How Safe?

[Mtns2Skies]

Hey, I was wondering if wood and/or fabric aircraft can really stand up to the rigors of flight. Are there any special procedures you must use for them in tie down or flight? I know they seems like dumb questions but I cant wrap my head around how something so flimsy or organic materials can support a plane and the rigors of landings, stalls etc.

 

[Trapper John]

Hey, I was wondering if wood and/or fabric aircraft can really stand up to the rigors of flight.

Sure they can, if you operate them within the type certificate limitations. Take a fabric covered Pitts Special or a Christen Eagle, for instance. If you can get a ride in one, you'll give up long before the plane does!

There are tons of Aeroncas, Citabrias ans Decathlons with wood wing spars and fabric coverings, plus gobs of old fabric covered Pipers, etc. Bellanca Vikings have wood wing structures that are like a work of art, if you're ever lucky enough to see one uncovered.

The B-29s that dropped the A-bombs had fabric covered control surfaces, even. It's really just a matter of using the chosen material appropriately to perform the needed function, whether it's wood, fabric, aluminum or a modern composite. Yeah, it's probably not the best thing to leave a fabric covered plane stored outside long term, but there are composites that have temperature limitations, too, so everything has its drawbacks.


Trapper John

 

[SCCutler]

Materials science guys will tell you that, with proper care, a wood structure is in many ways superior to aluminum. In particular, wood is not subject to repetitive-stress deterioration like aluminum.

 

[Mtns2Skies]

Materials science guys will tell you that, with proper care, a wood structure is in many ways superior to aluminum.

What type of proper care do you mean though?

 

[Geico266]

The B-29s that dropped the A-bombs had fabric covered control surfaces, even.
Trapper John

I'm pretty sure the same is true on the mighty and battle tested B-17's.

 

[Trapper John]

I'm pretty sure the same is true on the mighty and battle tested B-17's.

Yes, that's what I remember, too. Not sure about the B-24, though. I think Twin Beeches had some fabric control surfaces, but it's been too long since I've seen one up close to be sure.


Trapper John

 

[Trapper John]

What type of proper care do you mean though?

Basically you don't want to create conditions where the wood gets wet and stays wet which creates an environment favorable for microbial critters to start breaking down (rotting) the wood. That's not to say you can't fly in the rain, you just want to keep the wood dry as possible.


Trapper John

 

[Greg Bockelman]

What type of proper care do you mean though?

Basically keep it dry and out of the sun. Although with the modern materials, they CAN be tied down outside for a long time without any long term bad affects.

 

[steingar]

There are trees that have been around sitting out in the elements a lot longer than the airplane. Pound for pound silk is stronger than steel. Your premise is wrong Hawkeye, there is nothing at all flimsy about organic materials.

 

[dmccormack]

There are trees that have been around sitting out in the elements a lot longer than the airplane. Pound for pound silk is stronger than steel. Your premise is wrong Hawkeye, there is nothing at all flimsy about organic materials.

Yeah.. I'm 100% organic, and have withstood 47 years!

I'm not tied down outside, though.

 

[fgcason]

Ever watch a tree in a horrific storm? They can take an incredible amount of abuse before they start having problems. I watched a tornado go through some trees, destroy a building and keep going. Seeing the trees suddenly stop twisting and swaying and stand up straight like nothing ever happened with bricks thrown all over the place was fairly comical actually. It was like the trees were saying "what's the problem, that was nothing."

Strong isn't always about rigidity. Glass is rigid and strong, it just doesn't bend worth poop. Give me a wood ladder over an aluminum one anyday - wood warns you before it dumps you on the ground.

 

[Trapper John]

..there is nothing at all flimsy about organic materials.

Old Bellanca Viking promo pic:

Trapper John

 

[Mtns2Skies]

Old Bellanca Viking promo pic:

Trapper John

I certainly wouldn't do that with a 152. I see that wood is certainly fine to fly with.
Is it hollow or solid wood? And what about Fabric? I know piper Cubs have gone through a lot of stress but I've read stories about fabric wings buckling in heavy turbulence.

 

[fgcason]

Old Bellanca Viking promo pic:

Wasn't it a wooden Viking wing that broke a metal test rig used for finding the actual failure point of a wing?

 

[Greg Bockelman]

Is it hollow or solid wood?

Hollow? What, the spar? It MAY be routed out some, but just to make it lighter without compromising the strength, but hollow? No. And typically the ribs are aluminum, but some may be built up out of wood. Like a truss.

And what about Fabric? I know piper Cubs have gone through a lot of stress but I've read stories about fabric wings buckling in heavy turbulence.

What about fabric? The modern materials are much better than the original cotton and lasts a long time. They do not, however, contribute much if anything to the structural strength of the airframe. That is all done by the underlying structure. And a metal wing can and does buckle just like a fabric wing in heavy turbulence.

 

[Skip Miller]

One of the best fighter-bombers in the European theater of WWII: the wood De Havilland Mosquito. For its day, it was also stealthy, too, as the primitive radars couldn't see it very well.

http://www.historylearningsite.co.uk/mosquito.htm

-Skip

 

[Trapper John]

Is it hollow or solid wood?

Here's a Viking wing. It's mostly hollow, like an aluminum wing. The spars are solid material:

And what about Fabric? I know piper Cubs have gone through a lot of stress but I've read stories about fabric wings buckling in heavy turbulence.

Well, you can break the wings off anything, but as long as the fabric is in good condition and correctly attached to the underlying structure, the fabric isn't an issue at all.


Trapper John

 

[Trapper John]

Wasn't it a wooden Viking wing that broke a metal test rig used for finding the actual failure point of a wing?

That does sound vaguely familiar...

Back in the early '70s one of the Bellanca saleman's favorite tricks was to pour a big coffee can full of marbles onto the wing from a height of a few feet. Of course, the marbles bounced harmlessly off the taut fabric. Then he'd say, "So, should we try this on your [Piper, Cessna, Beech]?" It was a pretty effective demo...


Trapper John

 

[mantakos]

I certainly wouldn't do that with a 152.

Well, for a 1600lb 152 at 3.8Gs, the wings are generating 6000 lbs of lift. That's about the same force as having 46 Dale Evans lookalikes, in full regalia, spread out on the wings. Of course, the forces are distributed differently, but they're comparable in magnitude.
-harry

 

[Capt. Geoffrey Thorpe]

Hey, I was wondering if wood and/or fabric aircraft can really stand up to the rigors of flight. Are there any special procedures you must use for them in tie down or flight? I know they seems like dumb questions but I cant wrap my head around how something so flimsy or organic materials can support a plane and the rigors of landings, stalls etc.

This one defiantly stands up to the "rigors of flight"

A photos of the "guts"

 

[skyflyer8]

I think Twin Beeches had some fabric control surfaces, but it's been too long since I've seen one up close to be sure.

Yes, I can vouch for the Beech 18 having fabric control surfaces. The one at our field has them removed right now for recovering.

IIRC even the DC-3 has fabric control surfaces right?

 

[SCCutler]

Yes, I can vouch for the Beech 18 having fabric control surfaces. The one at our field has them removed right now for recovering.

IIRC even the DC-3 has fabric control surfaces right?

Yes, the DC-3 has fabric control surfaces.

 

[Everskyward]

Yes, the DC-3 has fabric control surfaces.

This is what happens when they are not maintained.

 

[gismo]

This one defiantly stands up to the "rigors of flight"

A photos of the "guts"

Hmmm, something about that airplane looks familiar

 

[gismo]

The only things about wood and fabric construction that might be of any concern are the potential for rot (easily avoided and pretty easily tested for) and flammability. It's always seemed silly that the FAA requires upholstery and other interior materials to be "self quenching" on an airplane that's made of wood, fabric, dope.

 

[fgcason]

It's always seemed silly that the FAA requires upholstery and other interior materials to be "self quenching" on an airplane that's made of wood, fabric, dope.

IMHO: Your ability to deal with an inflight systems/structural fire related emergency is directly related to whether YOU are on fire or not.

 

[gismo]

IMHO: Your ability to deal with an inflight systems/structural fire related emergency is directly related to whether YOU are on fire or not.

I can only imagine this but IMO a situation where the covering was burning off the wings might be a bit more difficult to "deal with" than a fire in the seats.

 

[Tom-D]

All you wanted to know about fabric covering systems and more.

http://www.airtechcoatings.com/faq.html

And it's fire proof

 

[wanttaja]

Hey, I was wondering if wood and/or fabric aircraft can really stand up to the rigors of flight. Are there any special procedures you must use for them in tie down or flight? I know they seems like dumb questions but I cant wrap my head around how something so flimsy or organic materials can support a plane and the rigors of landings, stalls etc.

I've owned a wooden airplane (with fabric covering) for about twelve years. Prior to that, I flew one that was over 30 years old. Certainly things you have to watch for, just like there are things you have to watch on metal airplanes, too. But the only real "care" mine gets is a hangar to keep it out of the rain.

It ain't flimsy, either. Here's a picture of the G-meter after one of my landings:

The positive-G recording needle is pegged at +4 Gs. My *back* hurt for a couple of days, but the plane was fine.

Ron Wanttaja

 

[Tristar]

That brings up another interesting question. Although a normal category airplane can stand up to positive 3.8 G's but how much can the landing gear take? Is it the same? more?

An even more difficult question is how much can the body take before something such as your spinal chord breaks?

 

[jesse]

That brings up another interesting question. Although a normal category airplane can stand up to positive 3.8 G's

Well, technically (part 23), it should survive another 50% before failing...provided the airplane is still structurally sound.

but how much can the landing gear take? Is it the same? more?

Well the landing gear isn't going to fall off at 3.8G's. You'll lose the wings before you lose the gear. With the exception, perhaps, of over-speeding the gear. As far as what they must be able to withstand during ground operations-- this is answered in 23.471-23.511.

An even more difficult question is how much can the body take before something such as your spinal chord breaks?

More than our airplanes can take. You're going to pass out before your body fails you. John Stapp during his testing survived 46.2Gs. 0-632 mph in 5 seconds followed by 632 to 0 mph in less than a second.

The human body can take a lot--but you're going to pass out / lose the ability to control the airplane before you hit the limit of the body.

 

[Capt. Geoffrey Thorpe]

I've owned a wooden airplane (with fabric covering) for about twelve years. Prior to that, I flew one that was over 30 years old. Certainly things you have to watch for, just like there are things you have to watch on metal airplanes, too. But the only real "care" mine gets is a hangar to keep it out of the rain.

It ain't flimsy, either. Here's a picture of the G-meter after one of my landings:

The picture didn't include the most important meter on the panel:

 

[gismo]

The picture didn't include the most important meter on the panel:

That gauge seems to imply that there's such a thing as too much fun. How can this be?

 

[gismo]

That brings up another interesting question. Although a normal category airplane can stand up to positive 3.8 G's but how much can the landing gear take? Is it the same? more?

An even more difficult question is how much can the body take before something such as your spinal chord breaks?

Landing gear strength requirements are defined in FAR 23.471-572. IIRC the minimum g limit for the gear is just under 3g measured at the cabin, but there are a lot of other factors. For one thing the real issue at hand is the vertical velocity of the airplane when the wheels first touch and the flexing/compression of the landing gear which spreads the deceleration over time. The resulting g force experienced in the cabin will be a function of that and the energy absorbing potential of the gear since deceleration (g force) is equal to the change in velocity divided by time.

Consider an airplane with no springs (nothing to flex or compress) between the wheels and the cabin and "rock hard" wheels. In that case the only things that flex at touchdown will be the pavement and the cabin itself. If both of those were really stiff a touchdown at a measly 1 ft/sec (60 ft/min) could theoretically result in a 10 g (320ft/sec^2) impact if the structures were stiff enough to stop the vertical motion in 3 milliseconds. Put in enough travel and flexion to spread that deceleration over a half second and you reduce the g force to 1/16 g.

Another easy too understand analogy is jumping off a 2 ft high stool onto a concrete floor with your knees locked vs bent and flexible. In the locked knee case you could injure your spine and possibly knock yourself out from the shock of impact but if you flex your knees upon landing the rest of your body will hardly feel it.

One thing is certain, the g force that an airplane is intended to experience upon landing cannot exceed the aerodynamic g force limits. If it did you could have an "acceptable" (i.e. within design limits) landing that would permanently deform the weakest structures.

 

[wby0nder]

you forgot to mention that this plane has WOOD LANDING GEAR struts with NO SHOCK ABSORBERS other than the tires. (unless it's been modified from the plans)

I've owned a wooden airplane (with fabric covering) for about twelve years. Prior to that, I flew one that was over 30 years old. Certainly things you have to watch for, just like there are things you have to watch on metal airplanes, too. But the only real "care" mine gets is a hangar to keep it out of the rain.

It ain't flimsy, either. Here's a picture of the G-meter after one of my landings:

The positive-G recording needle is pegged at +4 Gs. My *back* hurt for a couple of days, but the plane was fine.

Ron Wanttaja

 

[gismo]

Well, technically (part 23), it should survive another 50% before failing...provided the airplane is still structurally sound

I think Tristan is asking about impact loads during landing rather than the aerodynamic loading experienced in flight. That said, your statement is a bit misleading. The 50% "safety margin" only applies to catastrophic structural failures. There is no requirement for a margin beyond the design limits (3.8g for normal category etc) regarding permanent deformation that still allows the airplane to fly. IOW you can render a normal category airplane unairworthy by pulling 3.9g and if the landing gear on a normal category airplane is robust enough to survive a 4 g landing the rest of the airplane can be damaged by such a landing impact. Another thing to consider is the rate of g force application (aka jerk). The strength of many plastics is rate sensitive and the potential for damage increases significantly when the force is applied over a very short time. A classic example of this is a demonstration with a piece of PVC drainpipe. You can apply enough force to bend such a pipe into a 'U' shape without damage, but if you "smack" the pipe on something hard like concrete or steel with much less force, the pipe will shatter. I've always wondered if this is taken into consideration when designing a plastic airplane. It has been noted that with plastic bodied cars (e.g. Corvettes) the body doesn't absorb crash energy as well as metal bodied cars and that the fiberglass typically breaks into many small pieces rather than just deforming in a crash like metal does.

Well the landing gear isn't going to fall off at 3.8G's. You'll lose the wings before you lose the gear. With the exception, perhaps, of over-speeding the gear. As far as what they must be able to withstand during ground operations-- this is answered in 23.471-23.511.

Yep, except I think it goes beyond 23.511

The human body can take a lot--but you're going to pass out / lose the ability to control the airplane before you hit the limit of the body.

I think the body's ability to take g forces is also affected by the duration of the force and it's definitely affected by the direction of the force relative to the body as well as how much surface area it's applied over. IIRC you can withstand a lot more vertical force if you're lying on a conforming flat surface than if you're sitting in a chair, for instance.

 

[AcroBoy]

I thought the design limit loads were done with static loads, as mentioned in the previous post, as opposed to dynamic or cyclical loading?

Also, the guy who took all of those g's in a rocket sled took them along the x axis, not the z axis, which is where most of the g from acro (or hard landings) are seen. G loading along the x axis is much better tolerated than along the z axis, either positive or negative.

 

[Capt. Geoffrey Thorpe]

That gauge seems to imply that there's such a thing as too much fun. How can this be?

uuurrrppp

 

[bullwinkle]

More than our airplanes can take. You're going to pass out before your body fails you. John Stapp during his testing survived 46.2Gs. 0-632 mph in 5 seconds followed by 632 to 0 mph in less than a second.

The human body can take a lot--but you're going to pass out / lose the ability to control the airplane before you hit the limit of the body.

In a typical 35mph automobile crash, the human body is subjected to over 200Gs, but only for fractions of a second. people are pretty tough for squishy fluid bags.

.

 

[jesse]

In a typical 35mph automobile crash, the human body is subjected to over 200Gs, but only for fractions of a second. people are pretty tough for squishy fluid bags.

.

Sure--basically--what I was getting at..is the fact.. (from an in-flight load perspective) that our bodys are going to survive more than our GA airplanes will.