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Old 08-01-2013, 12:32 AM   #1
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Default A "park flyer"

Although my club has a first-class flying field, it's a 20 minute drive for me. So going there to fly means 40 minutes of driving + however long I'm there; not a trivial amount of time. I've got a couple of nice city parks within walking distance (almost), and I'd like to have a plane small and slow enough to fly in them. So...



... I've started on a new foamie . This evening I've cut the wing cores from blue foam. 4.75" chord, about 30" span, flat-bottom airfoil. I'm thinking it will have a little bit of dihedral, so it's not quite so "twitchy".

No plans yet, but I'm thinking in terms of a pod-mounted pusher arrangement with no landing gear. I've got some 350 mAh 2 cell batteries and 5 gram servos on the way. I have a Heads Up RC 250 Sport motor on hand, but I think I'll get a smaller one (and a tiny esc to match).

I want to be able to do simple aerobatics, but want to be able to fly it in a relatively small area...

More as things progress!

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Old 08-01-2013, 12:44 AM   #2
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Looks ok so far.
How many channels?
Is the wing going to be removable?
350mAh sounds a bit on the small size for a 3ft span.
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Old 08-01-2013, 03:20 AM   #3
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Originally Posted by quorneng View Post
Looks ok so far.
How many channels?
Is the wing going to be removable?
350mAh sounds a bit on the small size for a 3ft span.
Yup
You will want about 100-130 watts per pound of airplane on this, depending on how you want to fly it. So, if your model, ready to fly weighs in at one pound, that is about 125 watts. A three cell 350 Mah battery has about 10 Volts, so Amps equals watts/volts, or about 12.5 Amps. At full throttle, your battery will last a bit over a minute. If it doesn't burn up in the mean time.

Perhaps a 1300 Mah battery pack would be more reasonable. That battery pack would last about 6 minutes at full power. Or perhaps a battery pack with about 600-800 Mah would be even better.

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Old 08-01-2013, 02:51 PM   #4
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The wingspan is going to be around 28" - 30", not 36", and I don't expect it to weight more than 10 oz or so. But I'll make room for bigger batteries .

Ailerons & elevator, no rudder.

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Old 08-02-2013, 12:35 AM   #5
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Not much today, but I've inlaid an 1/8" x 3/16" pine spar in the wing halves and rounded off the wing tips. Tomorrow I'll do an 1/8" plywood joiner with 2 degrees dihedral (total) and glue them together, with that 3/32" plywood motor pylon captured between:



I've got the fuselage pretty well thought out, but not yet detailed on paper (to make sure all the electric bits, etc. fit). This one is going to be painted (rather than covered in packing tape as I've been doing) so I can have compound curves all over the place .

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Old 08-02-2013, 12:47 AM   #6
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We need to get you thinking "light" for electric. What is the weight goal?
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Old 08-03-2013, 12:08 AM   #7
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The weight goal is 8 - 10 oz. I think I'm doing OK; put the wing, the motor, the esc, a battery, some servos, and a chunk of foam about the size of the fuselage on a scale and it came in at 6.75 oz. Of course we've got push rods & control horns & what-not yet to go, but I think we're doing OK .

This afternoon I glued the wing halves & motor pylon up, but mainly worked on the fuselage. It turns out that all this wood working machinery I've got here works just fine on foam too. Here's the core & the sides:



I re-sawed the side pieces with a band saw and then got them to final thickness with my thickness sander .

I tapered the core piece and inlaid a couple of 1/8" square pint longerons:



...then glued on the side panels:



I've got a pretty huge "equipment tray" built in there, which should allow for moving things around to get the CG right.

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Old 08-03-2013, 12:39 AM   #8
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dgjessing
Nice work!
If you can keep to under 10oz you will be doing well. Not quite in the 'super light' class but still pretty good.

With modest power a pylon mounted motor above wing should be fine but if you really go up in power it can cause a strong nose down pitch as you open the throttle.

One solution is a pylon wing mount as it gets the centre of drag closer to the thrust line yet retains a fully protected prop. Its a configuration I have used quite a bit on my 'performance' park flyers.
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Old 08-03-2013, 02:18 AM   #9
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Yep pine has no place in light. I used to overbuild everything. Then I tried no spars and lighter stuff, then you start to get flutter and flexing and broken. Like the old bipe pilots that would start taking wires off till they wing flew off then they put one back. O yea...

Keep up the work - it is all trial and error, but it all adds up. Remember the radio, equipment, motor ESC and battery are about half of the weight.
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Old 08-03-2013, 11:38 PM   #10
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Here we are so far:



All that stuff weighs 5.5 oz.; I'm thinking an 8 oz. AUW is achievable without too much trouble.

Now that I can see most of it together I'm getting pretty concerned with the height of the motor/thrust line... Jamming on the power is going to have the potential to be violent . Getting the thrust angle right is going to be critical. Oh well, we'll see

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Old 08-04-2013, 01:16 AM   #11
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So far so good but it does look a bit short of dihedral for 3 ch 'rudder only' control particularly with the weight of the motor well above the wing.

There will be only so much you can do to limit the effect of its very high thrust line. In an ideal world you would like the motor axis to pass through the centre of drag but that is simply not possible in your configuration.

It reality all you can do is to simply try it to find out just how much of an issue it is.
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Old 08-04-2013, 04:16 AM   #12
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One way that may eliminate any concern for dihedral, is to use v-tail control instead of rudder. It would definitely make it an original also. The v-tail will initiate roll quite effectively on turns. I discovered how well it works accidentally, on a small HE162 30mm EDF with a high thrustline and not much dihedral, when the elevator joiner bar was flexing unacceptably. Roughly a mere 1/16" of mismatch of the elevator halves caused by pulling up on the elevator initiated a hard right turn, all while staying level at the same time. The first right turn here was purely unintentional, caused by pulling up on the elevator after launch:
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Old 08-04-2013, 10:04 PM   #13
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Default small foamy

I hope this picture goes though, the flying wt. 7.4 oz. 33" span, 180 1100kv motor from heads up. The plane is a EZ-1 pusher made from $ store foam, it could be close to what your building. I fly at a small park near my house,like you my club is 25 mi. away. This little plane fly's great.


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Old 08-04-2013, 10:10 PM   #14
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dgjessing- please add the ailerons- it would make me happy-lol
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Old 08-05-2013, 01:03 PM   #15
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Originally Posted by dgjessing View Post
Ailerons & elevator, no rudder.

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Old 08-05-2013, 11:46 PM   #16
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And here are those ailerons, plus the horizontal stab & elevator, and the vertical stab (w/o a rudder):



I've used those fiber hinges - just little 1/16" wide pieces so they are flexible enough. The leading edges of the stabilizers both have .038" dia. carbon fiber rods inlaid in slots cut in their edges (visible in the vertical one).

Even though I'd kinda decided to use the 250 size motor, I went ahead and ordered a 180 size, and it (and a 7 amp esc) arrived today. Now I'm thinking of using the smaller one... The 250 and it's esc weigh 1.75 oz. The 180 and it's esc weigh .9 oz. All of the airframe components, a receiver, battery, servos, and the small motor & esc weigh 5.45 oz at this point. I'm thinking about 7 oz AUW is possible with the small motor!

I'm locked into a 5" prop due to the height of the motor pylon. According to Heads Up Rc, with that limitation the 250 can put out about 7 oz thrust, the 180 about 5 oz thrust. Given that it would weigh almost an ounce more with the bigger motor, we're looking at not much difference in the thrust-to-weight ratio with the smaller motor. Hummmm....

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Old 08-06-2013, 01:26 AM   #17
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Just remember the smaller the plane the less efficient so relatively high thrust to weight ratios are a necessity.

On this basis the small motor should be ok but the performance will be ...er... quite modest and you likely to need close to full power most of the time.

I am still a bit concerned at the small battery. Do you know what the current draw is likely to be with the small motor? If it is any where near the maximum of your ESC (7A) a 360mAh battery will be exhausted in no more than 3 minutes.
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Old 08-06-2013, 01:45 AM   #18
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Originally Posted by quorneng View Post

I am still a bit concerned at the small battery. Do you know what the current draw is likely to be with the small motor? If it is any where near the maximum of your ESC (7A) a 360mAh battery will be exhausted in no more than 3 minutes.
Yeah
Here is some information on what's involved in short time discharge of those LiPo batteries, or any battery for that matter.
http://www.wattflyer.com/forums/showthread.php?t=65869

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Old 08-06-2013, 12:40 PM   #19
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Originally Posted by quorneng View Post
Do you know what the current draw is likely to be with the small motor?
I'll measure it today (got a meter, finally).

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Old 08-06-2013, 04:59 PM   #20
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Meh- I thought rudders were only to steer tail wheel anyway
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Old 08-06-2013, 06:57 PM   #21
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If you are trying to fly out of small areas a rudder would be very helpful. It will allow for fast, minor flight path adjustments, without the delay inherent in the "yank-and-bank" philosophy. Full, basic controls also give you options in crosswind landings, which are much more likely to occur when forced to fly out of small areas.
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Old 08-06-2013, 11:20 PM   #22
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OK, we're at 5.3 oz with everything pictured:



The only things missing that I can think of are the propeller, horns, pushrods, battery compartment cover, and whatever finish will eventually go on it. I think it will turn out to be under 7 oz AUW .

I put the motor on a watt meter this morning. With a newly charged battery it initially spikes to 7 amp, then settles in to about 6.2 amp within a few seconds, at full throttle. At ~80% throttle it draws around 4.5 amp.

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Old 08-07-2013, 12:08 AM   #23
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Really the question is how close to full power you will need to fly and you wont find that out until you try!

Anyway it certainly looks the part, well done!
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Old 08-07-2013, 01:04 AM   #24
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Well done - excellent job.
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Old 08-08-2013, 12:15 AM   #25
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Thanks!

Not much today - had a lot of "real" work to do .

But I did put that little piece of foam in front of the wing, which will be sanded to blend in with the wing and fuselage. I also mounted the motor, and stuck on a little piece of foam to form a streamlined "pod" in front of the motor.



Slowly but surely...

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