Scratch and Kit Built Aircraft Discuss and share your scratch built or kit built aircraft as well as building techniques, methods, mediums and resources.

My first "real" scratch build

Old 03-01-2015, 04:01 PM
  #1  
Bald Paul
Member
Thread Starter
 
Bald Paul's Avatar
 
Join Date: Mar 2011
Location: Boiling Springs, SC
Posts: 547
Default My first "real" scratch build

After building several available kits, doing a bit of "kit bashing" (Sig MidStar 40 conversion to twin electric power), and building from published plans (redrawn in CAD and laser cut for accuracy), I've decided to jump into the deep end, so to speak, and try my hand at designing my own plane.

Not being an aerodynamics genius, I'm using a set of 3-views of an Avro 625 Avian monoplane (as seen in Flying Models - July 2000 issue) as a basis for general layout. However, my design will be shoulder wing vs. low wing, and a simple box-style fuselage for simplicity, both in design and construction. I plan on incorporating something I've been thinking about for awhile in the design - a modular wing mounting system, that allows different wing designs (flat bottom, semi-symmetrical, symmetrical) with varying amounts of dihedral to change the flight characteristics of the model, without the need to re-design (and build) a fuselage to match the wing shape.

There are two rather important questions I'm still unsure of, though:

1) how do you estimate the weight of a model design that has yet to be built? The size of the motor, battery, and ESC have to be determined. I know construction and design will make a difference in the finished weight, and plan on building as light as possible while maintaining integrity.

2) how do you determine proper CG? I'd hate to go through all the design and build process, only to end up tail heavy and crash on the maiden flight. (The CG is not noted on the 3-views)

Some design goals - 48" wingspan with 8 1/4" constant chord, fuse length of 34". Aiming for somewhere around a 450 watt motor for power.

Thoughts? Am I on the right path, or about to walk off a cliff?
Bald Paul is offline  
Old 03-01-2015, 08:43 PM
  #2  
Abuelo
Member
 
Abuelo's Avatar
 
Join Date: Sep 2013
Location: Arizona desert
Posts: 311
Default

Hi Paul,

Good and valid questions.

To estimate your target weight, figure the wing loading and then the cubic wing loading. Multiply the wingspan by the chord. i.e. 48 * 8.25 = 396 sq inches (2.75 sq feet). Make an assumption on what wing loading you are after by the type of flying the plane will do, and multiply by the sq inches. In this size, a 12 - 16 oz wing loading will work, so 12 * 2.75 = 33 oz to 16 * 2.75 = 44 oz. These numbers are your target.

The cubic wing loading is a dimensionless number for comparing flight characteristics across various aircraft sizes.

This site will do the calculations for you plus there is a chart that broadly shows what to expect for the number generated. http://www.ef-uk.net/data/wcl.htm

The C/G will fall somewhere between 25% and 33% of the chord, probably around 28%-30% but exactly where will have to be determined by flight testing. Start at the 25% point (2" back from the leading edge) and gradually move the C/G back. As stated many times on this site, aircraft with a forward C/G fly poorly, with rearward C/G fly once.

Take a look at the Headsup R/C site for motor info and weights. They test and document what they sell and provide the data. The 450 watt motor could very well provide vertical performance with this size plane, depending on the prop. Remember that a too large motor can be throttled down for lower thrust but that does not reduce the weight, which might not be a problem in the larger planes but is in the smaller and lighter ones. http://www.headsuphobby.com/main.sc

There are some various guidelines stated occasionally, like the power system should be one half of the empty plane weight or one half of the total weight, etc. OK to keep them in mind but they are only guidelines.

You might want to check the ongoing and occasional build thread for the Cloud Kitten, a simple design almost the same size (48" span, 34" length) as what you are contemplating. Mine is coming in at under 32 oz, the upper limit for park flyers. http://www.wattflyer.com/forums/showthread.php?t=74762

Keep us posted on your progress, and photos are always appreciated.

Bill
Abuelo is offline  
Old 03-01-2015, 09:09 PM
  #3  
Bald Paul
Member
Thread Starter
 
Bald Paul's Avatar
 
Join Date: Mar 2011
Location: Boiling Springs, SC
Posts: 547
Default

Bill, thank you for the info. Good and useful stuff.

I usually go with Heads Up RC for my motors, batteries, and ESC, and have already done some preliminary research. Looking at the Emax BL2815/09 1000kv 450 watt motor with a 45 amp ESC and 11.1v 2200mah 50C battery.
With a 10X5 prop, it should do the job.

Funny, but my wife and I were looking at a move to South Carolina at first, but we're now focusing on Arizona instead (after the winter we've been through this season, we want to retire somewhere with zero chance of snow!) Maybe we'll bump into each other at a field somewhere once we get established in the new home.

This plane will be CAD designed. If all works well (meaning, things fit together as planned and it actually does fly) I'll make the files available for anyone who would like to have a short kit cut for themselves. I hope to create a build log as well. Right now, the house here is on the market, so nothing is on the building board. I don't think a partially built wing would survive the move very well.
Bald Paul is offline  
Old 03-01-2015, 10:55 PM
  #4  
quorneng
Super Contributor
 
quorneng's Avatar
 
Join Date: Jul 2008
Location: Cheshire, UK
Posts: 1,846
Default

Bald Paul
I suppose in reality nobody just sits down and designs a plane just from first principles. Invariably it is a progression from previous experience - or even somebody else's!
There is nothing wrong with starting from (copying?) a successful design!

In some respects scratch building for electric power has one great advantage. You have a pretty heavy lump of a battery whose location is not critical to its performance so just design and build in such a way that the battery can be positioned to achieve a 'safe' CofG after the rest of the plane is more or less complete.

Unless you are designing for an extreme of performance the detail of a planes aerodynamics is not that critical. Just look at what can be achieved with flat plate or KF aerofoils on models that simply do not work at all well at full size.

Unless you try something pretty bizarre it should fly. The clever bit is getting it to fly the way you intended it to rather than the way it does!

I will follow wit interest.
quorneng is offline  
Old 03-02-2015, 02:23 AM
  #5  
Bill G
Super Contrubutor
 
Bill G's Avatar
 
Join Date: Oct 2005
Location: West Central PA
Posts: 4,422
Default

The Avian's an interesting subject. I first saw it in a model magazine also. Not too long ago outerzone posted a plan for it. It's one of those one the forever list, with some enlargement from 24". 200% would be your 48" size.
http://www.outerzone.co.uk/plan_details.asp?ID=2922
Bill G is offline  
Old 03-02-2015, 12:09 PM
  #6  
Bald Paul
Member
Thread Starter
 
Bald Paul's Avatar
 
Join Date: Mar 2011
Location: Boiling Springs, SC
Posts: 547
Default

Originally Posted by quorneng View Post
Bald Paul
I suppose in reality nobody just sits down and designs a plane just from first principles. Invariably it is a progression from previous experience - or even somebody else's!
There is nothing wrong with starting from (copying?) a successful design!
You got me thinking. I will freely admit that I am a novice CAD user. My biggest difficulty in the past has been designing (or copying) wing ribs. I had been considering purchasing the Profili program, but only the Pro version allows exporting of files to CAD formats. I was having a hard time justifying the cost of the program to myself, just for the sake of being able to design (at least for now) one wing, just to see if the thing flys.

Started poking around a bit, and found that the Sig Four-Star 20 EP just happens to have a 48" wingspan and a wing area of 430 sq in, about the size I need. And, Sig will sell a wing kit for far less than the cost of Profili. I think it would be much simpler for me to design a modular mount for the wing, than to design a wing to fit the modular mount.

Scratch one problem off the list!
Bald Paul is offline  
Old 03-02-2015, 03:02 PM
  #7  
fhhuber
Super Contributor
 
fhhuber's Avatar
 
Join Date: Mar 2013
Posts: 4,786
Default

CG is always based on wing and horizontal stabilizer configuration.

Conventional (main wing in front and far smaller horizontal stabilizer well separated) the CG should be 25% to 35% of MAC. Generally safe to do the test flight at 20% to 30% then slowly move it back. Occasionally you can push the CG back as far as 45%.

Flying wing CG should be at 15% to 25% and ALWAYS do the test flight at 15% then adjust slowly
Consider planes with small horizontal stab or with the horizontal stab being very close to the main wing as if they were flying wings.
The plane in my profile pic needs CG more like a flying wing than a conventional aircraft. 20% to 25%... further back is very unstable.

Canard (horizontal stab in front) you "balance" the CG between the lifting surfaces 30% MAC. You want equal weight per sq inch area being supported on the foreplane (canard surface or horizontal stab) and the main wing.

**********

Weight target should be similar to the weight as if it was a single engine. "Stall speed" (minimum airspeed for sustained level flight) is based on wing loading.

You can get similar performance on slightly lower power with a twin (or more engines) than a single engine aircraft due to an efficiency factor of the props. But you should still aim for about the same power:weight ... 30 watt/lb will fly. 75 watt/lb for trainers and mild sport non-aerobatic. 100-150 watt/lb (or more) for sport aerobatic. 200 watt/lb or more for 3D.

Just add up the watts of all of the motors....
fhhuber is offline  
Old 03-02-2015, 03:53 PM
  #8  
quorneng
Super Contributor
 
quorneng's Avatar
 
Join Date: Jul 2008
Location: Cheshire, UK
Posts: 1,846
Default

Bald Paul
A tip. A CAD package is not the only way to produce wing rib profiles.
This set are accurately reproduced from a drawn NACA 63418 wing profile I found on the web.
Click image for larger version

Name:	OuterRibs1.jpg
Views:	198
Size:	233.1 KB
ID:	179725
They are correctly sized and also progressively reduce from 18% thickness at the root to 13% at the tip using nothing more than the 'select, copy, resize, paste' options of Microsoft "Paint" package which is free within Windows.
quorneng is offline  
Old 05-15-2015, 07:53 PM
  #9  
Bald Paul
Member
Thread Starter
 
Bald Paul's Avatar
 
Join Date: Mar 2011
Location: Boiling Springs, SC
Posts: 547
Default

I've been slowly teaching myself the needed CAD skills to get some plans started. Using the three view drawings of the Avian, I came up with a basic plan: fuselage length, wingspan, wing area, etc. I went with a shoulder wing design, and a taildragger. It's still a work in progress, but it looks encouraging so far. At least to me it does!
Attached Thumbnails Click image for larger version

Name:	Capture.JPG
Views:	216
Size:	40.8 KB
ID:	180767   Click image for larger version

Name:	Capture2.JPG
Views:	209
Size:	73.5 KB
ID:	180796  

Last edited by Bald Paul; 05-18-2015 at 08:44 PM.
Bald Paul is offline  
Old 05-15-2015, 10:54 PM
  #10  
mclarkson
Super Contributor
 
mclarkson's Avatar
 
Join Date: May 2010
Location: Wichita, KS
Posts: 3,958
Default

Way to go, Sir. I gotta say, though, that CAD skills are most definitely not needed for scratch building. (I actually have some, but rarely use them.)
mclarkson is offline  
Old 05-16-2015, 01:35 AM
  #11  
Bald Paul
Member
Thread Starter
 
Bald Paul's Avatar
 
Join Date: Mar 2011
Location: Boiling Springs, SC
Posts: 547
Default

Originally Posted by mclarkson View Post
Way to go, Sir. I gotta say, though, that CAD skills are most definitely not needed for scratch building. (I actually have some, but rarely use them.)
Thank you! CAD has always intrigued me. I guess in a way I'm designing the plane in CAD as a good excuse to force myself to learn to use the program.

In the past I have scratch built. (I admit, it was long ago - a C/L model) No matter how carefully I traced, cut, made templates, or sanded, having a part (or multiples of a particular part) laser cut from a CAD drawing is going to be more accurate 99.9% of the time for me. I just end up with a better, straighter airframe.

As a plus, if it flies well, and others would like to build one (or I end up trying to fly 5 feet below airfield level), it's easy to have another short kit cut.
Bald Paul is offline  
Old 05-16-2015, 02:44 AM
  #12  
mclarkson
Super Contributor
 
mclarkson's Avatar
 
Join Date: May 2010
Location: Wichita, KS
Posts: 3,958
Default

No doubt. Not dissing CAD skills or their utility at all. Especially if you have access to a laser cutter, 3D printer, or etc.
mclarkson is offline  
Old 05-20-2015, 02:37 PM
  #13  
Bald Paul
Member
Thread Starter
 
Bald Paul's Avatar
 
Join Date: Mar 2011
Location: Boiling Springs, SC
Posts: 547
Default

Progress made. All fuselage parts have been designed. I thought it might be prudent to build a 3D model out of cheap Dollar Store foamboard, just to help find any glaring errors I may have made.

Happy to say there were no glaring errors! I did pick up a few minor ones. I forgot to allow a cutout for the tailwheel mount. The servo tray cutouts for the servos were too far back, and the wing mount block would have prevented access to the rear mounting screws. The horizontal stab mount block had one corner that was rounded, instead of squared off. All issues easily corrected. I think I can confidently get the files layed out for the laser cutter. Then, on to the wing!
Attached Thumbnails Click image for larger version

Name:	[dateiso].jpg
Views:	232
Size:	193.8 KB
ID:	180818  
Bald Paul is offline  
Old 05-20-2015, 04:42 PM
  #14  
solentlife
Super Contributor
 
solentlife's Avatar
 
Join Date: Oct 2010
Location: Ex UK Brit now in Latvia west coast - Ventspils
Posts: 12,293
Default

I have a Laser Cutter ... as many know here ... having written up various repair threads etc.

There are three programs that are universal to my Laser work :

PDF reader and tiling program

MS Paint for producing JPG images for export

Moshidraw on the Laser to import the JPG's so I can vectorise for the cutter.

That's it ... no CAD at all ... I have AutoCAD enabled Laser machine - but when I tried - it proved to be such a learning curve just for the CAD - I gave up !! I tried all sorts of lesser CAD programs that export to AutoCAD ... etc.

I went back to Paint !!

If I look online for plans - I ignore the DWG / DXF files and only take the PDF versions.

Nigel
solentlife is online now  
Old 05-20-2015, 06:28 PM
  #15  
Bald Paul
Member
Thread Starter
 
Bald Paul's Avatar
 
Join Date: Mar 2011
Location: Boiling Springs, SC
Posts: 547
Default

Nigel, it's taken me a long time just to get to where I am now with CAD. I will admit, I had several aborted attempts at teaching myself. It is, indeed, a very steep learning curve. In fact, I was going about 'designing in CAD' all wrong!

Once I began to learn basic commands, the concept of layers in drawings, etc., I find it's either a matter of the program becoming more user friendly, or my brain becoming more program friendly. Either way, I'm enjoying working in CAD now. There are still a few commands that frustrate me a bit, but I either plug away and figure them out, or find a way to work around them.

I remember asking a local shop with a laser cutter to make up one piece for me - I think it was a servo tray. Shop called and told me it was done. Evidently, when he went to cut it, he neglected to select a 1:1 size ratio, and hit "fit to page" instead. I ended up with a tray with cutouts for servos about 10 inches long and 4 inches wide!
Bald Paul is offline  
Old 05-20-2015, 07:31 PM
  #16  
solentlife
Super Contributor
 
solentlife's Avatar
 
Join Date: Oct 2010
Location: Ex UK Brit now in Latvia west coast - Ventspils
Posts: 12,293
Default

Ahhhh Fit to Page !! Yep - that gets em all !

Its the first thing I do when printing or sending output -ACTUAL size .... is ticked !

Usual one is where you print the tiles to stitch for a plan .... Fit to Page distorts every page !!

Nigel
solentlife is online now  
Old 05-22-2015, 06:39 PM
  #17  
Bald Paul
Member
Thread Starter
 
Bald Paul's Avatar
 
Join Date: Mar 2011
Location: Boiling Springs, SC
Posts: 547
Default

All fuselage components, including horizontal and vertical stabilizer, elevator, and rudder, have been designed and are basically ready for laser cutting.

Now comes the dreaded wing.

Rather than attempt drawing up a semi-decent airfoil, I may invest in one of the programs available specifically for the task. I'm looking at both Profili Pro, and CompuFoil.

Both programs can be a bit pricey once all the 'add on' modules are selected that would be required to get CAD files generated. Has anyone used the programs (either one or both) and could you provide some insight as to accuracy, ease of use, what 'add on' modules are required, etc?

Thanks!
Bald Paul is offline  
Old 05-22-2015, 10:48 PM
  #18  
Brner
AMA16634
 
Join Date: Dec 2008
Posts: 554
Default

Pretty simple to write up a script file (like 4 or 5 lines of simple code) using the abundant (free) .dat files online and let your CAD program do the work....
Brner is offline  
Old 05-23-2015, 02:38 AM
  #19  
Bald Paul
Member
Thread Starter
 
Bald Paul's Avatar
 
Join Date: Mar 2011
Location: Boiling Springs, SC
Posts: 547
Default

Originally Posted by DEG View Post
Pretty simple to write up a script file (like 4 or 5 lines of simple code) using the abundant (free) .dat files online and let your CAD program do the work....
Even better! I found a huge database of DWG airfoils online - FREE. I can just find the one I want, import it, and then I can scale it to the necessary size. Just add LE, TE, spars, etc, and it's done!
Bald Paul is offline  
Old 05-23-2015, 02:40 PM
  #20  
Bald Paul
Member
Thread Starter
 
Bald Paul's Avatar
 
Join Date: Mar 2011
Location: Boiling Springs, SC
Posts: 547
Default

Done! Not the sleekest looking plane, but easy to build! Now, it's just a matter of having the short kit laser cut, and getting all the electronics and hardware needed to complete.

Oh, and saving up the $$ to do so....
Attached Thumbnails Click image for larger version

Name:	Overall Side View.JPG
Views:	168
Size:	97.0 KB
ID:	180837   Click image for larger version

Name:	Overall Top View.JPG
Views:	192
Size:	73.8 KB
ID:	180838  

Last edited by Bald Paul; 05-24-2015 at 12:32 PM.
Bald Paul is offline  
Old 06-01-2015, 10:09 PM
  #21  
Bald Paul
Member
Thread Starter
 
Bald Paul's Avatar
 
Join Date: Mar 2011
Location: Boiling Springs, SC
Posts: 547
Default

Motor, battery, ESC, and servos are here. Two sets of full size plans (fuselage/tail and wing) are being printed, and the CAD files are out for quote.

With no fully stocked LHS around, I had to be very careful putting together my list of 'bits' to complete this build. Along with landing gear, linkages and pushrods, and the obvious, I had to remember details like a pilot figure, paint for the pilot, brushes for the paint, thinner for the brushes.... it seemed like the list went on and on. I'm going to have to start stockpiling hardware for future builds, I guess.

Next up - the building board. I've located a local lumber yard that stocks Homasote board (which I prefer for building wings) and I have my magnetic board for fuselage building (IKEA metal shelf with CAD designed and laser cut uprights, with four magnets each - works well). The build should start soon. Must....remember.....to.....document....build!
Bald Paul is offline  
Old 06-02-2015, 08:50 AM
  #22  
solentlife
Super Contributor
 
solentlife's Avatar
 
Join Date: Oct 2010
Location: Ex UK Brit now in Latvia west coast - Ventspils
Posts: 12,293
Default

Have you thought of own Laser CNC ?

I decided to buy my own ... just the cheap $500 jobbie ... simply amazing ! So much so - that I am seriously considering a larger machine .... suitable to cut fuselage lengths etc.

There's actually no need to buy a complete machine - plenty of firms out there who do component style - so you can put together own set-up.

Nigel
solentlife is online now  
Old 06-02-2015, 09:01 AM
  #23  
solentlife
Super Contributor
 
solentlife's Avatar
 
Join Date: Oct 2010
Location: Ex UK Brit now in Latvia west coast - Ventspils
Posts: 12,293
Default

I appreciate you likely enjoy CAD ... me ? I hate it !! Could never get on with it.

So I tend to design on paper sketching ... then map out direct to foam sheet.... or scan and transfer sketch to MS PAINT. Paint imports into my Laser machine as a JPG file ... so no need for extra work. The Laser software then lets me vectorise for cutting or leave as JPG for engraving.

I am lucky in one sense that I can visualise in my head what I want ... so transferring to foam sheet direct is not a problem. The old "What looks right - Most likely is right" seems to work ok.

On the subject of 'inventory' .... that will build with time. And don't throw out stuff ... old engines have Magnets, shafts, wire ... crashed models have tails and bits that can be grafted to other, horns, clevis, push rods ... all salvageable.

You'll develop an 'eye' for things in shops ... now where can I use that ! People will look at you strange for searching through the Dolls section of Toy Shops ... BBQ skewers ... Tooth Picks ... Bicycle wheel spokes ....
Multi-core telephone cable to wire up multi servo installations ...
List goes on ...

Cheers
Nigel
solentlife is online now  
Old 06-02-2015, 09:04 AM
  #24  
solentlife
Super Contributor
 
solentlife's Avatar
 
Join Date: Oct 2010
Location: Ex UK Brit now in Latvia west coast - Ventspils
Posts: 12,293
Default

Don't be afraid to mix materials ... E-power has no fuel to worry about ... so using foam sheet to create ribs, formers, wing saddles, nose cowls, fairings etc .... all helping to keep weight down and easy to cut / work.

I have even 60 powered 'wood' Glow models with foam repair parts in ...

Nigel
solentlife is online now  
Old 06-02-2015, 03:27 PM
  #25  
Bald Paul
Member
Thread Starter
 
Bald Paul's Avatar
 
Join Date: Mar 2011
Location: Boiling Springs, SC
Posts: 547
Default

Originally Posted by solentlife View Post
Have you thought of own Laser CNC ?

I decided to buy my own ... just the cheap $500 jobbie ... simply amazing ! So much so - that I am seriously considering a larger machine .... suitable to cut fuselage lengths etc.

There's actually no need to buy a complete machine - plenty of firms out there who do component style - so you can put together own set-up.

Nigel
I really don't do enough cutting to justify buying my own machine. And, the budget and space restrictions don't really allow it, either.
Bald Paul is offline  

Quick Reply: My first "real" scratch build


Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.

Page generated in 0.12110 seconds with 18 queries