The Affordaplane, a poorly executed exercise in design.

The Affordaplane, a poorly executed exercise in design.

nerobro
nerobro

November 11th, 2011, 6:00 pm #1

Pfew. So, I got the plans, and I found some time.

Several things about the plans bugged me. So I started re-engineering the plans. Lemmie tell ya, things got all pear shaped rather quickly.

So, most of the frame is 2x2x.125wall aluminum tube. This tubing has a "cyclic loading rating" of 14,000psi. The plane weighs (or should weigh) 254lbs. Loaded, it shouldn't be more than 550. Even under a 10g static loading, you could still have a 10g vibration and you'd STILL be under the rated limit for that aluminum. And that's even derated to account for aluminum's lack of fatigue limit. The actual tensile strength of 6061-t6 tubing is something like 44,000psi.

Using the square tubing more or less doubled the weight of the fuselage. Versus round tubing. But fine, I understand square tubing is easier to deal with. And heck, the bottom, engine and rear cabin frames are loaded in bending at several points, so we'll leave those alone. Swapping out the windshield, roof, and tail tubing for .. uh.. tubes. 2" diameter, 1/16" wall. Would save 25lbs. Without even coming close to having the parts "just barely adequate" for the job.

The wings have a lot of excess weight as well. The foam for the ribs is left solid. There's no real good reason for that. And the inter rib webs could also be "fixed" By cutting the webs into X shapes, and carving out half the foam on the ribs.. you could save another four pounds.

And this is on top of the whole motor fiasco.

So..... I think I'm going to look at other designs. Though, since the wings and tail are proven on this, I may just have to make my own fuselage. ... perhaps clone something like the belite.
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Someone
Someone

November 11th, 2011, 6:29 pm #2

Drawings for things that are to be built by people I don't know, possibly with next to no experience, and with significant life safety hazards give me the willies.

If there are ways to make it simpler, and assumptions that give a bit of extra factor of safety, without pushing it over the top, I'd do it. That may mean my weight budget isn't fully optimized. That may mean solid ribs instead of ones with cutouts. Fatigue cracks at the corners of a cutout are a significant concern, especially if a builder decides my fillets aren't necessary and he can use square corners.

There's a reason that engineers stamp drawings and specs, and it's so that the lawyers know who to sue when things go wrong.
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Someone
Someone

November 11th, 2011, 6:40 pm #3

Pfew. So, I got the plans, and I found some time.

Several things about the plans bugged me. So I started re-engineering the plans. Lemmie tell ya, things got all pear shaped rather quickly.

So, most of the frame is 2x2x.125wall aluminum tube. This tubing has a "cyclic loading rating" of 14,000psi. The plane weighs (or should weigh) 254lbs. Loaded, it shouldn't be more than 550. Even under a 10g static loading, you could still have a 10g vibration and you'd STILL be under the rated limit for that aluminum. And that's even derated to account for aluminum's lack of fatigue limit. The actual tensile strength of 6061-t6 tubing is something like 44,000psi.

Using the square tubing more or less doubled the weight of the fuselage. Versus round tubing. But fine, I understand square tubing is easier to deal with. And heck, the bottom, engine and rear cabin frames are loaded in bending at several points, so we'll leave those alone. Swapping out the windshield, roof, and tail tubing for .. uh.. tubes. 2" diameter, 1/16" wall. Would save 25lbs. Without even coming close to having the parts "just barely adequate" for the job.

The wings have a lot of excess weight as well. The foam for the ribs is left solid. There's no real good reason for that. And the inter rib webs could also be "fixed" By cutting the webs into X shapes, and carving out half the foam on the ribs.. you could save another four pounds.

And this is on top of the whole motor fiasco.

So..... I think I'm going to look at other designs. Though, since the wings and tail are proven on this, I may just have to make my own fuselage. ... perhaps clone something like the belite.
2" square tubing, with a 0.125" wall, has an area of 0.9375 sq inches and an I of 0.55 in^4.
2" round tubing, with a 1/16" wall, has an area of 0.4 sq inches and an I of 0.19 in^4.

So in essence, your proposed substitution cuts the strength in tension and bending by more than half.

What is your life worth? Are you a qualified and licensed engineer with experience working in aircraft structures?
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pbjosh
pbjosh

November 11th, 2011, 9:02 pm #4

Pfew. So, I got the plans, and I found some time.

Several things about the plans bugged me. So I started re-engineering the plans. Lemmie tell ya, things got all pear shaped rather quickly.

So, most of the frame is 2x2x.125wall aluminum tube. This tubing has a "cyclic loading rating" of 14,000psi. The plane weighs (or should weigh) 254lbs. Loaded, it shouldn't be more than 550. Even under a 10g static loading, you could still have a 10g vibration and you'd STILL be under the rated limit for that aluminum. And that's even derated to account for aluminum's lack of fatigue limit. The actual tensile strength of 6061-t6 tubing is something like 44,000psi.

Using the square tubing more or less doubled the weight of the fuselage. Versus round tubing. But fine, I understand square tubing is easier to deal with. And heck, the bottom, engine and rear cabin frames are loaded in bending at several points, so we'll leave those alone. Swapping out the windshield, roof, and tail tubing for .. uh.. tubes. 2" diameter, 1/16" wall. Would save 25lbs. Without even coming close to having the parts "just barely adequate" for the job.

The wings have a lot of excess weight as well. The foam for the ribs is left solid. There's no real good reason for that. And the inter rib webs could also be "fixed" By cutting the webs into X shapes, and carving out half the foam on the ribs.. you could save another four pounds.

And this is on top of the whole motor fiasco.

So..... I think I'm going to look at other designs. Though, since the wings and tail are proven on this, I may just have to make my own fuselage. ... perhaps clone something like the belite.
It is designed to be cheap, not perfect.

I think this is something to be said for the square tubing, since it is just fine for gyrocopters, and otherwise.

But the wings (I think I mentioned this) were very heavy in construction, since they were designed to be easy and cheap to build. That is one place where some people have lost allot of weight. UL wings are generally more expensive since they use higher grade fabrics (light ans strong) instead of the cheap painted fabric ones in the Afford-a-plane.

And that is the bit about that plane- simple, cheap, to be done with minimal tools.

Other UL plane designs just used a simple single rear tube, partially because that was all that was needed. The tail boom on the UL planes see very little force, really.

In addition, there are people who built alternative wings for the A-plane and they shaved allot of weight off of it.

Good luck if you go forward -

Josh
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Skreemer
Skreemer

November 11th, 2011, 10:32 pm #5

Pfew. So, I got the plans, and I found some time.

Several things about the plans bugged me. So I started re-engineering the plans. Lemmie tell ya, things got all pear shaped rather quickly.

So, most of the frame is 2x2x.125wall aluminum tube. This tubing has a "cyclic loading rating" of 14,000psi. The plane weighs (or should weigh) 254lbs. Loaded, it shouldn't be more than 550. Even under a 10g static loading, you could still have a 10g vibration and you'd STILL be under the rated limit for that aluminum. And that's even derated to account for aluminum's lack of fatigue limit. The actual tensile strength of 6061-t6 tubing is something like 44,000psi.

Using the square tubing more or less doubled the weight of the fuselage. Versus round tubing. But fine, I understand square tubing is easier to deal with. And heck, the bottom, engine and rear cabin frames are loaded in bending at several points, so we'll leave those alone. Swapping out the windshield, roof, and tail tubing for .. uh.. tubes. 2" diameter, 1/16" wall. Would save 25lbs. Without even coming close to having the parts "just barely adequate" for the job.

The wings have a lot of excess weight as well. The foam for the ribs is left solid. There's no real good reason for that. And the inter rib webs could also be "fixed" By cutting the webs into X shapes, and carving out half the foam on the ribs.. you could save another four pounds.

And this is on top of the whole motor fiasco.

So..... I think I'm going to look at other designs. Though, since the wings and tail are proven on this, I may just have to make my own fuselage. ... perhaps clone something like the belite.
GS4**?
or you have another spare laying about?
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nerobro
nerobro

November 11th, 2011, 10:38 pm #6

2" square tubing, with a 0.125" wall, has an area of 0.9375 sq inches and an I of 0.55 in^4.
2" round tubing, with a 1/16" wall, has an area of 0.4 sq inches and an I of 0.19 in^4.

So in essence, your proposed substitution cuts the strength in tension and bending by more than half.

What is your life worth? Are you a qualified and licensed engineer with experience working in aircraft structures?
Yup, using half the material does reduce the strength. And that's completely ok.

The tail doesn't get latteral bending resistance from the tubing. It has guy wires to handle it's horizontal loads. The vertical loads are handled by the truss design. However the loads are then transfered to the middle of the rear cabin post, loading it in bending. Which.. is an interesting choice.

The tail, either through maximum lifting force, or through maximum pressure exerted, at speeds well above the VNe (40% higher...) can't exceed 350lbs. Basically the tail is built hilariously strong, and it seems the material choice was one of "hey I have this hanging around" versus "what's going to be strong enough".

So, you're just questioning it, am I wrong? Show me!

The person who designed the airplane is not a qualified or licensed aircraft engineer, if he was I'd expect his plane would have been built under 254 pounds. He was 10lbs overweight, and his recommendation to other buyers was to install a motor that's yet another 30lbs heavier. His "engine weight budget" is 80lbs, the recommended motor is 82, without prop.

My life is worth a lot to me. Airplanes, especially ones flying at less than 55mph, are very, very difficult to build to lightly. And very, very easy to build to heavy. Weight on these small planes matters much more than it does on larger aircraft. Have you seen how tight panties get over 5lbs on a certified GA plane?
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nerobro
nerobro

November 11th, 2011, 10:39 pm #7

GS4**?
or you have another spare laying about?
The GStwin motors are around 170lbs. I need 35-45hp in a 65lb package. Which ain't happening with a big ol suzuki.
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Hans
Hans

November 12th, 2011, 6:19 am #8

Pfew. So, I got the plans, and I found some time.

Several things about the plans bugged me. So I started re-engineering the plans. Lemmie tell ya, things got all pear shaped rather quickly.

So, most of the frame is 2x2x.125wall aluminum tube. This tubing has a "cyclic loading rating" of 14,000psi. The plane weighs (or should weigh) 254lbs. Loaded, it shouldn't be more than 550. Even under a 10g static loading, you could still have a 10g vibration and you'd STILL be under the rated limit for that aluminum. And that's even derated to account for aluminum's lack of fatigue limit. The actual tensile strength of 6061-t6 tubing is something like 44,000psi.

Using the square tubing more or less doubled the weight of the fuselage. Versus round tubing. But fine, I understand square tubing is easier to deal with. And heck, the bottom, engine and rear cabin frames are loaded in bending at several points, so we'll leave those alone. Swapping out the windshield, roof, and tail tubing for .. uh.. tubes. 2" diameter, 1/16" wall. Would save 25lbs. Without even coming close to having the parts "just barely adequate" for the job.

The wings have a lot of excess weight as well. The foam for the ribs is left solid. There's no real good reason for that. And the inter rib webs could also be "fixed" By cutting the webs into X shapes, and carving out half the foam on the ribs.. you could save another four pounds.

And this is on top of the whole motor fiasco.

So..... I think I'm going to look at other designs. Though, since the wings and tail are proven on this, I may just have to make my own fuselage. ... perhaps clone something like the belite.
LOL I wish you had asked about this 10 years ago, when I could still do some aerodynamics work. I'd be checking NACA profiles and all that good stuff for you, but there's no way I can do that now. I have to say, just from an initial look, I don't like the design either but for different reasons.

My main concern is lateral stability of the entire tailplane. It looks like it's overbuilt for the vertical stiffness but next to nothing laterally. I noticed that some builders put in bracing wires, not sure if the plans call for it, but you're going to need them. Unfortunately I don't know if the wing structure is strong enough to make any difference. I probably would also recommend against lightening the tubing though, that's a long bending moment for the structure. Definitely don't skin that ass end either if you don't have to, that's for sure, or you'll just increase the lateral loads as well as increasing suseptibility to side winds. I'd personally think about a triangular cross section with a smaller vertical height, but then that's a bit radical of a design change and essentially changes the whole layout of the aircraft.

Also be wary of you CG too. Start lightening areas of that aft structure too much, and you're going to get nose heavy. Don't let your weight reduction compromise the stability of the aircraft, nor its strength. If you REALLY want to go lighter, you may think about rectangular or I-beam type material in place of the 2x2 square, but don't reduce that lateral strength at all. And watch your CG every step of the way.

Don't even get me started on those foam core wings. I'd also suggest NOT channeling or carving out internally in the foam. I work with the stuff enough that I wouldn't trust its strength if you did so. Build them solid profile, or find a different material, you'll be surprised how much pressure that the skin will compress onto the ribs.

Lets not forget that you're flying an extremely light aircraft, not a performance aircraft. The only purpose is to get you up in the air safely and under control, and it looks like the design is fairly proven in that regard. If you're chasing anything else such as speed, maneuverability or any other performance factor.... you're looking at the wrong class of airplane.


-Hans


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Someone
Someone

November 12th, 2011, 6:27 am #9

Yup, using half the material does reduce the strength. And that's completely ok.

The tail doesn't get latteral bending resistance from the tubing. It has guy wires to handle it's horizontal loads. The vertical loads are handled by the truss design. However the loads are then transfered to the middle of the rear cabin post, loading it in bending. Which.. is an interesting choice.

The tail, either through maximum lifting force, or through maximum pressure exerted, at speeds well above the VNe (40% higher...) can't exceed 350lbs. Basically the tail is built hilariously strong, and it seems the material choice was one of "hey I have this hanging around" versus "what's going to be strong enough".

So, you're just questioning it, am I wrong? Show me!

The person who designed the airplane is not a qualified or licensed aircraft engineer, if he was I'd expect his plane would have been built under 254 pounds. He was 10lbs overweight, and his recommendation to other buyers was to install a motor that's yet another 30lbs heavier. His "engine weight budget" is 80lbs, the recommended motor is 82, without prop.

My life is worth a lot to me. Airplanes, especially ones flying at less than 55mph, are very, very difficult to build to lightly. And very, very easy to build to heavy. Weight on these small planes matters much more than it does on larger aircraft. Have you seen how tight panties get over 5lbs on a certified GA plane?
I haven't seen the plans, nor done any review of them. I can't say whether the tubing substitution you describe will work or not. If you do substitute, all of the risks of proceeding are on your head.

I've had clients ignore my design and recommendations. Sometimes they want to save money, sometimes they think they know a better way, and sometimes they're just stubborn. I can't force them to follow the design. I can let them know there may be large problems with what they're doing. They're also up the creek when they come to me later and want a letter saying everything was built in accordance with the plans and code requirements.

Proceed at your own risk.

"The person who designed the airplane is not a qualified or licensed aircraft engineer, if he was I'd expect his plane would have been built under 254 pounds."

Personally, if the set of plans is not signed and stamped by an engineer, I wouldn't use them. If something goes wrong, even if built exactly according to the plans, your recourses are severely limited without an engineer's approval of the plans. Especially in a life safety application.

Besides, it's not just a good idea, it's the law. There are penalties in every state for practicing engineering without a license, and designing a plane for people to build and fly is practicing engineering.
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nerobro
nerobro

November 12th, 2011, 8:01 am #10

LOL I wish you had asked about this 10 years ago, when I could still do some aerodynamics work. I'd be checking NACA profiles and all that good stuff for you, but there's no way I can do that now. I have to say, just from an initial look, I don't like the design either but for different reasons.

My main concern is lateral stability of the entire tailplane. It looks like it's overbuilt for the vertical stiffness but next to nothing laterally. I noticed that some builders put in bracing wires, not sure if the plans call for it, but you're going to need them. Unfortunately I don't know if the wing structure is strong enough to make any difference. I probably would also recommend against lightening the tubing though, that's a long bending moment for the structure. Definitely don't skin that ass end either if you don't have to, that's for sure, or you'll just increase the lateral loads as well as increasing suseptibility to side winds. I'd personally think about a triangular cross section with a smaller vertical height, but then that's a bit radical of a design change and essentially changes the whole layout of the aircraft.

Also be wary of you CG too. Start lightening areas of that aft structure too much, and you're going to get nose heavy. Don't let your weight reduction compromise the stability of the aircraft, nor its strength. If you REALLY want to go lighter, you may think about rectangular or I-beam type material in place of the 2x2 square, but don't reduce that lateral strength at all. And watch your CG every step of the way.

Don't even get me started on those foam core wings. I'd also suggest NOT channeling or carving out internally in the foam. I work with the stuff enough that I wouldn't trust its strength if you did so. Build them solid profile, or find a different material, you'll be surprised how much pressure that the skin will compress onto the ribs.

Lets not forget that you're flying an extremely light aircraft, not a performance aircraft. The only purpose is to get you up in the air safely and under control, and it looks like the design is fairly proven in that regard. If you're chasing anything else such as speed, maneuverability or any other performance factor.... you're looking at the wrong class of airplane.


-Hans

Hey Hans,

I didn't know you were an aero type. I've done lots of r/c and free flight stuff over the years, to say the least i'm aware of the effects of CG, and importantly having enough side area in the right places to give the plane spiral stability. Thankfully, a pilot can counteract that tendency.

So, the tail is wire braced to the wing tips, and then there's wire braces from the wing tips back to the nose. The wings also have drag links within them. The support struts are also crossbraced.

So as designed, I think they have the tip of the tail well triangulated.

Now what isn't designed for is torque on the tailplane. Say it's sideslipping, you can get some good torque on the tail. Box section tubing is less strong in twist than circular tubes. I think going to circular tubing would improve tailplane stiffness. (not that I'm prepared with the math to prove that right now..)

Quite honestly, I'm very close to saying to hell with it, and just redesigning the whole fuselage. Copying the belite in concept would be a big step forward. Going with a triangular tail would solve a lot of it.

The way the plans call the plane, it's going to be stupid nose heavy no matter what I do. But since I'd be building it myself, I have the option of moving the wing forward.

So tell me about your experience with foam ribbed wings? The sky pup uses foam ribs. The ribs will be covered in 1/16" ply cap strips, and a layer of fiberglass, so the foam isn't going to be carrying the load anywhere near directly.

Yeah, I do hope this will be an extremely light aircraft. The problem is it ends up being 50-100lbs heavier than the design weight. I think it would be safer to be lighter, rather than fly an overweight pig. ... Perhaps I'm scarred from what happens with r/c airplanes when you load them up. They start to wallow. They go from light, and fun, and easy to fly, to mushy, and reluctant. Part 103 is speed limited, so it's not like I can (legally) just fly faster.

As far as I'm concerned, making something fly is easy. A brick will fly with enough horsepower. Making something fly well, is another story entirely, and usually involves making it light.

I prefer the idea of flying a metal plane. Metal is a very consistent material that makes deciding if it's ok or not very easy. But if it comes down to it... the FP-606 seems to be a much more well thought out design. Though it's an all wood design.
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