DIY Glider

[malik98]

Hello all,
Today I had the urge to build a glider from DTF and was wondering if somebody could tell me whether or not it'll fly and what, if any, bad tendencies it'll have. The sizes are as follow,
Wingspan: 60 in
Wing Root Chord: 5 in
Wing Tip Chord: 5 in
Fuse Length: 30 in
LE to Prop Distance: 9 in
Motor: 2208/14
Prop: APC 8x6
Battery: 1300mah 3s
AUW: 386 Grams
Servos: (2) Basic 9 Grams
Transmitter: DX6i
Receiver: Ar600
Airfoil: KFM3
LE to Stabilizer: 12.5 in
Horz Stab Dimensions: 16.5 in x 4.5 in
Vert Stab Dimensions: 8 in x 3.5 in
Elevator Dimensions: 16.5 in x 1.5 in
Rudder Dimensions: 8 in x 1.5 in
CG: Approximately 1 in from LE

Cheers,
Malik

 

[Craftydan]

A lot of numbers, but hard to tell from the missing details.

60" span and 5" cord is a fairly thin wing. Depending on the airfoil build this could be good or bad, but generally the aspect ratio on a wing will trade slipperiness with lift. If it's a fairly thin airfoil it should cut through the air well with good lift. if it's fairly fat it'll have a lot of drag without much lift to make up for the cost.

CG at 1/5th chord sounds nose heavy to me. VERY nose heavy, especially for a KFm3. I'd expect neutral to be closer to 2-2.5" on a KFm3, but that's all by RoT. Frankly, nothing will beat a few gentle glide tests for finding the sweet spot.

~10" is in the nose, 5 in the wing, 5 in the stab so the length between the TE and the stab is a little over 10". 60" wing? Sounds VERY short coupled. a good chance she will be lightly unstable in pitch, and a good chance rudder will be anemic.

Overall how should she fly? My guess will be pitchy and nose heavy, but with an able pilot, just fine . . . but a lot of that will depend on details not mentioned and how you put her together.

 

[malik98]

Ok, my CG was just an estimate based on sight since I hadn't done the math yet. The TE to front of stabilizer is 7.5" and 12" to the rear of the stabilizer. With a thrust to weight over 1 will the power compensate for the bad tendencies as long as I move the CG back some and how well will it glide? My wing thickness at the thickest point is 0.5" and the airfoil is a KFM3 (Kline-Fogleman), how well will this allow it to glide?
I'll try to upload my pictures later.

Will expo fix the pitchiness?
It's a highwing with standard tail.

 

[Craftydan]

With the throttle on, it won't glide at all

Having a good thrust-to-weight will only help in launch. In glide you get no benefit, so don't count on it to overcome bad tendencies.

Again, it sounds short coupled -- that makes for lower stability in pitch, and lower authority in yaw.

As for glide performance, I've seen fatter wings than 1/2". I've seen thinner. The KFm series foils sacrifice pure clean lift performance for other benefits -- easier construction and wider CG envelope. They do fly. They can fly acceptably well. They won't beat a clean airfoil of the same chord and height, but those are hard to make. you've got a fair compromise with that selection. I would expect fair performance out of it if your LE is smooth.

Any dihedral? Polyhedral? Washout on the tips?

If you haven't yet, figure out how to turn on the brake feature in your ESC.

 

[malik98]

I have no polydihedral or dihedral but will probably add wingtips and maybe a bit of dihedral.

 

[malik98]

Link to picture of plane
https://lh3.googleusercontent.com/-.../zYfg_YwHY6o/w997-h561-no/20150407_231531.jpg

 

[Craftydan]

With no ailerons, you must. You will have NO control over roll without a self-righting wing.

My Recommendation:

Go about 1/6 of the span (~10") in and cut a narrow (1/8" at most) sliver out of the top of the wing, wider in the middle and narrowing to a point about 1/4"-1/2" from the LE. Then with the wing flat on the table bend up the wing tip around 3-4". How far up is really to your taste -- most designers will use previous wings or will build a few and see where enough is enough. lay it flat once more, inject hot glue into the slot and bend up again. as you are holding it up to let it dry, hold it from the trailing edge. The natural crush of the foam will warp that tip upward slightly giving you a little washout (helps prevent tip stalls).

Repeat on the other side taking GREAT care to duplicate it as accurately as you can.

The lift spilling off the polyhedral will keep the wing level, then the offset ruder will give you a gentle roll as it yaws the plane. release the rudder, and she'll gently return to level.

 

[malik98]

Will doing polydihedral better these effects and will this fix the problem of an anemic rudder?

 

[ghost civic]

from my experience the LE to nose should be 1 to 1.5 the length of the wing cord and the TE to tail should be 2-3 times the wing cord. For a glider I would assume that you would want a short nose with all the electronics in it and a longer, light tail.
given a 60'' wingspan, I would have at least a 6'' wing cord myself.
also, the planes I have made that used similar electronics and battery and that have had 30-40'' wingspans were usually 450-500grams. I do go a bit heavy on the tape and glue, but I don't want my planes snapping in half the first time I crash. and with any experimental design, you are almost always going to have 1 crash or bad landing.

 

[Craftydan]

Will doing polydihedral better these effects and will this fix the problem of an anemic rudder?

Not really. It'll fix a problem I didn't realize you had -- roll instability. You have nothing preventing the plane from slowly rolling over except the rudder so you'll constantly be yawing to try to keep the wings sorta level. That's no good.

Increasing the tail length will improve the pitch stability and elevator authority. RoT I've heard for elevator placement is 1/2-1/3 wingspan. which would be 20-30" for your wing -- A bit higher than the 10-ish you have now. the tail, which acts as a lever against the fulcrum at the CG is around 1/3 to 1/2 the length it would typically be. I won't say it won't work, just that it won't be particularly stable.

 

[malik98]

You were definitely correct about the anemic rudder. My first problem was the constant left roll. I moved the wing a bit left to fix the problem. Second the yaw was off so I trimmed it off, third it took about two right loops with left rudder to change the flight direction.
When I cut throttle it noses in a tad then glides perfectly, is this normal?

 

[Craftydan]

It's pretty typical for gliders to drop their nose a bit in the glide and nose up strongly in launch -- in a glide they *are* falling, just more forward than down. giving it a bit of down thrust will help tame the aggressive climb with throttle, but in fairness, that's pretty much what you want the airframe to do on launch anyways. You can also trim the elevator up a bit to bring the nose up and slow her down, but if you're trying to thermal that's not really to your benefit.

Typically glider pilots will have the "cruise" glide trimmed clean and fast -- if you're not in lift, you're falling, so make the best use of it. This way they can cover more ground in search of the next thermal to climb in. Find one, turn hard into it and switch to a "thermal" mode, which is slower so you stay in the lift as long as you can.

On a three channel power glider like yours, really the best you can do to slow down is click your elevator up a bit to slow down. Keep it above stall, but otherwise, it's hurry to the next thermal, hook it and take your time in the rising air.

If you're just wanting a rollercoaster, however, all you have to do is punch the throttle to get to the top of the hill, and enjoy the ride back down -- All the tweeking and trimming to optimize for lift is overkill.

 

[malik98]

I finally got my plane flying perfectly. I added 3.5" of polyhedral to each wingtip and that stopped my roll problem. It glides really well but I haven't gotten that high yet since I was battling a 15 mph wind. I downsized my motor to a 250 size and get about 200 grams of thrust at full throttle which will give 25 minutes of flight with a 1300MaH. Can you have spoilers flaps and ailerons all in one surface as I want to save weight by using only two servos?

 

[Craftydan]

You mean spoilerons? Flaperons? yup. you'll need two seperate channels on your radio -- one for each servo -- and your radio will need to be able to mix for you. The DX6i has a "flaperon" capability, so it's a matter of touching the right settings, and the difference between the two is only in direction.

They work better when the control surfaces are very long -- ailerons do most of their work the closer they are to the wingtip, and flaps do most of their work closer to the root.

All that being said . . . are you sure you want ailerons? Three channel gliders can perform VERY nicely, in some cases letting the wing handle the roll more efficiently than a pilot would. Flaps/spoilers change the way the wing works -- flaps trading drag for lift, spoilers cutting lift from the wing for shorter landings. It's a handy feature for thermal soaring and gliders on short fields. Ailerons are nice, but not as useful as you might think.

 

[ProfessorFate]

Hey Very interesting thread, My 1st plane was a kit Olympic 650, in 1986

I like gliders, used to have a Pulsar 2000, uses an RG15 airfoil with a chord length of 7.5 inches, I cannot find the wing but it's thickness was like 3/4" and it flew pretty fast just gliding. You might really look at the high performance gliders that are sold (Esprit Model), that you know work and examine their shape, AUW, airfoils they use. See youtube videos

Download Profili for free you might enjoy using it to draw your airfoils if you make wood wings
Link

http://www.profili2.com/eng/pannelli.asp

I'm glad to see your model flies good, looks kind of like something Ed from Experimental Airlines would build, only without the red/orange tape... he calls his the Photon. What you and Ed built both look good to me, I might want to build one too.

I thought maybe flaperons would be real good for this ( flat land thermaling ? sloping?), you could camber down with a mix knob to get more lift. Where I live it's flat, no slopes so we fly more thermals.

 

[malik98]

Thank you all for all of your help. Today my glider met it's demise when it started pouring as I was flying and as I walked back to my dorms flying it I landed in a tree, I got it back but it suffered severe water damage. Oddly enough I'm glad it happened because now I get to incorporate every bodies great advice into my next plane which will have a 51" fuselage and 70" polyhedral wing with flaps.

And to Professor Fate I'm glad you enjoy my airplane and may build one in the future, this is actually the first plane I've built without plans and I've learned that it's much more efficient. My plane was indeed based off of the Photon from ExAir and many others I looked at over the interwebs.

Cheers,
Malik

 

[malik98]

I've got new dimensions for the larger plane I've built, still haven't come up with a name yet:
Wingspan: 75 Inches
Root/Tip Chord: 6 Inches
Fuselage Length: 52 Inches
Horz. Stab. Dimensions: 20 in x 5.625 in
Elevator Dimensions: 20 in x 1.25 in
Vert. Stab. Dimensions: 9 in x 5 in
Rudder Dimensions: 9 in x 1.25 in
LE to Propeller: ~10.5 inches (Haven't mounted motor yet)
Motor Thrust: 4.5 lb
Prop Size: 12 Inches
CG: ~1.5 in - 1.98 in from LE
AUW: 27 oz

What do you all think of these dimensions?
My estimated Wing Cubic Loading is 5 and the chart states a glider is under 4, will it glide/thermal well?

I'll try to upload pictures later.

Cheers,
Malik

 

[canuck]

My first glider was the Windrifter a Tom Williams design. A wonderful 2 channel floater with polyhedral wing. I followed that one with the Aquila by Lee Renaud, a more agressive 3 channel glider with spoilers for spot landings.

windrifter


Aquila