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Old Jul 14, 2002, 12:41 AM
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Chipmunk - Building Project

Hey Jim,

I found this file http://www.fortunecity.com/marina/ma.../chipmunk.html . Is this the Chipmunk we are looking for?

Not to change what we talked about, but what about an Extra 330L or Cap 232? Simple lines and easy to make.
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Old Jul 15, 2002, 01:43 PM
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(After looking closer, I think the Chipmunk will be a good plane.)

I found enough drawings on the net that I can put together some profile drawings. I did do an check on the dimensions at scale. With a 50" wing span the area is 382 sq. in., length is 37" and the top fuselage projection width is 3.5". I haven't done the side profile yet, but it's easy to see with the tall fuselage that the bass wood blocks will not fit in any band saw we have. For that reason, I suggest that we do go ahead and try to make the plug one out of blue foam. It's a form of mold making that I've been wanting to master. I can make a vertical hot-wire cutter that will give us the clearances we will need. I've been wanting to make one anyway.

I did some rough performance numbers too. The printout is at home, but I remember these approx. numbers:

Weight 33 oz
Loading 12.5 oz/sq.ft.
Amps 20-22
70 watts per pound
~160 watts input
run time 3.75 minutes full power on 7 cell CP-1300
run time at 20 watts per pound (min power) 12 minutes.

It looks like a small Hacker or Aveox on a 4-4.4 to 1 gear box and about a 10" prop will be close to right. A 480 or 400 might fly it with lighter batteries and a good bit less performance. The wing weight was set on 50 sq. in / ounce so it will have to have a light built wing. Foam sheeted or built-up will be about the same weight at this size of a wing. With the constant tapper it will work well in foam. We could cut all the wing cores we need in one night. Sheeting will be lightest with 1/32" balsa but would recommend 1/16" for bottom surface because of landing gear. We can work this out latter.

I recommend we do the original DHC-1 and not the Super Chipmunk because of the lack of drawings on the net. I have about 50 pictures downloaded at home to show assorted color schemes. The more I look at them, the more I like this project. The plan form is simple and clean and the plane should fly well as a model at scale without having to fudge on the tail fins if the tail wheel is effective for ground rolls. If the tail wheel is not going to have enough grip, we might have to enlarge the vertical area by 15-20%, as the scale fin is a bit small, though it's because of a long tail moment. Horizontal fin is just right for models.

Access for batteries can be from the top under the canopy or from under the wing. Opening both up for access might leave the fuselage too weak, so we need to decide on which one we want before cutting a plug. I suggest a bottom access with a pin on the leading edge of the wing with one screw to hold the trailing edge to fuselage. We will have to work with this area anyway in order to install wing mounts. And so far as the lay up process, this offers the largest access hole to work with. The canopy area can come down to a "tray" area where a bust and dash can be placed. The canopy can be one of the two styles (framed in flat panels or bubble type) and still stay at scale without changing the fuselage mold. We will have to make a separate nose cowl to stay true to scale. The tail does get a bit tight in space, so we should stop the fuselage mold under under the fixed horizontal fin and block shape the tail cone separately because of the small size.

Gary
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Old Jul 16, 2002, 12:38 AM
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Quote:
I recommend we do the original DHC-1 and not the Super Chipmunk because of the lack of drawings on the net.
I like the Chipmunk but the Super Chipmunk seems like a better subject to me. I know that Shawn has his heart set on it as well.

If we can't find drawings for the Super Chipmunk then I'll be just as happy with the Chipmunk. The primary issue to me is that the Super Chipmunk is an aerobatic plane and the Chipmunk is a trainer. It makes more sense to me if we model the high-performance version of the aircraft.

I'd like to make sure the fuselage will accomodate up to 10 sub-C cells. I hate having to remove the wing to swap out the battery so if we can avoid that I'd like it.

Jim
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Old Jul 16, 2002, 01:39 PM
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My .02. I'm not planning on attending scale comps, so a recognizable scale model is fine with me. So, things like getting the cross section perfect at each station is not one of my requirements.

Gary, sounds like you are looking for more fidelity to scale. I don't have a lot of reference books with civilian stuff, but I'll see if I can find anything.

Maybe somebody has the Goldberg plans we could look at to help with fuselage cross sections and other details.

As far as performance differences between the "super" and "non-super" versions. How much is due to just going to a larger horsepower motor. Maybe a trainer just isn't cool enough for Jim
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Old Jul 16, 2002, 11:36 PM
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The basic fuselage is the same between the DCH-1 and the Supper Chipmunk. The cowls are obviously not the same, but we could just make a second set of cowls easy enough. Tails are not the same either. For what I'm doing right now, it is not an issue about which version we'll be making, except for the airfoil (addressed latter in this post).

I'm not looking for competition grade scale, but I was looking for some eye candy. The time and investment in making a plug warrants more detail with just a bit more planning and work. I think it will be far cry from Scale Masters quality.

I didnít think that top loading the battery would be that important. Understand that Iím not arguing for the bottom access because of scale esthetics. It's just an engineering point of view that suggests the easiest and strongest solution is not to have it. Still, I'm with you all, in that top access is on my "wish list" too. I'll take a bit to consider what it's going to take to make it happen that won't compromise the strength or performance. Can I get some thoughts about this too.

The carbon fiber reinforcements suggested might help if the surfaces were more rounded. Instead, the sides over the wing are almost flat. It would help with stress cracks around openings, but not that much in over all torsion strength of the fuselage. The best I can come up with is to add a ďtrayĒ inside the fuselage about half way down the side. But as mentioned before, this is a top heavy airplane, as it is, and having the batteries high up in the fuselage wonít make things better.

I need feedback to give me practical ideas on how to implement top access. Keep in mind that the widest fuselage section is 3.5" and the scale canopy width is only 2.5" at its max. and about (not measured yet) 6Ē long.

When I looked for 3-views I looked everywhere I could think of. I even checked every picture that Alta Vista had under pictures with the name Chipmunk. Can someone else give it a go? We need clear and usable 3-views of the Supper Chipmunk, especially for the cowl and wing/tail. Top views are more important at this time. I have enough for the DHC-1 to cad the fuselage profiles.

So far as performance, I can't see what the difference would be, unless they were to use different airfoils and power loading. Before you get your hopes up, a change in airfoil upper surface will interfere with the fuselage fit. The fuselage will be set for only one airfoil. I was working on a foil last night. It looks like a 2% camber, 11.5% thick will work at the 12-13 ounce wing loading quite well. It's an efficient airfoil for long flights and a wide speed range, but favors slower flight. The upper surface is based on the S3021. For aerobatics it will be ok, but other less cambered and thicker sections would be needed for all out pattern aerobatics.

This is something else we need to settle on before cutting the fuselage plug. In terms of 1 to 10, tell me what performance you want. (1 being slow, best efficient, long trainer like flying performance and 10 being flying fast, on rails and crisp aerobatics that Chip Hyde would be proud of)

Can I get some feedback on the pictures I e-mailed out. Any ideas whatís going on under the wing at the fuselage. The two surfaces donít seem to meet and I canít find a clear enough picture to tell what's going on.

I can start getting a plug ready when we decide on the canopy yes/no on the access and the type of airfoil section (targeting the performance we want). When we finalize these two items Iíll be quite for a while and give these book-long posts a rest.
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Old Jul 17, 2002, 01:07 PM
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Maybe this link will help

http://www.luftfahrtmuseum.com/htmi/ite/dhc1.htm
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Old Jul 17, 2002, 01:46 PM
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Thanks. This is one of the sites I've been all over before. Did I miss a Supper Chipmunk 3-view at this site?

Gary
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Old Jul 17, 2002, 05:53 PM
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Chris,

That is a cool site. Not to digress too much but I found a side view of an airplane my dad used to have: http://www.luftfahrtmuseum.com/htmi/itf/ha200.htm

Its a little twin engine 2 seater jet plane. He sold them for awhile so he owned a few of them and would take the kids joy riding once in awhile.

Back to the Chipmunk, I think we've established that its a lot easier to do the regular Chipmunk than the Super. So lets go with that.

The airframe is almost too simple for a composite build. Its going to be a lot like your handlaunch plug, Gary.

Jim
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Old Jul 17, 2002, 05:59 PM
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Forgive me for digressing further on that Saetta but it brought back some memories. One of the guys that my dad sold a plane to killed himself on the same day that he picked up the plane.

For fun I went to the NTSB page to do a search and I found the accident report: http://www.ntsb.gov/ntsb/brief.asp?e...12X23065&key=1

Jim
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Old Jul 17, 2002, 06:14 PM
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For the airfoil, I recommend a thick, cambered section. We do not need a fast airplane. I'd like it to be very gentle, actually. My big Spacewalker is one of the nicest flying airplanes I've ever had and its not a speed demon by any means. A nice fat wing and a low wingloading make for a nice combination.

The exact airfoil is not important to me. Left to my own devices I would just use the NACA 2412. But if you want to pick something different, Gary, then go ahead.

I'm really not picky about any of these decisions. I'm more interested in the process of making the plug and the mold.

Jim
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Old Jul 17, 2002, 06:43 PM
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Hi Jim,

The NACA 2412 is a good choice. It's fairly close to what I was working on. I'm glad to see you are not interested in symmetrical cross sections! I'll surprise you with the airfoil . I think you'll like it.

You're right, it is a very simple fuselage. A good starter I think. It's because of the long flat to shallow curves that I don't like the open hatch under the canopy. I'm suggesting that we don't open the canopy area up on the plug and we can cut it open afterwards if it is that important to some one. I'm still waiting to hear Chris chime in on this after my last post.

Are we wanting to work on the plug/mold on the next monthly meeting? Any thoughts about extra meetings to work on this so that it's done sometime in 2002?

Gary
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Old Jul 17, 2002, 06:55 PM
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Quote:
Originally posted by jbourke
For fun I went to the NTSB page to do a search and I found the accident report: http://www.ntsb.gov/ntsb/brief.asp?e...12X23065&key=1

Jim
Ouch! nuf said...
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Old Jul 17, 2002, 09:52 PM
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Quote:
Originally posted by Gary Warner
You're right, it is a very simple fuselage. A good starter I think. It's because of the long flat to shallow curves that I don't like the open hatch under the canopy. I'm suggesting that we don't open the canopy area up on the plug and we can cut it open afterwards if it is that important to some one.
Ok, I agree about the canopy hatch. Lets just drop that idea and concentrate on designing a clever way to quickly remove the wing. I'd like to use a spring-loaded latch. I did that on a model years ago and it worked really well.

Quote:

Are we wanting to work on the plug/mold on the next monthly meeting? Any thoughts about extra meetings to work on this so that it's done sometime in 2002?
That has been discussed but it has to wait until I stop travelling. We can talk it over at the August building session. I'll be done travelling at the end of this month so I think we can have several day-long sessions in August.

Jim
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Old Jul 18, 2002, 05:22 PM
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Some day-long meetings sounds good.

I started the design process for the vertical cutter last night. Wow, when you think of all that needs to be considered, it's going to take a bit to build. A small project I thought - right! It will take a week or two of free time. But when I get done I'll have a quality cutter that will last and perform forever.

How much preparation do you think I should plan on having done before the next meeting? I will have the cutter ready, a blank block or two of foam ready (gluing blocks together) and I'll have the templates made. After typing that all out, I think I just answered my own question. I'll have enough to do for next month.

We'll start with the profile cuts and shaping. We can also setup a coat of glass. I can take the plug from there and on my own add the additional finishes to the plug and get the drop-box cut and ready. Then the next meeting we can be ready to layup the mold. The second side of the mold might as well be done between meetings to. On the third meeting we should be ready to layup a fuselage. Subsequent layups can be done by passing the mold around and or members can come by my house and help with the layup of their fuslage. Before you know it it will be time for the wings and tail feathers.

Also, I was considering making a set of wings now so that the plug can be fitted tighly to the wings and it will make it much easier to add the wing fairings to the plug. I can do this on my own, or we might consider a wing building meeting before working on the fuselage. I was going to do a wood over foam wing.

Short on time right now... No time to spell check - sorry.

Gary
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Old Jul 19, 2002, 03:16 PM
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I had a bit of time today to look at web sites that used foam to make plugs/molds. I thought I had seen it all. Take a look at this Cozy engine cowling that this guy made. Is there no end to the possibilities?

Gary
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