Heavy thin v. thicker Light Material

LAST EDITED ON Nov-30-04 AT 03:16 PM (CST)
 
Reading Godzilla's message regarding his ultra-stiff sheeted flap, using 7 -8 lb sheeting got me thinking. I recall some other inputs regarding substitution of thinner, but heavier wood for thicker 4-6lb stuff, what is the thinking on where it is fesible?

For example:

on a 60size pipe plane designed for 1/8 fuse sides over 3/32 cross grain bulkheads, what about using 1/16 7lb material and adding some warren truss 1/16 intermediate bulkeads to control the dishing?

Or same skin but add some camber to the flat side panels to stiffen them up ?

Or subing 1/20th 7 lb for sheeting on stab and elevators like Brad was making?

I understand that the tensile strength goes up faster than the density but seems like it can still lead to flexing if the panel stiffener spacing isn't made closer.

Is there any current rule of thumb for construction scantlings?

It seems that we all need to be thinking in this regard given the forecast for lack of availability of really nice 4-5Lb wood.

Curt

EDIT: FYI
The reasoning here is to arrive at a LIGHTER structure that is adequate in strength. I'm not trying to make things stronger.

LAST EDITED ON Nov-30-04 AT 02:48 PM (CST)
 
>Is there any current rule of thumb for construction
>scantlings?
>
>It seems that we all need to be thinking in this regard
>given the forecast for lack of availability of really nice
>4-5Lb wood.

Some won't like what I'm saying but don't build to crash build to fly and land. You must build so it's rigid enough to take G forces. A reasonably light straight airframe is what you need IMO. I use 4-6 lb stock all over, lighter if I can find. My flaps are made of 3/8 4-6 lb wood sanded to shape and covered with .02 lite CF they come out at 14 grms. Done in this manner they are really stiff!

Stab is 1/2" cored and sheeted 3/4 oz. the finished stab and ele should weigh around 1.75 oz less horn and hinges for the T-Bolt.

LAST EDITED ON Nov-30-04 AT 03:14 PM (CST)
 
I agree completely with the built to fly not to crash premise.

Its just that it is getting more and more difficult to get 4 or 5 lb material. And I hate to take a nice piece of 1/2 x 4 4lb stock and core it all out--when I could do the same with sticks and sheeting.

I know alot of the material specs on plans are arrived at by gosh and by golly--many simply copied over from some long ago notions of how thick a fuse side should be for instance.

And I hate take the Formula 1 approach of kep making it lighter until it fails, then heavy it up just a tad.

My last project, Randy Smith's Katana had component weights in the neighborhood you suggest.

Curt

PS: I'm enjoying the pix and commentary on your builds--I get to touch and feel a P47 pretty often over the summer cause I fly with Bob McD.

"Don't build to crash, build to fly and land"

You hit it right on the head. No matter what you do it's going to break when you hit the ground, and if you build to hit the ground your building to heavy.

>"Don't build to crash, build to fly and land"
>
>You hit it right on the head. No matter what you do it's
>going to break when you hit the ground, and if you build to
>hit the ground your building to heavy.

Ron B.
F4Fguy

As I read it, the question had nothing to do with surviving crashes.It was about substitution of thinner and more dense materials to get the same or lower weight with without compromising structural strength.

The answer is,most certainly,yes. It may,as you've already surmised,require some redesign to retain stiffness but ,as you've also noted the strength goes up faster than the weight.I've been sheeting foam wings(up to 750 sq.in.) for years with 1/32" 8# balsa. the fuselage on the F4F is 1/16" 6# from the flap line back(love that fat, round shape for stiffness)the Zero is 1/20" 6# in the same area.Stab and fin in both cases is 1/32" 6-8#skin over 1# foam.

There are other areas too. For example, formers of two layers of laminated 1/32" 8# are stiffer and lighter than 1/8" solid 6#.

There's also the fact that uniform grain and density is easier to find in the heavier grades. You seldom see wind checks or "punky" spots in 8-12# balsa.

And that's only balsa,take a hard look at basswood, or spruce. Carbon fiber is self evident,as is aramid fiber. The wing on the F4F is carbon epoxy over foam with no other skin.

The big question is always the same ,do you invest the time and expense to pioneer,or tread the same ground over and over again?

Ron B.
Ron B.

My flaps are made of 3/8 4-6 lb wood sanded to
>shape and covered with .02 lite CF they come out at 14 grms.
> Done in this manner they are really stiff!

14 gram flaps made from one solid piece of 4-6 lb stock, covered in one layer of CF veil are possibly "stiff enough" for your uses but are not going to be stiff by Al Rabe standards.

A 2 oz flap, if built correctly will typically be twice as stiff as a 1 oz flap. Reagardless of construction (we are talking balsa).

> My flaps are made of 3/8 4-6 lb wood sanded to
>>shape and covered with .02 lite CF they come out at 14 grms.
>> Done in this manner they are really stiff!
>
>14 gram flaps made from one solid piece of 4-6 lb stock,
>covered in one layer of CF veil are possibly "stiff enough"
>for your uses but are not going to be stiff by Al Rabe
>standards.

I am building Billy's designs not Al's. So they must be ok for what I'm doing.

>> My flaps are made of 3/8 4-6 lb wood sanded to
>>>shape and covered with .02 lite CF they come out at 14 grms.
>>> Done in this manner they are really stiff!
>>
>>14 gram flaps made from one solid piece of 4-6 lb stock,
>>covered in one layer of CF veil are possibly "stiff enough"
>>for your uses but are not going to be stiff by Al Rabe
>>standards.
>
>I am building Billy's designs not Al's. So they must be ok
>for what I'm doing.

That is what I said.

Lighter is more flexible. Heavier is stiffer. Everything is a trade-off.

Balsa is good example. I think twice stronger wood makes only 1.5 higher density or so. It means that thin strong heavy wood will make lighter construction of the same strength. But problem is, that thin material tends to flex. So if you want make profit on hard wood you will come to more complex construction - which will need good knowledge how to design the construction.

I am lazy builder so I prefer light wood and simpler/quicker construction.

>Balsa is good example. I think twice stronger wood makes
>only 1.5 higher density or so. It means that thin strong
>heavy wood will make lighter construction of the same
>strength. But problem is, that thin material tends to flex.
>So if you want make profit on hard wood you will come to
>more complex construction - which will need good knowledge
>how to design the construction.
>
>I am lazy builder so I prefer light wood and simpler/quicker
>construction.

Igor....You're my kind of guy!

There ARE places where there could be a benefit, but it involves a LOT of work...Just ask Ron!!!

I agree with Al on this one...The flaps are one place where there almost isn't "too" stiff. The stiffer, the more linear the response. If you don't believe it, put on some real wimpy flaps and have fun in the wind! This is applicable to fairly high level competition ships. Sport planes are not trimmed out well enough to feel this most of the time!

You have some blue flaps that are quite stiff torsionally.

>You have some blue flaps that are quite stiff torsionally.

I would like to pursue that again with you.....AFTER YOU FINISH YOUR NEW PLANE!

>>>The flaps are one place where there almost isn't "too" stiff.<<<

No questions about it.

But I am asking myself if lighter / thicker flap is better or not. If thinner heavier wood of the same final weight leads to thinner flap, then may me thicker lighter wood of the same final weight can make stronger flap IN TORSION. Just because thiner body at the same torsion needs on surface heavier compression/expansion question is if hardness compensates it or not. Anyone has some relevant data?

>Reading Godzilla's message regarding his ultra-stiff sheeted
>flap, using 7 -8 lb sheeting got me thinking. I recall some
>other inputs regarding substitution of thinner, but heavier
>wood for thicker 4-6lb stuff, what is the thinking on where
>it is fesible?
>
>For example:
>
>on a 60size pipe plane designed for 1/8 fuse sides over 3/32
>cross grain bulkheads, what about using 1/16 7lb material
>and adding some warren truss 1/16 intermediate bulkeads to
>control the dishing?
>
>Or same skin but add some camber to the flat side panels to
>stiffen them up ?
>
>Or subing 1/20th 7 lb for sheeting on stab and elevators
>like Brad was making?
>
>I understand that the tensile strength goes up faster than
>the density but seems like it can still lead to flexing if
>the panel stiffener spacing isn't made closer.
>
>Is there any current rule of thumb for construction
>scantlings?
>
>It seems that we all need to be thinking in this regard
>given the forecast for lack of availability of really nice
>4-5Lb wood.
>
>Curt
>
>
>EDIT: FYI
>The reasoning here is to arrive at a LIGHTER structure that
>is adequate in strength. I'm not trying to make things
>stronger.

I realize it says nothing about a crash but what I was getting to was over kill. Yes 2 pieces of 1/16 laminated together with CF is stronger than 1/8 4 lbs stock but is it necessary?

X ing ribs and formers is the strongest and lightest way. IMO. Any time you can shorten up contact points it will be stronger and more rigid.

Larry's new fusalage structure for the Kit version of the Mo'Best is 1/64" ply over 1/8" warren truss like internals - claims lighter & stiffer. I'm building one that way and have finished a fusalage to the earlier 3/32" angled sheeting version so I will have a good back to back comparison. I'll report back when I have both to the same state.

Regards

Mike

LAST EDITED ON Dec-01-04 AT 04:56 AM (CST)
 
>>Reading Godzilla's message regarding his ultra-stiff sheeted
>>flap, using 7 -8 lb sheeting got me thinking. I recall some
>>other inputs regarding substitution of thinner, but heavier
>>wood for thicker 4-6lb stuff, what is the thinking on where
>>it is fesible?
>>
>>For example:
>>
>>on a 60size pipe plane designed for 1/8 fuse sides over 3/32
>>cross grain bulkheads, what about using 1/16 7lb material
>>and adding some warren truss 1/16 intermediate bulkeads to
>>control the dishing?
>>
>>Or same skin but add some camber to the flat side panels to
>>stiffen them up ?
>>
>>Or subing 1/20th 7 lb for sheeting on stab and elevators
>>like Brad was making?
>>
>>I understand that the tensile strength goes up faster than
>>the density but seems like it can still lead to flexing if
>>the panel stiffener spacing isn't made closer.
>>
>>Is there any current rule of thumb for construction
>>scantlings?
>>
>>It seems that we all need to be thinking in this regard
>>given the forecast for lack of availability of really nice
>>4-5Lb wood.
>>
>>Curt
>>
>>
>>EDIT: FYI
>>The reasoning here is to arrive at a LIGHTER structure that
>>is adequate in strength. I'm not trying to make things
>>stronger.
>
>I realize it says nothing about a crash but what I was
>getting to was over kill. Yes 2 pieces of 1/16 laminated
>together with CF is stronger than 1/8 4 lbs stock but is it
>necessary?
>
>X ing ribs and formers is the strongest and lightest way.
>IMO. Any time you can shorten up contact points it will be
>stronger and more rigid.

Ron B.
F4Fguy

Robert:

You don't need the CF. Two lams of 1/32" 8# will be both lighter and stiffer than 1/8" 4#. Even more so with some cross grain camber. I respectfully refer you to post #5 in this thread.

Curt,

IMHO, The best way to accomplish the goal of stiffer lighter structures is to use well engineered structures to begin with. While you can take a run of the mill design (structurally) and achieve a reduction in weight with some small increase in rigidity by using heavier thinner wood, I find it more productive to use better engineering and just less balsa. Double tappered wings and stabs are enormously more rigid than single tapered equivalents. Boxing in the trailing edge between ribs adds a lot of rigidity. When making fuselage formers use vertical grain in the vertical direction and cross grain only at the top and bottom. (less balsa than a wholy laminated sheet). Making flaps with thin sections will never produce the torsional rigidity of a thick leading edge with a quick taper to the trailing edge. Compound or "Tortured" shapes in the fuselage (only possible by moulding) will produce a structure so stiff you will not want to see just how far you need to go to twist it. As a general principal, think of closing all "open boxes". The application of Carbon veil in a matrix with dope is a non starter as it is the ability of the dope to hold the carbon fibers in isolation that produces (or not) the stiffness. Dope is just too soft. You simpy do not need carbon veil if the structure is properly engineered. Done religiously this approach will yield a plane that is very stiff and highly responsive to control input as it will not be flexing and vibrating.

Hope this helps.

I believe the thickness of wood wll play a role in the vibration dampning.

> I believe the thickness of wood wll play a role in the
>vibration dampning.

Definitely, stiffness is another major factor, which affects natural frequency (resonance) of the part. Stiffer structures have higher resonant frequencies, which may or may not be helpful.

[photo not recovered: 3e69bb3870f12ad5.jpg]

"When I played in the sandbox the cat kept covering me up." -Rodney Dangerfield

Well,

Generally you can use lighter and thinner balsa except for one area - that would be leading edge planking. If i'm using 1/16 LE sheeting I will use 7-8 lb stock. Of course, if you are sheeting the LE with 3/32 then the lightest 4-6 lb stock is what you want to use! 3/32 4-6 lb fuse sides are O.K. if good former usage is used. With modern smooth running engines doublers can be 3/32 maple ply or sometimes even 1/64 if you cross brace between the motor mounts with 1/2 inch balsa block. I once built an original "Northern California" design called the "Pegasus" 54" span and 610 Sq In. that weighed 32 ounces and powered it with one of the old series 100 Veco .19's. I'll probably do it again when I go to my 1st VSC - sometime in the future.

>You have some blue flaps that are quite stiff torsionally.
I would like to pursue that again with you.....AFTER YOU FINISH YOUR NEW PLANE!

P Walker

Paul,

What is amazing about Howard and his glacially slow building of PA models is that this time it is really ridiculous. In an event where engines even long out of production are gleefully used by many, Howard has managed to cycle through building the new model to a specific engine, upgrading to a bigger version of same engine, now shows serious signs of changing strokes on us with yet another engine for motive power.

Truly, this is a new high. Or low, depending upon one's perspective.

Glacially slow!!

You're a mean man, Dan! (I think of you as the Lewis Black of Stunt..)

[photo not recovered: 3e69bb3870f12ad5.jpg]

"The best coffee in Europe is Vienna coffee, compared to which all other coffee is fluid poverty." -Mark Twain

I am currently building a Cardinal -92 . (The big one) from Windy's kit. The balsa supplied for the flaps was very light (and soft) , so i mailed Windy about this.
His prompt answer was : " Light flaps are not a problem in my opinion" ... "SEALED HINGELINES are important,dont skip that item weather flaps are heavy or light"

After all, the pressure on the surface is very evenly distributed.
I have also read that the Beringer designs have very thin pianowire for the flaps horn to actually make it flexible.

>I am currently building a Cardinal -92 . (The big one) from
>Windy's kit. The balsa supplied for the flaps was very light
>(and soft) , so i mailed Windy about this.
>His prompt answer was : " Light flaps are not a problem in
>my opinion" ... "SEALED HINGELINES are important,dont skip
>that item weather flaps are heavy or light"
>
>After all, the pressure on the surface is very evenly
>distributed.
>I have also read that the Beringer designs have very thin
>pianowire for the flaps horn to actually make it flexible.

Al's planes have huge flaps. Small wings and huge flaps, they need to be stiff.

Berringer is essentially flying without flaps.

After getting the plans, which are 1/2 size except for the root and tip ribs, for the Berringer Sukhoi and Caudron (not the twin), to say they are unusual is putting it lightly. Yeah, pretty "tiny" flaps.
Bill <><

I'm with Kim Doherty on this subject. I've spent a good bit of time looking for places to use CF in our models, mainly 'cos it seems so right! And of course I sometimes hang around with Rush, and he is truly accomplished at this sort of thing.

Let's see, CF motor mounts sticks, although I don't use them. Good idea, however. Landing gear from CF; another good idea, one I may eventually take advantage of. CF pushrods, of course. That's a no-brainer.

Still, that's not a very long list of Good Stuff from CF.

Bad ideas abound. I personally think CF tuned pipes, while easy to fabricate, are hardly the best solution to a problem that, from a materials standpoint, is best handled by aluminum. Despite raving reviews of CF veil attached with dope, that just doesn't pass the laugh test, see Kim's response for why.

There are a good many other examples, but clean, well-designed structure fitted together with good glue joints, all the best techniques, is much more than adequate for our purposes.

In other words, we don't need trick stuff. We need good, basic stuff that has been proven to work for many years.

As to Norvaldo's response, I have always tried for two things in flaps: stiffness and equal stiffness left to right. If forced to choose, I think I'd rather have equal flex in left and right stabs over ultimate stiffness. But I don't have to choose, so regard both as important.