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Aug 22, 2014, 01:37 PM
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Build Log

BBCC5- Arrowhead-based "Baby Gryphon" flying wing RC Conversion

I am announcing my second BBCC5 contest, which will be built at the conclusion of my XP-55 project.

LINK to the BBCC5 contest thread here...

This is the Arrowhead FF flying wing glider, from the Outerzone website:

Original Arrowhead, designed by JT Holmes in 1947, for free flight

Arrowhead plans

This is only a 14" wingspan, but I saw potential in it.
When I printed up the plans, I selected 11x17 paper, which expanded the wingspan out to 22". I noticed that's not the original wingspan, but the rib spacing was still only about 2" apart, so it'll work. Then I started cooking up an idea.

This would make a great parkflyer wing. If I put a small motor in the nose with a small folding prop, and built out a new fuselage for it, to house the electronics, but use the original wing, I realized this would probably work.
However, if you notice, the designer used rounded trim tabs near the wingtips (elevons, if you will), to set at 20 degrees up-angle for reflex. Using this as reflex, the wing flew stable as a hand-toss glider.
So what I'm going to experiment with is a split elevon, along the entire length of the wing, instead of only at the wing tips. However, I plan to separate it into ailerons and split elevators, separate from the ailerons.
I'm doing this to experiment with reflex on a flying wing.
This also has an added advantage that a non-mixing radio can be used on this wing.

What the Arrowhead is going to look similar to is a baby parkflyer version of my Gryphon flying wing, except with a flat-bottom airfoil and a motor with folding prop in the nose. Here's a good picture of that, to give you all an idea.

The idea I've had for a long time is to make a removeable nose in the Gryphon to allow an outrunner motor with a folding prop, but still be able to revert back to a pure slope soarer just by changing out the nose cone.
I think this little Arrowhead will be an almost identical smaller version of the larger Gryphon, so this gives me a chance to test the concept.

The reason I am entering a second BBCC5 entry is because I really don't think the XP-55 project will take me very long. I'll get done in a month and have 5 months left. It's only been a week and I've already got the wing finished on the XP-55.
I liked the Arrowhead and got excited about it, but noticed someone else was talking about entering it into BBCC5. So I was originally going to just build it for myself. But when I asked him about it, he conceded and told me to go ahead because he is going to build something else. (So thank you...).

So this won't actually begin until my first BBCC5 project is about a month. In the meantime, I'll work out my modifications, including the new non-profile fuselage. It won't exactly be an Arrowhead when I'm done with it, because of my RC modifications.
I suppose it could also be called a Baby Gryphon.
Last edited by builderdude; Sep 18, 2014 at 10:24 AM.
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Aug 22, 2014, 07:51 PM
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With the broader fuselage, you could call it the Broadhead.

For the fuselage section, are you going to round it or do something like your interceptor?
Aug 22, 2014, 08:40 PM
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I was thinking I would do a full fuselage, because the one thing about the Interceptor was how difficult it was to access things. Even the battery was a tight fit. As great as that plane flew, it was really packed on the inside.

The XP-55 is going to be like the Interceptor, real narrow.
So I was thinking I'd put a normal fuselage on this one.
Thus, normal access to equipment.
To preserve the wing area of the original Arrowhead, I'll add to the wingspan to make up for the width of the fuselage.
It might turn out to be a 24" wingspan.

There just aren't too many options for the Arrowhead's original fuselage. As an RC glider, yes, the fuselage can be shaped like that. But I had to make changes anyway for motor provisions, etc.

I want this to be a parkflier flying wing.
It would be nice if I come away with it not weighing any more than 6 or 7 oz.
And it's possible, if I'm careful.

I need to work out what the wing area will be, so I can work on my target weight budget.
Aug 22, 2014, 09:58 PM
skumgummi dave

Looking forward to see what you do with the Arrowhead conversion.

Aug 22, 2014, 10:53 PM
I'd rather be flying.....
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Hi Builderdude - i'll be watching your arrowhead conversion ! I posted a link to a similar model from Air World I want to make rc sometime..... I think it's the same designer...

Best Regards,

Aug 23, 2014, 08:54 AM
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Thanks guys.
I'm actually going to start this project after I complete the XP-55. But that won't take long.
After that long and arduous Vampire jet build, I've been building smaller, simpler models.
I'll start it soon, but not yet.
Still working out some fuselage details...
Aug 24, 2014, 09:54 AM
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I think I might have settled on how to handle the fuselage.
I've drawn a sketch and it also copies how the Gryphon's fuselage was built.
The Gryphon has balsa sheet sides to its fuselage, with triangle stock in the corners, and then a solid balsa nose cone is carved for it. Then the wing is built as a single unit and meets in the middle, inside the fuselage, with fiberglass cloth around the joint.
And that big 5 1/2 foot flying wing is surprisingly light! I think the whole thing weighs just a little over a pound. Incredibly light wing loading.

Likewise, if I build my fuselage with 1/8" balsa siding and maybe 1/4" stick balsa in the corners, I'll have a fuselage that is strong and will have enough meat to sand the corners rounded like the Gryphon was done. For the nose cone, I'll probably handle that just like I made the nacelle on my Firefly 60" RC build not too long ago. I'll use 1/4" balsa sheet and build it up, and then sand it rounded. And that will be removable for motor access.
What I did on the Firefly that works well is that I used a couple of rare earth magnets to simply hold the cowl onto the nacelle. It's not going anywhere, because the prop is in front. And it was quite secure. I didn't have a problem.

For the motor, ESC, and receiver, I decided to use the Gunther Micro that I had in my Interceptor. That is a powerful little motor! 3850 kv, 60 W, and only 12 grams. I was very impressed with this motor in the Interceptor. (The Interceptor never flew right after it was crashed while doing near-ground aerobatics, so I salvaged it). I'll use the same 10 Amp ESC and 72 MHz receiver that I used in the Interceptor. It's 4 channel, but since I won't need a rudder this time, only 3 channels will be used. The Futaba radio I use cannot do elevon mixing, but I'm splitting the elevons on this Arrowhead into separate ailerons and a split elevator anyway, so I'll be okay.

All that's left is to sketch the simple fuselage.
The wing will have some dihedral for it, since this is going to have a flat-bottomed airfoil and will be mainly for park flyer duty. It won't be for aerobatics, although aerobatics are possible.
I can do some simple stuff, like rolls and loops, but I'm not very good at flying inverted yet anyway, so I won't build it for that.

The one thing that'll be difficult to find is a folding prop small enough.
5x3 is the smallest folding prop available, I believe. And that might work, although it won't give it the speed that a 4.5x4.5 would. But it would have better static thrust I would imagine. (Torque, in the automotive world). That might be okay. The Interceptor was kind of fast and jet-like with the 4.5" pitch prop, so I could sacrifice some of that speed for the ability to fly right out of my hand.

Or, I could get the folding prop from a Hitec eHawkeye, which is small enough.
I thought about using an eHawkeye motor, but the Gunther Micro is stronger and I already have it, salvaged from my old Interceptor.

I'll probably draw up some preliminary fuselage plans, just on small 8.5xll printer paper.
I'm thinking about a 2 inch width, because extending the wings out 2 more inches would give me exactly a 2 foot wingspan, which is a nice size for a park flyer and a good, round number.
It's also the same wingspan as my XP-55 will have.

I find that after my elaborate Vampire build, I just want to do simpler builds for now.
I could have built half a dozen smaller park flyers in the time it took to build that jet.
(It won't be the last time I build a jet, but I'm taking a break).

Actually, EDF would have been a really cool way to power this Arrowhead "Baby Gryphon".
But let's see how the prop job does first, and if I want, it's a simple enough build that I could revisit it in the future and do a micro EDF effort.
Aug 29, 2014, 04:25 PM
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Alright. Long weekend. XP-55 project is finishing up.
I think it's time to tackle this project and begin the construction.
So the wing is going to go on my building board this weekend.

I had to make a few modifications to make room for a fuselage, since the original Arrowhead only had a profile balsa fuselage. I am going for a 2" wide fuselage, and probably about a foot long. Therefore, I had to add one more segment to each wing. This makes the wing a true 24" span. The airfoil is pretty thin, though, so I'm going to have to go small on the aileron servos so that they fit. Also keep the aileron servos as inboard as possible.
I was kind of thinking about adding just a little more thickness to the airfoil, but not sure if I should. I don't want to mess too much with a proven design.

The bigger question in my mind is, since I'm splitting the elevons into separate ailerons and elevators, where do I make that split? I guess it depends on how much authority I want to give to the ailerons versus the elevators. (Elevators because it is a split elevator).

Something to think about while I build it.
If I thought the XP-55 went together quick, this build is probably going to be quicker!
There just isn't much to it.
Aug 29, 2014, 04:50 PM
skumgummi dave
What if you make it 36WS? Wings should be thick enough then...

Aug 29, 2014, 05:20 PM
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That's true, Dave.
Problem with that is that the motor, servos, ESC, and receiver I have for it was salvaged off of a plane that similar size when that crashed, and if I go up to 36" wing span, I'm not sure if that will start getting too big for the motor and stuff that I have.

I think the wings are thick enough for the small servos that I have. I'll have to double check that.
Aug 29, 2014, 11:42 PM
skumgummi dave
Oh silly me. I knew that when you first mentioned it.... Doh

Aug 30, 2014, 12:01 AM
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This looks like an interesting model. Might even be small enough for indoor flying this winter. I don't think a slightly thicker airfoil would be a disadvantage; it might even help.
Aug 30, 2014, 10:26 AM
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Originally Posted by JohnAV8R
This looks like an interesting model. Might even be small enough for indoor flying this winter. I don't think a slightly thicker airfoil would be a disadvantage; it might even help.
You might be right.
My Gryphon flying wing has a really thick airfoil.
I even have a foam plane I built myself using a Stryker wing, and that has a thick airfoil. The one thing a thicker airfoil will create, along with more lift, is more drag, which is okay if you're building a wing small enough for flying in a park...or even indoors. (And that was my thoughts as well, John).

Our flying club does use a large B-29 hangar in Plainview, Texas (I think that's the location), for winter flying. This wing would be a natural there. In fact, so would be my XP-55 I'm also building now. ...As well as a smattering of other small planes I've been building.

Okay, I'll increase the airfoil a couple percent. Now is the time to decide, before I actually create the print wood. (I might need to go pick up another sheet of 1/16" today, come to think of it).
Sep 01, 2014, 08:13 PM
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Know what I love about days off?
My building workshop!

I got all the printwood cut and built both wings in one day!

I'm going to let everything dry before attempting to join the wings. I also have to think about whether I want any dihedral, and if so, how much. The advantage to not having any dihedral is that it'll be better at aerobatics. But maybe for the sake of stability, I should put a little into it.

The notches were off on the plans. I'm glad I didn't cut them until I had the spars in place. I always check the notches before cutting them. But I got the angles cut about right.
Sep 02, 2014, 09:56 AM
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Tonight I will work on attaching the ailerons and split elevator, and will also attach the wing halves together and sand. I haven't decided yet if I'm going to try a single elevator servo, or two in the wings in parallel. It would be nice if I could just use a single servo. There are a few ways I can think of to do that, but it had better be a fairly strong servo I think.

Probably the best way is to have a music-wire link from a clevis on one elevator surface, through the fuselage and through the servo control horn in the middle, then back out the fuselage and into the clevis on the other elevator.

I know I could just wait until I get a new 2.4 radio with mixing capabilities, but part of this experiment is to see if I can use a non-programmable radio on a flying wing.
If not, how hard is it to glue the split elevator to the aileron and just go with a 2.4 radio? That would be a non-issue. And one advantage to doing that would be that I could set additional reflex on the ailerons.

Oh, by the way, I did thicken the airfoil by a few percent from that on the plans...just to allow myself ample room for servos in the wings.

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