Wooden aircraft and crash safety

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wren460

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You guys have just made the argument for a composite VP-1.


BJC
Exactly! Well sort of. It's entirely possible to build a pure synthetic composite VP-1 or VP-2 but probably it makes more sense to build an already proven all composite design? For me the VP attraction is the slow flying nature and simplicity of construction and most especially I like the really ugly look (Ha!) and I simply like wood materials better than I like aluminum or steel tube structures. My gravitation toward mixing in some synthetic composite materials has much to do with the insane prices of aircraft quality spruce and plywood not to mention the cost of shipping materials long enough to make longerons or wing spars in one piece and large sheets of plywood are not so easy to ship too. Therefore, I'm thinking more in terms of making use of alternate types of locally available wood and reinforce them with synthetic composite materials. Notice how I keep saying "synthetic composites"? That's because all wooden aircraft are already built from composites, just natures version. I just put a new MT propeller on my 182 and it's built with a wooden core wrapped in carbon fiber. MT calls it "MT-natural" composite. Man does this propeller perform better than the old aluminum one.

Anyhow, the wood species I have available locally and which I would consider for aircraft use based on NACA wood report 1941 are Douglas fir and Hem Fir. However, most of the available Douglass fir found in long lengths is of fast growth variety and full of knots. Sometimes available are 3/4"x 1-1/2" furring strips made from really close grained Douglass Fir but only in 8' lengths which means splicing material for long lengths which is not so good for wing spars and makes me a tad nervous for longerons. The available Hemlock fir is fairly close grained and rather consistent in grains structure and can be found fairly clear of knots but not always in long lengths and is somewhat low in density but Hem fir is pleasant to work with having almost no splintering (unlike Douglas Fir) and is very low in resins or oils making for low odor and good bonding of joints. The NACA report 1941 says Hem Fir's strength is similar to Sitka Spruce.

Suppose I frame up the basic fuselage structure (minus the plywood skins) using Douglass fir in the two main bulkheads as called for on the plans but I use Hem fir for the cross members and the longerons. I'm good with the main bulkheads made from Douglass Fir and with the short Hem Fir cross members but I'm not so comfortable with spliced longerons? What to do about it? I grab my router and cut a full length groove into the side of the longerons, perhaps 1/4" wide and maybe 3/8" or 1/2" deep. Now into that groove I laminate carbon fiber tow until said groove is plum full. I end up with a pretty good cross section of unidirectional reinforcement for not too much money and not too much work. Carbon fiber tow is perhaps the most economical and insanely strong form of composite reinforcement available yet not very seldom considered due to it being just a loose yarn. Want to take this idea a little bit farther? If we cut some cross grooves into the surface of the firewall and tail bulkhead which line up with the grooves in the longerons, now our carbon tow can tie the longerons into each end of the fuselage and wrap around in one continuous piece.

So many possibilites.
 

cluttonfred

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While quite a few VP 1/2s have been completed, I don’t think most rack up the flying time. Lack of rollover events can be lack of flying. Most build them for the low cost. Flying out in the open that exposed has probably scared more out of the cockpit than enticed. Yes some will gravitate to it, but not many. Just looking at these things come up for sale, almost none are passing pilot to pilot. Most seem to be estate sales of sorts to people trying to capitalize on it being an airplane they bought cheap, Ha, Ha. Not understanding it’s a cheap airplane. A hoop behind the head ruins the lines. I would not fly one without one, and I would never expect to crash.

I am not so sure about that. The VPs and other low and slow designs that make poor cross-country machines get less attention all around because so few show up at the big fly-ins. That doesn’t mean that they don’t get flown.
 

phil'flaugher

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My understanding (at least in boatbuilding) is that wood should not be sandwiched in fibreglass, as it may encourage rot should there be any water ingress. Is it different for airplanes?
Corvette wrapped balsa with sheet metal for stronger, quieter floorboards....
 

Vigilant1

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Therefore, I'm thinking more in terms of making use of alternate types of locally available wood and reinforce them with synthetic composite materials.
If a wood longeron (or spar, or other part) is wrapped in a composite layup that is stiffer than the wood and has a lower strain rate than the wood, then the wood itself may be taking very little of the load, especially since the synthetic composite is at the outer surface (where both tensile and compressive bending loads are concentrated).
 

TFF

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I don’t think the original revolution of homebuilts got flown all that much. So many adds for sale of a hundred hours on a thirty year old plane. Many a owner/ builder were surprised how high performance their small plane was. Too much effort put into it to sell; not enough practice to not be scared of it. Sold only at the builders death. Sadly these planes are running out now. Most get bought to pirate the engine and airframe thrown away. It’s where the comment of even if you are building, keep flying something and not get out of practice. Don’t just wait until your own plane is done to save money. Especially the smaller planes of the first generation. VPs, T2s, small biplanes.

Generally the original homebuilt formula was pick an engine and put it in half the airframe it originally came in. Even a Flybaby is a high performance cut down Cub, if all you have flown is Cubs. A Tcraft is what the first Pitts was scavenged from, fuselage, tail, engine, cowl. Cut out the bent parts and add some small wings. It freaked people out in 1945 when it first flew.

Today we have a better handle on training and options for training. A friend bought a 1963 Stits Playboy it only had 200 hours on it, another friend restored a Flybaby for someone, 40 hours; it too was an early plane. My plane had 98 hours before it was flipped and sat 20 years before I got it to repair.

I personally don’t think the VP needs anything to fly that’s different. It’s a normal airplane. My practicality and my neck don’t want to be the guy that hits a gopher hole and wonder why everything is upside down. Does it need a F1 race tub, no. That’s getting into silly world.
 
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Wanttaja

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At a glance, it appears that there is a correlation between fatality rate and speed.
You must be new here. :)
fatality_plot.JPG
In other words, MV^2 is a *****. Certainly most accidents don't occur when the aircraft is flying at its cruise speed, but it's an indication on how "slippery" a plane is...how much it might accelerate if some factor causes loss of control, or how less-forgiving it might be.
is there a breakdown of in which stage of flight the accidents happened?

Nearly 100% of the fatalities occurred when the aircraft hit the ground. The pilot losing control is a major factor in fatal accidents; when this happens at low altitude, it reduces the opportunity to recover from the situation. The classic loss-of-control situation is a stall/spin, and the majority of those occur during initial climb.

Ron Wanttaja
 

Wanttaja

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I am not so sure about that. The VPs and other low and slow designs that make poor cross-country machines get less attention all around because so few show up at the big fly-ins. That doesn’t mean that they don’t get flown.
As part of my accident statistics work, I perform a rough (ROUGH) estimation of the average annual hours flown by homebuilt aircraft involved in accidents. I compute the age of the aircraft, assuming each made its first flight on January 1st of the year of completion, and divide the aircraft total time by the computed age.

Yes, stupid metric. But I feel comparing this average annual utilization to that of all homebuilts gives us an indicator of the relative utilization of particular homebuilt types, and the errors caused by assuming a 1 January first-flight date will cancel out. Oddly though, too, my estimation of the average annual flight time of the whole homebuilt fleet is within a couple of hours of the FAA's estimate based on the annual survey.

This table shows a summary, for homebuilts that had 50 or more accidents in the 1998-2020 period.
Total AccidentsPercentage of Overall
Rand KR-25048%
Midget Mustang 25052%
Sonex5655%
Challenger6474%
Kitfox22381%
Zenair CH-7017783%
Rans S-125085%
Avid10187%
Rans14488%
Zenair22392%
Long-EZ5195%
Zenair CH-6018398%
All Homebuilts4560100%
Vans RV-4108100%
Searey63100%
Velocity63101%
Glasair104102%
Glastar65130%
Lancair 2-Seat114133%
Lancair201134%
Lancair IV69136%
Vans (all)634143%
Vans RV-770150%
Vans RV-6251150%
Vans RV-8107167%

You'll see there are no estimates for single-seat aircraft like the Volksplane or Fly Baby. This is because of the 50-accident threshold; both those types have around 17 accidents. Their average annual flight time computes to about 33% of the overall fleet.

Ron "Numbers, I got numbers" Wanttaja
 

cluttonfred

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Sorry, Ron, I don't quite follow. "Their average annual flight time computes to about 33% of the overall fleet." So the average homebuilt flies three times more per year than the average VP-1 or Fly Baby? Or single-seater flight time adds up to about 1/3 of the total flight time of the homebuilt fleet?
 

Wanttaja

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Sorry, Ron, I don't quite follow. "Their average annual flight time computes to about 33% of the overall fleet." So the average homebuilt flies three times more per year than the average VP-1 or Fly Baby? Or single-seater flight time adds up to about 1/3 of the total flight time of the homebuilt fleet?
33% of the average annual flight time for the homebuilt fleet. So yes, the average homebuilt flies three times more per year than the average VP-1 or Fly Baby. Using my rough approach, of course.

Ron Wanttaja
 

BJC

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Too much effort put into it to sell; not enough practice to not be scared of it. Sold only at the builders death. Sadly these planes are running out now.
I've seen some good deals in this category for someone wanting an airplane. Stitts Playboys, for example, have been offered at really good prices.
Nearly 100% of the fatalities occurred when the aircraft hit the ground. The pilot losing control is a major factor in fatal accidents; when this happens at low altitude, it reduces the opportunity to recover from the situation. The classic loss-of-control situation is a stall/spin, and the majority of those occur during initial climb.
Yes, and that issue - LOC at low altitude - is the result of inadequate piloting skills.


BJC
 

Wanttaja

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That sounds like a challenge. I bet you are personally improving the average for Fly Babies. ;-)
I try, but still nowhere near the average for the homebuilt fleet. As you said earlier, single-seaters aren't flown places, much. The pandemic killed most of the small fly-ins around, so I come up with fewer opportunities. I do try to raise the mean, but it's tough. I shoot a lot of video, now....

Reminds me of a story an Air Force buddy told me. He was in one of the last classes at the Air Force Academy that was all men. The staff was *trying* to reform the near-simian ways of young men in an all-male establishment in the '70s, but not having much luck.

So they brought in what was called at the time a "women's libber." to speak to the cadets at a group assembly. She didn't get a very good reception. She got mad at one point, and said, "It's a well-known fact that men your age [self-pleasure] themselves five times a week!"

One cadet shot to his feet. "All right...who's been holding down the mean?"

The man marched punishment tours until the day he graduated. But he DID graduate.....

Ron "Trying not to be mean" Wanttaja
 

tls258

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Some design thoughts:
The most significant factor in crash fatalities is the amount of energy the crash transfers to the human. The upper limit is about 30Gs. 10Gs are doable depending upon the occupant's position and movement during the crash. For design purposes, I think we really want to look at design strength from the perspective of making it as strong as necessary but not more. Rigidity is necessary to some extent to carry loads. Rigidity does not help an occupant during a crash when impact loads are nearly instantaneous. What you want in a crash is a structure that collapses and absorbs the impact energy, keeping that energy away from the human. Wrapping wood in glass will increase its rigidity and strength. That may reduce its ability to absorb crash energy before it gets to the human unless its component size is correspondingly reduced and retains the ability to absorb energy. Wood tends to bend substantially before it snaps. That bending absorbs energy similar to a bow and arrow. Metal will bend and absorb energy until it reaches the deformation limit of the structure when it will collapse. Does any of this make a difference in a crash? Depends. The analysis is complex: https://web.mit.edu/civenv/wtc/PDFfiles/Chapter IV Aircraft Impact.pdf

Basically, survivability depends on the crash energy that must be dissipated. A few years ago, a couple of guys were killed when they stalled their Breezy, the open air, aircraft- a very high drag aircraft. Everything is hanging out there in the breeze. But they died in the crash. Why? They were in a stall and fall. The dirty design did not really affect their impact energy because they were stalled. The design did not keep enough of the crash energy away from them to avoid fatal injuries. I can imagine that a wing is hitting and collapsing. I conceptualize this as falling from 400 feet wrapped in the aircraft. Can you survive that? Nearly always, the answer is no. Free falling from 400 ft with an initial acceleration of 0 (stall) results in an impact speed of 160 feet per second, which is 5 Gs. Your 200# becomes 1,000# that must stop in a millisecond in maybe 3 feet. You cannot survive regardless of the design because there is just too much energy in your falling body that has to be released upon impact. What we can survive are the low-speed crashes during landings or takeoffs where the pilot is able to continue flying into the crash which allows us to avoid critical impact energy transfer. We can learn something from the ATV industry where impact happens in a similar fashion.

So, we want two things to happen in the crash. We want the airframe to collapse, to deform, in the direction of the impact energy so that critical levels of energy are not transferred to the occupants. This is what contributes to head on collision survivablity in car crashs now. Most of our aircraft are not designed to absorb that sort of energy. Instead, our designs rely primarily on occupant restraint. Safer restraints use at least 4-point systems. The old lap belt designs are practically worthless in crashes. Two things happen. The torso catapults forward (really, the airframe suddenly stopping and the body still moving forward at the initial impact speed). Usually, the head and upper torso crash into the panel and any projections. Depending on the crash energy, as the torso is stopped by the impact with the panel, the spine continues moving forward in the body and causes severe internal injuries- laceration of the heart, lungs, etc. Same is happening in the skull as the brain slams into the skull, and bounces back and forth, which tears the tissues and maybe results in inter-cranial hemorrhage.

Good 4, 5, or 6 point harnesses prevent the upper body impact with the panel. But because most harnesses are "hard fixed", the body is snapped by the impact force. The ability of the airframe to absorb energy is the critical component in dissipating the impact energy. It is possible to design the belt attachment to a structural member, or the seat, that is designed with a little "give" to absorb energy by deforming without breaking. The give is not to the extent that allows the occupants to move about during the impact.

There is a critical design component that is missing in nearly every aircraft. it is an additional restraint that is used in ATV and NASCAR racing. It is head/helmet restraint that is tied to the whole body restraint. These are called "HANS" systems. These systems work with helmets that are strapped to the main body belts. The helmet straps keep the head from being catapulted forward as the body is restrained. If the head is not restrained, what happens is the head is still flying forward while the body is being restrained. This results in a severe spinal cord injury, possibly pulling the brain stem out of the base of the skull. This injury is almost always fatal when the crash would otherwise have been survivable. https://www.summitracing.com/search/part-type/head-and-neck-restraint-systems

Related, of course, is trying to avoid high-crash energy crash in the first place. If the engine coughs it up on TO, do not try to return to the runway unless you are at pattern altitude or close to it. Plan ahead to land straight ahead and fly it into the crash landing. Trying to return often results in a stall and fall. A "successful" "return" landing is going to contain higher energy because ground speed has increased (and energy squares) when landing downwind. So, if stall is 40 and headwind is 10, converting to a downwind is going to nearly double the ground speed energy at touchdown or crashdown. 50 vs 30. That energy difference can be a killer. A crash that is survivable at 30 is very likely not going to be survivable without significant injury at 50.

Happy thoughts.
 

Dan Thomas

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At a glance, it appears that there is a correlation between fatality rate and speed.
It appears that there is a correlation between money and fatalities.

It's been fairly well-known that faster airplanes, which are more expensive, are typically bought by "successful" people who fail to recognize that success and wealth in the professional occupations do not equate to skill and wisdom in aviation. They get into serious trouble, often by continuing VFR into IMC, where they lose control.

"More money than brains," we used to say.
 

wren460

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How about a lot of foam with wood...it would float at least.
That's a great idea. How about we also fill up the cockpit with packing peanuts before each flight. That would provide an additional measure of energy absorption before ones body impacts the interior of the fuselage. Ha, ha..........

Seriously though. There may be something to all that foam and fiberglass, check out this ultra hard Long EZ crash landing.
 
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