Chitty Chitty Bang Bang!

[altbob]

I'm getting a little better at the whole "groups" and "component" thing..by far my biggest problem are edges that refuse to form into faces, or faces that suddenly disappear when I am editing edges where the polygon looks closed yet somehow isn't. It's frustrating as all get out that Sketchup can't give better info/feedback to help users locate and correct these problems. Maybe the Pro version has some of this funtionality now. I'm just using Sketchup 8, which is the free version that everyone uses, and had to resort to redrawing a few parts to get around this and not go insane...

So back to Chitty, I now have all the parts done on and placed flat on two virtual foam boards. As you can see in the attached photo, I've cut out sections of the chassis to make the model as light as possible (especially the back half), cut a rudder out of the rear/trunk of the car, reduced the size of the front canard, made tabs to hold the canard in place which will allow me to experiment with its incidence (I'll try this both flat and with a 1 degree up angle if I need more lift in front...), and added enlarged Elevons to the rear stabilizer with tabs to hold that in place and a cut-out for the rudder to move. The wings are designed to have 5 degrees of dihedral. With their sweep, this should actually be the rough equivalent of 6 percent, which I hope will make it stable in roll. More dihedral than that and I think I'm asking for trouble. I'm not completely sure how to attach the wings at this point, but I'm planning on inserting them together in the airfoil-shaped slot on the chassis and using a bit of stiff, V-shaped wire on the final model to hold them up at the 5 degree angle. Tabs will probably be added at some point. I also imagine I will need some type of formers to hold the canard and rear stab at exactly 90 degrees. I need to do some research on how these foam board models are put together. This is my first attempt...

Having done all of the above, I'm going to make another proof of concept model now, resizing Chitty to work with a tiny pusher prop, namely the PowerUp Electric Paper Airplane Conversion kit:

http://www.amazon.com/PowerUp-Elect...81531&sr=8-2&keywords=power+up+paper+airplane

A 6" long Chitty model should put the balance point of the PowerUp right at the top curve of Chitty's wing, which I think is pretty close to where the CG should go. We shall see. Given the very unconventional shape of this airfoil, that's a guess as to where the CG should be.

I probably won't get time to actually cut paper and assemble all this until the weekend, so if someone out there in FliteTest land can help me more accurately figure out where the CG should go on Chitty's wing, you would have my everlasting appreciation! FWIW, here's the formula: MAC = rc x 2/3 x (( 1 + t + t2 ) ÷ ( 1 + t )) - Good luck with that!

Seriously, between the front taper, the rear sweep and the bat-like trailing edges, finding the mean aerodynamic chord on this wing is perhaps one of the biggest challenges for me at this point. Any help would be appreciated! I'm happy to email the Sketchup file of the wing components to anyone (or any other parts for that matter). Thanks!

 

[rcspaceflight]

The easiest way to figure out CG is to take your test model and toss it. Adjust the CG, then toss it again. I made an RC X-Wing (from Star Wars) and I had to do that, but I used the first version of it and I ended up ruining the plane by the time I got the CG correct.

The easiest way to attach your wings would be to make a small spar that has the dihedral cut into it. Make the spar just big enough to go through the fuselage and have a gluing point to hold the wings in place. If you're making the fuselage just one sheet of foam board thick, adding a slot for the wings will be useless. Keep the slot on the plans so you can draw guide lines onto the plane, but don't actually cut it out.

 

[altbob]

Can you point me to a foamboard plane with a dihedral spar on a single sheet undercambered wing so I can take a look at what you're describing? I am concerned with wreaking the airflow under the wing and creating unwanted turbulence. Thanks!

 

[rcspaceflight]

I was thinking of a spar similar to the FT Spitfire spar. Except that it would only be 4 inches long or so. It probably would create air turbulence, but you want to keep it super short it keep that to a minimal. It would only serve as a guide for installation and add a little bracing to the wings. You only need about an inch of length under each wing. That little bit of bracing could make all the difference for how well the wings stay on.

Or you could make some sort of brace that is removable. So after the wings are glued on with the proper dihedral, you could just remove the brace.

It was just a thought.

 

[altbob]

I checked out the FT Spitfire spar design and I don't think it will work well for a 2D body design, but I think I can modify it somewhat. I like the use of cheap bamboo BBQ skewers for spars as an alternative to wire or CF rods, if cost is an issue. I think the way to go is to laminate a couple extra foam sections under the wing so the spars have something to stick into and hold in place at the correct angle. I'll work on this a big in Sketchup and see what I can come up with. The added weight under the wing at the bottom edge of the chassis/fuse will actually be a good thing for roll stability, I think, given how high the vertical CG is on the body (indeed without adding a few strips of paper to my proof of concept model it wants to fly inverted).

 

[altbob]

Hi everyone!

It's been a busy couple of days for me, so I am just finally able to update this log today. And I'm happy to report that having built a 1/4 scale model of my 2D Chitty design from the Sketchup plan, I had a very successful series of test flights!

The aim of the testing was to verify the placement of the CG and the shape of the airfoil. I really am very encouraged now that a full sized (2.5 foot) model will fly at a reasonably leisurely/scale speed and look great in the air.

I have just a few refinements in mind, mainly to make it easier to assemble the wing with the proper dihedral and make the connection of the wing to the body stronger. The model was so stable in roll that I also think I will abandon plans at this point for elevons and just go with 3 channel control.

Here's a video I made of the flight testing for your enjoyment:

https://www.youtube.com/watch?v=_jWZXQDiVOE&feature=youtu.be

I'm thinking a 250 sized engine will give this plenty of power.

The CG is so far forward (as you can see in the photos, I needed a paper clip added before I got it on the red line) I am really thinking that a long shaft between the engine and pusher prop will be necessary. Does that rule out a brushless outrunner? Or is there a way to attach a long prop shaft to the motor, perhaps with heat shrink tubing?

I'm attaching photos of the build if anyone would like to build along with me - your comments and encouragement are always welcome!

 

[altbob]

Just a quick update - making some wing ribs now that I've figured out how to use the line intersect tool in Sketchup, so that getting the wings on in the proper airfoil shape and dihedral won't be such a headache when working with a hot glue gun. There will be a carbon spar on either wing, but having the wing root section five layers of foam board thick will help give the spar something solid to grab onto. Also possible that these spars could be balsa (or something harder) if the foam board proves to be too flimsy.

If this all proves to be too much of a pain, I will simplify further with a Kfm-1 or Kfm-2 airfoil, but I don't think I can test those with small paper models the way I did in the video posted earlier this week, and I think the cambered wing looks more classic early 1900's "chitty"...

Anyway, I would still love to hear some feedback, and ideas about how to lengthen the motor shaft so the motor can go up front. Might not be possible, but worth considering. Thanks for watching!

 

[altbob]

Oh, and before I leave you all for the weekend, my wife thought these screen captures from the test flights had to be shared. Weeee!

 

[altbob]

Carbon rods arrived in the mail yesterday and now I am just waiting on some time to get to work on the next prototype model.

FYI, The motor I am going to try first is a Turnigy AX-2203C 1400KV/60W Brushless Outrunner, simply chosen because of it's extremely low weight and small profile. It hasn't arrived from HobbyKing yet, but I couldn't resist playing with it virtually in Sketchup (yet another couple doodles attached). The Turnigy AX-2203C can spin a 7 x 3.8 prop, which will just fit in the space available.

Given the pusher prop and the fact that the rudder and elevator on this model are not directly in the prop wash, my sense is that this will make the model turn in a very, very wide arc. Actually I am not that worried about the elevator...but the rudder is a big concern, being actually in front of the prop.

I don't care about the model not being capable of acrobatics, but it does need to turn around withing about a space of 100 feet.

The option of adding a second rudder in the front, by way of making the front wheel turn, is still very much, therefore, on my mind. So much so that I drew a doodle of it yesterday (attached)...

 

[altbob]

On, and just in case you missed the video of the 1/4 scale prototype, here it is:

 

[altbob]

Started the build and just wanted to post a quick update. So far I am finding the sketchup plans printed with PosterRazor (after saving to PNG, and then converting to JPEG) to be hard to use because they are not vector graphics and full of jagged low-res edges (I will use an SVG printout in the future), and I've dulled quite a few x-acto blades cutting the foam (my self healing build board is packed somewhere and I can't find it!).

Also Elmer's foam board is proving to be a lot heavier than I imagined it would be. I may strip off some of the paper in places where structural stiffness isn't critical. I can't find any "Dollar Store" foam board around here, which I hear is lighter. If all else fails, I may just buy some Depron online and use that for the final version. But I don't want to spend a lot on a test model that is likely going to have the crap kicked out of it.

One cool discovery I thought I'd pass along - I found these fiberglass shaft "youth arrows" at Dick's Sporting goods - three for $9.99! I used a hacksaw to cut off the point and bowstring nook, and shaved off the feathers (made of soft rubber actually) and voilà! Cheap and light structural longeron for the fuselage! Not as light as carbon fiber, but $3 each for 1/4" dia. round shafts 30" long is a great deal, I think. Hopefully someone will find this useful.

 

[rcspaceflight]

I can't find any "Dollar Store" foam board around here, which I hear is lighter. If all else fails, I may just buy some Depron online and use that for the final version. But I don't want to spend a lot on a test model that is likely going to have the crap kicked out of it.

I never ordered DTFB straight to my door, but it's possible and is certainly an option. http://www.dollartree.com/office-sc...rds/610c567c567p16450/index.pro?method=search

 

[altbob]

Thanks RCSpace - I would, but I found that stripping most of the paper off to save weight while leaving it on in certain key areas for strength is a nice solution and AC Moore sells a slightly thinner Elmer's foam board for only $2.50 a sheet, which is close enough, especially when you avoid shipping charges.

I didn't stop to take photos during the build, but basically everything went together as expected from the Sketchup plans. I'm trying to keep the "all up weight" under 300 grams. I managed to strip about 80 grams off the airframe between the cut-outs that you see in these photos, and by peeling off paper. I left the blue paper on the front for reinforcement around the engine firewall (in the rear), the back wheel (which will be hitting the ground) and forward of the CG (where I want some crash resistance anyway). As for the red paper covering, I just couldn't resist leaving those stripes in place - it just looked so p-o-s-h, Posh!

The fiberglass arrow shaft also weighs more than a carbon rod, so in the final version I will strip a little weight off that way as well.

There is a "block" 5 foam boards thick under the wing to hold the CF spars in place. A 1/8" dia. brass rod is bent to a 16 degree angle to hold the spars in place. The rod is 2" long. Aluminum might have saved a couple grams there, but I couldn't find my aluminum stock over the weekend (it turned up in a box last night - so hooray for that!). The resulting 8 degree dihedral just looks better to me than the planned 5 degree dihedral, given how low the wings sit and the potential roll instability from the high mounted pusher motor - I am just guessing at this stuff, but I might as well take the best guesses I can on the first outing, right?

Some stats:

All up weight - 300g (...best guess, assuming a battery of around 50g); wing area 233 sq.in (15 dm); wing loading 6.55 oz/sq.ft; lift coefficient of 1 or 1.5 (...hard to say, as this wing is "Truly" sui generis!).

Assuming the above measurements and guesses are in the ballpark, I estimate a stall speed of between 10 and 12 mph. If so, the static pitch speed I need for a gentle flyer will be somewhere between 26 and 32. A lot of assumptions, I know, but based on these assumptions I ordered a slightly bigger motor than the one I was planning to use. Now waiting for a Turnigy AP19, 1580kv to arrive at my door. Supposedly that will have enough thrust for a model up to 350g, not that I believe it, but worth a try!

All calculations aside, I took the model and tossed it around the living room last night and I can report that it floats straight and level more or less the same way the 7.5" paper model did, so onward and upward we must quest, yes yes!

Comments welcome

 

[SnowRocker88]

Can't wait to see it in the air (with appropriate background music, of course). Lol

 

[altbob]

Given how eager YouTube seems to be to hit people with takedown notices for using music, you might have to settle for me humming the theme song under my breath

 

[SnowRocker88]

You could just set it as your ringtone and have someone coincidentally call you...

 

[altbob]

That's friggin brilliant! I'm totally going to do that!

Meanwhile, here's a video I made with "canned" no-rights music, showing the test flighting of a 1/4 scale paper model, before I decided to go ahead with the "full size" foam board RC version:

 

[SnowRocker88]

Haha, that thing flies pretty darn sweet! Looking forward to the first full scale!

 

[altbob]

I've got my model down to 260 grams with everything but the battery added (installed servos, motor, firewall, prop, esc, rx, pushrods, etc.) but unfortunately because of the pusher prop configuration I can't get the CG right without adding at least another 100 grams of weight to the nose (used a cup of water to test this) - resulting in a much heavier battery, and a much heavier model, than what I was aiming for. (no point just putting lead weights in the nose, is there?)

It may still fly slow if I fly it at the edge of a stall, so I may as well test it and crash it a few times at this point. My best guess is that I will have to fly this Chitty model fast to stay airborne.

With an AUW of 370+ grams including the battery, the Turnigy AP19 motor will almost certainly have to be used. Which will mean adding even more nose weight to maintain the CG where I need it - figure a 110-120g battery at the nose will be needed.

I feel like I am going back and forth on a see-saw here, trying to balance the front and back...

I believe I could rebuild the airframe using less glue and a carbon rod instead of a fiberglass rod, and maybe strip 20-50 grams off this thing...but I'll wait on that until I've flown it and crashed it a few times. No sense going back to square one now, before getting some actual data/experience flying this. But first, I will have to wait for it to dry out around here (sloppy, slushy snow fell last night) and I need to order a 2S battery weighing about 115g. Probably have to wait another week to 10 days for that to arrive, which will give me time to solder all the wires together.

I hope to get some video of the test flight, so I will need a buddy to join me at the flying field...

 

[rcspaceflight]

What is the wing cube loading for it?