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Old Aug 05, 2006, 12:05 PM
neu
inspired newbie ;)
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Hungary
Joined May 2006
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Beginner questions from a first time scratchbuilder :)

Hi flyers!

I've never scratchbuilt anything till now, but the more and more I read about working with foam, I'm more and more inspired. I just bought the book of Sparks and I've found it really useful.

So I decided to start building something to my existing power system (with Axi 2820/12). It should be something with about 1500mm (60') wingspan, and flying weight less than 2kgkg (70oz), would be nice to have it around 1.5kg (52oz).

The planes I'm thinking about: A-1 Skyraider, F6 Hellcat, P-47 Thunderbolt.

As I never bult anything I don't even know where to start. Okay, I have few ideas about the constructions, but I've got some questions:

- How you select the appropriate airfoil?
- How do you know which servo will be appropriate? (I know there are online calculators for this, but they are asking for paramters that I still don't know, and I have no idea how to get)
- How do you know the approximate wight of the model before it's finished?
- How do you know how to setup the engine? I don't know the exact term in english, but I'm talking about the angle difference between the planes axis and the motor's axis needed to compensate the torque?
- How do you know the wing's and stabs incidence settings?

I'd appreciate any help concerning this, and any further ideas! It would be great to start in and build it together, but I know it does not make too much interest to help a newbie step by step through a full construction, while he's asking stupid questions that he already should know

So Any help and ideas are appreciated!

Greetings, neu
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Old Aug 05, 2006, 12:48 PM
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United States, OH, Westlake
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Guten Tag Neu,

I am just starting to build my own as well. I am doing it by taking an existing known good airframe and modifying it. The feedback from the designer and others here helps. I consider it a kind of apprenticeship. At this point these are just profile blu-cor planes. I like that as the builds do not have a lot of time invested in them and I can quickly make a modification and try it out. A hot melt glue gun makes it easy to take things apart.

One of the threads on designing planes suggested a book called:

Basics of R/C model aircraft design : practical techniques for building better models / by Andy Lennon. ISBN 0911295402

Surprisingly my local public library had it. I think it will provide the level of information you are looking for.
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Old Aug 05, 2006, 12:53 PM
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Joined Nov 2005
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There's no such thing as a stupid question - none of us were born knowing this stuff.

I am posting the estimable Gene Bond's "Design Parameters for Slow Flyers/Parkflyers - it will give you plenty of guidance in beginning your project.
You'll need Adobe Acrobat to read it.

Despite a long and happy time spent in Free Flight my first two foamies were barely flyable. Besides Gene's guide, I would suggest looking through the build threads for planes similar to yours and see what kind of servos, airfoils, etc, those builders used. There are years of experience here in the Forums.

Finally, be patient with yourself and your project. Plan it all out before you cut a single piece: it's often the little seemingly unimportant things that turn out to be major headaches down the road.

And, viel Gluck!
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Old Aug 05, 2006, 02:28 PM
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Neu, one more thing. In order to fly, a plane must be posted in a build-thread here on RC-groups with photos, plans, and preferably a video.
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Old Aug 05, 2006, 04:06 PM
If it ain't broke...break it!
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South Coast, UK
Joined Jun 2006
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I started out scratch building other peoples designs and then followed something similar to Gene Bonds guide to build my own first design. Went together really quick (2 evenings) in Correx. It's an amazing feeling when you throw something into the air and it actually flies!
With the foam, maybe try wire cutting a couple of wings and then covering in brown paper and watered down PVA. Put these in a simple profile fuselage and see how they fly. Cheap and quick to put together, but I learnt loads about what works and how to put foam planes together. I've built a few scale WW2 planes now, but I'd still be a little cautious about taking on something larger than a 36" wingspan

Start simple, and if you've got the 'knack' and the time, just keep making them bigger and better!

Get yourself a smallish cheap BL outrunner, a couple of 9g servos, Li-Po and a couple of other lightweight bits and give it a go. Maybe save the 60" P-47 'til winter. You'll enjoy designing and building it a load more with a bit of confidence and practice.

I hope this doesn't sound patronising, just know that this is what worked for me
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Old Aug 05, 2006, 04:13 PM
If it ain't broke...break it!
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Joined Jun 2006
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Selecting airfoils depends on what you want the plane to do.

The Clark Y is a classic for slowflyers that need plenty of lift and don't mind a bit of drag.

The Eppler 374 is great for fast sports flyers.

http://www.aviationshoppe.com/airpla...ils.php?id=304 is a free airfoil plotter for making foam cutting templates. The library of different sections is here;

http://www.aviationshoppe.com/airpla...ils.php?id=303
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Old Aug 05, 2006, 05:28 PM
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- How you select the appropriate airfoil?

For parkfliers, your ability to hot-wire, fold or sand an aerofoil will probably have almost as much affect as the aerofoil you are trying to produce. You can get away with a flat 1/4" plate with rounded front and tapered rear (most of the time) because you're not worried about ultimate efficiency. At this level of building there are really only a handful of aerofoils to choose between:

Undercamber (bent foam sheet) for really slow flight
Flat bottomed Clark-Y style for gentle flight
Semi-symetrical for sport
Symetrical for patern aerobatics
Flat plate for when you can't be bothered to build anything else.

- How do you know which servo will be appropriate?

I check what GWS or other kits recommend for a similar plane (in size, weight and power)

- How do you know the approximate wight of the model before it's finished?

The airframe usually weighs about the same as the power system you fit, but again you can check kits of similar planes. Another technique is to estimate the amount of foam you'll use.

- How do you know how to setup the engine?

Really stable planes need more downthrust, aerobatic planes can need none. I'd make it adjustable and be ready to cope with minor trim changes for the first flight.

- How do you know the wing's and stabs incidence settings?

Again, stable designs need a few degrees, aerobatic designs need none. As long as you have a relatively powerful elevator you can trim it out in the air, then adjust the incidence afterwards.

Basically, you're thinking about it as if you want it to fly perfectly from the first flight, and own-designs rarely do. However the bit I really enjoy is converting a barely controllable pig into a nice enjoyable model by adjusting the CG, control throws, thrust angle, etc, over the first few dozen flights.

It can help to build a simple profile version first, and then a nicer model, and certainly don't bother with a beautiful paint job until after you've got it flying the way you want.
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Old Aug 06, 2006, 11:58 AM
If it ain't broke...break it!
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South Coast, UK
Joined Jun 2006
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Somewhat off topic, but Robin, I notice you're in Crawley. Just wondered where you fly locally. I'm down the other end of the A23 on the south coast.
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Old Aug 06, 2006, 12:07 PM
neu
inspired newbie ;)
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Hungary
Joined May 2006
57 Posts
WOW!

Thank you for the answers, and I don't mind if you try to save from disappointment by recommending simple builds. My only problem that I don't feel inspiration to build "2D" planes. I do scale plastic modeling, I'm a 3D Artist so I want to go for "3D". Theoretically I can build the airframe I want, probably I'll get help if I'm stucked, but I think I can do it at some levels. My concerns were about if this airframe can fly at all, and I'd like to understand the questions listed more.

Correct me if I'm wrong, but if a rc car and a rc grass trimmer can fly what has nothing to do with aircrafts, then something that looks like a real aircraft should fly too. Or I'm wrong? I thought that if I build the airframe in the desired size and weight, and put the cg to the right position, then it should fly. Shoudn't it? That's an other question how it flyes, and that's why I'd like to understand the listed questions clearly before I do anything.

The wing foil question is something I'm really interested to, as I know the basic types and what are they good for, but there are thousands of airfoiles, and I don't really understand the differences, and the process of selection for these. What I'd like to do is something that is flying stable, I don't want to do too much acrobatics, I'd rather prefer scale looking (and yes, I don't plan nice decals and coverign before it's flying good )

About hte incidence I thought that it can be adjustable as well, and can be experimented which setting is good. I've got an idea how to do it.

So I'm still an overinspired beginner with 0 knowledge, and I will not be able to make the airframe I theoretically can build to fly? Let 'say I can build the airframe in the desired size and weight, I select an airfoil that is close to the real one, and I have an appropriate power system. What are the issues that can make this configuration grounded and not to fly at all?

Don't get me wrong, I don't htink I'm more clever than others I appreciate any help, but I have to keep up my inspiration by doing something that I found really interesting!

Greetings, neu
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Old Aug 06, 2006, 09:09 PM
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Neu: I'd find all the plans I could on the Skyraider, Hellcat, and Tbolt. Study them all, and note the incidence angles that each designer uses. If you hit those angles in the middle (kinda like taking a vote, and averaging it all out) you should be pretty close to ideal. I'd suspect that you will see the tail at 0°, the wing at maybe +1°, and maybe -1° downthrust - if you see any at all. I think these would also be okay at 0-0-0.

Semisymmetrical 'foils would do you well with any of these - I guess you could also call them sport airfoils. Profili makes a another great 'foil program that's easy to use, and will let you determine the thickness of the skin if you decide to go with built up rather than hot wired. http://www.profili2.com/ Something in the NACA 24XX family would work well - maybe a 12% thickness? that would be the NACA 2412. If you want more lift (and drag) to give it slower flight, then bump up the thickness of the wing - the NACA 2415 is 15% thick.

A lot of my knowledge is "seat of the pants" - I've read tons of stuff over the years, taught myself how to read L/D "buckets" at various Reynold numbers, etc. etc. The only way to "know it" is to invest the time in "learning it". The Andy Lennon book that kdahlhaus pointed out in post #2 is an excellent place to start.

Brad
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Old Aug 08, 2006, 02:37 AM
neu
inspired newbie ;)
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Hungary
Joined May 2006
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Excellent! Thank you! I already downloaded Profili, it's a really handful stuff! Also thank you or the ideas, it helps a lot! Probably I'll go for the book too!

Greetings, neu
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Old Aug 08, 2006, 02:44 PM
If it ain't broke...break it!
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South Coast, UK
Joined Jun 2006
912 Posts
Model Aircraft Aerodynamics by Martin Simmons is a great reference for designing your own stuff.
I can only echo the above advice. Look at all the information you can get your hands on. Measure and compare other models. But what can be most useful is talking to the owner/ builder. We all love to talk about what we enjoy doing, so ask how it flies, how it was built, is there anything they'd change?
And if you think you're up to the challenge, go for it, I look forward to hearing about your experiences and seeing the results!

I found Profili a bit of a handful too! The one I suggested above is a lot simpler, but still allows you to change the camber and thickness of the section, as well as allow for skin thickness etc. Profili will proably be better for working out the elliptical wing on a P47 though!
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Old Aug 08, 2006, 04:18 PM
neu
inspired newbie ;)
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Hungary
Joined May 2006
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Well, I was in a home improvment center today, and they have tons of EPP and EPS sheets for buy, and even as trash So it's really cheap, I'll start experimenting with the building technics, and trying out ideas, as it's almost free.

My idea is to start a topic and build this thing, and report it step by step, so you guys can correct me or save me if I'm doing something wrong! There's no serious danger if I miss something, so I prefer to learn on the fly! What do you think about it? I'd post from the first basic plans...
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Old Aug 09, 2006, 08:05 AM
gpw
“There’s no place like Foam”
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Neu , you may want to re-post in the new and exciting DIY FOAMIES section ...
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