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Old Aug 16, 2014, 10:02 PM
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BBCC6- Curtiss XP-55 Ascender

I am rolling this BBCC5 entry into BBCC6 because I did not get the maiden flight done in time.
LINK to BBCC6 contest thread

Curtiss XP-55 Ascender



I built this once before, as a 4 channel RC plane.
It never maidened, mainly because I realized I had built a monster that I was not ready for. It also turned out heavy, and would need a very high speed on a long runway to take off, plus I feared the weight issue would make it difficult to control in flight and would need the speed kept high. So I stripped it of everything except the motor and put it on static display.



But I always wanted to try again.

I always thought I would end up trying again as a giant scale effort.
But then I wondered if this whole time I was going the wrong direction.
The AJ 404 Interceptor that I built was a really brilliant flyer, until the first crash, and the unsuccessful repair. But the Interceptor was built small as a faux profile plane, just wide enough to house small electronics.
Then I looked at the profile glider I built for testing the CG on a very different plane like this, and I realized I should be building something as a park flyer in a smaller scale.



I realized that because this mock up was built as a free flight glider, if I were to convert it to a balsa built-up RC plane, it would qualify for BBCC5. So earlier this year, I designed it, using exactly the same dimensions as the glider.
(See pics, below)

Nearest I can estimate is that it has about a 104 cubic inch wing area. It has a 24" wingspan and is 18" long. Pat Tritle thinks it's going to weigh about 7 oz, and I've learned to trust him. He nailed the Firefly when he estimated the weight.
If so, that gives me a 9.7 oz/sf wing loading.
That would give me a stall speed of 15.4 mph. So not too bad.
It's a little faster than the Interceptor was, but pretty reasonable for a park flyer, and a lot better than the heavy P-55 I built before.
This one is going to use a Clark-Y Airfoil for both the wing and canard.
It is going to be a 4 channel maybe, but not with the rudders, because having the vertical stabilizers out near the wingtips is problematic.

Instead, it will be a bank and crank plane, but the rudder channel will be for nose steering only.
You see, canards with pusher motors don't benefit from the prop-wash blowing over the control surfaces, so you have to build up that speed on the ground before it can lift off. It also cannot rotate very much or it will suffer a prop strike and the flight might be over real quickly. So the nose wheel steering is real important to keep it on the narrow asphalt runway at our flying field. (Just have to make sure I don't end up ground looping it). Or just hand launch and don't worry about it.

Oh by the way, I'll probably build a second version of this exact same plane out of dollar tree foamboard, just to experiment with how it would fly with lighter weight...as an indoor flyer. I could also verify the CG is correct on that indestructable version before applying it to the more fragile balsa version.
Because after losing that beautiful Interceptor the other day, I don't want to take those kind of chances again on an unknown.
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Old Aug 16, 2014, 10:31 PM
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The original P-55 I built was recorded in this old build thread from 3 years ago.

Just in case anyone wants to check that out...
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Old Aug 17, 2014, 09:35 AM
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One thing that I did differently on this XP-55 iteration is that I am keeping the dihedral out of the wings and building them straight. However, the winglets outside of the twin fins will have a steep dihedral. This way, I can use a continuous main spar for greater strength.

Today, I'll start working on the print wood.
One trick I've used before to make print wood was to use carbon paper and trace over the ribs, which of course, leaves nice black lines on the balsa sheet beneath. But that was back when typewriters were more popular and personal computers hadn't yet taken off. I wonder if carbon paper is still available. I might need to take a little trip to Office Max today and find out.

I will have to probably make the canopy. I wasn't able to find any canopies nor any plastic bottles anywhere that were even close to the shape I need. I could just carve the canopy out of balsa and paint it, but I think it would look nicer if it were clear and if I can get a small pilot head in it. So I'm going to have to build a vacuum forming tool during the course of this build (which of course I can use in the future too).

I have so much silkspan. I personally really like the "lost" art of silkspan, but finding the colored dope has gotten harder. Especially in spray cans. Like silkspan, there isn't much of a demand for this anymore, which is frustrating for those of us who like to build the occasional tissue-covered airplane. I take pride in getting a perfect skin out of silkspan, which is child's play anymore with the available iron-on films. The only thing is, I'll have to take care not to land in the fields around here, which will always poke holes through the wing on landing.
Thus, the landing gear.

By the way, the original P-55 you see above, was covered in silkspan too. But the silver paint is just chrome rattlecan paint. Adhesion is a little bit of a problem, but it works better than trying to paint over plastic iron-on skin.

Like the previous iterations of P-55, this one will have a full-flying canard.
I just hope that doesn't make it too sensitive.
And because the canard is the first thing that hits something if it does have to come down in a field, the leading edge will be a carbon-fiber rod.
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Old Aug 17, 2014, 01:10 PM
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Old Aug 18, 2014, 07:45 AM
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Old Aug 18, 2014, 09:20 AM
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The wing is basically an egg-crate design, and assembles quickly.
It's probably a little stronger than it needs to be, actually, but I justify it because it supports landing gear. I did all the printwood, using carbon paper that I did find at Office Max, and that didn't take long, so I cut them all out. THAT didn't take long either, so I thought, what the heck, and sat down and assembled the main wing last night.

I used 1/8" thick balsa for all the wing spars. I have two horizontal wing spars (at right angle to the fuselage), and one main diagonal wing spar (which follows the sweep angle of the wing). The spars are notched as well as the 1/16" thick wing ribs. In a few places, I got confused and cut angled notches in the horizontal spars, which I had to straighten out and fill in with thin pieces of balsa. (Darn it). Otherwise, it was accurate. On my plans, I left extra length, which then get trimmed to the length you need. When dealing with swept back wings, you're dealing with angles, which complicate things.

I do think I could have gotten away with using 1/16" sheet balsa for ALL the spars. The 1/8" spars might be a little stronger and thicker than I really needed. Pat Tritle often uses an egg-crate design, but he would have probably used all 1/16" balsa, and would have also hollowed out the spaces between the ribs; but that's difficult to do unless you're having the set laser cut. I punched a few holes in ribs, but only for the servo leads.

I had planned on using solid balsa ailerons, but I might go back and build them up instead. I'll save a lot of weight if I build them up.
I had also originally planned on D-boxing the entire leading edge, top and bottom, to the main spar, but here again, I changed my mind and will only sheet the top. The wing is designed a LOT stronger than it really needs, and I can shave some weight by not sheeting the bottom.

I have been using Sigment glue, which is very similar to the old Ambroid glue I used to prefer to use. However, for sheeting duties, I really prefer CA glue, because it sets up on a wet surface almost immediately. So I'll have to go pick up a new bottle of CA today.

I didn't want to post a pic of a half-done wing, so I'll post a pic tomorrow after I get more done, probably.
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Old Aug 18, 2014, 09:35 AM
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Old Aug 18, 2014, 09:41 AM
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I've also noticed on my plans that I did not account for a steerable nose wheel.
Okay then. Omitting that would make this a 3-channel plane.
Hmmm...I'll have to think about that. If I can come up with something, I might like to have a steerable nose wheel, but I don't HAVE to have one. (Just have to make sure it rolls straight).
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Old Aug 19, 2014, 09:35 AM
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Continued on last night. I built the winglets that mount on the outside of the rudders. And I began sheeting the wing from the spar to the leading edge. A note on this, I would have been fine omitting all of the sheeting on the wing. It appears strong enough and really doesn't need the extra strength. But being that I already cut relief into the ribs in anticipation of the sheeting, I went ahead with it. That's not done yet though.

The ailerons were going to be solid balsa, but in the interest of cutting weight, I believe I am going to build them up instead. Minor change. The ailerons are not yet done.
This wing is 24" in wingspan, and that's not very big, so it's going to need to be light.

I am at 0.8 oz so far, for the wing and winglets. That's about half the plane, so I am probably okay, but I also know it's not done yet.

The winglets on this design carry the dihedral. So the angles are weird. I started with a regular notch and modified it over the plans until the spar and the first rib were at the correct angles. This is one of those things you just have to eyeball.
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Old Aug 20, 2014, 11:05 AM
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Wow this is going together fast.
Except for sheeting the leading edge of the winglets, and some final sanding, the wing is done.
I've only spent 3 days on this project so far. I'm beginning to think I should have a second BBCC5 project.

I started building up the ailerons and then realized the weight I was saving was so marginal, that I just went with solid balsa ailerons after all. (Sorry for all the back and forth).
I don't really like cutting slots for hinges. I wish they made something for my dremel tool to do it. I could use a small cutting wheel I suppose. The slot will be a little wider than I need, but it'll work.

I worked on lining up the winglets. One winglet was about a centimeter higher than the other, so I've been sanding adjustments to get them about the same. I'll probably start on the twin vertical stabilizers tonight and try to finish up the wing, so I can move on to the fuselage.
The fuselage is simple. It's just pretty much stick construction.
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Old Aug 20, 2014, 11:08 AM
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On a side note, I don't know if I need to build any washout into the wingtips.
Since this is a canard design, if it stalls, the canard will stall first, then nose down the plane, building up speed and lift over the canard again so that it starts flying. That being the case, building washout into the wing is probably a moot point.
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Old Aug 20, 2014, 12:46 PM
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Washout is a good idea, even full-scale canards use it on the main wing.

Andy
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Old Aug 20, 2014, 01:37 PM
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Quote:
Originally Posted by AndyKunz View Post
Washout is a good idea, even full-scale canards use it on the main wing.

Andy
Okay.
The winglets are the best place to incorporate wash out...when I glue them in place.
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Old Aug 20, 2014, 05:26 PM
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I would do it from the root myself. It's easy enough to do now, before covering.

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Old Aug 20, 2014, 11:28 PM
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I don't put wash out on my wing roots. If I do it, it's on the tips only.
But you may have your own way of doing things, and that's fine. Thank you for the suggestion.

The wing parts are now fully assembled but the winglets aren't yet glued on.
They won't be until I've final sanded everything and I'm satisfied with it. But the wing will be completed this week. And it looks good.
The wing weighs 1.2 oz.
We'll see how the rest of the airplane turns out.
I've seen things seem to be heading in the direction of light, but once finished, it's heavier.
I need to learn how Pat Tritle uses helium to build.
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