Wooden aircraft and crash safety

[One Sky Dog]

While it is true that canard aircraft do not stall/spin there are still ways to screw it up. While in high AOA flight the drag curve goes exponential. Even with full power you cannot climb at low airspeed. The save would have been hold in ground effect, build airspeed and go around. Instead it appears he just kept adding drag and sunk into the ground.

 

[speedracer]

While it is true that canard aircraft do not stall/spin there are still ways to screw it up. While in high AOA flight the drag curve goes exponential. Even with full power you cannot climb at low airspeed. The save would have been hold in ground effect, build airspeed and go around. Instead it appears he just kept adding drag and sunk into the ground.

Sky Dog, you've obviously never flown an EZ and stalled it. With my 215 HP Long EZ holding the stick to the aft stop at full power it climbs at 2,000 FPM with the canard happily bobbing up and down 4", 4 times a second. And BTW, the canard bob speed is the same (around 65 MPH) at idle or full power.

 

[One Sky Dog]

True I have never flown an EZ . So what is your analysis if he could have climbed away at 2000 ft/ min? If certainly looked like drag greater than lift as he flopped it on. My opinion.

 

[cluttonfred]

One Sky Dog, what you described is exactly true for something like a Mignet Pou-du-Ciel. In that case, it can be a benefit in that it provides a way steepen the glide path on demand in a plane that cannot slip or spin. It does mean the pilot must learn *not* to land with full back stick and instead push forward to a less extreme angle to avoid a hard landing. That inability to control the glide path is something that David Thurston highlights as a pitfall of two-axis designs like the Ercoupe in his book Design for Safety. I suspect that the EZ is set up so that it never goes down the drag bucket, but that's why it needs the belly air brake and, IIRC, the ability to splay the rudders out as well for additional air brake function.

 

[TFF]

Back to the OPs problem is one can’t simply slap some special glass instead of buying plywood. One is stuck making a composite sandwich to take the place of the ply.

Now you have to figure out how strong that ply is in all the directions and then figure it out in the composite. Where it always runs off the road is some direction will not be strong enough. Because of that, the substitute material ends up being beefed up for that one problem and overkill for the others.

For me I’m not smart enough to figure out the actual numbers. Demonstrations of materials is probably the worst way to decide what you need. They are all biased to the best of that material. They don’t tell the whole story. If you can’t source the materials it’s probably the wrong plane. If you can’t do the structural analysis to figure out the problems, you shouldn’t change the design.

In the long run it will still be cheaper to buy the overpriced materials than spend all the time figuring it out, much less the sooner you could start building. A better thread would have been, how would one make a composite VP like plane? Easy to fly, cheap on materials, cheap to power. Then materials could be put to use correctly. New MT prop is not a good crutch to say stuff it too expensive.

 

[BJC]

With my 215 HP Long EZ holding the stick to the aft stop at full power it climbs at 2,000 FPM with the canard happily bobbing up and down 4",

How does a LE do with the stick held on the aft stop with a stock O-235?


BJC

 

[wren460]

Back to the OPs problem is one can’t simply slap some special glass instead of buying plywood. One is stuck making a composite sandwich to take the place of the ply.

Now you have to figure out how strong that ply is in all the directions and then figure it out in the composite. Where it always runs off the road is some direction will not be strong enough. Because of that, the substitute material ends up being beefed up for that one problem and overkill for the others.

For me I’m not smart enough to figure out the actual numbers. Demonstrations of materials is probably the worst way to decide what you need. They are all biased to the best of that material. They don’t tell the whole story. If you can’t source the materials it’s probably the wrong plane. If you can’t do the structural analysis to figure out the problems, you shouldn’t change the design.

In the long run it will still be cheaper to buy the overpriced materials than spend all the time figuring it out, much less the sooner you could start building. A better thread would have been, how would one make a composite VP like plane? Easy to fly, cheap on materials, cheap to power. Then materials could be put to use correctly. New MT prop is not a good crutch to say stuff it too expensive.

What is your issue with the MT prop? My point was the composition of it's construction and not the cost of it and not whether I can afford it though certainly my cheap ass nature is the reason I can afford it. Without a doubt the current times with the economy and supply chain issues, there will continue to be pressure put on alternate aircraft construction methods and materials. For some people I suppose the decision will be made to no longer consider building wooden aircraft designed around Sitka Spruce and aircraft grade plywood. Gee too bad. I thought we were talking about Home Built "Experimental" aircraft...

New engineering is theoretical until proven by testing. I thank the Wright Bros. (non engineers) for all their experimentation which helped to spark powered aviation in the first place. For sure any new construction methods will require some testing and perhaps some engineering too but, we don't have to do that, we can forget all about the Experimental part and all move over to the ENGINEERED AND CERTIFIED AIRCRAFT forum and talk about that recent Cessna cracked wing-strut-mount AD or perhaps the Cessna 177 and 210 cracked wing spar carry-through AD.

 

[Dan Thomas]

What is your issue with the MT prop? My point was the composition of it's construction and not the cost of it and not whether I can afford it though certainly my cheap ass nature is the reason I can afford it. Without a doubt the current times with the economy and supply chain issues, there will continue to be pressure put on alternate aircraft construction methods and materials. For some people I suppose the decision will be made to no longer consider building wooden aircraft designed around Sitka Spruce and aircraft grade plywood. Gee too bad. I thought we were talking about Home Built "Experimental" aircraft...

New engineering is theoretical until proven by testing. I thank the Wright Bros. (non engineers) for all their experimentation which helped to spark powered aviation in the first place. For sure any new construction methods will require some testing and perhaps some engineering too but, we don't have to do that, we can forget all about the Experimental part and all move over to the ENGINEERED AND CERTIFIED AIRCRAFT forum and talk about that recent Cessna cracked wing-strut-mount AD or perhaps the Cessna 177 and 210 cracked wing spar carry-through AD.

No need to get triggered. Some of us have been around this stuff a long time and have learned some things, and/or have seen others learn things, the hard way. They crashed. Some died. Many times it was because they assumed too much, or because wishful thinking got in the way of sober assessment, or because they cheaped out on materials or engineering. Some crashed because they wouldn't listen to friendly warnings. We can't resist warning new folks about some of the pitfalls. It would be irresponsible to stay silent.

As an aircraft mechanic and commercial pilot, I can say that there are a lot of things that can be learned from the defects of certified airplanes. I have seen cracked wing struts. I spent over 40 hours grinding and polishing corrosion pits out of a Cardinal's carrythrough spar, and recording the thicknesses noted by the NDT tech with the ultrasonic thickness measuring equipment. Hundreds of measurements. I did some work on a Cessna 182 with the SMA diesel in it, with its MT composite three-blade propeller, and I found a cracked prop blade. They're not perfect, you know. The vibration from that engine, at idle, is awesome, and that propeller didn't like it. Vibration that is a result of four huge cylinders making 230 HP instead of the usual six, doing it at a low redline, with a compression ratio of 15:1.

I restored and flew, for 20 years, a homebuilt wooden airplane. If I was to build another, it wouldn't be wood. Good wood has become expensive and scarce, and a wooden airplane should be hangared, which adds enormously to its cost. The last five years I had mine it sat outside, and it suffered. Didn't have a hangar anymore. And those five years were in a semi-desert area. In a rainy area, rot would have set in rapidly. When I restored it I had to deal with a bunch of rot that had only taken a few years to accumulate. There are some very good reasons why some builders choose aluminum or composites or even tube and rag.

The Wright Brothers were bicycle mechanics. They were working on the most common personal transportation machines of the day. Most everyone else was riding horses or horse-drawn wagons or carriages. So those boys were not ignorant of mechanics and physics. They built their own wind tunnel. They designed their own propellers that had efficiencies of 83%, a figure that is still respectable. They had their own machinist, who built their engine. They worked hard at educating themselves in a lot of areas BEFORE they flew that airplane. They had to; there were no textbooks on the subjects they were dealing with.

So yes, you can thank them for that effort and the risks they accepted. At the same time, nobody can rightly imply that they were equal to current, uninitiated homebuilders. They were not.

But you don't have to accept risks; you have access to 119 years of aviation experience to learn from, and that experience includes some spectacular failures that were completely avoidable. And some of those failures were in the certified world.

 

[TFF]

Nothing actual about an MT prop. Very nice hardware. It was about money. Bad luck with one in Phoenix last week that will generate an AD for some people. That’s just a part of flying.

As for the theoretical engineering. Some is theoretical like sodium cooled nuclear plants and some are more like not optimized, like substitute a piece of Doug fir for spruce. Why complicate a VP so much that it’s a nuclear power plant.

 

[wren460]

No need to get triggered. Some of us have been around this stuff a long time and have learned some things, and/or have seen others learn things, the hard way. They crashed. Some died. Many times it was because they assumed too much, or because wishful thinking got in the way of sober assessment, or because they cheaped out on materials or engineering. Some crashed because they wouldn't listen to friendly warnings. We can't resist warning new folks about some of the pitfalls. It would be irresponsible to stay silent.

As an aircraft mechanic and commercial pilot, I can say that there are a lot of things that can be learned from the defects of certified airplanes. I have seen cracked wing struts. I spent over 40 hours grinding and polishing corrosion pits out of a Cardinal's carrythrough spar, and recording the thicknesses noted by the NDT tech with the ultrasonic thickness measuring equipment. Hundreds of measurements.

Not triggered, but now I'm taking my toys home.........

 

[wren460]

Nothing actual about an MT prop. Very nice hardware. It was about money. Bad luck with one in Phoenix last week that will generate an AD for some people. That’s just a part of flying.

As for the theoretical engineering. Some is theoretical like sodium cooled nuclear plants and some are more like not optimized, like substitute a piece of Doug fir for spruce. Why complicate a VP so much that it’s a nuclear power plant.

I don't understand the "Money" comment. If I can afford 16K for a new composite prop, why am I whining about the high cost of aircraft grade (un-certified these days) Sitka Spruce? I happen to not revere throwing money away if there is an alternative path.

 

[wren460]

I figured I would get resistant comments. That's what a forum should be about, constructive criticism. I'm not asking anyone to agree. Even though I claim to be a cheap ass, I'm really just advocating for reasonable cost as compared to the unreasonable prices of so called certified Sitka spruce (which isn't even certified anymore) and the trucking expenses associated thereto. Are there species alternatives? Yes, actual ones identified by NACA. Everyone is fixated on the notion that all wooden aircraft must be made from Sitka Spruce..... Even Mr. Evans who designed the VP aircraft recognized this as evident by the several species he calls out in his plans. Even the Spruce Goose is not predominately made from spruce. I am very careful to not recommend anything specific. I'm just trying to spark some out of the box thinking.

 

[challenger_II]

The Army Air Force had a document that listed alternative materials to use in place of spruce. Listed densities, and other pertinent information.

 

[Ava]

 

[Dan Thomas]

Sitka is the ideal stuff. It's light and it's strong and it doesn't splinter easily and it takes glue well. Its chief drawback is its cost, and that's due to its scarcity, with so many of the big trees that the best wood comes from having been cut already, and the remaining trees in protected parks.

Sure, there are alternatives. Some of them are far away. Some of them are available here, but Douglas fir is seeing the same problem as spruce: big trees gone, smaller trees presenting knots and other defects. And it's heavier, and weight is always the enemy. That's one factor that newbies often don't take seriously, and their airplanes can end up as real dogs. There have been homebuilts that came out with empty weights that were close to their design gross weights. They are little more than lawn ornaments that cost years of work and a lot of dollars.

Certified wood? Who certifies it? Some guy that knows what to look for, that's all. FAA's AC3.13-1B has everything one needs to know about good wood. It doesn't come from some certified tree or certified logger, though if they plan to sell it as high-quality wood they'll be careful to avoid falling it across other logs and bending or bruising it. Airplanes aren't the only things made from it; violins and their bigger brothers, wooden ladders, some other stuff. You are competing with those people when you buy it. If you use an alternative wood, you have to apply similar criteria to it. You, the builder, essentially certify it.

I grew up in lumber country. My dad and grandfather and several uncles were in that industry. Grandpa was a lumber grader. I used to stand next to him while he inspected the lumber moving along the green chain, stamping each piece with a grade stamp. Used a crayon on some: C for clear (nice wood, some of it would have made airplanes) and X (cull, throwaway stuff, sold real cheap). X was truly bad wood. The #1, 2, and 3 grades were really nice. Went into house construction.

Now I go to Home Depot or any other lumberyard and look, sadly, at what they offer. A lot of it would have been marked "X" by Grandpa, and it's stamped #2. None of it is clear. Rounded on two corners where the bark used to be. Twisted or curved or split. Knots falling out of it, big pitch pockets. Clear stuff is in the fancy-wood section, and costs about 500 times as much as it did when I was a kid. None of it is any good for aircraft, as its grain is flat, not edge-grain or within 45° of edge.

Which is one reason I would choose aluminum or tube-and-rag for a new airplane.

 

[wren460]

The Army Air Force had a document that listed alternative materials to use in place of spruce. Listed densities, and other pertinent information.

https://ntrs.nasa.gov/api/citations/19930091423/downloads/19930091423.pdf

 

[wren460]

Sitka is the ideal stuff. It's light and it's strong and it doesn't splinter easily and it takes glue well. Its chief drawback is its cost, and that's due to its scarcity, with so many of the big trees that the best wood comes from having been cut already, and the remaining trees in protected parks.

Sure, there are alternatives. Some of them are far away. Some of them are available here, but Douglas fir is seeing the same problem as spruce: big trees gone, smaller trees presenting knots and other defects. And it's heavier, and weight is always the enemy. That's one factor that newbies often don't take seriously, and their airplanes can end up as real dogs. There have been homebuilts that came out with empty weights that were close to their design gross weights. They are little more than lawn ornaments that cost years of work and a lot of dollars.

Certified wood? Who certifies it? Some guy that knows what to look for, that's all. FAA's AC3.13-1B has everything one needs to know about good wood. It doesn't come from some certified tree or certified logger, though if they plan to sell it as high-quality wood they'll be careful to avoid falling it across other logs and bending or bruising it. Airplanes aren't the only things made from it; violins and their bigger brothers, wooden ladders, some other stuff. You are competing with those people when you buy it. If you use an alternative wood, you have to apply similar criteria to it. You, the builder, essentially certify it.

I grew up in lumber country. My dad and grandfather and several uncles were in that industry. Grandpa was a lumber grader. I used to stand next to him while he inspected the lumber moving along the green chain, stamping each piece with a grade stamp. Used a crayon on some: C for clear (nice wood, some of it would have made airplanes) and X (cull, throwaway stuff, sold real cheap). X was truly bad wood. The #1, 2, and 3 grades were really nice. Went into house construction.

Now I go to Home Depot or any other lumberyard and look, sadly, at what they offer. A lot of it would have been marked "X" by Grandpa, and it's stamped #2. None of it is clear. Rounded on two corners where the bark used to be. Twisted or curved or split. Knots falling out of it, big pitch pockets. Clear stuff is in the fancy-wood section, and costs about 500 times as much as it did when I was a kid. None of it is any good for aircraft, as its grain is flat, not edge-grain or within 45° of edge.

Which is one reason I would choose aluminum or tube-and-rag for a new airplane.

Blanks intended for longerons and wing spars used to be aircraft "certified" Nobody wants to stick their neck out anymore.

Yes, most lumber found in lumber yards is fast growth intended for general framing. I think I mentioned in an earlier post that Furring strips made from close grained Douglass fir can sometimes be found. I have pulled many such gems from the pile at Home Depot. These close grained sticks are the sort that arrow makers now cherish for the finest arrows. Only problem is they come in 8 foot lengths so are not ideal for wing spars and longerons unless one trusts their own splices.

Yes DF is heavier than Sitka spruce. It is also stronger which means you can use a smaller cross section which is basic engineering principle with data available. It is interesting to note that GENERALLY speaking lumber strength correlates to it's density. Even more interesting is the fact that Balsa wood is one of the lowest density woods available yet pound for pound is also one of the strongest.

I will mention that if you have a local wood truss manufacturer or a really good lumber yard you can get Select Structural lumber which is much more devoid or knots and defects.

 

[David L. Downey]

Sitka is the ideal stuff. It's light and it's strong and it doesn't splinter easily and it takes glue well. Its chief drawback is its cost, and that's due to its scarcity, with so many of the big trees that the best wood comes from having been cut already, and the remaining trees in protected parks.

Sure, there are alternatives. Some of them are far away. Some of them are available here, but Douglas fir is seeing the same problem as spruce: big trees gone, smaller trees presenting knots and other defects. And it's heavier, and weight is always the enemy. That's one factor that newbies often don't take seriously, and their airplanes can end up as real dogs. There have been homebuilts that came out with empty weights that were close to their design gross weights. They are little more than lawn ornaments that cost years of work and a lot of dollars.

Certified wood? Who certifies it? Some guy that knows what to look for, that's all. FAA's AC3.13-1B has everything one needs to know about good wood. It doesn't come from some certified tree or certified logger, though if they plan to sell it as high-quality wood they'll be careful to avoid falling it across other logs and bending or bruising it. Airplanes aren't the only things made from it; violins and their bigger brothers, wooden ladders, some other stuff. You are competing with those people when you buy it. If you use an alternative wood, you have to apply similar criteria to it. You, the builder, essentially certify it.

I grew up in lumber country. My dad and grandfather and several uncles were in that industry. Grandpa was a lumber grader. I used to stand next to him while he inspected the lumber moving along the green chain, stamping each piece with a grade stamp. Used a crayon on some: C for clear (nice wood, some of it would have made airplanes) and X (cull, throwaway stuff, sold real cheap). X was truly bad wood. The #1, 2, and 3 grades were really nice. Went into house construction.

Now I go to Home Depot or any other lumberyard and look, sadly, at what they offer. A lot of it would have been marked "X" by Grandpa, and it's stamped #2. None of it is clear. Rounded on two corners where the bark used to be. Twisted or curved or split. Knots falling out of it, big pitch pockets. Clear stuff is in the fancy-wood section, and costs about 500 times as much as it did when I was a kid. None of it is any good for aircraft, as its grain is flat, not edge-grain or within 45° of edge.

Which is one reason I would choose aluminum or tube-and-rag for a new airplane.

If you are willing to make many trips to the local "lumber yard" you can still find nice wood that meets the criteria to fly. Many times though, most of it ends up as sawdust while you reorient the grain to suitable orientation. Never want to use aged surfaces anyhow though...

 

[David L. Downey]

Blanks intended for longerons and wing spars used to be aircraft "certified" Nobody wants to stick their neck out anymore.

Yes, most lumber found in lumber yards is fast growth intended for general framing. I think I mentioned in an earlier post that Furring strips made from close grained Douglass fir can sometimes be found. I have pulled many such gems from the pile at Home Depot. These close grained sticks are the sort that arrow makers now cherish for the finest arrows. Only problem is they come in 8 foot lengths so are not ideal for wing spars and longerons unless one trusts their own splices.

Yes DF is heavier than Sitka spruce. It is also stronger which means you can use a smaller cross section which is basic engineering principle with data available. It is interesting to note that GENERALLY speaking lumber strength correlates to it's density. Even more interesting is the fact that Balsa wood is one of the lowest density woods available yet pound for pound is also one of the strongest.

I will mention that if you have a local wood truss manufacturer or a really good lumber yard you can get Select Structural lumber which is much more devoid or knots and defects.

Of course some of that optimization may not be possible if the design is compression crippling or buckling critical...that's where TLAR can get one in trouble...

 

[Ava]

Sitka is the ideal stuff. It's light and it's strong and it doesn't splinter easily and it takes glue well. Its chief drawback is its cost... there are alternatives... but Douglas fir is seeing the same problem as spruce... (a)nd it's heavier, and weight is always the enemy. That's one factor that newbies often don't take seriously, and their airplanes can end up as real dogs...

Weight was carefully accounted for when Andy and Geoff built Ole Piet to Bernard Pietenpol's original plans. However, at just an ounce or three under 670 pounds-- with water, engine oil, and unusable fuel-- it's 45 pounds heavier than BHP's printed specifications. Heavier wheels and tires account for a little bit of this; we have three more instruments; but like the propeller, new gauges are lighter than those in the original plan. On paper, the covering is lighter, but as applied it may have more coats-- as well as metal powders-- so it could actually be slightly heavier.

Individually, the empennage and wings-- and aluminum is lighter than Terne metal-- are just a little heavier than the original specifications. But the fuselage-- which has a lot of marine plywood instead of specified but unobtainable Haskelite; and like the wings and tail feathers are built up from fir instead of spruce, account for the largest weight increase, which is a 7% empty weight increase for the entire airframe.

On the other hand, we run a counterweighted, slightly higher compression engine-- that is only a pound or so heavier with its propeller and accessories-- than BHP's specifications. It makes more HP, torque, and thrust swinging a slightly longer blade in the same rpm range.