Airframe strength...

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DarylP

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Hi everyone,
I am a STOL fan, and have read most posts on this site and others on the design and function of my slow and able "mules of the air". I like the capability of being able to land in remote locations, and so I like many of you look at the bush plane for inspiration. My quest for knowledge brought me to a website that discussed the airframe strength of these type of aircraft. I wish I had bookmarked it, but alas I did not, otherwise I would provide the link.

The author of the site brought forth something that I had not thought of. That is to say that just because you put on big tires, a big engine, big prop it does not mean that the airframe can handle the rough landings. Of course it makes sense, as an aluminum frame will be stronger than a wood frame, much as a steel frame would be stronger than an aluminum frame. (Right?) Bouncing around on the ground over rocky ground exerts stresses on the airframe, which could be bad...to say the least. The manufacturer provides all sorts of specs, but what would you look for to assure that you are not overtaxing the design? I have seen big tires on ultralights, and watched them land them in some crazy areas. I see the G loads mentioned in the positive and negative, but I assume that is in flight. Bouncing a plane around on the ground, fast enough to take off, surely takes its toll on the airframe too...right? You guys always have great insight, so I thought I'd toss this out there.

So what makes an airframe suitable for STOL, and how do you know that you are pushing the boundaries...from an airframe point of view?

Thanks,
DarylP
 

WonderousMountain

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Well, there's a lot to consider here, even for an experianced engineer. First you have the tires, your air pressure at max-stress landing. Then there's your suspension, which can be of many different designs. Finish that off and there's still the exact attachment point.

Then once that's all settled you can get to the airframe. Generally speaking, wood can crack, fiberglass can bend, but to be strong it is made stiff, so it too has similar splintering, which is usually noticable. Aluminum and steel are very close performing metals, if they're te same weight. Usually steel will weigh more, probably because of standard sizes and the fact that those wanting lightweight commonly choose aluminum, where those wanting durability choose steel. Steel is very ductile, easy to weld, and considered good in a crash.

You know you're pushing the bondaries when you see flexing in members that shouldn't flex. Another thing to consider, load path. Your engine, wings, fuel, pilot/cargo/passenger, will all be supported in a hard landing. You'll want to focus on the spots when designing the airframe.

I should spend some time on this very subject, as I hope to be a bushman one day; and need a plane that can drop out of te sky and stick down, and come out of the mud without having to pull a flight of the pheonix;)


Wonderous Mountain
 

orion

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First of all, the comparison of wood, steel and aluminum is simply incorrect - if you properly design the airframe for the anticipated loads, it will not matter which material you select for the construction. Furthermore, any change you make to a standard airframe (engine, tires, prop, etc.) must be accompanied by proper engineering to make sure it can handle the loads you are going to be subjecting it to, either through service or through the modification. The changes you discuss are generally not made in a vacuum - they are done in order to enhance the performance but said performance and change are never free and other changes must accompany even the smallest modification in order to assure the functionality and safety of the design.
 

addaon

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Landing gear seems to be designed to 3 g or so (limit) as a general rule for small aircraft, from what I can tell.
 

TFF

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When designed right it does not matter the material, but when damaged, you will want something easy to repair. I would think steel tube the easiest to repair and when damaged have a good bit of strength still. Aluminum would be second; not bad to repair but requires some engineering; once the skin shape is gone it is not strong. Composites are easy to do but require more understanding to make a safe repair.
 

WonderousMountain

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Okay, I'm not going to try and decide what the best material is.....

Repairing is a matter of production capabilities. Repairing steel tube is fast and strong, but requires a welder to repair, or at least a hack saw and nuts and bolts. Fiberglass could be impossible in the wrong weather, so you may need to hobble that wonky ex-flying machine into a barn first and then do your best. Aluminum, has a slightly different nature than steel, but presumably if you built it, you know how to repair it.

Wood is actually the easiest to repair with the least setup time and cheapest tools to work with. Many of these could be carried inside the plane, and lumber/woodworking is almost everywhere in the world.

Also, I disagree about the when designed right, it doesn't matter theory. Quality materials are surprising close to each other in ability, but for a highly optimized structure, there is likely to be a victor.

For an ultralight, the most likely problem would arise from a heavy engine placed in tractor configuration, with improper bracing into the landing gear.

Let's look at failure modes: Airframe could fold either because of the nose load or in between front and back gear. The main landing gear could go strait through it's attachment point and cause a belly flop, or the gear could just, snap off, leaving you skidding across whatever is acting as a runway.


Never let the lack of good advice stop you from giving it.

Wonderous Mountain
 

TFF

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"Does not matter"was about the ability with heavy dose engineering, fabrication, and money you can make a plane out of anything. The old Honda F1 V6 was a cast iron block that weighed less than an aluminum one. I figured engineering the design would point to the best material, not picking an material and seeing if we can make it work, unless you have invented a new material.
 

Kristoffon

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as an aluminum frame will be stronger than a wood frame, much as a steel frame would be stronger than an aluminum frame.
Now try flying an RV into the Spruce Goose and tell me which is stronger. By mass, wood is actually stronger than aluminium by a tiny percentage.

what would you look for to assure that you are not overtaxing the design?
Shock absorption ability. A big fat tire is a good start. Also struts and springs with lots of travel to absorb it. And low stall speed. Of course you can't just take an ordinary design and strap a huge landing gear under it and call that a rough terrain aircraft because it won't be one.

Lastly, I think aluminium is a particularly poor material for the application because it suffers from fatigue. A steel frame would take the most continued punishment imho.
 

Dan Thomas

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All of the building materials have their drawbacks. A wood structure is glued together, and ungluing it means, usually, destroying it. If you were to crunch a lower longeron in a ply-skinned fuselage, or tear out landing gear mounts, you have a BIG job ahead of you to cut out the damage and scarf in new pieces. And that's after the fabric is removed. The repaired area will be 15 times larger than the original damage.

Steel tube is strong but can also be damaged, and welds do crack from time to time. You can't hacksaw it and bolt in a new chunk, either. That doesn't restore the necessary strength and rigidity. The damaged tubing must be cut out and replaced, or a new piece spliced in, and there are guidelines regarding what's safe and what's not. Fabric, again, must be removed over a large area (due to welding heat) and replaced afterward.

And steel also suffers fatigue. Wood, theoretically, does not.

Aluminum looks, to the uninitiated, easy to repair. It's not. Dents are seldom repairable. If they have sharp creases, the metal must be removed and replaced, and that can be a real headache on things like leading edges where the leading edge rivets were the first things installed in the spar, and everything else came afterward.

I've repaired all three types and none are easy. Repairing composites is a whole field of its own. Restoring a surface while restoring strength can be tough. It's easy to make it look good but have a weak repair.

On the FAA website there are Aviation Circulars. Look up AC43.13-1B and see what the procedures are for safe repairs to various structures. Very enlightening, and that publication (it's a big book in hard copy form) should be the homebuilder's bible.

Dan
 

orion

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By mass, wood is actually stronger than aluminium by a tiny percentage. Lastly, I think aluminium is a particularly poor material for the application because it suffers from fatigue. A steel frame would take the most continued punishment imho.
We've had this discussion here before so I wont go to far to comment however, the simple fact is that all materials are subject to fatigue issues due to load and/or geometry and while some materials behave such that they come across as more sensitive to the phenomenon than others, the choice of material for this type of application is still irrelevant since accounting for fatigue concerns must be a part of every design. As I said before, proper engineering will select the proper materials, material gauges and geometric details necessary to meet the load criteria at hand and to account for all service related issues.
 

wsimpso1

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By regulation, storebought airplane landing gear does its work in the 0-3g range.

If you want to suck up more vertical velocity, you gotta put in more travel in whatever energy absorbers you have. You also want the bird to have some ability to take a hard landing while protecting the people and carry some reasonable amount of g's in flight. Once you have designed in the gear travel to suck up the energy, the 19g occupant protection and the flight loads, the ground bumping around is really pretty minor.

And materials are not particularly at issue either. All of the mentioned materials make airplanes that are just fine. Pick what you feel comfortable with building, learn its limits and characteristics, and build. Or even design and build...

Billski
 

WonderousMountain

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So, it is true that you can't just stick great landing gear on the plane and call it STOL, but it's also true that you don't need to use any certain material to make it strong.

If you may damage the airframe, it's really important that you know how to fix it, supposing that you want to get out. Carrying welding, and advanced machinary for repairs may be difficult, especially for an ultralight.

I work in wood, but the easiest repair from my perspective would be fiberglass, because the large materials are only a bucket of resin, and some fabric. That said, if something happened to my bucket, I would be up a S*** creeek with no paddle.

This is probably worth it's own seperate discussion.

Wonderous Mountain
 

DarylP

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Hey thanks everyone,
I apologize if some of this stuff is elsewhere in the forum, and I truly do try and search for items of interest before posting.

I figured out the plane that the website was speaking of, a review of sorts. (I was searching for something else when I found it, but alas I still cannot find the site.) It was the Sherpa, (no...defiantly not an LSA) and the essence of the article was about the strength of the airframe with respect to rough off-field landings. Now that plane is so far from I would build that its not worth getting into specifics. However, the author was referring to the steel welded frame and its ability to handle loads like no other plane can. I have found this before on other sites where the thought process in building a bush type plane is to build a tank of a frame and then putting biggest engine you can. I suppose that if you are a professional bush pilot that would be the case, but I feel that intelligent design goes a long way too.

I appreciate the responses and agree that a well made plane with good landing gear, appropriate power and prop, could provide decent ability to land off field. I also see what your saying about the repairable nature of different materials. I sure do not want to go there again as repairing a damaged aircraft is a $#^&*%^@. I am sure that a wood structure that is well made, could withstand considerable stress. I appreciate the skill of the person that can build with wood, but I am not one of them. So I guess the ease of repair is highly dependent on what you are familiar with. My repair was in aluminum, and required replacing items whether they seemed to be bad or not. But I have another twist on my intent on this thread.

If you were to build and airframe, of the same design...exactly the same, (Hypothetical as the last time I checked you can't weld wood;)) and did a stress test on the design using the same landing gear and tires...suspension. Say you were to drag the frame, tires and landing gear, through a test field of ...well say rocks. With all things being equal, my thoughts would be that the steel fuselage would be stronger. Now yes...if and when they broke, the repairs would be different, but what would hold up better. Remember...all things being equal, so a common load of say 200 lbs, to simulate pilot weight.

What do ya say...you in?

DarylP
 

orion

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No, it would not be stronger - it would be as strong as the loads it was designer for, as would an airframe made out of any other material.
 

PTAirco

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No, it would not be stronger - it would be as strong as the loads it was designer for, as would an airframe made out of any other material.
I think what he is getting at is not the loads the designer thinks of when dealing with normal air and ground loads, but those kind of things that damage bushplanes from time to time; rocks thrown up by wheels, taxying over logs, getting a wingtip caught on a tree; that kind of thing.

Lets assume you throw a fist size rock at a steel tube/fabric frame, a plywood fuselage, a sheet metal one and a composite one- the sheet metal would fare the worst I imagine.

Fabric will bounce off a rock to a certain size and then simply give way - and easy patch. Steel tubes are fairly resistant to such damage, but bend fairly easily if you landed over a rock or log and whacked a longeron on it.

Glass fiber has a fairly tough surface and again things will bounce, sometimes without penetrating, but it could delaminate the skin form the core and you wouldn't see it. Kevlar has great impact resistance, but carbon fiber is lousy in that respect.

Plywood? Tough in thicker sections, but 1-1.5mm stuff is damaged pretty easily. (And coontrary to popular opinion, wood aircraft structures are not that easy to repair - damage spreads considerably and if you ever had to splice a new 3ft section of longeron into a wood fuselage or scarf-joint a new piece of leading edge onto a wing, you know sheet metal and tube is a lot easier to fix.)

The Cessna-type of metal structure does not take minor damage of this kind well; a rock that would bounce off fabric will leave a permanent dent.

For a bushplane my preference would still be steel tube and fabric, with the belly of the fuselage and other susceptible areas (tail surfaces) covered with kevlar panels, easily fixed or replaced. Same for leading edges - forget thin aluminum sheet - use a strong kevlar laminate or a tubular edge.
 

Dana

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For a bush plane, a critical aspect is the ability to do field repairs, possibly in very cold weather. For this, steel tube and fabric wins hands down. You can repair steel tube with a hacksaw, filem and a small oxyacetylene rig anywhere, and patch fabric with duct tape. If you're lucky, duct tape is the onlyrepair you'll need to fly out. Composites and glued wood require a warm work area, and aluminum repair requires more tools and often larger pieces than just a bundle of tubes, and any damage requires a proper structural repair.

-Dana

Help, I've fallen up and I can't get down
 

BBerson

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A steel tube and fabric airplane requires more manhours to build and sheet metal is the cheaper to make in a factory. But steel tube is preferred in bush operations like Alaska for the reasons stated here. Two seat bushplanes get minor landing damage more often than larger transport aircraft. Floatplanes are usually aluminum (Cessna 206, Beaver), less damage with floatplanes.
 

Dan Thomas

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I work in wood, but the easiest repair from my perspective would be fiberglass, because the large materials are only a bucket of resin, and some fabric. That said, if something happened to my bucket, I would be up a S*** creeek with no paddle.
Composites are all temperature-sensitive, some very much so. Repairs out in the open are usually really inconvenient or risky in terms of resultant strength. Cold will slow the cure to nearly nothing, and heat will make it cure far too quickly. Power tools are almost mandatory, too, for cutting out and scarfing the area to fix it. And serious structural damage isn't field-repairable at all. Taking the airplane apart and trucking it out is standard operating procedure for almost any significant damage. Sometimes it has to be helicoptered out. It's usually illegal to abandon it so it can get really expensive to haul it away. There's a big floatplane at the bottom of a really deep lake in a Provincial park in BC, my favorite spot, and the owners and their contracted salvage outfit haven't even been able to find it since it went under last summer. They MUST find it and remove it and I imagine it will cost gazillions.

Dan
 
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