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Old Apr 22, 2015, 06:20 PM
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Help!
Apprentice Flying Wing

So I had the bright idea of taking an Apprentice wing and making a flying wing. It didn't work out so well, and I'd like ask if anyone knows why. Here are the details:

Wing was cut w/hot wire @ 45 degree angle
Epoxy wing together
Added spare foam to rear for straight edge and glued balsa on top & bottom of rear center to mount the motor (Cheetah A2212-06) and prop (6x4). I don't assume the rest of the electronics is as relevant (I could be wrong).
Create winglets for vertical stabilizers on the ends of the wings, also at 45 degree - so looking at it from front, they are straight down the underside of the wing.

So I power up to full throttle, and toss this thing up, and it immediately rolls left. Initially didn't have full servo throws on my DX6i, but corrected that and tried again.

Second try, same throw, same result.

Every following attempt, does exactly the same thing.

I've read many posts & threads, but see reasons from motor torque to wing shape. I'm considering trying again, but obviously don't want the same problem.

Ideas & help would be awesome!
Thanks in advance!
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Old Apr 23, 2015, 12:25 AM
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Couple things to check - are the elevons moving the correct directions? Are the mechanical links for the elevons skewed at all? -- if nothing else you could adjust the mechanical links on the elevons to give it some roll right to see if that corrects the problem.

and.. where's the pics?
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Old Apr 23, 2015, 04:58 AM
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Post a video of the control movements with the transmitter inputs visible in the frame so that we can verify all is moving as it is supposed to.
Post pictures of the plane from various angles.
Give us FULL specs of the plane (wing span, chord, weight, CG location, etc.).
Post a video of the next launch (do it over tall grass so the plane doesn't get damaged. Try launching with less than full throttle.

This sort of thing has been done before, so you're just missing something critical - we'll figure it out.
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Old Apr 23, 2015, 05:09 AM
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Sorry, no pictures available as the last attempt broke the left wing in half. Once I get the new one ready to go, I'll post pictures.

The elevons were set correct.

Root Cord: 13"
Wing Tip Cord: 8.5"
Sweep Distance: 21"
Half span: 20"
MAC: 9"
Sweep @ MAC: 10"
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Old Apr 23, 2015, 07:52 AM
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CG is absolutely critical on these flying wings (much less tolerant range).
Post a drawing (annotated with dimensions) with the CG marked.
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Old Apr 23, 2015, 07:57 PM
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Here's the drawing. The numbers from the bottom were calculated on the website http://fwcg.3dzone.dk/

Hope this helps you help me!
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Old Apr 24, 2015, 01:01 AM
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What you showed in the drawing is not what the CG calculator in the link you provided draws (see image below for what the data you entered means geometrically).
An therein lies your problem (at least one of them).
Your actual planform requires a different CG (slightly more forward - and that can make all the difference in the world with flying wings, as you've no doubt heard/read).
By the way: You've got a lot of sweep - this will make for very sensitive/twitchy roll response. Typical sweep is between 20 and 35 degrees.

So there's a series of things you should do:

1. Move the CG forward in small increments (like 1/8 inch at a time) and glide test (power off chuck over tall grass or similar). When it seems to glide best, mark the CG.
2. Make trim adjustments (up/down, right/left) till your glide is as best as you can get it.
3. Do 1/4 power launches and re-trim for that (again, over tall grass).
4. Do half power launches and re-trim for that. Depending on your motor/prop, this may in fact be your "nearly optimal" launch power.
5. Repeat step 4 at higher power settings till you reach what feels "comfortable" for you.

After getting the plane to fly, you may want to consider reducing sweep.
Also, from the drawing you gave, it's not clear where the winglets are, what size they are and how exactly they are oriented.
Also, is there any dihedral to the wing?
What is the all up weight??? This is absolutely critical. Wing loading has a huge impact on the ease/difficulty of hand launching.
Again, pictures and video will be very useful. Otherwise, there's a lot of room for miscommunication.
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Old Apr 24, 2015, 01:36 AM
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Assuming the servos and control surfaces are set up correctly and working, some of the other things that could go wrong are:

1. CG balance: what %MAC are you balancing at? 20% should be good start point. Make sure the plane is just slightly nose heavy at 20%
2. Balance along the roll axis: Make sure the plane is balanced along roll axis and no wing tip is dropping when held with finger tips at nose and motor shaft.
3. Elevon Reflex: Measure and make sure the elevons are reflexed to same degree
4. Elevon travel: Make sure both elevons deflect to same angle as elevator. In other words when you are trying to input "up elevator" elevons aren't actually acting as aileron.
5. Thrust angle: Ensure motor is mounted parallel to wing root axis and not some skewed angle
6. Rates and Expo: with the sweep angle on your wing, it's going to be really sensitive for roll. Make sure you have dialed down the ailerons or have dual rate set to low for the maiden.
7. Launch: Finally, most important. When launching make sure motor is generating enough thrust to actually push the wing through the air and get to Vr without stalling. Before I let go of the plane, I get the throttle up to the point where it feels really light in the hand. Do you feel the motor is generating enough thrust? With the steep sweep angle you have, I'd expect stall speed to be somewhat higher. Also it's really important to keep the wing tips level when you let go, so as not to induce any bank.

BTW, the way you are showing tip chord in your picture is not right. Tip chord is measured parallel to the root chord. Better luck next time!
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Last edited by elewon; Apr 24, 2015 at 01:44 AM.
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Old Apr 24, 2015, 06:16 AM
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I've attached an updated picture. Remember, this is an Apprentice Wing, so I did not cut the wing tips but left them exactly as pictured.

The red line is where the winglets were. They stuck out from the bottom about 4-5" (I just eyeballed it to try and clear the prop when it landed).

I have added what the wing tip cord would be if I had cut it off.
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Old Apr 24, 2015, 06:23 AM
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Quote:
Originally Posted by nuteman View Post
What you showed in the drawing is not what the CG calculator in the link you provided draws (see image below for what the data you entered means geometrically).
Correct, but I was trying to be correct in what the wing was actually shaped like vs. what the drawing created.

Quote:
Originally Posted by nuteman View Post
By the way: You've got a lot of sweep - this will make for very sensitive/twitchy roll response. Typical sweep is between 20 and 35 degrees.
Should this be reduced, or can it fly as is? I'd rather cut the wing once vs. 2+ times.


Quote:
Originally Posted by nuteman View Post
So there's a series of things you should do:

1. Move the CG forward in small increments (like 1/8 inch at a time) and glide test (power off chuck over tall grass or similar). When it seems to glide best, mark the CG.
2. Make trim adjustments (up/down, right/left) till your glide is as best as you can get it.
3. Do 1/4 power launches and re-trim for that (again, over tall grass).
4. Do half power launches and re-trim for that. Depending on your motor/prop, this may in fact be your "nearly optimal" launch power.
5. Repeat step 4 at higher power settings till you reach what feels "comfortable" for you.

After getting the plane to fly, you may want to consider reducing sweep.
Also, from the drawing you gave, it's not clear where the winglets are, what size they are and how exactly they are oriented.
Also, is there any dihedral to the wing?
What is the all up weight??? This is absolutely critical. Wing loading has a huge impact on the ease/difficulty of hand launching.
Again, pictures and video will be very useful. Otherwise, there's a lot of room for miscommunication.
I'll try to answer these (and Elewon's) questions when I get the new wing and try again.
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Old Apr 27, 2015, 07:33 PM
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Quote:
Originally Posted by celtic_crasher View Post
Correct, but I was trying to be correct in what the wing was actually shaped like vs. what the drawing created.


Should this be reduced, or can it fly as is? I'd rather cut the wing once vs. 2+ times.




I'll try to answer these (and Elewon's) questions when I get the new wing and try again.
On wings I usually go for 28-29deg sweep. More sweep is good for stability but it also makes it more sensitive to roll.

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Old Apr 30, 2015, 02:55 AM
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Check both wings for parallelism. If they aren't perfectly parallel, that could cause it to roll.
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Old Yesterday, 09:11 PM
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Quote:
Originally Posted by elewon View Post
On wings I usually go for 28-29deg sweep. More sweep is good for stability but it also makes it more sensitive to roll.

I agree. I usually use the TLAR method and shoot for around 30 degrees. I've build several wings this way and all of them have been successful. Here's one of them...
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