take years to reate, etc.

Meanwhile, there is a thriving kit airplane industry that is offering decent to excellent options for any given mission. So please trust me when I say set a reasonable goal and then figure out a clever or superior or less expensive way to meet that goal.

With that, and with great joy sending you down some of HBA's deepest rabbit holes, please look up the discussion threads on the "21st Century Volksplane", the"Flying Motorcycle of the Air", and the "Ranger". These are very long-standing discussions and idea-festivals for many of us. The reason that I'm sending you on that much of a deep space trip is that they actually represent viable, usable, and achievable aircraft categories that you could consider developing.

For example, if you can come up with something that is a real contender for the "21st Century Volksplane" market niche, you have a very good chance of selling enough kits to make it worthwhile. The Flying Motorcycle is more risky, and will result in things along the lines of the Fly Nano, so if you can't stay away from the exotic side then this may be your best compromise. But make no mistake, you will sell fewer kits for the flying bike than you will with the modern volksplane.

The Ranger is definitely viable and people will want to build it, but although it started as a group discussion it is now mostly the intellectual proprty of one HBA participant... look at that thread for inspiration and ideas but don't hijack the design.

If you're still speaking with us after looking through these threads, and still have your sanity, we're all glad to help toss around all sorts of ideas :)
 

radfordc

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Megan you may want to contact James Wiebe and pick his brain about his experience doing something similar to what you are about. He designed, developed, and marketed the Belite ultralight. Here is his Facebook info... James Wiebe
 

Megan May

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Messages
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The Fly Nano is an outlier within that group. A mash-up of the GOAT, Airbike, and your sketch is possible, but mixing Fly Nano DNA into that equation is like a restaurant serving tuna fish topped with ice cream :) ALSO... beware the darkest, most evil force in the aircraft design universe: Mission Creep.

I truly don't want to constrain or deflate any of your creativity... honest. But myself (and many others here who have been around airplanes for a while) also want to prevent you from rapid bunout. Many many people with high intelligence and cleverness went around in circles chasing after competing design priorities and competing configurations, and lost their energy. Belive it or not, after seeing only the first handful of your posts, a lot of us here really don't want to risk losing you to burnout.

OR, to quote another friend of mine who occasionally posts on this forum... "Perfection is the enemy of Completion".

So being more or less the largest mouth in these parts, I'll be the first to give you advice that others will soon back up: Pick a specific mission that the airplane needs to do, or a specific problem your airplane will solve better than others. Then focus your rceativity on accomplishing that in the most direct manner. A thousand different airplanes would be "really cool". But nobody would buy it, or it would be too complicated to build, or it would cost too much, or the tooling to make the kit parts would take years to reate, etc.

Meanwhile, there is a thriving kit airplane industry that is offering decent to excellent options for any given mission. So please trust me when I say set a reasonable goal and then figure out a clever or superior or less expensive way to meet that goal.

With that, and with great joy sending you down some of HBA's deepest rabbit holes, please look up the discussion threads on the "21st Century Volksplane", the"Flying Motorcycle of the Air", and the "Ranger". These are very long-standing discussions and idea-festivals for many of us. The reason that I'm sending you on that much of a deep space trip is that they actually represent viable, usable, and achievable aircraft categories that you could consider developing.

For example, if you can come up with something that is a real contender for the "21st Century Volksplane" market niche, you have a very good chance of selling enough kits to make it worthwhile. The Flying Motorcycle is more risky, and will result in things along the lines of the Fly Nano, so if you can't stay away from the exotic side then this may be your best compromise. But make no mistake, you will sell fewer kits for the flying bike than you will with the modern volksplane.

The Ranger is definitely viable and people will want to build it, but although it started as a group discussion it is now mostly the intellectual proprty of one HBA participant... look at that thread for inspiration and ideas but don't hijack the design.

If you're still speaking with us after looking through these threads, and still have your sanity, we're all glad to help toss around all sorts of ideas :)
Regarding the FlyNano, I should've been clearer to say that just the cockpit area and the overhead engine pod location are what I liked/wanted to incorporate of that design. The staggered/looping wing is, like you wrote, different DNA.

Last night, I started into the Ranger thread--and got a fair ways. Also started into the 21st Century Volksplane thread. I'm only about 5 pages in. (It was late last night, and I was sleepy. LOL) A great discussion there--and a great example that I really need to nail down the "mission" of the craft, as you recommended. And I would say these are my goals, if you'll indulge me to list them here:

1. Overarching goal: Get into the air as quickly, cheaply, safely as possible--in a "real" powered airplane. (FAR 103 is vital in that goal from my perspective.)

2. Make the plans and/or kit attractive to people in a real "Hey, even I could build THAT" sense. I got this from the beginning pages of the 21st C. Volksplane thread. (Nothing would make me happier than to see people who've been putting off building and flying a "real" airplane all their lives--like yours truly--finally have at it--and finish it within a couple of months, at most, not years. And for not a lot of money. Safely. And with a bit of style. Who says fun can't be fashionable?) Perhaps composite construction is too far removed from this goal? I, too, am not really a big fan of the whole fill-and-sand idea.--even though I've done it during past motorcycle projects. Maybe laser-cut plywood covered in Oratex? Has anyone ever used vinyl film, like they wrap cars in, for covering? Might be too difficult to work with. Seems very durable. And the cost, if DIY, is fairly low. While I love the idea of an aluminum monocoque airframe, it's just too off-putting in the "even I could build that" department IMHO.

3. Regarding the criteria for my first flying project to be a "real" plane, and I don't mean to insult anybody, IDK, aluminum tubes covered in sailcloth just scream 1980s ultralight death machines to me. I know that that is such an unwarranted perception. I think I read that one of the earliest ultralights--Challengers maybe?--have THE safest flying record of all aircraft? Not sure where I read that. Anyway... They also seem sorta "temporary"? I'm not sure how to express this perception of mine. Like an overgrown hang glider? Something you pull some bolts and pins on, roll up, stuff in a tube on top of your VW bus and drive home to roast a bowl. (Hey, I'm not judging. I might be projecting. But I'm certainly not judging. LMAO) But not a "real" airplane. This is really possibly just my own prejudice. They also seem to have A TON of little brackets, bolts, nuts, pins that would tend to complicate the whole matter. (There is a company that did up/reengineered the Goat and made the SolidWorks files available. Even they said that the sheer number of parts in the Goat was a much bigger task than they'd imagined. And even though I've flown PPG, while I would call them "aircraft," I don't think I'd really ever consider them an "airplane,' you know?

4. In the vein of "Hey, even I could build THAT," I think making the powerplant very approachable is key. Big, expensive, heavy, complicated, etc. engines might scare off a lot of people. First, the expense. Then the handling of such a thing. To truly capture "hearts and minds," my own thinking has been sort of a divide-and-conquer-while-providing-redundancy approach. Hence, the centerline-thrust twin arrangement. First, it's safer than a more conventional twin arrangement, as yaw isn't adversely affected in an engine-out scenario. Second, you still have that redundancy. Third, two little engines are easier to heft into place--or down for servicing--than one bigger engine. This part, however, does drive up the cost. So on the front, it bucks my "cheaply" criterium.

I think I'm rambling here. Thank you for suffering me. I think the DubCub checks some of these boxes, but the composite construction might be "too much" or too "exotic" or "scary" or "new" or "perceived as difficult" for a lot of people. The 21st Century Volksplane is just too much airplane for what I have been thinking. The Ranger is much closer. But yeah, if someone else is already doing that, I don't want to steal any of their thunder. IDK if a DoubleEnder version of that style of airplane is a different-enough project to not step on any toes/dilute their customer base. I'm just getting into the Motorcycle of the Air thread--and is something closer to what I'm thinking.

One SUPER crazy idea I've toyed with is inflatable wings. I'm guessing people have seen the Woopy thing? And the Goodyear Inflatoplane? Those got me researching that idea a few months ago. But something more like the Big Blue Project--but for Earth. My idea was for individual "tubes/cells" running laterally the length of each wing with a centralized area overhead for the pump, inflation tubes, check valves, etc. I stopped myself going too far down that road, as it just seemed... well, crazy. And probably too unproven.
 

Megan May

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Megan you may want to contact James Wiebe and pick his brain about his experience doing something similar to what you are about. He designed, developed, and marketed the Belite ultralight. Here is his Facebook info... James Wiebe
You know, I've been an admirer of his SkyDock for a couple of years. I even incorporated the shape of its tailboom into my DubCub.
 

Megan May

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Here's a VERY rough (Be nice! LOL) old pseudo Q1-type sketch I did one afternoon several months ago. Lots more wing area to slow the plane down to FAR 103 territory.
 

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BBerson

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I designed a twin engine (two 10hp Tecumsehs) light motorglider in the 80’S. I thought two cheap engines made sense. Not so sure now.
It takes some time to accumulate the features of your personal design style, materials, etc.
Good luck with that!
Affordable and safe Ultralights are actually very difficult to design because of the weight and other limitations.
 

Megan May

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You know, I've been an admirer of his SkyDock for a couple of years. I even incorporated the shape of its tailboom into my DubCub.
You know, something that was a mashup of the BeLite SkyDock and this flying cycle design that was THE simplest/user-friendly construction possible. One thing that I think the SkyDock really lost along the way was the simple, open feeling--when they started fairing in the cockpit and adding a canopy.
 

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fgutier2

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You know, that was the primary concern of a friend of mine, Jim, who is, as we joke to each other, a bit "portly." (He's a big Weird Al fan.) LOL I think it all really just comes down to CL, CG, and wing loading, which could be adjusted to suit each pilot. Though wing loading would increase top speed. I should probably design for someone closer to 200, huh?
That's a good start, but I'm thinking closer to 220 I think people over that weight don't think about ultralights any longer. I'm thinking older people weight more now days than they did fifty years ago, but they can afford to pay for what they want the base may be very limited.
 

Victor Bravo

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Another very successful approach to ultralights is the Kolb Firefly. There are a LOT of very clever and well-developed ideas in that one, and it genuinely meets Part 103 with a "real airplane". There are also parts of that design series that I do not agree with at all, and would recommend changes to.

Don't be afraid of simple sheet metal. I was terrified of it for many years, now it's a preference for most of the things I sketch out.

Have a look at the Zenith / Zenair CH-701 and its derivatives. There is an excellent series of articles by the designer Chris Heintz that explains how and why certain design decisions were made. "Anatomy of a STOL Aircraft" or something similar to that... available on the Zenith website.

Part 103 is going to be difficult to accomplish with traditional sheet metal. It has been done, but not often.

However, there is far less reason or justification for Part 103 at present. The Sport Pilot rating, LSA category, and driver's license medical have taken down most of the barriers that made Part 103 desirable. There is very little real justification for somebody to need to start flying without getting a small amount of instruction. Unless there is a specific reason you need Part 103, give (most of us) a chance to steer you toward designing/building something that meets LSA. The biggest single reason is that you will be able to make something that is much much more of a "real airplane", with a structural safety margin, and enough performance to enjoy it as a "real airplane" too.
 

Megan May

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Another very successful approach to ultralights is the Kolb Firefly. There are a LOT of very clever and well-developed ideas in that one, and it genuinely meets Part 103 with a "real airplane". There are also parts of that design series that I do not agree with at all, and would recommend changes to.

Don't be afraid of simple sheet metal. I was terrified of it for many years, now it's a preference for most of the things I sketch out.

Have a look at the Zenith / Zenair CH-701 and its derivatives. There is an excellent series of articles by the designer Chris Heintz that explains how and why certain design decisions were made. "Anatomy of a STOL Aircraft" or something similar to that... available on the Zenith website.

Part 103 is going to be difficult to accomplish with traditional sheet metal. It has been done, but not often.

However, there is far less reason or justification for Part 103 at present. The Sport Pilot rating, LSA category, and driver's license medical have taken down most of the barriers that made Part 103 desirable. There is very little real justification for somebody to need to start flying without getting a small amount of instruction. Unless there is a specific reason you need Part 103, give (most of us) a chance to steer you toward designing/building something that meets LSA. The biggest single reason is that you will be able to make something that is much much more of a "real airplane", with a structural safety margin, and enough performance to enjoy it as a "real airplane" too.
I've read that Chris Heintz article. It had a lot of good stuff in it. :)

I kinda felt like Basic Med sorta gutted the whole LSA area, no? I mean, you do have to have your medical clearance first before you let it lapse and go Basic Med, I think.

I do agree that anyone even thinking of getting into the air take some lessons. But I guess it's the Libertarian tendencies in me (don't burn me at the stake) that just dislikes government red tape, etc. Being able to just go fly your little airplane when you want--as long as you know what you're doing--seems like a facet of the American Dream to me.
 
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