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BJC

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There are planes that will takeoff and land shorter than typical airplanes in a category. They are referred to as STOL. As far as I know, there is no definition of just what “Short” is. For example, my Sportsman will, with about 450 pounds of payload, takeoff in 400 +/- feet. That seems short compared to a C172, but not compared to a Carbon Cub. However, the standard landing gear for the Sportsman is marginal for landing on rough, sandy, rocky, or brush covered land. So, by one set of criteria, my airplane could be considered a STOL, but not a bush airplane. There is an alternative landing gear that is suitable for bush flying, that might begin to qualify the Sportsman as a bush airplane.

I would not want to use a typical aluminum fuselage for bush flying, even though there are some aluminum fuselage airplanes that have excellent STOL characteristics.

As a point of interest, I once asked a friend, who had made a living operating a hunting and fishing lodge and a charter air service in Alaska, which airplane was the best for bush flying. The answer was long, but the short version was that it depended on the specific mission. Over the years, he operated everything from Super Cubs, to Twin Otters. He said that the most consistent money maker in Alaska was the C206, complete with tricycle landing gear.


BJC
 

cluttonfred

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I figured it was something like that, really down to "short" vs. "rough" field operation, durability, maintainability, repairability, just wasn't sure if you were taking this in another direction.
 

Victor Bravo

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A "bush plane" is a working farm animal. It needs to take as large of a load as possible into and out of a short or unprepared strip. As mentioned, many "normal" airplanes are working bush planes, even the Cherokee Six works for a living in Alaska.

A STOL plane is the little Poodle that can balance a tennis ball on its nose, but it can't plow a field.

A STOL plane can also be a bush plane but it can also be a one-trick pony that is not useful or capable of making a living.

A good example is that the dirt bikes we see in dirt track and Motocross competitions can do things and go places where no 4 wheeled vehicle can go. But they are not a 1 ton 4x4 pickup truck that can get you and your equipment to the remote construction site.

The motocross bike is a STOL equivalent, and the big-tire pickup may be a STOL equivalent too... but only the big truck can be a Bush Plane equivalent because it can work for a living.

My little Ridge Runner project is a perfect example of a STOL sportplane. I want to go out and land in tiny spaces and in places where other planes can't go. But I am not trying to carry anything, or make a living with it. It's a fun toy.
 

Bradsopex

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Good analogies. I am not looking to make a living with it, so the only cargo is going to be what I want to take camping. I want to get in and out of tighter spaces, off airport. That's why good STOL is a priority.
 

TFF

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A lot of STOL flying depends on CG and weight but it also depends on stuff like landing gear. The extreme STOL fliers fly controlled crashes to the ground. The gear can take lots of Gs vertically. Gear is much like a Baja or Stadium Truck being able to dissipate vertical energy. Takeoff is usually the easiest. You either have enough power or you don’t. Wing is big enough or isn’t. You’re in the South, you will not get the he Alaska performance numbers. Too hot. Just watching the STOL competition at Oshkosh, and they probably double the Alaska distance. Summer in Oshkosh is like Spring in the south.
Build a Kitfox or like. The Just Aircraft are outgrowths and both are outgrowths of the Avid. If you are expecting heavier operations, you will be going at some sort of Cub clone. Bearhawk Patrol and the like are just idea shifted Cubs. The Zenith stuff has great performance but I think they are too fragile if you started slamming them down like you can do a Super STOL. You kind of have to pick a team on the type you want.
 

blane.c

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STOL, all things being equal density altitude, temperature, wind, and weight a bush plane should be able to take of in the distance it landed or less. A circus performer only needs to take off short with just a little gas in the tank with a skinny pilot wearing swim trunks.
 

Bradsopex

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Yeah, down here it gets hot and humid so performance isn't the same as it is out west in the Sierras or up in Alaska.

I've been looking hard at the Zenith because I can pick apart the kit and purchase portions, or buy the plans and purchase individual components as necessary. This is attractive because, as TFF mentioned, the Zenith can be a bit fragile when you start slamming it down. I have an idea for the suspension based on some more modern design, but implementing it on the Zenith will be tricky. I do enjoy challenges though. I have the ability to machine my own parts, so modification is that much easier. I will be switching from a metal skin to painted carbon fiber skin. This will give me lighter weight over the aluminum/paint method from the factory and still allow me to store outdoors if needed. Plus weight saved is always a bonus.
 

rv7charlie

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Yeah, down here it gets hot and humid so performance isn't the same as it is out west in the Sierras or up in Alaska.

I've been looking hard at the Zenith because I can pick apart the kit and purchase portions, or buy the plans and purchase individual components as necessary. This is attractive because, as TFF mentioned, the Zenith can be a bit fragile when you start slamming it down. I have an idea for the suspension based on some more modern design, but implementing it on the Zenith will be tricky. I do enjoy challenges though. I have the ability to machine my own parts, so modification is that much easier. I will be switching from a metal skin to painted carbon fiber skin. This will give me lighter weight over the aluminum/paint method from the factory and still allow me to store outdoors if needed. Plus weight saved is always a bonus.
Please consult an aero engineer before starting down that path. It's extremely unlikely that it'll be that simple, for a variety of reasons.
 

Victor Bravo

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The Cessna 170/180/206 series, the Beaver/Otter/Twin Otter series, the Canadaian Found Bros. series, and the Pilatus Porter are all sheet metal airplanes used as working bush planes in very harsh conditions, which include being slammed down onto rough landing strips.

The Zenith may be built a little lighter (to create a sport airplane using less power), but that is a design decision based on the intended use... not a fragile design. The Zenith was never designed to be a working bush plane.

If your mission requires a full size commercial grade F-350 4x4 truck, then you need that kind of vehicle, and it doesn't mean that the Jeep CJ is too fragile of a design.

With no disrespect or chastising intended whatsoever... make a realistic assessment of what your actual 90th percentile flight mission is, and build to that. If your mission really requires a heavy duty commercial grade bush plane, then buy or build one. If you are sport flying, and you want to take two people, fishing tackle, and a beer cooler for camping on a river sandbar, then the Zenith sportplane or equivalent is a good mix of high utility and low cost.

By the way, the Zenith can be tinkered with to be able to land on rougher ground. An articulated long-travel gear (Fieseler Storch, Pilatus, Just Super-STOL, et al) can be fitted to the Zenith without too much effort. Adapt the Pilatus style landing gear to the Zenith, put big low-pressure tires on it, use one of the whiz-bang modern air shock tailwheels, and you are most of the way to where you want to be.

Adding carbon skins to the Zenith structure is likely going to be more problematic than it is worthwhile.
 
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cluttonfred

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I would be leery of substituting carbon sheet for aluminum without some serious structural analysis. I would be far less concerned about using thin composite (maybe Kevlar for damage resistance?) in place of fabric covering like the Scott "Ol' Ironsides." There is, of course, nothing to stop you from storing a properly finished and maintained fabric-covered aircraft outside in any case.
 

BJC

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I will be switching from a metal skin to painted carbon fiber skin. This will give me lighter weight over the aluminum/paint method from the factory and still allow me to store outdoors if needed. Plus weight saved is always a bonus.
How will you attach the CF skin to the underlying structure?

How will you address the CF/aluminum corrosion issue?

How much CF will you need to match the structural properties, including the toughness needed to prevent damage due to a stone or two that hits the fuselage, of the aluminum that you will replace?

How much weight will you save by using CF rather than aluminum?

Thanks,


BJC
 

Victor Bravo

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How much will the Carbon Fiber skin "field repair kit" weigh that you take into the field with you?

A quart of resin and hardener, a small roll of CF cloth, a can of solvent, brushes and rollers, don't forget the vacuum pump and roll of bleeder cloth, the vacuum bag material, the gloves, the generator to run the vacuum pump... oh I almost forgot... the heater to help cure the resin overnight...
 

Bradsopex

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Please consult an aero engineer before starting down that path. It's extremely unlikely that it'll be that simple, for a variety of reasons.
I'm an engineer currently working in aerospace, so no worries on that.


With no disrespect or chastising intended whatsoever... make a realistic assessment of what your actual 90th percentile flight mission is, and build to that. If your mission really requires a heavy duty commercial grade bush plane, then buy or build one. If you are sport flying, and you want to take two people, fishing tackle, and a beer cooler for camping on a river sandbar, then the Zenith sportplane or equivalent is a good mix of high utility and low cost.

By the way, the Zenith can be tinkered with to be able to land on rougher ground. An articulated long-travel gear (Fieseler Storch, Pilatus, Just Super-STOL, et al) can be fitted to the Zenith without too much effort. Adapt the Pilatus style landing gear to the Zenith, put big low-pressure tires on it, use one of the whiz-bang modern air shock tailwheels, and you are most of the way to where you want to be

Adding carbon skins to the Zenith structure is likely going to be more problematic than it is worthwhile.
I appreciate all input! It may cause me to consider things overlooked. This plane is for fun, pure and simple. I want to design my own, but want to get my feet wet building a proven platform. Articulated suspension is exactly what I am thinking, maybe with inboard shocks. I could possibly do a Cabane V style setup on the main gear similar to the popular Cub design. I need to look at the fuselage structure to really see what is feasible, because what I have in my head may not actually work in reality.

I would be leery of substituting carbon sheet for aluminum without some serious structural analysis. I would be far less concerned about using thin composite (maybe Kevlar for damage resistance?) in place of fabric covering like the Scott "Ol' Ironsides." There is, of course, nothing to stop you from storing a properly finished and maintained fabric-covered aircraft outside in any case.
Carbon fiber (which is a composite) is pretty resilient depending on what weight of weave you use. I can use carbon/Kevlar where increased strength is needed. I have made carbon fiber structures before, so I'm not too concerned. I will probably use carbon fiber tubing (very light and strong) to reinforce where needed. And just to clarify, these won't just be panels, but a molded skin. This will reduce the hardware needed, and ounces can add up to pounds quick.


How will you attach the CF skin to the underlying structure?

How will you address the CF/aluminum corrosion issue?

How much CF will you need to match the structural properties, including the toughness needed to prevent damage due to a stone or two that hits the fuselage, of the aluminum that you will replace?

How much weight will you save by using CF rather than aluminum?

Thanks,


BJC
The carbon fiber will be attached using polyurethane isolators and washers, this will prevent the galvanic corrosion issue. Anywhere there isn't a fastener but carbon fiber could be touching the aluminum structure will be addressed as needed. As for how much, that depends on the weave I use. Since the carbon won't be exposed I can go with a denser weave. As for the weight and how many mils of thickness I will need, that will require some analysis I won't have until I get/make a model of the airframe so I can do some FEA on it.

I don't want anyone to think I am jumping into this haphazardly, and I appreciate the questions! I have used carbon fiber extensively in my motorsport hobbies/business, so I'm not new to it. When I get around to building, then I will know how much weight I will be saving, or if it was all a hogwash. Either way, it'll be cool!

I got the inspiration to do this from Mike Patey's build. He's the dude that built Draco from a Wilga, if any of you are familiar. He is doing a carbon fiber skin on his Carbon Cub, and it's pretty impressive. I encourage you all to check it out!

https://www.youtube.com/channel/UCSvdee86uThqIrloZjWwNVg/videos
 
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Victor Bravo

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I looked at it briefly, and I believe that the Storch / Super-STOL landing gear configuration will work best on a Zenair. The main gear will hinge at the lower longeron and lower front doorpost structure which is already there. The long compression member will attach to the wing root fitting structure already there at the top of the steel cage. A small three sided pyramid can be attached to the side of the fuselage and instrument panel cross-member to create a fixed node or junction where the top of the shock absorber will mount.

The only obstacle you might run into is that the large stock OEM Zenair doors will possibly interfere with the landing gear uprights. You might have to change the hinging of the door, or make it a sliding door like the Pilatus.
 

rv7charlie

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So, you're an engineer working in aerospace. Which vehicle you''ve seen has replaced a load bearing ('stressed') aluminum skin with a load bearing ('stressed') carbon skin on an underlying stamped aluminum structure, using polyurethane isolators and washers (implying attachment with fasteners, instead of bonding)?
 

Bradsopex

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I was wondering if you were going to go there, haha. And I must admit, I work on stuff that goes to space, not air-breathing aircraft, but I find there's some good cross knowledge.

I'm not sure what you think is going to happen, but what I'm doing is far from new. There a number of composite/aluminum airframes out there. I had a lot of experience on the CV-22, which is mostly a composite airframe with some aluminum components.

In my plan, the fuselage will have a top and bottom skin, split down the side of the airframe. Each side will be one continuous piece of CF.

The Zenith skin panels are not bonded to the airframe, they use blind rivets. I will be using fasteners, as I want the skin to be removable to access the frame and the components. I chose polyurethane because that is what we use in our racecars due to it's stiffness.

Now I know: "but the airframe wasn't designed for that!" and you are absolutely right! The airframe lacks bracing, as they designed the skin to absorb some of those forces. I will be introducing cross bracing based on what the FEA shows. I'm not afraid to break out the TIG if need be. I've built some pretty stiff roll cages, so I don't think this is too difficult.

I'm not sure exactly how well this work, that's why its EXPERIMENTAL. Sure, I can go "tried and true" but:

A) Where is the fun in that?
B) I like to push boundaries.
 

TFF

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Using carbon skin on a Cub is no big deal because it’s just a skin. Substitute carbon skin on a Zenith where the skin is the structure is going to take more thought. I bet it will be heavier than the aluminum one. At least first iteration. Removable and load bearing and a bunch of holes to attach all along with beefing up the internal to take removable fasteners is way crazy amount of change. I would think building one stock to see if it really makes sense would be time well spent. Once it is flying, then let the mod version out.

I must say too, and not trying to offend, but this group wants to see the money. Not saying you don’t have the talent to do this, but your nondescript explanations are coming off like a high school student talking about it. Telling us you are just going to put some nutplates on bulkheads and screw some flat carbon sheet on will have milk coming out of everyone’s noses.
 
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