740mm Scratch FPV Race Wing Doesn't Fly - RC Groups
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Sep 19, 2017, 08:55 PM
Fun until flying robots attack
carcynic's Avatar
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740mm Scratch FPV Race Wing Doesn't Fly


Backstory:

My daughter wanted to get into the hobby, but insisted on only flying FPV. She wants to race/fly around and under stuff.

I got her a Ares Alara, and installed a VTx and Camera. ( https://www.rcgroups.com/forums/show...5&postcount=21)

Yes, I made her do a few LOS Buddy Box flights, and then she was off flying FPV.

Not only was she proficient with it, but it very quickly became way too slow for her. She still wanted an FPV race wing.

Getting to the Point Faster than an Ares Alara (But not much):

I told her that if she wanted a new plane every month, it would have to be homebuilt from parts we have laying around. She said she wanted it to be small and fast, and sketched a bat-wing-thing that she said she wanted to be about 600-700mm.

However, by a series of events (I actually am trying to be brief here), she came into possession of an E-Flight Manta.

Problem solved. The Manta is a little bigger than what she wanted, but it is fast enough. She's all over the sky with it, and ready for our next FPV meet.

The 740mm Scratch FPV Race Wing

So I took her sketches, and changed them into something more buildable. It is made of dollar store foam board with a central hardwood spar. This gives it an airfoil shape, and strength. It's not really a true wing, as it has a rudimentary fuselage made of thin plywood and foam.

Here is a rough drawing that the parts are cut from:



Pictures:





Basic Specs:

Span: 740mm
Fuselage Length: 240mm
Battery: 1500 or 1800mAh 3S 50C
ESC: 20 Hobbywing Skywalker
Motor: Angel 2208 2600KV
Prop: 5x4 (currently 3 blade)

The Moon Pie inspiration came from a trip to Charleston for the Eclipse. While there, we visited the Moon Pie General Store. The clerk gave me a strip of Moon Pie Stickers (and I bought lots of Moon Pies!)

The problem is that it does not fly. It seems totally unstable on launch.

CG is set to 20% MAC using the Flying wing CG calculator. (Note measurement of 115mm from leading edge of wing.)

Yes, I have a slight up curve to the trailing edge of the wing, and some up trim in the elevons.

Elevons work correctly.

I have tried launching at full throttle (it has tremendous thrust, and yes, torque), and at half throttle.

I have tried launching it like a paper airplane, and by holding one wing.

I think that at this point, there is at least a 50% chance that the problem is in the launch.

Questions:

1. How do you launch a small FPV wing? I've watched several videos (Including IBCrazy's video about launching FPV wings), but they are all for much larger wings.

2. Note that I have drawn larger vertical stabilizers in the drawing. However given the size of the vert. stabs. on other wings particularly the STRIX ALATUS, I have a hard time believing that mine are too small.

3. Could it be the up sweep at the ends of the wing? Should I get rid of them?

Other suggestions?
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Sep 19, 2017, 11:14 PM
Registered User
Launch it for her with two hands. Toss it over your head with force. Throw that thing into the air so it dosnt stall and torque roll. this is the launch used at the races. Its good for stally props and high wing loads.

https://youtu.be/Ci5hDIO_I5c?t=23s
Sep 21, 2017, 10:48 AM
Fun until flying robots attack
carcynic's Avatar
To be clear, the "Moon Pie Racer" is mine.

My daughter is flying both the E-Flite Manta and the Ares Alara.

I built the Moon Pie Racer to use up parts we had laying around, and to have something so that I could fly/race with her.

I'm going to try having someone launch it overhead, and I am going to put a 2 blade, 5 inch prop on it to reduce torque.
Sep 21, 2017, 10:16 PM
Laughs at un-boxing videos...
basicguy's Avatar
I launch all my wings by holding on to the right wing tip nose up and letting the thrust pull the plane so that is hovering. If it isn't at full throttle I move it up all the way and turn it level and release.

Red Duck Plank flying wing 2 of 2 Side arm launch. (4 min 37 sec)

1 Meter Plank (3 min 55 sec)

plank foamie RC plane 20x12 (4 min 5 sec)

It works close to 100% of the time.

If it torque rolls hold a bit of right aileron as you release the plane then let off as the planes wings start taking effect.
Last edited by basicguy; Sep 21, 2017 at 11:22 PM.
Sep 22, 2017, 05:07 PM
We don't need no stinkin' tail
RPM314's Avatar
What exactly is it doing on launch? Pitch oscillations, flat spins, rolls? For pitch oscillations and immediate stalls you should move the CG forward maybe 5mm at a time and/or reduce elevator throws. If it wants to roll and spin immediately the problem is likely yaw stability and/or your launch.

1. I launch my wings sidearm like basicguy, but 10 to 20 degrees up from horizontal. It's important to keep your wrist from yawing the wing with this technique.

2. Hate to break it to ya, but those fins are probably doing close to squat for yaw stability, or at least less than the swept wing itself. The moment arm available for stabilization via the drag of the fins (i.e. the wingspan) is much larger than the moment arm available to the lift of the fins (~150mm in your drawing). So you could try moving the fins to the tips and toeing them in a few degrees.

3. The dihedral at the tips could be promoting a roll if you're yawing the plane when you launch it. It's not necessary for this wing, so try ditching it.

Have you built any washout into this wing?
Sep 23, 2017, 10:43 AM
Fun until flying robots attack
carcynic's Avatar
Quote:
Originally Posted by RPM314
What exactly is it doing on launch? Pitch oscillations, flat spins, rolls?
It's definitely a roll thing. It usually immediately rolled right. On the last launch, I gave it a lot of left roll trim, and it rolled left. The roll is immediate and fairly violent.

I don't have to take a step to pick up the pieces.

One significant note: As mentioned, I now launch my daughter's Manta overhead. (2 hands on wings in front of me, toss over my head and backward.) On at least 2 launches before I started doing it that way, I got a bad toss, and the Manta (AS3X, in Launch ("SAFE") mode) did exactly the same roll and crash. Fortunately, the Manta is a tough aircraft (and I am doing this in tall grass), and it does not even have a single dent.

When I launch the Manta two-hand-overhead, even though the launch is nearly vertical, it flies away beautify. My Daughter can take a break for a sandwich before she touches the sticks if she wants to.
Sep 23, 2017, 04:03 PM
Registered User
What is the wing loading of the airplane?
Is it significantly higher than your daughter's Manta?
It may simply be stalling on launch due to high wing loading. Which way it rolls when stalling will be highly dependent on which wing happens to stall first due to side slip.
I highly doubt your mildly up swept tips are contributing significantly to the problem.
You may want to enlarge vertical stabilizers for better yaw stability.
Also, have you done any power off glide testing? That will clear up whether or not your CG is reasonable.
Still, most often very iffy launch consistency is due to borderline/high wing loading...
Last edited by nuteman; Sep 23, 2017 at 04:12 PM.
Sep 25, 2017, 11:42 AM
Just call me Justin.
Quote:
Originally Posted by nuteman
What is the wing loading of the airplane?
Is it significantly higher than your daughter's Manta?
It may simply be stalling on launch due to high wing loading. Which way it rolls when stalling will be highly dependent on which wing happens to stall first due to side slip.
I highly doubt your mildly up swept tips are contributing significantly to the problem.
You may want to enlarge vertical stabilizers for better yaw stability.
Also, have you done any power off glide testing? That will clear up whether or not your CG is reasonable.
Still, most often very iffy launch consistency is due to borderline/high wing loading...
+1......

Also, try a torquer prop setup at first. Help you get of the ground. Then see how it does. Then see if you can do a more speed prop. Mabe a 6x4. Or something. ....
Nov 09, 2017, 02:58 PM
Fun until flying robots attack
carcynic's Avatar
Update:

It Flied. It Died.

With little time to work on this, but wanting to "gather data" (Code word for impatience), I made a small modification, and tried it again.

I cut the ends of the wings open (like filleting a fish), and re-glued them between 2 books -- in other words, I removed the upsweep from the wing tips.

I decided to try it even though I had not made larger Vertical Stabs.

I took it to the flying field on Sunday, and my son made two attempts to launch it, with me at the sticks. It landed in the grass right next to him. I decided we should trade places -- Him flying, me launching.

Again, back to the wind, and with the aircraft in front of me, I gave it a good, fast launch. It flew! My son mentioned that it was very sensitive, so another flyer suggested he reduce throttle, which my son did. He then commented that it was still sensitive, but flew pretty well.

At this point, my son decided that he was going to "land". I think he was lining up for a landing in the tall grass left of the runway, but overshot the turn. He reduced throttle just as it was approaching the paved runway. The instant it slowed, it entered a sharp spin, and hit the pavement.

The FPV camera took the worst of it. It was installed for weight and CG, but the FPV system was not powered for this flight.

While the wings survived, the fuselage (central body) would have to be completely rebuilt. I think the main problem was still the size of the Vertical Stabs. It may also have been just too heavy (high wing loading).

While I may still rebuild it, I think for now, I will try something different.

I have 2 wings from E-Flite/HH FJ2 Fury's, One of these will form the basis of a flying wing using the same components as the "Moon Pie Racer".

It will have significantly lower wing loading, and the Vertical Stabs will be larger and further out on the wing.
Nov 09, 2017, 06:49 PM
We don't need no stinkin' tail
RPM314's Avatar
Quote:
Originally Posted by carcynic
It Flied. It Died.
Might just be the best catchphrase in the history of RC right there. Shame about the plane though.

Honestly, sounds like your plane is just tail heavy. I've had plenty of wings fly squirrelly and ready to spin out at the drop of a hat, then calm down into an obedient puppy with the CG a few mils forward. Put the CG 5mm forward of where you think it needs to be next time around, and 5mm more if that don't work.

I touched on this earlier and maybe it got glossed over, but it bears repeating that washout is very important for wings like this to handle well at high lift. I'd reckon your plane, may it rest in pieces, would do well with 3-5 degrees washout. When I started washing out my wings, the difference in stall characteristics was night and day, and your launch problems and wing dropping could be largely because of this problem.

Wing loading is probably much less of a culprit. A properly balanced and washed out wing with a high loading will need to be going faster to launch, sure, but it won't want to spin out like a madman because of smaller wing alone. You can have a wing with a brick wing loading, but enough power to accelerate in launch, handle just fine with good balance, washout, and reasonably sized fins. I know cuz I've built a few. So I'd recommend not focusing on loading too much in your next plane.

Best of luck
Last edited by RPM314; Nov 09, 2017 at 07:00 PM.


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