Wooden aircraft and crash safety

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cluttonfred

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No, I don’t want to talk about Knute Rockne and the interwar Fokkers, just about safety of wooden airframes in light aircraft crashes.

I remember reading about wooden longerons being bound with cord around the cockpit to prevent splitting in a crash (and potential stabbing of the pilot) as early as WWI.

Personally, I am always drawn to wood since I like working with the material, and it gives me a sense of connection to all the early aviators with their wooden planes.

Does anyone have any data on whether wooden airframes are more or less dangerous than steel tube, aluminum tube, sheet aluminum, or composite airframes in a crash?

Cheers,

Matthew
 

henryk

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aluminum, or composite airframes in a crash?

-I was witness of DELFIN crash, when 3-wheel ultralight recumbant was strike at big tree,
when ranned at circa 100 km/h speed...

=ONLY small distorsion in upper part (made from styrofoam +carbon composite),
no "pilot" ingurance !
 

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TFF

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I think you have to narrow it down to type of airframe and it’s speed range and how you hit. A Piet probably has not much difference to a Baby Ace in a flip over in a field . A 200 mph plane bouncing off trees on a ballistic impact, wood isn’t going to be the best, but good chance nothing will in the end.

I would take a Falco over an RV if you are handing them out even if I thought the crash survivability is less. I would take an Emarude over a Stits. No offense to the Jungster guys on this forum, but I would take my Starduster over their Jungsters. That’s pure game show pick a winner; pure choice. Opportunity can change things. I want a Pitts. While not the same, my Starduster fell in my lap and is close enough for my abilities. I don’t feel bad.

Wood planes in my head are the Falco, Emarude, Flybaby, and Pietenpol. I know there is VPs, dainty French designs, Jungsters, CA-61/65s, GP-5s, but they really are not in my head. I would be thinking of Sopwith Camels and Albatross DVas before those. Just like any design, it’s a bag of pros and cons and you pick them and put them in order. The only thing that frustrates me about wood today is buying wood.
 

TarDevil

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Agree with TFF. If it's a controlled impact in a light/slow airframe I don't think you'd see much difference. Everything else is a crap shoot no matter the material.
 

Vigilant1

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It would be interesting to see as study or other days as Matt requests in the OP, but I doubt it exists (just too many factors to parse out to yield useful findings). I'd like to be wrong.
I wonder if NACA or other agencies ever produced an AGATE - like study comparing GA construction materials, or specifically wood.
I suspect we'll be left with highly accurate info on yield strengths of various materials and then be on our own to think of ways to use them to protect occupants (prevent crushing/intrusion of the places where people are, provide energy management to reduce deceleration loadings on occupants) .
FWIW, it seems to me that "wood aircraft" covers a lot of territory with a broad brush. Specifics will probably be important.
 

Wanttaja

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It's the kind of thing I originally hoped would come out of my homebuilt accident studies, but it hasn't been the case.

Ideally, you'd find two similar aircraft of opposing construction methods and compare their percentage of fatal accidents. First problem is FINDING two similar aircraft to compare.

For instance, I could take the Fly Baby (wooden construction) and compare it to another ~85 HP single-seat homebuilt made of aluminum. But those are rare on the ground; planes of the Fly Baby's category are overwhelmingly wood, or wood and steel tube. Could get do a comparison with a Baby Ace, which is wood AND steel tube. The trouble is that the Fly Baby is a low wing, and the Baby Ace is a high wing...which gives better pilot protection.

Yes, the Fly Baby has a ~41% fatality rate (41% of accidents kill the pilot) while the Baby Ace has a 11% rate. But how much of that is due to the Fly Baby's wing issues, vs. crash survivability? How much is due to the Baby Ace pilots being inside a structural cage, irrespective of whether that cage is wood or steel tube?

The Team series of aircraft (Mini-Max, High Max, etc.) are also all wood...and have a fatality rate about the same as the Baby Ace. Is that a reflection of construction material not being a major factor, or because the Team series is lighter and slower?

Beats the heck out of me.

The other factor to consider is the small sample size for most homebuilt aircraft types. When comparing Fly Babies to Team aircraft to Baby Aces, I'm looking between 10 to 20 accidents over a 23-year period. That's really not very much. When doing my analyses, I tend to want at least 50 accidents to ensure a good enough sample. I've been known to slip that down to 35, in some cases. I have a second spreadsheet that compares accident causes across about 30 specific homebuilt aircraft models. Only 19 of them exceed that 50-crash threshold.

My first thought, when I saw this thread, was to compare the Piel Emeraude with the RV-6. Similar configurations, similar engines and performance. The Piels have a ~45% fatality rate vs. the RV-6's 27%. But there are only 11 Emeraude accidents in my database, vs. 251 RV-6 accidents. Is the difference statistically significant?

Instead, let's compare the Emeraude to ANOTHER two-seat, all metal, low-wing homebuilt commonly flown with 150 HP engines: The Thorp T-18. The T-18 has a ~36% fatality rate...much closer to the Emeraude. And it almost meets my 50-accident threshold (47 accidents 1998-2020).

So: Are metal homebuilts safer than wooden ones? I don't think one can make that call.

Finally, consider one factor why spruce is the preferred aircraft-building wood: Its resistance to splitting and splintering. This is more related to workability rather than crashworthiness, but it may well be a factor.

Ron Wanttaja
 

Dan Thomas

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Wood planes in my head are the Falco, Emarude, Flybaby, and Pietenpol. I know there is VPs, dainty French designs, Jungsters, CA-61/65s, GP-5s, but they really are not in my head.
Jodels. Minicabs. There were a lot of wooden French airplanes.

The only thing that frustrates me about wood today is buying wood.

Yup. Note that the production Jodels went from wood to metal a long time ago. Suitable wood is now rare and expensive. Wood has to be carefully sealed, inspected regularly, and shielded from the elements. A bit of rot in a critical spot can mean an inflight structural failure, everybody's nightmare. Wood also means fabric cover in most cases, mandated in some countries to protect it. So add the weight and cost of the fabric.

Note that no manufacturer I know of is still building airplanes with wood as a structural material. I think the Bellanca Viking was about the last one, and that wing has ADs against it to force annual inspections for delamination and rot.

I like wood. Nice to work with. Makes a quiet airplane, too. But its cost and susceptibility to decay means that I wouldn't build another wooden airplane. Of course, at my age, it would rot more slowly than me and I would fall apart first:)
 

ToddK

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I am inclined to think is more about inertia then it is the material its made out of.

If choosing an airplane to fly to terrain without power, I think we would all be more interested in a light plane with a big fat wing, a slow power off stall speed, and high(ish) glide ratio, then an airplane made out of this or that.
 

ragflyer

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I have often wondered this too and am curious to hear actual field experience. But there is a dearth of actual data for sure and as Ron W. mentions compisons are difficult.

Given this, first principles can give some insights into this question:

1. All things such as impact angle, speed being equal, crash survival is about doing three things well:

First, absorbing as much of the energy by the structural elements as it fails and crumples- the crumple zone. The type of material and structural form will play a big role here. More on this in points 2 and 3..​
Second, maintaining the structural integrity and space within the pilot/passenger compartment so that the person is not crushed. This is about being strong enough and designing to high enough loads that the human body will fail ( due to high g force) before the compartment collapses. As such any suitable engineering material can be used here as long as it is designed for suitably high enough loads.​
Third, restraining the pilot/passenger within the compartment. This again is more about designing (seat belts and attach points etc.) for appropriate loads than structural materials as such.​

2. Energy can be absorbed by a structure in a number of ways. Plastic deformation ( as with steel) is what we most often think of. As the material crosses its yield stress it deforms plastically (bends) and absorbs a lot a of energy. Steel is great at this and this is why steel fuselages/roll cages are popular.

3. Wood on the other hand does not deform plastically appreciable but is still a very tough (actually an engineering measure) material pound for pound and absorbs a lot of energy before/as it fails unlike say a pane of glass. All the compression buckling and splintering will absorb a lot of energy. In many ways it is similar to a formula one carbon fiber structure that shatters and splinters spectacularly while the driver remains relatively safe in his compartment. The key is having a safe pilot/passenger compartment that maintains the integrity while other bits fly of and absorb energy.

4. One caveat though with wood is that its toughness is directly related to the grain slope and drops dramatically with spiral or diagonal grain- even more so than strength.

5. Early wooden fuselages that where braced with steel wire did fail and pose skewering risk but DeHaviland is purported to have solved that largely with plywood covered wooden fuselages.
 

ragflyer

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One other point re. my previous post. My sense is that in terms of importance I have it in inverse order with restraining the pilot being the most important followed by cockpit integrity, and finally energy absorption. The Human body can absorb a lot a G momentarily as long as it is not crushed and suitably restrained from flaying and causing blunt trauma.

In general typical steel fuselages have a lot more reserve strength than wooden once. Compare a 3/4" 0.035" steel longeron tube to a 3/4" square spruce piece and you will see a big difference in strength.
 

henryk

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2. Energy can be absorbed by a structure in a number of ways.

=the shorter the overload time, the greater can be for alive...=

=yellow="G"=500/ 5 V...

=Puls Bumper=



a= V^2/ 2 s = ???

V=sgr. [2gh

(h=10 m, s=0.3 m)

"The Human body can absorb a lot a G momentarily as long as it is not crushed and suitably restrained from flaying and causing blunt trauma. "
 

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Dan Thomas

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Steel tubing has been known to break and pierce the occupants. It's not especially better in a crash.

You know what's bad? Low-wing airplanes that can be difficult or impossible to get out of when they end up on their backs. Fuel leaking everywhere and you can't get the canopy open. There have been plenty of accidents where the occupants easily survived but died in the post-crash fire.
 

Pops

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My Falconar F-12 was all wood. With a 150 HP Lyc, they will cruise at 150--160 mph , according whether you have a climb or cruise prop. Smooth and quiet, felt more like you were flying a light twin that almost handled as well as a RV-4. I had a climb prop that cruised at 150 mph and 1700 fpm ROC at GW and at single place it had a 2200 fpm ROC.
At one time wood was the cheapest material to use in building. Not so now.
 

Tiger Tim

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It's the kind of thing I originally hoped would come out of my homebuilt accident studies, but it hasn't been the case.

Ideally, you'd find two similar aircraft of opposing construction methods and compare their percentage of fatal accidents. First problem is FINDING two similar aircraft to compare.
I don’t know what kind of accident data was kept in 1930s England but the DH 60 Moth was available in both wood and steel tube fuselages. Surely there are similar accidents that happened to both construction types that can be compared knowing that all else is pretty well equal.
 

Dan Thomas

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I don’t know what kind of accident data was kept in 1930s England but the DH 60 Moth was available in both wood and steel tube fuselages. Surely there are similar accidents that happened to both construction types that can be compared knowing that all else is pretty well equal.
The Tiger Moth was a steel-tube fuselage. The French Stampe SV4 was a very similar airplane, but, if I recall correctly, was made of wood. The numbers built were small, though, making comparisons difficult.
 

Wanttaja

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I don’t know what kind of accident data was kept in 1930s England but the DH 60 Moth was available in both wood and steel tube fuselages. Surely there are similar accidents that happened to both construction types that can be compared knowing that all else is pretty well equal.
That's a reasonable thought, but the question is data availability. Where in the world would one find British accident reports from the 1930s? For that matter, was there formal tracking of non-commercial aircraft accidents in Britain back then?

Heck, the NTSB downloadable accident database has only seven accidents prior to 1982.....

Ron Wanttaja
 

Old Koreelah

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…5. Early wooden fuselages that where braced with steel wire did fail and pose skewering risk but DeHaviland is purported to have solved that largely with plywood covered wooden fuselages.

I lined Jodel’s cockpit with Hoop Pine plywood, partly for added strength, but mainly to reduce the chance of being impaled by splinters.
Another advantage: I can rivet or screw fittings and equipment to this inner layer without going thru the skin.
 
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