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May 08, 2009, 06:47 PM
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Build Log

BYOB - Back Yard Outdoor Bipe: Fly in a small outdoor space! PLANS, VIDEO, PICS

This plane is a 26" wingspan biplane loosely based on a Pitts Special. With an AUW of 4-5 ounces and flying the 10g brushless motor from Hobbyking, it will make lazy circles or figure eights in your backyard AND still have the power and ability to do basic aerobatic maneuvers.

It took a long time to get here, too. I've been wanting a plane I could fly in my backyard space for a long while now. I didn't want a 3D hovering machine. I didn't want something that would have to fly high-alpha in order to stay "low and slow." So this plane uses the tried and true 4-40 type undercambered airfoil, and with a wing area of nearly two full square feet and an AUW of about 4.5 ounces, leaves with a super-light wing loading of just 2.25 ounces per square foot.

Here's the specs:

AUW: 4 to 5 ounces
Wing area: 286 square inches
Servos: 3.6g to 6 gram x 2
Receiver: 4 to 6 grams
Motor: 10g brushless outrunner
Prop: GWS 5030
Battery: 350 to 800 mAh 7.4V Lipo
Power: Approx 75 watts per pound, depending on final weight of plane

It took me three tries to get the design "just right" on this plane. The nice thing about having done that is I know it will fly with a variety of different materials and weights. For example, this latest one uses 2mm Depron for the wings, 3mm Depron for the wing supports, rudder, elevator, and "top profile" section of the fuselage, and 6mm Depron for the fuse. However, it probably is a bit stronger and doesn't weight much more using all 3mm foam in place of the 2mm. And if someone wanting to build this only has a heavier 800 mAh battery, well, I know that will work too!

Just keep in mind that the lighter it's built, the slower it can fly. I had my wife take some video of the plane flying in my backyard today. Note that the "flyable space" for my backyard is limited to about 40 feet x 50 feet, with a ceiling of about 60 feet for most of it, and as low as 35-40 feet in one corner. So, not a lot of space to fly! But with slightly oversized control surfaces and decent power from a tiny brushless motor, it will turn on a dime, or float around leisurely. Plus, it will fly in winds up to 7 to 8 miles per hour, though it's best if it's calm.

Here's the video:

BYOB - The Back Yard Outdoor Biplane (2 min 43 sec)

EDIT: 32.5" wingspan version for FFF is now available - see below for plans.

EDIT: Added untiled versions of both plans for those who wish to scale to different sizes and/or tile the plans themselves.
Last edited by magic612; May 08, 2010 at 12:48 PM. Reason: Added pictures, edited repetitive phrases, added 32.5" version PDF file, added 32.5" version pics
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May 08, 2009, 06:52 PM
magic612's Avatar
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Getting started

After cutting out all parts, I weighed the Depron to see where I was at - 1.5 ounces for the airframe. Not bad!

I cut angled sections out of the fuselage portions of the foam, to allow the elevator and rudder to turn using a simple, lightweight tape hinge.
May 08, 2009, 07:08 PM
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Fuselage construction

Slide the two fuselage pieces together. I prefer to place the angle for the elevator facing "down."

To save weight, I "spot" glued small dabs of hot glue along the length of the fuselage. I made sure there was plenty of hot glue at each end, particularly the front where the firewall attaches, but alternated "spots" in two-inch distances, offset by 1/2" along each inside corner. See photo for detail.

Next up, I taped the elevator and rudder sections onto the fuselage. Be sure to tape the elevator on FIRST, then the rudder. I prefer to use 3M brand Long Term Storage tape, but no matter what tape you do use, be sure to push down FIRMLY when applying it. These are pressure-sensitive adhesive tapes, and they are activated by pressure. Your hinges will stay on better and last longer if you push down on the tape when applying it.
May 08, 2009, 07:19 PM
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Forming the airfoil

Forming the airfoil for the wings is next. Now, I like a KFm airfoil as much as the next guy because 1) they’re easy to make and 2) they work well. But for this plane, I was really interested in saving weight while using a good, slow-flyer type airfoil. The undercambered 4-40 is good for this; it’s a 4% rise at 40% back from the leading edge. With either 2mm or 3mm Depron, it’s not hard to make at all.

First, find out which way the “grain” goes on your Depron. One direction is “stiffer,” and the other direction will bend more. We want the “stiff” direction (i.e. – the grain) to be in the same orientation as the length of the wings. So parallel to the grain is how will bend the airfoil. BE CERTAIN TO CUT THE DEPRON IN THIS DIRECTION WHEN CUTTING YOUR WINGS!

Since these are such small wings, just about any rounded surface will do. I used a small roll of paper I found lying around our house. It wasn’t even the full length of the wing, nor did it have to be. Just bend the Depron gently over the curved surface using your hands. 3mm Depron will “crackle” a bit more as it bends, so be careful with that thickness. 2mm takes a bit more effort, and must be bent fairly significantly to achieve the necessary curve. Bend the foam MORE than the curve needed – you can match the curve to the edge of one of the wing supports to see how your progressing.

First bend the foam at about 40% back from the leading edge. Give it a good bend there. Next, bend it about halfway between that point and the trailing edge. Finally, bend it halfway between the 40% bend and the leading edge. You may need to give it a little extra bend towards the leading edge – I just use my fingers to gently curl the foam up along the edge. If you have bent a little TOO much (see picture), you’ve got it right!

Now, cut the LOWER wing section in half. Be sure you know which direction is the leading edge and trailing edge.
Last edited by magic612; May 29, 2010 at 08:55 AM. Reason: Stupid mistake.
May 08, 2009, 07:31 PM
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Lower wing sections

Cut the lower wing in half. Using the 95 degree angle template provided in the plans, get ready to glue on one wing support. Check the full-view drawing to make sure you have the wing support in the correct direction, and that it’s orientation to the leading edge is also correct.

I prefer hot glue, so what I did was to make sure the glue gun was nice and warm, and apply a full-length bead of hot glue to the bottom edge of the wing support. Then, quickly lining up the support on the wing, I placed it onto the wing, and put the 95 degree angle template next to it, so the support is leaning AWAY from the center of the wing. The template will be on the INSIDE section of the wing. Regardless of what adhesive you use, make sure the adhesive has cured completely before letting go of the support (or removing any tape to hold it in place).

Do the same procedure for the other lower wing section.

Next, glue the wing sections to the fuselage, in the slot cut out for them. (MAKE SURE that when you cut that slot, it is the right size for your foam width!!) I folded the 95 degree template in half, so that my wing will have a 5 degree dihedral (each side). Be careful to only glue the first wing section into the fuselage ONLY HALFWAY into the fuselage foam, to leave room to glue the other half. I started by gluing to small dots of glue – one on the leading edge, and one on the trailing edge. This way I knew the wing wouldn’t slide in any further. THEN I placed a few nice long beads of hot glue along the bottom edge of the wing along the fuselage, and held the 95 degree template in place.

After hardening, I put some hot glue along the top. Then I repeated the procedure for the other lower wing half.

So far so good, right?
May 08, 2009, 07:42 PM
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Upper wing attachment.

Now it’s time to make the upper wing. If you’ve already bent the airfoil for the upper wing when you bent the lower wing, then cut the upper wing in half. If not, bend the wing to the airfoil shape using the same procedure as the lower wing.

This wing needs to be glue at a 170 degree angle, to make for a 5 degree angle on each wing on each side. Before gluing, I blocked up one wing (with the other flat on the table) so that my angle was correct using the 170 degree angle template. Then, leaving the blocked up wing in place, I put a full-length bead of glue along the edge of the other wing half, and pushed them together. Note that the curvature of the wing won’t allow them to fit together perfectly, so it’s okay to use more glue on the ends, and less in the middle. I actually added a little extra glue to each end after my initial bead had hardened.

Once this is ready, put a rather large bead of glue onto the top of the canopy, as shown. Put the center of the trailing edge of the upper wing into this bead, and hold it in place until it’s hardened. Don’t worry about the wing support sections yet – we’ll do those one at a time next.

Once dry, look to see where your wing sets in relation to the wing supports. It should be pretty well even, but if it’s slightly forward or back of the supports, that’s okay. Just note which direction it is so that both wings will be parallel when you glue them.

Holding up one end of one wing, run a full-length bead of glue along the top of a wing support. Push the wing down onto this support, sliding it slightly forward or back as needed based on your previous observation. Hold in place till cured. Now repeat the procedure on the other side.
May 08, 2009, 07:44 PM
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Airframe complete!

The empty plane should look like this now.
May 08, 2009, 07:48 PM
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Details, details...

Add the support dowels for the center of the wing between the fuselage and the upper wing. I used bamboo skewers, but simple .093” or .125” dowels will work too. Cut to length as per the plans, then glue in place. I found it easiest to glue the FORWARD support in place first, then the middle one.

This are important, as they help the upper wing keep it’s airfoil shape. Don’t neglect them!!
May 08, 2009, 07:59 PM
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Final touches

Next is adding the radio. A few points, as everyone’s system will be different, including speed controller, servos and such (though hopefully everyone will use the 10 gram motor – I can’t vouch for any other motor on this plane!).

Firewall is simply a 7/8" x 7/8" piece of .125" thick plywood, hot-glued to the "cross" in the cut out section of the nose.

I used 5 gram servos on my earlier versions, but wound up using 3.6 gram servos on my final version. They are still strong enough to much the controls. I’m not sure I’d go with 2.5 gram servos – they look like they’d not have the power. I soldered my esc to the motor leads, to save the weight of connectors. Also, the battery in my picture is actually a bit further back than I wound up actually placing it for proper balancing – so if you’re using a pack that is less than 1 ounce (28 grams), move your servos even farther forward, or you may have trouble balancing the plane.

I put my servos on the bottom of the 3mm fuse section, but this is not necessary. What will be necessary to keep the weight down though is to use .032” piano wire or carbon fiber rods for the pushrods. I used three small pushrod tubes to make sure my piano wire does not bend along that length.

I also prefer to use the Dubro Micro connectors on the control horns – these are on the pricey side, but they make life SO much easier when setting up the right trim on a plane, that I feel they’re worth it. But that’s merely my personal preference.

Also, if you’re going to add any color to the plane, do it before gluing anything together, and I’d suggest markers only – no paint or stickers, as they’ll just add weight (unless you’re not concerned with the AUW being too heavy when you’re done).

After that, the optimal CG I’ve found is ½” (12 mm) behind the leading edge of the LOWER wing.

Have fun, and please feel free to ask questions and/or post your planes if you build one!!!
May 08, 2009, 09:52 PM
John 3:16
Daddy-O's Avatar
I just placed an order this afternoon for some 3mm depron. I have all of the gear you mentioned also. I think I will build one of these when my depron arrives (about 1 month for me). Thank you.
May 08, 2009, 10:32 PM
magic612's Avatar
Thread OP
You're welcome, Daddy-O! It's been a fun plane to play around with - I actually played it pretty "safe" when flying in the video, but it's certainly capable of flying more intense aerobatics than what's on there. I'll try and get some more video posted soon that shows a bit more of it's capabilities. Hopefully the weather will cooperate this weekend.

The nice thing about this plane being so light is that you can brush up against a tree branch or side of the house, and as long as you have sufficient altitude to recover, you just gun the throttle and keep going. I've had to re-glue a wing support here and there, but other than that, it's a pretty tough and light little plane.
May 08, 2009, 11:00 PM
skumgummi dave
Nice design and great build tutorial. Thanks Magic612.

May 09, 2009, 12:20 AM
My plans are in my blog
Rusty-Gunn's Avatar
Excellent, Dave, excellent. I enjoyed the video.
May 09, 2009, 10:02 AM
Registered User
kdahlhaus's Avatar
This is just what I was looking for. I'm with you - the high-alpha fliers just are not all that appealing to me either. The closest to meeting the needs of a tight space flyers I've found so far has been Gene's blu dart, but it won't work in the small space of the backyward, so it goes out front. I've already a project for my depron on the board (KF Crossbow for use on a trip to the beach) but then I think your BYOB will be a wonderful build. Thanks for posting and including the great build guide.
May 09, 2009, 11:18 AM
still learning
aeroknot's Avatar
magic612 - Nice plane and build photos!

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