1/18 Scale Blue Box F4U-1A Corsair Modification

[Warbird Kid]

On 4/28/2021 at 2:02 AM, Citadelgrad said:

Hi Jay. I think shapeways is a 3d printing clearinghouse.  Ive never bought from there, but my understanding is it puts buys in touch with printers.

 

Bill

 

Shapeways (as far as I've experienced with them) 3D prints everything on site and in house and then sends it to you. It allows for designers to create profiles, upload designs and make them available to sell. Shapeways takes a percentage off the top since they are handling the production and shipping of the product, and the designer / seller receives whatever percentage they want ontop of it. Same process as my Warbird Wear shirt business on Redbubble.com. The other nice thing is they offer alot of different options as far as material goes. I bought a set of these wheels when they first came out. They're still in the bag untouched but one day I'll get around to them. 

 

On 4/27/2021 at 6:26 PM, JayW said:

What???   It's a rare day where I find 1/18 scale aftermarket stuff!  And Corsair wheels to boot!  Do you think these are 3D print?  He got the inside wheel halves wrong, but the outer halves look pretty good.  And the tires look great!

Jay, yes indeed it is 3D printed. But if you buy the more expensive material, it doesn't look 3D printed at all!

[JayW]

I have put in many many hours on Rhino developing the digital models for the main landing gear tires and wheels, taking advantage of the 3-month free trial I have.  That is why you see no progress on the cockpit.  And this will be my first foray into 3D printed parts.  For those of you who know a thing or two about 3D printing, I would like to get your comments and suggestions.  And also comments on the weighted tire:

 

 

First the tire:

 

 

 

What you see is a giant effort by a Rhino neophyte trying to model a tire that has a good diamond tread, and a good weighted bulge.  Cannot tell you how often I had to redo this to get it where it is now. 

 

Importantly, I would like opinions as to if the flat and the bulge are too much.  Profile drawings of the F4U show even more flat than what you see - typically they show a max gross weight aircraft with the right tire pressure (that writing along the ground line says "20% tire deflection"):

 

 

What I did to create the bulge was to first decide how flat to make the tire.  Mine is a bit less than what you see in the picture above (mine is 10% tire deflection).  Then I measured the length of the curve defining the unweighted tire cross section, and create a flattened shape that had the SAME CURVE LENGTH.  After all, a tire's cross section does not magically shrink or expand (given a certain tire pressure) just because it has weight on it.  I did this in a couple of places along the flat, and then had the program loft a shape.  Anyway that was my thinking - it forces the bulge.

 

But if any of you think it's just too wide, tell me and I will consider reshaping it.  I'm getting good at it!

 

Now the wheel halves:

 

 

Here is the front (the part I have been trying to machine on the lathe):

 

 

I am going to try to learn more about filleting, to round off some of those edges....otherwise it's ready to go.  I do not foresee issues with it.

 

The back halve, with all the innards, is more of a concern:

 

 

 

Again, I will try to fillet edges and corners if I can.  But my concern is the complexity compared to its size.  The outer diameter of the wheel is 1.00 inch, depth is about .5 inch, and the thickness of the internal ribbing is 0.015 inch.  Is this too much to ask of a good 3D print machine?  Did I get carried away?  If it is, then I can make the ribs myself and glue them in.  I have done that on my other 1/18 efforts with somewhat satisfactory results.  Comments please!

 

I am also concerned about the cost - these are going to be quantity of two or three only.  I am also concerned about accuracy.  I don't know much about how accurate the final part is.  Does it shrink?  Is the fidelity enough to well represent all that detail?  Will I see ridges?  I modelled a slight clearance fit between the tire and the wheel (a few thousandths of an inch) so it will hopefully slip in without a clash.  But is it enough....

 

I still have much to learn - what model type do I convert these models into, that printers deal with.  What material should I use.  Also, how to provide the data.  The file size for this thing is pretty big (currently 21 meg), and the fillets will add more.  Should I have separate models for each part (that would drop down the file size).

 

Thanks for looking in.   

 

 

Edited May 3, 2021 by JayW

[Ironwing]

Jay

 

This looks terrific and you're clearly becoming a pro at it. You're tempting me to give Rhino a go. I've got some crazy wheel pants to make. This might be the way to go. Or you could start a business doing this stuff for us!

Keep up the great work..looking forward to more. 

Regards

Geoff

 

[LSP_Kevin]

Is there nothing you can't do? Fantastic work, Jay! On the quiet, though, I reckon the bulge in the tyres is a bit extreme.

 

Kev

[bdthoresen]

Nice work Jay. I do agree with Kev, the bulge looks a bit much. I would reduce its width overall by about 50 percent each side, that way it’s less pronounced. I have always felt that tire bulge in scale doesn’t always translate right. It always seems that the tires are flat. But I would leave the flat spot as is. FWIW....

 

THOR    

[LSP_Ron]

Looks good,

I agree that the bulge is to much. I'd suggest no bulge at all, just a flat contact spot.

 

 

 

[RLWP]

9 hours ago, LSP_Ron said:

Looks good,

I agree that the bulge is to much. I'd suggest no bulge at all, just a flat contact spot.

 

 

 

 

 

 

Or provide a model of a man with a foot-pump

 

Richard

[dodgem37]

Nice renderings.

 

Sincerely,

Mark

[JayW]

Thanks all for comments on the tires and wheels!  After brooding over the size of the tire bulge, I decided to decrease the width of the bulge from .58 inch to .53 inch.  Unweighted tire width is .486 inch, so that is a substantial reduction. 

 

So I did some math for anyone interested (you know - a sanity check).   The bulge shape you see defines a footprint that is a near ellipse with a major radius of about .28 inch and a minor radius of .165 inch.   Ellipse area is major R x minor R x pi = .1454 sq in.  Full size would be .1454 x 18 x 18 = 47 sq in.  The Erection and Maintenance Manual specifies 90 PSI for main gear tires on land based aircraft.  So each tire thus weighted would have a load of 47 x 90 = 4232 lbs.  But there are two main gear tires - total load on the main gear would be 4232 x 2 = 8464 lbs.  Well now - empty weight of a F4U is 9000 lbs.  If you allow for a certain percentage of the aircraft weight to be taken by the tail wheel, then this is somewhat believable.  However, a fully loaded aircraft, by these calculations, would drive a larger flat and wider bulge.   

 

I will admit though that tire deformation under load is something I don't know alot about, so I am going to go with my highly experienced friends on LSP, who just about universally wanted the bulge decreased (alot).  How's this:

 

    

 

I am pretty sure that is going to look just fine!  Better than the previous effort.  Thanks for your valuable input.  Stay tuned!

Edited May 4, 2021 by JayW

[Landrotten Highlander]

I think this looks a lot better.  Now the impression of a flat tire has almost completely gone - if I were to see a bulge likethaton my car tires I would still feel safe to drive to the forecourt to check the pressure.  Before I would have reachedfor a foot pump

[RLWP]

Personally, I think that tyre is still flat. Look at the size of the contact area - it's huge!

 

This 'tyre bulge' thing seems to belong in the chronic AMS category, in reality it's pretty small as shown above:

 

 

Richard

[brahman104]

Brilliant and incredible result of your introduction to Rhino! I've been using this for upwards of 6 years now and still don't even know how to do a basic render...... you've greatly surpassed me in just the time you've been "dabbling."

 

Looks like you're going to go very far indeed with this CAD stuff.

 

Craig

[JayW]

7 hours ago, RLWP said:

Personally, I think that tyre is still flat. Look at the size of the contact area - it's huge!

 

This 'tyre bulge' thing seems to belong in the chronic AMS category, in reality it's pretty small as shown above:

 

 

Richard

It is my opinion the tire shown is overinflated, and the aircraft is at or near empty weight.  But who knows....

 

I did an experiment with tire bulge and my own car (a Honda Fit 2017).  I wanted to see if the tire contact area and the tire pressure corresponded with the curb weight of the car.   The tire contact area was tough to measure, but I got two pieces of paper and crammed them under the tires on either side of each tire in both directions and measured between the edges.  Then I assumed the patch was elliptical and calculated areas for the front pair, and the back pair (the backs have a smaller patch).  At 33 PSI, my calculations for total weight were fairly near the advertised curb weight of the car.  That told me that for the Corsair model tires, I have to have a contact patch that corresponds to the aircraft weight in real life.

 

Also I attempted to measure the "percent tire deflection" similar to what you see here on the top general arrangement drawing for the F4U-1A:

 

  

 

It says "ground line for static load IW & 20% tire deflection.."  Now that looks like a flat tire!  Here is a front view, just for kicks:

 

 

I think the draftsman really faked the shape of the bulge, but clearly the intent is to represent a bulge - a pretty big one.

 

Percent tire deflection for the Fit is about 7% (more on the front, less on the back).  Bulge width is about 110% of the basic tire width.  This is of course for a radial tire as opposed to a bias ply tire. 

 

There are things I know and things I don't know.  I know the empty weight of the aircraft (9K lbs), and I know the max gross weight (approx 14K lbs).  I know ground operation tire pressure is supposed to be 90 psi, as opposed to deck operation (112 psi).   And I know that the sum total of the tire contact area times tire pressure needs to be approximately if not exactly equal to the weight of the vehicle.  I do not know the details of the shape of the bulge.  The F4U tire was not doubt a bias ply tire which will have the same contact area, but probably a different shape as for a radial tire.  And I don't know what percent of the aircraft weight is on the tail wheel (it will be pretty small though).

 

Lastly I know that the cross section of a weighted tire - a wireframe curve of its outer surface, and that of the unweighted tire, must be about the same length.  Different shape, but same length.

 

So whatever I do with this tire, I am going to give it a contact area that the math suggests, or close to.  There are an infinite number of shapes that will do the trick; I just have to pick one.  Which I did.  To be perfectly honest, it's a little shy on contact area.  It ought to be a bit larger!  The tire shape I have right now corresponds to an empty airplane with at least 90 psi pressure.  That is 10% tire deflection and a 109% bulge width.   

[RLWP]

Contact patch looks pretty small to me:

 

 

Of course, that's just landed so it's light, even so

 

Richard

[RLWP]

The thing is, having a big deflection in the tyre is incredibly inefficient and absorbs lots of power. That's very undesirable, especially when you need lots of power - like at take off

 

You can confirm this if you want. Let one of your car tyres down until it has the exaggerated bulge, let the parking brake off, then try pushing it - it's hard work. With the tyres properly inflated, it's pretty easy. if you drive the car a bit, you'll find the tyre gets hot just through the work done in compressing the sidewalls

 

I'm off to google Vought F4U tyre inflation pressures... 

 

Richard