comparative weights

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StRaNgEdAyS

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Here's an interesting question.
Foam sandwich style construction of a fuselage a-la Rutan method versus spruce and plywood construction with foam sandwich style fore and aft decking a-la KR2.
Which is lighter?
I'll put my money on the latter, but then I figure it's worth an ask.
Anyone got an opinion on this?
 

orion

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The answer here is really - "it depends". The configuration and structural arrangement will drive many of your options, the combination of which in turn results in the final weight. It also depends on which part of the structural makeup carries all the load.

Several years ago I did a structural analysis for a gentleman who built a KR2 like wooden structure, covered in glassed foam to provide the shape. He intended to use the wood as the primary flight structure and the foam/glass as nothing more than the fairing. He called me after about fifty hours of flying when he discovered cracks in one or two areas of his glass job.

To review a basic premise of structural design, the load always goes to the stiffest part of any structure. It is therefore the stiffest part of the structure that encounters the highest concetration of stresses. The stiffness of any structure is a function of two characteristic values - the Modulus of Elasticity (flexibility) of the material (E) and the structure's cross sectional moment of inertia (I). The term "E x I" is often referred to as the structure's spring constant and the higher the number, the stiffer the structure.

In the case of the foam/glass over wood configuration, the fiberglass has a substantially higher Modulus of Elasticity as compared with the wood so that alone would direct most of the load into the glass - at that point the wood only provides support for the overlying foam.

The Moment of Inertia term is a function of the height, cubed. This means that for even small increases in height, the term "I" increases significantly. Since the fiberglass is further out from the neutral axis than the wood (greater "height"), it will see a dramatically higher amount of the flight load than the interior wooden framework. Yes, there might be more wood in the framework's cross-section, but that is usually not enough to overcome the larger inertia term, epsecially when it is also combined with greater material stiffness.

In the case of my customer, although he designed a good underlying wooden framework, all his loads were being absorbed by the glass skin. Since he however did not design the skin for those loads, it fatigued and cracked. The result was not unsafe, since the wooden framework then carried the load, but it did look quite unsightly.

So, to answer your question, on a strength-to-weight basis, the structure that concetrates most of its mass at the outer extremes becomes the lighest for the given amount of load carrying capability. As such, the pure glass/foam sandwich is more efficient than one that uses an underlying wooden framework.

Also, wood is significantly weaker than glass so you need substantially more of it. So, even though the glass on a cubic inch basis is heavier, you generally use substantially less than when working in wood.

But overall, keep in mind that this is a very simplistic answer. The application of any material should include an analyis of the suitability of that material for that particular structure, as well as many secondary issues that address everything from manufactuirng to assembly and maintenance. Even though the wood/foam/glass is heavier, there may be benefits to its use over the Rutan carved foam methods.
 

StRaNgEdAyS

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Thank for that! :D
So, if I was to build my KR2S boat out of 5/8" (or maybe even 1/4")foam with 2 layers of BID on each side, the resultant structure could well be lighter AND stronger than the standard spruce/ply construction?
 

mtorzews

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StrangeDays

If you increase the foam thickness you increase the stiffness of the structure. Strength is unaffected. The outer glass layers will carry the majority of the load.

Increasing the thickness is great way to optimize weight while acheiving a specified stiffness.

If the section is limited by strength you can add additional layers, use a heavier weave, or use a stronger fiber to add strength.
 

orion

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Yes, designed right, the foam/glass KR2 could be stronger and lighter than the combination of wood/foam/glass. However, just blankly assuming a two-core-two layup is oversimplyfying the case. The laminate would have to reflect the flight loads, the locallized stress concetrations, etc.

For glass application, 1/2" foam should be more than sufficient. That should be about a 6 pcf density at a minimum, although I tend to prefer something a bit denser, say about 8 pcf usually. However, with the higher density you can reduce the thickness a bit, to maybe 7/16" or even 3/8", depending on the actual application.

As far as the glass is concerned, rather than simple BID, I pretty much like to standardize on 7781 material as the properties are better. The handling and layup qualities are also better than for most light BID fabrics.
 

StRaNgEdAyS

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Thanks :D
So I can use this method, as long as I take into account the loading areas for the spars and make appropriate provision to support the weight and stresses of the engine mounting on the firewall.
I did have thought of using the Rutan style construction throughout the whole aircraft, wings and spars included, but I think for now I'll stick to using the plans method for the spars and such and work on the fuselage, where I figure I can make significant weight reductions without impacting too much on the rest of the properties of the plane.
 

Largeprime

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The inside layer requires so little strength that one layer of glass is more than enough.

Anything more would not be nessary
 

orion

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No, no, no ...

Two rules of thumb - First, in any primary structure you never, ever want to use only one layer of anything. This is extremely poor practice from a structural standpoint as well as a service one. Second, as often as you can, you should try to achieve a symmetrical laminate.

Furthermore, in most sandwich applications, when designed right, both skins take the load - you do not want to concetrate the load in only one - there are negative stability issues that would possibly degrade the laminate and structural integrity.
 

AVI

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Orion

Most of these glass/foam/glass sandwich fuselages seem to incorporate spruce longerons - Rutan Long EZ, Cozy, Vision, for example. However, the SQ2000 apparently has a thicker (approx 3/4") foam sandwich without longerons. What advice do you have concerning the use of wood longerons or carbon longerons? How are the calculations made?
 

StRaNgEdAyS

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Graphite longerons would be the bomb!:ban:
But unfortunately I'm working on a firecracker budget, so spruce longerons it is.:rolleyes:
I'm thinking of 4 of 5/8" spruce longerons, adding an extra section of 14"at the rear, and stretching the nose by another 5"just in front of the main spar, which should give me a little better pitch stability and help maintain the C.G. position.
I'll dispense with the header tank altogether to mimise aft C.G. shift as the fuel load lightens in favor of bigger wing tanks afforded by the AS5048/5 airfoils. :D
I have considered using a winglet extending up from the end of the spar, some 8" inside the wingspan to a height of 8" at a 70° angle to help reduce induced drag, but this may be a little overkill. :ponder:
Slotted or split flaps, will move from the stub wings to the main wing section and shorter, deeper, friese style ailerons will sit further out along the wingspan. The use of the friese syle ailerons will allow much more aerodynamic mass balancing as more weight can be fitted into the LE of the aileron. :p:
Finally, on the wing section again, :whistle: the dihedral is pretty large for a plane of it's size and speed, so it could probably come down by an inch and lend a little more responsiveness to the roll, while washout will probably remain at the 2.25° that is indicated with the AS5045/8 airfoil set, 2° is probable ample so that might be getting revised a little as well.
A fair few of these mods have already been successfully been incorporated into KR's, some are my own and I'd like to try to get the most out of what is a fairly quick and responsive little plane.
That's a rundown of what ideas I have for my KR2S(SD)???
Any thoughts Ladies and Gentlemen?
 

StRaNgEdAyS

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Hmmm after drawing the wing up, I noticed that the new airfoil narrows the tip, so the plans 5" dihedral does not look so pronounced as it does with the old RAF48 airfoil.
I've got one of the wings with the winglet drawn, which is the AS5045 tip airfoil reduced to 7.5% thick. all up it doesn't look too bad at all!:ban:
 

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orion

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The use of wood or graphite stringers really depends on the application and on what the designer is looking to achieve. As such, any comments that I might make as to their use would only be second-guessing.

In past examples, the use of imbedded stringers was to provide locallized stiffness or some form of hard point for mounting other structure or for providing a bit of locallized reinforcement in order to take care of loads imposed by service. The most common example of this is providing a structural longeron below the canopy. This provides geometric control for the canopy frame, as well as a mounting point for the accompanying hinges. The longeron also provides local reinforcement in that area for the loads imposed by a person getting in and out of the airplane.

Regarding the KR modifications - the lengthening of the fuselage is generally a good idea since that will provide you with added horizontal authority (needed when using slotted flaps), a wider allowable CG range, and just better overall handling qualities. Do however make sure to account for the changes necessary in your structure. Regardless of the body geometry, the tail loading requirements stay the same (a function of wing loading for analysis purposes). When your tailcone is longer, it imposes a longer arm for the tail to act through and thus you will be generating a higher bending moment into the fuselage structure (highest roughly at the spar carry-through location).

Wing twist and dihedral really depend on the design issues of the airplane. Washout is usually used when the design imposes a selection of airfoils that would cause inadvertent boundary layer seperation at the tip (tip stall). The selection of the correct airfoils should prevent this, but the washout is a good piece of insurance. Two degrees is usually enough.

The amount of dihedral depends on the type of control quality you are after. Personally, in my design work I prefer a small amount, usually no more than about three degrees, measured along the span-wise chord plane. My Cherokee has five degrees - this makes the airplane quite stable but sort of sluggish to aileron responce (need good amount of rudder input).

As far as the wing tips are concerned, my own opinion is that for small general aviation applications, the winglets will most likely not provide any significant measurable benefit to your performance. For larger airplanes with higher wing loadings, swept-up winglets do provide a benefit for climb and cruise. The overall benefit is on the order of a small percentage but in a commercial airplane, even fractional percentages result in tens of thousands of dollars (or more) of operational cost savings over the life of the airframe. In GA, the benefit is generally imesurable.

Also, the design of the winglest must be correct and specific to the wing. Installing just any winglet design may actually result in a decrease in performance since it is added wetted area (that produces no beneficial lift) and the angle between the winglet and the wing produces a bit of interference drag.

Personally, given the amount of work necessary to build the tooling for this bit of structure, my recommendation is usually to avoid this and just install a more standard, well designed tip. But, if you really like the look of the tip winglet, well, that's something else and if your aesthetic drive requires that bit of structure, that's of course another story - and in that case I'd say go ahead, but don't expect any great improvements in efficiency and performance.
 

AVI

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StrangeDays

Have you checked out Mark Langford's KR2 website?

http://home.hiwaay.net/~langford/

And, Orion, perhaps this is elementary, but how about further clarification concerning the use of spruce/graphite longerons (not necessarily for KR2 application). Are you saying that a true monocoque can be constructed of foam/glass sandwich without incorporating spruce/graphite longerons? (SQ2000 apparently does not have these longerons)

I'd previously believed that the longerons were required for structural reasons.
 

StRaNgEdAyS

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OK I've been considering the modifications and I've decided that I'll go with the foam sandwich method for the fuselage.
It took a bit of working out, and I still have to tweak the concept a little before I'll be 100% happy, but I'll be starting work on building a jig for the fuse in the next couple of weeks. :ban: Fortunately living in a timber town, jigging wood is practically free. Pity I can't say the same for the mahogany ply and spruce I'll be using for the spars and other assorted components.:rolleyes:
I had to check the dihedral a couple of times to be sure I had it right, when the end of the spar is lifted the plans requisite 5", the dihedral along the centre line of the spar actually worked out to be 0.5°!
I went with 2° of washout, in my opinion the plans 3° was way too much, and since the winglets do produce usable lift (yes, I checked:whistle: ), I decided to keep them, 'cos they do look good! (I think I can give in a little to vanity :gig: )
The fuse. has been widened to 40" at the shoulder area and stretched 19"overall, 14" of that behind the aft spar.
On another note, I got my pulsejet tube back from being welded, **** THAT THING IS HUGE! Dust reckoned it's not a tailpipe, it's a sewer pipe! :gig: picture if you will, 3M long at 14" diameter at the exhaust (narrowest) end... :eek:
It does weigh a little more that I anticipated, (only like 500grams over estimate) so I have to start on the valve plate next.
 

orion

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Hi Alex;

The use of the longerons really depends on what you hope to achieve. If all the longerons are doing is providing shape, then of course there are other ways of doing that so their use is not all that critical. And to answer your question, yes, you can build a monocoque structure without the logerons. That's actually the structural method of most composite kit aircraft - they are composed of molded sandwich shells that are reinforced and/or stabilized with frames or bulkheads, only as needed, for the structure and service design.

The design of the monocoque however has to be correct so that the skins act not only in unison but also in a predictable way. You also have to understand the sandwich behavior in order to be able to design in the necessary hard points, and distributing those concetrated loads into the surrounding structural skins.

The latter is an important consederation since at times, the longerons provide a convenient hard point for attaching the engine mount, as well as other systems that require the type of reinforcement the longerons can provide.

In short, you have to design for the service, for the structural efficiency you hope to gain and for the manufacturing methodology you wish to use. As such, much of this is an issue of personal choice and limitations of the materials and techniques at hand.
 

wsimpso1

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Longerons - Well, they do contribute something structurally, but I am convinced that the designers included them mostly to hold the shape until you get a layer of glass on each side of the foam. The rest of the reason to have the wooden longerons is to give an attachment location for canopy hardware. You could design just as well without them, and not have an intrusion into the cockpit space right where you would like to have more shoulder room.

Wing Aerodynamics - Why do we continue to see people making thinner sections at the tip than at the root? I know that we want to taper chord towards the tip to tailor spanwise loading, reduce drag and improve roll response, but going with a thin section at the tip gives problems that I don't want in my ship and I can not understand in other people's ships...

Thin foils don't really reduce drag measurably until you get into compressibility. How many of our ships fly at 0.7 Mach? Use 12% thick foils for the tail and enough wing thickness to fit the necessary fuel.

Second - Stall behaviour of the families of foil sections get worse as the foil gets thinner. Stall behaviour of almost all foils also gets worse as the Reynolds number drops. So what you are doing is lowering the Reynolds number by tapering the wing in plan view AND thinning the wing at the same time. "Doctor, Doctor, it hurts when I do this!" "Well don't do that. That will be $5" (The Marx Brothers).

How to avoid all of this trouble? Use one foil section for the whole wing, scale the whole section to the chord that you want at each station, and then if you really want insurance, you can add some washout to make sure that the tip stalls last. It has been done without the washout and the airplanes that do have marvelous handling, good straight ahead stalls, and yes, some are very efficient. Piper Comanche, many Rutan ships including the Boomerang for one... And this way, you can avoid having to cover your wing with vortex generators, drooping leading edges, etc.

Billski
 
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