Upscaling WAR plans

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vhhjr

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I have read through many posts and responses about WAR plans and have built several, scaled WWII aircraft mockup cockpits. My conclusoion is that 50% is just a bit small especially when you consider the size of most of us. Redrawing plans based on upscaled RC models requires lots of engineering.

How about upscaling a set of WAR plans a modest amount, say 10% - 15%? Some engineering would still be necessary, but going to the next size hardware, say from AN3 to AN4 bolts or from 1/4 inch to 3/8 inch plywood increases the structural strength by factors of 1.8 to 2.25. Attention would have to be paid to weight control , but it's all doable. The upside is the cockpit seating gets 2 - 3 inches wider and 4 to 6 inches taller. Also, the wing area goes up by 15% to 30% reducing the wing loading considerably (assuming you watch the build weight). It is my undersrtanding that aircraft stability increases with upscaling

The easiest way to accomplish this is to get the plans in a digital format and import them into a suitable CAD system. If digital isn't available the drawings for components, such as bulkhaeds and wing ribs, can be enlarged by your local blueprint shop. That's how I made the plans for all three of the mockups I built. The P-39 plans started out as a Pepino giant scale RC model and a trip to Kinkos produced the mockup plans. The ME-262 and B-25 were also giant scale RC plans and were imported into Draftsite or Progecad, rescaled and patterns were printed. One downside is that any parts such as canopys, cowlings and fuselage/wing parts would have to be made in the larger scale thus increasing the build time for such a project.

Vince Homer
 

wsimpso1

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Several issues may become really significant.

When we make a solid bigger its dimensions go up with the first power of the scale, the areas (as in wing area and frontal area and profile drag) go up with the second power of scale, and weight goes with the third power of scale. 15% scale has 1.32x the surface area and 1.52x the weight. If we just scale up all dimensions, we have to accept that it will be 1.52 times as heavy.

A lot of things in those WAR plans are probably at min gage. That is to say, if they were sized for just flight loads they would be thinner, but for some practical reasons, they were built thicker. Typical parts that are min gaged are skins and some other underlying structures. Usually min gage is about either material availability or ability to build/handle/operate the airplane without damaging it. Parts built at min gage in the original might be kept at same thicknesses in the scaled up airplane. I am thinking the skins of the fuselage, wings, and tailplanes. While I do not know what cloth covers these parts in the WAR airplanes, the LongEz and derivatives plus the Defiant are skinned in three lies of 7 oz UNI or some equivalent. Other parts may have to be increased in thickness. We will not know which is which unless proper analysis is performed, although a smart person might look at airplanes the size of the intended airplane and see what thicknesses they use.

Typically lift possible (and drag produced) at any given airspeed goes up with area, which goes with scale squared (1.15x size is 1.32x lift). Simple scaling may take drag above intent and this reduces cruise speed below intent. It may also adjust stall speed and thus runway required by substantial amounts, while also adjusting Va substantially. Scaling an airplane frequently requires some adjustments in wing and tail sizing then adjusting how that area is presented to keep the look and feel correct.

Total lift must go up with airplane weight... With lift going up and wing area going up to give similar stall speeds and runway performance, and with span increasing, the shear, bending moment, and pitching moments also increase. Total lift will scale with wing area, but bending moment will increase with both lift and span - count on scale squared being a minimum on both.

Spar sizing should be reviewed carefully - Yes, wing spar bending and torsional stiffness goes with depth cubed, but strength only goes with depth squared. Control system forces will also change substantially. Forces at the control stick and pedals will be different and perhaps gradients too, with all of the control harmony issues that might also happen. I good review of the control forces and harmony of the original and of the new bird might be in order. Once the first guess on new airplane forces in the system are acquired, review all linkage elements for strength and buckling resistance. Yeah, even if the forces in the elements are the same, they are longer now - pushrod buckling strength goes down with the square of pushrod length - 1.15 scale means buckling strength using the same tube is 0.75x the buckling load.

Even the method of construction may be adjusted. If the wooden wing spars get too heavy or too expensive after scaling them up, you may want to shift the construction from wood spars and ribs to Rutan style hot wired foam with composite spars and skins. This too will adjust not only weights, but CG.

Then I have one more caution. Just because the WAR designs seemed to have no nasty habits or weak spots does not mean that it did not have any areas that might have been marginal. We do not know ... So much of the effort in scaling up a design is in designing the the new airframe and making sure everything has safe margins at the new size.

This is a new airplane design and needs to be approached as such. This process is not so simple as one might think - it will require much of the effort of a new design.

Billski
 

vhhjr

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I don't disagree with any of the well written discusion from Billski. The intent was to put forward the possibility of a starting point other than a clean sheet design. Shurely, designing a whole new aircraft would be a bigger task than modifying, as suggested, an existing one.

It would be interesting to be privy to the design notes from projects that started with reduced scale, manned test aircraft. During WWII the British made several such aircraft and, more recently, Rutan did the same.

Vince Homer
 

Victor Bravo

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But it is a very very very slippery slope for several reasons. Several things about the original WAR airplanes are long known to be less than perfect. So much so that this whole idea started with the idea to fix certain issues.

Then you have the very significant advances that have been made in composite construction, mold-making, composite materials and build processes, the improvements available with the use of CNC, newer engines and redrives, and a better understanding of what aerodynamic and construction parameters yield better flying airplanes.

Then you have the fact that material costs have done a lomcevak, and airplane or even marine quality wood is very possibly not the cheapest way to do the sub-structure any more.

So there is a very real possibility that clinging to the original WAR construction method (from 1975) is holding you back and making things harder and more expensive than they could otherwise be.

Heck, even the old-school moldless hotwire Rutan style construction has been tweaked and streamlined to result in smoother surfaces with drastically less sanding and bondo. Plain old lowbrow foam and glass, with a few hundred feet of pulltrusion in the spar caps and longerons, may give you the fastest and cheapest at this point.
 
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Shurely, designing a whole new aircraft would be a bigger task than modifying, as suggested, an existing one.
This seems logical - at first - until you actually go down this road. I have. All I really wanted to add was folding wings.

What I found is that I ended up starting from scratch to reverse engineer the original design from flight loads so I knew what the original designer (now dead) intended. For the most part* my numbers matched the original hardware/dimension. From there I could then size my modified parts as needed.

By the time you go thru all of this you have done most of the work needed for a clean sheet design. I had my notes from the had calculations and putting them into Excel sheets was fairly easy. Would have been easier if I had better handwriting.

* Surprisingly well for most of the design. The area that has failed in the past, due to poor workmanship, has a FOS of only about 1.1, by my calculations.
 

wsimpso1

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I don't disagree with any of the well written discusion from Billski. The intent was to put forward the possibility of a starting point other than a clean sheet design. Shurely, designing a whole new aircraft would be a bigger task than modifying, as suggested, an existing one.

It would be interesting to be privy to the design notes from projects that started with reduced scale, manned test aircraft. During WWII the British made several such aircraft and, more recently, Rutan did the same.

Vince Homer
Scaled's usual approach to scaled down airplanes was aerodynamic similitude accepting whatever payload restrictions that wrought, then do structures and controls.

No doubt the design progression would be fascinating.

The processes may be short cut to some extent in that you start with a scheme that you know works, but once you start, you still have the absolute imperative to make structures, controls, stability, and w&b work. That is still a bunch of work.

Billski
 

BJC

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I agree with the comments about the the structure and, possibly, the aerodynamics being an all-new design.

However, the control system, electric and or hydraulic systems, instrumentation scheme, canopy attach, firewall forward details etc., likely are good starting points for the different scale aircraft.


BJC
 

wsimpso1

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This seems logical - at first - until you actually go down this road. I have. All I really wanted to add was folding wings.

What I found is that I ended up starting from scratch to reverse engineer the original design from flight loads so I knew what the original designer (now dead) intended. For the most part* my numbers matched the original hardware/dimension. From there I could then size my modified parts as needed.

By the time you go thru all of this you have done most of the work needed for a clean sheet design. I had my notes from the had calculations and putting them into Excel sheets was fairly easy. Would have been easier if I had better handwriting.

* Surprisingly well for most of the design. The area that has failed in the past, due to poor workmanship, has a FOS of only about 1.1, by my calculations.
Which just points out the danger in not doing a downtown job during the redesign. Parts that are overbuilt get more overbuilt and overweight, while parts that are marginal or worse can be easily made dangerous.
 
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Billski:

What design were you wanting to add folding wings to?


BJC
AV-36. The plans do have removable wings as an option but folding with auto connect meant having to go back thru the aileron loads as well......and the attach brackets...........and the switch from cable to push/pull...........
SPT-1 for test-.JPG SPT-1 for test-b.JPG One of the reasons I was asking questions about fatigue cycles a couple of years ago.


I'll probably never be able to build one myself, but a downsized Pelican clone is still possible. Having the numbers, and work notes, from the big sister is making the design of the little brother a little easier.
 
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Victor Bravo

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Vince, sorry to take this further off topic, but Hot Wings the idea I finally used in my sketching (for the new aluminum Pelican I want to do) was to put a piano hinge on the upper wing skin, keeping the folding mechanism completely separate from the spar pins. You would remove a gap cover strip, reach in and pull the forward and rear spar pins, then fold the outer wing panel up and over using the piano hinge. This removes all trailering and folding loads/wear off of the important parts. The weight of the piano hinge is your only penalty, and it can be a light hinge because it doesn't carry any flight loads. You can even pull the hinge pin to disassemble the wing. Automatic aileron connect is very possible with paddles or "cup and ball" fittings on the end ribs at a slight weight penalty.
 

Bill-Higdon

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AV-36. The plans do have removable wings as an option but folding with auto connect meant having to go back thru the aileron loads as well......and the attach brackets...........and the switch from cable to push/pull...........
View attachment 130048 View attachment 130049 One of the reasons I was asking questions about fatigue cycles a couple of years ago.


I'll probably never be able to build one myself, but a downsized Pelican clone is still possible. Having the numbers, and work notes, from the big sister is making the design of the little brother a little easier.
I'd love to see a Pelican clone
 
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