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Jan 19, 2009, 08:37 PM
Running jokes into the ground
b-29er's Avatar
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Scratch-Build-Off III: Pond Racer


if anyone has been over to electric plane talk, you may have heard of the plane im trying to build into a balsa kit.

the plane almost everyone decided on is the Pond racer, a twin-engine racer for a 35" scale.

my question is this-what is a good way to scale down the dimensions on the plane? wingtip-to-wingip will be 35 inches.

by the way, my first parts will be designed on monday. plane will be doen in 6 weeks..
Last edited by b-29er; Jan 21, 2009 at 09:45 PM.
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Jan 19, 2009, 09:29 PM
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scratchnhover's Avatar
You need to take the measurement on the full size and divide the desired size measurement by it. For example, if the full size is 35 feet, 4 inches in wingspan, and you want a 35" wingspan model, here's what you do. Mutilply 35 feet by 12 (inches per foot) to get the result in inches= 420 inches. Add the 4 inches (over 35 feet) to it= 424 inches. Now, divide 35 inches by 424 inches and you get .0825, or 8-1/4% scale.
Everything you measure on the full size, you multiply by .0825 to get the length it should be on the model.

Now, if you have plans, you can do the same thing. Instead of the full size, substitute the length of the span on the plans to get the percentage you need the plans reduced (or enlarged) to. Follow the math above to get the result. Then take them to a copy/blueprint/engineering supply place and they can do it with their equipment. My engineering supply has a handy little round do-dad that they can use to give them the percentage- but the math is more accurate.

Mike
Jan 20, 2009, 04:47 AM
North East England
...or if you don't mind cutting up your original plan, chop it into A4 bits, scan them, do the scale-down factor then re-print them and have a pleasant 20 mins with the sellotape

Steve
Jan 20, 2009, 09:31 AM
Neophyte hacker
portablevcb's Avatar
Are you talking about just scaling a set of plans or 3 views? Or, are you talking about which parts should not be scaled exactly, like maybe tail surfaces?

So, don't really understand what you are trying to accomplish. Are you going to design a plane that can be kitted or just something to fly? Carve it from foam, or built up balsa?

FYI, there were some German guys who built a large electric Pond Racer a year or so ago. Looked really nice.

From your question it sounds like you have not done a scratch design before? If not you might want to read through some of the threads in here. Especially the ones for the three scratch build contests. A bunch of design work and a lot of different techniques you could learn from.

charlie
Jan 20, 2009, 08:44 PM
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b-29er's Avatar
Thread OP
i am trying to make a balsa kit with all formers laid out.
Jan 20, 2009, 09:01 PM
Neophyte hacker
portablevcb's Avatar
I said that wrong. Are you trying to make parts for yourself or a kit that can be sold to others? If for yourself then I encourage you to keep going.

If for selling a kit I might be able to help a bit more offline.

charlie
Jan 20, 2009, 09:16 PM
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b-29er's Avatar
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i am hoping to make a kit for myself, and plans i can distribute online for free use. this should be a high-quality kit. i am hoping to make a good design on this that even a first or second-time builder can understand and accomplish.
Jan 25, 2009, 12:35 AM
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b-29er's Avatar
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ok. i am going to begin design monday. thus far, here are the design features, spinner to rudder.

1. the spinner will be blended with the fuselage for a scale look
2. twin bl inrunners with a 2-piece cowl, held on by magnets for easy engine maintenance/exchange
3. 3/16" formers held in place by 4 1/4" supports that run the length of the nacelles.
4. 2 esc's and batteries held near the joint between the wing and the nacelle for greater support
5. retractable landing gear with scale guards
6. scale wing with a symmetrical chord for better performance
7. flaperons with enhanced surface area for better performance and slower landing
8. solid beam stretching between the nacelles in the wing to hold the nacelles together, along with a solid leading edge for more durability.
9. cockpit designed for a manequin or a camera in the seat
10. taileron for better turning (ailerons are larger, but performance even with these may be mediocre at best
11.steerable tailwheel
12. wing lights for scale look and evening/night flights
is there anything else you would like to see in the design? any concerns
Jan 26, 2009, 11:16 AM
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theKM's Avatar
Quote:
Originally Posted by b-29er
ok. i am going to begin design monday. thus far, here are the design features, spinner to rudder.

1. the spinner will be blended with the fuselage for a scale look
2. twin bl inrunners with a 2-piece cowl, held on by magnets for easy engine maintenance/exchange
3. 3/16" formers held in place by 4 1/4" supports that run the length of the nacelles.
4. 2 esc's and batteries held near the joint between the wing and the nacelle for greater support
5. retractable landing gear with scale guards
6. scale wing with a symmetrical chord for better performance
7. flaperons with enhanced surface area for better performance and slower landing
8. solid beam stretching between the nacelles in the wing to hold the nacelles together, along with a solid leading edge for more durability.
9. cockpit designed for a manequin or a camera in the seat
10. taileron for better turning (ailerons are larger, but performance even with these may be mediocre at best
11.steerable tailwheel
12. wing lights for scale look and evening/night flights
is there anything else you would like to see in the design? any concerns
Symmetrical airfoil sections are mainly just a compromise made for aerobatic purposes. Speed machines rarely go inverted and be best served with a quick airfoil like the MH30, MH32 or their like. Heres the MH32, a very slippery customer indeed...
http://forums.flyesl.com/uploaded/fr...AG40toMH32.gif

There's a reason why pylon racers don't use symmetrical sections. Airfoils like the MH32 reduce overall drag by producing lift without induced incidence, increase energy retention in corners, etc etc.


Ailerons... you wont need tailerons with ailerons that size. High performance machines don't need much input from any surface when they get up and go. If you've ever seen a model pylon racer in person, the amount of throw they use is ridiculously small. Before computer radios, it was a challenge with my small pylon models to reduce the throw enough mechanically and still get a solid connection without slop. Another fact is that more surfaces deflecting amounts to drag, and drag is naturally the enemy of performance. If you set up the ailerons to the scale drawings, and use dual rates (you'll want dual rates or a boat load of expo), the low rates for speed flying will only need a few mm in each direction at most. For normal rates, a deflection of 20-25 degrees should be ample for increased control for landing.


(...oops, I think I just hijacked the thread again)
Jan 26, 2009, 11:57 AM
The wings are not on fire
W@CC0's Avatar
Quote:
Originally Posted by theKM
Symmetrical airfoil sections are mainly just a compromise made for aerobatic purposes. Speed machines rarely go inverted and be best served with a quick airfoil like the MH30, MH32 or their like. Heres the MH32, a very slippery customer indeed...
http://forums.flyesl.com/uploaded/fr...AG40toMH32.gif

There's a reason why pylon racers don't use symmetrical sections. Airfoils like the MH32 reduce overall drag by producing lift without induced incidence, increase energy retention in corners, etc etc.
The MH32 is a very nice profile but it doesn't like building errors very much. If you're building a glassfiber/epoxy plane, then you can go with the 30 or 32.
If you're building a wooden plane on the other hand, the profile that you actually build isn't that accurate. I would go for the MH43 then.
The MH43 isn't affected that much by building errors and is also a fast profile, designed for pylon racing and fast hotliners.

http://www.mh-aerotools.de/airfoils/mh43koo.htm

Tom
Jan 26, 2009, 12:41 PM
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theKM's Avatar
Quote:
Originally Posted by [email protected]
The MH32 is a very nice profile but it doesn't like building errors very much. If you're building a glassfiber/epoxy plane, then you can go with the 30 or 32.
If you're building a wooden plane on the other hand, the profile that you actually build isn't that accurate. I would go for the MH43 then.
The MH43 isn't affected that much by building errors and is also a fast profile, designed for pylon racing and fast hotliners.

http://www.mh-aerotools.de/airfoils/mh43koo.htm

Tom
Yup, all good info. Even an MH30/32 with build errors would be better than symmetrical though, and the first challenge of calling out specific airfoils like this is simply getting the points accurately into CAD let alone the build
Jan 26, 2009, 05:01 PM
Running jokes into the ground
b-29er's Avatar
Thread OP
Quote:
Originally Posted by theKM
(...oops, I think I just hijacked the thread again)
you dodnt hijack the thread. you hijacked it when you were being a depron door-to-door mormon.

but on serious notes, i may use a 43. it isnt going to be an accurate build. however, where i fly, tailerons are kind of a requirement, unless you have a good sink rate. i intend to land this model slowly, so i may just use a 43. but where i fly, there are two kinds of landings- in the wind, nearly scraping trees, or a long, slow slope landing where thin ailerons work poorly.

what does the 43 stand for, 43% of dihedral?
Last edited by b-29er; Jan 26, 2009 at 05:29 PM.
Jan 26, 2009, 07:26 PM
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b-29er's Avatar
Thread OP
also, built my first former today. it is the former for the area behind the wing in the nacelles. tomorrow, i am making the area behind this. this is-

the rear of the nacelles
v-tail thing on the plane

this will be made from 3 boards that slide together, and 2 formers that slid over these. also, a plate to hold the elevators, and the battery bay (where the black composite material is)

after that, ill be making the top of the battery bay, and the wing support beam (1/4x1/2 strip). the wings will be formed into this using precision holes.

and i hope to have the kit laser cut.

if anyone wants to start early, i can have prelims up in about 1 week of the nacelle.

also, anyone know where i can get a 2.75" plastic spinner?
Jan 26, 2009, 08:23 PM
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theKM's Avatar
Quote:
Originally Posted by b-29er
it isnt going to be an accurate build. however, where i fly, tailerons are kind of a requirement, unless you have a good sink rate. i intend to land this model slowly, so i may just use a 43. but where i fly, there are two kinds of landings- in the wind, nearly scraping trees, or a long, slow slope landing where thin ailerons work poorly.
The wing loading of the plane governs how fast the plane is going when it stalls, not the ailerons/tailerons. The only way to reduce the effective landing speed would be flaps but this puppy is not destined to come in slow. You will need to be giving the maiden for this plane in a field where you can have a long and shallow approach so you can wash off the speed in a safe manner. Some things are a contradiction, rocket fast lead sleds and confined flying fields would be one such example.

Quote:
what does the 43 stand for, 43% of dihedral?
The 43 in MH43, like most numbers in airfoils, is just an arbitrary number of the design. This being the 43rd airfoil that Martin Hepperle released. Unsure of the "release" process, but most airfoils are just simply named/numbered by their designers.
Jan 26, 2009, 09:07 PM
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b-29er's Avatar
Thread OP
i am not talking about stall speed. i'm talking about angle of attack. the plane has either a 5 degree landing slope or about a 45 unless i can turn within the 40 feet of endzone.


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